Packet combining with adaptive retransmission control in DS-CDMA random access networks

Packet combining with adaptive retransmission control (ARC) in DS-CDMA random access networks has been investigated. The proposed system uses diversity packet combining while adaptively adjusting the retransmission probability of backlog users to optimize packet retransmissions. A soft decision DS-CDMA analysis is used to derive the optimum channel input for ARC. Simulation results show that packet combining with ARC can significantly increase the throughput and maintain the performance very close to its optimum in the high traffic region.     

Joint frequency-domain differential detection and equalization for DS-CDMA signal transmissions in a frequency-selective fading channel

It has been revealed that direct sequence code-division multiple access (DS-CDMA) can achieve a good bit-error rate (BER) performance, comparable to multicarrier CDMA (MC-CDMA), by using coherent frequency-domain equalization (FDE) instead of coherent Rake combining. However, coherent FDE requires accurate channel estimation. Pilot-assisted channel estimation is a practical solution, but its accuracy is sensitive to the Doppler spread. In this paper, a frequency-domain differential encoding and detection scheme is proposed for a DS-CDMA mobile radio. Joint frequency-domain differential detection and equalization (FDDDE) based on minimum mean-square error (MMSE) criterion is presented, where a simple decision feedback filter is used to provide a reliable reference signal for MMSE-FDDDE. Also presented is an approximate BER analysis. It is confirmed by both approximate BER analysis and computer simulation that MMSE-FDDDE provides good BER performance close to the coherent MMSE-FDE and shows high robustness against the Doppler spread; it outperforms coherent MMSE-FDE for large Doppler spreads. The proposed MMSE-FDDDE can also be applied to MC-CDMA. A performance comparison between uncoded DS- and MC-CDMA shows that DS-CDMA with MMSE-FDDDE achieves better BER performance than MC-CDMA with MMSE-FDDDE for small spreading factors.     

Maximum diversity in single-carrier frequency-domain equalization

In broadband wireless communications, multipath propagation often results in an overall channel with a long impulse response that could span tens or even hundreds of symbol intervals. To equalize such long channels, conventional single-carrier time-domain equalization becomes infeasible due to high computational complexity. Relying on the use of fast Fourier transform, single-carrier frequency-domain equalization (SC-FDE) offers low-complexity equalization as well as other desirable features. In this correspondence, we prove that SC-FDE is also capable of collecting full multipath diversity even without channel coding. As far as we are aware, this is the first analytical proof of the diversity gain of SC-FDE. This conclusion justifies those existing simulation results regarding the performance comparison between SC-FDE and orthogonal frequency-division multiplexing (OFDM), and offers important guidelines for further improving SC-FDE and OFDM     

Single-carrier frequency-domain equalization with decision-feedback processing for time-reversal space-time block-coded systems

In this letter, we propose to introduce frequency-domain equalization with decision-feedback processing (FD-DFE) for time-reversal space-time block-coded (TR-STBC) systems with multiple transmit antennas to effectively achieve both spatial diversity and multipath diversity over frequency-selective channels. This letter will also present a theoretical performance analysis result that relates the minimum mean-squared error (MMSE) to a new tight upper bound for the probability of error. It is shown that the new bound is not only valid for the TR-STBC systems, but also valid for single-input single-output, single-input multiple-output, and multiple-input multiple-output systems with MMSE equalization.     

少模光纤通信系统中的自适应频域均衡算法

少模光纤模式复用存在模式耦合和差分模式时延,必须通过自适应均衡算法补偿。为了降低长距离少模光纤通信系统中自适应均衡算法的复杂度,采用基于变步长-频域块最小均方算法的多输入多输出均衡器对2×2模分复用系统解复用。利用频域块最小均方自适应算法修正均衡器权系数,并通过变步长函数调整步长因子,兼顾算法收敛速度和收敛性能。算法可通过快速傅里叶变换降低计算复杂度。在112Gbit/s的1000km少模光纤高速通信仿真系统中,保证相同收敛速度情况下,提高信号Q2因子3.7dB,并在可编程现场门阵列上验证了100km少模光纤通信系统时的算法性能。结果表明,该算法能够实现模分复用系统的信号解复用,达到快速收敛、低稳态失调的目的。     

频域均衡的单载波传输方案研究

无线信道的信道干扰主要表现为多径衰落和多普勒衰落对信道的影响。我们可以采用均衡技术来补偿信道中由于多径效应产生的码间干扰（ISI）。现引入频域均衡的单栽波传输（SC／FDE）方案，探讨了其实现机理，并对采用SC／FDE的高速率无线通讯系统实例进行系统整体建模和仿真计算。     

发射分集OFDM系统中子载波频域均衡接收机设计

针对发射分集下的OFDM系统,提出了一种子载波频域均衡接收机设计方法。在保持子载波数恒定的条件下减少循环前缀的长度,并推导出最优分集合并系数。仿真结果表明,在发射分集条件下,本文提出的子载波频域均衡器有效地抑制了OFDM系统中由于循环前缀小于信道最大时延扩展所造成干扰和各个信道信号之间的互干扰,获得了较好的系统性能。     

