Evolution map from TD-SCDMA to FuTURE B3G TDD

The rapid growth of mobile applications in recent years has created a need for wideband wireless communication. The only predictable trend is that data-rate and QoS requirements will increase rapidly. This demand is pushing time-division-synchronous code-division multiple access (TD-SCDMA) to evolve in order to provide higher and higher data rates. This article presents the evolution map from TD-SCDMA to future terrestrial universal radio environment (FuTURE) TDD in China. The evolution includes four phases: low chip rate (LCR), high-speed downlink/uplink packet access (HSxPA)/TD-SCDMA EV 1x, long-term evolution (LTE) TDD, and FuTURE beyond 3G TDD. The main features of each phase are described in detail. By introducing the new technologies into the system step by step, for example, multiple input and multiple output (MIMO), orthogonal frequency-division multiplexing (OFDM), TD-SCDMA system can evolve to FuTURE B3G TDD smoothly, and provide high-data-rate services with low cost, low latency, and improved coverage and capacity.     

Multiuser MIMO-OFDM for Next-Generation Wireless Systems

This overview portrays the evolution of orthogonal frequency division multiplexing (OFDM) research. The amelioration of powerful multicarrier OFDM arrangements with multiple-input multiple-output (MIMO) systems has numerous benefits, which are detailed in this treatise. We continue by highlighting the limitations of conventional detection and channel estimation techniques designed for multiuser MIMO OFDM systems in the so-called rank-deficient scenarios, where the number of users supported or the number of transmit antennas employed exceeds the number of receiver antennas. This is often encountered in practice, unless we limit the number of users granted access in the base station's or radio port's coverage area. Following a historical perspective on the associated design problems and their state-of-the-art solutions, the second half of this treatise details a range of classic multiuser detectors (MUDs) designed for MIMO-OFDM systems and characterizes their achievable performance. A further section aims for identifying novel cutting-edge genetic algorithm (GA)-aided detector solutions, which have found numerous applications in wireless communications in recent years. In an effort to stimulate the cross pollination of ideas across the machine learning, optimization, signal processing, and wireless communications research communities, we will review the broadly applicable principles of various GA-assisted optimization techniques, which were recently proposed also for employment in multiuser MIMO OFDM. In order to stimulate new research, we demonstrate that the family of GA-aided MUDs is capable of achieving a near-optimum performance at the cost of a significantly lower computational complexity than that imposed by their optimum maximum-likelihood (ML) MUD aided counterparts. The paper is concluded by outlining a range of future research options that may find their way into next-generation wireless systems.     

Editorial - Advanced signal processing for wireless and mobile communications

Wireless and mobile communications have developed rapidly in recent years due to the endless demand for broadband multimedia Internet access and wireless connection anywhere and any time. Several 4G standards have been proposed such as WiMax, LTE and IEEE 802.11n. Signal processing plays a key role in the evolution of wireless and mobile communications. With the emergence of various 4G enabling technologies, advanced signal processing has become ever increasingly important for low complexity, low power and efficient physical implementations of future multimedia communication systems. Multi-antenna, multi-frequency, multi-user and cooperative technologies have emerged as powerful enablers for high date rate and highly reliable broadband communications. To harvest the benefits of these technologies maximally, both excellent opportunities and significant challenges are present in the research of signal processing. The aim of this special issue was to publish some most recent advances in the signal processing aspects of these wireless and mobile communication technologies. Five papers have been selected for the special issue. These papers cover various multi-antenna topics including MIMO detection, spacetime equalization, MIMO channel prediction and estimation, and space-time coding. Some papers also consider cooperative relay communications, OFDM, and multiuser systems.     

Blind Multiuser Detection with Array Observations

Cochannel interference is usually a major limitation to the performance of mobile wireless systems. Examples of different forms of cochannel interference include multi-access interference in CDMA systems and cochannel interference resulting from frequency reuse in TDMA systems. In order to mitigate the interference from other users we present a blind multiuser receiver which utilizes array observations and performs both spatial and temporal processing of the received signal. The presented technique is completely blind in the sense that no signature sequences, channel state or spatial location needs to be known a priori, nor use of a training sequences, channel state or spatial location needs to be known a priori, nor use of a training sequence is required for the adaptation. The diversity introduced by the array observations can be efficiently combined with the use of CDMA signature sequences. After initial convergence, a reliable estimate of the combined temporal and spatial signature for each user is provided that can be employed by a multiuser receiver of lower complexity.     

