Outage capacities and optimal power allocation for fading multiple-access channels

We derive the outage capacity region of an M-user fading multiple-access channel (MAC) under the assumption that both the transmitters and the receiver have perfect channel side information (CSI). The outage capacity region is implicitly obtained by deriving the outage probability region for a given rate vector. Given a required rate and average power constraint for each user, we find a successive decoding strategy and a power allocation policy that achieves points on the boundary of the outage probability region. We discuss the scenario where an outage must be declared simultaneously for all users (common outage) and when outages can be declared individually (individual outage) for each user.     

Outage Minimization and Rate Allocation for the Multiuser Gaussian Interference Channels With Successive Group Decoding

We consider a memoryless Gaussian interference channel (GIC) where $K$ single-antenna users communicate with their respective receivers using Gaussian codebooks. Each receiver employs a successive group decoder with a specified complexity constraint, to decode its designated user. It is aware of the coding schemes employed by all other users and may choose to decode some or all of them only if it deems that doing so will aid the decoding of its desired user. For a GIC with predetermined rates for all transmitters, we obtain the minimum outage probability decoding strategy at each receiver which satisfies the imposed complexity constraint and reveals the optimal subset of interferers that must be decoded along with the desired user. We then consider the rate allocation problem over the GIC under successive group decoding and design a sequential rate allocation algorithm which yields a pareto-optimal rate allocation, and two parallel rate allocation algorithms which yield the symmetric fair rate allocation and the max-min fair rate allocation, respectively. Remarkably, even though the proposed decoding and rate allocation algorithms use “greedy” or myopic subroutines, they achieve globally optimal solutions. Finally, we also propose rate allocation algorithms for a cognitive radio system.      

频谱有效的差分空时编码CDMA系统

该文针对码分多址(Code-Division Multiple-Access,CDMA)系统提出了频谱有效的差分空时传输方案。考虑包含M个同步共道用户的多用户环境,每一用户配备双发射天线。若接收端配备N2个接收天线,该方案将采用解相关检测器和接收天线分集分离M个用户。基于平坦Rayleigh衰落信道,给出了非相干译码器,它可为每一共道用户提供2(N-1)的最小分集增益。与已有的差分空时编码CDMA系统相比,该方案具有两大优势:第一,仅需增加单个接收天线,该方案可在频谱效率提高1/3的条件下显著地改善系统性能;第二,译码仅具有线性复杂度。     

Algebraic properties of space-time block codes in intersymbol interference multiple-access channels

In this paper, we study the multiple-access channel where users employ space-time block codes (STBC). The problem is formulated in the context of an intersymbol interference (ISI) multiple-access channel which occurs for transmission over frequency-selective channels. The algebraic structure of the STBC is utilized to design joint interference suppression, equalization, and decoding schemes. Each of the K users transmits using M/sub t/=2 transmit antennas and a time-reversed STBC suitable for frequency-selective channels. We first show that a diversity order of 2M/sub r/(/spl nu/+1) is achievable at full transmission rate for each user, when we have M/sub r/ receive antennas, channel memory of /spl nu/, and an optimal multiuser maximum-likelihood (ML) decoder is used. Due to the decoding complexity of the ML detector we study the algebraic structure of linear multiuser detectors which utilize the properties of the STBC. We do this both in the transform (D-domain) formulation and when we impose finite block-length constraints (matrix formulation). The receiver is designed to utilize the algebraic structure of the codes in order to preserve the block quaternionic structure of the equivalent channel for each user. We also explore some algebraic properties of D-domain quaternionic matrices and of quaternionic circulant block matrices that arise in this study.     

