A low-complexity beamforming-based scheduling to downlink OFDMA/SDMA systems with multimedia traffic

In this paper, we propose a low-complexity beamforming-based scheduling scheme utilizing a semi-orthogonal user selection (SUS) algorithm in downlink orthogonal frequency division multiple access (OFDMA)/space division multiple access (SDMA) systems to support multimedia traffic. One of the challenges in the multi-dimensional (space, time, and frequency) radio resource allocation problem for OFDMA/SDMA systems is its high complexity, especially to simultaneously satisfy the quality of services (QoS) requirements for various traffic classes. In the literature, the SUS algorithm is usually applied to the single-class traffic environment, but extending the SUS algorithm to the multimedia environment is not straightforward because of the need to prioritize the real-time (RT) users and the non-real-time (NRT) users. To solve this problem, we propose the concept of urgency value to guarantee the fairness of the NRT as well as the best effort (BE) users while satisfying the delay requirement for the RT users. Simulation results show that, when traffic load is greater than 0.5, the proposed scheduling algorithm can improve the fairness performance by more than 100% over the most recently proposed algorithms.     

多业务MIMO-OFDMA/SDMA系统跨层调度与动态资源分配

对多业务MIMO-OFDMA/SDMA 系统下行链路跨层调度与动态资源分配问题进行了研究.首先,在满足各种约束条件的前提下,以最大化系统吞吐量为目标建立了相应的优化模型;然后,提出了一种基于业务类型和子空间距离的用户分组算法,该算法采用聚类分析的方法在每个子载波上对配置有多根接收天线的用户进行分组,从而降低了调度时所需搜索的用户空间的维数;接着,基于所提出的用户分组算法并结合不同业务的优先级提出了一种新的跨层调度和资源分配算法,该算法充分利用跨层信息为每个子载波调度相应的用户组,并为调度到的用户分配相应的系统资源,从而通过最大化每个子载波的吞吐量近似实现了系统整体吞吐量的最大化.仿真结果表明,与现有的方案相比,所提算法更好地满足了不同业务用户的QoS要求,并获得了更好的吞吐量性能.     

Low complexity resource allocation and scheduling algorithms for downlink SDMA systems

The downlink zero-forcing beamforming strategy in the case of random packet arrivals is investigated. Under this setting, the relevant fairness criterion is the stabilization of all buffer queues which guarantees a bounded average delay for all users. It has been shown that allocating resources to maximize a queue-length-weighted sum of the rates is a stabilizing policy. However, the high complexity of user selection and the feasible rates determination for optimal scheme may prevent the real-time scheduling operation. Two low complexity algorithms are provided taking the channel state, queue state and orthogonality into account. In particular, the authors pick the first user with the largest product between channel gain and queuing length, and select the remaining users to construct candidate user set based on the greedy user selection method or channel orthogonal user selection method. Then, the power and rate allocation for the selected users are implemented based on the modified water-filling method. The complexity of the proposed algorithms is analyzed. The average delay and average throughput are studied in homogeneous scenarios and heterogeneous scenarios, respectively. Simulation results show that the proposed algorithms can take full advantage of the multi-user diversity gain and provide average delay (or throughput) and fairness improvement compared with channel-aware-only schemes.     

Performance of a nonblocking space-division packet switch withcorrelated input traffic

This work studies the performance of a nonblocking space-division packet switch in a correlated input traffic environment. In constructing the input traffic model, the author considers that each input is a time division multiaccess (TDM) link connecting to multiple sources. Every source on a link supports one call at a time. Each call experiences the alternation of ON and OFF periods, and generates packets periodically while in ON period. The stochastic property of each call does not have to be identical. Packets from each individual call are destined to the same output. The output address of each call is assumed to be uniformly assigned at random. The author derives both upper and lower bounds of the maximum throughput at system saturation. His study indicates that, if the source access rate is substantially lower than the link transmission rate, the effect of input traffic correlation on the output contentions can generally be ignored. Also, the analysis of each input queue becomes separable from the rest of the switch. The same study is carried out with nonuniform call address assignment     

