Table of Contents
ISRN Communications and Networking
Volume 2011, Article ID 147292, 7 pages
http://dx.doi.org/10.5402/2011/147292
Research Article

A Blind Two-Stage Receiver for MIMO-CDMA Systems

1Department of Optoelectronics and Communication Engineering, National Kaohsiung Normal University, 62 Shenjhong Road, Yanchao Township, Kaohsiung County 824, Taiwan
2Department of Electronic Engineering, National United University, 1 Lien-Da, Kung-Ching Li, Miaoli 360, Taiwan

Received 23 February 2011; Accepted 24 March 2011

Academic Editors: H. Ikeda, A. Maaref, and H.-L. Wei

Copyright © 2011 Fuh-Hsin Hwang and Tsui-Tsai Lin. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

A blind two-stage multiple-input multiple-output (MIMO) receiver is proposed for code-division multiple access (CDMA) systems utilizing space-time block coding (STBC) over a multipath fading channel. Specifically, in the first stage, a signal-blocked (SB) interference-blocked (IB) detector is first constructed for collection of the desired signals and suppression of multiple access interference (MAI). In the second-stage, a decision-directed scheme is developed to alleviate desired signal cancellation. Computer simulations demonstrate that the proposed blind receiver can achieve a reliable output signal-to-interference-plus-noise ratio (SINR) performance approximating that of the optimal minimum mean squared error (MMSE) receiver and can exhibit the robustness against the finite data sample effect.

1. Introduction

Code-division multiple access (CDMA) with the advantages of high transmission rates and capacity potential, and an intrinsic resistance to narrowband interference have been the leading solutions and technical standard specifications for the wireless network applications [1]. In spite of these advantages, however, signals from other users, referred to as multiple access interference (MAI), with higher transmission power caused by the near-far problem cannot be efficiently suppressed because of insufficient processing gains. This leads to a substantial degradation in performance.

The multiple-input multiple-output (MIMO) system [24] using the space-time block coding (STBC) scheme [3] to provide spatial diversity can promise an enormous increase in system capacity and link quality without sacrificing the radio frequency resource. Recently, CDMA systems employing the MIMO and STBC techniques have been proposed and discussed to support the tremendous growth in various high-rate services. The physical layer impediments in the design and implementation of the CDMA MIMO receiver are MAI and channel dispersion impairment. Especially for the STBC system, the MAI resulting from the interference between antennas is more severe than that in single-antenna systems. The minimum mean squared error (MMSE) receiver has been extensively discussed and used as a remedy to combat MAI [2]. However, its performance is sensitive to both the mismatches between the true and user channel state information (CSI) matrix and the finite sample effect. In order to correctly estimate the CSI matrix, a larger amount of preknown pilot symbols is required and leads to a significant decrease in the system throughput [5]. As an alternative, a nondata aided signal-blocked (SB) interference-blocked (IB) receiver [6] was proposed for direct sequence CDMA system. It has been shown that the SBIB receiver can successfully collect the desired signal and suppress MAI for a single-input single-output (SISO) system over a multipath fading channel. Partially adaptive (PA) filtering [7], which uses a portion of the degree of freedom available in the weight adaptation process, is a computational efficient scheme for various signal processing applications. The subspace-based two-stage receivers [8] adopt the generalized sidelobe canceller (GSC) incorporating with the PA algorithm to improve performance of channel estimation and symbol detection for Alamouti’s STBC CDMA MIMO system over a one-path fading channel.

In this paper, over a multipath fading channel, we proposed a blind two-stage MIMO receiver that can achieve a similar output signal-to-interference-plus-noise ratio (SINR) as the optimal MMSE receiver but got rid of requirement of the preknown pilot symbols. Specifically, the design of the proposed scheme involves the following procedure. In the first stage, an SB transformation is first developed before processing MAI suppression for circumventing desired signal cancellation. An IB transformation is then constructed based on the SB transformed data and is used to suppress the MAI blindly. Finally, the desired signals buried in the IB transformed data are combined in accordance with maximizing the output signal-to-noise ratio (SNR) criterion. In the second stage, the subspace MMSE (S-MMSE) receiver [9] incorporating with a decision-directed scheme is developed to mitigate desired signal cancellation resulting from the finite sample effect. Simulation results indicate that the proposed receiver exhibits significant performance improvement when compared with conventional methods.

