Antenna diversity in multiuser data networks

We consider the use of multiple antennas at the transmitter and/or the receiver to provide open-loop spatial diversity in a multiuser wireless data network. With channel quality information (CQI) available to the transmitter, and by always scheduling the transmission to the active user having the best channel conditions at the time of scheduling, another form of diversity, termed multiuser diversity, is obtained in a data system. This paper provides an analysis of the interaction between these two forms of diversity. From a network point of view, we prove that the asymptotic sum rate, in the limit of a large number of active homogeneous users and subject to the same average total transmit power, is inversely related to the number of transmit antennas for independent and identically distributed (i.i.d.) flat Rayleigh fading channels. In the case of i.i.d. flat Rician fading, the asymptotic sum rate also depends inversely on the number of transmit antennas, but directly on the number of receive antennas. Numerically, we show that the total diversity gain is also constrained by finite CQI quantization and channel fading statistics.     

A dual-mode multiuser detector for DS-CDMA systems

We introduce a dual-mode multiuser detector that dynamically switches its detection mode between matched-filter and decorrelator operations based on the channel characteristics. This detector significantly reduces the overall computational requirement while maintaining similar performance as that of the decorrelator. The switching mechanism of our dual-mode detector is designed by exploiting the performance-complexity tradeoff between the decorrelator and the matched-filter. Extensions of this idea to other types of multiuser detectors are also proposed     

Preselection-based iterative multiuser detector for DS-CDMA systems

A low-complexity iterative multiuser detector is presented that uses adaptive preselection to minimise complexity while maintaining system performance very similar to that achieved when using the optimal maximum likelihood receiver.     

The Cholesky-iterative detector: a new multiuser detector forsynchronous CDMA systems

The decorrelating decision-feedback (DDF) multiuser detector based on Cholesky factorization has been shown to alleviate the performance degradation of the users in the detection process, especially for relatively low cross-correlation values between user signals. A new detection concept for multiple users described in this paper employs two triangular matrices (upper and lower) and soft output information to improve the data estimates. Simulation results show that significant performance gains can be achieved over the DDF. Also, the users tend to have their bit error probabilities clustered. Thus, the performance of a given user is less dependent on its position in the detection process than for the DDF detector     

Diversity order gain for narrow-band multiuser communications withpre-combining group detection

In narrow-band multiuser communication systems with fading diversity, it is shown that pre-combining group detection can bridge the diversity-order gap in performance between the optimum and linear detectors. For a system with M diversity channels, the group detector diversity order is M-|G|, where |G| is the interfering group size, a design parameter. Group detection thus provides a more substantial improvement in performance in narrow-band channels over linear detection than in wide-band channels in which the diversity orders of the optimal and linear detectors are equal. Here, the complexity of the receiver is a new parameter which, in addition to the number of antennas, can be used to control the diversity order. Exact formulas for the pairwise-error probabilities and bounds for the bit-error rate are obtained, and numerical results are shown     

Robust blind adaptive multiuser detector

A modified version of the constrained mean output energy (CMOE) detector, called a multiuser detector (MUD), is presented. With estimation of the channel noise variance, a significant reduction in performance can be avoided when signature mismatch exits, which is one of main problems of the CMOE detector. Numerical results are presented which confirm the validity of the method     

Adaptive induced fluctuations for multiuser diversity

This paper investigates multiuser diversity in the context of cellular networks, with emphasis on the gains that can be achieved by adaptively inducing fluctuations in the environment. In a cellular system that at any time schedules for service the user with the best channel at that time, the expected service rate increases with the variability of the channel. Controlling the fluctuations using available feedback can further increase the expected service rate. This paper proposes a scheme for controlling fluctuations using only the feedback required to exploit multiuser diversity. Fluctuations are induced by introducing at the base station another transmit antenna that sends out the same signal but at a different phase from the first one, and then adaptively varying the phase difference. The performance of the scheme (for the same total transmitted power) is evaluated when the users are infinitely back-logged or have finite queues, and when the channels are Rayleigh or Ricean distributed. Fairness issues and performance of the scheme under an additional fairness mechanism are also investigated in the context of users with finite queues. In all scenarios the performance is better when fluctuations are adaptively induced than when the fluctuations are randomly induced or not induced at all.     

