A New ML Based Interference Cancellation Technique for Layered Space-Time Codes

This paper presents a new and simple decoding algorithm for layered space time block codes such as the two independent Alamouti's codes which are also called the double space-time transmit diversity (DSTTD) system. By using group interference suppression and successive interference cancellation, we can treat DSTTD as two independent space-time block codes (STBC). We can then decode both of these STBC's through a simple maximum likelihood (ML) detector with null space-based interference cancellation. We also compare the proposed interference cancellation (IC) scheme with the conventional MMSE IC scheme. The performance of the proposed IC scheme is comparable to that of the MMSE IC scheme while the complexity reduction factor of the proposed scheme can be up to 5 compared to the MMSE IC scheme.     

迫零准则下的V-BLAST检测算法比较

在采用分层空时编码V-BLAST的MIMO系统中,笔者对基于迫零准则的四种信号检测算法的性能进行了仿真分析。系统接收端分别使用迫零算法ZF、排序加干扰抵消的迫零算法ZF-OSIC、QR分解算法和排序的QR算法SQRD检测信号,通过仿真得到误码率曲线。对算法复杂度及相应误码率综合比较,得出了ZF-OSIC算法下系统接收端性能最好,SQRD算法为最可行的结论。     

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.     

Multistratum-permutation codes for MIMO communication

Multiple-input-multiple-output (MIMO) systems offer much capacity gain over single antenna approaches. The recently proposed multistratum space-time code (MSSTC) is such a MIMO-scheme among many others. In this paper, a generalization of the multistratum idea is presented and leads to the new family of multistratum-permutation codes. It is shown, that this scheme can achieve a higher link capacity than the MSSTC or Vertical-Bell Labs Layered Space-Time (V-BLAST). Scheme examples are presented for a 4/spl times/4 antenna system transmitting four data streams in parallel. They also show a promising performance for successive interference cancellation (SIC) receiver algorithms with a realistic complexity.     

基于QR分解V-BLAST检测算法研究和比较

分层空时码是一类具有可执行解码复杂度的空时编码技术,最大似然检测在误比特率最小的意义下是最优的接收,但是其复杂度不可实现。在D.Wubben提出的基于Gram-Schmidt正交化排序QR分解检测算法的基础上,提出了另外2种可以排序的QR分解检测算法,取得了和基于Gram-Schmidt正交化QR分解算法相同的性能。与V-BLAST算法相比,避免了多次矩阵求逆的计算,以很小的性能损失为代价,降低了复杂度。     

Efficient adaptive receivers for joint equalization and interference cancellation in multiuser space-time block-coded systems

This paper develops low-complexity adaptive receivers for space-time block-coded (STBC) transmissions over frequency-selective fading channels. The receivers are useful for equalization purposes for single user transmissions and for joint equalization and interference cancellation for multiuser transmissions. The receivers exploit the rich code structure of STBC codes in order to deliver recursive-least-squares (RLS) performance at least-mean-squares (LMS) complexity. Besides reduced complexity, the proposed adaptive receivers also lower system overhead requirements.     

Space-Time Codes for Multiple Access Systems with Low MMSE Decoding Complexity

We study a class of space-time codes constructed by linear dispersion encoding that allow low-complexity linear minimum mean square error (MMSE) decoding for a multiple access system. Considering that the information symbols are drawn from square QAM constellations, we optimize rate-one space time code designs with MMSE decoding to achieve minimum bit error rate (BER) for any channel realization. In addition, the performance is further improved with a simple MMSE successive interference cancellation (MMSE-SIC) method. Simulation results compare the proposed design with existing rate-one designs and show that simple rate-one circulant designs are also good candidates for deployment in multiple access systems.     

Simple space-time coded SS-CDMA systems capable of perfect MUI/ISIelimination

In this letter we propose a very simple strategy for combining space-time block coding (STBC) with code division multiple access (CDMA) to be used in single-user links or in broadcasting over time-dispersive channels. The proposed system provides the desired space diversity gain and is capable of perfect cancellation of multiuser interference (MUI) and intersymbol interference (ISI), using a very simple receiver structure. The advantage is obtained by simply incorporating cyclic prefixes in the user codes. We provide a theoretical analysis, verified via simulation results, and a comparison between CDMA and OFDMA systems incorporating STBC     

MIMO-OFDM for wireless communications: signal detection with enhanced channel estimation

