Capacity and Fairness in Multiuser Diversity Systems with Opportunistic Feedback

An opportunistic feedback protocol is proposed for multiuser diversity systems with proportional fair scheduling and maximum-throughput scheduling. The uplink feedback period is divided into two subperiods and users randomly access the feedback minislots according to their scheduling metric. An integrated analytical model is developed for evaluating the proposed feedback protocol.     

Spatial Transmission Mode Switching in Multiuser MIMO-OFDM Systems With User Fairness

Multiantenna radio systems allow accessing the channel in diversity or spatial multiplexing (SMUX) mode. Adequate switching between these modes according to current channel conditions was shown to yield significant performance improvements while requiring little feedback from the receiving side. We present a transmission concept for the downlink of a multiuser multiple-input–multiple-output orthogonal frequency-division-multiplexing (MU-MIMO-OFDM) system aiming at high user rates with limited feedback demands. An extended score-based scheduling (SB) approach ensures fair-resource allocation to the users, whereas transmission mode switching is used to guarantee high user rates. The degree of fairness of the scheduler can be adapted by adequately configuring a weighting function for the scores. Comparison with single-mode schemes reveals substantial throughput gains of the adaptive switching concept. Furthermore, targeting maximum throughput, we show that a considerable proportion of the capacity of the MIMO broadcast channel (BC) can be achieved with a comparatively low amount of required feedback.      

Multiuser Capacity and Fairness Evaluation of Channel/QoS-Aware Multiplexing Algorithms

In this work multiplexing of variable-rate sources over shared generic multiple fading channels is addressed. In general, the performance of existing multiplexing algorithms has been compared in very specific environments, making it difficult to draw general conclusions. In this article we propose a generic variable-rate multiuser and multichannel system model that can be applied to any orthogonal channel multiplexing mechanism (e.g., TDM, OFDM, CDM, or SDM) through parametric modeling. Based on the model, the performance of different multiplexing algorithms proposed in the literature is evaluated over moderately fast fading snared channels. Physical channel conditions, random traffic arrival, and QoS requirements are taken into consideration in the study. Multiuser capacity and fairness are assessed and evaluated for different channel-aware and/or QoS-aware user multiplexing algorithms. Results show the joint diversity gain and QoS in terms of the throughput and delay experienced by different user information flows     

采用天线选择的多用户MIMO-OFDM系统容量研究

对于采用线性预编码进行空分复用的多用户MIMO-OFDM系统的下行链路,分析了使用户间干扰完全消除的预编码矩阵设计实现过程,提出了用户端联合天线选择算法,以增大发送的数据流数,提高系统和容量,在此基础上提出了逐次递增确定用户天线的选择算法.仿真结果表明,本文所提的天线选择算法使系统和容量远高于未经天线选择时的和容量,并且次优的逐次选择算法在计算量可接受的情况下与最优联合选择算法相比获得的容量增益相差不大.     

Spatial Scheduling With Interference Cancellation in Multiuser MIMO Systems

This paper proposes a novel downlink spatial scheduling algorithm in multiuser multiple-input-multiple-output (MIMO) systems, which selects a good combination of terminals and base station's (BS's) transmit beams so that the BS's beams nullify interstream interference at the selected terminals. In the derivation process, we reveal the new property that the optimization problem of downlink spatial scheduling is equivalent to that of uplink scheduling under the BS's zero-forcing beamforming. Using this property, an efficient downlink scheduling algorithm is presented, applying the principle of an uplink scheduling algorithm. Numerical results show that the presented spatial scheduling achieves a much higher system throughput than a multiuser MIMO system without spatial scheduling or with conventional spatial scheduling by linear processing. We also present a realistic control structure to achieve the uplink and downlink spatial scheduling in time-division duplex systems.     

