This paper introduces a novel beam tracking scheme for full-duplex ground user terminals aiming to transmit uplink and receive downlink from two low Earth orbit (LEO) satellites at the same time and same frequency. Our proposed technique leverages phenomena observed in recent measurements to strategically select transmit and receive beams which couple low selfinterference across the satellites' trajectories, thereby enabling in-band full-duplex operation. By taking a measurement-driven approach, our scheme circumvents the need for explicit selfinterference channel estimation and can inherently account for hardware nonidealities. We show that our proposed scheme reliably selects beams which spatially cancels self-interference to below the noise floor, avoiding the need for digital/analog cancellation. Simulation results using satellite and orbital parameters published in 3GPP and FCC filings show that this substantial reduction in self-interference does not prohibitively compromise beamforming gain, allowing user terminals to attain near-maximal SINRs, thus unlocking full-duplex operation.

This paper investigates a hybrid beamforming structure for wideband satellite communication (SatCom) systems and proposes a novel compression-based hybrid precoding (CHP) method for low computational complexity. Unlike cellular systems with multipath fading channels, SatCom systems, which have a single path channel, show high correlation between precoding vectors generated at adjacent frequencies. The key idea of our approach is to leverage this unique correlation characteristic. By leveraging this, we first derive two compressed precoding matrices that have marginal compression loss and are significantly smaller in size compared to the original precoding matrix. We then design the analog and digital precoding matrices to implement these compressed matrices, considering the constant modulus constraint and computational complexity in the base-band processing. Simulation results verify the superiority of the proposed solution in terms of average spectral efficiency, system throughput, and computational complexity. In particular, the results show that the proposed solution can reduce the computational complexity to 15.5% of what is required by existing methods.

A null-space expansion-based beamforming (NSE-BF) algorithm was recently proposed to mitigate leakage signals from desired users caused by channel variation over time. Studies on the NSE-BF algorithm have revealed that the leakage decreases as the number of previously removed leakage channels increases, but the greater the number of previous leakage channels removed, the smaller the remaining signal components for the desired users. These findings have indicated that increasing the number of previously canceled leakage channels, denoted by Q, does not always lead to improved system performance. Building upon these insights, a new beamforming solution called leakage-based beamforming (L-BF) is introduced. The L-BF solution is designed to achieve the theoretical upper bound of the average spectral efficiency for a given Q. Additionally, a closed-form expression approximating the signal to leakage plus noise ratio (SLNR) performance of the proposed solution, which is directly related to the upper bound, is derived. Using this approximation, a local search algorithm is developed to find the optimal Q that approaches the upper bound. Simulation results confirm the superiority of the L-BF solution in terms of average spectral efficiency, emphasizing the significance of our proposed algorithm.

Frequency-modulated orthogonal frequency division multiplexing (FM-OFDM) is a recently proposed waveform which overcomes the notoriously high peak-to-average power ratio (PAPR) of traditional OFDM wireless systems. Employing this new FM-OFDM waveform, however, will demand a redesign of certain signal processing mechanisms, since those tailored to traditional OFDM waveforms may no longer be optimal or even applicable. This paper proposes two novel power allocation schemes for FM-OFDM systems, both of which take into account this new waveform's unique interactions with noise and with the propagation channel. We analytically derive and optimize closed-form expressions for both of the proposed power allocation schemes, the first of which improves average bit error rate (BER) while the second maximizes spectral efficiency. Our simulation results illustrate the improvements in BER and in data rate when employing our power allocation schemes over conventional benchmarks but also underscore what regimes they should be employed to maximize these gains.

In a wireless communication system, channel aging produces a mismatch between the estimated channel and the channel state at the time of data transmission. This mismatch enhances the interference to users and degrades system capacity. In this paper, we first define a channel aging metric (CAM) to reflect the influence of channel aging on user scheduling. We then propose a CAM-based semiorthogonal user scheduling (CAM-SUS) algorithm that adaptively adjusts the number of serving users according to the user's CAM. Specifically, the CAM-SUS algorithm reduces the number of serving users to mitigate intra-user interference (lUI) when channel aging is severe, and otherwise increases the number to increase the multiplexing gain. Simulation results show the proposed algorithm outperforms the SUS algorithm in a time-varying channel in terms of signal-to-interference-plus-noise ratio by 6–7 dB, and the achievable rate by 39–41 %.

This paper proposes a new access link (AL) and backhaul link (BL) resource management technique for use in 3-dimensional (3D) aerial networks. Increased traffic resulting from the emergence of various applications for unmanned aerial vehicles (UAVs) has resulted in a scarcity of resources. The non-orthogonal shared resource allocation (NSRA) framework proposed here solves the problem of resource scarcity by allocating resources to be shared non-orthogonally between the AL and the BL. In addition, we propose a new selective AL-BL resource sharing algorithm assisted by successive interference cancellation (SIC). We also define the resource sharing allowable zone of the 3D aerial network based on air-to-air channel characteristics. Numerical results show that the proposed algorithm enhances the data rate of the 3D network compared with orthogonal resource allocation.

The low earth orbit (LEO) satellite communication (SatCom) system can provide global communication services. However, LEO SatCom has a larger channel estimation overhead compared to the terrestrial network because of the higher number of equipped antennas and greater number of users being served simultaneously. In this paper, we propose a new global navigation satellite system (GNSS)-based channel estimation strategy that can reduce channel estimation overhead. The proposed strategy exploits the GNSS information that can be obtained in LEO SatCom. We also propose a new grouping algorithm using GNSS-based spatial information, followed by a channel estimation and spatial information updating scheme that utilizes the small and constantly changing angle of arrival (AOA) variation in LEO channels. Simulation results show that the proposed strategy has lower overhead than the conventional least squares (LS) method in the LEO environment.

This paper investigates an asynchronous non-orthogonal multiple access (NOMA) scheme that intentionally generates a frequency offset between users in the orthogonal frequency division multiplexing (OFDM) system. We begin by proposing frequency asynchronous NOMA (FA-NOMA) as a basic system. The characteristics and challenges of FA-NOMA are then described. However, the challenges hinder the utilization of FA-NOMA in a practical environment. To further improve and utilize FA-NOMA in practice, we propose a novel sub-band filtered FA-NOMA (SBFA-NOMA) system. We also provide a new transceiver to implement SBFA-NOMA. Simulation results show that the proposed system can achieve performance improvement via gains in received power and degrees of freedom (DoF) compared to the power-domain NOMA (P-NOMA) system.

Cell-free massive multi-input multi-output (CF-mMIMO) systems assume that a single central processing unit (CPU) communicates with a large number of user equipments (UEs) using the same time-frequency resources. However, allocating the same frame configuration to all the UEs makes it difficult to meet the traffic demands of each individual UE. In this paper, we propose using dynamic-time division duplex (D-TDD) in CF-mMIMO systems. To mitigate the cross-link interference (CLI), we propose a new UE-grouping algorithm. Simulation results show that the implementation of D-TDD reduces the median value of the UL packet delay and the DL packet delay by up to 75% and 30% respectively. Furthermore, the proposed UE-grouping reduces the average packet delay by 50% compared to static-TDD (S-TDD).

Multiple-access edge computation offloading (MECO) systems have been highlighted as a solution for extending the battery life and computation capability of mobile devices. However, in scenarios where information data and computation offloading (CO) data coexist, CO users and information users affect each other. It means that the communication resources for information data transmission are inevitably reduced by the communication resources allocated for CO in the conventional half-duplex (HD) based systems. Hence, improving the spectral efficiency of MECO systems in coexistence scenarios is essential, and we investigate a full-duplex (FD)-aided MECO (FD-MECO) system. A step-wise resource configuration is proposed to improve the performance of computation offloading under the information data rate constraint. The main idea is to improve spectral efficiency by maximizing the opportunity of operating in FD mode. By comparing all the communication phases in FD-MECO systems, the proposed resource configuration maximizes the amount of delivered CO data while guaranteeing the required information data rate. The simulation results show that FD-MECO systems with the proposed resource configuration always outperform HD-MECO systems.

Precise positioning in indoor environments has recently begun to take on greater significance. The characteristics of new radio (NR), such as high frequency-based small cells and broader system bandwidth, can improve the accuracy of observed time difference of arrival (OTDOA) positioning. However, OT-DOA positioning still cannot distinguish the estimated positions of adjacent user equipments (UEs) due to the resolution limitation caused by the sampling period. In this paper, we introduce a method for updating the estimated position of OTDOA using accurate beamforming. Simulation results show the proposed method can improve the ability to distinguish the estimated position of adjacent UEs as well as OTDOA positioning accuracy.

Dynamic heterogeneous networks (HetNets) in which small cells operate in dynamic time division duplex (D-TDD) systems enable the instantaneous traffic load to be handled with greater spectral efficiency than conventional HetNets. However, combining HetNets and D-TDD systems causes cross-link interference (CLI) both within the same tier and across tiers, negatively impacting system performance. In this paper, we point out that small cells are underprivileged in terms of interference mitigation under the conventional spectrum partitioning usage. Accordingly, we propose employing a novel spectrum partitioning scheme that brings up interference scenarios where we can take advantage of multiple-input multiple-output (MIMO) compatibility from macro base stations. Simulation results show that the proposed spectrum usage in combination with general MIMO schemes achieves significant performance enhancement in terms of small cell capacity at the cost of a relatively small degradation in macro cell capacity.

The grant-free non-orthogonal multiple access (GF-NOMA) system, in which users autonomously transmit uplink data signals without carrying out a complicated random access process, offers significant advantages for increasing the efficiency of resource usage with low signaling overhead. In the proposed GF-NOMA system, since the collision resolution process is omitted and resources are not pre-assigned to users, multiple users may use same resource, without the base station (BS) being able to identify these collided users. In addition, in the absence of the UL synchronization procedure generally performed in the random access process, multiple users' signals are received asynchronously. To solve these problems, we have developed a new transceiver structure for the GF-NOMA system consisting of a secondary preamble and a multiuser-multisymbol (MUMS) detector. The purpose of the secondary preamble is to successfully detect user access and estimate channel information even when no collision resolution process is being performed. The MUMS receiver is then proposed as a way to effectively mitigate the severe interference caused by the asynchronous transmission. Simulation results show that the proposed preamble structure and the receiver structure are superior to the conventional schemes. Furthermore, we show that the GF-NOMA system with the proposed transceiver structure achieves much better performance than other GF-NOMA systems in terms of reliability and successfully detected bits.

In this paper, we propose a novel location-based operation strategy for Device-to-Device (D2D) communication which exploits successive interference cancellation (SIC). First, we investigate the characteristics of SIC-enabled D2D, and then propose a new resource selection scheme to fully exploit cellular uplink (UL) resources and downlink (DL) resources using only location information. Second, we propose a new power allocation scheme designed to overcome the problem of excess outages in cellular users. Simulation results show that the proposed schemes show good D2D throughput performance compared to the existing schemes, while guaranteeing reliability for cellular users.

In the centralized mode of the cellular-based vehicle-to-vehicle (C-V2V) system, the base station performs resource allocation based on location information from the vehicle user equipments (VUEs). To implement this location-based resource allocation, the base station and VUEs frequently exchange driving information with each other. This causes serious control overhead which includes the signaling that occurs between the base station and VUEs and the computing power needed to perform location-based resource allocation. In this paper, we propose a new hybrid resource allocation scheme designed to reduce control overhead while satisfying high communication reliability. The proposed scheme is composed of two phases, a centralized phase and a distributed phase. In the centralized phase, the base station performs location-based resource allocation. In the distributed phase, VUEs autonomously select the resource occupied by the VUE in front of them. This procedure fixes the geographical locations of the resources to specific locations and consequently reduces the frequency with which the base station performs resource allocation. Simulation results show that the proposed resource allocation scheme reduces the control overhead of the conventional resource allocation with centralized mode.

As an approach to reduce the computational complexity of Filtered-Orthogonal Frequency Division Multiplexing (F-OFDM), we removed restrictions on filter lengths in F-OFDM and then applied linear pre- and post-processing for a transmitter and receiver using the singular value decomposition (SVD) method in order to solve the data distortion caused by the use of filter lengths much shorter than the symbol duration. This new F-OFDM has been given the designation SVD-based F-OFDM (SF-OFDM). Numerical results show that SF-OFDM has much lower complexity than F-OFDM, especially when transmitting small numbers of resource blocks. SF-OFDM also has nearly the same bit error rate performance as conventional F-OFDM. Moreover, SF-OFDM retains a much lower out-of-band emission than cyclic prefix (CP)-OFDM, while at the same time having a similar level of peak to average power ratio compared to CP-OFDM.

The following topics are dealt with: cellular radio; 5G mobile communication; MIMO communication; vehicular ad hoc networks; probability; telecommunication power management; wireless channels; telecommunication network reliability; resource allocation; optimisation.

In this paper, we consider the user fairness in partial non-orthogonal multiple access (P-NOMA) system which was proposed in our previous work. In the P-NOMA system, the interference of NOMA can be managed by adjusting the overlap ratio between users. In our previous works, we considered the enhancement of sum of achievable rate. In this paper, we investigate the performance of P-NOMA in terms of user fairness. For reflecting user fairness on P-NOMA systems, we first define the metric to represent user fairness and then observe the target performance metric according to overlap ratio. In addition, we propose a feasible operating method to achieve the user fairness.

Dynamic time division duplex (D-TDD) maximizes resource utilization by dynamically allocating resources to the downlink and uplink according to variations in traffic conditions. The new 5th generation (5G) mobile communication standard has reflected D-TDD in the frame structure. However, D- TDD has inherent cross-link interference (CLI) problems. Although many CLI mitigation schemes have been proposed as part of the standardization of the 5G system, none of these schemes have solved the multifaceted CLI problem. In this paper, we analyze the CLI mitigation schemes by dividing them into proactive and reactive scheme categories considering operational principles. Through this categorization process, we found that combining CLI mitigation schemes can solve the CLI problem more effectively. The performance of the various CLI mitigation schemes and combinations thereof is evaluated using a system level simulator. The best combination of CLI mitigation schemes is also derived.

In a long term evolution based vehicle-to-vehicle (LTE-V2V) system, vehicle user equipment units (VUEs) broadcast their driving information to neighboring VUEs. However, they cannot receive packets from others during transmission because they operate in half-duplex (HD) mode. Moreover, adjacent channel interference further degrades the communication reliability. To overcome these problems, we propose a permuted resource allocation (RA) scheme based on the grouping of neighboring VUEs. Through a theoretical analysis for the proposed scheme, a closed-form expression of the approximated reliability is derived. Based on this, we also suggest an adaptive resource size control mechanism for the proposed method. Numerical results show that the proposed method achieves higher reliability than conventional geo-based RA in practical road environments.

We consider a mixed numerology spectrum sharing (SS) system where the users have different subcarrier spacing (SCS). Unlike in single numerology SS systems, mixed numerology SS systems suffer from inter-numerology interference (INI). We first derive the interference pattern and find that the variance of interference energy increases due to the difference in SCS. This increase in variance negatively affects decoding performance, since the interference energy is unbalanced between subcarriers. However, previous works on this issue did not take this interference pattern into account for interference management. In order to suppress the increase in variance of the interference energy, we propose a transceiver structure for large SCS users by using simple cyclic shift and frequency shift operations. The proposed transceiver disperses the effect of INI, reducing the variance in the interference energy. From the derived distribution characteristics of the interference energy in closed-form expressions, we show that the proposed transceiver achieves better decoding performance.

