非理想信道估计下的放大转发协同通信系统性能分析

针对两跳多中继放大转发协同通信系统,分析了信道估计误差对系统性能的影响。考虑机会中继和全参与中继两种中继策略,分别推导了系统误码率的闭式表达式,并分析了不同信道估计质量阶数下的系统分集增益。平均信噪比损失率分析表明机会中继较全参与中继对信道估计误差更为敏感。仿真结果验证了理论分析。     

Optimizing antenna configuration for MIMO systems with imperfect channel estimation

We study the optimal antenna configuration (i.e. number of transmit and receive antennas) for multiple-input multiple-output systems in pilot-symbol-assisted modulation schemes with imperfect channel estimation. We assume block flat-fading channels and focus on a practical range of high signal-to-noise ratio. An ergodic capacity lower bound is used as the objective function to be maximized. We analytically study the capacity gain from adding extra antennas to the transmitter or to the receiver in two different scenarios. Our numerical results show that the optimal antenna configuration under imperfect channel estimation can be significantly different from that under perfect channel estimation assumption. In addition, we investigate the capacity gain from optimizing antenna configuration and find that the gain can be larger than that achieved by optimizing transmit power over pilot and data symbols, particularly for large block lengths.     

EXIT chart analysis of BICM-ID with imperfect channel state information

We analyze the convergence of iteratively decoded bit-interleaved coded modulation with imperfect channel state information using the extrinsic information transfer (EXIT) chart. A canonical analysis model is adopted, where the power correlation coefficient between the fading and its estimate becomes the key parameter affecting the extrinsic transfer characteristics of the demapper and hence the convergence of iterative decoding. We further illustrate that decoding convergence can be triggered by tradeoff between the quality of channel estimation and code rate.     

Capacity analysis in CDMA distributed antenna systems

In this letter, the effect of maximal ratio combining (MRC)-based macrodiversity on the reverse-link and forward-link capacity in code division multiple access (CDMA)-distributed antenna systems is analyzed. The concept of virtual cell is illustrated, and the analytical outage probability expressions are derived. The present investigation shows that on the reverse link, the interference can be suppressed greatly with macrodiversity, which leads to a significant increase in capacity. However, on the forward-link, it is proven that if simulcasting is used in CDMA-distributed antenna systems, the forward-link capacity cannot increase with macrodiversity whatever power allocation scheme is adopted. Based on the analysis of the cause of capacity loss, a new transmission scheme is further presented and the optimal power allocation scheme is derived. It is shown that, in this case, the forward-link capacity increases rapidly with the number of involved distributed antennas.     

分布式天线系统中的下行容量分析

针对分布式天线系统(DAS)的下行容量问题展开研究。首先建立了包含路径损耗、阴影衰落和瑞利衰落的复合信道模型;然后,基于该信道模型,分别对各态历经容量和中断容量进行了分析。考虑到小区内移动台的分布特点,在选择分集传输策略下,推导了各态历经容量和中断容量的近似解析表达式。仿真结果表明,所推导的2个近似解析表达式均能准确地评估分布式天线系统的系统性能。     

Performance analysis of maximum ratio transmission with imperfect channel estimation

Maximal ratio transmission (MRT) is designed assuming the availability of perfect channel state information (CSI) at both the transmitter and the receiver. However, perfect CSI is not available in practice. This paper investigates the impact of Gaussian estimation errors on the MRT performance in independently and identically distributed (i.i.d.) Rayleigh fading channels. We derive the cumulative distribution function (cdf), the probability density function (pdf) and the moment generating function (mgf) of the MRT output signal-to-noise ratio (SNR) with imperfect CSI, enabling the evaluation of some useful performance metrics such as the average error rate and the outage performance. Numerical and simulation results are provided to show the impact of imperfect CSI on the MRT performance.     

部分信道状态信息下多天线系统的最优发送

在实际的单用户多天线平坦衰落通信系统中,接收端往往具有理想的信道状态信息,而发送端只有来自接收端的部分信道状态信息反馈,因此在发送端信道模型假设为复高斯随机矩阵.在发射端具有信道协反差反馈或者均值反馈的情形下,对达到最大的信道容量即信息论角度的最优化问题进行了理论分析,研究了系统的最优发送方案.对目前的关于单方向发射的最优条件进行扩展,进一步推导了沿任意多个方向发送达到信道容量的条件.数值结果验证了分析结论.     

Antenna location design for generalized distributed antenna systems

A squared distance criterion for antenna location design in generalized distributed antenna systems (GDAS) is proposed to maximize the cell averaged ergodic capacity. The criterion requires the antenna port locations minimize the expectation of the squared distance between a randomly distributed user and the nearest antenna port. This is equivalent to codebook design in vector quantization. For uniform user distribution, we can easily derive analytical expressions for antenna locations. For more general user distribution, we can obtain numerical results using the codebook design algorithm. Applying the proposed criterion to circular-layout GDAS with uniform user distribution and linear cell with non-uniform user distribution, we achieve near optimal performance.     

