Performance Analysis of Distributed Space-Time Block-Encoded Sensor Networks

Sensor networks are comprised of nodes with minimal processing and radio-frequency functionalities. In such networks, it is assumed that a source sensor communicates with a target sensor over a number of relaying sensors by utilizing distributed but cooperative low-complexity space-time encoding techniques, thereby achieving highly robust communication links. Each relaying stage is hence comprised of a given number of cooperating sensor nodes, which may or may not exchange additional data. The contribution of this paper is the derivation of throughput-maximizing resource-allocation strategies for various sensor network configurations. The case of full data exchange at each relaying stage is analyzed first and then relaxed to the case of partial data exchange. Monte Carlo simulations are used to numerically verify the theoretically derived performance of distributed sensor networks. It is shown that notable power savings can be achieved compared to traditional single-link and nonoptimized sensor networks     

Performance analysis of orthogonal space-time block codes in spatially correlated MIMO Nakagami fading channels

Orthogonal space-time block coding (STBC) is an open-loop transmit diversity scheme that decouples the multiple-input multiple-output (MIMO) channel, thereby reducing the space-time decoding into a scalar detection process. This characteristic of STBC makes it a powerful tool, achieving full diversity over MIMO fading channels, and requiring little computational cost for both the encoding and decoding processes. In this paper, we exploit the single-input single-output equivalency of STBC in order to analyze its performance over nonselective Nakagami fading channels in the presence of spatial fading correlation. More specifically, we derive exact closed-form expressions for the outage probability and ergodic capacity of STBC, when the latter is employed over spatially correlated MIMO Nakagami fading channels. Moreover, we derive the exact symbol error probability of coherent M-PSK and M-QAM, when these modulation schemes are used along with STBC over such fading channels. The derived formulae are then used to assess the robustness of STBC to spatial correlation by considering general MIMO correlation models and analyzing their effects on the outage probability, ergodic capacity, and symbol error probability achieved by STBC.     

Space–Time Power Schedule for Distributed MIMO Links Without Instantaneous Channel State Information at the Transmitting Nodes

A space-time optimal power schedule for multiple distributed multiple-input multiple-output (MIMO) links without the knowledge of the instantaneous channel state information (CSI) at the transmitting nodes is proposed. A readily computable expression for the ergodic sum capacity of the MIMO links is derived. Based on this expression, which is a non-convex function of power allocation vectors, a projected gradient algorithm is developed to optimize the power allocation. For a symmetric set of MIMO links with independent identically distributed channels, it is observed that the space-time optimal power schedule reduces to a uniform isotropic power schedule when nominal interference is low, or to an orthogonal isotropic power schedule when nominal interference is high. Furthermore, the transition region between the latter two schedules is seen to be very sharp in terms of nominal interference-to-noise ratio (INR). For MIMO links with correlated channels, the corresponding space-time optimal power schedule is developed based on the knowledge of the channel correlation matrices. It is shown that the channel correlation has a great impact on the ergodic capacity and the optimality of different power scheduling approaches.     

On the Capacity of Underlay Multihop Cognitive Relaying Over Generalized-K Composite Fading Channels

Cognitive radio (CR) is one of the candidate enabling technologies for future wireless communication systems. This paper is devoted to analyze the capacity of underlay cognitive multihop relaying over independent and non-identically distributed generalized-K fading channels. In doing so, we derive upper and lower-bounded expressions for the ergodic capacity and the outage probability of the secondary user (SU), respectively. By using these expressions, new insights in the performance of the cognitive multihop amplify-and-forward relaying are revealed. The obtained results provide interesting details on the joint effect of shadowing and multipath fading on the capacity of the SU in relay-assisted underlay CR networks. The analytical results are verified by Monte-Carlo simulations for different fading conditions.     

Performance analysis of distributed space-time coded protocols for wireless multi-hop communications

In resource limited, large scale sensor networks, cooperative communication over multiple hops offers opportunities to save power: intermediate nodes between source and destination act as cooperative relays. In order to exploit spatial diversity, protocols coupled with space-time coding strategies are proposed herein and analyzed for distributed cooperative communication. In contrast to prior work, multi-hop (versus two-hop) schemes are developed and analyzed for amplify-andforward type of communication protocols. First, the Alamoutibased two-hop scheme proposed by Hua et al and analyzed by Jing & Hassibi is generalized to an arbitrary number of hops L, and a general approximation for the pairwise error probability (PEP) at high SNR is obtained. This expression is used to provide a close approximation to the achievable diversity gain of the scheme. It is further shown that the diversity decreases with L, for large, but finite signal-to-noise ratio (SNR). This motivates the subsequent development of new distributed multihop protocols to mitigate the diversity losses and, hence, yield improved performance. This work presents two such strategies as well as their diversity characterization, which are analyzed for the specific case of L = 3 hops and shown to exhibit improved performance at high SNR. These schemes are based on the structure of the rate-half codes proposed by Tarokh and the square-matrix embeddable codes of Tirkkonen & Hottinen.     

