Abstract For wireless communications, the multiple‐input multiple‐output (MIMO) system efficiently makes use of the spectrum and enhances the transmission throughput. In this work, the maximum‐likelihood (ML) detection for the MIMO system is studied, and two ML detection algorithms are first considered for the MIMO system, including the sphere decoding (SD) algorithm and an algorithm based on differential metrics (DMs). Each of the two algorithms has its advantages and disadvantages. The two algorithms are first modified such that they are based on the same signal model. Then, a new ML detection algorithm is proposed for the MIMO system based on the hybrid operation of the two modified algorithms on the tree search process, in which both the branch‐and‐bound principle and indicative functions are applied to remove unnecessary searches. The proposed algorithm can attain the ML detection with lower average complexity over low and high ranges of signal‐to‐noise ratio (SNR), as the authors verify by simulations. The proposed ML detection can also generate soft output, and anti‐phase sequences are exploited to further reduce the complexity.

In wireless communications, the multiple-input multiple-output (MIMO) system efficiently can make use of the spectrum and enhance the transmission throughput. The sphere decoding (SD) is an efficient algorithm that enables the optimal maximum-likelihood (ML) detection for the MIMO system. However, the SD algorithm has the complexity that increases rapidly with decreasing signal-to-noise ratio (SNR). Another MIMO detection algorithm that is based on differential metrics (DMs) can also attain the exact ML detection without the need of QR decomposition and matrix inversion. The complexity of the DM-based algorithm does not increase with decreasing SNR as the SD algorithm. On the other hand, the SD algorithm has lower complexity at high SNR, especially for large modulation constellation like the quadrature amplitude modulation (QAM). In this paper, we propose a new ML detection algorithm for the MIMO system based on the hybrid operation of both the SD and DM-based algorithms. We first modify both of them such that they are based on the same signal model. Then we apply both the two modified algorithms in the tree search process, with the bit-level and symbol-level operations, respectively. Simulation shows that the proposed hybrid algorithm attains the ML detection, with the same bit-error rates (BER) as the SD algorithm. It also maintains the advantages of both algorithms at high and low ranges of SNR with lower average complexity.

In multi-cell networks, coordination multi-point (CoMP) is an effective means to enhance received signal strength through joint transmission (JT) or null out undesired interference via coordinated beamforming. This work theoretically analyzes the performance of CoMP with both JT and interference nulling (IN) using stochastic geometry. To enable cooperation across cells, multiple base stations (BSs) are grouped in a cluster predetermined by the network operator, which we call the network-centric clustering. Due to the irregular topology of BS clusters and independent design of beamforming weights across different clusters, exact signal and interference distributions are difficult to obtain. To simplify performance analysis, previously work assumes either a regular topology of BS clusters or a certain scheduling mechanism in place to eliminate intra-cluster interference. In this work, we consider the irregular topology of BS clusters and allow the cooperating BSs to serve multiple users by JT and cancel intra-cluster interference through IN. We derive the coverage probability and ergodic rate of network-centric clustering. Numerical results are presented to validate the analysis accuracy and demonstrate the impact of key system parameters, such as cluster size, BS load, number of antennas, to the performance of the considered system.

In the device-to-device (D2D) communications, the strong mutual interference between the uplink cellular users (CUs) and D2D pairs is fatal to its successful application. It is well known that a well designed Stackelberg game approach can alleviate the impact of the mutual interference. However, in the general game-based approaches, a stronger protection mechanism for CUs is lacked, especially for the uplink spectrum sharing problem. Thus, in this paper, we incorporate the capacity- oriented restricted (CORE) region into the conventional Stackelberg game approach such that the CUs can be well protected. Moreover, to fully utilize the CUs' resource, the conventional capacity oriented resource allocation (CORAL) sub- algorithm is applied to boost the D2D pair's capacity. By combining these three techniques, we propose the fairness-and-safety capacity oriented resource allocation (FAST CORAL) scheme. Via the simulation results, the superior performance of the overall system capacity is demonstrated.

During the past dozen years, numerous approaches have been proposed to enhance the efficiency of the cooperative spectrum sensing (CSS) for the cognitive radio (CR) systems. Reducing the signaling overheads and energy consumption during the reporting period are the main targets of these works. In this paper, we propose an efficient reporting and accumulating (ERA) scheme to jointly improve the sensing and reporting efficiency. However, owing to individually reducing the sensing period for each secondary user (SU), the undesired collisions can be incurred while reporting the local decisions. To alleviate the collision problem, a Gaussian retreat (GR) scheme is developed. As verified by the simulation results, higher energy efficiency and spectrum utilization can be achieved. Moreover, the miss-detection probability can become negligible by using the GR scheme.