A Unified Capacity Analysis for Wireless Systems With Joint Multiuser Scheduling and Antenna Diversity in Nakagami Fading Channels

A Unified Capacity Analysis for Wireless Systems With Joint Multiuser Scheduling and Antenna Diversity in Nakagami Fading Channels In this paper, we present a cross-layer analytical framework to jointly investigate antenna diversity and multiuser scheduling under the generalized Nakagami fading channels. We derive a unified capacity formula for the multiuser scheduling system with different multiple-input multiple-output antenna schemes, including: 1) selective transmission/selective combining (ST/SC); (2) maximum ratio transmission/maximum ratio combining (MRT/MRC); 3) ST/MRC; and 4) space–time block codes (STBC). Our analytical results lead to the following four observations regarding the interplay of multiuser scheduling and antenna diversity. First, the higher the Nakagami fading parameter, the lower the multiuser diversity gain for all the considered antenna schemes. Second, from the standpoint of multiuser scheduling, the multiple antennas with the ST/SC method can be viewed as virtual users to amplify multiuser diversity order. Third, the boosted array gain of the MRT/MRC scheme can compensate the detrimental impact of the reduced amount of fading gain on multiuser scheduling, thereby resulting in greater capacity than the ST/SC method. Last, employing the STBC scheme together with multiuser diversity may cause capacity loss due to the reduced amount of fading gain, but without the supplement of array gain.     

基于应用时间窗多用户MIMO-OFDM系统中的比例公平算法

该文针对基于延时信道状态信息的多用户MIMO-OFDM系统,在用户比例速率要求和功率限制的情况下,以最大化时间窗内系统吞吐量为目标,提出了一种基于应用时间窗比例公平算法。该算法首先设计各子载波上满足用户误比特率要求的星座距离,然后把系统中每个用户按照其比例映射为相应数目的虚拟用户,最后根据影子价格把子载波最优地分配给虚拟用户。仿真结果表明,该算法在保证用户公平性的基础上,有效地提高了系统吞吐量。     

Opportunistic Feedback for Multiuser MIMO Systems With Linear Receivers

A novel multiuser scheduling and feedback strategy for the multiple-input multiple-output (MIMO) downlink is proposed in this paper. It achieves multiuser diversity gain without substantial feedback requirements. The proposed strategy uses per-antenna scheduling at the base station, which maps each transmit antenna at the base station (equivalently, a spatial channel) to a user. Each user has a number of receive antennas that is greater than or equal to the number of transmit antennas at the base station. Zero-forcing receivers are deployed by each user to decode the transmitted data streams. In this system, the base station requires users' channel quality on each spatial channel for scheduling. An opportunistic feedback protocol is proposed to reduce the feedback requirements. The proposed protocol uses a contention channel that consists of a fixed number of feedback minislots to convey channel state information. Feedback control parameters including the channel quality threshold and the random access feedback probability are jointly adjusted to maximize the average throughput performance of this system. Multiple receive antennas at the base station are used on the feedback channel to allow decoding multiple feedback messages sent simultaneously by different users. This further reduces the bandwidth of the feedback channel. Iterative search algorithms are proposed to solve the optimization for selection of these parameters under both scenarios that the cumulative distribution functions of users are known or unknown to the base station     

Cross-Layer Design for Multiuser MIMO MRC Systems With Feedback Constraints

Cross-layer techniques represent efficient means to enhance throughput and increase the transmission reliability of wireless communication systems. In this paper, a cross-layer design of aggressive adaptive modulation and coding (A-AMC), truncated automatic repeat request (T-ARQ), and user scheduling is proposed for multiuser multiple-input–multiple-output (MIMO) maximal ratio combining (MRC) systems, where the impacts of feedback delay (FD) and limited feedback (LF) on channel state information (CSI) are also considered. The A-AMC and T-ARQ mechanism selects the appropriate modulation and coding schemes (MCSs) to achieve higher spectral efficiency while satisfying the service requirement on the packet loss rate (PLR), profiting from the feasibility of using different MCSs to retransmit a packet, which is destined to a scheduled user selected to exploit multiuser diversity and enhance the system's performance in terms of both transmission efficiency and fairness. The system's performance is evaluated in terms of the average PLR, average spectral efficiency (ASE), outage probability, and average packet delay, which are derived in closed form, considering transmissions over Rayleigh-fading channels. Numerical results and comparisons are provided and show that A-AMC combined with T-ARQ yields higher spectral efficiency than the conventional scheme based on adaptive modulation and coding (AMC), while keeping the achieved PLR closer to the system's requirement and reducing delay. Furthermore, the effects of the number of ARQ retransmissions, numbers of transmit and receive antennas, normalized FD, and cardinality of the beamforming weight vector codebook are studied and discussed.      