This paper investigates a grant-free non-orthogonal multiple access (NOMA) system where massive number of users wake up and send data right away without performing a grant-based initial access procedure. Since no scheduling grant is used in this system, the base station (BS) does not know which user is transmitting on each resource. Also, due to the lack of an uplink (UL) timing control process, multiple user's signals are asynchronously received at the BS. This causes asynchronous interference, or multiuser inter-carrier and inter-symbol interference. We propose two ideas to address these two problems. We first propose an auxiliary preamble structure to successfully detect the user activity, even in the presence of a massive number of users. We, then, propose a modification to the interleave-division multiple access (IDMA) receiver to mitigate asynchronous interference. The simulation results show that the proposed scheme significantly improves the preamble detection performance and BER performance compared to the conventional schemes. Furthermore, we show that the proposed grant-free NOMA system can achieve much better performance than the grant-based NOMA system in terms of transmission time and signaling overhead.

In this paper, we propose joint power allocation and modulation and coding scheme (MCS) selection for downlink non-orthogonal multiple access system considering max-min user fairness. The proposed scheme determines the power allocation factor according to the MCS level of the far user. It allocates the maximum fraction of power to the near user while guaranteeing the decoding of the far user, so that it can improve data rate of both users and achieve user fairness. Also, since there is a finite number of MCS levels in the practical system, the complexity of the proposed scheme is greatly reduced. Finally, the simulation results show that the proposed scheme achieves the performance close to the maximum of minimum user rate within a small number of iterations.

Sparse code multiple access (SCMA) is the most prominent non-orthogonal multiple access (NOMA) scheme considered as massive connectivity. Because SCMA users transmit information using the same time-frequency resources, properly estimating each user's channel information is a challenging issue. In this paper, we propose a channel estimator in the uplink SCMA system. We design a pilot structure based on a cyclically shifted Zadoff-Chu (ZC) sequence. The proposed algorithm using the autocorrelation property of the ZC sequence separates each user's channel information and estimates each user's channel frequency response. In addition, we calculate the number of required training blocks and prove that the number of training blocks in the proposed algorithm is lower than the number of needed in conventional channel estimation techniques. In simulation results, we compare the mean squared error (MSE) of the proposed algorithm with conventional approaches.

This paper proposes a new multi-user detection scheme for sparse code multiple access (SCMA) system in grant- free scenario. SCMA systems use a message passing algorithm which considers all user equipments (UEs) to be active. However, in a practical system, UEs do not always request resources. A multi-user detection procedure enables the decoder to mitigate interference from proper active UEs and to ignore the signal from inactive UEs. Therefore, we propose a multi-user detection scheme for SCMA system. The simulation results present that the proposed SCMA receiver structure achieves a remarkable enhancement in false alarm probability, miss detection probability.

5G New Radio is expected to employ multiple numerologies where diverse subcarrier spacings coexist to satisfy the requirements and channel characteristics of users. Since conventional autocorrelation-based time synchronization methods are designed for orthogonal frequency division multiplexing (OFDM) systems with a single numerology, they cannot be applied in OFDM-based multiple numerologies. We propose a new preamble structure for use in OFDM system with multiple numerologies. The proposed preamble is generated by overlapping multiple preamble components which have a repetitive structure and an amplified cyclic prefix. With these features, a different time metric corresponding to each numerology can be independently obtained from the proposed preamble. Then, each user can perform time synchronization process with the system parameter corresponding to their numerology, regardless of other user's numerology.

This paper proposes a new generalized frequency division multiplexing (GFDM) system that eliminates the effects of intrinsic interference and makes it possible to insert a pilot for channel estimation without interference. We express inter-subsymbol interference (ISI) and inter-carrier interference (ICI), which represent the intrinsic interference in GFDM systems, in a matrix form. The proposed GFDM system can remove the ISI through pre-processing and post-processing, which are done by eigendecomposition. We analytically derive the sufficient condition for the ICI removal. In this way, the proposed GFDM system is able to eliminate the effects of both the ISI and ICI. Furthermore, we investigate the prototype filter structures of the proposed GFDM system transformed by pre-processing and post-processing. We verify that the changed prototype filter structures are able to insert pilot symbols that are orthogonal to data symbols. Hence, the pilot symbols for channel estimation can be clearly observed. Simulation results demonstrate that the proposed system has better BER performance than conventional GFDM systems when the channel estimation process is considered.

In this paper, we propose a linearly processed filter-bank multicarrier (LP-FBMC) system which employs faster than Nyquist signaling to remove residual intrinsic interferences. We represent the FBMC transceiver in a matrix form and then apply singular value decomposition-based linear transformation in order to convert the overlapped FBMC data into parallel independent interference-free data. This interference-free characteristic of the proposed LP-FBMC system enables the use of conventional multiple antenna configurations. Performance evaluations show that out-of-band emission performance of the proposed LP-FBMC system is superior to those of conventional quadrature amplitude-based FBMC (FBMC-QAM) and orthogonal frequency division multiplexing systems. Furthermore, the proposed LP-FBMC system outperforms the conventional FBMC-QAM in terms of bit error rate performance when high order modulation is employed under time-varying channel and multiuser uplink environment.

In this paper, we consider a new interference problem caused by idle small cells, which have no associated user equipment in ultra-dense small cell long-term evolution (LTE) networks. Specifically, we investigate the effect of idle small cells on the signal to interference and noise ratio (SINR) of the cell-specific reference signal (CRS) and the data signals. We confirm that CRS interference from idle small cells produces uneven interference pattern across CRS and data signals and eventually causes an SINR mismatch between CRS and data signals as well as between data signals with and without CRS symbols. In addition, these phenomena become severe with cell densification. In order to solve this mismatch problem, we propose a simple link adaptation framework, which utilizes clustered CRS assignment and hybrid SINR measurement. The numerical results show that the proposed method improves the average sum throughput compared with the conventional approaches. Overall, this paper sheds new light on investigating and coping with the interference problem coming from idle small cells in future ultra-dense small cell LTE networks.

Full-duplex (FD) systems ideally double the system capacity compared to conventional half-duplex (HD) systems. However, we can obtain that goal only when FD systems always have both downlink (DL) and uplink (UL) traffic loads. This paper sheds new light on whether FD can achieve this promising performance enhancement in more realistic traffic settings. We first provide the sum capacity of the FD system when DL and UL traffic loads are different. In addition, we define the FD gain according to traffic loads and compare FD performance with the HD system which is based on time-division duplex (TDD). We then analyze the effect of traffic loads and residual self- interference (SI) on the FD system. Simulation results and analysis reveal that FD performance decreases in a limited traffic setting.

This paper proposes a subband-wise filtering with different filter orders in filter bank multi-carrier (FBMC) system. The objective of the proposed scheme is to improve the time and frequency efficiency of the FBMC for short burst transmission in future communication systems. The proposed scheme divides the band into two subbands, which are at the inner and the outer of the band, and uses the high filter order to the outer subband and the low filter order to the inner subband. Numerical results show that the proposed scheme has the high time and frequency efficiency when transmitting short burst. Moreover, the proposed scheme also has the nearly same bit error rate performance compared to the conventional FBMC.

Recently, the orthogonal code-based block transmission (OCBT) systems have been proposed to fulfill the explosive growth of traffic demands. The multi-carrier-based structure of OCBT, however, requires accurate synchronization to combat inter-symbol interference (ISI) and inter-carrier interference (ICI). Fortunately, the repetitive pattern contained in the OCBT symbol enables this synchronization without preambles. In this paper, we propose a novel time-frequency offset estimation method based on the repetition characteristic. Simulation results demonstrate that the proposed estimator provides acceptable root-mean-square error (RMSE) performance. Moreover, we verify that the OCBT systems with the proposed method achieve remarkable throughput enhancement compared with conventional OFDM systems.

This paper investigates timing and frequency synchronization applicable for filter bank multicarrier (FBMC) systems. Filtering and symbol overlap make designing a training period that has a specific block pattern quite difficult. To address this, a receiver architecture that exploits synchronization structure in a simple way is proposed. Specifically, the receiver performs observation windowing and weighted correlation processes in order to overcome the problems from overlap and filtering, respectively. Simulation results show that the proposed method achieves accurate synchronization while utilizing a minimum of resources.

Ultra-dense small cell networks have been proposed as a way of addressing the explosive growth of data traffic. However, this approach often results in empty cells which only end up causing interference instead of serving UEs because they do not have any associated UEs. In this paper, we investigate the empty cell problem using system-level simulation and propose a dual layer on/off strategy for solving the empty cell problem. Systemlevel simulation results verify that the proposed scheme achieves performance enhancement in terms of capacity compared with conventional ultra- dense small cell networks.

When two nodes communicate using in-band full-duplex (FD) with orthogonal frequency division multiplexing (OFDM), the self-interference (SI) and desired signals experience a time and frequency misalignment problem. In this article, we introduce the effect of timing and frequency offsets in FD systems. A conventional FD node simultaneously receives two signals: the desired signal and the SI. The FD node then synchronizes based on the desired signal, and the SI is distorted by timing and frequency offsets. These timing and frequency offsets break orthogonality and create inter-symbol interference and inter-channel interference that degrade the performance of the self-interference cancellation (SIC). To address this, we design a modified digital SIC approach that considers the timing and frequency offset and synchronizes the desired signal and the SI in consecutive order.

The dynamic time division duplex (TDD) system has been proposed as a way to meet today's asymmetrically and dynamically changing traffic demand. However, this approach causes cross-link interference, because neighboring base stations and user elements transmit in opposite directions. In this paper, we investigate and analyze the characteristics of cross-link interference in dynamic TDD systems. Based on this observation, we propose an aligned reverse frame structure to utilize and cancel the cross-link interference. Mathematical analysis and numerical results verify that the proposed scheme achieves performance enhancement in terms of capacity compared with conventional dynamic TDD systems.

This paper presents the performance analysis of the self-interference cancellation (SIC) methods in full-duplex large-scale multiple-input multiple-output systems. To mitigate self-interference (SI), we assume that the full duplex-base station (BS) uses SI-subtraction or spatial suppression. Analytical and numerical results confirm that the SI-subtraction outperforms the spatial suppression for SIC in a perfect channel estimation case. It is also concluded that the uplink and the overall ergodic rates performance of the spatial suppression are respectively better than those of the SI-subtraction in an imperfect channel estimation case under a given system constraint such as uplink and downlink sum rates.

In this paper, we investigate the effect of channel estimation (CE) error on the performances for the ultra-dense small cell networks. Specifically, we examine its effect on the distribution of signal to interference and noise ratio (SINR) and trend of spectral efficiency (SE), and outage probability. From these, we found out that the SINR degradation due to the CE errors becomes larger, as the density ratio of small base station (BS) to UE increases. We derived the theoretical lower bound of the outage probability, which is the function of CE correlation coefficient. Finally, numerical results demonstrate that the CE errors diminish the increasing scaling of SE, as the small BS density increases.

Filter-bank based multi-carrier systems have recently attracted lots of interest as a promising approach for the next generation wireless communication systems. Due to high complexity and long symbol duration, however, these systems need to evolve in a new multi-carrier technique, universal-filtered multi-carrier (UFMC). In conventional UFMC systems, a fixed size of resource block (RB) reduces the flexibility of spectrum utilization and leads to high computational complexity. This problem highlights the need for RB size control that will efficiently allocate frequency resource to satisfy users' resource demand. In this paper, we propose a generalized UFMC system for uplink scenario. First, we analyze the impact of RB size on the UFMC spectrum utilization and complexity. Then, we observe the effect of filter length on symbol error rate (SER). Based on this observation, the proposed UFMC system controls RB size and filter length according to users' demand. Finally, we demonstrate that the proposed system tremendously improves throughput per sub-carrier without SER performance degradation.

In this paper, we propose a new sequence generation methodology to make a constant amplitude zero autocorrelation (CAZAC) preamble available in offset Quadrature Amplitude Modulation-based filter bank multicarrier (FBMC) synchronization. The role of the new sequence is to compensate the filtering and symbol overlap of the FBMC structure so as to finally be able to make a CAZAC preamble in time domain. To derive the new sequence, we formulate a symbol overlap model that shows the relation between the new sequence and a CAZAC preamble. On the other hand, the offset modulation format is a constraint that needs to be considered in the design of the new sequence. By using a property of this offset modulation constraint, we derive a closed-form expression of the new sequence from the symbol overlap model. In addition, we extend an advanced model to create a pseudo-CAZAC preamble that is not data-dependent, thus enabling convenience of implementation. Simulation results show the superiority of the proposed model with respect to synchronization accuracy, out-of-band radiation, and peak to average power ratio. Moreover, we verify that the proposed model is well-suited for use in the cellular environment through complexity and resource efficiency comparisons.

A filter-bank multicarrier-quadrature amplitude modulation (FBMC-QAM) system with two prototype filters for transmitting QAM symbol is proposed, and the orthogonality conditions for the FBMC-QAM system without the intrinsic interference is derived. The proposed transmitter performs the individual filtering for the even and odd-numbered sub-carrier symbols, respectively. In order to satisfy the derived orthogonality conditions, we perform a sub-block wise reverse ordering procedure for the outputs of the odd-numbered sub-carrier filter. Also, this paper shows that the proposed FBMC-QAM system satisfies the suggested orthogonality conditions and can use the multiple-input multiple-output transmission schemes as the conventional orthogonal frequency division multiplexing (OFDM) systems. The signal-to-interference power ratio and the bit error rate (BER) for the proposed FBMC-QAM system are evaluated. Numerical results show that the proposed FBMC-QAM system has almost the same BER performance compared with the FBMC-OQAM and OFDM systems.

In this paper, we investigate the impact of outdated channel state information (CSI) on the capacity of an opportunistic relaying system. We derive the analytical expressions of the channel capacity and its associated outage probability of reactive relay selection for four adaptive transmission techniques. Analytical and numerical results show that the rate adaptation policy achieves better performance compared to the sub-optimal channel inversion in the presence of outdated CSI. In addition, design insights and useful observations for relay selection can be deduced from these results, implying that CSI accuracy in the source-relay link plays a significant role in determining the relay selection strategy for given system parameters.

In in-band full-duplex employing conventional frame structure, pilot contamination is inevitable due to asyn-chronism between the nodes. Asynchronism can occur for several reasons such as propagation delay and synchronization error. Pilot contamination increases channel estimation error for both the desired and self-interference channels. We propose a new frame structure that avoids interference during pilot transmission. The proposed frame structure utilizes maximum time offset information in order to cover all the possible time offsets. We then derive the mean square error (MSE) of the channel estimation when the proposed frame structure is employed. Furthermore, based on the MSE analysis, we deliver superior conditions for the proposed frame structure compared to the conventional one. Finally, we show that sacrificing pilot length to avoid interference during pilot transmission guarantees channel estimation performance when the power of the interference is large.

Licensed band communication systems have recently begun trying to utilize unlicensed bands. Due to lack of coordination, however, this spectrum utilization results in strong interference to other unlicensed band equipment (UBE). This problem highlights the need for coexistence scenario that will prevent UBE performance degradation. In this paper, we propose a new access control mechanism for downlink cellular networks using the unlicensed band. First, we analyze the impact of the cellular networks on the UBE outage performance. Based on the analysis, we then obtain a decision threshold for the unlicensed band access control. In the proposed control mechanism, the decision threshold is adaptively controlled by considering the density of the UBE. The numerical results demonstrate that the proposed access control mechanism guarantees the UBE's target performance under various network circumstances.