In this paper, we revisit the frequency allocation scheme (FAS) for the homogeneous cellular networks by taking the non-uniform distribution of users into account. Owing to the non-uniformly distributed user density from sector to sector, the traffic loads become highly unbalanced across whole network. Thus, to satisfy users' requests, the dynamic user-oriented FAS (Duo-FAS) is proposed to allocate different amount of subcarriers to sectors according to the variant user densities. However, the Duo-FAS technique can partially remove the innate benefit of interference avoidance via frequency planning, i.e. the conventional three-sectored cell architecture. That means some extra inter-cell interference (ICI) can be produced by using Duo- FAS. Fortunately, the network multiple-input and multiple-output (MIMO) technique can effectively alleviate the impact of ICI. Thanks to Duo-FAS, the number and positions of the cooperative cells when using the network MIMO technique can be significantly different from the conventional schemes. Thus, properly selecting the cooperative cells becomes a pivotal issue. The key is that for any particular user, the neighboring cells to form a cooperative network MIMO group should be adaptively selected so that the strong ICI can be eliminated. The simulation results demonstrated the higher achievable cell capacity.

Device-to-Device (D2D) communications underlaying cellular networks can improve the network capacity and spectrum efficiency by sharing the cellular resources in the direct-transmission mode. However, the stringent interference between D2D and cellular systems can seriously repress the overall performance improvement. In this paper, we aim to solve this dilemma by applying the concept of differentiated priority into the resources sharing problem between the D2D pairs and cellular users. Firstly, we modified the conventional partial time-frequency resource allocation scheme to let each D2D pair select its sharing comrades, i.e. the cellular users, based on its own preference list such that less amount of mutually interference can be incurred. Secondly, the differentiated priority scheme is furthered implemented into the cellular users, i.e. the so-called bipartite differentiated priority. Then, the association problem between the D2D pairs and cellular users is solved by using the Gale-Shapley algorithm. The simulation results show that the proposed schemes can greatly enhance the system stability and capacity, while accommodating more D2D pairs.

Nowadays, the emerging hyper-dense heterogeneous network (HetNet) is an effective solution to building an easy-access wireless communication environment. In the hyper-dense HetNet, however, the inescapable handoff procedure is the key to its success. An effective handoff algorithm can not only achieve the goal of load-balancing but also enhance the overall system capacity. In this paper, we proposed an efficient load-aware vertical handoff (VHO) algorithm for HetNets. With the aid of the system load information from the neighboring cells, the computational complexity during the VHO procedure can be significantly reduced. Moreover, by effectively off-loading the cells with relatively light traffic loads (i.e. non-worst conditioned cells), the dropping rate can be significantly reduced. Most importantly, the user and the overall system capacity can be greatly enhanced as well.

To meet the explosive demands of wireless access, existing cellular networks are under a revolutionary change from macro-only homogeneous networks to heterogeneous cellular networks (HCNs) with small cells added into current infrastructure. Since small-cell base stations (SBSs) transmit with a much small power than the macro base stations (MBSs), the cell association scheme based on received signal strength will underuse SBSs and thus the notion of cell range expansion (CRE) has been proposed as a de facto technique for future HCNs. In CRE, a constant range expansion bias (REB) is added to the received signal strength of SBSs to increase the footprint of small cells and in turn offload the MBS onto the SBSs overlaid in the same geographical area. Without taking the cell load into consideration, cell offloading may not be effectively performed because an overloaded MBS needs to offload users more urgently than an underloaded MBS. In this work, we propose a load-based cell association scheme, where a bias adjustment function is introduced to determine the timing and the magnitude of offloading based on the load of individual MBS. Using 3GPP long term evolution advanced (LTE-A) compliant parameters, extensive simulation results are presented to show that the proposed load-based cell association scheme outperforms the existing one in terms of both average user rates and association successful probability.

Combating the intersymbol interference is an important issue in single-carrier block transmission systems. In this paper, we present a nonlinear equalizer that combines frequency-domain (FD) pre-filtering with time-domain tree search detection. Simulation results show that the proposed algorithm achieves a detection performance that is very close to that of the conventional tree search equalizer (which employs the QRD-M algorithm), but with a much lower complexity. In addition, the proposed algorithm performs significantly better than the linear equalizers and decision-feedback equalizers, with only a modest increase in complexity.

