Mean vs. variance trade-off analysis in multi-antenna multi-user channels

In multi-user communications, the access point (AP) has several alternatives for distributing the scarce resources among users. Since there exists a trade-off between the global performance and the individual needs, an analytical framework to study fairness is derived, which completes the scope given by the existing fairness indexes in the literature. The framework proposed in this paper is a way to interpret fairness that has been inspired by portfolio selection; basically, it analyzes the mean vs. standard deviation trade-off. In this work, the target application is a multi-antenna AP transmitting simultaneously to several single-antenna terminals, although this framework is valid to analyze other procedures in multi-user communications.     

MMSE techniques for space diversity receivers in OFDM-based wireless LANs

This paper studies the application of the minimum mean square error (MMSE) beamformer to orthogonal frequency division multiplexing (OFDM)-based wireless local area networks. The questions here addressed are mainly the design with finite-length data and the choice of the OFDM signal domain where the beamformer is applied, either frequency or time. As OFDM signals need more samples than other modulations to stabilize the estimation of the signal statistics, how to exploit the finite-length training sequence provided for the design of equalizers becomes an important issue. The paper also shows that the usual frequency processing in OFDM is not always the best choice for the spatial beamforming, mainly for channels with a very high delay spread. Then, time processing turns out to be the best suited approach in terms of the tradeoff between performance and complexity. Additionally, novel modifications of the MMSE spatial filter are proposed to improve the raw bit-error rate performance: 1) a temporal semiblind approach that exploits the cyclic prefix and 2) windowing in the frequency domain.     

Cross-coupled DOA trackers

A new robust, low complexity algorithm for multiuser tracking is proposed, modifying the two-stage parallel architecture of the estimate-maximize (EM) algorithm. The algorithm copes with spatially colored noise, large differences in source powers, multipath, and crossing trajectories. Following a discussion on stability, the simulations demonstrate an asymptotic and tracking behavior that neither the EM nor a nonparallelized tracker can emulate     

Robust Design of Spatial Tomlinson-Harashima Precoding in the Presence of Errors in the CSI

In this letter, we study spatial Tomlinson-Harashima precoding (STHP) from an information theoretic point of view encompassing both the single and the multiuser scenarios. Since the performance of any communications preceding scheme relies on the quality of the channel state information (CSI) that is made available at the transmitter side, in this work we focus our attention on the analysis of the robustness of STHP in terms of rate loss when the CSI is imperfect. Precisely, we present a robust design of the set of moduli used in STHP and, consequently, of the corresponding power allocation, that maximizes the achievable rates for the worst-case errors in the CSI in the small errors regime.     

Spatial scheduling in multiuser wireless systems: from power allocation to admission control

Given a zero forcing transmit beamforming, we focus on how the multi-antenna access point distributes the scarce resource (power) among the single-antenna terminals. Since there is a clear trade-off between the satisfaction of the individual needs and the global performance of the cell, several criteria are proposed, ranging from a classical physical layer point of view of capacity (rate) maximization to bit error rate (BER)-based cost functions, which are closer to the second layer of the protocol stack. Between two traditional techniques, namely the uniform power allocation and the equal BER and rate, a new one is proposed, which ultimately provides an intermediate performance. Then, we add BER (or signal to noise ratio) constraints so that the admission control problem has to be solved. Among traditional options, we propose a new mechanism to balance the above-mentioned trade-off between the total performance and the particular user behavior. The results in terms of fairness are presented by a mean vs. variance plot and by the Gini plot.     

Practical Implementation of Bit Loading Schemes for Multiantenna Multiuser Wireless OFDM Systems

This paper provides useful insights into the practical design of bit allocation algorithms in multiantenna multiuser orthogonal frequency-division multiplexing (OFDM) systems. With the degrees of freedom obtained with multiple antennas and multiple subcarriers, the performance might be enhanced at the expense of a higher complexity. Since the scheduling with realistic integer mappings is an NP-complete combinatorial problem, suboptimum solutions based on the scalar product are shown to be good candidates for yielding a realizable scheduler at a wireless physical layer. Additionally, a power reuse strategy is proposed to lower the computational requirements of such a system. Besides the tradeoff between performance and complexity, there exists the tradeoff between performance and signaling. Therefore, we show that the required signaling might be reduced either by a user-subcarrier clustering or by using a scheme that forces an equal mapping for all the users at the same subcarrier, which might be well-suited if instantaneous fairness is required. The proposed strategies are evaluated for typical OFDM-based wireless LAN scenarios.     