In this paper, we propose a uplink (UL) non-orthogonal multiple access (NOMA) systems equipped with multiple antennas to enhance the sum capacity of UL. In the proposed UL NOMA system, a base station (BS) with N antennas can support 2N users by sharing the space resource. However, sharing the space resource leads to interference between users. Therefore, to mitigate the interference and improve the sum capacity, we additionally propose a set selection algorithm and power control scheme. The set selection algorithm, which selects 2N users based on orthogonality and gain-difference between their channels, reduces the interference between users. Furthermore, the power control scheme can maximize the sum capacity with ensuring minimum target rate. Numerical results show that the proposed system improves the sum capacity over that of the conventional orthogonal multiple access (OMA) system.

In this paper, we consider IEEE 802.15.4 based wireless body area sensor networks(WBASN) in the healthcare environment. Connectivity is a critical constraint in the healthcare environment. In particular, disconnection from a sensor hub is a significant problem that leads to sensor node orphaning. To address this, we propose to use a sub hub designed to enhance WBASN connectivity. The sub hub understudies the original (main) sensor hub and acts to resolve the problems associated with disconnection. We also propose two protocols for use in operating the sub hub. Each protocol prevents the sensor nodes from being orphaned, and forwards data from the sensor nodes to the original sensor hub. In addition, we evaluate the connectivity performance of our proposed protocols and analyze the disconnection tolerance and the number of packet drops. To demonstrate the performances of the proposed protocols, we developed an IEEE 802.15.4 compliant system level simulator with an OPNET Modeler. Simulation results show the individual merits and the demerits of each of the protocols in terms of connectivity. In addition, the energy efficiency of our proposed protocol is evaluated.

Cognitive radio networks using relays have been introduced to enhance secondary network performance by having secondary users (SUs) transmit in two hops. This paper develops a framework for a random cognitive relay network (RCRN) where the primary, secondary, and relay nodes are randomly distributed according to Poisson point processes (PPPs) to investigate the effect of the random nature of the relays on secondary outage performance. We first derive the optimal relay location in terms of the outage probability of the secondary network. In addition, we provide the outage probability of the RCRN based on the optimal relay selection strategy that compares the instantaneous channel state information (CSI) of all relays. We then propose an efficient relay selection strategy that reduces the selection complexity and the feedback burden by limiting candidate relays to those located close to the optimal relay location. This paper sheds new light on the design of an efficient RCRN with respect to elements such as the effects of network parameters and the candidate relay set size on the RCRN outage probability.

A filter-bank multicarrier - quadrature amplitude modulation (FBMC-QAM) system with two prototype filters for transmitting QAM signal is proposed. The proposed transmitter performs separate filtering for the even-numbered sub-carrier symbols and the odd-numbered sub-carrier symbols. We derive the orthogonality conditions for a FBMC-QAM system without the intrinsic interference. In order to satisfy the suggested orthogonality conditions, we perform a kind of block interleaving for the odd-numbered sub-carrier filtering. The receiver structure is the counterpart to the transmitter. Numerical results showed that the proposed FBMC-QAM system has almost the same bit error rate (BER) performance compared to the FBMC-OQAM and the orthogonal frequency division multiplexing (OFDM) system. with the proposed FBMC-QAM, multiple-input multiple-output transmission schemes and channel estimation schemes can be utilized similarly as in OFDM.

We examine small cell deployment strategies in two- tier cellular networks. Specifically, we propose a joint interference and load deployment (JD) strategy, reflecting a realistic interference environment and traffic load. In this JD, the location of a small base station (s-BS) is sequentially decided in greedy manner by exploiting both the spatial user equipment (UE) distribution and the received signal strength indicator (RSSI). Furthermore, we propose a deterministic deployment (DD), which does not require this RSSI measurement. In DD, the deployed pattern of s-BSs is the same for each macro cell. System level simulation (SLS) results demonstrate that the proposed schemes yield a cell throughput gain of up to 61%, when the UEs are densely placed near a m-BS. Moreover, 5%-tile UE throughput and median UE throughput are improved by up to 134% and 76%, respectively, when UEs are densely distributed far from the m-BS. Furthermore, the SLS results provide intuition on between load and interference in the s-BS deployment as follows: 1) load is more important than interference in the macro edge region, 2) interference is more significant than load when the s-BS is deployed close to the m-BS.

In this paper, we investigate the impact of outdated channel state information (CSI) on the channel capacity of an opportunistic relaying system in conjunction with adaptive transmission techniques. We derive the closed-form expressions of the channel capacity and its associated outage probability for four adaptive transmission techniques: (1) optimal rate adaptation with constant transmit power, (2) optimal transmit power and rate adaptation, (3) channel inversion with fixed rate, and (4) truncated channel inversion with fixed rate. Analytical and numerical results show the effects of outdated CSI on the capacity of the optimal power and rate adaptation and the sub-optimal channel inversion techniques. In addition, we compare the channel capacity among various adaptive transmission techniques in the presence of outdated CSI, thereby illustrating the intuitive tradeoff between the achievable capacity and implementation complexity.

We consider an energy-harvesting cognitive radio system where the secondary transmitter harvests energy. This system operates under an energy causality constraint mandating that the average energy consumption must not exceed the average harvested energy, and a collision constraint mandating the protection of the primary system. The purpose of this paper is to identify the optimal pairing of the sensing duration and the energy detector's sensing threshold in order to maximize the average throughput of the secondary network. Since the sensing duration and sensing threshold are intertwined with the energy causality constraint, they need to be redesigned with the purpose of conserving energy in mind. Hence, the sensing duration must be shorter while still satisfying the collision constraint. The numerical results show that the optimal sensing duration is determined based on which constraint, collision or energy causality, needs to have priority. In addition, the simulation results show a pairing of the optimal sensing duration and sensing threshold provided by the coordination between the two constraints, which gives insight into how to design them.

In cognitive radio, spectrum sensing is used to find the white spectrum or protect the primary user from interference caused by the secondary user (SU). There are two conventional spectrum sensing approaches: quiet and active. However, these conventional approaches have several problems. In quiet sensing, the quiet period degrades the SU capacity. With active sensing, the SU capacity is also degraded by the need for additional resource consumption and the mismatch in feedback information. In order to mitigate these problems, the structure of simultaneous PU sensing and data transmission is introduced. This structure is equipped with antenna isolation and self-interference cancellation in which the communication and the sensing radios are already assumed to be significantly isolated. This approach is designed so that the SU transmitter can sense PU signals and transmit data signals at the same time by dividing its spatial resources. Expanding on this work, we propose a concept of "TranSensing" which adaptively uses spatial resource according to the surrounding environments. To effectively use TranSensing, we propose a two-stage algorithm (TSA). Finally, the impact of residual interference on TranSensing is investigated. Simulation results show that TranSensing with TSA enhances the SU capacity over the conventional quiet or active sensing.

We consider energy harvesting cognitive radio networks to improve both energy efficiency and spectral efficiency. The goal of this paper is to analyze the theoretically achievable throughput of the secondary transmitter, which harvests energy from ambient sources or wireless power transfer systems while opportunistically accessing the spectrum licensed to the primary network. By modeling the temporal correlation of the primary traffic according to a time-homogeneous discrete Markov process, we derive the upper bound on the achievable throughput as a function of the energy arrival rate, the temporal correlation of the primary traffic, and the detection threshold for a spectrum sensor. The optimal detection threshold is then derived to maximize the upper bound on the achievable throughput under an energy causality constraint and a collision constraint. The energy causality constraint mandates that the total consumed energy should not exceed the total harvested energy, while the collision constraint is required to protect the primary network from secondary transmission. Analytical results show the temporal correlation of the primary traffic to enable efficient usage of the harvested energy by preventing the secondary transmitter from accessing the spectrum that may be occupied by the primary network.

We consider a clustered wireless network composed of energy-harvesting access points (APs) to improve both spectrum efficiency and energy efficiency. Each AP only uses the harvested energy for transmission and becomes inactive when the harvested energy is less than the required energy for transmission. The number of inactive APs increases as the number of users being served by an AP increases. Considering the relationship between the number of served users and the active AP density, we propose a spectrum-efficient operating policy designed to ensure that an AP serves the closest n users rather than all users in the cluster. We derive the outage probability and the area spectral efficiency (ASE) for an energy-harvesting clustered wireless network in closed form using stochastic geometry. Numerical results show that there is an optimal number of served users per AP that maximizes the ASE, which verifies that the ASE can be increased by the proposed spectrum-efficient operating policy.

This paper proposes an optimal transmit power control policy designed to maximize the average throughput of an energy-harvesting secondary network. As the transmit power increases, the energy-harvesting secondary network can enhance its throughput during a slot, but the probability of being active decreases due to the energy shortage. The probability of colliding with the primary signal also decreases as the transmit power increases. The optimal transmit power can be obtained by analyzing the effect of the transmit power on performance, especially as a function of statistics of harvested energy.

Portable unlicensed devices operating in TV white space must adhere to FCC rules designed to protect licensed devices. These rules decrease the achievable throughput for the portable unlicensed devices because the portable unlicensed devices are required to perform a channel availability query (CAQ) before transmitting. In order to reduce the number of CAQs, the Federal Communications Commission allows a region based mobility procedure (RMP). In this paper, we propose a new-RMP using directional information in order to enhance the achievable throughput. Directional information enables an unlicensed portable device to increase its achievable throughput by considering a uni- directional region that is smaller than an omni-directional region. Simulation results show that the proposed RMPs enhance the achievable throughput compared to the conventional RMPs. In addition, the achievable throughput for the proposed RMP using a navigation system is greatly enhanced compared to the conventional RMPs.

This paper analyzes large-scale networks that share the spectrum with interference cancellation (IC). The efficiency of spectrum sharing is determined primarily by interference, which in turn depends on the spatial densities, the interference cancellation method, and the spectrum sharing method, i.e., underlay or overlay. By assuming the Poisson distribution for transmitters, equal transmission power in the same system, and an interference-limited environment, this paper finds the performance gain from IC in terms of spectrum-sharing transmission capacity (S-TC), defined as the number of successful transmissions per unit area while guaranteeing the target outage probabilities of all coexisting systems. The effectiveness of IC is characterized by the coefficient of cancellation (CoC), and specific CoC values are derived for two simple IC scenarios, the strong interferer and the close interferer cancellation, with the assumption of having perfect information for channel states of interfering links and interferer locations. The sum S-TC optimal spatial densities of the two systems are given. Finally, CoC conditions to determine the superiority of an underlay or overlay method are presented. We verify that the underlay method could be preferred depending on the CoCs of coexisting systems; that is starkly different from the case without IC, in which the overlay method is always better.

In this paper, we investigate the effects of outdated channel state information (CSI) on an opportunistic relaying scheme (ORS) consisting of relay selection and data transmission phases. The outage probability and average symbol error rate (ASER) of the ORS are derived in a closed-form expression for an arbitrary link signal-to-noise ratio (SNR) when the CSIs in the source-relay and relay-destination links are outdated. We also analyze the diversity order and coding gain in terms of the outage probability and ASER. Analytical and numerical results show that when the CSIs in either the source-relay or relay-destination links are outdated, the diversity order is reduced to one. The overall performance of the ORS is mainly degraded by inaccurate CSI in the relay-destination link. Conversely, CSI accuracy in the source-relay link has a negligible effect on the performance of the ORS unless the CSI is perfect.

In this paper, we propose an optimal tone space selection scheme for orthogonal frequency division multiple access with variable tone spaces (OFDMA-VTS) in carrier aggregation based on user-specific channels. We begin by analyzing inter-symbol interference (ISI) and inter-carrier interference (ICI) in non-contiguous carrier aggregation, which differ according to the delay and Doppler spreads associated with each user, as well as the tone space that each user utilizes. Based on that, we propose an optimal tone space selection scheme as a way to minimize both ISI and ICI. We also derive an upper bound on OFDMA-VTS capacity when it is applied to the proposed scheme. Furthermore, we investigate the effect of inter-user interference (IUI) in contiguous carrier aggregation and show that the proposed scheme can be applied to OFDMA-VTS in contiguous carrier aggregation as a suboptimal solution. Numerical results demonstrate that the proposed scheme maximizes OFDMA-VTS capacity by minimizing the total ISI and ICI interference, thereby enhancing the system capacity over what is possible with conventional OFDMA.

We consider energy harvesting cognitive radio networks in which a secondary transmitter harvests energy from ambient sources or wireless power transfer systems while opportunistically accessing the spectrum licensed to the primary network. The primary traffic is modeled as a time-homogeneous discrete Markov process, and the secondary transmitter may not be able to operate continuously due to sporadic and unstable energy sources. At the beginning of each time slot, the secondary transmitter thus needs to determine whether to remain idle so as to conserve energy, or to execute spectrum sensing to acquire knowledge of the current spectrum occupancy state. It also needs to configure the spectrum sensor detection threshold to achieve an effective tradeoff between false alarms and misdetections. This sequential decision-making, done to maximize the expected total throughput, requires the joint design of a spectrum sensing policy and a detection threshold under the energy causality and collision constraints. We formulate this stochastic optimization problem as a constrained partially observable Markov decision process (POMDP), and then convert it to a computationally tractable unconstrained POMDP. Numerical results show that the proposed approach enables efficient usage of the harvested energy by exploiting the temporal correlation of the primary traffic.

We consider a cognitive radio network with an energy-harvesting secondary transmitter to improve both energy efficiency and spectral efficiency. The goal of this paper is to determine an optimal spectrum sensing policy that maximizes the expected total throughput subject to an energy causality constraint and a collision constraint. The energy causality constraint comes from the fact that the total consumed energy should be equal to or less than the total harvested energy, while the collision constraint is required to protect the primary user. We first show that the system can be divided into a spectrum-limited regime and an energy-limited regime depending on where the detection threshold for the spectrum sensor lies. Assuming infinite battery capacity, we derive the optimal detection threshold that maximizes the expected total throughput subject to the energy causality constraint and the collision constraint. Analytical and numerical results show that the system is energy-limited if the energy arrival rate is lower than the expected energy consumption for a single spectrum access. They also show that a decreasing probability of accessing the occupied spectrum does not always result in decreased probability of accessing the idle spectrum in the energy-limited regime.

This paper investigates an optimal mode selection policy for cognitive radio sensor networks powered by RF energy harvesting. The RF energy harvesting enables the sensor node to operate with a potentially perpetual lifetime. We assume that the sensor node harvests RF energy received from the primary network and it cannot carry out RF energy harvesting and opportunistic spectrum access at the same time. Therefore, the sensor node should decide whether to access the spectrum or to harvest RF energy in each time slot to maximize an expected total throughput. We develop the optimal mode selection policy by casting this decision making problem in the framework of partially observable Markov decision process (POMDP). Numerical results show that the developed optimal policy finds a balance between obtaining the immediate throughput and harvesting the RF energy for future use.

Cooperative spectrum sensing improves sensing accuracy in primary user detection, but can be threatened by malicious users. Malicious users may try to falsify the sensing result to indicate that the primary user exists even when there is no primary user in order to monopolize the spectrum usage, thereby depriving other users of their spectrum opportunities. To address this, we propose a malicious user detection scheme where the malicious users are identified and cut off from the cooperative sensing process. The proposed scheme exploits the Anderson-Darling (AD) goodness-of-fit technique which tests whether the empirical distribution of the sensing data from each secondary user fits the expected distribution for a malicious user. In addition, we derive false alarm and detection probabilities for when malicious users are cut off by the malicious user detection scheme. Simulation results show that the proposed goodness-of-fit-based malicious user detection significantly improves sensing performance in comparison with conventional outlier detection-based schemes.

