STBC MIMO CDMA系统CMVDR接收机研究

提出了STBC MIMO CDMA系统中一种新的基于多用户ML估计的带制约的最小方差无失真响应(CMVDR)接收机，能有效抑制多址干扰(MAD和符号间干扰(ISI)。给出了多用户、多径的STBC MIMO CDMA系统信道模型及ML参数估计，并给出了分部加权和整体加权两种CMVDR接收机，本接收机也适用于SIMO CDMA系统。进行了系统模拟，并对分部加权和整体加权两种CMVDR接收机的性能以及自适应CMMSE接收机的性能进行了比较分析。     

Iterative receivers for multiuser space-time coding systems

Space-time coding (STC) techniques, which combine antenna array signal processing and channel coding techniques, are very promising approaches to substantial capacity increase in wireless channels. Multiuser detection techniques are powerful signal processing methodologies for interference suppression in CDMA systems. In this paper, by drawing analogies between a synchronous CDMA system and an STC multiuser system, we study the applications of some multiuser detection methods to STC multiuser systems. Specifically, we show that the so-called “turbo multiuser detection” technique, which performs soft interference cancellation and decoding iteratively, can be applied to STC multiuser systems in flat-fading channels. An iterative multiuser receiver and its projection-based variants are developed for both the space-time block coding (STBC) system and the space-time trellis coding (STTC) system. During iterations, extrinsic information is computed and exchanged between a soft multiuser demodulator and a bank of MAP decoders, to achieve successively refined estimates of the users' signals. Computer simulations demonstrate that the proposed iterative receiver techniques provide significant performance improvement over conventional noniterative methods in both single-user and multiuser STC systems. Furthermore, the performance of the proposed iterative multiuser receiver approaches that of the iterative single-user receiver in both STBC and STTC systems     

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。     

Performance Investigation of Space‐Time Block Coded Multicarrier DS‐CDMA in Time‐Varying Channels

In this letter, we evaluate the system performance of a space‐time block coded (STBC) multicarrier (MC) DS‐CDMA system over a time selective fading channel, with imperfect channel knowledge. The average bit error rate impairment due to imperfect channel information is investigated by taking into account the effect of the STBC position. We consider two schemes: STBC after spreading and STBC before spreading in the MC DS‐CDMA system. In the scheme with STBC after spreading, STBC is performed at the chip level; in the scheme with STBC before spreading, STBC is performed at the symbol level. We found that these two schemes have various channel estimation errors, and that the system with STBC before spreading is more sensitive to channel estimation than the system with STBC after spreading. Furthermore, derived results prove that a high spreading factor (SF) in the MC DS‐CDMA system with STBC before spreading leads to high channel estimation error, whereas for a system with STBC after spreading this statement is not true.     

一种满速率空时分组编码CDMA系统及多用户接收方案

通过引入满速率空时分组码方案, 该文给出一种满速率空时分组编码CDMA系统模型, 并针对现有空时编码CDMA系统过高的译码复杂度, 提出一种低复杂度的多用户接收方案。该方案在通过类似多用户检测方法有效抑制多用户干扰后, 充分利用空时分组码的复正交性来简化原有方案高译码复杂度。与原有指数性译码复杂度相比, 该方案有着线性复杂度, 而且与满分集空时分组编码CDMA系统相比, 可实现满速率、低复杂度和部分分集, 有着相对多的空间冗余信息, 从而级联信道编码后可有效弥补部分分集所带来的性能损失。仿真结果表明在相同系统容量和级联码的情况下, 所给系统比相应的满分集空时编码CDMA系统有着低的误比特率。     

An adaptive CMMSE receiver for space-time block-coded CDMA systems in frequency-selective fading channels

A technique that can suppress multiple-access interference (MAI) in space-time block-coded (STBC) multiple-input-multiple-output (MIMO) code-division multiple-access (CDMA) systems is developed. The proposed scheme, called a constrained minimum mean square error (CMMSE) receiver, is an extension of the CMMSE receiver for a single-input-single-output system to MIMO systems. It is shown that the complexity of the proposed CMMSE receiver is almost independent of the number of transmitter antennas. The advantage of the proposed receiver over the existing receivers for STBC CDMA systems is demonstrated by comparing the closed-form expressions of the signal-to-interference plus noise ratio and simulated bit error rates. The results indicate that the proposed CMMSE receiver can provide a significant performance improvement over the conventional receivers and that the gain achieved by suppressing the MAI can be larger than that from increasing the transmitter or receiver diversity.     