Robust Multiuser Interference Cancellation for OFDM Systems With Frequency Offset

In orthogonal frequency division multiple access systems clusters of subcarriers are assigned to different users for parallel data transmissions. The subcarriers are overlapped, but orthogonal to each other such that there is no intercarrier interference (ICI). However, synchronization errors among users cause the loss of the orthogonality and introduce ICI resulting in multiple-access interference. Synchronization between users is particularly difficult in the uplink channel where the user signals are potentially asynchronous and affected by different frequency offsets due to misalignment in carrier frequencies and Doppler shifts. This paper proposes a method to lower the effects of different frequency offsets among user signals in an OFDMA uplink system. The multiple access interference due to the user frequency misalignments is reduced by reconstructing and removing the interfering signals in the frequency domain. An approach based on the selective cancellation method, is proposed and its performance is analyzed by means of theoretical analysis and computer simulations. The effectiveness of the proposed system has been evaluated in the case of ideal and no-ideal frequency offset estimation and has been compared with that of the classical successive and parallel cancellation schemes. Simulation results show that the proposed approach allows performance close to the ideal case, i.e., with ideal frequency synchronization among users, with a low increase of the implementation complexity. Moreover, it is also highlighted here, that the successive cancellation method slightly outperforms the selective scheme, at the expense of a higher computational complexity and processing delay     

Effect of Carrier Frequency Offset on the Channel Capacity in Multiuser OFDM-FDMA Systems

Orthogonal Frequency Division Multiplexing (OFDM) is a very robust transmission procedure in multipath and frequency selective radio channels. A Frequency Division Multiple Access (FDMA) resource allocation technique offers the opportunity of a detailed link adaptation scheme. The combination of these transmission- and multiple access techniques in OFDM-FDMA is an ideal and very strong candidate for the downlink of future fourth generation (4G) mobile communication systems. This technical combination offers high cell capacities by exploiting the inherent multiuser diversity effect of the system. To apply OFDM-FDMA in the uplink, the time and carrier synchronization accuracy becomes very important. Non-ideal synchronization of the user signals to the carrier frequency of the base station leads to intercarrier interferences (ICI). In this paper, an analytical model for the ICI consideration in the uplink of a multiuser OFDM-FDMA based system is derived. The impact of the carrier frequency offset (CFO) on the performance of a cellular multiuser system with respect to different subcarrier allocation schemes is analyzed.

Design of Multiuser Downlink Linear MIMO Precoding Systems With Quantized Feedback

We treat the problem of sum-rate maximization via scheduling and linear precoding in multiuser (MU) multiple-input–multiple-output (MIMO) systems with quantized feedback. We formulate the optimal quantized linear precoder design problem and provide the numerical procedure for finding the solution. We also provide a simple scheduling scheme that exploits the MU diversity gain. With the aim of sum-rate maximization, we further address the quantization codebook design based on the capacity measure by introducing a new distance metric. It is demonstrated that, with or without scheduling, the proposed optimal linear precoding scheme achieves significant gain over the conventional linear precoder designs with similar feedback overhead, such as the zero-forcing precoder. Moreover, although the proposed quantization codebook design improves the performance of other existing MU-MIMO precoding schemes, the performance gain offered by it is typically much higher when it is coupled with the proposed linear precoding design.      

Degrees of Freedom in Wireless Multiuser Spatial Multiplex Systems With Multiple Antennas

This letter investigates the structure of the optimal spatial multiplex scheme in a multiuser multiantenna wireless fading environment. Based on a sum-capacity criterion, this letter shows that the optimal transmission strategy in an uplink or downlink channel with$n$antennas at the base-station involves more than$n$users at the same time. In particular, when remote users are equipped with$m$antennas each, the maximum number of data streams is shown to be upper bounded by$n^2$, with each user transmitting or receiving up to$m^2$data streams. This gives a dimension-counting interpretation for multiuser diversity. Multiple antennas at the base-station increases the total number of dimensions, thus allowing more users to transmit and receive at the same time. By contrast, multiple antennas at the remote terminal allow a single user to occupy multiple dimensions, which increases its transmission rate, but also has the potential effect of precluding simultaneous transmission by other users.     