Performance considerations of code division multiple-access systems

The performance of code division multiple-access (CDMA) systems is determined using direct-sequence spectral spreading. Asynchronous users are assumed so that there is no network control. Under relatively ideal conditions, the degradation in system performance as a function of the number of users is shown to have a threshold effect. This basic limitation in the number of users of the system is further limited if the powers are unequal. For two users, system performance as a function of their power ratios also has a threshold effect. System performance as a function of the amount of spectral spreading is determined. The performance of both coded and uncoded systems is predicted.     

An efficient scheduling scheme for hybrid TDMA and SDMA systems with smart antennas in WLANs

Wireless Networks - Smart antennas are an emerging technology that exploits spatial diversity and allows an access point (AP) to simultaneously transmit downlink packets to multiple stations...     

Performance analysis of multicarrier frequency-hopping (MC-FH) code-division multiple-access systems: uncoded and coded schemes

In this paper, we provide multiuser performance analysis of a multicarrier frequency-hopping (MC-FH) code-division multiple-access system as first introduced in the work of Lance and Kaleh. We propose to use a practical low-rate convolutional error-correcting code in this system, which does not require any additional bandwidth than what is needed by the frequency-hopping spread-spectrum modulation. We provide multiuser exact performance analysis of the system for both uncoded and coded schemes in additive white Gaussian noise and fading channels for a single-user correlator receiver. We also derive the performance analysis of the system based on a Gaussian distribution assumption for multiuser interference at the receiver output. Our numerical results first indicate that the coded scheme significantly increases the number of users supported by the system at a fixed bit error rate, in comparison with the uncoded MC-FH scheme. Moreover, it shows that the Gaussian analysis in some cases does not accurately predict the number of users supported by the system.     

Performance of Orthogonal Beamforming for SDMA With Limited Feedback

On the multiantenna broadcast channel, the spatial degrees of freedom support simultaneous transmission to multiple users. The optimal multiuser transmission, which is known as dirty paper coding, is not directly realizable. Moreover, close-to-optimal solutions such as Tomlinson-Harashima precoding are sensitive to channel state information (CSI) inaccuracy. This paper considers a more practical design called per user unitary and rate control (PU2RC), which has been proposed for emerging cellular standards. PU2RC supports multiuser simultaneous transmission, enables limited feedback, and is capable of exploiting multiuser diversity. Its key feature is an orthogonal beamforming (or precoding) constraint, where each user selects a beamformer (or precoder) from a codebook of multiple orthonormal bases. In this paper, the asymptotic throughput scaling laws for PU2RC with a large user pool are derived for different regimes of the signal-to-noise ratio (SNR). In the multiuser interference-limited regime, the throughput of PU2RC is shown to logarithmically scale with the number of users. In the normal SNR and noise-limited regimes, the throughput is found to scale double logarithmically with the number of users and linearly with the number of antennas at the base station. In addition, numerical results show that PU2RC achieves higher throughput and is more robust against CSI quantization errors than the popular alternative of zero-forcing beamforming if the number of users is sufficiently large.     

一种基于码分多址的改进性空分多址性能分析

为解决SDMA中下行波束交叠引入的不同波束覆盖下用户群间相互干扰,研究一种结合CDMA的改进性SDMA多址技术。基站对下行链路的赋形波束进行实时的相关性检测,为高相关性的SDMA波束动态分配正交的PN码,以CDMA方式来识别不同波束,实现码分和空分的有效结合。仿真结果表明改进的SDMA多址技术在波束高度相关,也即是用户群之间干扰较强的情况下仍能获得较好性能。     