Multistage multiuser detector for DS-CDMA multipath fading channel

The proposed multistage multiuser detector for DS-CDMA communication over a multipath fading channel consists of a bank of matched filters and a linear equaliser whose outputs are diversity combined to produce initial data estimates. Using this data, multiuser interference is removed from the matched filter bank output. Diversity combining is then applied again, which produces the final decision statistics. Simulation results indicate that this detector is near-far resistant     

Decorrelating decision-feedback multiuser detector for synchronouscode-division multiple-access channel

A decorrelating decision-feedback detector (DF) for synchronous code-division multiple-access (CDMA) that uses decisions of the stronger users when forming decisions for the weaker ones is described. The complexity of the DF is linear in the number of users, and it requires only one decision per user. It is shown that performance gains with respect to the linear decorrelating detector are more significant for relatively weak users and that the error probability of the weakest user approaches the single-user bound as interferers grow stronger. The error rate of the DF is compared to those of the decorrelator and the two-stage detector     

异步CDMA系统中的CM多用户检测器

本文将恒模（CM）算法应用于异步CDMA系统的多用户信号检测。该CM检测器只要求了解目标用户的信息，运算较简单，且由于采用了自适应方法，检测器无需估计信号的幅度。分析及仿真结果均表明，CM检测器的误码性能和抗-近效应能力均优于传统检测器，且对于用户数目的变化有很好的适应性。     

Decision-feedback blind adaptive multiuser detector for synchronousCDMA system

We develop a blind adaptive multiuser detector for synchronous code-division multiple access (CDMA) with a noise-whitening filter. The triangular structure of the noise-whitened model ensures complete resolution of detection ambiguities. To further improve the symbol error probability performance, we introduce decision feedback in our detector similar to the decorrelating derision-feedback detector (DDFD), thus forming the decision-feedback blind adaptive multiuser detector (DFBD). Simulations indicate that the performance of the DFBD is very close to that of the DDFD in additive white Gaussian noise (AWGN) channels. In Rician fading channels, the DFBD can track the slowly varying channels well and has a symbol error probability performance approaching that of the DDFD, which requires the knowledge of users' energies. The blind adaptive and decision-feedback blind adaptive multiuser detectors proposed here do not, however, require that knowledge     

基于干扰空间的盲自适应多用户检测器

在现有盲自适应多用户检测技术基础上，本文提出一种更具有实际应用价值的基于干扰空间线性MMSE盲自适应多用户检测器及其相应的基于干扰自相关矩阵C1跟踪算法。数学分析和仿真结果表明：与现有盲自适应多用户检测器相比，该检测器具有更低的实现复杂度、更快的跟踪速度和更强的健壮性。     

Decorrelator-based neighbour-searching multiuser detector in CDMAsystems

A new decorrelator-based neighbour-searching (DENS) multiuser detector is proposed for synchronous CDMA systems. It is shown that the DENS multiuser detector with neighbour-searching radius r=2 can achieve good bit-error-rate performance close to that of an optimum multiuser-detector     

A novel blind space-time multiuser detector for DS-CDMA system

In this letter, a novel blind space-time multiuser detector, called space-time constrained minimum output energy (ST-CMOE) detector, is proposed. It can achieve the same performance as the training based space-time minimum mean square error (ST-MMSE) detector. An adaptive version of the proposed ST-CMOE detector is also presented. Simulation shows that, the adaptive ST-CMOE detector can work well in a dynamic multiple-access channel.     