MIMO CDMA系统中一种新的循环空时分组编译码方法

该文提出了MIMO CDMA系统中一种新的空时分组编译码方法,称为循环空时分组码(Cyclic Space-Time Block Code,C-STBC),它是将输入信息进行分组循环编码,然后对循环编码后的码字通过不同的天线分别采用不同的扩频码扩频后发射出去。这种CDMA系统下的循环空时分组码对任意的发射天线数都能达到满编码速率和满分集度,且其译码与传统的空时分组码一样简单。仿真结果验证了这种循环空时分组码优于传统的空时分组码。     

Space-Time Block Coding for Uplink Single-Carrier CDMA with Joint Detection in the Frequency Domain

Single-carrier code-division multiple access (SC-CDMA), also named cyclic-prefix CDMA in the literature, is a promising air interface for the uplink of the 4G cellular wireless communication systems. It enables the high capacity intrinsically offered by CDMA by making the equalization of the multipath channels and the mitigation of the resulting interference possible at a low complexity. This paper proposes a new air interface that combines SC-CDMA with space-time block coding (STBC) across multiple transmit antennas in order to make the link more robust. Contrary to existing air interfaces that perform the STBC at the chip level, making them only applicable to the downlink, the STBC is performed at the symbol level, making it also applicable to the uplink. In order to optimally detect the different antenna and user signals, a linear joint detector optimized according to the minimum mean square error (MMSE) criterion is designed. By exploiting the cyclic properties of the channel matrices, the complexity of the joint detector is significantly reduced. Furthermore, it is shown analytically that the inter-antenna interference is canceled out at the output of the first stage of the linear MMSE joint detector, consisting of a matched filter. By space-time coding the signal through two antennas at each transmit mobile terminal, a significant gain in signal-to-noise-ratio can be achieved. However, the spatial diversity gain of the proposed system is limited by the multiuser interference (MUI), that is increasing with the user load. Higher complexity non-linear receivers are needed to better compensate the MUI and still benefit from the spatial diversity at high user loads.     

Successive interference cancelation for space-time block codes over time-selective channels

In this letter, we propose a very simple successive interference cancelation (SIC) based signal detector for space-time block codes (STBC) to combat time-selective fading. The main idea is to treat un-detected symbols as noise using a Gaussian approximation. Simulation results show that our scheme can provide performance very close to ML decoding with extremely low computational complexity     

On the comparisons of system performance and capacity of asynchronous STBC MC‐CDMA multirate access schemes

Three multirate access schemes, multicode, variable spreading gain (VSG), and spectral overlaid multiple‐symbol‐rate (MSR), for asynchronous space‐time block coded (STBC) multicarrier code division multiple access (MC‐CDMA) systems are proposed. The three possible spectral overlaid configurations for MSR systems are also investigated. The expressions to evaluate the multiple access interferences, bit error rate (BER) performances, and system capacities of a antenna STBC MC‐CDMA using the three multirate access schemes are obtained. Transmit power allocation is adjusted according to the service rates and the number of active users in each service class to maintain the link quality and to improve the system capacity. Our numerical results show that systems with multicode access scheme using orthogonal Gold spreading codes and with VSG access scheme have similar system performance and capacity, and both perform in general better than systems with MSR access scheme of any spectrum configurations. In case when non‐orthogonal Gold codes are used, multicode access scheme shows degradation in the system capacity as compared to VSG, due to the presence of larger self‐interference (SI) among the codes used by each user. The achievable capacities for the three spectral overlaid configurations of MSR multirate systems are also compared. Copyright © 2007 John Wiley & Sons, Ltd.     