Performance enhancement of the DSRC system using frequency-domain equalization for 5.9 GHz DSRC applications

This paper proposes the use of equalization concepts in frequency domain that exploit the frequency domain channel matrix to combat inter-carrier interference (ICI) instead of inter-symbol interference (ISI) in 5.9 GHz Dedicated Short Range Communications (DSRC) systems. The physical layer (PHY) of DSRC is currently being developed by work group of IEEE 802.11p. The conventional system currently assumes static channel characteristics. Channel tracking schemes were investigated and the Viterbi-aided channel estimation scheme was proposed for DSRC systems that did not explicitly exploit the ICI components caused by the time-varying channels. The performance of the DSRC system is investigated for a time-varying channel using a conventional DSRC model, a decision-directed (Viterbi-aided) channel estimation model, and the frequency-domain equalization design. It is shown that the DSRC system with the frequency-domain equalization achieves a considerable performance enhancement compared to both the conventional and the Viterbi-aided channel estimation schemes in terms of both packet error rate (PER) and bit error rate (BER) at relatively high and low velocities.     

Adaptive frequency-domain equalization and diversity combining forbroadband wireless communications

We introduce a new kind of adaptive equalizer that operates in the spatial-frequency domain and uses either least mean square (LMS) or recursive least squares (RLS) adaptive processing. We simulate the equalizer's performance in an 8-Mb/s quaternary phase-shift keying (QPSK) link over a frequency-selective Rayleigh fading multipath channel with ~3 μs RMS delay spread, corresponding to 60 symbols of dispersion. With the RLS algorithm and two diversity branches, our results show rapid convergence and channel tracking for a range of mobile speeds (up to ~100 mi/h). With a mobile speed of 40 mi/h, for example, the equalizer achieves an average bit error rate (BER) of 10 ^-4 at a signal-to-noise ratio (SNR) of 15 dB, falling short of optimum linear receiver performance by about 4 dB. Moreover, it requires only ~50 complex operations per detected bit, i.e., ~400 M operations per second, which is close to achievable with state-of-the-art digital signal processing technology. An equivalent time-domain equalizer, if it converged at all, would require orders-of-magnitude more processing     

EFFICIENT CYCLIC PREFIX RECONSTRUCTION USING DECISION FEEDBACK FILTER FOR CODED SINGLE-CARRIER SYSTEMS WITH FREQUENCY-DOMAIN EQUALIZATION （SC-FDE）

In this paper, an efficient Cyclic Prefix （CP） reconstruction scheme is proposed for Single-Carrier systems with Frequency-Domain Equalization （SC-FDE） that employ insufficient length of CP at the transmitter. By utilizing a decision feedback filter to cancel the residual InterSymbol Interference （ISI） in the equalized signal, the proposed scheme can effectively lower the low bound of performance for the CP reconstruction schemes and can greatly improve the Bit Error P~te （BER） performance of SC-FDE systems. In addition, the existing methods and the proposed scheme are also optimized. It is shown in the simulation results that, when the Signal-to-Noise Ratio （SNR） exceeds a certain threshold, the proposed scheme can achieve the low bound of performance for the existing methods. Moreover, by increasing the number of iteration or through optimization, the low bound can be outperformed.     

Blind equalization for broadband access

This article discusses the general principles of blind equalization and its use in emerging broadband access applications such as FTTC and xDSL. New results obtained for these applications are also presented     

Single-carrier frequency-domain equalization for space-timeblock-coded transmissions over frequency-selective fading channels

We propose an Alamouti-like scheme for combining space-time block-coding with single-carrier frequency-domain equalization. With two transmit antennas, the scheme is shown to achieve significant diversity gains at low complexity over frequency-selective fading channels     

On frequency-domain equalization and diversity combining for broadband wireless communications

This paper is concerned with the use of frequency-domain equalization (FDE) and space diversity within block transmission schemes for broadband wireless communications. The expected performance with both multicarrier (MC) and single-carrier (SC) modulations is emphasized, when a cyclic prefix, long enough to cope with the maximum relative channel delay, is appended to each transmitted block. A set of numerical results is presented and discussed, with the help of appropriate, analytical performance bounds which are conditional on a given channel realization. These bounds are used to explain the performance advantage of the SC/FDE option, the benefits of space diversity, and the impact of the criterion for computing the FDE parameters.     

