Satellite communication systems (SCSs) have recently evolved into mega-constellations with thousands of non-geostationary orbit (NGSO) satellites. They offer an advantage of global coverage with low latency for communication and information access but impose an increased risk of harmful radio frequency interference (RFI) to a ground-based radio astronomy system (RAS). RFI assessment for such a scenario is crucial and the existing approaches which are based on simulations do not scale well with a large NGSO satellite constellation size and a long RAS observation time. To overcome such limitations, this paper develops analytical models of RFI from the NGSO SCSs to the ground RASs under various system conditions. These models are extended to future scenarios with sky radio quiet zone deployments. General applicability and accuracy of the analytical models are corroborated by comparing them with simulation-based results for several NGSO SCSs, RAS locations and RAS observation directions.

Spaceborne passive sensing (SPS) has been crucial in obtaining the Earth’s environmental and meteorological information. As its signal levels are significantly lower than those of communication systems, passive sensing bands need to be protected from radio frequency interference (RFI) of other transmission sources. Here, we investigate the RFI of inter-satellite links (ISLs) of an emerging mega-constellation medium earth orbit (MEO) satellite communication system to nadir-pointing low earth orbit (LEO) SPS. We develop accurate static and dynamic analytical RFI models by introducing static and time-varying virtual satellite distribution functions and the regions of line-of-sight RFI paths. Performance comparison between the proposed analytical approach and the simulation-based approach for various scenarios of MEO satellites and LEO SPS system settings corroborates the accuracy, flexibility, and generality of the proposed analytical models, and reveals insights into the corresponding RFI characteristics. The results also indicate that it is possible for MEO ISLs to utilize the SPS spectrum while keeping the RFI to the SPS at an acceptable level.

In this paper, we consider three ground-aircraft communication issues, namely, ground-centric connectivity, aircraft-centric connectivity, and link capacity analysis. Particularly, we propose the continuous connection probability over at least τ seconds, termed as τ-connectivity, for the ground-centric link and aircraft-centric link respectively, which are derived in the expression of several key parameters such as the aircraft/ground station density, the aircraft velocity, and the connectable elevation angle threshold. Theoretical tradeoff between ergodic outage capacity and the number of connected aircraft is analyzed for low and high transmit power regimes. The hypotheses in the ground-centric τ-connectivity model are tested, and we observe a good match between the theoretic probability and the real world data-based probability although some real world aircraft statistic characters do not match the ideal assumption perfectly. Next, the aircraft-centric τ-connectivity is analyzed theoretically. Approximate yet practically accurate closed form aircraft-centric connection probabilities are derived and verified with Monte Carlo simulations. Furthermore, considering atmospheric gas and cloud attenuation, we compare the link budget and capacity between the ground-centric ground-to-aircraft link and the ground-to-satellite link under the τ-connectivity and demonstrate the advantage of ground-aircraft communication.

Large-scale non-geostationary orbit (NGSO) satellite communication systems (SCSs) are gaining interest from industries because of their ubiquitous wireless access and backhaul capabilities. However, the NGSO SCSs’ global downlink transmission can cause radio frequency interference (RFI) to the radio astronomy system (RAS) on earth. Thus, this paper first investigates RFI impacts of a large-scale NGSO SCS. Our RFI analyses show that a large-scale low earth orbit (LEO) SCS completely disrupts RAS’s continuum observation within or adjacent to the SCS downlink bands, which limits coexistence and growths of both SCS and RAS. To overcome such limitation, we propose a new paradigm where SCS and RAS are integrated into the NGSO satellite system, thus effectively creating large-scale telescopes in orbit. This integrated system not only avoids SCS’s RFI to RAS but also offers more spectrum access opportunities to both SCS and RAS. In addition, this paper addresses two related problems of the new paradigm, namely, the spectrum resource allocation problem and the RAS data transport problem. Our performance evaluation illustrates the advantages of the proposed paradigm in terms of accessible spectrum bands, RAS observation performance, and SCS maximum mean supportable data rate as well as enabling coexistence and growths of both types of services.

For a radio resource-limited multi-tier machine-type communication (MTC) network, controlling random access congestion while satisfying the unique requirements of each tier (type) and guaranteeing fairness among nodes is always a challenge. In this paper, we study the network dimensioning and radio resource partitioning for the uplink of an MTC network with the signal-to-interference ratio-based clustering and relaying, where the MTC gateways (MTCGs) capture and forward the packets sent from MTC devices (MTCDs) to the base station (BS). Specifically, under transmission outage probability constraints, we investigate the trade-off between network utility (in terms of transmission capacity and revenue) and resource allocation fairness. With both outage probability constraints and minimum MTCD density constraints, we propose approaches to maximize the weighted sum of quality of experience of different tiers of MTCDs. Furthermore, a transmit power control strategy for MTCG-to-BS link is proposed to achieve a constant data rate.

In this paper, we consider a downlink orthogonal frequency division multiplexing system from a base station to a high-speed train equipped with fully/partly calibrated massive uniform linear antenna-array (ULA) in wireless environments with abundant scatterers. Multiple Doppler frequency off- sets (DFOs) stemming from intensive propagation paths together with transceiver oscillator frequency offset (OFO) result in a fast time-varying frequency-selective channel. We develop an angle domain carrier frequency offset (CFO, a general designation for DFO and OFO) estimation approach. A high-resolution beamforming network is designed to separate different DFOs into a set of parallel branches in angle domain such that each branch is mainly affected by a single dominant DFO. Then, a joint estimation algorithm for both maximum DFO and OFO is developed for fully calibrated ULA. Next, its estimation mean square error performance is analyzed under inter-subarray mismatches. To mitigate the detrimental effects of inter-subarray mismatches, we introduce a calibration-oriented beamforming parameter and develop the corresponding modified joint estimation algorithm for partly calibrated ULA. Moreover, the Cramér-Rao lower bound of CFO estimation is derived. Both theoretical and numerical results are provided to corroborate the proposed method.

This paper proposes a new paradigm to accommodate the rapid growth of non-geostationary orbit (NGSO) satellite communications system (SCS) and radio astronomy system (RAS). Traditionally, SCS and RAS are operated in different frequency bands to avoid radio frequency interference (RFI). However, the prospects of large-scale NGSO satellites cast a distressful RFI scenario to the ground-based RAS observations. By integrating SCS and RAS into the NGSO satellite system, the proposed paradigm mitigates the RFI significantly, enables plenty of new observation opportunities and capabilities for RAS, provides higher throughput for SCS, and creates potential new services/revenues for SCS. Potential deployment scenarios are discussed. In addition, the paper addresses two related problems of the new paradigm, namely the spectrum resource allocation problem and the RAS data transport problem. Simulation results demonstrate the performance advantages of the proposed paradigm and related solutions.

In this paper we consider a densely deployed phantom cell system providing a low latency network for massive connectivity. Non-orthogonal pilot designs serve as a promising solution to support a large number of users but fast collision detection at the receiver is needed for low latency networks. Recently, a nice solution based on an on-off type non-orthogonal pilot design with collision detection capability has been proposed. It can serve more users than the orthogonal pilot design but at the cost of degraded channel estimation performance compared to the orthogonal optimum pilot design. We propose a new non-orthogonal pilot design with collision detection capability and improved channel estimation performance. An optimum threshold based detection criteria is developed. We further show that a dynamically calculated optimum threshold outperforms a fixed threshold based detection. Next, we investigated non-orthogonal pilot designs for fractional bandwidth allocation. We propose two new non-orthogonal pilot designs for physical resource block (PRB) based resource allocation. Both of the new designs support fast collision detection at the receiver. Performance evaluation results show that the proposed schemes provide equivalent or better channel estimation performance and support much more users than the orthogonal pilots defined in the current 4G standards.

Tera-Hertz (THz) transmission can offer several attractive applications, yet developing low-cost energy-efficient THz devices is at an early stage. The most promising low-cost THz transmitter architecture in the literature is the so-called frequency-multiplier-last architecture but it is incapable of transmitting quadrature amplitude modulation (QAM) due to the architecture's inherent nonlinear distortions. We study such nonlinear THz communication systems by incorporating the non-linearity aspects of the low- cost THz devices into the signal model. Then we propose novel pre-compensation schemes to compensate the nonlinearity effect, thus enabling QAM-capable frequency-multiplier-last architecture for THz systems. Numerical results show that the proposed pre-compensation schemes overcome the prominent problems experienced in the existing scheme, namely severe nonlinear distortions of the modulation symbols as well as spectral spreading and/or large spectrum sidelobes.

The expansion of wireless communication systems across frequency, time, and space has exacerbated the radio frequency interference (RFI) issue to radio astronomy systems (RAS). A recent solution based on a coexistence paradigm offers some RFI- free durations for RAS but further expansion of RAS observation faces with strong RFI. To enhance spectrum access opportunities for both wireless communications and RAS, we explore a new approach where both systems simultaneously access the spectrum for some durations and RAS exploits RFI cancellation. The main challenge is nonlinear distortion caused by strong RFI. We study this nonlinearity's effect on RAS observation performance and develop a nonlinear-RFI cancellation scheme and a modified RAS signal power estimation scheme. We also design a channel estimator for the nonlinear system based on the unscented Kalman filter. Simulation results in flat stationary channels and multipath Rayleigh fading channels show the proposed approach yields a great improvement in RAS signal power estimation performance even at strong RFI levels.

Owing to its reduction in hardware complexity and power consumption, hybrid beam-forming has become one of the most promising substitutes for fully digital beam-forming (FDB) in large antenna array systems. In conventional hybrid beam-forming (CHB) architecture, phase shifters are designed to operate at RF band and inaccuracies of the RF phase shifts are unavoidable in practice. Furthermore, these RF phase shifters still consume considerable power if implemented as active devices or a large space/area on dice (form factor) if implemented as passive devices. This issue becomes more critical for massive MIMO systems where lower power consumption and smaller form factor are much desired due to the large number of antenna elements. Aiming at reducing power consumption and form factor of the receiver, we investigate a new method to insert phase shifts via adjusting sampling-offset on sample and hold (S&H) circuit. Based on this method, a new receiver architecture with RF under-sampling technique and adjustable S&H circuit is proposed. We also develop a method to find out the required phase shifts for the beam-forming in the proposed receiver architecture. Performance evaluation at 28 GHz band shows that the proposed technology gives practically the same bit error rate (BER) performance as the CHB architecture but yields advantages in terms of power consumption, chip area, and beam-forming phase errors, which makes it appealing for massive MIMO systems.