Multiuser capacity in block fading with no channel stateinformation

Consider M independent users, each user having his own transmit antenna, that transmit simultaneously to a receiver equipped with N antennas through a Rayleigh block-fading channel having a coherence interval of T symbols, with no channel state information (CSI) available to either the transmitters or to the receiver. The total transmitted power is independent of the number of users. For a given coherence time T, we wish to identify the best multiaccess strategy that maximizes the total throughput. If perfect CSI were available to the receiver, it is known that the total capacity would increase monotonically with the number of users. If the CSI is available to both the receiver and to all transmitters, the throughput maximizing strategy implies for N=1 that only the single user who enjoys the best channel condition transmits. In the absence of CSI one is forced to a radically different conclusion. In particular, we show that if the propagation coefficients take on new independent values for every symbol (e.g., T=1) then the total capacity for any M > 1 users is equal to the capacity for M=1 user, in which case time division multiple access (TDMA) is an optimal scheme for handling multiple users. This result follows directly from a recent treatment of the single-user multiple antenna block-fading channel. Again, motivated by the single-user results, one is lead to the following conjecture for the multiple-user case: for any T > 1, the maximum total capacity can be achieved by no more than M = T users. The conjecture is supported by establishing the asymptotic result that, for a fixed N and a constant M/T for large T, the total capacity is maximized when M/T→0, which yields a total capacity per symbol of N log(1 + ρ), where ρ is the expected signal-to-noise ratio (SNR) at the receiver. We further support the conjecture by examining the asymptotic behavior with large to for fixed M, T, and N ⩽ T     

一种未知信道下的发射分集差分检测新方法

本文提出一种收发端均不知道信道状态信息情况下发射分集差分检测新方法。该方法将发射天线分组,对每组进行独立的差分空时分组编码以降低编码复杂度,并对每组天线信号进行正交扩频,以便接收端分离各组天线信号实现解码。这种方法不仅编译码简单、编码速率高,而且仍然保持了差分空时分组码的发射端和接收端不需要知道状态信息的优点,同时由于引入了正交的扩频码,系统可获得扩频增益。     

多天线频谱共享认知网络保密中断概率分析

针对单输入多输出认知无线电网络,研究了瑞利衰落信道下采用最大比合并时保密中断性能。在所研究的系统中,次用户发射机发送机密信息给另一个次用户接收机,次用户接收机配备多个天线并且采用最大比例合并多个接收信号。同时,拥有多个天线窃听者也采用最大比合并方案偷听次用户发射机和次用户接收机之间传送的信息。频谱共享下次用户发射机工作时必须保证主用户的服务质量。推导了保密中断概率的精确表达式,分析了系统保密中断概率渐近性能。仿真结果验证了分析的正确性。     

Concatenation of trellis coded modulation and space-time block codes for high data rate on wireless systems using multiple antenna elements

Performance results using concatenation of high-rate pragmatic TCM (trellis coded modulation) codes with a simple high-rate space-time block code operating on a multiple input/multiple output (MIMO) channel with 4 transmit and 4 receive antennas are presented. Four TCM encoders feed 4 data streams consisting of 32-QAM symbols into a simple Alamouti — like space-time code, spreading the data over 4 transmit antennas. In this way an overall data rate of 8 information bits per channel use is obtained. Perfect channel state information (CSI) at the receiver is assumed for all investigations. Using 4 receive antennas with a low complexity zero-forcing (ZF) receiver we get diversity order of approximately 6. Compared with coded V-BLAST (Foschini, 1996) operating on the same information bit rate and decoder complexity, our system performs much better for all types of spatially correlated and uncorrelated MIMO channels under investigation.     

Outage-based design of robust Tomlinson–Harashima transceivers for the MISO downlink with QoS requirements

We consider a broadcast channel in which the base station is equipped with multiple antennas and each user has a single antenna, and we study the design of transceivers based on Tomlinson–Harashima precoders with probabilistic quality of service (QoS) requirements for each user, in scenarios with uncertain channel state information (CSI) at the transmitter. Each user's QoS requirement is specified as a constraint on the maximum allowed outage probability of the receiver's mean square error (MSE) with respect to a specified target MSE, and we demonstrate that these outage constraints are associated with constraints on the outage of the received signal-to-interference-plus-noise-ratio (SINR). We consider four different stochastic models for the channel uncertainty, and we design the downlink transceiver so as to minimize the total transmitted power subject to the satisfaction of the probabilistic QoS constraints. We present three conservative approaches to solving the resulting chance constrained optimization problems. These approaches are based on efficiently solvable deterministic convex design formulations that guarantee the satisfaction of the probabilistic QoS constraints. We also demonstrate how to apply these approaches in order to obtain computationally efficient solutions to some related design problems. Our simulations indicate that the proposed methods can significantly expand the range of QoS requirements that can be satisfied in the presence of uncertainty in the CSI.     