Performance of group ordered successive interference cancellation for multiuser detection in GSTBC SFH/MC DS‐CDMA system

In this paper, we present a novel group space‐time block coding slow frequency‐hopping multicarrier direct‐sequence code division multiple access (GSTBC SFH/MC DS‐CDMA) system over frequency selective fading channels. The proposed scheme greatly improves the bandwidth‐efficient through assigning the users employing the same frequency‐hopping (FH) pattern. Moreover, the users employing the same FH pattern are assigned into different virtual groups, in which the users are assigned to the different spreading codes, while the users are assigned to the same spreading code in the same virtual group. Then, a novel group detection scheme that we denote by group ordered successive interference cancellation (GOSIC) is presented to suppress the interference between the different virtual groups. Our proposed scheme consists of ordering group at the receiver side in order to maximize the overall system performance, and carrying BLAST‐STBC (LSTBC) detection for the users in the same virtual group. We define and derive the optimal group order based on the post group signal to interference plus noise ratio (PGSINR). We also propose another suboptimal group order in order to overcome the complexity issues. Finally, we compare the performance of our proposed GOSIC with conventional group successive interference cancellation (GSIC), conventional STBC multiuser detection (MUD), and LSTBC‐MUD, and show that significant improvement is introduced. Finally, it is shown that the proposed scheme is robust to the imperfect channel estimation. Copyright © 2006 John Wiley & Sons, Ltd.     

空时分组编码CDMA系统的性能分析

空时分组编码 ( STBC)可有效的应用于无线系统中 ,提高系统的容量。 STBC采用最大似然译码算法 ,译码过程中需要信道信息。本文利用导频辅助的方式获得信道信息 ,分析了Rayleigh信道下随着移动台速度的变化 STBC- CDMA系统的误码率性能     

Selection of gold and kasami code sets for spread spectrum CDMA systems of limited number of users

Spread spectrum code-division multiple-access (CDMA) systems are currently considered as very attractive alternatives to the more familiar FDMA and TDMA systems, especially in the presence of multipath fading and external interference. The problem of code selection in a CDMA system with a finite number of users is addressed in this paper. A simple and efficient method for the selection of finite code sets with relatively high processing gain from Gold and Kasami code families of relatively large sizes is described in detail. Selected code sets of period 63 are presented along with their overall average performance parameters for different number of users.     

Low-complexity turbo equalization and multiuser decoding for TD-CDMA

The authors propose a low complexity multiuser joint parallel interference cancellation (PIC) decoder and turbo decision feedback equalizer for code division multiple access (CDMA). In their scheme, an estimate of the interference signal (both multiple-access interference and intersymbol interference) is formed by weighting the hard decisions produced by conventional (i.e., hard-output) Viterbi decoders. The estimated interference is subtracted from the received signal in order to improve decoding in the next iteration. By using asymptotic performance analysis of random-spreading CDMA, they optimize the feedback weights at each iteration. Then, they consider two (mutually related) performance limitation factors: the bias of residual interference and the ping-pong effect. The authors show that the performance of the proposed algorithm can be improved by compensating for the bias in the weight calculation, and they propose a modification of the basic PIC algorithm, which prevents the ping-pong effect and allows higher channel load and/or faster convergence to the single-user performance. The proposed algorithm is validated through computer simulation in an environment fully compliant with the specifications of the time-division duplex mode of third-generation systems, contemplating a combination of time-division multiple access and CDMA and including frequency-selective fading channels, user asynchronism, and power control. The main conclusion of this work is that, for such application, soft-input soft-output decoders (e.g., implemented by the forward-backward BCJR algorithm) are not needed to attain very high spectral efficiency, and simple conventional Viterbi decoding suffices for most practical settings.     