For sustainable growth and coexistence of cellular wireless communications (CWC) and radio astronomy system (RAS), a coordinated shared spectrum access paradigm was recently introduced. Embracing such paradigm, this paper proposes a distributed radio telescope (DRT) system which can geographically and spectrally coexist with CWC while offering additional capability or performance enhancement to RAS. Theoretical performance analysis of the DRT system with different quantization resolutions is presented, and approximate closed-form expressions are obtained. Furthermore, an analytical expression for the DRT system parameters under the shared spectrum access paradigm to achieve the same performance as the existing single-dish RAS with a radio quiet zone is developed to provide guidance in the DRT system design. The numerical and simulation results illustrate feasibility and potentials of the proposed DRT system.

For a radio resource limited multi-tier Machine-type Communication (MTC) network, controlling random access congestion while satisfying the unique requirements of each tier and guaranteeing fairness among nodes is always a challenge. In this paper, we propose three approaches for network dimensioning and radio resource partitioning for the uplink of an MTC network with signal-to-interference ratio (SIR)-based clustering and relaying, where MTC gateways (MTCGs) capture and forward the packets sent from MTC devices (MTCDs) to the base station (BS). Assuming that each tier has a different outage probability constraint, our first resource allocation method achieves maximum sum network capacity, while the other two methods achieve suboptimal sum network capacity with higher degrees of fairness. The proposed methods are also applicable when the objective is the network operator's total revenue instead of sum network capacity.

Millimeter-wave (mmWave) large-scale antenna systems typically apply hybrid analog-digital precoders to reduce hardware complexity and power consumption. In this paper, we design the hybrid precoder to maximize the average sum rate of mmWave multiuser downlink systems with partial channel knowledge in contrast to the full channel knowledge assumption of the existing approaches. We design the zero-forcing (ZF) hybrid precoder with partial channel knowledge, and provide the necessary conditions for its application. If these conditions are not satisfied, we propose another hybrid precoder design which is based on the signal-to-interference- and-noise ratio (SINR) and signal-to-leakage-and-noise ratio (SLNR). Performance results corroborate advantages of the proposed hybrid precoder designs over a benchmark design.

In this letter, we propose a hybrid analog-digital precoder design to enhance the physical layer security of millimeter-wave (mmWave) multiple-input single-output (MISO) systems with partial channel knowledge. We derive a closed-form expression for the average signal-to-interference-and-noise ratio (SINR) of the eavesdropper (Eve) as a function of the hybrid precoder. Using the average SINRs of Eves, we derive a secrecy rate lower bound. Then, we propose a low-complexity artificial-noise-aided (AN-aided) hybrid precoder design to maximize the secrecy rate lower bound. Numerical results show that the proposed AN-aided hybrid precoder achieves comparable performance to that of the fully digital precoder, with much lower hardware complexity. Moreover, the proposed AN-aided hybrid precoder outperforms the existing hybrid precoder design. Effect of finite-resolution phase shifters on the proposed precoder is also investigated.

This paper proposes a new paradigm for spectrum sharing between cellular wireless communications (CWC) and radio astronomy systems (RAS). In contrast to the existing paradigm of geographical and spectral isolation between CWC and RAS, this paper develops a three phase spectrum access, which enables geographical and spectral coexistence between CWC and RAS. Shared spectrum access zone (SSAZ) is created around the RAS site and CWC cells within the SSAZ follow the three phase spectrum access scheme while those outside the SSAZ have full spectrum access. In addition, system characteristics-based improved spectrum sharing is developed. Furthermore, a built-in fine tuning mechanism is presented for addressing mismatches between design and practical environments as well as for facilitating service evolutions. Performance evaluation results demonstrate that the proposed paradigm offers 1) certain guaranteed spectrum access to RAS, which is impossible in the existing paradigm, 2) capability to handle higher peak and mean traffics to CWC under spectrum restructuring of both CWC and RAS bands, and 3) overall improved spectrum utilization.

Millimeter-wave large-scale antenna systems typically apply hybrid analog-digital precoders to reduce hardware complexity and power consumption. In this paper, we design hybrid precoders for physical-layer security under two types of channel knowledge. With full channel knowledge at transmitter, we provide sufficient conditions on the minimum number of RF chains needed to realize the performance of the fully digital precoding. Then, we design the hybrid precoder to maximize the secrecy rate. By maximizing the average projection between the fully digital precoder and the hybrid precoder, we propose a low-complexity closed-form hybrid precoder. We extend the conventional projected maximum ratio transmission scheme to realize the hybrid precoder. Moreover, we propose an iterative hybrid precoder design to maximize the secrecy rate. With partial channel knowledge at transmitter, we derive a secrecy outage probability upper-bound. The secrecy throughput maximization is converted into a sequence of secrecy outage probability minimization problems. Then, the hybrid precoder is designed to minimize the secrecy outage probability by an iterative hybrid precoder design. Performance results show the proposed hybrid precoders achieve performance close to that of the fully digital precoding at low and moderate signal-to-noise ratios (SNRs), and sometimes at high SNRs depending on the system parameters.

Large-scale multi-input multi-output (MIMO) technique has drawn considerable research interest recently. However, multiuser transmissions in large-scale MIMO systems would face challenging estimation and compensation for multiple carrier frequency offsets (CFOs) that co-exist at the receiver. In this paper, we design a new frequency synchronization scheme in multiuser orthogonal frequency division multiplexing uplink with the aid of scattered pilot symbols. We specifically consider a base station with a large number of antennas and propose to assign a number of scattered pilot subcarriers to each user. With sufficient spatial dimensions offered by the large number of antennas, the designed scheme could estimate CFO for each user individually and eliminate the necessity of the multidimensional search. Moreover, after the CFO estimation, the receive beamforming matrix is further designed for inter-user interference cancelation, which yields an equivalent single user transmission model. The conventional single user channel estimation and data detection can then be performed. Finally, the numerical results are provided to verify the proposed studies.

The emerging millimeter-wave (mm-wave) MIMO systems are subject to strong radio frequency (RF) distortions and their compensation is crucial to realize such systems. In the existing literature, several estimation and compensation schemes have been proposed for RF distortions, such as carrier frequency offset, phase noise, and in-phase and quadrature amplitude and phase imbalance (IQI). However, in-phase and quadrature timing mismatch (IQTM) is largely ignored. This paper investigates the effect of the IQTM on mm-wave system performance and reveals that IQTM causes a specific image rejection ratio characteristic, which is substantially different from the regular frequency-dependent IQI and it substantially prolongs the effective channel length. If not compensated, the IQTM can degrade system performance significantly. As a solution to this IQTM problem, this paper proposes novel pilot designs for transmit and receive IQTM estimation, and develops corresponding estimators, transmission protocol, and compensation schemes. MIMO averaging is also proposed, which substantially enhances the IQTM estimation performance. Simulation results show that our proposed pilot designs and estimators offer an efficient solution to the IQTM problem.

This paper proposes a new paradigm of shared spectrum access between cellular wireless communications (CWC) and radio astronomy systems (RAS). Traditionally, most spectrum chunks are separately allocated to CWC and RAS, with a few shared spectrum without guaranteed spectrum access for RAS. Furthermore, RAS sites are generally set up at remote locations, protected by radio quiet zones. In view of growths in spectrum demands from both systems, in population, and in applications and service deployments, the existing paradigm of geographical and spectral isolation between CWC and RAS will become inefficient. This paper develops a three phase spectrum access which enables geographical and spectral coexistence between CWC and RAS. Additionally, traffic statistics based improved spectrum access is developed. Furthermore, a built-in fine tuning mechanism is presented for addressing mismatches between design and practical environments as well as for facilitating service evolutions. Performance evaluation results demonstrate advantages of the proposed paradigm.

Massive MIMO systems offer large spectrum efficiency improvements but when applied to systems with noncontiguous bands as commonly encountered in practice, their performance gains are largely offset by the overhead in acquiring channel state information (CSI). This paper presents three schemes to enable overhead-efficient massive MIMO in noncontiguous bands with frequency-selective channels. By incorporating CSI errors, CSI overhead, pilot contamination, and guard interval cost, achievable rate or throughput expressions of uplink and downlink of the proposed schemes are derived. Novel resource adaptation mechanisms between uplink and downlink are proposed and analyzed. Compared with the conventional frequency division duplexing scheme, analytical/numerical studies elaborate substantial spectrum efficiency enhancement, and flexible resource adaptation capability of the proposed schemes.

Massive multi-input multi-output (MIMO) systems require channel state information (CSI) at base stations. In multicell environments, adjacent cells using the same spectrum cause a pilot contamination problem and degrade CSI quality. Under such practical environments, it is important to assess achievable rate of the system for a proper system design. There exist achievable rate bounds of uplink (UL) systems in the literature, but general closed-form rate bounds for downlink (DL) systems are not available yet. This paper studies a practical massive MIMO DL system with or without pilot-aided coherent detection under the scenario of pilot contamination, and derives closed-form approximate achievable DL rate expressions for both maximum ratio combining (MRC) and zero forcing (ZF) precoders. For the case with DL pilot, a corresponding DL effective channel estimator is also developed. Numerical simulation results corroborate accuracy of the closed-form rate expressions and they also show performance characteristics of pilot-contaminated massive MIMO DL systems with MRC and ZF precoders.