Multiple transmit and receive antennas can be used to form multiple-input multiple-output (MIMO) channels to increase the capacity by a factor of the minimum number of transmit and receive antennas. In this paper, orthogonal frequency division multiplexing (OFDM) for MIMO channels (MIMO-OFDM) is considered for wideband transmission to mitigate intersymbol interference and enhance system capacity. The MIMO-OFDM system uses two independent space-time codes for two sets of two transmit antennas. At the receiver, the independent space-time codes are decoded using prewhitening, followed by minimum-Euclidean-distance decoding based on successive interference cancellation. Computer simulation shows that for four-input and four-output systems transmitting data at 4 Mb/s over a 1.25 MHz channel, the required signal-to-noise ratios (SNRs) for 10% and 1% word error rates (WER) are 10.5 dB and 13.8 dB, respectively, when each codeword contains 500 information bits and the channel's Doppler frequency is 40 Hz (corresponding normalized frequency: 0.9%). Increasing the number of the receive antennas improves the system performance. When the number or receive antennas is increased from four to eight, the required SNRs for 10% and 1% WER are reduced to 4 dB and 6 dB, respectively. Therefore, MIMO-OFDM is a promising technique for highly spectrally efficient wideband transmission.     

Space-time power optimization of variable-rate space-time block codes based on successive interference cancellation

Based on a zero-forcing with successive interference cancellation (ZFSIC) method, we construct variable-rate space-time block codes (STBCs) for two, three, and four transmit antennas in Rayleigh fading channels. Considering the error-propagation effect in the ZFSIC scheme, we analyze the bit-error rate (BER) and optimize the transmit power so that the average BER is minimized. Unlike the approach based on zero-forcing (ZF), the method adopted in this paper can construct variable-rate STBCs even when the receive antennas are fewer than the transmit antennas. In addition, to improve the BERs further, we propose space-time power optimization. Numerical results show that the codes presented in this paper provide much lower BERs, compared with the codes developed by Kim and Tarokh.     

Space-time processing for broadband wireless access

We present an overview of research activities on space-time coding for broadband wireless transmission performed at AT&T Shannon Laboratory over the past two years. The emphasis is on physical layer modem algorithms such as channel estimation, equalization, and interference cancellation. However, we also discuss the impact of space-time coding gains at the physical layer on throughput at or above the networking layer. Furthermore, we describe a flexible graphical user interface attached to our physical layer simulation engine in order to explore the performance of space-time codes under a variety of practical transmission scenarios. Simulation results for the EDGE cellular system and the 802.11 wireless LAN environment are presented     

Space-time iterative and multistage receiver structures for CDMAmobile communication systems

We propose novel space-time multistage and iterative receiver structures and examine their application in code division multiple access (CDMA) mobile communication systems. In particular we derive an expression for weighting coefficients in parallel interference cancellers (PICs) in a system with a large number of users, where decision statistics bias is pronounced. We further examine the parameters in this expression and show how to obtain a practical partial cancellation method that allows on-line estimation of the weighting coefficients. In the proposed multistage PIC, the coefficients are calculated by using only the variances of the detector outputs. We also examine an iterative PIC and observe that this receiver has similar limitations as the multistage PIC. The application of the novel parallel interference cancellation strategy in the iterative receiver structure results in a spectacular system capacity improvement with a negligible complexity increase relative to the standard iterative receiver. The performance of the proposed receivers is further enhanced by receiver adaptive array antennas and space-time processing     

Semiorthogonal Space–Time Block Codes

In this paper, a new class of full-diversity, rate-one space-time block codes (STBCs) called semiorthogonal algebraic space-time block codes (SAST codes) is proposed. SAST codes are delay optimal when the number of transmit antennas is even. The SAST codeword matrix has a generalized Alamouti structure where the transmitted symbols are replaced by circulant matrices and the commutativity of circulant matrices simplifies the detection of transmit symbols. SAST codes with maximal coding gain are constructed by using rate-one linear threaded algebraic space-time (LTAST) codes. Compared with LTSAT codes, SAST codes not only reduce the complexity of maximum-likelihood detection, but also provide remarkable performance gain. They also outperform other STBC with rate one or less. SAST codes also perform well with suboptimal detectors such as the vertical-Bell Laboratories layered space-time (V-BLAST) nulling and cancellation receiver. Finally, SAST codes attain nearly 100% of the Shannon capacity of open-loop multiple-input-single-output (MISO) channels.     