This work investigates the effects of full duplexity on delay and throughput in interference-limited multi-hop networks. It is assumed that channel state information is not available at transmitters and beamforming is employed as the transmission scheme. A closed-form expression is developed for hop success probability in networks of this type. The delay and throughput of multi-hop networks employing full duplex and half duplex beamforming are than compared. The delay and throughput are studied in two cases: when full duplexity is used for full duplex relay (FDR), and bi-directional beamforming (BBF). For FDR, employing full duplex beamforming is more beneficial if a low rate constraint is required or if interferers are rare. However, this benefit diminishes as the required data rate or the interferer density increases. For BBF, the use of full duplex beamforming is always more beneficial because it leads to a reduction in the required rate constraint. Specifically, this benefit becomes more significant as the required rate rises and interferers become rarer.

Recent developments in various devices and applications require pairwise communications in an ad-hoc manner. Such systems are mostly interference-limited and suffer from capacity degradation caused by two-way communication. This paper investigates improvement of capacity performance in pairwise two-way communication through bi-directional use of spatial resources when beamforming is employed as the transmission scheme and instantaneous channel information is not available. This system is called a 'bi-directional beamforming' system. The primary approach to showing improvement in capacity performance in the bi-directional beamforming system involves focusing on the scaling of its achievable sum rate. Lower and upper bounds on the achievable sum rate are derived to evaluate the scaling of the achievable sum rate. Then, a condition is derived under which the bi-directional beamforming system outperforms the conventional beamforming system in the pairwise two-way communication environment. As a result, when the bi-directional beamforming system is employed, results show that the achievable sum rate scales twice as fast as that when conventional beamforming is employed.

This paper investigates the use of full duplex relaying (FDR) in cognitive radio systems. Cognitive full duplex relay networks (CogFRNs) offer the advantage not only of increasing spectral efficiency by spectrum sharing but also of extending coverage through the use of relays. Concurrent transmissions at the source and relay in CogFRNs can overcome a loss of resource efficiency in a way that conventional half duplex relay (HDR) systems cannot. However, in CogFRNs, the primary user experiences interference from the secondary source and relay simultaneously due to the effects of full duplexing. Satisfying the interference constraint by simply reducing transmission power results in performance degradation for the secondary user. What is therefore needed is a way to optimize the transmission powers at the secondary source and relay. To address this need, we propose an optimal power allocation scheme based on minimizing the outage probability in CogFRNs. We then analyze the outage probability of the secondary user in the noise-limited and interference-limited environments. In addition, we also propose an outage-constrained power allocation scheme which does not require the instantaneous channel state information (CSI) for the link between the primary and secondary users. Simulation results show that the proposed schemes achieve performance improvement in terms of outage probability.

Conventional investigations on the capacity of a secondary link in spectrum sharing environments have assumed that a secondary user knows perfect channel information between the secondary transmitter and primary receiver. However, this channel information may be outdated at the secondary user because of the time-varying properties or feedback latency from the primary user. If the secondary user allocates transmission power using this outdated channel information, the interference power to the primary receiver will not satisfy the predetermined interference constraint. In this paper, we investigate the impact of outdated channel information between secondary and primary users in spectrum sharing environments. We begin by deriving the ergodic capacity of secondary user along with the optimum power allocation under the average received-power constraint. We also provide a closed-form expression for the ergodic capacity without interference from the primary transmitter, and the capacity bounds with interference from the primary transmitter. Moreover, we provide the power margin required to satisfy the interference outage probability at the primary user under the peak received-power constraint. Lastly, we derive the secondary user's ergodic capacity with and without interference from the primary transmitter. Comparisons done using simulations show the effects of the uncertainty of channel information and interference from the primary transmitter under both constraints.

In this paper, we investigate a power allocation method without feedback of the instantaneous channel gain between a secondary transmitter (ST) and a primary receiver (PR) in spectrum sharing systems where channel information between the ST and the PR is outdated. We propose a mean value-based power allocation scheme (MVPA), which uses only the mean value of the channel gain of the ST-PR link. The proposed MVPA satisfies the interference outage constraint at the PR without feedback of the instantaneous channel information, which enables MVPA to reduce feedback burden. We also derive the ergodic capacity based on MVPA in closed-form and compare the ergodic capacity based on MVPA with that of a power allocation method using the outdated channel information instantaneously. Through the comparison, we convincingly demonstrate that MVPA without feedback of instantaneous channel information provides more reliable and superior capacity performance by reducing capacity loss from which the power allocation method using the outdated channel information severely suffers.

This paper considers an increase in the achievable rate of beamforming systems in two-way communication, resulting from bi-directional use of the spatial resources. The concept of bi-directional use of the spatial resources is first introduced. Then, a condition is derived under which bi-directional beamforming (BB) has larger achievable rate than that of conventional beamforming operating via frequency division duplexing (UB-FDD). In addition, the variation in the achievable rate of bi-directional beamforming is investigated when echo interference is pre-cancelled in the air. Simulation results show that the achievable rate of BB is always larger when the echo is ideally eliminated, and is larger over a certain level of SNR and below a certain level of fading correlation than that of UB-FDD.

In this paper, we investigate the effects of outdated channel state information (CSI) in opportunistic relaying systems on the ergodic capacity. The CSI in the data transmission instant may be different from that used in the relay selection instant due to channel variation. When the CSI in the relay-destination link is outdated, the average signal-to-noise ratio (SNR) and the ergodic capacity of the reactive decode and forward (DF) relaying system are derived. In addition, the effective ergodic capacity which is the spectral efficiency of the successful transmissions excluding the outage events is also derived. The numerical results show that if the CSI in the relay-destination link is outdated, the loss in the ergodic capacity is remarkable compared to the perfect CSI case. However, the level of the inaccuracy of the CSI does not affect the performance of the ergodic capacity in the reactive DF relaying system. In other words, no capacity degradation occurs even if the CSI inaccuracy becomes much larger.

The efficiency of spectrum sharing can be improved by interference suppression and/or cancellation. This paper analyzes the performance of spectrum sharing networks with interference cancellation (IC) based on the spectrum-sharing transmission capacity (S-TC), defined as the number of successful transmissions per unit area while guaranteeing all target outage probabilities of spectrum-sharing systems. The effectiveness of IC can be characterized by the coefficient of cancellation (CoC) and a specific CoC for strong interferer cancellation (S-IC) method is derived. It is verified that the performance of S-IC is affected solely by system parameters of its own system and the S-TC region can increase in various forms depending on CoCs, which should be considered in analysis of spectrum-sharing efficiency.

Distributed space-time-frequency coding (DSTFC) is a method to achieve diversity from both multipaths and user cooperations. The DSTFC has been researched by assuming that the multipath profiles of relays are the same (symmetric). However, the symmetric frequency- selective channel is limited in the wireless relay communications. Thus, in this paper, we propose a new DSTFC for relay-based amplify-and-forward (AF) cooperative scheme over the asymmetric frequency selective channels. The proposed scheme is based on the sub-carrier grouping in OFDM transmission. By analyzing pairwise error probability (PEP), diversity order of the proposed DSTFC is derived. In addition, a necessary condition of sub-carrier grouping to achieve the maximum diversity and minimize the decoding complexity is investigated. Simulation results verify that the proposed scheme can achieve the maximum diversity gain for a given asymmetric multipath profile.

In this paper, we propose a new detection algorithm for single carrier-frequency division multiple access (SC-FDMA) systems called duo maximum likelihood detector with transmit power allocation (DMLD-TPA). The proposed scheme is based on iteratively detecting two symbols out of Q transmit symbols by an orthogonal projection. It can make the detection complexity less than that of the maximum likelihood detector (MLD). In addition, we also propose a transmit power allocation scheme without the channel information feedback to enhance the BER performance. As a result, DMLD-TPA can keep the complexity low and its BER performance approaches that of the MLD.

In multiple antenna systems, spatial fading correlation causes a significant degradation in performance. In this work, a method for reducing the sensitivity to spatial fading correlation through the bi-directional use of spatial resources is investigated. Firstly, the concept of bi-directional use of spatial resources is introduced and the fading characteristics of the proposed bi-directional multiple-input multiple-output (B-MIMO) system are investigated. The achievable rates of the B-MIMO system are then investigated and compared with those of conventional MIMO systems for beamforming and full rank transmission. Based on those results, conditions needed for the B-MIMO system to demonstrate a larger average achievable rate are evaluated. B-MIMO systems are found to efficiently reduce the sensitivity to spatial fading correlation, while supporting comparable achievable rates in spatially white environments.

This paper analyzes spectrum sharing between multiple systems. The efficiency of spectrum sharing is determined primarily by interference, which is a function of the spatial densities of the transmitters in systems dependent on the chosen spectrum sharing method. One method is underlay, which allows all systems to concurrently use the whole spectrum, and the other is overlay, in which a system only utilizes its own assigned spectrum. We define the spectrum-sharing transmission capacity (S-TC) as the number of successful transmissions per unit area subject to outage probability constraints for each system. To prevent some systems from monopolizing access to the spectrum, we also propose a fair coexistence constraint and derive the optimal spatial densities and relative transmission powers both with and without this constraint in terms of the sum S-TC. Through analytical results, the overlay and underlay methods are compared, verifying that the overlay method is generally preferred, and the underlay method is equally good only for optimal transmission power ratios under a fair coexistence constraint.

This paper evaluates the outage probability of cognitive relay networks with cooperation between secondary users based on the underlay approach, while adhering to the interference constraint on the primary user, i.e., the limited amount of interference which the primary user can tolerate. A relay selection criterion, suitable for cognitive relay networks, is provided, and using it, we derive the outage probability. It is shown that the outage probability of cognitive relay networks is higher than that of conventional relay networks due to the interference constraint, and we quantify the increase. In addition, the outage probability is affected by the distance ratio of the interference link (between the secondary transmitter and the primary receiver) to the relaying link (between the secondary transmitter and the secondary receiver). We also prove that cognitive relay networks achieve the same full selection diversity order as conventional relay networks, and that the decrease in outage probability achieved by increasing the selection diversity (the number of relays) is not less than that in conventional relay networks.

A new interference management strategy is proposed to enhance the overall capacity of cellular networks (CNs) and device-to-device (D2D) systems. We consider M out of K cellular user equipments (CUEs) and one D2D pair exploiting the same resources in the uplink (UL) period under the assumption of M multiple antennas at the base station (BS). First, we use the conventional mechanism which limits the maximum transmit power of the D2D transmitter so as not to generate harmful interference from D2D systems to CNs. Second, we propose a δD-interference limited area (ILA) control scheme to manage interference from CNs to D2D systems. The method does not allow the coexistence (i.e., use of the same resources) of CUEs and a D2D pair if the CUEs are located in the δD-ILA defined as the area in which the interference to signal ratio (ISR) at the D2D receiver is greater than the predetermined threshold, δD. Next, we analyze the coverage of the δD-ILA and derive the lower bound of the ergodic capacity as a closed form. Numerical results show that the δD-ILA based D2D gain is much greater than the conventional D2D gain, whereas the capacity loss to the CNs caused by using the δD-ILA is negligibly small.

A new interference management scheme is proposed to improve the reliability of a device-to-device (D2D) communication in the uplink (UL) period without reducing the power of cellular user equipment (UE). To improve the reliability of the D2D receiver, two conventional receive techniques and one proposed method are introduced. One of the conventional methods is demodulating the desired signal first (MODE1), while the other is demodulating an interference first (MODE2), and the proposed method is exploiting a retransmission of the interference from the base station (BS) (MODE3). We derive their outage probabilities in closed forms and explain the mechanism of receive mode selection which selects the mode guaranteeing the minimum outage probability among three modes. Numerical results show that by applying the receive mode selection, the D2D receiver achieves a remarkable enhancement of outage probability in the middle interference regime from the usage of MODE3compared to the conventional ways of using only MODE1or MODE2.

This paper evaluates the capacity of the reactive decode-and-forward (R-DF) scheme, which considers the reduced relay candidates based on the signal-to-noise ratio (SNR) of the source-relay link for relay selection in underlay cognitive relay networks. The performance of R-DF scheme is compared with that of the proactive DF (P-DF) scheme. It has been proven that R-DF scheme achieves the similar capacity of P-DF scheme as the primary receiver is located further from the secondary source. Since the transmission power of a secondary user is constrained by the location of the primary receiver, the performance of a cognitive relay network varies depending on the considered link for determining the relay candidates, between the source-relay and the relay-destination links. Hence, we also analyze the performance of the backward R-DF (BR-DF) scheme which the relay candidates are determined based on the relay-destination link. It is also verified that the performance of BR-DF scheme is better than that of R-DF scheme when the primary receiver is located close to the secondary source.

With the quantized channel state information (CSI), space division multiple access (SDMA) can extract the multiplexing gain under the limited feedback burden. However, huge signaling burden of feedback can still suffer SDMA system because the total feedback data of SDMA is linearly dependent on the number of users. Hence, we propose the user restriction scheme to control the feedback load. In this scheme, the cut-off level, which restricts the feedbacks of poor conditioned users, is suggested for the reduction of the feedback burden without the performance loss. From simulation results, then, we show that the proposed feedback scheme can achieve not only the sum-rate gain but also the reasonable feedback reduction.

This paper evaluates the achievable transmission capacity of the secondary system in cognitive radio networks, defined by the spatial density of successful transmissions while guaranteeing the target outage probabilities of the secondary and the primary systems. By using stochastic geometry, the effects of the spatial densities and the transmission powers on the achievable transmission capacity is presented. Subsequently, the optimal spatial density of the secondary system and the optimal transmission power ratio of the primary system to the secondary system are derived. Furthermore, the maximum achievable transmission capacity is defined using the derived optimal values. From the theoretical results, it is shown that the optimal transmission power ratio is affected by not the density of the primary system, but the system parameters including the target outage probability. In addition, the achievable transmission capacity of the secondary system decreases as the spatial density of the primary system increases at the decreasing rate determined by the system parameters of the primary system.

In this paper, we propose a new grouping-maximum likelihood detector (GMLD) for single carrier-frequency division multiple access (SC-FDMA) systems. The proposed detector performs local maximum likelihood (ML) detection by grouping the received symbol block based on orthogonal projection to reduce the complexity of ML detector. As a result, the proposed detector offers lower complexity than the ML detector, while its performance approaches that of the ML detector. In addition, the efficient group size to guarantee the lowest complexity and the BER performance close to the ML is also presented.

In multiple-input multiple-output (MIMO) broadcast channel, the lattice-reduction-aided (LRA) precoder is the modified zero-forcing beamforming (ZFBF) scheme which perturbs data symbols to reduce the transmit power. In this paper, we propose the optimal user group selection (UGS) scheme for the LRA-precoder to maximize the sum-rate in MIMO broadcast systems. We derive the approximated sum-rate of the LRA-precoder and the optimal UGS scheme. We also propose a suboptimal UGS scheme which requires low-complexity. From the simulation results, we show that the sum-rate of the proposed LRA-precoder with the optimal UGS scheme outperforms that of the conventional ZFBF with its optimal UGS scheme, especially when the number of users is small. In addition, we also show that the proposed LRA-precoder with the suboptimal UGS scheme has a considerably larger sum-rate than the conventional ZFBF with its suboptimal scheme, especially when the transmit correlation exists.