We present a new approach for enhancing spectrum efficiency by means of multisystem spectrum channelization. The proposed approach incorporates impact between spectrum channelization and exploiting channel state information at transmitter (CSIT). We create disparities in accuracy of CSIT for different systems through frame offsets between the systems such that the time segments of the best CSIT accuracy for different systems are disjoint. We assign more spectrum resources to each system during its duration of best CSIT accuracy. This yields systematic and periodic changing of spectrum boundaries (in contrast to the fixed boundaries of the existing approach) while providing better CSIT accuracy level and larger degrees of freedom in resource allocation and hence higher spectrum efficiency for each system. We formulate the given collaborative multisystem channelization problem, derive the analytical performance metrics (postscheduling average data rate under time-varying CSIT accuracy for several schedulers), and develop the optimally channelized resource maps of the systems. In the basic form, the proposed channelization is designed offline once. We also generalize it into adaptive channelization to accommodate disparities of traffic demands for different systems. Analytical and simulation results corroborate substantial performance gains of the proposed scheme over the existing approach.

We develop new transmission schemes for multipoint-to-multipoint (M2M) networks which, unlike the existing approaches, address duplexing/multiplexing and multiple access jointly and possess true M2M characteristics in the physical layer. In the proposed schemes, all forward and reverse links of the M2M network share the overall spectrum concurrently in an orthogonal frequency division manner. This provides increased degrees of freedom and enhanced diversity in scheduling and resource allocation, thus leading to performance enhancement. We first discuss practical implementation issues. Next, we develop our proposed schemes for both centralized and distributed access scenarios. Under the centralized access, we illustrate advantages of the proposed approach for M2M networks using a scheduling algorithm and provide a closed-form analytical throughput upper-bound. Next, we extend the proposed approach to multicasting and present a new scheduling strategy and its closed-form analytical approximate throughput expression. Under the distributed access, we develop a generalized multi-channel carrier sensing multiple access/collision avoidance scheme for M2M networks and propose a new scheme which exploits local channel information. Closed-form analytical throughput expressions for a specific scenario are also presented. Simulation results corroborate substantial performance gains of the proposed schemes over the conventional schemes in both centralized and random access scenarios.

Interference alignment (IA) is known to significantly increase sum-throughput at high SNR in the presence of multiple interfering nodes, however, the reliability of IA is little known, which is the subject of this paper. We study the error performance of IA and compare it with conventional orthogonal transmission schemes. Since most IA algorithms require extensive channel state information (CSI), we also investigate the impact of CSI imperfection (uncertainty) on the error performance. Our results show that under identical rates, IA attains a better error performance than the orthogonal scheme for practical signal to noise ratio (SNR) values but is more sensitive to CSI uncertainty. We design bit loading algorithms that significantly improve error performance of the existing IA schemes. Furthermore, we propose an adaptive transmission scheme that not only considerably reduces error probability, but also produces robustness to CSI uncertainty.

Orthogonal multicarrier division duplexing (MDD) is a new duplexing scheme that gains the advantages of both time division duplexing and frequency division duplexing. Resource allocation for multipoint-to-multipoint (M2M) MDD is considered in this paper. We propose joint subcarrier and power allocation for M2M MDD that maximizes the sum rate of the network under either total or individual power constraint. Simulation results show that MDD outperforms conventional TDD and TDD-OFDMA systems. Some insights on M2M MDD under different channel conditions are given and convergence of the proposed algorithm is shown through simulation results.

Traditional communication systems adopt either time division duplexing (TDD) or frequency division duplexing (FDD). TDD requires guard interval and FDD needs guard band. Orthogonal multicarrier division duplexing (MDD) is a new duplexing scheme, which can avoid both guard interval and guard band. In this paper, we propose a new MDD scheme with a group-wise subcarrier partition, which is more robust to the inter-carrier interference (ICI) than the subcarrier-wise scheme, while keeping the same or better outage performance. We introduce somewhat modified block outage probability and block outage capacity metrics which are defined in such a way to facilitate the analysis and design of duplexing schemes in point-to-point (P2P) system. We give the closed-form of the block outage probability and derive the maximal capturable frequency diversity order based on outage probability. By analysis and simulation, it is verified that in P2P system, the proposed MDD scheme outperforms the existing schemes.

To find an efficient and valid alternative of polysomnography (PSG), this paper investigates real-time sleep apnea and hypopnea syndrome (SAHS) detection based on electrocardiograph (ECG) and saturation of peripheral oxygen (SpO(2)) signals, individually and in combination. We include ten machine-learning algorithms in our classification experiment. It is shown that our proposed SpO (2) features outperform the ECG features in terms of diagnostic ability. More importantly, we propose classifier combination to further enhance the classification performance by harnessing the complementary information provided by individual classifiers. With our selected SpO(2) and ECG features, the classifier combination using AdaBoost with Decision Stump, Bagging with REPTree, and either kNN or Decision Table achieves sensitivity, specificity, and accuracy all around 82% for a minute-based real-time SAHS detection over 25 sleep-disordered-breathing suspects' full overnight recordings.

Interference control is crucial in cognitive radio networks and the existing related works focus on quasi-static channels. The expansion of wireless services and applications in everyday life has necessitated support of higher mobility, as evident from the emerging wireless standards. This results in time-varying fading channels within a transmission frame for which the interference power control schemes and power allocation strategies have not been developed yet. This paper proposes several interference control and power allocation strategies with different criteria for underlay cognitive radios in time-varying frequency-selective fading channels and extend the transmission schemes to multiple secondary systems. The transmission schemes are evaluated in terms of interference statistics at the primary receiver and the ergodic capacity of the secondary user link. Simulation results over the wide Doppler range show how the proposed schemes with different criteria shape the interference at the primary receiver and the capacity of the secondary user. The results also illustrate better interference control capability of the proposed approaches over the existing scheme.

Carrier frequency offset (CFO) causes spectrum misalignment of transmitter and receiver filters. This misalignment leads to energy loss and distortion of received signal, resulting in performance degradation of the whole system. However, these issues are often overlooked by existing works. This paper presents an exact signal model for OFDM that takes into account the CFO-induced spectral misalignment and related aliasing effects. Incorporating these practical issues, we also propose a new preamble structure and a new CFO compensation approach to mitigate their negative effects. Theoretical analysis and simulation results substantiate the importance of the exact signal model and the advantage of the proposed CFO compensation scheme.

We consider the effective degrees of freedom (DoF) achieved by interference alignment when channel state information (CSI) is acquired by training and feedback. For a flat block- fading K × K interference channel with power P per transmitter and M antennas per node, we show that interference alignment achieves higher DoF than orthogonal transmission (e.g., TDMA) only if the channel coherence time is large and the capacity of feedback link is at least as Θ(log P). Under this condition, to maximize the effective DoF, each receiver needs to feed back CSI via (M2-1)log P bits per coherence interval; smaller growth rate of feedback bits will decrease the effective DoF. We also show that in the presence of training and feedback cost, K = 3 achieves the optimal DoF for a broad range of channel characteristics; with larger number of user pairs, the DoF falls short of its optimum and beyond a certain point becomes a decreasing function of K.

LTE-Advanced systems have adopted the concept of Carrier Aggregation (CA) to meet the data rate requirements of IMT-Advanced systems. The CA approach opens up several new implementation challenges as well as opportunities for further performance improvement. Existing works on CA focus on implementation issues, but exploitation of CA-specific features for performance improvement has not been addressed yet. In this paper, we fill this gap and investigate the enhanced frequency diversity exploitation in CA based systems. New interleaving schemes for both downlink OFDMA and uplink SC-FDMA of LTE-Advanced systems are proposed to capture CA-specific enhanced frequency diversity with simple implementation. The proposed schemes also provide full backward compatibility with the LTE system. Performance evaluation via simulation in a simplified LTE system environment corroborates that the proposed approach provides performance advantages in both downlink and uplink.

Next generation wireless systems require much higher data rate supports and hence larger bandwidths than the existing systems. Due to the compatibility requirement with the existing legacy systems and the constraints on the availability of contiguous spectrum, bandwidth (or carrier) aggregation has recently emerged as a practical means for supporting high data rate requirement of next-generation systems. A practical issue associated with the bandwidth or carrier aggregation is the significant increase of the peak-to-average power ratio (PAPR) of the time-domain signal. This paper proposes a low-complexity PAPR reduction method for bandwidth aggregated systems with orthogonal frequency division multiplexing / multiple-access (OFDM/OFDMA) or single-carrier frequency division multiple access (SC-FDMA). The proposed approach adopts the existing selective mapping concept, but with some modifications for substantially reducing complexity and signaling overhead, while adapting to carrier-aggregated systems. Performance evaluation under 3GPP LTE-Advanced environment shows PAPR reduction advantage of the proposed method while not requiring any signaling overhead.

We propose multiple antenna transmission methods for fast fading channel conditions. Proposed methods allocate alternative multiple antenna transmission modes depending on the SNR, modulation order and Doppler frequency to increase reliability, i.e., decrease bit-error-rate (BER). A major difference of our approach from previous works is to allow different transmission modes during a single frame by considering the channel variations within the frame for high mobility scenarios. First method effectively makes use of available channel knowledge by allocating beamforming (BF) as long as channel knowledge is not outdated and switches to space frequency block coding (SFBC) afterwards. Approximate BER expressions for BF and SFBC that are functions of SNR, modulation order, Doppler frequency and initial channel knowledge are derived to be used as a decision metric for mode allocation throughout the frame. Second method adapts rate and transmission mode across symbols in a frame by considering that average channel power is monotonically decreasing as a function of time within the frame when multiuser diversity is exploited. First method performs as good as the better of BF and SFBC over all SNR values. Second method provides additional performance gain over the first method due to more efficient use of better channel conditions.