Optimal Resource Allocation for Multiuser MIMO-OFDM Systems With User Rate Constraints

With the proliferation of wireless services, personal connectivity is quickly becoming ubiquitous. As the user population demands greater multimedia interactivity, data rate requirements are set to soar. Future wireless systems, e.g., multiple-input multiple-output orthogonal frequency division multiplexing (MIMO-OFDM), need to cater to not only a burgeoning subscriber pool but also to a higher throughput per user. Furthermore, resource allocation for multiuser MIMO-OFDM systems is vital for the optimization of the subcarrier and power allocations to improve overall system performance. Using convex optimization techniques, this paper proposes an efficient solution to minimize the total transmit power subject to each user's data rate requirement. Using a Lagrangian dual decomposition, the complexity is reduced from one that is exponential in the number of subcarriers $M$ to one that is only linear in $M$. To keep the complexity low, linear beamforming is incorporated at both the transmitter and the receiver. Although frequency-flat fading has been known to plague OFDM resource allocation systems, a modification, i.e., dual proportional fairness, seamlessly handles flat or partially frequency-selective fading. Due to the nonconvexity of the optimization problem, the proposed solution is not guaranteed to be optimal. However, for a realistic number of subcarriers, the duality gap is practically zero, and optimal resource allocation can be evaluated efficiently. Simulation results show large performance gains over a fixed subcarrier allocation.      

一种多用户MIMO系统的公平调度算法

多用户多输入多输出(MIMO)系统中需要对用户进行合理调度,一般基于信道增益的调度算法忽略了用户之间的公平性。基于此,提出了一种公平的多用户调度算法,其思想是先根据信道条件数初选出用户,然后统计每个用户被选择的次数,如果次数之差的绝对值大于设定门限,则选择次数少的用户。这样既能获得多用户分集增益又兼顾了公平性。仿真结果表明,提出的调度算法在系统性能无明显损失的情况下保证了用户公平性。     

Interference Identification and Blind Multiuser Detection for Asynchronous CDMA Systems With Multipath Fading

We propose a blind user identification detection (UID) scheme based on linear prediction algorithm for asynchronous direct-sequence code-division multiple-access systems over multipath fading channels. Numerical results show that both the false alarm rate and bit error rate of the proposed algorithm are better than those of the existing subspace-based UID algorithms.     

空时CDMA多用户信号检测

本文利用最小均方误差(MMSE)和最大似然(ML)准则,推导了空时CDMA多用户检测算法,分析了空时多用户检测与单用户检测的关系,并提出一种空时CDMA多用户检测接收机结构.计算机仿真结果说明了本文提出的空时CDMA多用户检测算法,与单用户检测方法相比其误码性能有相当的改进.     

Space-Time Blind Multiuser Detection for Multiuser DS-CDMA and Oversampled Systems

Novel space-time blind multiuser detection method for multiuser DS-CDMA and oversampled systems is presented. To achieve signal recovery, Khatri-Rao product structure of the space-time channel matrix of received signals and rank-1 projection strategy are exploited. Two blind signal detection algorithms based on Khatri-Rao product decomposition are proposed. Uniqueness of the Khatri-Rao decomposition is discussed. Simulation results show that the proposed algorithms have performance close to the non-blind Zero Forcing algorithm and still work well in the case of only few antennae in the receiving end.     

基于比例公平原则的多用户MIMO-OFDM系统资源分配

提出了一种基于比例公平原则提高MIMO-OFDM系统吞吐量的子载波和功率分配算法.它在总功率和误比特率的约束下,以获取最大系统吞吐量为目标,同时为兼顾用户间资源享用的公平性,根据用户速率成比例推导出了子载波分配限制准则.仿真结果表明,本算法不仅可满足不同用户的速率要求,而且平衡了容量最大化和用户间公平性的矛盾,同时计算...     

Iterative Group-Blind Multiuser Detection and Decoding With Signal Rank Estimation for Coded CDMA Systems

In this paper, a turbo receiver structure is proposed for the uplink of coded code-division multiple-access (CDMA) systems in the presence of unknown users. The proposed receiver consists of two stages following each other. The first stage performs soft interference cancellation and group-blind linear minimum mean square error (MMSE) filtering, and the second stage performs channel decoding. The proposed group-blind linear MMSE filter suppresses the residual multiple-access interference (MAI) from known users based on the spreading sequences and the channel characteristics of these users while suppressing the interference from other unknown users using a subspace-based blind method. The proposed receiver is suitable for suppressing intercell interference in heavily loaded CDMA systems. Since the knowledge of the number of unknown users is crucial for the proposed receiver structure, a novel estimator is also proposed to estimate the number of unknown users in the system by exploiting the statistical properties of the received signal. Simulation results demonstrate that the proposed estimator can provide the number of unknown users with high accuracy; in addition, the proposed group-blind receiver integrated with the new estimator can significantly outperform the conventional turbo multiuser detector in the presence of unknown users.