A joint performance measure for the decorrelating multiuser detector

We analyze the performance of the decorrelating multiuser detector for a wireless communication system. Instead of looking at the error probability for each user, we focus on the probability distribution of the number of correctly decoded users at the receiver and the average number of correctly decoded users under the conditions of equicorrelated signatures and perfect power control. The approach is motivated by the fact that the error events for different users are not independent. Numerical results show that when the number of users is large, instead of using a set of long signatures with a very low negative cross-correlation, we can use a set of shorter signatures with a reasonably high positive correlation without sacrificing the performance. It is also found that the average number of correctly decoded users increases with increase in signal-to-noise ratio and decreases when the cross-correlation between users becomes more negative or more positive.     

Suboptimal multiuser detector for frequency-selective Rayleighfading synchronous CDMA channels

Proposes a suboptimal low-complexity multiuser receiver for synchronous CDMA frequency-selective Rayleigh fading channels. In contrast to the conventional RAKE receiver, which suffers from near-far effects due to channel fading, the proposed multiuser receiver is shown to alleviate the near-far problem while preserving multipath diversity gain. This is demonstrated by comparing the symbol error probability and asymptotic multiuser efficiency of the proposed multiuser detector and RAKE receiver     

An overlapping window decorrelating multiuser detector for DS-CDMAradio channels

A new multiuser detector for direct-sequence code-division multiple-access communication systems is described. The proposed detector divides the received signal stream into a number of overlapping windows and decorrelates them window by window. This scheme is justified by an analysis of the convergence and decay rate of the impulse response. Based on the results, a signal-adapted criterion is developed which enables one to determine the window length according to the near-far situation of a practical system. A performance analysis and simulation results show that a small to moderate window length is usually sufficient to yield a performance that is close to or even better than that of the ideal decorrelating detector     

A novel iterative multiuser detector for complex modulation schemes

A novel multiuser detector for direct sequence code division multiple access is proposed. The receiver performs iterated soft decision interference cancellation (ISDIC) based on multiuser interference suppression filters designed for minimization of the mean-square error. Assuming a complex modulation format, we show that the multiuser interference becomes rotationally variant in the course of the iterations. Regarding this rotational variance in the design of the multiuser interference suppression filter, the presented iterative multiuser detector achieves significant performance gains compared with conventional ISDIC employing a standard minimum mean-squared error filter which is optimum only for rotationally invariant multiuser interference     

A low complexity multiuser detector for synchronous CDMA system

In a code-division multiple-access (CDMA) system, multiuser detection (MUD) can exploit the information of signals from other interfering users to increase the system capacity. However, the optimum MUD for CDMA systems requires the solution of an NP-hard combinatorial optimization problem. It is well known that the computational complexity of the optimum multiuser detector is exponential with the number of active users in the system. In this paper, we apply a hybrid algorithm to develop a suboptimal MUD strategy. The result of symmetric successive overrelaxation (SSOR) preconditioned conjugate-gradient method is first used to initialize the reduced-complexity recursive (RCR) multiuser detector. Then, RCR algorithm is applied to detect the received data bit by optimizing an objective function in relation with the linear system of decorrelating detector. Simulation results for the synchronous case show that the performance of our proposed SSOR-RCR multiuser detector is promising and outperform the decorrelator and linear minimum mean squared error multiuser detector with lower computational complexity.     

异步DS-CDMA系统中的最优时空线性多用户检测器

该文给出了一种适用于异步DS-CDMA系统的通用线性时空二维多用户检测方法,它将所有线性多用户检测器归结到一个统一的数学模型之下。基于此模型,该文将时空线性多用户检测问题转化为一个权矢量的优化问题,并在线性约束最小方差(LCMV)准则下给出了权矢量的优化解。对现有的几种典型线性多用户检测器的分析表明,这些方法都可以看作是该文提出的通用时空线性多用户检测方法的特例,且只能在某种限定条件下得到局部最优解,因此其抑制多址干扰和噪声的能力都低于文中的最优时空线性多用户检测器。最后,给出了数值仿真结果。