Multiple antenna systems: their role and impact in future wireless access

Multiple antennas play an important role in improving radio communications. In view of this role, the area of multiple antenna communication systems is in the forefront of wireless research. This article reviews two key related aspects of multiple antenna communication systems: multiple access interference mitigation at the receiver via multi-user beamforming; and space-time modulation and coding for MIMO systems. It is shown that both multi-user and MIMO receivers share similar signal processing and complexity tradeoffs.. Following that, a general unified framework for assessing different types of space-time modulation for MIMO systems is introduced. These space-time modulation methods are then compared in terms of Shannon capacity over multipath channels. Key MIMO system performance and implementation issues are also highlighted.     

Delay-Tolerant Distributed Linear Convolutional Space-Time Code with Minimum Memory Length under Frequency-Selective Channels

In cooperative communication networks, the performance of distributed space-time code will be severely degraded if the timing synchronization among relay nodes is not perfect. In this letter, we propose a systematic construction of the so called distributed linear convolutional space-time code (DLCSTC) for multipath fading channels that does not require the synchronization assumption. We derive sufficient conditions on the code design such that the full cooperative and multipath diversities can be achieved under the minimum memory length constraint. Then we design DLCSTCs that both have the traceorthonormality property and achieve the full diversity. We show that the proposed codes can also achieve the full diversity for asynchronous cooperative communications with ZF, MMSE and MMSE-DFE receivers under frequency-selective channels. Finally, various numerical examples are provided to corroborate the analytical studies.     

Capacity and optimal resource allocation for fading broadcastchannels .I. Ergodic capacity

In multiuser wireless systems, dynamic resource allocation between users and over time significantly improves efficiency and performance. In this two-part paper, we study three types of capacity regions for fading broadcast channels and obtain their corresponding optimal resource allocation strategies: the ergodic (Shannon) capacity region, the zero-outage capacity region, and the outage capacity region with nonzero outage. We derive the ergodic capacity region of an M-user fading broadcast channel for code division (CD), time division (TD), and frequency division (FD), assuming that both the transmitter and the receivers have perfect channel side information (CSI). It is shown that by allowing dynamic resource allocation, TD, FD, and CD without successive decoding have the same ergodic capacity region, while optimal CD has a larger region. Optimal resource allocation policies are obtained for these different spectrum-sharing techniques. A simple suboptimal policy is also proposed for TD and CD without successive decoding that results in a rate region quite close to the ergodic capacity region. Numerical results are provided for different fading broadcast channels     

Closed-form capacity expressions of orthogonalized correlated MIMO channels

Space-time block codes are known to orthogonalize the multiple-input multiple-output (MIMO) wireless channel, thus reducing the space-time vector detection to a simpler scalar detection problem. The capacity over orthogonalized ergodic correlated Rayleigh and Ricean flat-fading MIMO channels has so far only been given in integral form. This letter derives a closed form capacity expression over such channels, hence avoiding numerical integrations or Monte Carlo simulations.     

Coded differential space-time modulation for flat fading channels

In this paper, powerful coding techniques for differential space-time modulation (DSTM) over Rayleigh flat fading channels and noncoherent detection without channel state information at the receiver are investigated. In particular, multilevel coding, bit-interleaved coded modulation, and so-called hybrid coded modulation (HCM) are devised and compared. For improved noncoherent reception multiple-symbol differential detection (MSDD) is adapted to DSTM. In order to reduce the computational effort required for MSDD, a low-complexity version of MSDD is applied. Evaluating the ergodic channel capacity for the different schemes as appropriate performance measure, HCM with simplified MSDD is shown to offer a favorable tradeoff between complexity and achievable power efficiency. Simulation results employing turbo codes in properly designed HCM schemes confirm the predictions from information theory.     