Performance analysis of multi-antenna relay networks with imperfect channel estimation

In this paper, we present the performance of multi-antenna selective combining decode-and-forward (SC-DF) relay networks over independent but non-identical Nakagami-m fading channels with imperfect channel estimation. The outage probability, moment generating function (MGF) and symbol error probability (SEP) will be derived in closed-form using the SNR statistical characteristics. To make the analysis trackable, we have derived the MGF and SEP for integer values of fading severity, m. Also, to make the relations more simple, we develop high signal to noise ratio (SNR) analysis for the performance metrics of our system. Subsequently, we propose optimal and adaptive power allocation algorithms along with the equal power allocation method. Finally, for comparison with analytical formulas, we perform some Monte-Carlo simulations.     

Power-allocation policy and optimized design of multiple-antenna systems with imperfect channel estimation

We here focus on the optimized design and performance of Rayleigh-faded multiple-antenna multiple-input-multiple-output systems when both transmitter and receiver share imperfect (i.e., error impaired) channel estimates computed via training sequences. In particular, according to the emerging principle of the so-called synchronized detection, for this operating scenario we propose a simple water-filling (WF)-like strategy for allocating power over transmit antennas and test its actual performance. Afterward, we introduce and evaluate some related figures of merit summarizing the overall system's performance and use these last for noting effective system's design guidelines. In particular, some optimized solutions for power-versus-bandwidth efficiency tradeoff are presented and their validity limits are debated. All the developed results explicitly take into account the actual reliability of the achievable channel estimates.     

基于不完美信道估计的闭环MIMO-MRC跨层设计

为提高无线通信的频谱效率,提出一种MIMO-MRC(multiple-input multiple-output-maximal ratio combining)跨层方案。是物理层的自适应调制(adaptive modulation,AM)和数据链路层的自动重传(automatic repeat request,ARQ)协作,发射端利用估计信道信息反馈,自适应调节调制模式,选择最优发射权矢量和自动重传发射数据。分析了估计误差对MIMO-MRC跨层系统的影响,给出了MIMO-MRC系统在信道估计存在误差时的频谱效率和中断概率的闭合表达式。通过仿真实验证明,对比SISO(single-input single-output)跨层系统和A lamouti's跨层系统,MIMO-MRC跨层系统的性能有明显提高,可获得约3 d B的分集增益。     

Adaptive transmitted distributed antenna selection strategy in distributed antenna systems with limited feedback beamforming

An adaptive transmitted distributed antenna (DA) selection strategy in distributed antenna systems (DAS) with limited feedback (LFB) beamforming is proposed. By maximising the upper bound of ergodic capacity in DAS with LFB beamforming, the proposed strategy can adaptively adjust the number of transmitted DAs. Simulation results show that the proposed strategy achieves greater ergodic capacity than the strategy with a fixed number of transmitted DAs.     

Performance analysis for transmit antenna diversity with/withoutchannel information

Transmit antenna diversity (TAD) for the downlink channel has been investigated to improve the performance of wireless communications using multiple transmit antennas. We present a performance analysis for TAD with/without channel information. For performance analysis, we use the pairwise error probability and cutoff rate. For the closed-loop TAD in frequency-division duplex (FDD) mode, the downlink channel information should be transmitted to the base station from the mobile station. The feedback channel information may be imperfect because of the feedback delay, quantization error, and feedback error. The impact of imperfect channel information has been considered to analyze the performance of the TAD. We also present a performance analysis for a TAD technique that does not use the channel information, called space-time transmit diversity (STTD). Looking at the cutoff rates for the TAD techniques, the trellis-coded modulation (TCM) has been considered for better performance. Simulation results are shown to see the impact of the TCM on TAD systems     

Leakage rate analysis with imperfect channel state information for cooperative nonorthogonal multiple access networks

This paper investigates the imperfect channel state information that is caused by the channel estimation error and feedback delay effects on the leakage rate analysis for the cooperative nonorthogonal multiple access networks. The investigation considers a dual hop one‐/two‐way nonorthogonal multiple access‐based information exchange process with the aid of half‐/full‐duplex untrustworthy wireless relaying network for the leakage rate analysis. The channel estimation error causes system coding gain losses while the feedback delay does not have any effect on the users' outage performance at untrustworthy relay terminal in low signal‐to‐noise ratio regimes. Conversely, the channel estimation error effects become negligible while the feedback delay causes system coding gain losses on the users' outage performance at untrustworthy relay terminal in high signal‐to‐noise ratio. Results also reveal that the untrustworthy relay terminal, which is under the effect of the channel estimation error and feedback delay, is being active between and ‐  dB. Beyond ‐  dB, the untrustworthy relay terminal becomes out of order and saturates. The Monte Carlo–based simulation results are in agreement with the analytical and asymptotic derivations.     