Transmit power adaptation for multiuser OFDM systems

In this paper, we develop a transmit power adaptation method that maximizes the total data rate of multiuser orthogonal frequency division multiplexing (OFDM) systems in a downlink transmission. We generally formulate the data rate maximization problem by allowing that a subcarrier could be shared by multiple users. The transmit power adaptation scheme is derived by solving the maximization problem via two steps: subcarrier assignment for users and power allocation for subcarriers. We have found that the data rate of a multiuser OFDM system is maximized when each subcarrier is assigned to only one user with the best channel gain for that subcarrier and the transmit power is distributed over the subcarriers by the water-filling policy. In order to reduce the computational complexity in calculating water-filling level in the proposed transmit power adaptation method, we also propose a simple method where users with the best channel gain for each subcarrier are selected and then the transmit power is equally distributed among the subcarriers. Results show that the total data rate for the proposed transmit power adaptation methods significantly increases with the number of users owing to the multiuser diversity effects and is greater than that for the conventional frequency-division multiple access (FDMA)-like transmit power adaptation schemes. Furthermore, we have found that the total data rate of the multiuser OFDM system with the proposed transmit power adaptation methods becomes even higher than the capacity of the AWGN channel when the number of users is large enough.     

A dynamic channel assignment algorithm for a hybrid TDMA/CDMA-TDDinterface using the novel TS-opposing technique

It has been demonstrated that code division multiple access (CDMA) provides great flexibility by enabling efficient multiuser access in a cellular environment. In addition, time division duplex (TDD) as compared to frequency division duplex (FDD) represents an appropriate method to cater for the asymmetric use of a duplex channel. However, the TDD technique is subject to additional interference mechanisms compared to an FDD system, in particular if neighboring cells require different rates of asymmetry. If TDD is combined with an interference limited multiple access technique such as CDMA, the additional interference mechanism represents an important issue. This issue poses the question of whether a CDMA/TDD air-interface can be used in a cellular environment. The problems are eased if a hybrid time division multiple access (TDMA)/CDMA interface (TD-CDMA) is used. The reason for this is that the TDMA component adds another degree of freedom which can be utilized to avoid interference. This, however, requires special channel assignment techniques. A notable example of a system which uses a TD-CDMA/TDD interface is the Universal Mobile Telecommunications System (UMTS). This paper presents a novel centralized dynamic channel assignment (DCA) algorithm for a TD-CDMA/TDD air-interface. The DCA algorithm exploits a new technique which is termed “TS-opposing technique.” The key result is that the new DCA algorithm enables neighboring cells to adopt different rates of asymmetry without a significant capacity loss     

广义正交传输系统及其高效实现方法

基于子带变换理论构建的广义正交传输复用模型，不仅适用于TDMA、FDMA和CDMA多用户系统，也适用于像OFDM之类的多载波（MC）系统以及由OFDM与CDMA综合而成的MC-CDMA系统。该模型在结构上等效于均匀DFT滤波器组，可以采用FFT结合多相滤波器组的方式高效实现。本文实现的多载波系统具有子载波间隔可伸缩性特点，而复杂性与OFDM加窗处理相接近。在相同的仿真环境下得到的结果表明：基于软脉冲响应的多载波系统（MCSIS）误码率性能优于OFDM；对于相同衰落信道，MC-CDMA系统抗衰落特性明显优于OFDM。     

数字移动通信中的抗多径衰落技术

移动信道的主要特征是由移动和多径传播所产生的多径衰落现象。在高速移动通信系统中，必须采用抗多径衰落技术，才能实现有效传输。本文首先概述了移动信道的数学模型及其典型形式，在此基础上，分别讨论了时分多址（ＴＤＭＡ）和码分多址（ＣＤＭＡ）系统的抗多径衰落方法──自适应信道均衡技术及多径分集接收技术，并简述了近年来较为活跃的多用户检测技术和自适应天线分集技术。     

Space-time communication for OFDM with implicit channel feedback

We consider wideband communication (e.g., using orthogonal frequency-division multiplexed (OFDM) systems) over a typical cellular "downlink," in which both the base station and the mobile may have multiple antennas, but the number of antennas at the mobile is assumed to be small. Implicit channel feedback can play a powerful role in such systems, especially for outdoor channels, which typically exhibit narrow spatial spreads. A summary of our findings is as follows: a) Implicit channel feedback regarding the covariance matrix for the downlink space-time channel can be obtained, without any power or bandwidth overhead, by suitably averaging uplink channel measurements across frequency. Since this approach relies on statistical reciprocity, it applies to both time-division duplex (TDD) and frequency-division duplex (FDD) systems. Using such covariance feedback yields significantly better performance at lower complexity than conventional space-time or space-frequency codes, which do not employ feedback; b) We provide guidelines for optimizing antenna spacing in systems with covariance feedback. Theoretical investigation of a hypothetical system with completely controllable channel eigenvalues shows that the optimal number of channel eigenmodes is roughly matched to the (small) number of receive antenna elements. Thus, while antenna elements in conventional systems without feedback should be spaced far apart in order to ensure uncorrelated responses, the optimal antenna spacing with covariance feedback is much smaller, thereby concentrating the channel energy into a small number of eigenmodes.     