Iterative frequency-domain channel estimation and equalization for single-carrier transmissions without cyclic-prefix

Compared to conventional time-domain equalization, frequency-domain equalization (FDE) presents a computationally efficient alternative for the reception of single carrier (SC) transmissions. In this paper, we consider iterative FDE (IFDE) with explicit frequency-domain channel estimation (FDCE) for non-cyclic-prefixed SC systems. First, an improved IFDE algorithm is presented based on soft iterative interferencecancellation. Second, a new adaptive FDCE (AFDCE) algorithm based on per-tone Kalman filtering is proposed to track and predict the frequency-domain channel coefficients. The AFDCE algorithm employs across-tone noise reduction, exploits temporal correlation between successive blocks, and adaptively updates the auto-regressive model coefficients, bypassing the need for prior knowledge of channel statistics. Finally, block-overlapping is used to facilitate the joint operation of IFDE and AFDCE. Simulation results show that, compared to related IFDE and adaptive channel estimation schemes, the proposed schemes offer lower mean-square error (MSE) in channel prediction, lower bit error rate (BER) after decoding, and robustness to non-stationary channels.     

A blind equalization with the sign algorithm for broadband access

This paper presents an optimal reduced constellation point of sign reduced constellation algorithm (SRCA) for square and nonsquare carrierless amplitude and phase (CAP)/QAM signal constellations. Convergence characteristics of the SRCA algorithm are analyzed and compared to those of the RCA algorithm     

Wide-band digital subscriber access with multidimensional blockmodulation and decision-feedback equalization

The author investigates the potential transmission performance of pair-wire subscriber lines at the higher rate of 800 kb/s, with particular reference to digital subscriber line transmission for ISDN (integrated services digital network) basic access. Block modulation schemes of 1-4 dimensions, at a rate of 2 bits per dimension, are considered. Time-division multiplexing is used to combine the multiple dimensions for transmission over a single-waveform channel, namely, the subscriber line. The channel noise is assumed to be additive and dominated by near-end crosstalk. MMSE (minimum mean-squared error) decision-feedback equalization is used to deal with the noise and the intersymbol interference. Using the theory developed, the potential performance of some simple lines is calculated. The coding gain of a multidimensional modulation scheme is found to be fully preserved after transmission if the equalizer is infinite in length. However, the gain realized can be much lower, or none at all, if the equalizer is only moderate in length. This latter phenomenon is due to the fact that the noise at the decision point is coloured, due to the inability of the equalizer to whiten it sufficiently     

Packet labeling technique using electronic code-division multiple-access for WDM packet-based access networks

We propose and experimentally demonstrate the feasibility of a packet labeling technique using electronic code-division multiple-access for a wavelength-division-multiplexing (WDM) packet-based access network, whereby each wavelength is assigned a unique electronic code-based label on a radio-frequency subcarrier. Such a technique allows individual wavelength channels to be electronically identified without requiring the use of a WDM demultiplexer. We experimentally demonstrate this technique with two WDM channels each with 1.25-Gb/s payload data and 10-Mb/s header coded onto an electronic code at 160 Mb/s. The experimental results and theoretical analysis show that this technique has the potential to support large numbers of WDM channels.     

双选择性信道条件下MIMO-OFDM系统中的ICI抑制及频域Turbo均衡

分析了双选择性信道条件下MIMO-OFDM系统的ICI产生原理,提出了一种以最大化缩短信干噪比(MSSINR,maximum shortening the signal to inter-carrier interference plus noise ratio)为目标的频域ICI抑制算法及其简化算法;在此基础上将Turbo均衡技术应用到MIMO-OFDM系统中,提出了一种基于MSSINR频域ICI抑制的频域Turbo均衡(FTE,frequency-domain Turbo equalization)算法,并进行了数值仿真和比较分析。     

Packet access mechanisms for cellular radio

The paper reviews the findings of work completed in RACE Mobile Telecommunications Project 1043 on the suitability of packet access mechanisms for mixed services in the Universal Mobile Telecommunication System. A first order comparison of random and scheduled access is provided and conditions under which these mechanisms provide an attractive alternative to channel allocation for call duration are specified. It is concluded that packet reservation multiple access (PRMA) is an attractive candidate for mixed services over a range of cell sizes as it can accommodate fixed channel access, reserved channel access and random channel access     

Performance analysis of DS-CDMA with slotted ALOHA random access for packet PCNs

In this paper, we present a discrete time Markov chain based analytical framework for the study of Direct-Sequence Code-Division-Multiple-Access (DS-CDMA) with slotted ALOHA random access protocols (DS-CDMA-S-ALOHA) for packet Personal Communications Networks (PCNs). It incorporates both the random access and the random errors associated with DS-CDMA-S-ALOHA protocols into a unified framework. The key feature is that it distinguishes between the two stages in the transmission process, namely the access stage and the reception stage, which characterize the random access and the random errors associated with DS-CDMA-S-ALOHA protocols respectively. Two DS-CDMA-S-ALOHA protocols are presented and analyzed. The performance of the protocols and the effects of the design parameters, namely the packet retransmission probability and the forward error correction code rate of the Bose-Chaudhuri-Hocquenghem (BCH) block codes are evaluated numerically and compared with a bandwidth equivalent conventional multi-channel slotted ALOHA system. The results show that, by proper design, the DS-CDMA-S-ALOHA protocols can double the throughput with respect to that of a bandwidth equivalent conventional multi-channel slotted ALOHA system.