The notations used in this paper follow that the usual convention-bold capital letters denote vectors or matrices, 𝐈𝑛 is an 𝑛×𝑛 identity matrix, 𝟎𝑛 is an all-zero vector of size 𝑛×1,  𝐎𝑚×𝑛 is an all-zero matrix of size 𝑚×𝑛,   is the Kronecker product, and (), ()𝑇, and ()𝐻 are complex conjugate, transpose, and Hermitian transpose of (), respectively. We also use 𝐸{} to denote ensemble average, || for absolute operator, for Euclidean norm, and tr{} for the trace operator. arg{} and Arg{} [10] denote an argument and the argument of a complex number, respectively. Finally, the operators mod{} and dec{} stand for computing the modulus after division and the data decision operator, respectively.

2. Data Model

Consider herein a 𝐾-user CDMA system equipped with two transmit antennas (TXs) and 𝑁 receive antennas (RXs) over a multipath fading channel with 𝐿-resolvable paths, in which the Alamouti’s STBC scheme is adopted to provide transmit diversity. Each modulated signal of 𝑘th user is spread with a unique spreading code 𝐜𝑘(1)  (𝐜𝑘(2)) of length 𝑀 for TX1 (TX2) [4]. After spreading, a zero-padding of 𝑁𝑔 samples, assumed to be longer than the expected maximum excess delay 𝐿, is added to combat intersymbol interference (ISI) caused by multipath. Finally, the resultant data is fed into the multiple access channel. At the receiver end, the received signal is chip matched filtered, sampled at the chip rate over the entire observation interval of chips, and then punctured the 𝑁𝑔𝐿+1 message-free samples. According to the Alamouti's STBC scheme utilizing two transmit antennas, the 𝑃×1 received data vectors, where 𝑃=𝑀+𝐿1, corresponding to the 𝑛th receive antenna over two consecutive symbol intervals are written by [4]𝐱𝑛(2𝑖1)=𝐾𝑘=1𝐡(1)𝑛,𝑘𝑑𝑘(2𝑖1)+𝐡(2)𝑛,𝑘𝑑𝑘(2𝑖)+𝐧𝑛𝐱(2𝑖1),𝑛(2𝑖)=𝐾𝑘=1𝐡(1)𝑛,𝑘𝑑𝑘(2𝑖)+𝐡(2)𝑛,𝑘𝑑𝑘(2𝑖1)+𝐧𝑛(2𝑖),(1) where 𝑑𝑘(𝑖) is the 𝑖th transmitted symbol of user 𝑘 assumed to be independent and identically distributed (i.i.d.) with zero-mean and unit variance, and the vector 𝐧𝑛(𝑖) is the spatially and temporally white noise of the covariance matrix 𝜎2𝑛𝐈𝑃, with 𝜎2𝑛 being the noise power. Finally, the 𝑃×1 vector 𝐡(𝑚)𝑛,𝑘 denotes the effective composite signature vector (CSV) of user 𝑘 from TX𝑚 to RX𝑛 given by𝐡(𝑚)𝑛,𝑘=𝜎𝑘𝐿𝑙=1𝑔(𝑚)𝑛,𝑙𝐜(𝑚)𝑘,𝑙=𝜎𝑘𝐂𝑘(𝑚)𝐠𝑛(𝑚),𝑚=1,2,(2) where𝐂𝑘(𝑚)=𝐜(𝑚)𝑘,1,𝐜(𝑚)𝑘,2,,𝐜(𝑚)𝑘,𝐿,𝐠𝑛(𝑚)=𝑔(𝑚)𝑛,1,𝑔(𝑚)𝑛,2,,𝑔(𝑚)𝑛,𝐿𝑇.(3) The scalar 𝜎2𝑘 is the transmit power of the 𝑘th user, 𝑔(𝑚)𝑛,𝑙 is the complex channel gain from TX𝑚 to RX𝑛 of the 𝑙th path, and 𝐜(𝑚)𝑘,𝑙=[𝟎𝑇𝑙1,𝐜𝑘(𝑚)𝑇,𝟎𝑇𝐿𝑙]𝑇 is the augmented signature vector associated with the 𝑙th path of user 𝑘. According to the STBC structure and stacking the two consecutive received data samples in (1), we have𝐲𝑛𝐱(𝑖)=𝑛𝐱(2𝑖1)𝑛=(2𝑖)𝐾𝑘=1𝐇𝑛,𝑘𝐝𝑘(𝑖)+𝐯𝑛(𝑖),(4) where𝐇𝑛,𝑘=𝐡(1)𝑛,𝑘𝐡(2)𝑛,𝑘𝐡(2)𝑛,𝑘𝐡(1)𝑛,𝑘,𝐝𝑘𝑑(𝑖)=𝑘(𝑑2𝑖1)𝑘,𝐯(2𝑖)𝑛𝐧(𝑖)=𝑛𝐧(2𝑖1)𝑛.(2𝑖)(5) Without loss of generality, we assume that the first user is the desired user. The received array data vector can be expressed as𝐲𝐲(𝑖)=𝑇1(𝑖),𝐲𝑇2(𝑖),,𝐲𝑇𝑁(𝑖)𝑇=𝐾𝑘=1𝐇𝑘𝐝𝑘(𝑖)+𝐯(𝑖),(6) where 𝐇𝑘=𝐇𝑇1,𝑘,𝐇𝑇2,𝑘,,𝐇𝑇𝑁,𝑘𝑇,𝐯𝐯(𝑖)=𝑇1(𝑖),𝐯𝑇2(𝑖),,𝐯𝑇𝑁(𝑖)𝑇.(7)