Diversity exploitation is a proven efficient means for addressing detrimental effects of wireless channel fadings. User channelization, where the frequency and time resources are structured into several user channels, can substantially help to capture the diversity, but it has not been well investigated. In this paper, we develop theoretical formulation of optimal resource structures under different performance metrics and then analyze them in frequency-selective time-varying channels. We propose a simplified system model which maintains the main features, and based on which, we develop a new practical approach of user channelization or user resource structuring which captures better diversity for OFDMA systems in multiuser time-varying frequency-selective fading channels. In contrast to the existing user resource structures where relative locations of the resources in the time-frequency grid remain the same throughout the transmission frame, the proposed resource structures evolve within the frame to enjoy better diversity gain. Example designs of the proposed evolving structures are presented from the 3GPP LTE perspective. Simulations done under LTE system environments validate that the proposed practical approach can obtain a near-optimal structure design, and provide significant performance advantage over the existing structures at the mobile speeds ranging from 3 km/h to 350 km/h.

We have developed a low-cost, real-time sleep apnea monitoring system ''Apnea MedAssist" for recognizing obstructive sleep apnea episodes with a high degree of accuracy for both home and clinical care applications. The fully automated system uses patient's single channel nocturnal ECG to extract feature sets, and uses the support vector classifier (SVC) to detect apnea episodes. "Apnea MedAssist" is implemented on Android operating system (OS) based smartphones, uses either the general adult subject-independent SVC model or subject-dependent SVC model, and achieves a classification F-measure of 90% and a sensitivity of 96% for the subject-independent SVC. The real-time capability comes from the use of 1-min segments of ECG epochs for feature extraction and classification. The reduced complexity of "Apnea MedAssist" comes from efficient optimization of the ECG processing, and use of techniques to reduce SVC model complexity by reducing the dimension of feature set from ECG and ECG-derived respiration signals and by reducing the number of support vectors.

Single-carrier frequency-division multiple-access (SC-FDMA) has been adopted in the uplink of the LTE standard due to its lower peak-to-average-power ratio (PAPR) compared to orthogonal frequency-division multiple access (OFDMA). Recent activities in the LTE-advanced (LTE-A) standardization have focused on effective uplink transmit diversity schemes with low PAPR but without considering the performance degradation due to Doppler despite the fact that LTE-A is required to support high mobility. In this paper, we present an open-loop space-frequency block coding (SFBC) scheme for the SC-FDMA uplink and demonstrate its robustness to high Doppler and large multipath delay spread while enjoying full spatial diversity, low PAPR and practical decoding complexity by a suitable design of the frequency span of each SFBC codeword. In this paper, we study the design tradeoffs involved in SFBC design for mobile SC-FDMA.

Multiple input multiple output (MIMO) orthogonal frequency division multiplexing (OFDM) systems facilitate high data rate wireless communications, and require reliable channel estimates to fully materialize their advantages. The semiconductor downscaling trend has exacerbated device impairments such as inphase and quadrature (I/Q) imbalances which cause inter-carrier interferences in OFDM systems which cannot be remedied by increasing signal power. Different RF chains of MIMO branches can cause different I/Q imbalances which further complicates MIMO OFDM channel estimation. This paper proposes several pilot designs for the estimation of the combined responses of MIMO frequency-selective channels and frequency-dependent I/Q imbalances. The proposed designs require much smaller pilot overhead than the existing designs, and also provide estimation mean-squares error optimality (under white noise) and general applicability to preamble as well as pilot-data-multiplexed symbols in MIMO systems with or without null guard tones. Performance analyses and simulation results corroborate advantages of the proposed designs.

This paper addresses initial ranging (uplink synchronization and power control) for TDD OFDMA systems. Exploiting the channel knowledge from the downlink channel together with initial power control, we develop a novel initial ranging method which provides multiuser diversity gain and significant power saving for the subscriber stations. We present new ranging signal designs which enable multiuser diversity gain and facilitate new efficient low-complexity algorithms for multiuser ranging signal detection, timing estimation and power estimation. The advantages of the proposed approach over existing methods in terms of the ranging signal detection performance, the number of ranging frames required to finish the ranging process (latency at the network entry), the timing and power estimation performance, the ranging transmission power saving at the subscriber stations, and the complexity saving at the base station are illustrated by analytical and simulation results.

Recent trends in clinical and telemedicine applications highly demand automation in electrocardiogram (ECG) signal processing and heart beat classification. A patient-adaptive cardiac profiling scheme using repetition-detection concept is proposed in this paper. We first employ an efficient wavelet-based beat-detection mechanism to extract precise fiducial ECG points. Then, we implement a novel local ECG beat classifier to profile each patient's normal cardiac behavior. ECG morphologies vary from person to person and even for each person, it can vary over time depending on the person's physical condition and/or environment. Having such profile is essential for various diagnosis (e.g., arrhythmia) purposes. One application of such profiling scheme is to automatically raise an early warning flag for the abnormal cardiac behavior of any individual. Our extensive experimental results on the MIT-BIH arrhythmia database show that our technique can detect the beats with 99.59% accuracy and can identify abnormalities with a high classification accuracy of 97.42%.

OFDM data detection in doubly-selective fading channels requires high complexity due to intercarrier interferences (ICI). We present a low-complexity receiver consisting of a semidefinite relaxation (SDR) based detector and parallel interference cancellation (PIC). The entire band is divided into clusters of adjacent subcarriers. SDR is applied on each cluster while PIC tackles ICI from other clusters. An upper bound of ICI power is derived and used to omit far-away clusters in performing PIC. Finally, an adaptive detector based on PIC, PIC-based SDR and the snap-shot SNR in channel is proposed to achieve a better tradeoff between complexity and performance.

Several variations of Cramer-Rao bounds for carrier frequency offset (CFO) estimation in frequency-selective fading channels have been used to benchmark practical estimators' performance or to design training signals for CFO estimation. Among them, the extended Miller-Chang bound (EMCB) provides a tighter bound than the CRB for locally unbiased estimators. However, there is no closed-form expression of the EMCB for the CFO estimation in frequency-selective fading channels with an arbitrary training signal. In this letter, we derive a closed-form exact average CRB (the EMCB) valid for any training signal and any signal structure for the CFO estimation over frequency-selective Rayleigh fading channels with uncorrelated or arbitrarily correlated taps. The accuracy and generality of the proposed average CRB expression, and its advantages over the existing expressions are corroborated by numerical and simulation results.

Direct-conversion multiple-input multiple-output orthogonal frequency division multiplexing (MIMO-OFDM) transceivers enjoy high data rates and reliability at practical implementation complexity. However, analog front-end impairments such as I/Q imbalance and high mobility requirements of next-generation broadband wireless standards result in performance-limiting inter-carrier interference (ICI). In this paper, we study the effects of ICI due to these impairments for OFDM with space frequency block codes and spatial multiplexing, derive a generalized linear model and propose a non-iterative reduced-complexity digital baseband joint compensation scheme. Furthermore, we present a pilot scheme for joint estimation of the channel and the I/Q imbalance parameters and evaluate its performance through simulations. Our proposed scheme is effective in estimating and compensating for frequency-independent and frequency-dependent transmit and receive I/Q imbalances even in the presence of a residual frequency offset.

In this paper, we derive optimal pilot power allocation for OFDM systems suffering from in-phase and quadrature-phase (IQ) imbalances. Existing works in literature on IQ imbalances optimize for pilot spacings and pilot designs. However, in all these works, optimal power allocation between pilot and data symbols has not been considered. Using a lower bound on the average channel capacity as a metric, we optimize for the pilot and data power allocations. Simulations show that the resulting optimal pilot power allocation increases the channel capacity along with lowering the bit error rate (BER). We further show that the power allocation is flexible in the sense that several power allocation choices exist that improve capacity compared to the equal power allocation scenario.

Efficient pilot designs are vital as they improve system performance and resource utilization, especially in multi-input multi-output (MIMO) orthogonal frequency division multiplexing (OFDM) systems. Most of the existing pilot designs do not consider in-phase and quadrature (I/Q) imbalance whose detrimental effects become more severe since the technology and applications are pushing towards smaller semiconductor node, smaller chip, cheaper equipment, and larger modulation order. This paper studies the impact of I/Q imbalance on the pilot designs, and presents several new pilot designs for equivalent channel estimation of MIMO OFDM systems with frequency-dependent I/Q imbalances. Advantages of the proposed designs are illustrated through simulation and analytical results in terms of estimation accuracy, error rate, overhead efficiency, and generality with the transmission formats (preamble-based or pilot-data-multiplexed systems, with or without null guard tones).

In this paper, we investigate polynomial expansion approximation to reduce matrix inversion complexity as encountered in the design of the minimum mean squared error decision feedback equalizer (MMSE-DFE). The scaling factor needed in this polynomial expansion is optimized for a fixed polynomial approximation order so that the received signal to interference plus noise ratio (SINR) is maximized. The BER performance of the reduced-complexity MMSE-DFE is comparable to that of the direct matrix inversion based MMSE-DFE and outperforms earlier approaches reported in the literature.

All practical OFDM systems insert null guard bands to facilitate easy filter implementation and emission spectrum control. The null guard bands introduce a constraint for optimal pilot designs in MIMO OFDM systems. Semiconductor downscaling and the use of high modulation order have signified the importance of pilot designs for OFDM systems in the presence of I/Q imbalances, but existing designs have not addressed the use of guard bands, the overhead efficiency, the estimation optimality, and general applicability in pilot data multiplexed format and/or MIMO setup. This paper proposes a pilot design for channel estimation of MIMO OFDM systems with guard bands and both transmit and receive frequency-dependent I/Q imbalances, which overcomes the limitations of the existing designs. Simulation and numerical results corroborate better performance, overhead advantage and applicability in pilot-data-multiplexed MIMO setup of the proposed design over the existing MIMO OFDM pilot design with I/Q imbalances.

It is a significant challenge to support high data rate services at high mobile speeds. In this paper, an efficient diversity-exploiting user resource structure, namely evolving- type resource structure, for fast time-varying frequency-selective fading channel environments is designed. This newly proposed resource structure enjoys the advantages of both band-type and interleaved-type structures and avoids their disadvantages by exploiting both the multiuser diversity and the frequency diversity under fast time-varying channels. Simulation results corroborate the performance improvement of the new resource structure over the existing ones under various mobility scenarios.

