POMDP-Based Coding Rate Adaptation for Type-I Hybrid ARQ Systems over Fading Channels with Memory

We address the issue of optimal coding rate scheduling for adaptive type-I hybrid automatic repeat request wireless systems. In this scheme, the coding rate is varied depending on channel, buffer and incoming traffic conditions. In general, we consider the hidden Markov model for both time-varying flat fading channel and bursty correlated incoming traffic. It is shown that the appropriate framework for computing the optimal coding rate allocation policies is partially observable Markov decision process (POMDP). In this framework, the optimal coding rate allocation policy maximizes the reward function, which is a weighted sum of throughput and buffer occupancy with appropriate sign. Since polynomial amount of space is needed to calculate the optimal policy even for a simple POMDP problem, we investigate maximum-likelihood, voting and Q-MDP policy heuristic approaches for the purpose of efficient and real-time solution. Our results show that three heuristics perform close to completely observable system state case if the fading and/or traffic state mixing rate is slow. On the other hand, when the channel fading is fast, Q-MDP heuristic is the most throughput-efficient among considered heuristics. Also, its performance is close to the optimal coding rate allocation policy of fully observable system state case. We also explore the performances of the proposed heuristics in the bursty correlated traffic case and show that maximum-likelihood and voting heuristics consistently outperform the non-adaptive case     

Cross-Layer Analysis of Joint Rate and Power Adaptation in Nakagami Fading Channels with Multiple-User Contention

Adaptively adjusting transmit rate and power concurrently to enhance goodput and save energy is a challenging issue in a wireless local area network (WLAN) because goodput enhancement and energy saving are usually two contradictory goals. In this paper, we propose channel-driven rate and power adaptation (CDRPA) schemes and develop a physical (PHY)/medium access control (MAC) cross-layer analytical method incorporating the impacts of Nakagami fading channel and the carrier sense multiple access (CSMA) MAC protocol. The CDRPA scheme has much lower computation complexity than the energy-optimal complete-search scheme. In a multiuser contention scenario, we analyze the energy efficiency and the goodput of the power-first and rate-first CDRPA schemes as well as the energy-optimal complete-search adaptation scheme. At the cost of lower goodput, the power-first scheme has better energy efficiency than the rate-first CDRPA scheme, whereas if the goodput is the main concern, the rate-first CDRPA scheme shall be chosen due to better goodput performance. More interestingly, we find that the power-first CDRPA scheme can achieve about the same goodput and energy efficiency as the energy-optimal complete-search link adaptation scheme.     

A Cross-Layer Design for Rateless Coding with Optimal Packet Size over Block Fading Channels

Rateless codes are popular erasure-filling codes, which are capable of providing the potentially infinite amount of redundancy at the packet level. However, the optimal packet size has not been carefully studied in a wireless communication system from both the application layer and the physical layer. In this paper, a cross-layer approach is proposed to find an optimal rateless-coded packet size in order to maximize the goodput of a wireless system delivering fixed-length messages over block-fading channels. We analyze the system performance and identify the impacts of packet size on the reliability and efficiency. Moreover, the goodput can be improved using the proposed approach by balancing reliability and efficiency. We also compare the performance of the rateless coding system with the conventional system utilizing channel codes only. A better performance of the rateless coding system is shown at relative low signal-to-noise ratios and the goodput is enhanced by about 20 % at 0 dB.     

A Receiver with Memory for Fading Channels

In this paper, an adaptive receiver with memory for fading communication channels is considered. The receiver consists of an estimator and a detector. The estimator is a finite-memory MMSE estimator with decision-feedback which minimizes the probability of error of the receiver. Asymptotic approximations are employed to derive an adaptive decision role based on the estimate. An example is presented where the receiver with memory is shown to perform better than an optimum receiver without memory.     

Symbol Error Rate of MIMO MRC Systems over Time-Varying Fading Channels

The outdated channel state information (CSI) at the transmitter and receiver, which is resulted from time variation of channels, is considered in multiple-input multiple-output systems employing maximal ratio combining. Closed-form expressions for the cumulative distribution function of output signal-to-noise ratio and symbol error rate under time-varying channels are presented to evaluate the effect of outdated CSI. The analytical results are compared with Monte Carlo simulations and a good agreement is obtained.     