High peak-to-average power ratio (PAPR) is a well-known drawback in orthogonal frequency-division multiplexing (OFDM). In this article, the pilot arrangement in OFDM is exploited to remedy this difficulty. A PAPR reduction scheme is proposed in this article which implements by imposing different linear phases to subcarriers within the pilot tones and selecting the phase vector with the lowest PAPR for transmission. The receiver can recover the phase rotation during the channel estimation procedure, thus it does not require additional side information to recover the signal. The scheme is implemented in Digital Video Broadcasting-Terrestrial (DVB-T) systems for simulations, and the results show that after reducing PAPR, it remains the same bit-error rate (BER) performance over the additive white Gaussian noise (AWGN) channel and fading channels without any side information. In addition, the proposed scheme improves the efficiency of the PAPR reduction compared to the conventional methods, for example, partial transmit sequences (PTS) and selected mapping technique (SLM), with the same number of candidate signals.

In the Long Term Evolution (LTE) system, the initial cell search needs not only to achieve the timing synchronisations of symbols, slots and frames, but also to detect the sector identity (ID), the cell ID group and the carrier frequency offset (CFO), which includes an integer part (ICFO) and a fractional part. To accomplish this cell search process, two synchronisation signals, the primary synchronisation signal (PSS) and the secondary synchronisation signal, are periodically broadcasted from base stations in the LTE system. The PSS is mainly used for sector ID detection. In this study, an innovative algorithm for detecting the sector ID and ICFO together via the PSS‐matching process is presented. In the proposed scheme, the phases of the differential correlation instead of the absolute values are adopted to determine the ICFO. Furthermore, a new set of PSS is proposed in order to improve the success probability of ICFO detection. Compared with conventional schemes, the proposed scheme can detect both the sector ID and ICFO with higher accuracy and much lower complexity.

Abstract-It is known that multiple-symbol differential detection can enhance the performance of coded differential phase shift keying (DPSK) systems over a correlated flat fading channel, but have exponential complexity. In this paper, the proposed scheme, Forward/Backward-MSDSD (FB-MSDSD), consisted of MAP-MSDSD, channel estimation and soft demodulation can reduce the complexity by dividing the original detection interval into forward and backward processes. The simulation runs show that compared with SISO MSDSD scheme, the proposed scheme has similar bit error rate but lower complexity.

The performance of multiple access interference (MAI) cancellations for the asynchronous M-ary pulse position modulation (PPM) time hopping (TH) ultra-wideband (UWB) systems are investigated. Parallel interference cancellation (PIC) and successive interference cancellation (SIC) are the most popular interference cancellation schemes. However, both the PIC and SIC have some shortcomings that the PIC suffers an error-propagation effect due to the erroneous MAI cancellation and the SIC imports a long processing delay caused by the large number of user. Therefore, a low-complexity iterative interference cancellation (IIC) scheme is proposed in this paper. The IIC scheme is based on the concept of iteratively cancelling MAI from those users whose symbols have been reliably detected. The performances of these interference cancellation schemes are compared in terms of the symbol error rate (SER) for the asynchronous M-ary PPM TH-UWB systems. The simulation results show the proposed IIC scheme performs better than PIC and SIC detectors in a MAI dominant environment.

In this paper, we study medium access control (MAC) protocols with quality-of-service (QoS) support, that is, topology-independent link activation transmission scheduling, for mobile code-division multiple-access (CDMA) ad hoc networks. QoS provisioning for each communication link is guaranteed without the need to adopt transmission schedules in mobile environments. An interference model, which captures the difference between transmission and interference ranges, is considered. Under this interference model, an approach to guaranteeing conflict-free transmission slots in each frame (QoS provisioning) for each communication link is proposed. Compared with the previously known method, superior performance is obtained. We then present a topology-independent link activation scheduling framework based on the theory of group-divisible (GD) designs. By the mathematical properties of GD designs, the proposed framework guarantees conflict-free transmission slots in each frame for each communication link, without the overhead due to the recomputation of transmission schedules when the network topology changes. With the proposed framework, we study and evaluate one series of GD design constructions. Based on the results derived, topology-independent link activation scheduling algorithms are then presented. The proposed schemes are designed for different objectives: maximizing the minimum system throughput and/or minimizing the schedule frame length. Numerical results show that the proposed algorithms outperform previously known schemes. The average performance of the proposed schemes is also derived.