A linear receiver is designed a 2NP × 2 weight matrix 𝐖1 to extract and remove 𝐇1 to retrieve the transmitted data 𝐝1(𝑖) from the received signal. Among them, the MMSE receiver [2], which is widely and popularly employed for CDMA systems, minimizes the mean squared error (MSE) between the receiver output and known pilot symbols 𝐝1(𝑖):min𝐖1𝐸𝐖𝐻1𝐲(𝑖)𝐝1(𝑖)2min𝐖1𝐖𝐻1𝐑𝐖1𝐖𝐻1𝐇1𝐇𝐻1𝐖1+𝐈2,(8) whose solution is given by𝐖1=𝐑1𝐇1,(9) where 𝐑 is the data correlation matrix defined by𝐑=𝐸𝐲(𝑖)𝐲𝐻=(𝑖)𝐾𝑘=1𝐇𝑘𝐇𝐻𝑘+𝜎2𝑛𝐈2NP.(10) The MMSE receiver has been shown to successfully detect the transmit data of the desired user and deal with MAI as long as the CSI matrix 𝐇1 can be distortionlessly estimated beforehand. However, both the estimation error in 𝐇1 and the finite sample effect will induce desired signal cancellation. This significantly drops the system performance. As a remedy, a larger amount of pilot symbols is necessary for the MMSE-based receiver, especially for high input SNR. Unfortunately, in practice, this is impossible due to the limitation of radio frequency resources [5].

3. Proposed Two-Stage MIMO Receiver

In this section, the proposed blind two-stage MIMO receiver is developed. In the first stage, the SB and IB transformations are constructed to alleviate desired signal cancellation and to resist interfering signals, respectively. An optimal combiner obtained in accordance with maximizing the output SNR is then performed on the IB transformed data for collecting the path signals. In the second stage, a decision-directed scheme is developed to enhance the robustness against the finite sample effect.