Performance analysis of STT-RAM with cross shaped free layer using Heusler alloys

We have investigated the performance of a spin transfer torque random access memory (STT-RAM) cell with a cross shaped Heusler compound based free layer using micromagnetic simulations. We have designed a free layer using a Cobalt based Heusler compound. Simulation results clearly show that the switching time from one state to the other state has been reduced, also it has been found that the critical switching current density (to switch the magnetization of the free layer of the STT RAM cell) is reduced.     

16-QAM BICM-ID in fading channels with imperfect channel state information

We analyze the impact of imperfect channel state information (CSI) on the performance of bit-interleaved coded modulation with iterative decoding (BICM-ID) over fading channels. We develop a general, accurate and efficient theoretical error-free feedback bound (EF bound) to analyze the asymptotic bit-error rate (BER) of BICM-ID with imperfect CSI, and predict the BER floor due to channel estimation error. The convergence to the EF bound and the accuracy of the BER floor prediction are verified by simulation with various sets of code and channel parameters. These results are canonical, in that they apply to a variety of system configurations. Pilot symbol assisted modulation is used as a particular example for Rayleigh-fading channels.     

分布式MIMO系统小区平均遍历容量分析

针对分布式MIMO系统的圆形小区平均遍历容量展开研究.文章首先建立了包含快衰落、阴影衰落和路径损耗的复合衰落信道模型;然后,对分布式天线采用覆盖式（BT）传输策略,并在高信噪比条件下,导出给定移动台位置时,区上、下行点对点链路遍历容量表达式.最后,考虑移动台在小区内任意分布特点,进一步推导出小区平均遍历容量闭合近似表达式.仿真结果表明,所推导的近似表达式可很好的反应系统的实际性能.导的近似表达式可很好的反应系统的实际性能.     

On the information throughput and optimized power allocation for MIMO wireless systems with imperfect channel estimation

In this paper, we focus on the throughput analysis, outage evaluation and optimized power allocation for Multiple-Input Multiple-Output (MIMO) pilot-based wireless systems subject to short-term constraints on the radiated power and equipped with a feedback-path for communicating back to the transmitter the imperfect MIMO channel estimates available at the receiver. The case of the ergodic throughput for Gaussian distributed input signals is analyzed, and the conditions for the (asymptotical) achievement of the Shannon capacity are pointed out. The main contributions of this work may be so summarized. First, we develop closed-form analytical expressions for the computation of the ergodic information throughput conveyed by the considered MIMO system for the case of ideal feedback link. Second, we present an iterative algorithm for the optimized power allocation over the transmit antennas that explicitly accounts for the imperfect MIMO channel estimates available at the receiver. Third, after relaxing the assumption of ideal feedback link, we test the sensitivity of the proposed power allocation algorithm on errors possibly introduced by the feedback channel, and then, we numerically evaluate the resulting throughput loss. Finally, we develop closed-form upper and lower bounds on the outage probability that are asymptotically tight.     

Capacity analysis of distributed antenna systems in MIMO Nakagami fading multicell environment

In this paper, the downlink capacity analysis of distributed antenna systems (DAS) with multiple receive antennas in Nakagami-m fading multicell environment is presented. According to the performance analysis, and using mathematical manipulation, we derive the probability density function (PDF) of the effective signal to interference plus noise ratio of the system. Based on the obtained PDF, an exact closed-form expression of DAS capacity is derived. This expression includes the existing capacity expression as a special case, and avoids the error in the latter. Thus, it can provide good theoretical performance evaluation for downlink DAS. Computer simulation shows that the theoretical analysis is in good agreement with simulation result. Moreover, the increase of receive antenna and/or Nakagami fading parameter m can improve the capacity effectively.     

Performance analysis of smart antenna test-bed operating in awide-band CDMA channel

In this paper, we present a performance analysis of a smart antenna system operating in a wide-band CDMA wireless-local-loop channel using a beam-forming module (BFM) that has been implemented on a digital signal processor (TMS320C6701) board. We first show the results of computer simulations obtained from the modeled received (RX) signals through a test-bed system consisting solely of baseband signal processing parts, i.e., modeled RX data-generating PC, BFM for computing the optimal weight vector and interfacing module. A test-bed system of the entire base station is then implemented to evaluate the adaptive beam-forming function with the actual wireless signals. This test-bed system includes several subscribers, as well as the array antenna, RF modules, and other receiving parts required at the cell site