An adaptive power distribution algorithm for improving spectral efficiency in OFDM

In this paper, we propose an adaptive power distribution algorithm which aims at improving the spectral efficiency in orthogonal frequency division multiplexing (OFDM) under the condition that the target bit error rate (BER) should be maintained and the transmit power threshold should not be exceeded. This algorithm is based on the iterative mechanism. In every iteration, the power and the corresponding number of bits are loaded to the subcarrier at which there is a modulation mode demanding the minimum average incremental transmit power per incremental bit to guarantee the pre-designated bit error rate (BER) performance. The iterative procedure terminates once the transmit power threshold is reached. The performance investigations show the proposed adaptive power distribution algorithm always outperforms the scheme of equal power distribution in a large dynamic range of transmit power threshold. Moreover, the performance improvement is very large when the transmit power threshold is required to be low.     

MIMO空间复用系统的最小BER比特分配

该文基于最小误比特率(BER)准则,提出了多输入多输出(MIMO)空间复用系统的贪婪比特分配算法和基于二分法的比特分配算法。在总比特速率和每个发射天线分配相等功率的约束条件下,通过比特分配优化每个发射天线的调制方式,改善了系统的BER性能。仿真结果表明,与传统的MIMO系统相比,比特分配的MIMO系统可获得显著的信噪比(SNR)增益;与功率分配相比,比特分配在性能损失很小的情况下减少了每个发射天线的功率放大器的动态范围。     

Robust Uniform Channel Decomposition and Power Allocation for MIMO Systems with Imperfect CSI

In point to point MIMO systems, uniform channel decomposition (UCD) has been proven to be optimal in bit error rate (BER) performance and strictly capacity lossless when perfect channel state information (CSI) are assumed to be available at both the transmitter and the receiver side. However, in practice, CSI can be obtained at the transmitter if there is reciprocity between the forward and reverse channels in time division duplex (TDD) systems or can be conveyed from the receiver to the transmitter via a feedback channel. In any case, channel estimation error is inevitable. In this paper, a novel robust UCD scheme and corresponding optimal robust power allocation are proposed, which are capable of improving the BER performance in the context of imperfect CSI compared with the conventional UCD scheme and the robust precoding scheme proposed by Amir D. Dabbagh and David J. Love. Simulation results show that the MIMO channel capacity of the proposed robust UCD scheme is higher than that of the conventional UCD scheme. By deriving and analyzing the MIMO channel capacity lower bound of the robust UCD scheme, we prove that our proposed robust UCD scheme is capacity lossless in a channel estimation error existing MIMO system.     

Variable rate and variable power MQAM system based on bayesian bit error rate and channel estimation techniques

The impact of inaccurate channel state information at the transmitter for a variable rate variable power multilevel quadrature amplitude modulation (VRVP-MQAM) system over a Rayleigh flat-fading channel is investigated. A system model is proposed with rate and power adaptation based on the estimates of instantaneous signal-to-noise ratio (SNR) and bit error rate (BER). A pilot symbol assisted modulation scheme is used for SNR estimation. The BER estimator is derived using a maximum a posteriori approach and a simplified closed-form solution is obtained as a function of only the second order statistical characterization of the channel state imperfection. Based on the proposed system model, rate and power adaptation is derived for the optimization of spectral efficiency subject to an average power constraint and an instantaneous BER requirement. The performance of the VRVP-MQAM system under imperfect channel state information (CSI) is evaluated. We show that the proposed VRVP-MQAM system that employs optimal solutions based on the statistical characterization of CSI imperfection achieves a higher spectral efficiency as compared to an ideal CSI assumption based method.     

Threshold-Based Substream Selection for Closed-Loop Spatial Multiplexing

We propose a low-complexity closed-loop spatial multiplexing method with limited feedback over multi-input-multi-output (MIMO) fading channels. The transmit adaptation is simply performed by selecting transmit antennas (or substreams) by comparing their signal-to-noise ratios to a given threshold with a fixed nonadaptive constellation and fixed transmit power per substream. We analyze the performance of the proposed system by deriving closed-form expressions for spectral efficiency, average transmit power, and bit error rate (BER). Depending on practical system design constraints, the threshold is chosen to maximize the spectral efficiency (or minimize the average BER) subject to average transmit power and average BER (or spectral efficiency) constraints, respectively. We present numerical and Monte Carlo simulation results that validate our analysis. Compared to open-loop spatial multiplexing and other approaches that select the best antenna subset in spatial multiplexing, the numerical results illustrate that the proposed technique obtains significant power gains for the same BER and spectral efficiency. We also provide numerical results that show improvement over rate-adaptive orthogonal space-time block coding, which requires highly complex constellation adaptation. We analyze the impact of feedback delay using analytical and Monte Carlo approaches. The proposed approach is arguably the simplest possible adaptive spatial multiplexing system from an implementation point of view. However, our approach and analysis can be extended to other systems using multiple constellations and power levels.     