3.1. Signal-Blocked Interference-Blocked Receiver

Conventional MMSE receiver is found to be sensitive to the errors in the CSI estimation, which will cause desired signal cancellation. To avoid such degradation, a 2NP × 2NP linear transformation matrix 𝐓SB executing on the received data before processing MAI suppression is designed to remove the desired signal corresponding to 𝑑1(2𝑖1):𝐓SB𝟎𝑇2(𝑛1)𝑃,𝐜(1)𝑇1,𝑙,𝟎𝑇[2(𝑁𝑛)+1]𝑃𝑇=𝟎2NP,𝐓SB𝟎𝑇(2𝑛1)𝑃,𝐜(2)𝑇1,𝑙,𝟎𝑇2(𝑁𝑛)𝑃𝑇=𝟎2NP,𝑙=1,2,,𝐿;𝑛=1,2,,𝑁,(11) such that the SB transformed data vector can be expressed by𝐓SB𝐲(𝑖)=𝐓SB𝐇1(,2)𝑑1(2𝑖)+𝐓SB𝐾𝑘=2𝐇𝑘𝐝𝑘+𝐓SB𝐯(𝑖),(12) where 𝐇1(,𝑖) denotes the 𝑖th column of the matrix 𝐇1. As expected, the SB transformation can completely remove the desired signal 𝑑1(2𝑖1) because of preknown spreading codes 𝐜1(𝑚) for 𝑚=1,2.

Comparing (12) with (6) indicates that, except for the desired signal, the only difference between them is the term involving the self-interference 𝑑1(2𝑖), the uncorrelated interference, and noise. Thus, in order for the weight vector working on the transformed data to suppress the interference, the remaining degree of freedom in 𝐓SB should be exploited to minimize the error𝐸𝐓SB𝐲(𝑖)𝐲(𝑖)2𝐓trSB𝐈2NP𝐑𝐓SB𝐈2NP𝐻.(13) In corporation of the linear constraints of (11) in the minimization of (13) leads the following constrained problem: min𝐓SB𝐸𝐓SB𝐲(𝑖)𝐲(𝑖)2,subjectto𝐓SB𝐂=𝐎2NP×2LN,(14) whose solution is given by𝐓SB=𝐈2NP𝐂𝐂𝐻𝐑1𝐂1𝐂𝐑1,(15) where 𝐂𝐂=1(1)𝐎𝑃×𝐿𝐎𝑃×𝐿𝐂1(2)𝐈𝑁.(16) With the assumption of sufficient degree of freedom in computing 𝐓SB, the SB transformed data can be approximately expressed by𝐓SB𝐲(𝑖)𝐇1(,2)𝑑1(2𝑖)+𝐾𝑘=2𝐇𝑘𝐝𝑘+𝐯(𝑖).(17) According to (17), the interference-plus-noise subspace can be extracted from the SB transformed data, chiefly consisting of interference and noise. Under the condition of (2𝐾1)<2NP, the SB transformed data correlation matrix can be eigen-decomposed as 𝐓𝐑=𝐸SB𝐲(𝑖)𝐲𝐻(𝑖)𝐓𝐻SB𝐓SB𝐑𝐓𝐻SB𝐇1(,2)𝐇𝐻1(,2)+𝐾𝑘=2𝐇𝑘𝐇𝐻𝑘+𝜎2𝑛𝐈2NP=2𝐾1𝑖=1𝜆𝑖𝜎2𝑛𝐞𝑖𝐞𝐻𝑖+𝜎2𝑛𝐈2NP,(18) where 𝜆𝑖 for 𝑖=1,2,,2𝐾1 in the decent order denote the (2𝐾1) largest eigenvalues of 𝐑 with the corresponding eigenvectors 𝐞𝑖 for 𝑖=1,2,,2𝐾1. Obviously, the interference part associated with the SB transformed data correlation matrix in (18) can be expressed in terms of the (2𝐾1) principal eigenvectors 𝐞𝑖 for 𝑖=1,2,,2𝐾1. On the other hand, the complement of the interference-plus-noise subspace spanned by {𝐞1,𝐞2,,𝐞2𝐾1} can be used to suppress the interference. Hence, the general solution of the IB transformation 𝐓IB is suggested by𝐓IB=𝐈2NP𝐄𝐄𝐻,(19) where 𝐄=[𝐞1,𝐞2,,𝐞2𝐾1]. Note that 𝐓IB is an idempotent and Hermitian matrix such that 𝐓2IB=𝐓IB and 𝐓𝐻IB=𝐓IB. According to (6) and (19), the IB transformed data, containing only the desired signal and noise, can be expressed by𝐓IB𝐲(𝑖)𝐓IB𝐇1(,1)𝑑1(2𝑖1)+𝐓IB𝐯(𝑖).(20)