For real-time traffic requirements in mobile ad hoc networks a novel and effective QoS admission control routing protocol (QACRP) is proposed in this paper. With a limit amount of extra overhead, QACRP can avoid network congestion by a simple and precise admission control to block most of overloading flow-requests in route discovery process. System simulations show that this proposed scheme can increase the system throughput and reduce both the dropping rate and the end-to-end delay when compared with previous QoS routing schemes like QoS-aware routing protocol and CACP protocol. Hence, QACRP is surely an effective routing protocol to provide QoS guarantee for real-time traffic.

IQ signal processing is widely utilized in today's communication receivers. However, it usually faces a common problem of mismatch of the amplitudes and phases on I and Q branches because of the gain and phase imbalance between quadrature mixers. In this paper, we propose a novel method to estimate and compensate for the IQ imbalances for communication channels with frequency offset by using a preamble of repeated training sequence. The proposed method, as it jointly adapts schemes in the system of estimation, is more robust and gives better performance than the earlier results. The performance of such a scheme has been validated by computer simulation on IEEE 802.11a signals.

In this paper, we study topology-independent link activation transmission scheduling protocols for multihop code-division multiple-access (CDMA) packet radio networks. We focus on quality-of-service (QoS) provisioning for each communication link in mobile environments. An interference model for wireless multihop packet radio networks, which is more practical than that adopted in earlier literature, is considered. Under this interference model, an approach to guaranteeing conflict-free transmission slots in each frame for each communication link is proposed. We then present a topology-independent link activation scheduling framework based on the theory of group divisible (GD) designs. The proposed framework guarantees conflict-free transmission slots in each frame for each communication link by mathematical properties of GD designs. With the proposed framework, we study and evaluate one series of GD design construction. We then propose topology-independent link activation scheduling algorithms based on the results derived. The proposed schemes are designed for different objectives: maximizing the minimum system throughput and minimizing the schedule frame length. Analysis results show that the proposed algorithms outperform previously known schemes.

We study topology-transparent link activation transmission scheduling protocols for multihop CDMA ad hoc networks. We focus on quality-of-service (QoS) provisioning for each communication link, particularly when node mobility is considered. An interference model, which is more practical than that adopted in earlier literature, is proposed. Under the presented interference model, two methods are proposed to guarantee conflict-free transmission slots for each communication link in each frame. After that, we present two frameworks for topology-transparent link activation scheduling which are based on resolvable balanced incomplete block (RBIB) designs and group-divisible (GD) designs. respectively. These schemes address and resolve primary and secondary conflicts through the mathematical properties of block designs. The proposed schemes maximize the minimum system throughput and analysis results show that the proposed algorithms can outperform the previously known algorithm.

The bit-error probability (BEP) is evaluated for a fast frequency-hopping binary frequency-shift-keying (FFH/BFSK) spread-spectrum communication system over Rician fading channels in the presence of worst-case multitone jamming (MTJ) and additive white Gaussian noise (AWGN). A diversity reception scheme of product-combining is applied at the receiver. Two types of MTJ models, namely n=1-band MTJ and independent MTJ, are considered in this letter. To obtain the final decision statistics, the closed-form or a single finite integral expressions of cumulative distribution functions (CDFs) of the ratio of two square-law detectors' outputs per hop are derived. According to the numerical results, it is shown, unlike the known knowledge for the AWGN channel, that independent MTJ is more harmful than n=1-band MTJ for the product-combining receivers (L/spl ges/2) in fading channels.

Hierarchical cellular systems are usually adopted in the non-uniform traffic environment, especially when hot spots exist. Considering the spatial fluctuation of traffic load, three schemes to adjust the coverage area of a microcell, namely ATP (adjusting total transmission power of microcell base station), ARP (adjusting the ratio of pilot signal power to the total transmission power), and ABP (adjusting both of them), are mathematically analyzed in this paper. The system performances of these schemes are also shown by the numerical examples.

The bit-error probability (BEP) is evaluated for a fast frequency-hopping/binary frequency-shift keying spread-spectrum communication system over a frequency-nonselective, slowly fading channel with worst-case band multitone jamming and additive white Gaussian noise. A diversity reception technique with ratio-statistic combining is applied at the receiver. Both square-law and envelope detectors are utilized and analyzed. Based on circularly symmetric signal theory, the paper obtains the closed-form expressions of probability density function and cumulative distribution function of the ratio-statistic output. It is shown from the analytical results, and verified by simulation, that the BEP performance of the ratio-statistic receiver is sensitive to the fading effect on the desired signal, but is insensitive to that on the jamming tones. It is also shown that the envelope detector provides better performance than the square-law detector.