Nested cooperative encoding protocol for wireless networks with high energy efficiency

Recently, distributed space-time code designs with high cooperative diversity for wireless communication networks, such as ad hoc and sensor networks, have received much attention. Amplify forward and decoding forward are widely used protocols for the cooperative diversity in the wireless communication networks. In both protocols, the information received by relay terminals are "forwarded" to destination or next relay terminals. Since the signals transmitted by relay terminals and those transmitted from the source terminal are correlated, there is information redundancy. To improve the energy efficiency of cooperative networks, we propose an encoding protocol, which is referred to as a nested cooperative encoding protocol. In our proposed protocol, the received signal at each relay terminal is divided into several sub-signals with the nest lattice structure of source information. Each of the sub-signals contains only a partial information with a smaller size of constellation compared to the original information sent by the source terminal. Do a new encoding or modulation by using these sub-signals before transmitting at relay terminals. It is shown that the proposed new protocols can achieve both high cooperative diversity and high energy efficiency.     

Cooperative Strategies and Capacity Theorems for Relay Networks

Coding strategies that exploit node cooperation are developed for relay networks. Two basic schemes are studied: the relays decode-and-forward the source message to the destination, or they compress-and-forward their channel outputs to the destination. The decode-and-forward scheme is a variant of multihopping, but in addition to having the relays successively decode the message, the transmitters cooperate and each receiver uses several or all of its past channel output blocks to decode. For the compress-and-forward scheme, the relays take advantage of the statistical dependence between their channel outputs and the destination's channel output. The strategies are applied to wireless channels, and it is shown that decode-and-forward achieves the ergodic capacity with phase fading if phase information is available only locally, and if the relays are near the source node. The ergodic capacity coincides with the rate of a distributed antenna array with full cooperation even though the transmitting antennas are not colocated. The capacity results generalize broadly, including to multiantenna transmission with Rayleigh fading, single-bounce fading, certain quasi-static fading problems, cases where partial channel knowledge is available at the transmitters, and cases where local user cooperation is permitted. The results further extend to multisource and multidestination networks such as multiaccess and broadcast relay channels.     

编码的多载波CDMA系统中Turbo时空多用户检测

联合MAP多用户检测和信道译码的迭代多用户检测技术可显著提高多载波CDMA系统的容量和性能,本文给出了结合智能天线和迭代MAP多用户检测的Turbo时空多用户检测算法,该方法进一步提高了系统的性能.Turbo时空多用户检测算法不仅极大减小了传统最优MAP多用户检测算法的运算量,而且,此算法性能在AWGN和频率选择性衰落信道中都能逼近单用户编码多载波CDMA系统多天线接收的性能.     

Performance analysis and design optimization of LDPC-coded MIMO OFDM systems

We consider the performance analysis and design optimization of low-density parity check (LDPC) coded multiple-input multiple-output (MIMO) orthogonal frequency-division multiplexing (OFDM) systems for high data rate wireless transmission. The tools of density evolution with mixture Gaussian approximations are used to optimize irregular LDPC codes and to compute minimum operational signal-to-noise ratios (SNRs) for ergodic MIMO OFDM channels. In particular, the optimization is done for various MIMO OFDM system configurations, which include a different number of antennas, different channel models, and different demodulation schemes; the optimized performance is compared with the corresponding channel capacity. It is shown that along with the optimized irregular LDPC codes, a turbo iterative receiver that consists of a soft maximum a posteriori (MAP) demodulator and a belief-propagation LDPC decoder can perform within 1 dB from the ergodic capacity of the MIMO OFDM systems under consideration. It is also shown that compared with the optimal MAP demodulator-based receivers, the receivers employing a low-complexity linear minimum mean-square-error soft-interference-cancellation (LMMSE-SIC) demodulator have a small performance loss (< 1dB) in spatially uncorrelated MIMO channels but suffer extra performance loss in MIMO channels with spatial correlation. Finally, from the LDPC profiles that already are optimized for ergodic channels, we heuristically construct small block-size irregular LDPC codes for outage MIMO OFDM channels; as shown from simulation results, the irregular LDPC codes constructed here are helpful in expediting the convergence of the iterative receivers.     

