On the feedback error compensation for adaptive modulation and coding scheme

In this paper, we consider the effect of feedback error on the performance of the joint adaptive modulation and diversity combining (AMDC) scheme which was previously studied with an assumption of perfect feedback channels. We quantify the performance of two joint AMDC schemes in the presence of feedback error, in terms of the average spectral efficiency, the average number of combined paths, and the average bit error rate. The benefit of feedback error compensation with adaptive combining is also quantified. Selected numerical examples are presented and discussed to illustrate the effectiveness of the proposed feedback error compensation strategy with adaptive combining. Copyright © 2011 John Wiley & Sons, Ltd.     

偏振模色散(PMD)自适应反馈补偿系统

文章报导了对2.5Gbit/s光纤通信系统的偏振模色散(PMD)快速自适应反馈补偿实验系统,该系统补偿55ps群时延所需补偿时间仅为1～2s.     

Multiantenna adaptive modulation with beamforming based on bandwidth-constrained feedback

Adaptive modulation has the potential to increase system throughput considerably by adapting transmission parameters to the time-varying channel characteristics. Crucial to adaptive systems is the requirement of a feedback channel, that is often capable of carrying only a limited number of bits. Under such a bandwidth-constrained feedback link, we aim to optimize a multiantenna system based on transmit beamforming and adaptive modulation, where the transmit power, the signal constellation, the beamforming direction, and the feedback strategy, are designed jointly. Our proposed nested iterative approach leads to an approximate, yet practical, solution. Simulation results demonstrate considerable improvement in transmission rate, as the number of feedback bits increases.     

An adaptive technique for transmit antenna diversity with feedback

Transmit antenna diversity (TAD) has been employed for third-generation wireless systems. We propose an adaptive technique for TAD with feedback. It is shown that the proposed adaptive technique can efficiently increase the number of transmit antennas without significantly increasing the feedback bit rate under ideal conditions. As a result, we can increase the number of transmit antennas and expect a performance improvement. Unfortunately, the proposed adaptive technique suffers from channel variation and error propagation due to feedback error. From this, the use of the adaptive technique shall be limited: it can be effective for indoor communications and for pedestrians with a reliable feedback channel     

Decision feedback detection with error compensation for hybrid space-time block codes

Spatial division multiplexing (SDM) techniques increase the total throughput by transmitting independent information streams through multiple transmit antennas whereas space time coding (STC) techniques utilize diversity gain. Hybrid space-time block code (STBC) schemes proposed combine the above two techniques to maximize the link performance. We propose a decision feedback detection method to improve the performance of the hybrid STBC scheme for orthogonal frequency division multiplexing (OFDM). In this scheme, we take the error propagation effect into account to enhance the detection performance. Simulation results show that the proposed method outperforms the conventional hybrid STBC detection algorithm by more than 3dB at 1% frame error rate for frequency selective fading channels.     

Blind adaptive modulation systems for wireless channels with binary feedback

In this paper, we investigate the performance of a blind adaptive modulation scheme that does not require any channel knowledge and just uses binary feedback, thereby decreasing feedback load. Retransmission of erroneous packet is not considered. In particular, we present an analytical framework for the performance evaluation of a simple wireless system in terms of receiver and transmitter structure. The system requires no knowledge of the channel and relies on a binary feedback. Slow and fast Rayleigh fading channel conditions are considered. The paper includes the derivation of the closed‐form expressions of the spectral efficiency. In some cases, closed‐form expression for packet error rate (PER) are derived. Our results show relatively high PER but some applications can still operate in a satisfactory fashion in these conditions, such as voice communication. Using coded modulation with high coding gain and increasing the number of blocks per time slot decrease the PER even more. An advantage of this system is that it uses a low complexity receiver, which sends binary feedback. Copyright © 2006 John Wiley & Sons, Ltd.     