Performance of BPSK Pre-detection MRC Systems over Two-Wave with Diffuse Power Fading Channels

The bit-error rate (BER) analysis for BPSK pre-detection maximal ratio combining (MRC) systems in two-wave with diffuse power (TWDP) fading is presented. In TWDP fading, the received signal is composed of two specular components in addition to the diffuse signal. The analysis shows that in TWDP fading, the BER of the MRC system is given by the weighted sum of the BERs of a number of MRC systems in Rician fading. The methodology used in the analysis is verified through simulation.     

Rice衰落信道下选择中继的性能分析与功率分配算法

本文针对无线传感器网络,研究了Rice衰落信道下基于选择中继的两跳译码转发系统的协同中继传输性能与功率分配算法.文中利用概率密度函数推导了选择中继协同传输的中断概率、误符号率的闭式表达式,并给出了高信噪比时的渐近性表达式;然后根据渐近性能进行源与中继节点间的功率优化,进一步提升系统传输性能;最后用蒙特卡洛仿真验证了结果的有效性.仿真结果显示,理论值与仿真结果基本一致,优化功率分配后的传输性能接近于搜索法得到的传输性能,但计算复杂度明显降低.     

A Blind Maximum Likelihood Carrier Frequency Offset Correction Approach for OFDM Systems over Multipath Fading Channels

In orthogonal frequency division multiplexing (OFDM) systems, carrier frequency offset (CFO) destroys the orthogonality between subcarriers and hence introduces intercarrier interference. In this paper, a maximum likelihood CFO correction (ML-CFOC) approach is proposed to compensate for the deleterious effects of CFO. The ML-CFOC method exploits the independence between the desired signals and is based on a novel simplified independent component analysis algorithm. The proposed ML-CFOC approach is successfully implemented for OFDM systems over multipath fading channels without requiring training sequences. Computer simulations are given to illustrate the optimal performance of the ML-CFOC.     

MIMO系统在慢衰落信道中的实用反馈方案

提出了一种可变码长编码方式,其应用于慢衰落信道中的基于有限反馈迫零预编码的多用户MIMO系统.该方案利用了慢衰落信道中每个配置单天线的用户连续两次从码本中选择的量化信道矢量的相关性来对码本进行有效编码,从而减少了需反馈的统计平均比特数,节省了信道带宽.在满足一定条件的慢衰落信道中,在需要反馈的比特数相同的情况下,新方案比应用基于量化的天线合并算法每用户多天线的多用户MIMO系统在信道总容量方面具有更好的性能,且该编码对应于二叉树形结构的叶子,故基站可自动识别其编码,可用于实际的通信系统之中.     

Cooperative Dual-Hop Relaying Systems with Beamforming over Nakagami-m Fading Channels

In this paper, we investigate the end-to-end performance of dual-hop relaying systems with beamforming over Nakagami-m fading channels. Our analysis considers semiblind (fixed-gain) relays with single antennas, and source and destination nodes equipped with multiple antennas. Closed-form expressions for the outage probability (OP), moment generating function (MGF), and generalized moments of the end-to-end signal-to-noise ratio (SNR) are derived. The proposed expressions apply to general operating scenarios with distinct Nakagamim fading parameters and average SNRs between the hops. The influence of the power imbalance, fading parameters, and antenna configurations on the overall system performance are analyzed and discussed through representative numerical examples. Furthermore, the exactness of our formulations is validated by means of Monte Carlo simulations.     

频率选择性信道下高速数据传输中的单载波频域均衡技术

本文利用空时联合检测的方法，提出了在频率选择性信道下多天线发送的单载波频域均衡方案，它可以提供和一个发送天线多个接收天线单载波系统相同的分集增益，和单载波时域均衡相比具有低的复杂度。分析了信道估计误差对系统的影响，类似于基于迫零算法的线性均衡器的情形，在低信噪比下，信道估计误差对系统影响很大，给出了一种简单的克服方法。在不增加发射功率的情况下，使用信道编码可以进一步改善系统的性能。本方案的系统性能在6径典型城市信道模型下进行了评估，仿真的结果证明了本方案的有效性。     

Error Rate Analysis for Coded Multicarrier Systems Over Quasi-Static Fading Channels