On spectral efficiency of low-complexity adaptive MIMO systems in Rayleigh fading channel

Adaptive MQAM modulation is used to maximize spectral efficiency of Multiple-Input Multiple-Output (MIMO) systems while keeping bit error rate (BER) under a target level. Closed-form expressions of the average spectral efficiency, coined as discrete-rate spectral efficiency (DRSE), are derived for adaptive modulation MIMO systems using different algorithms. To further enhance the spectral efficiency, a low complexity adaptation scheme is suggested to switch across different algorithms based on the DRSE. In the current letter, we investigate the adaptation scheme that switches between Orthogonal Space-Time Block Codes (OSTBC) and spatial multiplexing with zero-forcing (ZF) detection for MIMO systems with two transmit antennas. Two types of operating environment are considered: flat Rayleigh fading channel without spatial correlation and spatially correlated Rayleigh fading channel with transmit correlation.     

Analysis of Adaptive MIMO Transmit Beamforming Under Channel Prediction Errors Based on Incomplete Lipschitz–Hankel Integrals

This paper provides new results for a family of incomplete Lipschitz–Hankel integrals (ILHIs) which also lead to the evaluation of certain integrals involving the generalized Marcum Q function. These mathematical results are then applied to analyze the bit error rate (BER) of adaptive modulation over multiple-input–multiple-output (MIMO) fading channels under imperfect channel state information (CSI). A novel exact closed-form expression for the average BER of adaptive modulation under MIMO transmit beamforming with maximal ratio combining, assuming prediction errors at the receiver for the adaptation CSI required by the transmitter, is obtained. The benefit of this result with respect to previous analysis is threefold. First, the expression is an exact closed form. Second, it is applicable to any antenna configuration, and third, it allows a design improvement of the cutoff SNR thresholds, which leads to better system performance in terms of average spectral efficiency at no extra cost.      

Quality of Service Aware Inter Carrier Interference Mitigation and Antenna Selection Schemes for Beyond 4G Systems

Future broadband wireless communication systems demand high quality of service (QoS) for anytime anywhere multimedia applications. The standards which use orthogonal frequency division multiplexing (OFDM) coupled with multi input multi output (MIMO) are expected to rule the future wireless world. Time selective nature of the channel introduces inter carrier interference (ICI), which is the major performance limiting parameter in OFDM based systems. ICI causes loss in spectral efficiency and results in poor bit error rate (BER) performance, affecting the QoS of MIMO-OFDM systems. The conventional single input single output (SISO)-OFDM-flexible subcarrier spacing (FSS) system offers better performance than the fixed subcarrier spacing systems in terms of ICI mitigation. But BER and spectral efficiency performance of SISO-OFDM-FSS is not good enough to satisfy the requirements of future wireless broadband services. To improve the BER performance, SISO-OFDM system is replaced by space frequency block coded (SFBC)-OFDM system, which adds spatial and frequency diversity benefits to the conventional system. More number of antennas in the MIMO scheme increases the hardware cost, computational complexity and percentage of overhead. In the present study, to improve the spectral efficiency and to reduce the complexity and cost, optimal transmit antenna selection (OTAS) is combined with the SFBC-OFDM-FSS scheme. The simulation results prove that the proposed SFBC-OFDM-FSS-OTAS scheme offers better QoS than the conventional SISO-OFDM-FSS scheme.     

一种基于错误传播补偿的V—BLAST自适应比特和功率分配算法

本文探讨了V-BLAST系统在总发射功率受限和一定目标误码率要求的前提下,基于自适应调制时间块的比特、功率分配问题,分析和比较了系统在采用不同串行干扰抵消顺序时的抗信道时变性能和抗错误传播性能,找出了在BER和速率两方面性能最优的串行干扰抵消顺序。针对错误传播对系统BER性能的冲击,提出了一种新的基于错误传播补偿的V-BLAST自适应比特、功率分配方案。仿真结果表明,该方案能显著提高系统在时变信道下的BER性能,有效增大自适应调制块长度,减小反馈开销。     