Cognitive radio (CR) has been studied as a useful solution for efficient utilization of scarce radio spectrums. For it to succeed, two conflicting challenges are imposed on the secondary users: one is to ensure the quality of service (QoS) of the primary link, and the other is to maximize their own transmit throughput. To balance this tradeoff, beamforming and power control employing the multi-antenna in the base station (BS) of the CR network have been introduced. In the perfect beamforming situation, the power control algorithms suitable in the CR network with a multi-antenna BS (MBS) have been proposed in previous works. However, those algorithms are meaningless for realizing a practical CR network with the MBS since perfect beamforming is impossible. Therefore, unlike previous works, this paper proposes a joint beamforming and power control algorithm as a more practical strategy for realizing a CR network with the MBS. The algorithm is proposed so as to maximize the sum-rate of secondary users, while not degrading QoS for the primary link. Numerical results verify its effectiveness in a CR network with the MBS.

This paper investigates the effects of asynchronous multiple access interference (MAI) on the performance of orthogonal frequency-division multiple access (OFDMA) uplink systems in frequency-selective Rayleigh fading environments. We first analyze the MAI and its statistical characteristics using the Gaussian approximation. Then the analytical expression for the uncoded average symbol error probability (SEP) is obtained with various modulation schemes. Comparison with simulation results shows a tight agreement of the proposed analysis over a wide range of asynchronous timing errors and for various numbers of asynchronous users. Numerical examples show that when asynchronous timing errors reside within the guard interval, a performance of uncoded SEP- 10-3 can be feasible even in high dispersive environments. Furthermore, the effect of the Gaussian approximation is examined, and the performance comparison between interleaved- and block-type subcarrier allocation schemes is provided.

This paper investigates the performance of two-tier femtocell networks with cochannel femtocell deployment while considering cellular geometry and cross-tier interference in downlink. We derive the per-tier outage probability by introducing a simplified mathematical model that provides closely approximate femtocell interference distribution. Based on the outage probability analysis, we also derive the transmission capacity that represents the total capacity of the cochannel two-tier networks with outage constraints. The performance analysis provides an accurate characterization of the outage probability and the transmission capacity, accounting for the density of randomly scattered femtocells and femtocell transmission power.

The number of coefficients for estimating time-varying channel is generally much larger than the maximum number of observable data. Linearly time-varying channel (LTV) models have been widely used as a way of reducing the number of these channel parameters. This approach allows estimation using the channel slope and channel frequency response between symbols. A fatal problem with this approach, however, is that before it is even initiated, the estimated channel in the frequency domain has already been degraded by inter-carrier interference (ICI). We demonstrate this by analyzing the channel estimation mean square error (MSE) and signal to interference and noise ratio (SINR) after ICI cancellation with respect to the LTV model. We then propose a new channel estimation technique using dual-ICI cancellation which resolves this problem by pre-canceling ICI for the channel estimation using the channel of the previous symbol and post-canceling ICI for symbol detection with the more accurate channel information. The process is initialized using the first symbol as a preamble to estimate the channel in the time domain. Performance comparisons of MSE and SINR show that the proposed method is better suited to fast-fading channels than the conventional method, where ICI dominates the interference and noise power.

Error propagation in the source-relay link limits the performance of demodulation-forward (MF) relay systems. In order to solve this problem and efficiently exploit the relay, we propose log likelihood ratio (LLR)-based symbol selective transmission with a near-optimum threshold to minimize the bit error probability (BEP). In the proposed scheme, the source information is forwarded by the relay only if it is likely to be detected correctly, as determined by a comparison of the LLR with the near-optimum threshold. To derive the near-optimum threshold, the BEP of the proposed scheme is analyzed in the case of binary phase shift keying (BPSK) signals. Simulation results confirm the accuracy of the analysis and prove that the proposed scheme efficiently mitigates error propagation.

This paper examines an orthogonal random beamforming-based cross-tier interference reduction scheme for two-tier femtocell networks. In order to improve the immunity of both macrocell and femtocell users to cross-tier interference, we adopt a macrocell beam subset selection strategy. This beam subset selection strategy maximizes the throughput of the macrocell by optimizing the trade-off between the multiplexing gain and the multiuser interference. Simultaneously, the max-throughput scheduler suppresses the cross-tier interference with an adaptively reduced number of beams. Since the average cross-tier interference from the macrocell gradually decreases as the number of beams decreases, the beam subset selection strategy is capable of providing a spatial opportunity to the femtocell network. This spatial opportunity enables the femtocell network to take advantage of the selectivity arising from the correlation between the interference channel and the precoding matrix. Therefore, we also propose opportunistic channel selection and distributed power control strategies for the femtocell network. Both analytical and numerical results show that the proposed strategies collaboratively reduce the cross-tier interference in two-tier femtocell networks.

Previous investigations on capacity of secondary users in spectrum-sharing environments have determined the capacity of a secondary link based on the interference power threshold set at the primary receiver. In contrast to these previous works, we show that the capacity of a secondary link is determined based on a geographical relationship expressed as the ratio of the distance between the primary receiver and secondary transmitter to the distance between the secondary transmitter and receiver. Proceeding from that and in an effort, to enhance the capacity of the secondary user, which is limited by this distance-ratio, we adopt a secondary transmitter with M antennas. Furthermore, we analyze the capacity achieved using a simple antenna selection process.

In this paper, we design a zero-forcing (ZF) Tomlinson-Harashima precoding (THP) and a minimum mean square error (MMSE) THP for multiuser MIMO relay system with direct link. Based on these precoding structures, we propose the optimal combining weights of the ZF THP and the MMSE THP in half duplex relay system, and the sub-optimal weights of MMSE THP to reduce feedback channel state information (CSI). Through the Monte Carlo simulation, we verify that the optimal weights combining method outperforms the constant weights combining method. In addition, we confirm that the performance of the sub-optimal weights combining method is close to the optimal method, while reducing the feedback CSI simultaneously.

In this paper, we focus on single-selection opportunistic relaying with the decode-and-forward protocol over Rayleigh fading channels. The end-to-end average signal-to-noise ratios (SNR) and ergodic capacities of both proactive and reactive opportunistic relaying are analyzed as a closed form in a high source-to-relay SNR regime where all link qualities satisfy the required SNR. The derived results are then used to prove that the end-to-end average SNR for the proactive scheme is greater than or equal to that of the reactive scheme. In addition, it is proven that the SNR and ergodic capacity differences of these schemes are broaden as the number of relays increases. The end-to-end ergodic capacities of the two methods are investigated both with and without the threshold using simulations.

This paper considers the problem of sensing and transmission strategy of multiple parallel channels owned by the primary user, referred as stochastic multichannel sensing. The traffic parameters follow the Markovian traffic assumption and are not identically distributed among the channels. In order to obtain the optimal probabilities of channel selection for sensing, we formulate a maximization problem for the secondary user throughput with interference constraints to the primary user. The solution to the problem is obtained via linear programming. Numerical results show that the proposed stochastic sensing achieves higher normalized effective throughput and lower average collision probability than the conventional deterministic sensing in a non-identical traffic environment. Additionally, the proposed method greatly reduces computational overheads and memory space.

This paper proposes a rate allocation scheme for throughput maximization considering rate fairness in wireless relay systems. The throughput maximization problem is formulated to two link data rate balancing problems: global balancing and local balancing. And the rate fairness is converted to data rate balancing of each destinations. Based on this, we construct a feasible rate region (FRR) which is the set of achievable data rates. Since FRR is a polyhedra, rate allocation is performed by searching the intersection points of FRR. In addition, if the system is infeasible, the proposed rate allocation removes the destination for system feasibility. Simulation results reveal that the proposed rate allocation maximizes the throughput with satisfying rate fairness, not only for the feasible case but also for the infeasible case.

We investigate a non-linear precoding techniques for multi-user multiple-input multiple-output (MU-MIMO) downlink. When assuming full knowledge of the channel state information at the transmitter, a vector perturbation (VP) is a promising precoding scheme that approaches sum capacity and has simple receiver. However, its encoding is nondeterministic polynomial time (NP)-hard problem. In the MU-MIMO downlink, this encoding problem becomes harder because the proper user selection must be considered. In this paper, we propose a new vector perturbation scheme based on eigenvalue normalization (VPEVN), which converts the NP-hard problem into a channel condition-based finite integer problem, resulting in a reduction of encoding complexity. In addition, it can select a good user set by simple maximum norm criterion owing to its orthogonalization effect.

Demanding on higher data rate in wireless communications, femto-cell has been proposed to implement that with a low cost. The femto-cell can be realized by deploying a femto-base-station connected to the core network via a wired-backhaul. Through the wired-backhaul, a downlink and uplink data packets are delivered to the femto-base-station and the core network, respectively. Although the femto-cell with a low transmission power has a small cell coverage, co-channel interference problem between macro-cell and femto-cell has been an important research issue to deploy the femto-cell in the real world since the femto-cell reuses the macro-cell spectrum. As a solution to the co-channel interference problem, we propose a new time-division-duplex (TDD) frame structure called reversed-pair frame (RPF). The RPF consists of two TDD frames with a reversed order of downlink and uplink. Using the RPF and a wired-backhaul-delivered downlink data, capacity enhancement can be achieved through a simple interference mitigation process. We evaluate the performance of the RPF through analyzing the capacity of the two-user case, which is applied to multi-user case. The results show that adopting the RPF in a OFDMA femto-cell systems greatly enhances the uplink capacity of both the macrocell and the femto-cell, and even downlink capacities of them than using the conventional TDD frame.

Two new pilot patterns and channel estimation methods robust to inter-channel-interference (ICI) caused by fast fading channels are proposed which are applicable to 802.16m standards. A number of clusters of pilot subcarriers are used for channel estimation. In addition, the number of needed parameters for channel estimation is reduced by some properties of ICI in the time and frequency domains. Simulation results show that the proposed channel estimation has good bit-error- rate (BER) performance compared to conventional method based on 802.16m standards at the velocity of 350 km/h and has the same performance at the velocity of 3 km/h. Moreover, if the target BER is lower than the supportable BER of conventional method with uncoded 16 QAM, then the proposed method can be better option compared to diminish the modulation scheme as 8 PSK considering both BER and throughput.

This paper describes the derivation of detection and false-alarm probabilities for energy detectors in cognitive radio networks when a sensing node of the secondary system has correlated multiple antennas. The sensing performance degradation due to the antenna correlation is then investigated based on the performance analysis. The conclusions of the analysis are verified by numerical simulation results.

In this paper, we propose an asymmetric diversity modulation (ADM) scheme for a single-source relay system that utilizes the relay's higher transmission ability as a form of diversity. To achieve this, the proposed method transmits multiple source bits over a high-order modulating relay as a way to provide additional time diversity. The spatial and time diversity then undergo 'bit'-based combining at the destination. Using the proposed 'bit'-based channel combining method, we derive the theoretical bit error rate (BER) for such a system. Moreover, we investigate the fact that the proposed scheme shows a performance trade-off between bit power and time diversity resulting from the reduced bit power caused by a high-order modulating relay.

This letter presents a new full duplex relay (FDR) system that features more efficient use of time resource and antennas. The proposed FDR system is constructed based on time and antenna-sharing and elimination of underlying interference. The underlying interference caused by time- and antenna-sharing is eliminated using precoding. We propose a design for the precoder and decoder, and investigate the achievable rate of the proposed FDR system when the precoder and decoder are applied. Simulation results show improved achievable rate and BER performances with the proposed FDR system compared to half duplex and conventional full duplex relay systems.

The authors analyze the cause of bias in a fast Fourier transform (FFT)-based frequency domain signal-to-noise ratio (SNR) estimator by deriving the upper bound of the bias. The analysis is then used to propose a new frequency domain estimator using discrete cosine transform (DCT), which has lower bias. In addition, a criterion is proposed for use when maximum Doppler frequency information is available; it is based on the derived upper bound and can be used to improve the mean squared error (MSE) performance of the proposed DCT-based estimator. Simulation results show that the proposed estimator reduces the MSE remarkably by diminishing the bias..

In the paper, we deal with a single-selection opportunistic relaying with the decode-and-forward (DF) protocol over Rayleigh fading channels. The exact end-to-end average signal-to-noise ratios (SNR) and ergodic capacities of both proactive and reactive opportunistic relaying are derived as a closed-form for arbitrary link SNR. In addition, the effective ergodic capacity satisfying the minimum required data rate without outage is also identified for both relaying schemes. The analysis results are used to demonstrate which relaying scheme outperforms the other for given system parameters.

A critical problem in open-spectrum communications is fairness with respect to the coexistence of heterogeneous systems with different resource units and traffic models. In addition, the sensing performances of different systems can also lead to unfair resource utilization between systems. To address this problem, we derive a continuous-time Markov chain model to show the effect of sensing performance on system coexistence. The analysis derived from this model is then used as the basis for a sensing threshold control (STC) scheme to achieve fairness. The proposed STC determines the sensing threshold for each system as a way of balancing resource utilization among systems, while guaranteeing target detection probability. Numerical results on the amount of resource utilization by each system demonstrate that the proposed STC achieves a full degree of fairness.

The amplify-and-forward (AF) relaying system considered here consists of a source node with M transmit antennas, and relay and destination nodes with a single antenna each. The closed-form expressions for the outage probabilities for maximum-ratio-transmission (MRT) at the source are presented for some specific values of M. Numerical results show that increasing M from 1 to 2 can achieve roughly a 3dB SNR gain, guaranteeing the same outage probability. However, we discover that for M ≥ 3, the additional gain compared to M = 2 decreases quickly as the SNR increases. Therefore, increasing M above 3 does not achieve significantly greater gain over M = 2 at high SNR.

This letter presents our study of throughput enhancement achieved by multiuser diversity (MUDiv) in a multiuser system using a new asymptotic approach. The MUDiv gain is evaluated by deriving a new asymptotic formula with a closed form, which is flexible to the number of MUDiv orders. To do this, we simplify a Puiseux series instead of a extreme value theory used in the previous researches. The formula shows that the MUDiv gain is independent of the signal-to-noise ratio, and it is applied to analyze proportional fair scheduling. Finally, this analysis is verified using Monte-Carlo simulations of scheduling algorithms.

We propose a new robust timing recovery algorithm with small symbol timing jitter for orthogonal frequency-division multiplexing (OFDM) systems. The concept of coarse FFT window selection and adaptive reference pilot generation are employed in this paper. This proposed scheme avoids inter-symbol interference (ISI) as well as delayed locked loop (DLL) tracking failure over frequency selective time-varying channels. In addition, the proposed algorithm has much smaller and smoother steady state DLL tracking error than the conventional algorithm.

Most existing works on orthogonal frequency division multiple access (OFDMA) multihop cellular networks have assumed half-duplex relay, which suffers from spectral inefficiency since it requires additional resources for relaying. In order to alleviate this half-duplex loss, a new relay transmission scheme using dirty paper coding (DPC) is proposed. By alternately forwarding the messages to two mobile stations (MSs) via two fixed relay stations (FRSs) for each subcarrier, the base station (BS) predicts and pre-cancels the inter-relay interference between FRSs in advance. This proposed DPC-aided transmission scheme matches well with the nature of the quasistatic fading channel between FRSs. We also propose a two step resource allocation algorithm to select the appropriate MSs and to maximize the achievable sum rate. Numerical results demonstrate that the achievable sum rate of the proposed DPC-aided multihop cellular networks is significantly increased by utilizing the multi-user and multi-relay diversity gain as well as pre-cancelling the inter-relay interference between FRSs.

In multicell OFDMA systems, throughput is severely degraded by other cell interference (OCI), especially at cell edge. To satisfy the performance requirement, OCI must be mitigated effectively. Several schemes have been proposed to handle this problem by coordinating the way to allocate spectrum resource to each cell. Since the allocation of spectrum has effects on OCI mitigation and the spectral efficiency, an wise strategy to consider those two factors is required. In this paper, we propose a new scheme named as power division reuse partitioning scheme (PDRPS). PDRPS balances OCI mitigation and spectral efficiency by adopting interference cancellation (IC) scheme. The simulation results show that PDRPS has more throughput and lower outage probability than the conventional schemes.