一种基于高阶累积量的正交空时分组码盲识别方法

针对全盲条件下正交空时分组码(OSTBC)的正交性识别问题,提出一种基于高阶累积量的OSTBC识别方法.通过对接收信号白化,在不需要信道估计的情况下,消除信道对识别结果的干扰,既减少了算法复杂度,又提高了低信噪比条件下OSTBC的识别概率;考虑实际系统中噪声对识别结果的影响,利用高阶累积量(阶数大于2)的去噪声特性,消除噪声对于STBC正交特性识别的影响;分析了两种四阶累积量的OSTBC识别特性,选取了适用于OSTBC识别的四阶累积量作为特征参数.仿真结果表明,所提出方法对于OSTBC识别问题区分性能较好.     

多小区STBC系统中基于自干扰抵消的鲁棒线性接收算法

在多小区有未知共道干扰存在的STBC系统中,针对已有Capon类线性接收算法在有限样本情况下会造成STBC解码符号间自干扰的问题,该文提出了一种鲁棒的线性接收算法。该算法结合STBC的正交结构,首先通过正交投影的方法去除同一码字各符号间的自干扰,然后再利用Capon接收算法结构进行未知干扰的抑制。仿真结果表明,结合STBC正交结构和Capon接收结构的线性接收算法比传统的Capon接收算法有更好的干扰抑制性能。     

Interference bounds in power controlled systems

Capacity in a code-division multiple access (CDMA) cellular system is often referred to in terms of the maximum number of users that can be supported within a carrier to interference ratio above a specified quality threshold. This paper presents a simple analysis showing that the total mean interference in the reverse channel, in a multi-tier scenario of a CDMA system with power control, is finite regardless of the number of surrounding tiers. Riemann-Hurwitz analysis is introduced in order to obtain upper and lower interference bounds     

Receive Transformation for Diversity Based MIMO System Under Multi-user Interference and Correlated Fading

Receive transformations for diversity based MIMO systems under multi-user interference and correlated fading are studied. We derive the maximum-SINR based receive transformation (MSRT), which maximizes the symbol-wise SINR of the MIMO receiver, for maximizing the achievable capacity of the diversity scheme. We further derive the achievable capacities for the matched filtering (MF) based STBC (space time block code), the MSRT based STBC and the MMSE based STBC under multi-user interference. Analytical results and simulation results both show that the MSRT based STBC results in significant gains of achievable capacity under multi-user interference and correlated fading. Moreover, analytical results and simulation results both show that correlated fading does not make impact on the achievable capacity of the MF based STBC while the achievable capacity of the MSRT based STBC increases with the correlations of correlated fading.

     

Analysis of adaptive multistage interference cancellation for CDMAusing an improved Gaussian approximation

We consider a simple model for adaptive multistage interference cancellation within a CDMA system, and seek to develop an accurate analytical expression for the performance of this system. Previous work on interference cancellation has relied heavily on simulation techniques or a simple Gaussian approximation (GA). The standard GA ran lead to bit-error rate (BER) results which are optimistic for the conventional receiver, and this also occurs when the approximation is applied to the interference cancellation problem. Additionally, this approximation does not allow the second order effects of the multiple access interference (MAI) to be included in the performance estimates. Several improvements on the standard GA have been suggested which result in accurate performance results for a standard CDMA receiver. This paper presents an analytical expression for the probability of bit error for an adaptive multistage interference canceller, using an improved Gaussian approximation (IGA) for MAI. The BER at any stage of interference cancellation can be recursively computed from the signal-to-noise ratio (SNR), the statistics of the random powers of users, and the processing gain of the CDMA system. The performance of the resulting EER expression is compared with simulation results. Since the second order effects of MAI can be included, the analytical framework presented here can also be used to evaluate the performance of multistage interference cancellation in arbitrary fading environments, and we present results for the performance of interference cancellation in lognormal fading environments     

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.