非同步扩频系统中空时分组码的阵列处理

针对非同步DS-CDMA系统提出了一种基于空时分组码的阵列处理干扰对消的技术。通过对发射信号进行空时分组编码,可以使发射信号在瑞利衰落信道中传输时信息不会畸变,并取得最大分集增益和码增益。利用空时分组码的性质有效抑制干扰的同时,大大简化了空时分组码的解码。仿真结果表明,这里提出的方法与传统的抑制干扰的方法相比具有更低的误比特率,并且有更快的解码速率。     

不完全信道状态信息条件下多用户Turbo-BLAST迭代检测算法

该文将空时多用户检测技术和Turbo-BLAST方案相结合,构造了基于CDMA技术的多用户Turbo- BLAST系统模型,提出了不完全信道状态信息条件下的解相关算法和迭代检测算法。在发送端将V-BLAST结构与CDMA技术相结合实现多路复用,在接收端利用空时多用户检测算法去除用户间干扰,得到期望用户的接收信号,然后采用考虑信道估计误差的软干扰抵消迭代检测算法,对期望用户的接收信号进行检测以去除天线间干扰。仿真结果说明,所提算法对于多用户Turbo-BLAST系统是有效的,可在不增加复杂度的条件下达到良好的迭代效果。     

Noncoherent Spectral/Time/Spatial Optical CDMA System Using 3-D Perfect Difference Codes

In this paper, we propose a family of newly constructed codes, named three-dimensional perfect difference codes (3-D PD codes), and a corresponding system structure for spectral/time/spatial optical code division multiple access (OCDMA). The 3-D PD codes, which are generated from the perfect difference set, can further suppress the phase-induced intensity noise (PIIN) and possess the multiuser interference (MUI) cancellation property. Compared with the OCDMA systems using modified quadratic congruence codes (MQC codes), maximal-area matrices codes (M-matrices codes), or 2-D PD codes, the proposed system can effectively suppress the PIIN and eliminate MUI. Hence, the proposed system not only has better performance, but also can accommodate more users.     

A co-channel interference cancellation technique using orthogonalconvolutional codes

This paper proposes a new parallel co-channel interference cancellation technique which utilizes orthogonal convolutional codes. Co-channel interference (CCI) limits the performance of a spread spectrum multiple access communication link. Several CCI cancellation techniques have been proposed to remove this interference. Of particular interest are techniques which do not require the receiver to have knowledge of the cross-correlation between user sequences. These techniques reconstruct the CCI based on the initial decisions regarding the signals from the other users. However, these techniques leave residual interference after cancellation caused by errors in these initial decisions. To improve the initial decisions and reduce the residual interference, our proposed scheme utilizes the error correcting capability of orthogonal convolutional codes. This paper evaluates the performance of this scheme. We show that, given a processing gain of 128 for up to about 40 users, the performance of the proposed CCI canceller approaches the performance of a system without multi-user interference. We also show that the proposed CCI canceller offers an improvement in capacity by a factor of 1.5~3 over that of a conventional canceller     

An efficient decoding algorithm for QO-STBCs based on iterative interference cancellation

We propose an efficient decoding algorithm targeted for quasi-orthogonal space-time block codes which can be viewed as an iterative extension of the parallel detection algorithm using iterative interference cancellation. The essence of the algorithm is in the realization of the fact that the number of candidate symbols at the end of each interference cancellation iteration reduces very quickly, and that no more than two iterations are required.     

Space-Time Optimum Combining for CDMA Overlay for Personal Communications Systems

The requirement to suppress narrowband interferences in CDMA communication stems from the overlay concept, i.e., coexistence of different types of signals in the same frequency band. This paper describes the performance analysis of a direct-sequence (DS) CDMA personal communication system sharing a common spectrum with narrowband microwave radio links in the 1.85 to 1.99 GHz band. The coexistence of these two systems within the same frequency band will improve the overall spectrum efficiency, but will also cause interference to both systems. In this paper it is shown that joint spatial and temporal optimum combining provides an efficient means of improving the performance of the DS-CDMA system through cancellation of the narrowband signal and the co-channel interferences. The proposed space-time architecture provides degrees of freedom for both diversity and interference cancellation. It is shown that the joint space-time receiver is robust with respect to the narrowband interference signal bandwidth and its carrier frequency offset from the DS-CDMA carrier frequency.     

Interference suppression for space-time coded CDMA via decision-feedback equalization

A single-user receiver structure is proposed for space-time coded code-division multiple-access (CDMA) downlink in a multiuser frequency-selective channel. This structure is a two-dimensional (2-D) decision-feedback equalizer (2D-DFE) whose filters are optimized based on the MMSE criterion to mitigate noise, intersymbol interference (ISI), and multiuser interference (MUI) with a moderate complexity. By modeling the spreading codes of the interfering users as random sequences, system performance was evaluated using the Gaussian approximation. Two models for the desired user's spreading sequence have been considered and compared. Our numerical results show that in both cases the 2D-DFE exhibits significant performance improvement over the standard space-time coded RAKE, especially in interference-limited conditions. It is also observed that the gain obtained by using DFE in a MISO channel is less that in a SISO channel and this problem can be solved by providing diversity at the receiver.