On power allocation strategies for maximum signal to noise and interference ratio in an OFDM-MIMO system

Orthogonal frequency division multiplexing (OFDM) has been recently established for several systems such as HiperLAN/2 and Digital video/audio broadcasting, due the easy implementation of the modulator/demodulator and the equalizer. Moreover, also increasing interest is currently being put on multiple-input multiple-output (MIMO) channels, based on the use of antenna arrays at both the transmitter and the receiver. Here, we propose two joint beamforming strategies of low computational load for systems combining OFDM and MIMO. The ultimate objective is the maximization of the signal-to-noise and interference ratio (SNIR) over the carriers subject to a total transmit power constraint. Specifically, the maximization of the harmonic SNIR mean and the minimum SNIR over the subcarriers are proposed. The asymptotic behavior of the proposed methods is analyzed to provide a complete comparative and general view of the most relevant and already known transmit power allocation strategies. Finally, a theoretical analysis of the performance degradation of these techniques is carried out for the case in which the channel state information (CSI) is not perfect. Monte Carlo simulation results for the system bit-error rate and performance degradation with imperfect CSI are provided.     

简明的联合收发端天线选择算法

For reducing the computational complexity of the problem of joint transmit and receive antenna selection in Multiple-Input- Multiple-Output (MIMO) systems, we present a concise joint transmit/receive antenna selec-tion algorithm. Using a novel partition of the channel matrix, we drive a concise formula. This formula enables us to augment the chan-nel matrix in such a way that the computational complexity of the greedy Joint Transmit/Receive Antenna Selection (JTRAS) algorithm is reduced by a factor of 4nL, where nL is the number of selected antennas. A de-coupled version of the proposed algorithm is also proposed to further improve the efficien-cy of the JTRAS algorithm, with some capacity degradation as a tradeoff. The computational complexity and the perform-ance of the proposed approaches are evalu-ated mathematically and verified by computer simulations. The results have shown that the proposed joint antenna selection algorithm maintains the capacity perormance of the JTRAS algorithm while its computational complexity is only 1/4nL of that of the JTRAS algorithm. The decoupled version of the proposed algorithm further reduces the computational complexity of the joint antenna selection and has better performance than other decoupling-based algorithms when the selected antenna subset is small as compared to the total number of antennas.     

A Cross-Layer Approach to Heterogeneous Multi-User Diversity with Link Adaptation

Multi-user diversity (MUD) scheduling is based on the idea that access to the channel should be given to users with better channel state. In a system with many users whose channels vary independently, such strategy maximizes the probability that the channel of the scheduled user is near its peak. Consequently, the total ergodic capacity of the system is maximized. However, from a network point of view, performance is usually evaluated in terms of average throughput and average delay rather than ergodic capacity and fairness. Furthermore, information-theoretic rates are not achievable at the PHY layer and more practical solutions must be considered. In this paper the MUD problem is approached from a MAC layer perspective and the throughput maximal MUD scheduling policy, i.e., the policy that maximizes the total average throughput is investigated. We also study the average throughput region and show the suitability of optimal information-theoretic approaches in practical environments. The average throughput region is also contemplated in wireless networks where Heterogeneous multi-user diversity (HMUD) can be exploited. Finally, the average throughput region is related to the stability region and some average delay bounds for different ARQ mechanisms are provided.     

Energy efficient communications over the AWGN relay channel

This paper addresses the energy efficiency analysis of the relay channel under additive white Gaussian noise. We consider the rate bounds given by decode and forward and the cut set bound and assume that resources are optimally allocated to maximize the spectral efficiency according to the channel information and the sum network energy. The low energy analysis tools are used to compute the maximum rate per energy (RPE) and the slope of the spectral efficiency as a function of the energy per bit. Using these metrics, the energy efficiency benefit of several capabilities at terminals is investigated. Specifically, we take into account: i) the phase synchronization between transmitters, ii) the full duplex capability at the relay and iii) the channel access via superposition.