基于球形译码的联合多用户检测

针对采用码分多址接入（CDMA,Code Division Multiple Access）的多输入多输出系统（MIMO,Multiple Input Multiple Output）,建立多个用户同步上行接入基站的信道模型,收发端都采用2根天线。在基站端,由于扩频码非完全正交产生多个用户之间干扰和信道干扰影响信号检测的问题,分析并给出了多用户信号的最大似然函数,利用球形译码算法实现多用户的联合检测（Joint MUD,Joint Multiple User Detection）,并行处理信道干扰和多址干扰。仿真结果表明,提出的接收机有较好性能,同时降低了运算的复杂度。     

A soft decoding scheme for vector quantization over a CDMA channel

The optimal decoding of vector quantization (VQ) over a code-division multiple-access (CDMA) channel is too complicated for systems with a medium-to-large number of users. This paper presents a low-complexity, suboptimal decoder for VQ over a CDMA channel. The proposed decoder is built from a soft-output multiuser detector, a soft bit estimator, and the optimal soft VQ decoding of an individual user. Simulation results obtained over both additive white Gaussian noise and flat Rayleigh fading channels show that with a lower complexity and good performance, the proposed decoding scheme is an attractive alternative to the more complicated optimal decoder.     

Zero-Forcing Successive Decoding Under Nakagami-<Emphasis Type="Italic">m</Emphasis> Fading Channels

In this paper, the performance of a multiuser distributed spatial multiplexing system is analytically investigated. The system operates under a Nakagami-m fading environment. All the users, equipped with single antennas, are simultaneously transmitting their streams to a multi-antenna receiver. Zero-forcing is applied along with successive interference cancellation as a means for efficient detection of the received streams. New exact closed-form expressions with regards to some important performance metrics, namely, outage probability and ergodic capacity of each stream are presented. Both the analytical expressions and the simulation results show the impact of channel severity and scale of antenna array to the overall system performance. A particular emphasis on the scenario of the emerging massive multiple input-multiple output systems is provided.     

On the Outage Capacity of a Practical Decoder Accounting for Channel Estimation Inaccuracies

The optimal decoder achieving the outage capacity under imperfect channel estimation is investigated. First, by searching into the family of nearest neighbor decoders, which can be easily implemented on most practical coded modulation systems, we derive a decoding metric that minimizes the average of the transmission error probability over all channel estimation errors. Next, we specialize our general expression to obtain the corresponding decoding metric for fading MIMO channels. According to the notion of Estimation-induced outage (EIO) capacity introduced in our previous work and assuming a block Rayleigh-fading channel, we characterize the maximal achievable information rates using Gaussian codebooks associated to the proposed decoder. These achievable rates are compared to the rates achieved by the classical mismatched maximumlikelihood (ML) decoder and the ultimate limits given by the EIO capacity. Numerical results show that the derived metric provides significant gains, in terms of achievable EIO rates and bit error rate (BER), in a bit interleaved coded modulation (BICM) framework, without introducing any additional decoding complexity. However, the achievable rates of such metric are still far from the EIO capacity.     