Effect of feedback delay error on adaptive digital predistortion

The effect of feedback delay error on adaptive digital predistortion is analysed, i.e. a linearisation technique for RF power amplifiers is developed. A method for calculating achievable adjacent channel interference level with certain delay error is presented. The method and results can be utilised in estimating the required accuracy for delay compensation     

Joint clock recovery and baseband combining for the diversity radiochannel

Multipath fading is one of the major impairments encountered in terrestrial digital radio. A common countermeasure to limit the outage time due to multipath is the space diversity technique, which takes its effectiveness from the low correlation between the field samples of two well separated antennas. A very simple and effective blind receiver is proposed for the diversity radio channel. The novelty of the present study is the application of the constant modulus algorithm to joint clock recovery and baseband combining. The effectiveness of our proposal relies upon the synergic action of clock recovery and adaptive baseband combining, which allows optimal equalization of the two-ray diversity channel     

Performance analysis of joint switched diversity and adaptive modulation

In this paper, a simple and practical system based on a switched diversity scheme with adaptive modulation is presented. This system provides a reduced number of channel estimation while offering the optimum spectral efficiency given by a selection diversity system. In addition, the switching threshold is easily manipulated so as to make an efficient use of the tradeoff between spectral efficiency and channel estimation overhead. An extension of switched diversity into a multiuser scheduling is later also considered. This switch-based multiuser access scheme results in a lower average feedback load than that for the optimal selection-based multiuser scheme. Numerical results show that we can obtain a trade-off between spectral efficiency and the feedback load by choosing the switching threshold appropriately.     

Fast adaptive equalization/diversity combining for time-varyingdispersive channels

We examine adaptive equalization and diversity combining methods for fast Rayleigh-fading frequency selective channels. We assume a block adaptive receiver in which the receiver coefficients are obtained from feedforward channel estimation. For the feedforward channel estimation, we propose a novel reduced dimension channel estimation procedure, where the number of unknown parameters are reduced using a priori information of the transmit shaping filter's impulse response. Fewer unknown parameters require a shorter training sequence. We obtain least-squares, maximum-likelihood, and maximum a posteriori (MAP) estimators for the reduced dimension channel estimation problem. For symbol detection, we propose the use of a matched filtered diversity combining decision feedback equalizer (DFE) instead of a straightforward diversity combining DFE. The matched filter form has lower computational complexity and provides a well-conditioned matrix inversion. To cope with fast time-varying channels, we introduce a new DFE coefficient computation algorithm which is obtained by incorporating the channel variation during the decision delay into the minimum mean square error (MMSE) criterion. We refer to this as the non-Toeplitz DFE (NT-DFE). We also show the feasibility of a suboptimal receiver which has a lower complexity than a recursive least squares adaptation, with performance close to the optimal NT-DFE     

Cross-layer combining of power control and adaptive modulation with truncated ARQ for cognitive radios

To maximize throughput and to satisfy users＇ requirements in cognitive radios, a cross-layer optimization problem combining adaptive modulation and power control at the physical layer and truncated automatic repeat request at the medium access control layer is proposed. Simulation results show the combination of power control, adaptive modulation, and truncated automatic repeat request can regulate transmitter powers and increase the total throughput effectively.     

SR ARQ for adaptive modulation systems combined with selection transmit diversity

Adaptive automatic repeat request (ARQ) schemes are quite effective for throughput enhancement in time-varying mobile channel environments. In this paper, both throughput and packet error rate are analyzed for a selective-repeat ARQ scheme based on a constant-power variable-rate adaptive M-QAM system combined with selection transmit diversity over multiple-input multiple-output Markovian-Nakagami channels. In this analysis, the impact of using outdated and/or imperfect channel state information on the performance of the system is considered.     