In multiuser MIMO system with multiuser diversity (MUDiv) scheduler, spatial diversity plays a role to affect the scheduling gain. In order to investigate this phenomenon, we derive a new asymptotic capacity and an outage capacity of multiuser MIMO system with MUDiv scheduler using the extreme value theory. From our analysis, we define MUDiv gain by scheduling, and investigate the relationship between the spatial diversity and MUDiv. As a result, we find that the average capacity in multiuser MIMO system with MUDiv scheduler doesn’t increase linearly with the minimum number of antennas at high signal to noise ratio (SNR).

This paper presents a new duplex approach which does not require guard resources suitable for indoor 2times2 MIMO environments. In the proposed duplex, two virtual channels in the spatial domain are generated by precoding and postcoding MIMO channels, not to use guard resources in either the time or frequency domain. In addition, this system can achieve selection diversity since the generation of virtual channels is not unique for the given channels information. We will show the generation of the virtual channel, and that the capacity and reliability of the communication links improves when the problem of the guard resource is addressed and selection diversity is utilized.

In this paper, we propose a robust channel estimation method in OFDM systems over fast fading channels. In consideration of the effect of null subcarriers, the proposed channel estimation is first performed in the frequency domain. By making efficient use of channel correlation and the property of ICI to reduce the number of needed parameters for channel estimation, robust channel estimation for ICI is accomplished in parallel, pilot cluster by pilot cluster. It is then transformed into the time domain to obtain a gain in noise reduction and to reduce interpolation error as well. Simulation results show that the proposed channel estimation has good performance compared to previous works.

In cognitive radio systems, spectrum sensing schemes for detecting primary users in a licensed spectrum are essential to utilize the spectrum effectively. As a matter of fact, sensing accuracy is regarded as the most important factor for spectrum sensing with the intension of sufficient protection for the primary user. In this paper, we propose a new spectrum sensing scheme which exploits the cyclic prefix of an orthogonal frequency-division multiplexing (OFDM) symbol as well as idle period containing no information symbols. Redundant information in the cyclic prefix enables spectrum sensing to be more accurate and efficient. With analysis and simulation, we show that the proposed scheme can improve sensing accuracy and increase system throughput compared with the conventional system which uses only idle period.

In this paper, we compare the timing synchronization performance of the adaptive filter to that of the correlator in direct sequence spread spectrum (DS/SS) systems. The test variables used in the code synchronization are statistically analyzed for both schemes, and then the obtained results are used in calculating the detection and false alarm probabilities. Based on the derived probabilities, the synchronization performance is compared and the simulation is followed. Analysis and simulation results show that the correlator outperforms the adaptive filter under most acquisition environments.

This letter presents a call admission control (CAC) strategy for system throughput maximization in wireless uplink systems. This strategy considers not only the call-level quality of service (QoS) (i.e., blocking probability) but also the packet-level QoS (i.e., outage probability). Using the statistical co-channel interference (CCI) model and state diagram, the outage probability and the blocking probability are investigated as a function of the relative traffic load. We formulate the CAC strategy problem based on relative traffic load, and suggest a solution. The numerical results show that maximum system throughput can be achieved by controlling the relative traffic load. In addition, we illustrate the region where system throughput is constrained by call- and packet-level QoSs. This examination shows that the call and packet-level QoSs must be considered together to achieve maximum system throughput.

Equidistance in pilot spacing is an essential condition for discrete Fourier transform (DFT)-based channel estimation in OFDM systems. However, virtual subcarriers break this condition, degrade the estimation performance, and cause the interference (called ''leakage') because the orthogonality of Fourier matrix is broken. To solve this problem, we first analyze the leakage using the DFT-inverse DFT process. The pilot subcarriers inside virtual subcarriers area are estimated by the inverse of the estimated leakage. Thus, the equidistance condition is satisfied. The proposed estimator operates well in realistic environment such as IEEE 802.16, and it is robust to an increase of virtual subcarriers.

We propose a new channel estimation technique to mitigate interference with the aid of pilot density discrepancy in a cellular OFDM system, especially for 3GPP systems. The proposed estimator applies interference mitigation to pilot symbols, using repeated interference features according to the pilot subcarrier distance. In order to use this property, the proposed estimator employs a specific pilot structure which consists of two types of pilot symbols with different pilot density. The combination of interference alleviation and pilot rearrangement not only makes the channel estimation robust to the time-selectivity of the channel but also reduces the number of pilot subcarriers needed to estimate the channel. To clarify the advantages of the proposed method, the average mean square errors (MSE) of the frequency channel estimate are derived for the proposed estimator with unequal pilot density, and it is compared with general estimators with equal pilot density. Numerical analysis and simulation results confirm that the proposed estimator with unequal density relationship outperforms the estimators over practical time-varying environments in terms of MSE performance and pilot overhead efficiency.

This paper considers the problem of efficient usage of subcarriers in downlink orthogonal frequency-division multiple access (OFDMA) multi-hop cellular networks. Multi-hop transmission can either save or waste subcarriers depending on quality of service (QoS) requirements, interference from other cells, and the location of a given mobile station (MS). Preventing unnecessary usage of multi-hop transmission requires the use of a transmission mode selection (TMS) scheme, and we propose two types of TMS in this paper. The first of these is distance-based TMS (TMS-D), which determines the transmission mode based on the MSiquests location, and the second is subcarrier-based TMS (TMS-S), which selects whichever transmission mode uses fewer subcarriers. Numerical results on blocking probability demonstrate that TMS improves overall subcarrier usage efficiency, meaning that more MSs can be supported with low blocking probability within a cell. Furthermore, the performance of TMSS, even given its higher complexity, is shown to be superior to that of TMS-D.

Opportunistic scheduling algorithms with link adaptation can increase system throughput, but have the drawback of requiring the channel quality information (CQI) to be transmitted from all the candidate users. In order to alleviate this problem, we propose a new feedback reduction scheme that preserves CQIs over all channels. The proposed scheme binds similar CQIs in groups to represent them more effectively, and uses a simple method for fast grouping. Simulation results show that the proposed scheme with this simple grouping achieves close to perfect feedback performance with respect to the original CQIs under reasonable feedback loads.

In this paper, a problem for efficient use of subcarriers in downlink OFDMA multi-hop cellular networks is studied based on quality of service (QoS) guarantee of each mobile station (MS). We consider three possible transmission schemes in multi- hop networks: single-hop, multi-hop, and multi-hop with spatial reuse of subcarriers. We verify that the required number of sub-carriers to guarantee the target BER and the target data rate is changed according to the transmission scheme and QoS requirements as well as the channel condition. Based on this result, we propose a new transmission scheme selection algorithm to select the best performance scheme for the efficient utilization of the limited subcarriers. Through the numerical results, the performance of this algorithm is demonstrated by the number of supported MSs in the system and the blocking probability.

An opportunistic scheduling is adopted to improve the capacity of the system by exploiting the multiuser diversity of multiuser MIMO (MU-MIMO)systems. However, it requires a large amount of feedback carrying the channel quality information (CQI) of each user to the transmitter. The considered per user unitary and rate control (PU2RC) needs to feedback the preferred precoding index and its CQIs, and it has a defect in scheduling the streams for the grouped user. In order to overcome these drawbacks, a novel feedback reduction scheme is proposed and is modified for OFDMA systems in this paper. It employs transmitter controlled precoding and opportunistic feedback (TCP-OFB) in conjunction with frequency scheduling. The simulation results demonstrate that TCP-OFB/OFDMA shows comparable performance to PU2RC while it only requires far reduced feedback load.

This paper presents closed-form expressions for the end-to-end performance of wireless dual-hop systems with multiple amplify-and-forward relays and multiple users over Rayleigh flat fading channels. These results are based on the statistics of the harmonic mean of two random variables related to exponential distribution. Two generalized scheduling policies are considered in the systems for selection diversity from multirelay and multiuser: the centralized scheduling (CS) and the distributed scheduling (DS). Numerical results show that the performance of DS can be degraded when large number of relays are used relative to the number of users. Additionally, it is shown that DS is quite competitive with CS at small number of relays that can be considered to be desirable.

In the distributed space-time block coded (DSTBC) orthogonal frequency division multiplexing (OFDM) system (DSTBC-OFDM), there may exist distributed carrier frequency offsets (CFOs) by sharing the transmission antennas belonging to different nodes. In this paper, the signal model of the DSTBC-OFDM system with distributed CFOs is presented. Then we propose a new equalization technique via the modification of the QR decomposition (MQRD) in order to eliminate ICI and ISI which are caused by CFOs. The proposed MQRD scheme guarantees transmit diversity gain. Several kinds of equalization techniques are introduced and compared in terms of bit error rate (BER) performance and transmit diversity order. Simulation results show that the proposed method outperforms conventional equalization techniques and achieves equal transmit diversity order with perfectly synchronized DSTBC-OFDM system.

In this paper, a resource management for fixed relay stations (FRS) with the limited capability is considered in down-link OFDMA multi-hop cellular networks. All of MSs which want to use a FRS in multi-hop transmission can not be supported when the capability of the FRS is insufficient. Hence, the multi-hop user admission (MUA) is required to determine the admitted MSs to use the FRS because the gain obtained from multi-hop transmission is different to each MS. The problem of MUA is formulated to maximize the total multi-hop gain which the FRS can achieve from the admitted MSs, and this problem is proven to be NP-hard. Hence, two efficient heuristic algorithms for MUA are proposed. One is the balanced link-MUA (BL-MUA) algorithm which gives priorities of admission with attaching importance to the load balance between the BS-FRS link and the FRS-MS link. The other is the focused link-MUA (FL-MUA) algorithm which considers only the load state of the FRS-MS link in admission. Through the numerical results, it is verified that the FL-MUA algorithm obtains similar performance to the BL-MUA algorithm with less complexity. In addition, the total number of saved subcarriers in a FRS increases and more MSs can be supported in a cell with low blocking probability by the proposed algorithms.

An opportunistic scheduling is adopted to improve the capacity of the system by exploiting the multiuser diversity of multiuser MIMO (MU-MIMO) systems. However, it requires a large amount of feedback carrying the channel quality information (CQI) of each user to the transmitter. The considered per user unitary and rate control (PU2RC) needs to feedback the preferred preceding index and its CQIs, and it has a defect in scheduling the streams for the grouped user. In order to overcome these drawbacks, a novel feedback reduction scheme is proposed and is modified for OFDMA systems in this paper. It employs transmitter controlled preceding and opportunistic feedback (TCP-OFB) in conjunction with frequency scheduling. The simulation results demonstrate that TCP-OFB/OFDMA shows comparable performance to PU2RC while it only requires far reduced feedback load.

A new adaptive resource allocation scheme is proposed in orthogonal frequency-division multiple access (OFDMA) systems with rate proportionality constraints. The problem of maximizing the overall system capacity with constraints on bit-error rate, total transmission power and rate-proportionality for user requiring different classes of service is formulated. Since the optimal solution to the constrained fairness problem is extremely complex to obtain, a low-complexity suboptimal algorithm that separates subchannel allocation and power allocation is proposed. Firstly, the number of subchannels to be assigned to each user is determined based on the users' average signal-to-noise ratio and rate-proportion. Subchannels are subsequently distributed according to the modified max-min criterion. Lastly, iterative rate-proportionality tracking algorithm is proposed for maximizing the capacity together with maintaining the rate proportionality constraint. Simulation results demonstrate that the proposed method can distribute the capacity more efficiently and fairly among users than other existing method.

This paper analyzes the degradation effect of null subcarriers in orthogonal frequency division multiplexing (OFDM) systems on the time-domain maximum likelihood (ML) estimation performance. The analysis is used as the basis for a proposal for a new channel estimation method that can overcome performance degradation caused by null subcarriers. The accuracy of the proposed channel estimation is guaranteed by using an interpolation of the estimated channel in the frequency domain that comes before the channel estimation in the time domain.

An asymmetric pilot subcarrier allocation scheme (APSA) is proposed for orthogonal frequency division multiple access (OFDMA) systems which use regenerative relays. To achieve the best performance of regenerative relay systems, the performance of each hop should be balanced. When there is a direct link between the source and destination, APSA allocates pilot subcarriers asymmetrically to each hop to balance the performance. We show that APSA can improve the performance of the system based on the symbol error rate (SER) analysis and simulation results.

In this paper, we propose a new preamble structure for channel estimation in a MIMO OFDM-based WLAN system. Both backward compatibility with IEEE 802.11 a and low overhead are considered for the preamble design. Simulation results show that the proposed preamble has low overhead and good performance gain for channel estimation.

On estimating channel information in multicell OFDM(A) system, the received pilot signal is interfered by other cell's signals. In this paper, we propose a new channel estimation technique based on orthogonal pilot structures in multicell OFDM(A) system. The proposed pilot structure in time domain has zero padding of the channel length to minimize the noise enhancement and use orthogonal sequences to eliminate other cell's interference. Especially, we design orthogonal sequences to give robustness to propagation delay. Simulation results confirm that mean square error (MSE) performance of the proposed channel estimation is better than that of the conventional methods in multicell environment.

A new timing estimator using phase difference between adjacent subcarriers is proposed for interleaved orthogonal frequency-division multiple access (OFDMA) uplink systems. The phase difference of adjacent subcarriers is represented as the summation of the information of timing offset which we want to estimate and the phase difference of channel frequency responses which can cause performance degradation. Thus, a method to reduce the phase difference of channel frequency responses is applied in time domain, and then timing offset information is estimated from phase difference in frequency domain. Simulation results confirm that the proposed estimator can decrease the mean square error (MSE) compared with channel impulse response (CIR) based method when the information of channel length is unknown.

We propose a new channel-estimation method in a multicarrier-code division multiple access system with chip interleaving. This new method works with code-multiplexed pilot channels and data-dependent pilot symbols, where the reliability of the estimated channel gain is increased by the noise averaging effect. Simulation results show that the performance of the proposed method is comparable to the minimum mean-square error estimation method with less computation complexity

A novel feedback reduction scheme is proposed for dynamic channel allocation (DCA) in OFDMA systems. Applying this delta-modulation (DM)-based scheme to the channel gain of adjacent subcarriers reduces the feedback information to 1 bit per subcarrier. Since this reduced feedback information can lead to performance degradation in a bit error rate (BER), a modified amplitude craving greedy (ACG) method and improved DM are jointly adopted to minimize the loss. Simulation results verify that the proposed scheme can decrease feedback information by a factor of 1/M while retaining performance comparable to the full feedback case, which requires M bits per subcarrier

In this paper, we propose an adaptive time, frequency and code allocation algorithm to guarantee heterogeneous quality of service (QoS) of users in multicarrier code division multiple access (MC-CDMA) systems. It is separated into two stages. First, time slots and frequency bands are allocated based on the proposed signal-to-noise ratio (SNR) threshold which reflects channel information and the QoS requirements of each user. Next, we propose a cost function which represents sum delay of users and then the codes are allocated by minimizing the cost function while guaranteeing the required bit error rate (BER) of users. Simulation results show that the proposed algorithm yields lower outage probability and higher throughput compared with the existing algorithms.