Generalized layered space-time codes for high data rate wireless communications

We present the architecture of generalized layered space-time codes (GLST) as a combination of Bell Labs layered space-time (BLAST) architecture and space-time coding (STC) in multiple-antenna wireless communication systems. This approach provides both spectral and power efficiency with moderate complexity. The framework is to partition all the available transmit antennas into groups and apply STC on each group as component codes. Based on the mappings from coded symbols to transmit antenna groups, we can construct different GLST systems. Particularly, horizontal mapping and diagonal mapping are introduced and referred to as HGLST and DGLST respectively. The basic decoding of GLST, under quasi-static flat Rayleigh fading environments and assuming perfectly known channel state information (CSI) at the receiver, combines group interference suppression and group interference cancellation techniques. As a result, the individual STC on each group is decoded serially. To improve the overall system performance, we derive the optimal power allocation among all space-time codewords without requiring the knowledge of CSI at the transmitter and suitable for all GLST systems. We also derive the optimal serial decoding order based on the channel realizations at the receiver for HGLST systems without power allocation. Simulation results show that both can provide much improvement. To further enhance the system performance, we propose a low complexity hard-decision iterative decoding method. This method efficiently exploits full receive antenna diversity and, hence, dramatically improves the system performance which is confirmed by simulation.     

Capacity of multiple-antenna systems with both receiver and transmitter channel state information

The capacity of multiple-antenna systems operating in Rayleigh flat fading is considered under the assumptions that channel state information (CSI) is available at both transmitter and receiver, and that the transmitter is subjected to an average power constraint. First, the capacity of such systems is derived for the special case of multiple transmit antennas and a single receive antenna. The optimal power-allocation scheme for such a system is shown to be a water-filling algorithm, and the corresponding capacity is seen to be the same as that of a system having multiple receive antennas (with a single transmitter antenna) whose outputs are combined via maximal ratio combining. A suboptimal adaptive transmission technique that transmits only over the antenna having the best channel is also proposed for this special case. It is shown that the capacity of such a system under the proposed suboptimal adaptive transmission scheme is the same as the capacity of a system having multiple receiver antennas (with a single transmitter antenna) combined via selection combining. Next, the capacity of a general system of multiple transmitter and receiver antennas is derived together with an equation that determines the cutoff value for such a system. The optimal power allocation scheme for such a multiple-antenna system is given by a matrix water-filling algorithm. In order to eliminate the need for cumbersome numerical techniques in solving the cutoff equation, approximate expressions for the cutoff transmission value are also provided. It is shown that, compared to the case in which there is only receiver CSI, large capacity gains are available with optimal power and rate adaptation schemes. The increased capacity is shown to come at the price of channel outage, and bounds are derived for this outage probability.     

"Turbo DPSK" using soft multiple-symbol differential sphere decoding

Coded interleaved differential M-ary phase-shift keying (M-DPSK) with iterative decoding, the so-called "Turbo DPSK," is known as a power-efficient transmission format. Due to the rotational invariance of DPSK, it particularly enables detection without channel state information (CSI). However, the soft-input soft-output (SISO) component decoder for DPSK is the computational bottleneck if performance close to the ideal case of perfect CSI is desired. In this paper, we take a fresh look at SISO decoding without CSI and apply sphere decoding (SD) to reduce complexity. In particular, we devise a maximum a posteriori probability (MAP) multiple-symbol differential sphere decoder (MSDSD) which efficiently solves the high-dimensional search problem inherent to detection without CSI. Together with a soft-output generation device the MAP-MSDSD algorithm forms a new SISO-MSDSD module for iterative decoding. We analyze the extrinsic information transfer (EXIT) characteristic of the novel module, by means of which we are able to design powerful encoder and decoder structures. For, respectively, the additive white Gaussian noise (AWGN) and the continuously time-varying Rayleigh-fading channel without CSI these designs operate within 1.7-1.9 and 2.3-2.5 dB of channel capacity assuming perfect CSI. These figures compare favorably with results available in the literature, especially for reasonably high data rates of 1-2 bit/channel use. Simulation studies of the average and the maximum complexity required by SISO-MSDSD demonstrate the advantageous performance versus complexity tradeoff of our approach.     