The advances in semiconductor downscaling have empowered communication devices with significantly enhanced signal processing capability, but with the side effect of larger device impairments. Among those impairments, the I/Q imbalance is a critical one as it causes intercarrier interferences in orthogonal frequency division multiplexing (OFDM) systems. The I/Q imbalances in general exist at both transmitter and receiver, and they can be frequency-dependent. They may vary over time (e.g. due to sudden changes in temperature or the dynamics of the cooperating/relaying users). Under such scenarios, estimation and compensation of the combined effect of the channel and I/Q imbalances are crucial. This paper presents efficient pilot designs for such tasks. The advantages of the proposed designs over the existing ones in terms of estimation performance and overhead efficiency are corroborated by analysis and simulation results.

In this paper, we derive exact closed form bit error rate (BER) or symbol error rate (SER) expressions for orthogonal frequency division multiplexing (OFDM) systems with carrier frequency offset (CFO). We consider the performance of an OFDM system subject to CFO error in additive white Gaussian noise (AWGN), frequency flat and frequency selective Rayleigh fading channels. The BER/ SER performances of BPSK and QPSK modulation schemes are analyzed for AWGN and frequency-flat Rayleigh fading channels while BPSK is considered for frequency-selective Rayleigh fading channels. Our results can easily be reduced to the respective analytical error rate expressions for the OFDM systems without CFO error. Furthermore, the simulation results are provided to verify the accuracy of the new error rate expressions.

Multiuser systems can provide multiuser diversity gains by assigning channels to users with higher channel gains. To avoid the extensive information exchange with the access point for the uplink access in centralized approaches, we propose in this paper a distributed opportunistic access scheme. Through a judicious design of a novel backoff mechanism to utilize the channel information and reduce collisions, significant multiuser diversity gains are achieved. To a user, the higher the channel gain is, the smaller the backoff time-slot and, hence, the higher the access priority of that user is. In addition, for heterogeneous systems, our proposed scheme can realize multiuser diversity gains and achieve fairness among the users at the same time. Finally, we design two distributed opportunistic access schemes for OFDMA systems. Users contend on all sub-channels in the first scheme and only on several strongest sub-channels in the second scheme. Compared with traditional centralized OFDMA systems and other distributed access schemes, our proposed schemes reduce overhead and achieve a higher throughput.

In this paper, we derive closed form bit error rate (BER) expressions for orthogonal frequency division multiplexing (OFDM) systems with residual carrier frequency offset (CFO). Most of the published work treats CFO as a nonrandom parameter. But in our study we consider it as a random parameter. The BER performance of binary phase shift keying (BPSK) OFDM system is analyzed in the cases of additive white Gaussian noise (AWGN), frequency-flat and frequency-selective Rayleigh fading channels.We further discuss how these expressions can be related to systems with practical estimators. The simulation results are provided to verify the accuracy of these error rate expressions.

In-phase and quadrature-phase (I/Q) imbalance is a major performance-limiting impairment for direct- conversion orthogonal frequency division multiplexing (OFDM) transceivers. I/Q imbalance degrades the signal-to-noise-ratio in an OFDM system by causing inter-channel-interference (ICI) between image subcarriers. Doppler spread due to mobility also causes ICI but mainly between adjacent subcarriers. In this work, we investigate the combined effects of mobility and I/Q imbalance in a multiple-input-multiple-output-OFDM system with the alamouti space-frequency block-coding scheme and design an effective compensation technique. Furthermore, we propose a novel pilot allocation and channel estimation scheme, and exploit the problem structure to reduce complexity.

In this paper, we propose new ranging signal designs for time-division duplexing multiple input multiple output (MIMO) orthogonal frequency division multiple access (OFDMA) systems. Exploiting the channel knowledge from the downlink channel together with initial power control, we develop three ranging transmission schemes which provide multiuser and multi-antenna diversity gains and significant power saving for the subscriber stations. The advantages of the proposed approach over existing methods in terms of ranging performance, energy saving, and complexity saving are illustrated by analytical and simulation results.

In this paper, we propose a generalized utility function model for cooperative slotted Aloha in a distributed system. Multiuser diversity gains are achieved by appropriately designing the Nash equilibrium thresholds for the selfish users. The network enforces fairness among different users by employing a pricing policy that favors equal access probabilities. This generalized game model provides the flexibility in designing the utility function associated with QoS. Our study on the impact of different utility function models shows that the multiuser diversity gains achieved by different models remain constant under the throughput optimization framework and vary slightly under the revenue optimization framework.

In cellular networks, a mobile station (MS) may be located in the coverage hole or out of the coverage of the base station (BS). In such a scenario, one or more relay stations (RSs) can be used to facilitate the transmission. Here, a threshold- based opportunistic cooperative ARQ transmission scheme is proposed. Based on this scheme, the transmission between the BS and the MS can be separated into two parts: firstly, the transmission between BS and RSs (BS-RSs link) and secondly, the transmission between RSs and MS (RSs-MS link). The proposed scheme is different from the conventional ACK/NACK used for unicast transmission, in which two new types of ARQ messages are introduced for multicast transmission, namely, the relay-associated ACK/NACK (i.e., R-ACK/R-NACK) and the cooperative ACK/NACK (i.e., C-ACK/C-NACK), for BS-RSs link and RSs-MS link, respectively. A pre-defined threshold is applied to evaluate the reliability of the BS-RSs link. If the number of reliable RSs is larger than the threshold value, the reliable RSs will transmit the packet to the MS in a cooperative manner. Since reliable transmission exists in the BS-RSs link, the BS will release the packet and further transmission of this packet occurs only between reliable RSs and the MS. Therefore, the proposed scheme can reduce the latency of cellular networks and the processing burden on the BS as well as achieve higher throughput as shown in the performance analysis and simulation results.

In this paper, we compare the downlink capacity and throughput between the variable spreading factor-orthogonal frequency and code division multiple access (VSF-OFCDMA) with one-cell frequency reuse, and the orthogonal frequency division multiple access (OFDMA) with conventional three-cell frequency reuse. Since the multiuser diversity gain in OFDMA systems can be achieved by approximately designing its access schemes, while it is non-trivial for the VSF-OFCDMA systems, the effect of both access schemes on the link capacity are investigated. Our simulation results reveal that although the multiuser diversity gain achieved by OFDMA systems can compensate the loss of low frequency reuse at low to mid range of signal-to-interference-plus-noise ratio (SINR) values, the achievable throughput of VSF-OFCDMA systems is much higher than the OFDMA systems. Furthermore, the impact of user’s location on the inter-cell-interference is also reported.

In this paper, we investigate training aided frequency offset estimation for multiple-input multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) systems via polynomial rooting over frequency selective fading channels. By designing the training sequences properly, both integer and fractional carrier frequency offsets (CFO) can be estimated from the polynomials corresponding to the cost function. Moreover, we show through analysis that rooting the cost function is equivalent to rooting the first-order derivative of the cost function in the considered MIMO case. Simulation results verify the good performance of our new CFO estimator and the corresponding analytical results.

In this paper, we derive closed-form symbol error rate (SER) expressions for orthogonal frequency division multiplexing (OFDM) systems with residual carrier frequency offset (CFO). We treat CFO/residual CFO as a random parameter in this study. In particular, we consider channel-independent as well as channel-dependent random residual CFOs. We derive SER expressions for 4-quadrature amplitude modulation (4-QAM) OFDM systems in the cases of additive white Gaussian noise (AWGN) and frequency flat Rayleigh fading channels. The simulation results are provided to verify the accuracy of the new SER expressions.

I/Q imbalance and high mobility in OFDM systems result in performance-limiting intercarrier interference (ICI). However, the nature of ICI due to each of them is quite different. Unlike previous works which considered these two impairments separately, we develop a unified mathematical framework to characterize and mitigate ICI when both impairments are present. In addition, we exploit the special ICI structure to design efficient OFDM channel estimation and digital baseband compensation schemes for I/Q imbalance under high-mobility conditions.

We address joint estimation of frequency offsets and channel responses in OFDMA uplink. A cyclically equal-spaced, equal-energy interleaved pilot preamble is proposed by which two iterative estimators are developed. In the first estimator, we develop a modified SAGE (space alternating generalized expectation-maximization) method by incorporating multiuser interference cancellation in both time and frequency domains into the SAGE method. The proposed modified SAGE method yields a faster convergence rate, a better estimation performance, and a lower complexity than the existing conventional SAGE method from [10]. In the second method, we propose a very low complexity ad hoc method to replace the high complexity frequency offset estimator of the first method. The proposed ad hoc method is developed based on the time-domain characteristics of the pilot preamble and the frequency-domain power concentration property of OFDMA systems. It achieves almost the same estimation performance as the first proposed method but requires significantly lower complexity. Both proposed methods are not only better in estimation performance, convergence rate, and complexity, but also more robust to the number of active uplink users than the existing method while requiring only one OFDM training symbol and no restriction of the data subcarrier assignment scheme.

This paper presents pilot designs for consistent frequency offset estimation of MIMO OFDM systems in frequency selective fading channels. We derive the sufficient consistency condition for the pilots in MIMO OFDM systems to yield unambiguous estimation, and present corresponding consistent pilot designs. We discuss robustness of the frequency offset estimation against outliers at low to moderate SNR values and present an efficient criterion to choose robust and consistent pilots. Furthermore, we develop pilot designs which satisfy both consistency over a limited frequency offset estimation range and the optimal channel estimation condition in MIMO OFDM systems. Simulation results corroborate that both the consistent pilot design condition and the robustness criterion are efficient in choosing pilot patterns yielding better frequency offset estimation performance.

We address low-complexity, highly-accurate frequency offset estimation for multi-band orthogonal frequency division multiplexing (MB-OFDM) based ultra-wide band systems in time-invariant as well as time-variant channels. We investigate the unique characteristics of MB-OFDM systems, namely, different frequency offsets, channel responses, received energies, and preamble structures in different frequency bands. Utilizing them, we develop frequency offset estimators based on the best linear unbiased estimation principle. If compared to the reference estimators, our proposed methods achieve significantly better estimation performance (4 to 6.4 dB (5 to 20 dB) estimation mean-square error advantage in the time-invariant (time-variant) channels) for all preamble patterns of the MB-OFDM system in [8].