偏振模色散补偿系统中自适应反馈控制技术的研究

偏振模色散（PMD）已成为发展下一代高速长距离光纤传输系统的主要制约因素，本文中分析了分析了PMD补偿系统中几种自适应反馈控制技术，并详细比较，讨论了各自的特点，认为偏振度测量法是一种首选技术。     

Step-out detection and error compensation for a micro-stepper motor using current feedback

We propose a method to detect step-outs for a micro-stepper motor and a way to compensate the missing steps to maintain appropriate operation. Coil currents and their behaviors are investigated to determine the onset and termination of a step-out. Exact step-out values are identified, and a corresponding compensation scheme is proposed. Even though the use of an encoder to measure position error provides a means of compensating for the error, the primary cause of the error due to the step-outs may not be clearly identified. Rather, the coil current feedback is used in place of encoder measurements, we can clearly determine amount of the step-outs which results in the position error compensation.     

On the impact of SNR estimation error on adaptive modulation

In this letter we investigate the impact of signal-to-noise ratio (SNR) estimation on the performance of adaptive modulation. The adaptive modulation technique uses an estimate of the SNR at the receiver to choose a modulation scheme so that a given performance target is met. By adapting the modulation scheme, the link is able to improve the spectral efficiency as compared to static modulation. An analytic expression for the impact of SNR estimation error is derived and it is shown that SNR estimation has minimal impact on the performance.     

Symbol error probability of low-order orthogonal signalings in Rayleigh fading with general diversity combining

New analytical expressions for the symbol error probability of 3-ary and 4-ary orthogonal signaling and 6-ary and 8-ary biorthogonal signaling with general diversity combining in Rayleigh fading are derived.     

Joint LLR-CRC error mitigation for two-way adaptive denoise-and-forward network coding

Network Coding (NC) brings correlation between the coded signals from different sources, which makes the system more vulnerable to the decode error at relay. Conventional Cyclic Redundancy Code (CRC) has been implemented for error bit detection. However, its error correction is simply ignored. To fully exploit this feature, this paper proposes a novel joint Log-Likelihood Ratio (LLR) CRC error mitigation for NC two way relay channel. Specific thresholds are designed to estimate the error number of data block and identify those which can be recovered if the number is within the error correction scope of CRC. We examine two modes of the thresholds, one based on the average Bit Error Rate (BER) of source-relay link, while the other based on that of instantaneous one. We provide the full analysis for the Pair-wise Error Probability (PEP) performance of the scheme. A variety of numerical results are presented to reveal the superiority of the proposed scheme to conventional CRC NC under independent Rayleigh fading channels. Moreover, the efficiencies of the proposed thresholds are also validated.     

Joint MF combining and turbo equalization for wireless communications over ISI channels with diversity reception

A new method called joint Matched Filter (MF) combining and turbo equalization is proposed for wireless communications over Inter-Symbol Interference (ISI) channels with diversity reception. This method takes diversity combining and equalization as integrity and need just one turbo equalizer for all diversity branches. Computer simulations prove that our method can take advantage of turbo equalization and diversity reception to combat fading of wireless channels.     

A Laguerre polynomial-based bound on the symbol error probability for adaptive antennas with optimum combining

We derive a simple closed-form upper bound on the symbol error probability for coherent detection of M-ary phase-shift keying using antenna arrays with optimum combining, in the presence of multiple uncorrelated equal-power cochannel interferers and thermal noise in a Rayleigh fading environment. The new bound, based on Laguerre polynomials, is valid for an arbitrary number of antenna elements as well as arbitrary number of interferers, and it is proven to be asymptotically tight. Comparisons with Monte Carlo simulation are also provided, showing that our bound is useful in many cases of interest.     

多误码率固定速率OFDM自适应调制比特功率分配方法

一些应用中需要多误码率的固定速率OFDM自适应调制.本文首先对该多载波自适应调制需求中的比特功率优化分配问题建模,并进一步限定各OFDM子信道采用正交幅度调制(QAM),基于子信道调制比特的任意正实数和无限粒度假设详细分析了子信道集合划分和比特功率分配问题,由此得到一种实用的次最优子信道集合划分和比特功率分配方法.分析和仿真表明由于可得到比特分配的闭式表达,因此该方法计算复杂度低,且性能良好,是一种实用的多误码率固定速率OFDM自适应调制比特功率优化分配方法.