多径信号对MIMO系统信道容量的影响

一般的MIMO系统,MIMO-OFDM系统和MIMO MC-CDMA系统是未来宽带无线通信业务发展的候选技术,MIMO技术的研究已经成为通信理论的前沿领域。多径传输会引起频率选择性衰落,码间干扰等许多问题,分别在单用户和多用户条件下,检验多径信号传输对MIMO系统信道容量的影响。     

A Low-Complexity QoS-Aware Proportional Fair Multicarrier Scheduling Algorithm for OFDM Systems

Orthogonal frequency-division multiplexing (OFDM) systems are the major cellular platforms for supporting ubiquitous high-speed mobile applications. However, a number of research challenges remain to be tackled. One of the most important challenges is the design of a judicious packet scheduler that will make efficient use of the spectrum bandwidth. Due to the multicarrier nature of the OFDM systems, the applicability and performance of traditional wireless packet-scheduling algorithms, which are usually designed for single-carrier systems, are largely unknown. In this paper, we propose a new quality-of-service (QoS)-aware proportional fairness (QPF) packet-scheduling policy with low complexity for the downlink of multiuser OFDM systems to allocate radio resources among users. Our proposed algorithm is based on a cross-layer design in that the scheduler is aware of both the channel (i.e., physical layer) and the queue state (i.e., data link layer) information to achieve proportional fairness while maximizing each user's packet-level QoS performance. The simulation results show that the proposed QPF algorithm is efficient in terms of average system throughput, packet-dropping probability, and packet delay, while maintaining adequate fairness among users with relatively low scheduling overhead.      

Performance Analysis of Space-Time Coded CDMA System Over Nakagami Fading Channels with Perfect and Imperfect CSI

In this paper, the performance of multiuser CDMA systems with different space time code schemes is investigated over Nakagami fading channel. Low-complexity multiuser receiver schemes are developed for space-time coded CDMA systems with perfect and imperfect channel state information (CSI). The schemes can make full use of the complex orthogonality of space-time coding to obtain the linear decoding complexity, and thus simplify the exponential decoding complexity of the existing scheme greatly. Moreover, it can achieve almost the same performance as the existing scheme. Based on the bit error rate (BER) analysis of the systems, the theoretical calculation expressions of average BER are derived in detail for both perfect CSI and imperfect CSI, respectively. As a result, tight closed-form BER expressions are obtained for space-time coded CDMA with orthogonal spreading code, and approximate closed-form BER expressions are attained for space-time coded CDMA with quasi-orthogonal spreading code. Computer simulation for BER shows that the theoretical analysis and simulation are in good agreement. The results show that the space-time coded CDMA systems have BER performance degradation for imperfect CSI.     

海面的红外被动成像

本文在研究海面的红外成像特性中,建立了基于刚体的双尺度成像模型,从被动成像方面讨论了海面辐射特性的变化规律.该模型仿真结果很好地说明了风向对海面红外被动成像特性的影响.     

Decentralized dynamic power control for cellular CDMA systems

The control of transmit power has been recognized as an essential requirement in the design of cellular code-division multiple-access (CDMA) systems. Indeed, power control allows for mobile users to share radio resources equitably and efficiently in a multicell environment. Much of the work on power control for CDMA systems found in the literature assumes a quasi-static channel model, i.e., the channel gains of the users are assumed to be constant over a sufficiently long period of time for the control algorithm to converge. In this paper, the design of dynamic power control algorithms for CDMA systems is considered without the quasi-static channel restriction. The design problem is posed as a tradeoff between the desire for users to maximize their individual quality of service and the need to minimize interference to other users. The dynamic nature of the wireless channel for mobile users is incorporated in the problem definition. Based on a cost minimization framework, an optimal multiuser solution is derived. The multiuser solution is shown to decouple, and effectively converge, to a single-user solution in the large system asymptote, where the number of users and the spreading factor both go to infinity with their ratio kept constant. In a numerical study, the performance of a simple threshold policy is shown to be near that of the optimal single-user policy. This offers support to the threshold decision rules that are employed in current cellular CDMA systems.     