高速铁路无线通信中基于正交空时码的格型正交重构算法

分析了基于正交空时码的开环和闭环MIMO系统,并着重研究了高铁场景下速度对正交空时码的影响：高速移动导致的快时变信道将会破环正交空时码的正交结构,降低由此获得的分集增益,从而引起了误码率性能的降低。提出了格型正交重构算法,通过givens变换对正交空时码进行码内正交重构;算法在恢复码内正交性的同时,也改变了发射端波束成形方向。因此,在高速移动场景下,所提算法使发射端获得了波束成形的阵列增益以及与用户静止时相同的分集增益。从系统性能仿真中看出,所提算法提升了高铁场景下基于正交空时码MIMO系统的误码性能。     

Robust power allocation algorithms for MIMO OFDM systems with imperfect CSI

This paper presents a Bayesian approach to the design of transmit prefiltering matrices in closed-loop schemes robust to channel estimation errors. The algorithms are derived for a multiple-input multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) system. Two different optimization criteria are analyzed: the minimization of the mean square error and the minimization of the bit error rate. In both cases, the transmitter design is based on the singular value decomposition (SVD) of the conditional mean of the channel response, given the channel estimate. The performance of the proposed algorithms is analyzed, and their relationship with existing algorithms is indicated. As with other previously proposed solutions, the minimum bit error rate algorithm converges to the open-loop transmission scheme for very poor CSI estimates.     

Analysis of Transmit Antenna Selection/Maximal-Ratio Combining in Rayleigh Fading Channels

In this paper, we investigate a multiple-input-multiple-output (MIMO) scheme combining transmit antenna selection and receiver maximal-ratio combining (the TAS/MRC scheme). In this scheme, a single transmit antenna, which maximizes the total received signal power at the receiver, is selected for uncoded transmission. The closed-form outage probability of the system with transmit antenna selection is presented. The bit error rate (BER) of the TAS/MRC scheme is derived for binary phase-shift keying (BPSK) in flat Rayleigh fading channels. The BER analysis demonstrates that the TAS/MRC scheme can achieve a full diversity order at high signal-to-noise ratios (SNRs), as if all the transmit antennas were used. The average SNR gain of the TAS/MRC is quantified and compared with those of uncoded receiver MRC and space-time block codes (STBCs). The analytical results are verified by simulation. It is shown that the TAS/MRC scheme outperforms some more complex space-time codes of the same spectral efficiency. The cost of the improved performance is a low-rate feedback channel. We also show that channel estimation errors based on pilot symbols have no impact on the diversity order over quasi-static fading channels.     

Performance analysis of uplink spatial multiplexing MIMO MT‐CDMA over multipath fading channels

In this paper, we consider multiple‐input multiple‐output (MIMO) multi‐tone code division multiple access (MT‐CDMA) uplink transmission over multipath fading channels. The zero‐forcing vertical Bell Laboratories layered space‐time architecture (ZF V‐BLAST) algorithm and maximum ratio combining scheme are applied at the receiver. The average bit error rate (BER) expression is derived provided that the number of receive antennas is not less than that of transmit antennas. The BER expression is verified by simulations. Numerical results show that the numbers of transmit and receive antennas have significant effects on the BER performance of the considered system. Spatial and path diversity show different capabilities to improve the BER performance. The MIMO MT‐CDMA system based on the ZF V‐BLAST algorithm is capable of achieving a better BER performance and a higher capacity than the conventional MT‐CDMA system. Copyright © 2011 John Wiley & Sons, Ltd.     

基于贪婪算法自适应比特功率分配的MIMO-OFDM系统性能

OFDM技术、MIMO技术和自适应调制技术的结合可以有效的抗无线信道的各种衰落,最大限度提高系统的容量和传输质量。借助于贪婪算法的最佳比特功率分配,本文深人研究了基于贪婪(greedy)算法自适应比特功率分配的MIMO- OFDM系统性能,对贪婪算法、等比特功率分配算法及CHOW算法下的系统误比特率(BER)进行比较,并对不同天线配置下的MIMO-OFDM系统的误比特率(BER)进行比较,推导及仿真结果表明：基于贪婪算法自适应比特功率分配的MIMO-OFDM系统BER要好于采用传统比特功率分配算法的系统BER,其频谱效率也要优于另外两种算法；同时,在采用相同的贪婪算法自适应动态比特分配策略下,MIMO-OFDM系统的BER要好于SISO-OFDM系统的BER。     