Several standards such as IEEE 802.11a/g, IEEE 802.16, and the European Computer Manufacturers Association (ECMA) multiband orthogonal frequency division multiplexing (MB-OFDM) for high data-rate ultra-wideband employ bit-interleaved convolutionally coded multicarrier modulation over quasi-static fading channels. Motivated by the lack of appropriate error rate analysis techniques for this popular type of system and channel model, we present two novel analytical methods for bit error rate (BER) estimation of coded multicarrier systems operating over frequency-selective quasi-static channels with nonideal interleaving. In the first method, the approximate performance of the system is calculated for each realization of the channel, which is suitable for obtaining the outage BER performance (a common performance measure for, e.g., MB-OFDM systems). The second method assumes Rayleigh distributed frequency-domain subcarrier channel gains and knowledge of their correlation matrix, and can be used to directly obtain the average BER performance. Both methods are applicable to convolutionally coded interleaved multicarrier systems employing quadrature amplitude modulation, and are also able to account for narrowband interference (modeled as a sum of tone interferers). To illustrate the application of the proposed analysis, both methods are used to study the performance of a tone-interference-impaired MB-OFDM system.     

On Optimal Power Allocation over Time-Varying Rayleigh Fading Channels for Maximum-Ratio Combining in Diversity Systems with Partial CSI

Wireless Personal Communications - This paper studies the problem of optimal power allocation (OPA) over independent but not necessarily identically distributed time-varying Rayleigh fading...     

An Adaptive Receiver with Memory for Slowly Fading Channels

We consider the problem of designing an adaptive receiver with anM-bit memory for binary orthogonal signaling over a slowly fading Rayleigh channel. Both the cases of decision-feedback and no decision-feedback are considered. We present a problem formulation by defining the contents of the receiver's memory. The structure of the Bayes receiver that makes optimum use of the memory information is then established. The LRT that defines the receiver is obtained explicitly, and it dictates a detector-estimator receiver structure. The detector can be interpreted as being partially coherent, with coherence being achieved asymptotically in the limit of perfect estimation of the fading process. Simulation results are given to show the improved error rate performance of the adaptive receiver. Our results also indicate that decision-feedback should be employed in a manner dependent on the SNR.     

Unified Analysis for SIR-Based Power and Rate Control in Multipath Fading Channels

In this paper, we present a unified analysis on various signal-to-interference power ratio (SIR)-based power and rate control schemes in independent and nonidentical multipath fading channels. We study wireless direct sequence code division multiple access (DS-CDMA) systems with RAKE reception and derive new expressions for a mobile user's average SIR, average transmission power, and average data rate in terms of the probability distribution of the SIR. The performance of SIR-based combined power/rate control, power control, rate control, truncated power control, and truncated rate control schemes in independent and nonidentical Rayleigh fading channels is presented and compared. By substituting appropriate SIR distributions, this general mathematical framework can be applied to other fading channels such as the Nakagami, Rician, and lognormal channels     

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.     

A Fuzzy MOE Receiver for Uplink MC-CDMA Systems with Carrier Frequency Offset over Multipath Fading Channels

A novel fuzzy minimum output energy (MOE) detector is proposed for uplink multicarrier CDMA (MC-CDMA) systems with carrier frequency offset (CFO) over multipath fading channels. The proposed receiver involves the following stages. First, the fuzzy CFO constrained MOE detector after coarse CFO estimation is proposed to suppress multiple access interference and combat the degradation problem of the conventional MOE detector caused by the CFO effect. Next, using the signal subspace projection technique, the proposed detector can further reduce the enhanced noise due to the fuzzy CFO constrained detector. Finally, the output data obtained from these detectors are coherently combined to offer multipath diversity gain in accordance with the maximum ratio combining criterion. Furthermore, the proposed single input single output (SISO) robust detector can be easily extended for a multiple input multiple output (MIMO) MC-CDMA system with a high rate of performance. Simulation results show that the proposed SISO detector, which offers a similar performance as the optimal detector, can provide robustness against CFO and outperform the conventional detectors. The proposed MIMO detector with spatial multiplexing gain also exhibits excellent performance.     

Statistical Models for Fading and Shadowed Fading Channels in Wireless Systems: A Pedagogical Perspective

A unified analysis of statistical models for describing fading, shadowing, and shadowed fading channels is presented from a pedagogical viewpoint. The different probability density functions such the Rayleigh, Nakagami, gamma, generalized gamma, Weibull, lognormal, Nakagami-lognormal, K distribution, generalized K distribution, and Nakagami inverse Gaussian distribution are presented and the relationships among them are detailed. These density functions are compared in terms of two quantitative measures, namely the amount of fading and outage probability. A general approach to fading and shadowed fading channels using a cluster based approach is also presented to link several of the distributions. It is expected that this overview will be very helpful to students and educators who are engaged in the study of wireless systems and the adverse impact of fading and shadowing in wireless data transmission.