This paper addresses carrier frequency offset (CFO) estimation and training sequence design for multiple-input multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) systems over frequency selective fading channels. By exploiting the orthogonality of the training sequences in the frequency domain, integer. CFO (ICFO) is estimated. With the uniformly spaced non-zero pilots in the training sequences and the corresponding geometric mapping, fractional CFO (FCFO) is estimated through the roots of a real polynomial. Furthermore, the condition for the training sequences to guarantee estimation identifiability is developed. Through the analysis of the correlation property of the training sequences, two types of sub-optimal training sequences generated from the Chu sequence are constructed. Simulation results verify the good performance of the CFO estimator assisted by the proposed training sequences.

This paper addresses preamble-based low complexity synchronization, channel estimation and equalization for Zero-padded (ZP) MB-OFDM based UWB systems. The proposed synchronization method consists of sync detection, coarse timing estimation, fine timing estimation, and oscillator frequency offset estimation. The distinctive features of MB-OFDM systems and the interplay between the timing and carrier frequency hopping at the receiver are judiciously incorporated in the proposed synchronization method. In order to apply the low complexity one-tap frequency-domain equalizer, the required circular convolution property of the received signal is obtained by means of an overlap-add method after the frequency offset compensation. The proposed low complexity channel estimator for each band is developed by first averaging the over-lapadded received preamble symbols within the same band and then applying time-domain least-squares method followed by the discrete Fourier transform. We develop an MMSE equalizer and its approximate version with low complexity. We also derive the probability density functions of the UWB channel path delays, and using them we present several optimization criteria for our proposed synchronization, channel estimation, and equalization. The effectiveness of our proposed methods and optimization criteria are confirmed by computer simulation results.

Training signals for OFDM channel estimation should possess low PAR to avoid nonlinear distortions at the transmit amplifier and at the same time they should be robust against frequency offsets. In this letter, we show that the above two requirements of the training signals are conflicting. We propose two new training signal designs for MIMO OFDM frequency-selective channel estimation. Our proposed training signals achieve more robust channel estimation performance against frequency offsets while satisfying the PAR constraints compared to training signals designed to achieve a fixed low PAR but without any consideration for robustness to frequency offsets.

Multiuser diversity gains can be achieved by assigning channels to users with better channel quality in multiuser systems. To avoid the extensive information exchange required for centralized approaches, we propose a distributed fair pricing strategy for a slotted Aloha system in which users act selfishly to improve their own utilities for both the collision model and the multipacket reception (MPR) model. Based on a game theoretic framework, we show that multiuser diversity gains can be achieved by appropriately designing the Nash equilibrium thresholds for the selfish users to preserve the throughput and revenue achieved in the classical slotted Aloha systems. The network enforces fairness among different users by employing a pricing policy that favors equal access probabilities. Our simulation results show that significant multiuser diversity gains are achieved in terms of energy consumption and/or spectral efficiency.

Opportunistic space-time block codes (STBC) have been investigated for flat-fading channels in the literature. In this paper, we present a novel scheme for integrating them with orthogonal frequency division multiplexing (OFDM) for frequency-selective channels with an unequal error protection capability making it attractive for multimedia and multicasting applications. In addition, our new scheme optimizes the tradeoff between coding gain and peak-to-average ratio (PAR) while minimizing inter-carrier interference in the presence of carrier frequency offset. We show that our scheme achieves a 2 dB reduction in PAR over the conventional scheme.

In this paper, we propose a robust STBC transmission scheme to combat timing synchronization errors over frequency-selective multiple-access channels. We start by deriving the equivalent channel model in the presence of timing synchronization errors. Based on this correlated equivalent channel model, the statistical channel power gain profile is analyzed and shown to exhibit unequal channel power gains on different subcarriers. Furthermore, a robust statistical bit loading algorithm is proposed to optimize the BER performance in scenarios where link adaptation based on instantaneous channel information is infeasible or undesirable. Simulation results show that our proposed scheme is robust to timing synchronization errors, providing around 7.5 dB SNR advantage at the BER of 10-4over the conventional scheme in various channel environments.

This paper presents low-complexity preamble-based channel estimation and one-tap frequency-domain equalization for zero-padded MB-OFDM based UWB systems. The required circular convolution property of the received signal is obtained by means of an overlap-add method. The proposed low-complexity channel estimator for each band is developed by first averaging the over-lap-added received preamble symbols within the same band and then applying time-domain least-squares method followed by the discrete Fourier transform. By using the probability density functions of the UWB channel path delays, we also derive the statistics of the UWB channel models, and utilizing them we present several optimization criteria for our proposed channel estimation and equalization in the presence of timing offset. The effectiveness of our proposed methods and optimization criteria are confirmed by computer simulation results.

This paper addresses initial ranging (uplink synchronization and power control) for TDD OFDMA systems. Exploiting the channel knowledge from the downlink control channel together with initial power control, we develop a novel initial ranging method which provides multiuser diversity gain and significant power saving for the subscriber stations. We present a new ranging signal design which provides multiuser diversity gain and facilitates new efficient low-complexity algorithms for multiuser ranging signal detection, timing estimation and power estimation. The advantages of the proposed approach over existing methods in terms of the ranging signal detection performance, the number of ranging frames required to finish the ranging process, the timing and power estimation performance, the ranging transmission power saving at the subscriber stations, and the complexity saving at the base station are illustrated by analytical and simulation results.

This paper presents pilot designs for consistent frequency offset estimation of MIMO OFDM systems in frequency-selective fading channels. We derive the sufficient consistency condition for the pilots in MIMO OFDM systems to yield unambiguous estimation, and present corresponding consistent pilot designs. We develop pilot designs which satisfy both consistency over a limited CFO estimation range and the optimal channel estimation condition in MIMO OFDM systems. Simulation results corroborate that the consistent pilot design condition is efficient in choosing pilot patterns yielding better frequency offset estimation performance.

In [(S. Diggavi et al., 2003),(S. Das et al., 2005, 2006)] opportunistic space-time block codes were presented for flat-fading channels. In this paper, we propose an opportunistic STBC-OFDM for frequency-selective channels with an unequal error protection capability making it attractive for multimedia applications. Our new scheme optimizes the tradeoff between coding gain and peak-to-average ratio while minimizing inter-carrier interference in the presence of carrier frequency offset. We show that our scheme achieves a 2 dB reduction in PAR over the conventional scheme.

In multiuser systems, multiuser diversity gains can be achieved by assigning channels to users with higher channel quality. To avoid the extensive information exchange required for centralized approaches, we propose a distributed fair pricing strategy for a slotted ALOHA system in which users act selfishly to improve their own utilities. Based on a game theoretic framework, we show that multiuser diversity gains can be achieved by appropriately designing the Nash equilibrium thresholds for the selfish users, such as to preserve the throughput and revenue achieved in the centralized schemes. The network enforces fairness among different users by employing a pricing policy that favors equal access probabilities. Our simulation results show significant multiuser diversity gains achieved in terms of energy consumption and/or spectral efficiency.

This paper presents a preamble-based low complexity synchronization method for MB-OFDM based UWB systems. The proposed synchronization method consists of sync detection, coarse timing estimation, fine timing estimation, and oscillator frequency offset estimation. The distinctive features of MB-OFDM systems and the interplay between the timing and carrier frequency hopping at the receiver are judiciously incorporated in the proposed synchronization method. We also derive the probability density functions (pdfs) of the delays of the UWB channel paths which are used in optimizing our synchronization method. These pdfs are also useful in evaluating or optimizing the energy capture of other UWB systems. The effectiveness of our proposed synchronization method and optimization criteria are confirmed by computer simulation results.

This paper addresses a low complexity frequency offset estimator for multiple-input multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) systems over frequency selective fading channels. By exploiting the good correlation property of the training sequences, which are constructed from the Chu sequence, carrier frequency offset (CFO) estimation is obtained with great complexity reduction through factor decomposition for the derivative of the cost function. The CFO estimate's variance and Cramer-Rao bound (CRB) are developed to optimize the parameter of the simplified estimator and also to evaluate the estimation performance. Simulation results verify the good performance of the training-assisted CFO estimator.

We address low-complexity, highly-accurate frequency offset estimation for MB-OFDM-based UWB systems in time-variant channels. We present a simple channel model capturing an abrupt channel change characteristic of UWB systems. We devise an abrupt channel change detector and develop an oscillator frequency offset estimator based on best linear unbiased estimation principle while taking into account the time-variant channel behavior as well as the MB-OFDM characteristics such as different carrier frequency offsets, different channel responses, different channel energies, and different preamble structures in different frequency bands. If compared to the conventional estimators using correlation, our proposed method achieves significantly better estimation performance for all preamble patterns of the MB-OFDM system.