Performance of FHSS multiple-access networks using MFSK modulation

The main concern of this paper is to estimate the symbol error probabilities of synchronous and asynchronous frequency-hop spread-spectrum multiple-access (FHSS-MA) networks through semi-analytic Monte Carlo simulations. We concentrate on systems transmitting one M-ary (M⩾2) FSK modulated symbol per hop with noncoherent demodulation. The usual practice when analyzing the performance of such networks is to upper-bound the probability of symbol error when a hop is hit by K^'⩾1 interfering users with (M-1)/M or 1. Previous work on the derivation of accurate approximations to this probability for the case when M=2 has indicated that the aforementioned bound not only gives excessively pessimistic results but may also lead to wrong tradeoff decisions. Using the simulated values for the error probabilities, we show that a similar argument holds for the cases when M>2 as well. Also, by employing a normalized throughput measure that takes into account the bandwidth and time expansion associated with the modulation order M, we find that there exists an optimum value of M that achieves the maximum possible throughput for the cases when binary and M-ary error correcting codes are employed. Throughput results are also given for the case when the signals from the active users in the network suffer from independent Rayleigh fading     

Performance of DS-UWB multiple-access systems with diversity reception in dense multipath environments

Ultrawideband (UWB) technology is characterized by transmitting extremely short duration radio impulses. To improve its multiple-access capability, the UWB technology can be combined with traditional spread-spectrum techniques. This paper demonstrates the influence of spatial and temporal diversities on the performance of direct-sequence (DS) UWB multiple-access systems in dense multipath environments. Numerical results show that the bit error rate performance of the DS-UWB system can be improved significantly by increasing the number of antenna array elements and/or by adding more multipaths coherently at the receiver. Furthermore, this paper studies the impact of array geometry on system performance and shows that a rectangular array can capture more energy and thus can offer better performance than a uniform linear array.     

Performance of binary and quaternary direct-sequencespread-spectrum multiple-access systems with random signature sequences

The performance of synchronous and asynchronous, binary and quaternary (with and without offset) direct-sequence spread-spectrum multiple-access (DS/SSMA) communication systems using random signature sequences and arbitrary chip waveforms is investigated. The average probability of error at the output of the correlation receiver is evaluated using a characteristic-function approach for these systems. Numerical results are presented that illustrate performance comparisons between systems using random and deterministic signature sequences, synchronous and asynchronous systems, systems with rectangular or sinewave chip waveforms, and binary and quaternary systems with the same data rates and bandwidth. In all cases, the accuracy of the Gaussian approximation is also examined     

多用户MIMO系统中基于快速匹配调度的有限反馈SDMA

提出了一种能高效利用多用户分集和空分复用增益的有限反馈空分多址(SDMA,space division multiple access)方法。首先利用子空间扰动的方法构造了一种具有分簇结构的多用户预编码码本。基于该码本,进一步给出了一种新型的多用户机会调度算法,该算法利用码本的簇结构实现对信道条件匹配的用户组进行快速的机会调度,同时确定被调度用户的首选码字。仿真结果表明,在蜂窝网络中,与传统的有限反馈SDMA方法相比,所提方法可以在不明显增加反馈开销的前提下显著提高系统吞吐量。     

Asymptotic Analysis of SDMA Systems with Near-Orthogonal User Scheduling (NEOUS) under Imperfect CSIT

In this paper, we focus on the asymptotic cross layer analysis of multi-antenna systems with transmit MMSE (Tx- MMSE) beamforming, near orthogonal scheduling and outdated CSIT. To capture the effect of the potential packet outage, we introduce the average system goodput, which measures the average b/s/Hz delivered to the mobiles successfully, as the system performance objective. We derive closed-form expressions for the optimal power and rate allocations as well as a low complexity near orthogonal user scheduling (NEOUS) algorithm to solve the cross-layer optimization problem.We derive the asymptotic order of growth in system goodput for general CSIT error variance σ₂ and found that for sufficiently large n_T (number of antennas at the base station) and K (number of users) where K = g?1(n_T ) for some strictly increasing function g(x) = o(x), the the system goodput grows in the order of nT log[(1-σ₂ ) logK] when σ₂ < 1. This is the same order of growth as the optimal order of growth in broadcast channels with perfect CSIT and hence, the NEOUS is order-optimal. On the other hand, we need exponentially larger K to compensate for the penalty in multiuser diversity gain due to CSIT errors.     