Adaptive MIMO Transmission for Exploiting the Capacity of Spatially Correlated Channels

We consider a novel low-complexity adaptive multiple-input multiple-output (MIMO) transmission technique. The approach is based on switching between low-complexity transmission schemes, including statistical beamforming, double space-time transmit diversity, and spatial multiplexing, depending on the changing channel statistics, as a practical means of approaching the spatially correlated MIMO channel capacity. We first derive new ergodic capacity expressions for each MIMO transmission scheme in spatially correlated channels. Based on these results, we demonstrate that adaptive switching between MIMO schemes yields significant capacity gains over fixed transmission schemes. We also derive accurate analytical approximations for the optimal signal-to-noise-ratio switching thresholds, which correspond to the crossing-points of the capacity curves. These thresholds are shown to vary, depending on the spatial correlation, and are used to identify key switching parameters. Finally, we propose a practical switching algorithm that is shown to yield significant spectral efficiency improvements over nonadaptive schemes for typical channel scenarios     

On the capacity and BER performance of multiuser scheduling over MIMO Nakagami-m fading channels

This paper presents a performance analysis for evaluating multiuser and spatial diversities achieved in multiuser MIMO systems employing orthogonal space-time block coding (OSTBC) over Nakagami-m fading channels. Two multiuser scheduling schemes are considered, absolute SNR-based scheduling (AS) and normalized SNR-based scheduling (NS) schemes for both heterogeneous and homogeneous wireless networks. Analytical expressions are derived for the average channel capacity and average bit error rate (BER) of these systems. The considered scheduling schemes are numerically evaluated and compared in terms of average capacity, average BER and fairness. It is shown that in the heterogeneous case, unlike the AS scheme, the NS scheme can guarantee fairness to the users. It is also shown that in the heterogeneous case, the AS scheme yields a higher average capacity and a lower average BER compared to the NS scheme and to the homogeneous case.     

空时编码在单天线UWB通信系统中的应用

超宽带(UWB)适用于基带多用户通信、战场无线通信和高数据率多媒体业务等通信系统,其数据传输速率高、功耗低、多径分辨能力强。但超宽带脉冲信号时域支撑区极窄,信道为密集多径,将空时编码技术引入超宽带通信系统,能够提升无线通信系统的信道容量与抗误比特率性能。在对UWB空时分组编码系统模型性能理论分析的基础上,对空时分组码在单天线UWB系统应用方案与UWB空时分层码方案进进行了简要介绍,利用Matlab对IEEEUWB信道模型进行仿真,提出了空时编码在UWB通讯技术中应用后提升短距高速率无线通信的性能的结论 。     

A comparative study of distributed frequency assignment algorithms for wireless sensor networks

Wireless sensor networks are composed of energy constrained nodes embedding limited transmission, processing and sensing capabilities. The main research efforts in this area sought to prolong the network lifetime by reducing energy consumption of network operations. Data gathering mechanisms such as clustering have been shown to achieve significant energy savings. However, such benefits can be obtained only if neighboring clusters operate on different frequencies (channels). As the salient characteristics of wireless sensor networks favor a distributed approach, we analyze the performance of several distributed frequency assignment algorithms with a focus on energy consumption. In this context, we find that a heuristic may achieve better results than backtracking-based algorithms.     

Distributed differential space-time coding for wireless relay networks

Distributed space-time coding is a cooperative transmission scheme for wireless relay networks. With this scheme, antennas of the distributive relays work as transmit antennas of the sender and generate a space-time code at the receiver. It achieves the maximum diversity. Although the scheme needs no channel information at relays, it does require full channel information, both the channels from the transmitter to relays and the channels from relays to the receiver, at the receiver. In this paper, we propose a differential transmission scheme, which requires channel information at neither relays nor the receiver, for wireless relay networks. As distributed space-time coding can be seen as the counterpart of space-time coding in the network setting, this scheme is the counterpart of differential space-time coding. Compared to coherent distributed space-time coding, the differential scheme is 3dB worse. In addition, we show that Alamouti, square real orthogonal, and Sp(2) codes can be used differentially in networks with corresponding numbers of relays. We also propose distributed differential space-time codes that work for networks with any number of relays using circulant matrices.     

A Simple Distributed Antenna Processing Scheme for Cooperative Diversity

In this letter the performance of multiple relay channels is analyzed for the situation in which multiple antennas are deployed only at the relays. The simple repetition-coded decodeand- forward protocol with two different antenna processing techniques at the relays is investigated. The antenna combining techniques are maximum ratio combining (MRC) for reception and transmit beamforming (TB) for transmission. It is shown that these distributed antenna combining techniques can exploit the full spatial diversity of the relay channels regardless of the number of relays and antennas at each relay, and offer significant power gain over distributed space-time coding techniques.