Signal-to-noise ratio (SNR) measurement is required in many channel-adaptive techniques and radio resource management methods for mobile radio communication systems. In this paper, the factors that degrade the MSE performance of the conventional frequency domain SNR estimator in high SNR region are analyzed. Based on this analysis, we propose two improved frequency domain SNR estimators which employ windowing and discrete cosine transform (DCT) to suppress the leakage of the received fading signal's frequency spectrum. The simulation results show that the proposed methods remarkably improve the MSE performance for high SNR region (ges15 dB) in mobile fading channels

The location tracking of a mobile station is required to support the location-based services and the radio resource management. The improved location tracking algorithm which uses the Kalman filter with the velocity renovation process is proposed. The velocity renovation process consists of a velocity estimator and a direction finder. By this process, the proposed algorithm can use accurately estimated velocity in the location estimation. The proposed algorithm reduces more than 66% of the location estimation error and shortens the transient time of estimation compared to the Kalman filtering method in [5]. The performance of the proposed algorithm is verified in the aspect of the outage probability and the location error ratio.

In this letter, we compare the code acquisition performance of the adaptive filter to that of the correlator in direct sequence spread spectrum (DS/SS) systems. The test variables used in the code acquisition are statistically analyzed for both schemes, and then, the obtained results are used in comparing the detection and false-alarm probabilities. Analysis and simulation results show that the correlator outperforms the adaptive filter under most acquisition environments

This letter presents an analysis of asynchronous multicarrier code-division multiple-access (MC-CDMA) systems with a guard period (GP) in the form of a cyclic prefix over frequency-selective multipath fading channels, which results in closed-form bit-error rate performance. Furthermore, it confirms that the frequency diversity of MC-CDMA systems is identical to the path diversity. The analytical approach proposed here is extended to the case without a GP. The derived analytical results show that a GP is required for MC-CDMA systems in order to mitigate not only the effect of intersymbol interference and intercarrier interference, but also the desired signal power degradation.

The authors present an analysis of the effect of timing offset on channel estimation for comb-type pilot-aided orthogonal frequency division multiplexing (OFDM) systems. Residual timing offset does not negatively affect the channel estimation of the pilot subcarrier, but does corrupt the channel information obtained via interpolation. This paper provides the mean square error (MSE) channel estimation performance when a linear interpolation technique is used in a comb-type pilot-aided OFDM system. Analysis shows that the performance degradation of the channel estimator due to imperfect frame synchronization is dependent on the frequency correlation of the channels and the amount of timing offset

A novel feedback-reduction scheme is proposed for dynamic channel allocation (DCA) in OFDMA systems. Applying this delta-modulation (DM)-based scheme to the channel gain of adjacent subcarriers reduces the feedback information to 1 bit per subcarrier. Since this reduced feedback information can lead to performance degradation in the form of an increased bit error rate (BER), a modified amplitude craving greedy (ACG) method and improved DM are jointly adopted to minimize the loss. Simulation results verify that the proposed scheme can decrease feedback information by a factor of 1/M while retaining performance comparable to the full feedback case, which requires M bits per subcarrier as feedback information.

In this paper, when a timing offset occurs within a cyclic prefix, we describe a linear interpolation with phase compensation for preventing a performance degradation due to the timing offset in the comb-type pilot-aided OFDM systems. With the performance analysis, the phase value which is needed for the phase compensation is derived. By means of the simulation results, we show that the channel estimation is accurate, using the phase compensation with optimal phase value.

Information of the maximum Doppler frequency enable to optimize many channel-adaptive techniques and radio resource management methods for mobile radio communication systems. In this paper, we propose four maximum Doppler frequency estimators which are based on the level crossing rate (LCR), the zero crossing rate (ZCR), the covariance function (COV) and the autocorrelation function (ACF). To eliminate the influence of additive noise, we analyze the conditions for the estimators independent of the signal-to-noise ratio (SNR) and implement the conditions with a downsampling process in the time domain. The proposed methods achieve good SNR-independent performance in the Rayleigh fading channels.

In this letter, we propose a novel channel estimation method using two consecutive subcarriers (CE-TCS) that is based on pilot channel-based channel estimation (PCCE). The proposed CE-TCS alters the system structure in order to maintain orthogonality between data and pilot channels over two consecutive subcarriers. Contrary to conventional PCCE, this new CE-TCS can obtain both accurate channel state information and diversity gain in multicarrier code division multiple access (MC-CDMA) system with chip interleaving (CI). Simulation results verify that the proposed method outperforms pilot tone-aided channel estimation (PTCE).

We investigate an analysis of channel estimation using an interpolation technique in orthogonal frequency division multiplexing systems. Then a criterion for the adaptive interval of pilot subcarriers is derived. The mean-square error (mse) of comb-type estimators is dominated by the length of channel delay and the interval of pilot subcarriers. Analysis shows that the mse performance of comb-type estimators with the adequate interval of pilot subcarriers is better than that of block-type estimator for indoor channels.

In order to increase throughput using high order data modulations in wide-band code-division multiple-access (WCDMA) downlink, it is necessary to eliminate multipath interferences. In a multipath interference canceller (MPIC) was proposed, which enables the use of high order modulations in multipath fading channels. However, only packet channels and pilot channels were considered as transmitted code channels. Code channels dedicated to other users and remaining common code channels coexist in practice, and such unknown code channels degrade the performance of MPIC significantly. In this letter, we propose a multipath interference mitigation scheme that removes all the intracell interferences by orthogonal variable spreading factor (OVSF) code characteristics. Simulation results show that the proposed scheme makes it possible to improve total throughput using high order modulations in WCDMA downlink.

In this letter, we propose two maximum Doppler frequency estimators that are based on the level crossing rate (LCR) and the covariance function (COV). To eliminate the effect of additive noise, we analyze the conditions for the estimators independent of the signal-to-noise ratio (SNR) and implement the conditions with a simple downsampling process. The proposed methods achieve good SNR-independent performance.

This paper derives a tight union bound on the symbol error probability of maximum likelihood detection (MLD) in downlink multiple-input-multiple-output (MIMO) multicarrier code division multiple access (MC-CDMA) systems. The analysis is performed in multiuser and frequency-selective fading environments. The bound of MLD is used to demonstrate the optimum performance of MIMO MC-CDMA systems with respect to the number of users, subcarriers and antennas.

Exploiting the effect of frequency diversity based on the frequency-domain channel correlation of OFDM systems, we propose an improved space-time block coding with frequency diversity for OFDM systems in frequency selective fading channels. With only two transmit antennas, the proposed scheme can achieve a fourth-order diversity gain at low complexity. Simulation results for the proposed scheme demonstrate that this joint exploitation of spatial diversity and frequency diversity leads to a remarkable improvement in BER performance.

Future wireless systems will need to employ blind estimation to achieve better spectral efficiency. In this letter, we introduce a blind synchronization algorithm for jointly estimating timing and frequency offset in orthogonal frequency division multiplexing (OFDM) systems. The proposed estimator exploits the second-order cyclostationarity of received signals and then uses the symbol-timing and carrier frequency offset information appearing in the cyclic correlation. Because it is a blind estimator, no channel impulse response information is required. Simulations demonstrate that the proposed estimator performs at a sufficiently high level, thus indicating its suitability for use over fading channels.

In this paper, we evaluate the performance of 1/spl times/ evolution data and voice (1/spl times/EV/DV) systems with multiple antennas at both the transmitter and the receiver. The multicode interference (MCI) in 1/spl times/EV/DV systems is hard to compensate in multipath fading channels. Especially, the effect of MCI is more severe when using multiple antennas. We can improve the receiver performance by using multipath interference cancellation (MPIC) scheme. Simulation results show that 1/spl times/EV/DV systems with the multiple antennas and the MPIC scheme can achieve higher data rate than single antenna system in the frequency selective fading channel.

This paper contains our proposal for a new decoding algorithm for an iterative decoded V-BLAST system. The proposed algorithm is based on maximum a posteriori (MAP) decision criterion. In a V-BLAST system concatenated with channel coding, the extrinsic information from the soft output channel decoder can be utilized as a priori probability, making it possible to apply MAP decision criteria to the V-BLAST decoding process. The MAP decision criterion is applied to the V-BLAST ordering and slicing procedure, resulting in a considerable gain in bit error performance. As the iteration rate increases, the proposed system exhibits performance similar to that of systems with ideal sliced.

One WLAN cell results in unintentional interference to the adjacent cells using the channel of which center frequency is insufficiently separated. The interference affects the adjacent cells as partial band jamming (PBJ). This paper presents the effect of partial band jamming on orthogonal frequency division multiplexing (OFDM) systems. PBJ is modeled on the rectangular spectrum with uniformly distributed random phase over [0, 2/spl pi/). An upper bound of bit error probability (BEP) performance based on the effective signal to noise power ratio (SNR) is provided for OFDM systems in Rayleigh fading channels in addition to AWGN. Finally, coding gain over partial band jamming is analyzed in an OFDM-based wireless local area network (WLAN).

In this paper, asynchronous multicarrier-code division multiple access (MC-CDMA) systems with a carrier-frequency offset for the uplink is analyzed over multi-path fading channels. Moreover, SINR degradations caused by a carrier-frequency offset are evaluated. Derived and simulation results show that the performance of asynchronous MC-CDMA systems is sensitive to both the correlation among sub-carriers and the carrier-frequency offset. Furthermore, it is verified that a guard period is required for MC-CDMA systems in order to mitigate inter-symbol interference (ISI).

We propose a blind decision feedback equalizer (DFE) that is characterized by the fact that it does not require channel estimation. Because the output of the optimized multistep prediction error filter (PEF) can be represented as a product of the channel partial impulse response and the transmitted sequence, a backward multistep PEF can be used as the blind DFE feedforward filter. The corresponding feedback filter is obtained from the symbol-rate partial channel impulse response. The proposed algorithm has several advantages over existing blind channel estimation techniques, including stable performance without the necessity of exact channel order estimation.

In this paper, we propose a simple frequency-domain interpolation technique for the discrete Fourier transform (DFT). This interpolation technique can significantly improve the frequency and phase resolution capabilities of the DFT without increasing the size of the DFT (the number of points used for the DFT). This new technique employs a dividing point in the amplitude and phase spectra. Suitable areas of application include joint estimation of fine frequency and phase offsets in burst-mode digital transmission.

For wideband code-division multiple-access systems, the paper introduces a multicarrier modulation scheme that performs the spreading simultaneously in the time and frequency domains. This scheme attains higher flexibility and spectrum efficiency because system parameters can be selected at will. The performance is compared with that of a single carrier RAKE system by calculating the probability of error over a frequency-selective Rayleigh fading channel. The proposed scheme outperforms the single carrier RAKE system if the system parameters are selected properly for given conditions, such as bandwidth and delay spread.

This paper proposes a novel detection scheme using a radial basis function (RBF) network in a multi-input multi-output (MIMO) environment. In order to evaluate the performance of the proposed MIMO-RBF receiver, simulations are performed over the rich-scattering fading channel. Simulation results confirm that the proposed scheme shows the similar bit-error rate (BER) performance of a maximum likelihood detection (MLD) and outperforms Vertical-Bell Laboratories Layered Space-Time using minimum-mean-square-error nulling (VBLAST-MMSE) as well as VBLAST using zero-forcing nulling (VBLAST-ZF). Moreover, we investigate the effect on the performance of the number of RBF center with two modulation formats (i.e. BPSK and QPSK) and different number of transmit and receive antennas. The performance of the proposed detector is verified with respect to an initialization-rate of RBF centers.

We present a novel timing offset estimation method for orthogonal frequency division multiplexing(OFDM) systems. The proposed estimator is designed to avoid the ambiguity in timing offset estimation. The performance of proposed scheme is presented in terms of mean and mean-square error (MSE) sense. The simulation results show that the proposed estimator is unbiased and has significantly smaller MSE.

We propose an edge sidelobe suppressor (ESS) for the uplink of OFDMA systems which mitigates the interference introduced by different frequency offsets between a desired user and other users. The performance of this ESS scheme is evaluated using both the average signal to interference power ratio (SIR) and the average bit error rate (BER). The simulation results confirm that the proposed ESS scheme improves system performance by approximately 10 dB SIR as compared to conventional OFDMA systems, and without increasing the level of transmitter and receiver complexity.

In this paper, we provide the performance analysis of pilot symbol assisted modulation (PSAM) for transmit diversity with space-time block codes (STBC) in time-varying Rayleigh fading channels. For the performance evaluation, the average bit error rate of transmit diversity with STBC is derived when the channel estimation error exists. Using the result, the average BER in time-varying channels is evaluated when the PSAM is employed.

In this paper, the average bit error rate (BER) and the average throughput (BPS) are derived for adaptive modulation (AM) combined with transmit diversity (TD) in a frequency selective Rayleigh fading channel. The effects of multi path fading and TD on AM are investigated based on the derived BER and BPS expressions. The results show that the diversity effect by multipath fading can be positive or negative depending on the average SNR and that the performance of AM can be improved by combining TD scheme, especially when there are a few multipath components.

We propose a blind decision feedback equalizer (DFE) using second-order statistics (SOS). The proposed blind decision feedback equalization technique does not require channel estimation and obtains the equalizer coefficients directly from SOS of the received signals. In the case of SIMO channels, autoregressive (AR) process is also moving average (MA) process. Therefore, the output of the optimized multistep prediction error filter (PEF) can be represented as a product of channel partial impulse response and transmitted sequence. It is shown that a backward multistep PEF can be used as the feedforward filter (FFF) of blind DFE. The corresponding feedback filter is obtained from the symbol-rate partial channel impulse response. The proposed algorithm has several advantages which is not shared with existing blind channel estimation techniques. The proposed DFE acheives the stable performance without the exact channel order estimation. The performance of the equalizer can be improved by the nonlinearity of the decision device which alleviates the inaccuracy of the multistep PEF.

This paper concentrates on developing and analyzing advanced space-time multiuser receivers for WCDMA forward link. Through exploring the multiuser interference and utilizing the space-time diversity, an optimum diversity receiver, called a constrained adaptive space-time diversity combiner (C-ASTDC), is proposed. To mitigate the disadvantages of the full-rank multiple antenna system, such as computational complexity and increased difficulty to accurately tune the algorithm, a reduced form of C-ASTDC is also proposed.

In this paper, we examine the performance degradation due to the imperfect channel information in orthogonal frequency division multiplexing (OFDM)-based wireless LAN (WLAN). The average effective SNR and average bit error probabilities are derived in Rayleigh fading channels.

The average frame error rate (FER) and throughput expressions are derived for adaptive modulation combined with transmit/receive diversity schemes in a Rayleigh fading channel. Based on these derived expressions, the effect of transmit/receive diversity on the link performance of adaptive modulation is investigated. Two transmit diversity schemes are considered for analysis: space-time transmit diversity (STTD) and selection transmit diversity (STD). Our results show that the performance of adaptive modulation can be improved by combining STD, especially when there is a few receive antennas. However, STTD gives little benefit on adaptive modulation.

This paper studies the channel estimation schemes of a time division duplexing (TDD) code division multiple access (CDMA) system with a parallel interference canceller (PIC) in multipath fading channels. Furthermore, the effective interpolation method which maintains the flexibility of Universal Mobile Telecommunications System (UMTS) Terrestrial Radio Access (UTRA) TDD mode is proposed. By Monte Carlo simulations, it is verified that the proposed interpolation method can be used in order to obtain the proper performance of a multistage PIC and in order to reduce the required Eb/No for a given bit error rate.(BER).

This paper concentrates on developing a complexity reduced adaptive space-time interference canceller for multiuser WCDMA forward link. Through exploring the multiuser interference and utilizing the space-time diversity, a sub-optimum diversity receiver, called a reduced-rank constrained adaptive space-time interference canceller (R-CASTIC), is proposed. The proposed scheme mitigates the disadvantages of the full-rank multiple antenna system, such as computational complexity and increased difficulty to accurately tune the algorithm. R-CASTIC performs similar to the full-rank receiver within 1 dB at the BER of 10/sup -3/ in less than 60% of total cell capacity, with only an eighth of the complexity of a full-rank receiver.