基于TR-RASK的多输入多输出UWB方案

本文提出了一种基于时间反演接收天线移位键控的多输入多输出超宽带（time reversal receive antenna shift keying multiple input multiple output ultra wide band,TR-RASK-UWB）方案,较好地解决了MIMO-UWB系统中多天线干扰和多径干扰的问题。首先,该方案利用RASK调制将发射天线阵列形成空间波束,同时使用TR技术聚焦发射信号到达目标接收天线;其次,通过最大接收功率检测算法理论推导了该方案在UWB多径信道下的误比特率（bit error rate,BER）表达式,并通过仿真得到验证;最后结果表明,在发射天线和接收天线相同的情况下,所提方案的性能相比于MIMO-UWB有较大提升。      

Differential spatial modulation scheme based on orthogonal space-time block coded

Focusing on the problem that differential spatial modulation (DSM) couldn’t obtain transmit diversity and has high decoding complexity,a new differential spatial modulation scheme based on the orthogonal space-time block code was proposed and the proposed scheme is called OSTBC-DSM.There were two matrices in this scheme:the spatial modulation matrix and the symbol matrix.The former was aimed to activate different transmit antennas by setting the position of nonzero elements,and the latter structured symbolic matrix by using orthogonal space-time block codes (OSTBC) as the basic code block.The proposed scheme could obtain full transmit diversity and higher spectral efficiency compared with the conventional DSM schemes.Moreover,the OSTBC-DSM supported linear maximum likelihood (ML) decoding.The simulation results show that under different spectral efficiencies,the proposed OSTBC-DSM scheme has better bit error rate (BER) performance than other schemes.     

Performance of variable-power adaptive MQAM with transmit antenna selection and delayed feedback in Nakagami Fading channel

In this paper, the performance of MIMO systems with variable-power (VP) adaptive MQAM and transmit antenna selection (TAS) over Nakagami-m fading channel is presented. The optimum switching thresholds for attaining maximum spectrum efficiency (SE) subject to a target bit error rate (BER) and an average power constraint are derived. It is shown that the Lagrange multiplier in the constrained SE optimization will be unique if the existence condition for MIMO system with TAS is satisfied. A practical iterative algorithm based on Newton's method for finding the Lagrange multiplier is proposed. By using the switching thresholds, closed-form expressions of the SE and average BER are obtained. Besides, BER expressions with delayed feedback are derived for VP and constant-power (CP) systems, respectively. With these expressions, the impact of feedback delay on the BER performance is effectively assessed. Computer simulation for SE and BER shows that the theoretical analysis and simulation are in good agreement. The results show that the VP adaptive modulation (AM) system with TAS provides better SE than the CP counterpart and VP AM system with space-time coding, and has slight BER performance degradation for the normalized time delay less than 0.01.     

A Differential Modulation Scheme for MIMO Links with Distributed Transmit Antennas

This letter proposes a differential en/decoding scheme for multiple-input multiple-output (MIMO) communication links with distributed transmit antennas, in which neither the transmitter nor the receiver knows channel knowledge. The proposed scheme is based on the layered differential space-time codes we present. Simulation results show that the bit error rate (BER) performance is nonidentical in different channel propagation delay cases. The optimum delay time for two transmit antennas system, for example, is about 0.6T_s (where T_s denotes symbol period) when the links achieve better performance.     

V-BLAST系统中一种简单的发射功率分配方案

采用发射功率分配方案可以有效地提高V-BLAST系统的性能。一方面,采用注水发射功率分配方案,可以有效地提高系统容量,另一方面,采用合理的发射功率分配方案,可以在固定数据速率时,改善误码率性能。本文针对第二种应用,给出了一种基于等信干噪比的发射功率分配方案,计算复杂度低,需要的反馈信令少,而且该功率分配方案与调制方式和检测算法无关。通过对MMSE串行干扰抵消和MMSE软干扰抵消检测算法仿真验证,其性能远好于不采用发射功率分配方案,同时接近基于拉格朗日极值法的功率分配方案性能。     

Energy efficiency in wireless cellular networks based on dynamic power allocation

Energy efficiency is increasingly vital for wireless cellular systems due to the limited battery resources of mobile clients. Among previous work, many studies suggest different methods to reduce the transmission power in case of a fixed condition. However, according to the mobility of users and continuous variation of cellular network environment, the traffic load of base station (BS) and the channel state information (CSI) are varying. In this paper, we proposed a dynamic power allocation schedule to assign transmit power to mobile users in the downlink, resulting in optimal energy saving. The novel power allocation scheme was based on the coordination of neighbouring cells. The results showed that the scheme dramatically reduce the average bit error rate (BER) and total tranmitted power of the system, with the improvement of network capacity.