A resource-allocation strategy in an integrated voice/data cellular CDMA network is presented and analyzed. The delay-sensitive voice service is considered to have higher priority to use the channel resource, while the error-sensitive data traffic has higher priority to use the power resource. The concept of guard channel is used to improve service reliability. Three power resource allocation schemes are adopted and compared: equal power scheme, equal service-reliability scheme, and robust data scheme. Moreover, the effect of the data packet length is also investigated.

A bit error probability (BEP) expression is derived for the fast frequency-hopping binary frequency-shift-keying spread-spectrum communication system with self-normalizing combining receiver over a slowly Rician fading channel under independent multitone jamming (MTJ) and AWGN environment. In our study, the close-form or a single finite integral expression of cumulative distribution function (cdf) of the self-normalized output is derived and exact analysis of BEP performance is obtained. Our analytic results, validated with the aid of simulations, show that independent MTJ is more harmful than the corresponding n=1-band MTJ when diversity is taken into account (L>2).

The automatic gain control (AGC) loops are usually used in communication systems to stabilize the performance. In this paper, the combined performance of digital delay-locked loop (DDLL) and AGC loop for direct-sequence spread-spectrum systems is analyzed. Especially, the inherent coupling effects between DDLL and AGC loop are developed. The numerical results show that the DDLL with AGC loop can offer a reliable and satisfactory performance.

The performance of first- and second-order non-coherent digital delay lock loops (DDLL) for direct-sequence spread-spectrum (DS-SS) signals is investigated in the mobile radio environment. The mobile radio channel is first characterized by Rayleigh fading and Doppler shift. A closed-form expression for the timing error transition probability density function of the Chapman-Kolmogorov (C-K) equation is proposed. The probability density function of the steady-state timing error for the first- and second-order DDLL is obtained by solving the C-K equation numerically, and the results are confirmed by computer simulations. Furthermore, the mean time to lose lock (MTLL) of the first-order loop is evaluated, and some numerical results and simulation results are reported. Finally, the steady-state timing error and MTLL of the first-order loop for DS-SS signals in the log-normal fading environment are also presented, and the results are compared with those of Rayleigh and AWGN channels.

The code-division multiple-access (CDMA) scheme has been considered as one possible choice of the future standards for cellular networks because of its various advantages. Since there can be only one carrier frequency being used in CDMA systems, a handoff scheme with diversity, a so-called "soft handoff", was proposed for higher communication quality and capacity. A mathematical model is developed to analyze the soft handoff process. Markov's concept is applied to describe the system's steady state statistical behavior. System performance such as blocking probability, handoff refused probability, and channel efficiency are also determined. It is concluded that the larger the area the soft handoff region is, the better users in the cellular network will feel.

This paper considers discrete time analyses of digital delay lock loops (DDLL), and presents the results of an investigation concerning the performance degradation due to Doppler. The performance measures evaluated include the steady-state timing error probability density function (PDF) and the mean time to lose lock. The linear approximations are also provided and compared to the numerical results and simulations. The delay lock loop is a well-known technique to track the pseudo-noise (PN) codes for spread spectrum systems. Under a severe Doppler environment, such as in the low Earth orbit (LEO) satellite communication, it will be difficult to track the PN code for the first-order loop. Since the digital systems are more compact, lower cost, and more stable than their analog counterparts, we focus our attention on the digital first and second order code tracking loops that suffer from severe Doppler.

In this paper a tractable model is developed to analyze the soft handoff process. A Markovian conception is applied to describe the system's statistic behavior in steady state. Theoretical performances such as channel efficiency, blocking probability and handoff refused probability are also determined.

Power control (PC) is an important issue in the DS-CDMA cellular mobile radio system. For the IS-95 standard, the reverse-link PC strategy is based on fixed-step power change and it needs only one command bit per update. We call this strategy DM-PC. From the point of view of digital communication, ADM will have better performance with the same number of control bits compared to DM. So it appears to be a good idea to improve the DM-PC and be compatible with the current system as IS-95. However, the pure ADM-PC is not better than DM-PC from our earlier simulation results. Hence, we propose another PC strategy called modified ADM-PC. The simulation results show that it has better performance than DM-PC.

In a recent paper, a generalization of the slotted ALOHA random access scheme is considered in which a user transmits multiple copies of the same packet. In frequency diversity ALOHA, multiple copies of the same packet are simultaneously transmitted on different channels. We found the analysis for one of the proposed schemes, namely, channel selection without replacement, to be incorrect. These comments contain a counterexample to the published results and a corrected version of the analysis.