The weight vector constructed based on the IB transformed data is designed to be capable of constructively combining the desired signal introduced by multipath. To achieve an optimal performance, the corresponding weight vector can be determined in accordance with maximizing the output SNR: max𝐖1𝐸||𝐖𝐻1(,1)𝐓IB𝐇1(,1)𝑑1||(2𝑖1)2𝐸||𝐖𝐻1(,1)𝐓IB||𝐯(𝑖)2𝐖𝐻1(,1)𝐓IB𝐇1(,1)𝐇𝐻1(,1)𝐓IB𝐖1(,1)𝐖𝐻1(,1)𝐓IB𝐖1.(,1)(21) Unfortunately, 𝐇1(,1) in the above equation is not available in practice. Since the MAI can successfully eliminate by the IB transformation, the combiner output power is approximately equal to the signal-plus-noise power. According to the fact that maximizing the signal-plus-noise-to-noise ratio (SNNR) is effective the same as to maximize SNR, (21) can be rewritten as max𝐖1𝐸||𝐖𝐻1(,1)𝐓IB||𝐲(𝑖)2𝐸||𝐖𝐻1(,1)𝐓IB||𝐯(𝑖)2𝐖𝐻1(,1)𝐓IB𝐑𝐓IB𝐖1(,1)𝐖𝐻1(,1)𝐓IB𝐖1(,1).(22) The solution is given by the dominant mode (the eigenvector associated with the largest eigenvalue) of the eigen-equation 𝐓IB𝐑𝐓IB𝐖1(,1)=𝜂𝐓IB𝐖1(,1),(23) where 𝜂 denotes the largest eigenvalue. Ideally, the degradation of output SINR is mainly caused by the components in the complement of the desired signal subspace such that the optimal weight vector formed by projecting the weight vector 𝐖1(,1) onto the desired signal-plus-noise subspace (complement of the interference-plus-noise subspace) can be given by [11]𝐖1(,1)=𝐓IB𝐖1(,1).(24)

3.2. Decision-Directed MMSE Receiver

In order to further improve the robustness against the finite data sample effect, we proposed a decision-directed MMSE receiver. For a moderately good condition, the receiver output is approximately expressed by𝑧1(𝐖𝑖)=𝐻1(𝐖,1)𝐲(𝑖)𝐻1(,1)𝐇1(,1)𝑑1𝐖(2𝑖1)+𝐻1(,1)𝐯(𝑖).(25) It is interesting to note that the receiver output in (25) and the transmit information data 𝑑1(2𝑖1) differ by a complex scalar 𝐖𝐻1(,1)𝐇1(,1). With the assumptions of BPSK-modulated signal and the successful suppression of MAI, the phase of 𝐖𝐻1(,1)𝐇1(,1), 𝜑1, can be obtained based on the receiver output, leading to the following result:𝜑1=𝑁𝑠𝑖=1𝑧modarg1(𝑖),𝜋𝑁𝑠𝐖arg𝐻1(,1)𝐇1(,1),(26) where 𝑁𝑠 is the number of data samples. For high output SINR, the phase estimator in (26) performs well with a small MSE. In the extreme case, as the value of output SINR goes infinite, the proposed scheme is consistent in the mathematical sense [6]. However, an argument of a complex number arg{} is infeasible in implementation. An alternative approach uses the principal angle Arg{} instead of arg{}, in which value for Arg{} lies between 𝜋 and 𝜋. According to the assumptions of high output SINR and 𝐖𝐻1(,1)𝐇1(,1) in the first or second quadrant, that is, 0𝜑1<𝜋, we have the principal angle of the receiver output 𝑧1 [10]:𝑧Arg1=𝜑1,𝑑1𝜑(2𝑖1)=11𝜋,𝑑1(2𝑖1)=1.(27) Thus, the phase estimate is given by 𝜑1𝐖=Arg{𝐻1(,1)𝐇1(,1)}, which means that the phase 𝜑1 can be correctly obtained by (26). On the contrary, for 𝜋<𝜑1<0, the output of the argument of the receiver output is 𝜑1 and 𝜑1+𝜋 for 𝑑1(2𝑖1)=1 and −1, respectively, such that the phase estimation of the proposed scheme is 𝜑1+𝜋. It is noteworthy that there is a 𝜋-phase ambiguity involving the phase estimate. Without the further assumptions, we cannot resolve the 𝜋-phase ambiguity. Fortunately, in existence of the digital communications systems, a few predetermined bits, for example, the preamble, parity check, synchronization, or reserved bits [1214], can be used to get rid of this problem. Furthermore, the phase estimator in (26) can be applied to the phase shift keying (PSK) modulated signals with a little modification.