Performance of the limiter-discriminator-integrator detector infrequency-hop spread-spectrum multiple-access networks

This paper evaluates the performance of the limiter-discriminator-integrator detector (LDID) in asynchronous frequency-hop spread-spectrum multiple-access (AFHSS-MA) networks using binary frequency shift-keying (BFSK) modulation via Monte Carlo simulations. Both additive white Gaussian noise (AWGN) and Rayleigh-fading channels are considered and we find that the LDID offers a performance that is slightly worse than or comparable to that of the matched filter detector for the nonfading and Rayleigh-fading cases respectively. Also, hard limiting of the integrator input is proposed which significantly improves the performance of the LDID in AFHSS-MA networks resulting in a higher throughput than that achievable with a matched filter detector     

Space Division Multiple Access (SDMA): An Editorial Introduction

Wireless Personal Communications -     

Performance of a nonblocking space-division packet switch in a timevariant nonuniform traffic environment

The authors study the performance of a nonblocking space-division packet switch, given that the traffic intensities at the switch not only are nonuniform but also change as a function of time. A finite-state Markov chain is used as an underlying process to govern the time variation of traffic for the entire switch. The packet arrivals at each input form an independent Bernoulli process modulated by the underlying Markov chain. The output address of each packet is independently and randomly assigned with probability distributions, which are also modulated by the Markov chain. Provided that the traffic on each output is not dominated by individual inputs the service time of each output queue for sufficiently large switches can be characterized by an independent Markov modulated phase-type process. A matrix geometric solution for the resultant quasi-birth-death type queuing process is presented. The maximum throughput is obtained at the system saturation. The performance of the switch is numerically examined under various traffic conditions. A contention priority scheme to improve the switch performance is proposed     

Time-spread multiple-access (TSMA) protocols for multihop mobileradio networks

This paper introduces a novel technique called protocol threading, yielding a deterministic protocol that gives a guaranteed upper bound on the transmission delay of each packet at every node in a multihop mobile network. By eliminating the maximum degree constraint, the new method improves upon existing time-spread multiple-access (TSMA)-type protocols while preserving the advantages of the deterministic operation and topology transparency. We introduce the protocol threading solution, derive the maximum delay bound in a mobile topology, and analyze the performance of the protocol     

Performance of UWB multiple-access impulse radio systems with antenna array in dense multipath environments

In this letter, the influence of temporal and spatial diversities on the performance of ultra-wideband time-hopping pulse-position modulated multiple-access impulse radio (IR) systems is analyzed. We investigate how an antenna array can be used at the receiver to improve the bit-error rate (BER) performance and can cope with the effects of multiple-access interference of IR system in dense multipath environments. Analytical and simulation results show that the BER performance of the IR systems can be improved when the number of array elements is increased. The performance can be further improved by coherently adding more multipaths at the receiver.     

The Role of the Queueing Process in the Performance of Downlink SDMA Systems

The queueing process plays a crucial role in the performance that a Downlink SDMA system can achieve as it interacts with other system parameters such as the number of antennas, the traffic load and the number of active mobile nodes (MNs). This paper analyzes these interactions from the link-layer perspective, which has been traditionally ignored in the analysis of such systems. As a reference, a finite-buffer upper-bound queuing model able to predict the optimal system performance in terms of throughput (blocking probability) and system delay is presented. A comparative analysis between the considered system performance and the performance provided by the upper-bound queueing model allows to foresee the situations in which a Downlink SDMA system is underperforming and understand the reasons that cause this low performance. This knowledge is essential for the design of packet-based scheduling algorithms in order to maximize the system performance in a broad range of situations.