This paper presents pilot designs for consistent frequency-offset estimation of orthogonal frequency-division multiplexing systems in frequency-selective fading channels. We describe two design approaches, namely, consistency in the probabilistic sense and absolute consistency. Existing preambles and pilot designs in the literature do not guarantee the absolute consistency. We derive general criteria for both approaches, present sufficient conditions on the pilot structures over the maximum carrier frequency offset (CFO) estimation range (half of the sampling rate), and derive simple pilot designs satisfying these conditions. We also extend the sufficient conditions to any arbitrary but fixed CFO estimation range, and present some generalized design patterns. Furthermore, the CFO estimation performances of distinct consistent pilot designs can be quite different at moderate or low signal-to-noise ratio (SNR) due to different statistics of outliers which also yields a link failure. We develop efficient pilot-design criteria that provide both consistency and robustness against outliers at moderate-to-low SNR. Our consistent pilot designs facilitate flexible and economical implementation, while our robust pilot designs enable wireless links with less outage and better resilience

We present a new initial ranging method for OFDMA systems such as IEEE 802.16-2004. First, a new orthogonal ranging signal design is proposed by which an efficient, low-complexity multi-user ranging signal detection is developed. Based on the ranging signal detector results, power estimation for the detected ranging subscriber stations is performed. Then, by utilizing the proposed orthogonal ranging signal design together with the redundancy introduced by cyclic prefixes, a new iterative multi-user timing offset estimator is presented. Compared with the existing methods utilizing the CDMA-type ranging codes (in frequency-domain) defined in IEEE 802.16-2004, the proposed method achieves a better performance and greater robustness against multi-user interference and multi-path fading channels

In data-pilot-multiplexed OFDM schemes, data-interference to pilot tones causes degradation in frequency offset and channel estimation. This paper derives the best data-pilot-multiplexed scheme in terms of minimizing data-interference on pilot tones. This design gives a correlated data insertion scheme (CD) (data on the left and right adjacent tones of each pilot are correlated) and a null data insertion scheme (ND) (no data are transmitted on the left and right adjacent tones of each pilot). Both schemes outperform conventional data-pilot-multiplexed schemes (CV) significantly (over 7 dB SNR advantage at the uncoded BER of 2 times 10-2and pilot-to-data energy ratio (PDER) of 5 dB). With the BLUE frequency offset estimator, the CD scheme yields a better performance than the ND scheme while having a slight advantage in data throughput. A modified CD scheme is also presented which transmits the same data rate at similar complexity as the CV scheme does while achieving a significant BER improvement (over 3 dB SNR advantage at the uncoded BER of 2 times 10-2and PDER of 5 dB)

This paper proposes a novel iterative joint estimation of channels and frequency offsets for uplink OFDMA systems. The proposed method is based on the iterative line search algorithm with a trust region. The objective function minimizes the mean-square distance between the received vector and the reconstructed received vector based on the estimated parameters. In each iteration, the parameters are updated based on the minimum mean square error criterion. We also present a stop criterion for the iterative algorithm. The simulation results show the fast convergence rate of the proposed algorithm with asymptotic performance approaching the average CRBs (averaged over the random frequency offsets and fading channels). Our proposed method achieves a much lower computational complexity and better bit error rate performance than the existing SAGE method.

We address joint estimation of frequency offsets and channel responses in OFDMA uplink. A cyclically equal-spaced, equal-energy interleaved pilot preamble is proposed by which two iterative estimators are developed. In the first estimator, we develop a modified SAGE (space alternating generalized expectation-maximization) method by incorporating multiuser interference cancellation in both time and frequency domains into the SAGE method. The proposed modified SAGE method yields a faster convergence rate, a better estimation performance, and a lower complexity than the existing conventional SAGE method. In the second method, we propose a very low complexity ad-hoc method to replace the high complexity frequency offset estimator of the first method. The proposed ad-hoc method is developed based on the time-domain characteristics of the pilot preamble and the frequency-domain power concentration property of OFDMA systems. It achieves almost the same estimation performance as the first proposed method but requires significantly lower complexity. Both proposed methods are not only better in estimation performance, convergence rate, and complexity, but also more robust to the number of active uplink users than the existing method while requiring only one OFDM training symbol and no restriction of the data subcarrier assignment scheme.

This paper proposes a novel iterative joint estimation of channels and frequency offsets for uplink OFDMA systems. The proposed method is based on the iterative line search algorithm with a trust region. The objective function is minimizing the mean-square distance between the received vector and the reconstructed received vector based on the estimated parameters. In each iteration, updating the parameters is based on the minimum mean square error criterion. We also present a stop criterion for the iterative algorithm. We derive the Cramer-Rao lower bounds (CRBs) of the considered multi-user joint parameter estimation. The simulation results show that our proposed algorithm converges fast and the estimation performance gradually approaches to the average CRBs (averaged over the random frequency offsets and fading channels). Our proposed method achieves a much lower computation complexity and better bit error rate performance than the existing SAGE method.

In this paper, we propose a distributed opportunistic access scheme for uplink OFDMA systems. The sub-carriers are grouped into several sub-channels and users access those sub-channels through a distributed access scheme without requiring extensive information exchange with the access point. Our proposed scheme allows several parallel contention subchannels to exploit multiuser diversity. Using knowledge of all the sub-channels, obtained through a periodically-transmitted beacon signal from the access point, all users contend on their strongest sub-channel(s). The proposed scheme applies a novel backoff mechanism utilizing this sub-channel knowledge to yield further throughput improvement in addition to the throughput gain obtained by the collision reduction design. To a user, the better the sub-channel gain is, the smaller the backoff time on that sub-channel, and hence, the higher the access priority of the user on that sub-channel. Compared with the traditional centralized OFDMA systems, our proposed scheme reduces overhead significantly and achieves a better spectral efficiency as corroborated by the simulation results.

We address low-complexity, highly-accurate frequency offset estimation for MB-OFDM-based UWB systems. We discuss unique characteristics of the MB-OFDM systems, namely, different carrier frequency offsets, different channel responses, different channel energies, and different preamble structures in different frequency bands. Utilizing them, we develop frequency offset estimators based on the best linear unbiased estimation principle. If compared to the conventional estimators using correlation and averaging, our proposed methods achieve better estimation performance for all preamble patterns and the improvement is more significant for the preamble patterns 3 and 4 of the MB-OFDM system in [2].

This paper presents pilot designs for consistent frequency offset estimation of OFDM systems in frequency-selective fading channels. We describe two design approaches, namely consistency in the probabilistic sense and absolute consistency. Existing preambles and pilot designs in the literature do not guarantee the absolute consistency. We derive general criteria for both approaches, present sufficient conditions on the pilot structures, and derive simple pilot designs satisfying these conditions. Absolute consistency should not be compromised in emergency-related or other critical communication scenarios and our proposed consistent pilot designs address this need.

We derive on the impulse response and input signal of a frequency-selective finite impulse response channel to be diagonalized by the DCT into parallel, decoupled, and memoryless subchannels. We show how these conditions can be satisfied in a practical multicarrier transceiver through a novel design of the guard sequence and the front-end prefilter. This DCT-based design results in complete elimination of interblock and intercarrier interference without channel knowledge at the transmitter and at the same guard sequence overhead, compared with DFT-based multicarrier transceivers. Extensions to multi-input multi-output frequency-selective channels are also described. Finally, we present numerical examples from wireline and wireless communications scenarios to illustrate the viability and practicality of the DCT as a modulation/demodulation basis for baseband and passband signaling over frequency-selective channels.

We derive conditions on the impulse response and input signal of a frequency-selective finite-impulse response channel to be diagonalized by the discrete cosine transform (DCT) into parallel, decoupled, and memoryless subchannels. We show how these conditions can be satisfied in a practical multicarrier transceiver through a novel design of the guard sequence and the front-end prefilter. This DCT-based design results in complete elimination of interblock and intercarrier interference without channel knowledge at the transmitter and at the same guard sequence overhead, compared with DFT-based multicarrier transceivers. Extensions to multiinput multioutput frequency-selective channels are also described. Finally, we present numerical examples from wireline and wireless communications scenarios to illustrate the viability and practicality of the DCT as a modulation/demodulation basis for baseband and passband signaling over frequency-selective channels.

We develop robust mean-square error (MSE)-optimal training signal designs for multiple-input multiple-output orthogonal frequency-division multiplexing channel estimation with frequency offset and phase noise (PN), and present analytical and simulation results for the frequency-offset and PN effects on channel estimation. The proposed designs are more advantageous for moderate-to-high values of signal-to-noise ratio (SNR), residual frequency offset, and PN level. At SNR = 10 dB, the normalized MSE reductions of our proposed training signals at normalized frequency offset |v|=0.1,0.5 are about 9 and 19 dB, respectively, for one transmit antenna, and 6 and 11 dB for two transmit antennas

This letter addresses training-signal-based combined timing and frequency synchronization and channel estimation for orthogonal frequency-division multiplexing systems. The proposed scheme consists of two stages. At the first stage, coarse timing and frequency-offset estimates are obtained. Based on these estimates, a (coarse) channel response estimate is obtained. The timing and frequency-offset estimates at the second stage are obtained by maximum-likelihood (ML) realization based on a sliding observation vector. Then ML channel estimation is performed. A means of complexity reduction by an adaptive scheme is also presented. The simulation results show that the proposed combined approach performs quite well, and circumvents the problem of mismatch among individual synchronization tasks.

This paper addresses an optimal periodic training signal design for frequency offset estimation in frequency-selective multipath Rayleigh fading channels. For a fixed transmitted training signal energy within a fixed-length block, the optimal periodic training signal structure (the optimal locations of identical training subblocks) and the optimal training subblock signal are presented. The optimality is based on the minimum Cramer-Rao bound (CRB) criterion. Based on the CRB for joint estimation of frequency offset and channel, the optimal periodic training structure (optimality only in frequency offset estimation, not necessarily in joint frequency offset and channel estimation) is derived. The optimal training subblock signal is obtained by using the average CRB (averaged over the channel fading) and the received training signal statistics. A robust training structure design is also presented in order to reduce the occurrence of outliers at low signal-to-noise ratio values. The proposed training structures and subblock signals achieve substantial performance improvement.

This paper presents general classes of optimal training signals for the estimation of frequency-selective channels in MIMO OFDM systems. Basic properties of the discrete Fourier transform are used to derive the optimal training signals which minimize the channel estimation mean square error. Both single and multiple OFDM training symbols are considered. Several optimal pilot tone allocations across the transmit antennas are presented and classified as frequency-division multiplexing, time-division multiplexing, code-division multiplexing in the frequency-domain, code-division multiplexing in the time-domain, and combinations thereof. All existing optimal training signals in the literature are special cases of the presented optimal training signals and our designs can be applied to pilot-only schemes as well as pilot-data-multiplexed schemes.