By exploiting the phase estimate in (26), the transmit information symbols can be decoded by a data decision device, given by𝑑1𝑧(2𝑖1)=dec1(𝑖)𝑒𝑗𝜑1.(28) A simple and nature way to obtain 𝐇1(,1) would be to use 𝑑1(2𝑖1) as pilot symbols [5]:𝐇1(,1)=𝑁𝑠𝑖=1𝑑𝐲(𝑖)1(2𝑖1)𝑁𝑠.(29) Finally, the CSI estimate in (29) can be incorporated into the S-MMSE scheme [9] to obtain the first column of the weight matrix. Note that, according to the STBC structure, the second column of the weight matrix W 1 can be constructed by its first column such that its corresponding derivation is omitted for simplicity [4].

4. Computer Simulations

Computer simulations were conducted to ascertain the efficacy of the proposed two-stage receiver in a synchronous ten-user (𝐾=10) CDMA system using two receive antennas (𝑁=2). For all users, the signals were spread by random codes of length 𝑀=32 through a four-path (𝐿=4) Rayleigh fading channel, in which the path delays were uniformly distributed and the path gains were i.i.d. unit variance complex Gaussian random variables. The input SNR and near-far ratio (NFR) were defined as SNR=10×log10(𝜎21/𝜎2𝑛) and NFR=10×log10(𝜎2𝑘/𝜎21), respectively. All MAI power was assumed to be identical for simplicity. We assumed that the 𝜋-phase ambiguity can be solved by using the preknown data bits [1214]. In each simulation, a total of 500 independent trials were executed to compute the output SINR values, with each trial using a different set of spreading codes and data/noise sequences. For performance comparison, we included the results obtained with the S-MMSE receiver [9] using the true CSI matrix and the time-multiplexed pilot symbol assisted MMSE receiver [5], in which 𝑁𝑝 pilot symbols were used to estimate the CSI matrix. Note that the fully-known data-aided S-MMSE receiver, that is, 𝑁𝑝=𝑁𝑠, is referred to as the optimal receiver.

In the first set of simulations, the output SINR performance was evaluated by varying input SNR with NFR = 10 dB and 𝑁𝑠=500. The corresponding results shown in Figure 1 indicated that the proposed receiver achieved a similar performance as the optimal one and outperformed the others. This confirmed that the proposed receiver can successfully collect the desired signals and suppress MAI. On the other hand, the pilot symbol assisted receivers cannot correctly estimate the CSI matrix due to strong MAI, which led to a poor performance.

147292.fig.001
Figure 1: Output SINR performance versus input SNR with 𝑁=2, 𝑀=32, 𝐿=4, 𝐾=10, NFR = 10 dB, and 𝑁𝑠=500.