Blind synchronization algorithm is developed for estimating jointly timing and frequency offset of OFDM signals transmitted through a frequency-selective fading channel. The proposed estimator exploits the second-order cyclostationarity of received signals. As a blind estimator, the information of impulse response of channel is not required. The performance of the estimator is evaluated through computer simulations.

An adaptive channel precoded transmitter for space-time block coding based transmit diversity (STTD) is developed for a 3GPP WCDMA TDD system. Since all the functions of precoding are implemented in the base station, the channel equalization process can be left out in the mobile station. This results in the minimization of hardware complexity of the portable unit. Simulation results show that the adaptive channel precoded transmitter gives a close performance to the conventional Rake combiner.

A new space-time code employing a serially concatenated code structure is proposed. The proposed structure has L independent layers, and the serially concatenated turbo encoding is performed in each layer. The key structure, a 2-dimensional (time and space) interleaver, is placed between the inner encoder and the outer encoder and reorders the encoded bits in time and space according to its interleaving pattern. By using the 2D interleaver, the temporal and spatial diversity gains are sufficiently achieved. Moreover, it reduces the correlations between the inputs and the outputs of the interleaver, so that it protects against error propagation from the inner decoder to the outer decoder. By adding the proposed scheme, the bit error performance is remarkably improved, while complexity increases only negligibly.

We consider interference free decision feedback channel estimation (DFCE) for M-ary orthogonal modulation in direct sequence/code division multiple access (DS/CDMA) systems. The proposed DFCE uses the maximum combiner output in a RAKE receiver as the decision feedback information, enabling M-ary orthogonal signals to be demodulated coherently and a RAKE receiver to use a maximal ratio combining (MRC) scheme. Under multiuser environments, a multistage parallel interference cancellation (PIC) scheme is combined with the DFCE in order to cancel the interference. Accurate knowledge of the channel coefficient estimated by the proposed DFCE is used to regenerate the signal of each user for the multistage PIC scheme. According to the results of our simulations, the performance of coherent demodulation using the proposed system is significantly improved in comparison with conventional noncoherent demodulation.

We propose a new suboptimal detector that uses orthogonal polynomial approximation to determine the stage weighting factors at each interference cancellation (IC) stage. The performance of the new detector using the proposed scheme is compared with that of other sub-optimal detectors in the near-far and additive white Gaussian noise (AWGN) environments. Simulation results show that the average convergence rate of the proposed method for obtaining the optimum solution through estimation of the minimum (and maximum) eigenvalue of the code correlation matrix is faster than for other sub-optimal detectors based on IC. In addition, the method described here is also able to obtain better BER performance, with the same number of stages, than conventional parallel interference cancellation in the above environments.

An adaptive channel precoded transmitter for space-time block coding based transmit diversity (STTD) is developed for a 3GPP WCDMA TDD system. Since all the functions of precoding are implemented in the base station, the channel equalization process can be left out in the mobile station. This results in the minimization of hardware complexity of the portable unit. Simulation results show that the adaptive channel precoded transmitter gives a close performance to the conventional RAKE combiner.

A noncoherent adaptive code acquisition scheme is proposed by employing a differential detection technique. The carrier phase offset in the PN signal component of the filter input is removed by a differential detector. The numerical results for the detection and false alarm probabilities, and the mean acquisition time are derived. The performance of the proposed system is verified by simulations. The proposed system is compared with the conventional system using a matched filter by considering the mean acquisition time and the false alarm probability simultaneously. Simulation results show that the proposed system has a good performance under overall channel conditions, especially at low SNR, and outperforms the matched filter by 4-6 dB under an AWGN channel for 16 taps.

A new space-time code employing a serially concatenated code structure is proposed. The proposed structure has L independent layers, and the serially concatenated turbo encoding is performed in each layer. The key structure, a 2-dimensional (time and space) interleaver, is placed between the inner encoder and the outer encoder, and reorders the encoded bits in time and layer according to its interleaving pattern. By using the 2D interleaver, temporal and spatial diversity gains are sufficiently achieved. Moreover, it reduces the correlations between the inputs and the outputs of the interleaver, protecting against error propagation from the inner decoder to the outer decoder. By adding the proposed scheme, the bit error performance is remarkably improved, with only negligible complexity increase.

We propose a new coarse frequency offset estimation algorithm in orthogonal frequency-division multiplexing systems, which performs robust operation in the presence of a symbol timing offset within an allowed range. The effect of a symbol timing offset on estimating a coarse frequency offset is analyzed, and the coherence phase bandwidth according to the allowed symbol timing offset is introduced. The proposed algorithm adapts the concept of the coherence phase bandwidth for the purpose of weakening the effect of the symbol timing offset.

The authors propose a multiuser detector with channel estimation capability using a radial basis function (RBF) network in a synchronous multicarrier-code division multiple access (MC-CDMA) system. The authors propose to connect an RBF network to the frequency domain to effectively utilize the frequency diversity. Simulations were performed over frequency-selective and multi-path fading channels. These simulations confirmed that the proposed receiver can be used both for the channel estimation and as a multi-user receiver, thus permitting an increase in the number of active users.

We proposes a blind equalization algorithm that enables spatio-temporal diversity in OFDM systems. The proposed algorithm is based on the least-squares approach, which exploits the OFDM signal structure and multiple outputs of an antenna array. The sensitivity of the proposed equalizer to the input signal and noise are presented, and the proposed algorithm is evaluated in simulation results.

A new coarse frequency offset estimation algorithm is proposed which performs robust operation in the presence of a symbol timing offset in an allowed range. The effect of a symbol timing offset on estimating a coarse frequency offset is analyzed and the coherence phase bandwidth according to the allowed symbol timing offset is introduced. In addition, a new technique for performing the coarse frequency estimation using the coherence phase bandwidth is described. Simulation results show that the proposed algorithm estimates a coarse frequency offset well under both AWGN and multipath channels even in the presence of the symbol timing offset within an allowed range.

We consider the use of an adaptive sigmoid function for a pipelined successive interference cancellation (PSIC) in a direct sequence/code multiple access system (DS/CDMA). The PSIC is a pipelined modification of a successive interference cancellation (SIC). To compensate the decoding delay of the SIC, the PSIC was proposed. Because the decision variable of the latter stage is more reliable than that of the early stage in the PSIC, the wrong derision of the early stage affects the decision of the final stage. So, we use a tentative decision function, which is adjusted depending on the variance of the multiple access interference (MAI). The analysis and computer simulation results of the adaptive PSIC (ADP-SIC) show that the ADP-SIC outperforms both of the SIC and the PSIC with almost same decoding delay.

The performance of an MC-CDMA system with carrier frequency offsets in correlated multipath fading is mathematically analyzed. The bit error rate is obtained from such analysis for the uplink Rayleigh fading channel. The derived results show that the performance of the MC-CDMA system is sensitive to the frequency offset, and the correlation of the subcarriers. That is, the performance degradation of the MC-CDMA system is caused by both carrier frequency offset and correlation among subcarriers. From the derived results, however, the performance degradation due to correlation among subcarriers is more severe than effect of frequency offset.

An OFDM signal with large peak-to-average power ratio (PAPR) can cause power degradation (inband distortion) and spectral spreading (out-of-band radiation) by being clipped passing through a power amplifier. The PAPR characteristics are analyzed in two different aspects. From the foundation of the analysis, we propose Hadamard transform, which is ascribed to the relationship between the autocorrelation function and PAPR. Extensive computer simulations show that Hadamard transform is an effective technique to reduce PAPR.

Various studies have shown that adaptive antenna array techniques are good one to cancel multipath and multiuser interference. However, many of them need training signals to estimate the desired signal or have heavy complexity to compute the algorithmic processing. An antenna array receiver in a direct sequence/code division multiple access (DS/CDMA) system with M-ary orthogonal modulation is proposed. At every antenna element a channel estimation technique is used, which refers to the maximum correlator output of a RAKE receiver. The estimated channel coefficients enable the signal to be detected coherently with no need of training signals. Moreover, the antenna weights, which are determined by the estimated channel coefficients, make it possible to focus the receiver on the desired signal and to suppress the interference signal. Consequently, the proposed receiver has a 2D (dimensions) RAKE structure which can exploit both spatial diversity and temporal diversity. Simulation results show that the performance of the proposed scheme improves 1.5 dB in E/sub b//N/sub 0/ at a BER=10/sup -3/ over the receiver of noncoherent detection. The results also show that at a BER=10/sup -2/ the proposed receiver improves about 2.3 times and 3 times the system capacity with 3 and 5 antenna elements, respectively.

A combined diversity scheme of multiple turbo codes and multiple antennas for a synchronous direct sequence-code division multiple access (DS-CDMA) system over Rayleigh fading mobile radio channels is considered to improve the BER performance. In spite of the multiple structures, by employing a parallel mode decoding scheme the time delay becomes equivalent to that of a single turbo code system. Furthermore, the truncated trellis structure applied to MAP reduces the number of iterations from K for the original BCJR MAP to (K+1)/2 for the proposed MAP.

In order to improve the performance and simplify the structure of an equalizer using a bilinear recursive polynomial perceptron (BRPE), a new equalizer, which is an equalizer using a bilinear recursive polynomial perceptron with decision feedback (BRPDFE), is proposed. The proposed BRPDFE is compared with the BRPE in terms of mean square error (MSE). The performance is compared with the conventional decision feedback equalizer (DFE), the multilayer perceptron decision feedback equalizer (MRPDFE) and the BRPE in terms of bit-error rate (BER) in true data transmission systems. They ar compared on a digital communication system in which the dominant distortion factor is intersymbol interference and a digital storage system in which the primary interference element is nonlinear distortion. The results from analysis and simulation show that the proposed BRPDFE is superior to the other equalizers. Moreover, the BRPDFE has the simpler structure than the other equalizers.

In order to reduce the complexity of a radial basis function (RBF) network as a multiuser demodulator and an equalizer, we propose a simplified hybrid neural network architecture. The proposed neural network, which is called RN, has the structure of combining a radial basis function network with multilayer perceptrons (MLPs). The RBF network yields the linear combining output of the hidden layer while the proposed hybrid neural network produces the output using nonlinear combining techniques. From computer simulation results, the RN with the reduced structure from about 50% to about 70% over the RBF network shows better than or almost equal performance to the RBF network as a multiuser demodulator and an equalizer.

A multi-user detector with an ability of channel estimation using a radial basis function (RBF) network is proposed in a multicarrier-code division multiple access (MC-CDMA) system. In the proposed scheme, the RBF network is connected to the frequency domain to effectively utilize the frequency diversity. Simulations were performed over the frequency selective and multipath fading channel. From these simulations, the proposed receiver is verified to be used for both the channel estimation and the multi-user receiver that permits more active users.

The effect of carrier frequency on the performance of an MC-CDMA (multi-carrier code division multiple access) system is considered. The bit error probability of the downlink of this system is analyzed taking into account the effect of frequency offset. This paper is focused on the derivation of the bit error probability mathematically. The derived results show that the performance of MC-CDMA is very sensitive even to a relatively minute degree of frequency offset. Parameters such as the degree of carrier offset, number of subcarriers, number of users, method of combining, and type of channel are varied so that the influence of each parameter can be assessed.

The effect of carrier frequency offset on the performance of an MC-CDMA (multi-carrier code division multiple access) system is considered. Bit error probability of the downlink of an MC-CDMA system is analyzed taking into account the effect of frequency offset. Derived results show that the performance of an MC-CDMA system is very sensitive even to a relatively small frequency offset. As a countermeasure against carrier frequency offset, pulse shaping is proposed and analyzed. Comparison of the proposed method using different pulse shapes to conventional MC-CDMA transmission scheme shows that a considerable degree of performance improvement is possible.

In this paper, the average bit error probability of an uplink multi-carrier code division multiple access (MC-CDMA) system using maximal-ratio combiner (MRC) is derived for a frequency-selective slow fading channel. In contrast to a conventional multicarrier channel model in which the subcarriers experience independent subchannels, the subcarriers in MC-CDMA system undergo the correlated fading channel in a general tapped-delay line (TDL) model with variable channel parameters. Numerical results are shown based on the derived result, to show that conventional multicarrier channels tend to overemphasize the performance of an MC-CDMA system.

In order to compensate for severe intersymbol interference (ISI) and combat nonlinear distortions in digital magnetic recording systems, Bayesian equalizer using the radial basis function (RBF) network, REQ, is applied as a channel equalizer to a data storage system using partial erasure (PE) model. It can be seen from various computer simulation results that the REQ has a more complex structure than a conventional linear equalizer (LE) while the bit-error-ratio (BER) of the REQ is lower than that of the LE. Using various simulation parameters such as the width of the basis function, the updating algorithm for the centers in REQ, the recording density of a data storage system, and so forth, the performance in terms of BER of the REQ is examined.

Among the useful blind equalization algorithms, stochastic-gradient iterative equalization schemes are based on minimizing a nonconvex and nonlinear cost function. However, as they use a linear FIR filter with a convex decision region, their residual estimation error is high. In this paper, four nonlinear blind equalization schemes that employ a complex-valued multilayer perceptron instead of the linear filter are proposed and their learning algorithms are derived. After the important properties that a suitable complex-valued activation function must possess are discussed, a new complex-valued activation function is developed for the proposed schemes to deal with QAM signals of any constellation sizes. It has been further proven that by the nonlinear transformation of the proposed function, the correlation coefficient between the real and imaginary parts of input data decreases when they are jointly Gaussian random variables. Last, the effectiveness of the proposed schemes is verified in terms of initial convergence speed and MSE in the steady state. In particular, even without carrier phase tracking procedure, the proposed schemes correct an arbitrary phase rotation caused by channel distortion.

A "stop-and-go" decision-directed blind equalization scheme is newly proposed. This scheme uses the structure of complex-valued multilayer feedforward neural networks, instead of the linear transversal filters that are usually used in conventional LMS-type blind equalization schemes. A complex-valued activation function composed of two real functions is used. Each real activation function has a multi-saturated output region in order to deal with QAM signals of any constellation size. Also, the complex backpropagation algorithm is modified for the proposed scheme. Computer simulations are performed to compare the proposed scheme with the conventional "stop-and-go" algorithm in terms of the convergence speed, the MSE value in the steady state and the constellation of QAM signals after the initial convergence. Simulation results demonstrate the effectiveness of the proposed scheme.

A new neural-network architecture called the parallel, self-organizing, hierarchical neural network (PSHNN) is presented. The new architecture involves a number of stages in which each stage can be a particular neural network (SNN). At the end of each stage, error detection is carried out, and a number of input vectors are rejected. Between two stages there is a nonlinear transformation of input vectors rejected by the previous stage. The new architecture has many desirable properties, such as optimized system complexity (in the sense of minimized self-organizing number of stages), high classification accuracy, minimized learning and recall times, and truly parallel architectures in which all stages operate simultaneously without waiting for data from other stages during testing. The experiments performed indicated the superiority of the new architecture over multilayered networks with back-propagation training.

Two neural network architectures involving nonlinear spectral transformations are described. The first architecture involves generalization of nonlinear matched-filtering techniques, yielding a network that is very fast in learning and recall as well as highly accurate in classification. The second architecture is hierarchical with a number of stages; after each stage, error detection is carried out, followed by nonlinear spectral transformations when the error measure is above threshold.>