We present a new initial ranging method for OFDMA systems such as IEEE 802.16a. First, a new orthogonal ranging signal design is proposed by which an efficient, low-complexity multi-user ranging signal detection is developed. Based on the ranging signal detector results, power estimation for the detected ranging subscriber stations is performed. Then, by utilizing the proposed orthogonal ranging signal design together with the redundancy introduced by cyclic prefixes, a new iterative multi-user timing offset estimator is presented. Compared with the existing methods utilizing the CDMA-type ranging codes (in frequency-domain) defined in IEEE 802.16a, the proposed method achieves a better performance and greater robustness against multi-user interference and multi-path fading channels.

For emerging high data-rate communication systems in highly dispersive channels such as ultra-wideband systems, possible frequency offsets could be larger than the estimation range of the existing methods using training signals with several identical parts. This paper presents a novel improved maximum likelihood frequency offset estimator which can handle at least twice the estimation range of the existing methods using training signals with identical parts and achieves a better estimation performance. Based on the likelihood metric, a new design metric is introduced which is a pair-wise error probability (PEP) between the correct frequency offset point and a trial frequency offset point. The proposed PEP metric gives more theoretical insights on the performance of practical maximum likelihood estimators. How to design the PEP to achieve both a larger estimation range and a better estimation performance in fading channel environments is also presented and the corresponding estimator implementation is described.

We derive conditions on the impulse response and input signal of a frequency-selective FIR channel to be diagonalized by the DCT into parallel, decoupled, and memoryless subchannels. We show how these conditions can be satisfied in a practical multi-carrier transceiver through a novel design of the guard sequence and the front-end prefilter. This DCT-based design completely eliminates inter-block and inter-carrier interference at a lower complexity and without incurring any additional guard sequence overhead, compared to DFT-based multicarrier transceivers. Extensions to multi-input multi-output frequency-selective channels are also described.

All existing training signal designs for channel estimation in OFDM systems assume no frequency offsets. In practice, frequency offset is unavoidable and seriously degrades the performance of OFDM systems. In this paper, we address the problem of designing optimal training signals for the estimation of frequency-selective channels in MIMO OFDM systems with frequency offsets. The mean square error advantage of our proposed optimal training signals can be quite significant for moderate-to-high values of SNR and frequency offsets.

This paper introduces a novel differential modulation scheme for a two-user cooperative diversity system which does not require channel state information at either the users or the destination. The performance of fixed decode-and-forward and selection relaying protocols is evaluated in both symmetric and asymmetric interuser-channel cases. The lower bound on the performance of the decode-and-forward protocol is given, while the exact bit-error probability of the selection relaying protocol is thoroughly derived. The decode-and-forward relaying protocol achieves a performance gain when the signal-to-noise ratios in the interuser channels are symmetric and sufficiently high. The selection relaying protocol shows a larger performance gain and does not exhibit an error floor like in the case of the decode-and-forward protocol. In addition, it is robust to the asymmetric interuser channels.

This paper addresses an optimal training-signal design for frequency-offset estimation. Based on minimizing the Cramer-Rao lower bound for frequency-offset estimation with constraints on the peak and the total training signal energies, and the training block length, the optimal training-signal structure is developed. An approximate version of the optimal training-signal structure is proposed, which has practically the same performance as the optimal one, and provides convenience in training-signal generation and estimator derivation. Two robust reduced-complexity frequency-offset estimation methods for the proposed training structures are presented. In order to handle larger frequency offsets, modified training-signal structures are proposed. Frequency-offset estimation methods suitable for these training signals are also derived, based on the best linear unbiased estimation principle. Analytical and simulation results show that the proposed training-signal structures improve the estimation performance significantly.

We address the initial ranging process in OFDMA systems such as IEEE 802.16a. The proposed ranging method includes three main tasks - timing estimation, multi-user ranging code detection, and power estimation. All tasks are performed based on a bank of correlators corresponding to ranging codes. The timing estimation scheme is based on the peak of correlator outputs. The multi-user ranging code detection is based on the correlator outputs and an adaptive threshold. A novel adaptive threshold setting is proposed. A simple user-power estimator is derived based on the correlator outputs. A scheme of initial power adjustment at the random access users is also proposed which brings in a significant improvement in ranging code detection performance. The simulation results show that the proposed method works well even in the presence of several simultaneous random-access users and is robust to other data-users' interference. The simplicity of the proposed method is also quite appealing.

This work presents general classes of optimal training signals for estimation of frequency-selective channels in MIMO OFDM systems. Basic properties of the discrete Fourier transform are used to derive the optimal training signals which minimize the channel estimation mean square error. Both single and multiple OFDM training symbols are considered. Several optimal pilot tone allocations among the transmit antennas are presented and classified as frequency division multiplexing, time division multiplexing, code division multiplexing in frequency-domain, code division multiplexing in time-domain, and combinations thereof. All existing optimal training signals in the literature are special cases of the presented optimal training signals and our designs can be applied to pilot-only schemes as well as pilot-data multiplexed schemes.

Cooperative transmission is an alternative approach to achieve diversity in wireless systems. So far the research has focused on the situation where perfect channel state information is available and coherent detection is employed at at the destination receiver. This paper proposes a differential modulation scheme for a two-user cooperative diversity system which does not require channel state information at either users or destination. The relaying protocols such as fixed decode-and-forward, selection relaying, and incremental relaying protocols are compared. The proposed scheme achieves spatial diversity gains when the signal-to-noise ratios between cooperative users are sufficiently high.

The idea of combined timing and frequency synchronization and channel estimation is quite desirable since the synchronization and channel estimation tasks can affect each other. This paper addresses training signal based combined timing and frequency synchronization and channel estimation for OFDM systems. The proposed scheme consists of two stages. At the first stage, coarse timing and frequency offset estimates are obtained. Based on these estimates, a (coarse) channel response estimate is obtained. The timing and frequency offset estimates at the second stage are obtained by a maximum likelihood (ML) realization based on sliding observation vector. Then a ML channel estimation is performed. The simulation results show that the proposed combined approach performs quite well and circumvents the problem of mismatch among individual synchronization tasks.

This paper presents a training signal based improved timing offset estimation method in frequency selective fading channels. A coarse timing offset estimate is first obtained by maximizing a normalized correlation metric. A coarse frequency offset estimate is similarly obtained by a correlation metric. Then a sliding observation vector based maximum likelihood approach (SOV-ML) for joint timing and frequency offset estimation is applied. This SOV-ML requires the knowledge of channel impulse response (CIR). The CIR estimate is usually affected by the coarse timing and frequency offset estimates. The key requirement for SOV-ML is to obtain a CIR estimate which is not affected (or minimally affected) by the coarse timing and frequency offsets. This paper addresses how to obtain this CIR estimate and presents how this channel estimation assists in improving the timing offset estimation. A way of complexity reduction by an adaptive scheme is also presented.

This paper addresses optimal periodic training signal design for frequency offset estimation in frequency selective multipath Rayleigh fading channels. For a fixed transmitted training signal energy and within a fixed length block, the optimal periodic training signal structure (the optimal location of identical training subblocks) and the optimal training subblock signal are presented. The optimality is based on the minimum Cramer-Rao bound (CRB) criterion. Based on the snap-shot CRB, the optimal periodic training structure is derived. The optimal training subblock signal is obtained by utilizing the average CRB and the received training signal statistics. The optimal training structure with optimal training signals achieves substantial performance improvement over nonoptimal training structure with nonoptimal training signals.

Tarokh's differential space-time block codes (DSTBC) can provide a full spatial diversity and be decoded without channel state information. The bit error rate performance of a conventional differential detection has 3-dB degradation compared to a coherent detection in a quasi-static fading channel. Differential detection experiences more degradation in a fast fading channel and error floor appears at high SNR. In [E. Chiavaccini and G.M. Vitetta, May 2002], an approximate maximum likelihood receiver was derived and it reduced the error floor significantly. In this paper, we modify the receiver in [E. Chiavaccini and G.M. Vitetta, May 2002] and apply the concept of multistage decision-directed estimator with intrablock-interference cancellation to obtain further improvement. The proposed receiver works efficiently at moderate to high SNR and at the fading rate between f/sub d/T = 0.02 and f/sub d/T = 0.07.

A robust symbol-timing and carrier-frequency synchronization scheme applicable to orthogonal frequency-division-multiplexing systems is presented. The proposed method is based on a training symbol specifically designed to have a steep rolloff timing metric. The proposed timing metric also provides a robust sync detection capability. Both time domain training and frequency domain (FD) training are investigated. For FD training, maintaining a low peak-to-average power ratio of the training symbol was taken into consideration. The channel estimation scheme based on the designed training symbol was also incorporated in the system in order to give both fine-timing and frequency-offset estimates. For fine frequency estimation, two approaches are presented. The first one is based on the suppression of the interference introduced in the frequency estimation process by the training symbol pattern in the context of multipath dispersive channels. The second one is based on the maximum likelihood principle and does not suffer from any interference. A new performance measure is introduced for timing estimation, which is based on the plot of signal to timing-error-induced average interference power ratio against the timing estimate shift. A simple approach for finding the optimal setting of the timing estimator is presented. Finally, the sync detection, timing estimation, frequency estimation, and bit-error-rate performance of the proposed method are presented in a multipath Rayleigh fading channel.

A reduced complexity channel estimation for OFDM systems with transmit diversity is proposed by exploiting the correlation of the adjacent subchannel responses. The sizes of the matrix inverse and the FFTs required in the channel estimation at every OFDM data symbol are reduced by half of the existing method for OFDM systems with nonconstant modulus subcarrier symbols or constant modulus subcarrier symbols with some guard tones. The complexity reduction of half FFTs size and some matrix multiplications is still achieved for constant modulus subcarrier symbols with no guard tones. The price for the complexity reduction is a slight BER degradation and for the channels with small relative delay spreads, the BER performance of the reduced complexity method becomes quite comparable to the existing method. An alternative approach for the number of significant taps required in the channel estimation is described which achieves a comparable performance to the case with the known suitable number of significant taps. A simple modification which reduces the lost leakage of the nonsample-spaced channel paths is also proposed. This modification achieves a substantial performance improvement over the existing method without any added complexity.