In the second set of simulations, robustness of the proposed two-stage receiver against MAI was investigated as a function of different NFR values with SNR = −10 dB and 𝑁𝑠=500. As shown in Figure 2, the proposed scheme performed like the optimal receiver, which demonstrated an excellent near-far resistance in cancelling strong MAI. It is noteworthy that, in the case of NFR = 0 dB, the proposed receiver produced a slight degradation about 0.28 dB in output SINR. This is because the weaker MAI put less emphasis on the SB transformation which caused a transformation error. Fortunately, the effect of the weaker MAI can be significantly alleviated by means of the inherent property of the direct-sequence CDMA systems. Thus, this problem can be neglected. In addition, the existence of strong MAI can be determined by the strong interference detector [15].

147292.fig.002
Figure 2: Output SINR performance versus NFR with 𝑁=2, 𝑀=32, 𝐿=4, 𝐾=10, SNR = −10 dB, and 𝑁𝑠=500.

In the third set of simulations, the convergence behavior was examined by varying the data sample size 𝑁𝑠 with SNR = −10 dB and NFR = 10 dB. The results given in Figure 3 demonstrated that the proposed scheme with a similar performance as the optimal receiver converged in about 𝑁𝑠=500 data symbols. On the contrary, the other S-MMSE receivers cannot provide a reliable performance even in the case of 3000 data samples because of the finite data sample effect again.

147292.fig.003
Figure 3: Output SINR performance versus sample size 𝑁𝑠 with 𝑁=2, 𝑀=32, 𝐿=4, 𝐾=10, SNR = −10 dB, and NFR = 10 dB.

In the fourth set of simulation, the BER performance was examined by varying input SNR with NFR = 10 dB and 𝑁𝑠=500. The results depicted in Figure 4 showed that the proposed receiver can possess a performance with a little degradation as the optimal one. It is noteworthy that the proposed scheme did not perfectly decode the transmit symbols in low-output SNR, leading to a slight inferiority to the optimal receiver. On the other hand, all the other receivers once again did not provide a reliable performance due to desired signal cancellation.

147292.fig.004
Figure 4: BER performance versus input SNR with 𝑁=2, 𝑀=32, 𝐿=4, 𝐾=10, NFR = 10 dB, and 𝑁𝑠=500.

In the final set of simulations, the ISI effect was evaluated by setting the length of zero-padding equal to zero, that is, 𝑁𝑔=0, with NFR = 10 dB and 𝑁𝑠=500. The received data was reconstructed according to [16]. For comparison, the result obtained with the optimal receiver in an ISI-free case was also included, denoted by “Optimal (W/O ISI).” In order to combat the ISI, the (3×2𝐾1) largest eigenvectors in (18) were chosen to determine the IB transformation. Figure 5 showed the curve of output SINR versus input SNR. It is observed output SINR increased with the input SNR, confirming the efficacy of the proposed two-stage receiver to combat both the MAI and ISI. As compared with the ISI-free case, the proposed two-stage receiver performed with a slight degradation about 0.51 and 0.44 dB for input SNR equal to −20 and −10 dB, respectively. This is because that the more degree of freedom was required to suppress ISI and resulted in noise enhancement.

147292.fig.005
Figure 5: The ISI effect on the compared receivers when 𝑁=2, 𝑀=32, 𝐿=4, 𝐾=10, NFR = 10 dB, and 𝑁𝑠=500. The length of zero-padding is zero.

5. Conclusion

In this paper, a blind two-stage MIMO receiver with enhanced MAI suppression has been proposed. Numerical results demonstrate that the proposed scheme without requirement of pilot symbols can effectively improve the robustness against the finite data sample effect and can provide a similar performance as to the fully data-aided MMSE (optimal) receiver.

Acknowledgments

This paper was sponsored jointly by the National Science Council, Taiwan, under Grant NC-99-2221-E-239-021. This paper was presented in part at the 2008 International Workshop on Antenna Technology on Small Antennas and Novel Metamaterials, Chiba, Japan, March 2008.

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