Co-ordinate Interleaved Spatial Multiplexing with Channel State Information

Performance of spatial multiplexing multiple-input multiple-output (MIMO) wireless systems can be improved with channel state information (CSI) at both ends of the link. This paper proposes a new linear diagonal MIMO transceiver, referred to as co-ordinate interleaved spatial multiplexing (CISM). With CSI at transmitter and receiver, CISM diagonalizes the MIMO channel and interleaves the co-ordinates of the input symbols (from rotated QAM constellations) transmitted over different eigenmodes. The analytical and simulation results show that with co-ordinate interleaving across two eigenmodes, the diversity gain of the data stream transmitted over the weaker eigenmode becomes equal to that of the data transmitted on the stronger eigenmode, resulting in a significant improvement in the overall diversity. The diversity-multiplexing tradeoff (DMT) is analyzed for CISM and is shown that it achieves higher diversity gain at all positive multiplexing gains compared to existing diagonal transceivers. Over rank n MIMO channels, with input symbols from rotated n-dimensional constellations, the DMT of CISM is a straight line connecting the endpoints (0,N_tN_r) and (min{N_t,N_r}, 0), where N_t, and N_r} are the number of transmit and receive antennas, respectively.     

Closed-form blind channel identification and source separation inSDMA systems through correlative coding

We address the problem of blind identification of multiuser multiple-input multiple-output (MIMO) finite-impulse response (FIR) digital systems. This problem arises in spatial division multiple access (SDMA) architectures for wireless communications. We present a closed-form, i.e., noniterative, consistent estimator for the MIMO channel based only on second-order statistics. To obtain this closed form we introduce spectral/correlation asymmetry between the sources by filtering each source output with adequate correlative filters. Our algorithm uses the closed form MIMO channel estimate to cancel the intersymbol interference (ISI) due to multipath propagation and to discriminate between the sources at the wireless base station receiver. Simulation results show that, for single-user channels, this technique yields better channel estimates in terms of mean-square error (MSE) and better probability of error than a well-known alternative method. Finally, we illustrate its performance for MIMO channels in the context of the global system for mobile communications (GSM) system     

Delay and throughput characteristics of TH, CDMA, TDMA, and hybridnetworks for multipath faded data transmission channels

The new concepts of adaptive time hopping and variable frame code division (CDMA) multiple access are introduced. By a unified analysis, the probabilities of bit and packet errors in multipath fading environment for five time division (TDMA), code division, and time hopping (TH) related multiaccess networks are obtained; namely, TDMA, CDMA, CDMA/TDMA, Adaptive CDMA/TH, and variable frame CDMA/TDMA networks. The delay and useful throughputs of the five systems are also evaluated for data and voice traffic. All systems compared have the same channel power and bandwidth and support the same traffic. Though implementation issues are not covered, CDMA systems are put at a disadvantage (compared to cellular-type FDMA networks, for example) by ignoring such inherent advantages as voice silence utilizations and automatic frequency reuse. Nontheless, two CDMA systems outperform TDMA systems at low and medium input traffics     

On strategies of multiuser MIMO transmit signal processing

In this letter, we introduce five different strategies of linear transmit signal processing for multiuser multiple-input multiple-output (MIMO) systems and provide performance comparisons in terms of maximum throughput in both uncorrelated and correlated channels when the number of transmit antennas is much larger than the number of receive antennas. It is shown that the multiuser MIMO schemes are preferable to time-division multiple-access (TDMA)-based MIMO schemes, hence demonstrating the power of multiuser MIMO signal processing. Our work also indicates possibilities for future research in finding efficient suboptimal algorithms. As an example, we show that our multiuser MIMO decomposition scheme can improve the maximum throughput compared to TDMA-based MIMO schemes for large number of transmit antennas or high transmit power.     

A Simulation Study on Channel Estimation for MIMO-OFDM Based Beyond 3G Mobile Systems

1 Introduction Future wireless cellular communication networks areexpected to fully support multi-media services on a real-ti me basis ,therefore pose a challengingissue of develop-ing high-speed access and high data-rate transmissiontechnologies , which are not satisfied by current 2nd-and3rd-generation cellular mobile communications net-works[1]. Orthogonal Frequency Division Multiplexing(OFDM) has become a popular technique for high datarate wirelesstransmission duetoits many advantagesfo…     

Guest Editorial Multiuser Detection for Advanced Communication Systems and Networks

The thirteen papers in this special issue focus on multiuser detection for advanced communication systems and networks. The papers can be divided into three thematic groups: Multiuser detection (MUD) in i) CDMA; ii) MIMO and Multicarrier CDMA/OFDM; and iii) Cooperative Communications.