Cross layer design of uplink multi-antenna wireless systems with outdated CSI

This letter studies the performance of uplink cross layer design for multi-antenna systems with outdated channel state information (CSI). We consider a multi-user system with one base station (with n/sub R/ receive antennas) and K mobile users (each with single transmit antenna). The multi-user physical layer is modeled based on information theoretical framework and the cross layer design can be cast as an optimization problem. In view of the high computation complexity in the optimal solution, we propose a low complexity genetic algorithm as suboptimal solution. We found that with outdated CSI, there is significant degradation in the spatial multiplexing and multi-user diversity gain due to potential packet transmission outage as well as misscheduling. To address the poor performance in the presence of outdated CSI, we propose two simple but effective empirical solutions, namely the rate quantization and rate discounting, to tackle the packet outage problem. For instance, it is well-known that rate quantization imposes system capacity loss in systems with perfect CSI. However, we found that rate quantization can enhance the robustness of system capacity with respect to outdated CSI.     

Antenna combining for the MIMO downlink channel

A multiple antenna downlink channel where limited channel feedback is available to the transmitter is considered. In a vector downlink channel (single antenna at each receiver), the transmit antenna array can be used to transmit separate data streams to multiple receivers only if the transmitter has very accurate channel knowledge, i.e., if there is high-rate channel feedback from each receiver. In this work it is shown that channel feedback requirements can be significantly reduced if each receiver has a small number of antennas and appropriately combines its antenna outputs. A combining method that minimizes channel quantization error at each receiver, and thereby minimizes multi-user interference, is proposed and analyzed. This technique is shown to outperform traditional techniques such as maximum-ratio combining because minimization of interference power is more critical than maximization of signal power in the multiple antenna downlink. Analysis is provided to quantify the feedback savings, and the technique is seen to work well with user selection and is also robust to receiver estimation error.     

Double turbo equalization of continuous phase modulation with frequency domain processing

In this paper, a doubly-iterative linear receiver, equipped with a soft-information aided frequency domain minimum mean-squared error (MMSE) equalizer, is proposed for the combined equalization and decoding of coded continuous phase modulation (CPM) signals over long multipath fading channels. In the proposed receiver architecture, the front-end frequency domain equalizer (FDE) is followed by the soft-input, softoutput (SISO) CPM demodulator and channel decoder modules. The receiver employs double turbo processing by performing back-end demodulation/decoding iterations per each equalization iteration to improve the a priori information for the front-end FDE. As presented by the computational complexity analysis and simulations, this process provides not only a significant reduction in the overall computational complexity, but also a performance improvement over the previously proposed iterative and noniterative MMSE receivers.     

Zero-forcing methods for downlink spatial multiplexing in multiuser MIMO channels

The use of space-division multiple access (SDMA) in the downlink of a multiuser multiple-input, multiple-output (MIMO) wireless communications network can provide a substantial gain in system throughput. The challenge in such multiuser systems is designing transmit vectors while considering the co-channel interference of other users. Typical optimization problems of interest include the capacity problem - maximizing the sum information rate subject to a power constraint-or the power control problem-minimizing transmitted power such that a certain quality-of-service metric for each user is met. Neither of these problems possess closed-form solutions for the general multiuser MIMO channel, but the imposition of certain constraints can lead to closed-form solutions. This paper presents two such constrained solutions. The first, referred to as "block-diagonalization," is a generalization of channel inversion when there are multiple antennas at each receiver. It is easily adapted to optimize for either maximum transmission rate or minimum power and approaches the optimal solution at high SNR. The second, known as "successive optimization," is an alternative method for solving the power minimization problem one user at a time, and it yields superior results in some (e.g., low SNR) situations. Both of these algorithms are limited to cases where the transmitter has more antennas than all receive antennas combined. In order to accommodate more general scenarios, we also propose a framework for coordinated transmitter-receiver processing that generalizes the two algorithms to cases involving more receive than transmit antennas. While the proposed algorithms are suboptimal, they lead to simpler transmitter and receiver structures and allow for a reasonable tradeoff between performance and complexity.