Digital Transmission Performance on Fading Dispersive Diversity Channels

The reception and detection of a single digit under known channel conditions are investigated. The probability of error for an optimum one-shot receiver instantaneously matched to the channel state is averaged over an ensemble of dispersive diversity channels. The average probability of error as a function of energy to noise ratio is found to be solely dependent on the ratio of rms dispersion width to data symbol width. For these dispersive channels an implicit diversity effect is qualitatively explained in terms of eigenvalues that depend on the ensemble statistic. The one-shot receiver performance provides a bound for practical receivers. In a comparison with a decision feedback equalizer, it is shown that on moderately dispersive channels the equalizer nearly achieves optimum one-shot performance. Since an adaptive version of this equalizer exists, this means data transmission on slowly fading channels is possible at rates above the natural rate suggested by the channel dispersion spread without bandwidth expansion and with small intersymbol interference penalty. The use of one-shot receiver performance curves can also be used as estimates of equalizer performance in situations where computation of the latter is impractical.     

Optimum space-time processors with dispersive interference: unifiedanalysis and required filter span

We consider optimum space-time equalizers with unknown dispersive interference, consisting of a linear equalizer that both spatially and temporally whitens the interference and noise, followed by a decision-feedback equalizer or maximum-likelihood sequence estimator. We first present a unified analysis of the optimum space-time equalizer, and then show that, for typical fading channels with a given signal-to-noise ratio (SNR), near-optimum performance can be achieved with a finite-length equalizer. Expressions are given for the required filter span as a function of the dispersion length, number of cochannel interferers, number of antennas, and SNR, which are useful in the design of practical near-optimum space-time equalizers     

Performance of digital DECT radio links based on semianalyticalmethods

We report a very efficient semianalytical approach for the performance evaluation of differential detection schemes for GMSK signals of the DECT standard. Precisely, for a given channel, the performance is determined by means of an analytical procedure which includes the saddlepoint approximation. We consider both static channels (with impulse response generated by the simulation program SIRCIM) and two-ray Rayleigh and log-normal fading channels. As a departure from previous works, our receiver includes an all-digital part after the analog differential detection scheme. The digital part includes: (1) a block for the estimation of both the optimum sampling phase and the nonlinear channel coefficients (by making use of the DECT training sequence), (2) a one-tap decision feedback (DF) equalizer, and (3) a block for the evaluation of the approximate optimum bias level (γ _e) in the threshold detector. Both the DF equalizer coefficient and γ_e are based on the nonlinear channel coefficients estimate. For channels with a normalized delay spread up to 0.2, the use of the optimum threshold together with the DF equalizer permits a gain of about 2 dB at BER=10^-6 with respect to a receiver without equalization and a zero-level decision threshold. In addition, we discover that, in indoor environments, the 2-bit GMSK detector performs roughly the same as the 1-bit detector. The threshold optimization is also effective in the presence of channels affected by fading. To support this statement, we report the performance of the 1-bit differential detection scheme combined with antenna selection diversity in the presence of a two-ray log-normal and Rayleigh fading channel     

Adaptive linear equalization combined with noncoherent detectionfor MDPSK signals

A novel noncoherent receiver for M-ary differential phase-shift keying signals transmitted over intersymbol interference channels is presented. The noncoherent receiver consists of a linear equalizer and a decision-feedback differential detector. A significant performance gain over a previously proposed noncoherent receiver can be observed. For an infinite number of feedback symbols, the optimum equalizer coefficients can be calculated analytically, and the performance of the proposed receiver approaches that of a coherent linear minimum mean-squared-error equalizer. Moreover, a modified least mean square and a modified recursive least squares algorithm for adaptation of the equalizer coefficients are discussed     

Optimization of tap spacings for the tapped delay line decisionfeedback equalizer

In this letter, we optimize tap spacings in a tapped delay line equalizer. We derive a set of nonlinear equations for the optimum tap spacings and tap weights for the decision feedback equalizer. A recursive numerical technique is used to obtain the optimum settings. With optimized tap spacings, the proposed solutions provide a performance upper bound for the decision feedback equalizer for a given number of taps     

电阻加载环形谐振器在微带均衡器中的应用

文中提出了一种薄膜电阻加载的微带环形谐振器,并用其设计了一种新型均衡器子机构。研究表明:通过电阻加载的环形谐振器不仅具有中心频率和衰减幅度调节功能,而且能够对电路的品质因数进行调节,为均衡器的设计提供了极大的方便。最后给出了利用这种形环谐振器设计的均衡器的典型曲线。     

薄膜电阻对微带均衡器的影响研究

为了使微带均衡器满足衰减特性与给定的误差曲线逐点拟和的要求,就必须使其频率可调、衰减可调、品质因数可调。因此,本文通过大量仿真和实验,研究了加载了薄膜电阻的微带谐振器及其在微带均衡器中的应用,研究分析了薄膜电阻对微带均衡器传输特性及驻波特性的影响。研究结果表明将薄膜电阻加载到微带谐振器上构造微带均衡器,可以通过改变谐振器的尺寸使其频率可调,通过改变电阻阻值及位置使其衰减可调、品质因数可调。大量HFSS仿真及实验证明这种方式很适合微带均衡器的设计制造。本文利用这种结构设计制作出了一个宽带微带均衡器,得到了很好的实验结果。     

Multichannel MLSE equalizer with parametric FIR channelidentification

We propose a parametric finite impulse response (FIR) channel identification algorithm, apply the algorithm to a multichannel maximum likelihood sequential estimation (MLSE) equalizer using multiple antennas, and investigate the improvement in the overall bit error rate (BER) performance. By exploring the structure of the specular multipath channels, we are able to reduce the number of channel parameters to provide a better channel estimate for the MLSE equalizer. The analytic BER lower bounds of the proposed algorithm as well as those of several other conventional MLSE algorithms in the specular multipath Rayleigh-fading channels are derived. In the derivation, we consider the channel mismatch caused by the additive Gaussian noise and the finite-length channel approximation error. A handy-to-use simplified BER lower bound is also derived. Simulation results that illustrate the BER performance of the proposed algorithm in the global system for mobile communications (GSM) system are presented and compared to the analytic lower bounds     

The Estimate Feedback Equalizer: A Suboptimum Nonlinear Receiver

A recursive nonlinear equalizer has been developed. The Bayes estimation theory has been used to obtain an unrealizable nonlinear minimum mean-square error (MMSE) receiver for the reception of binary pulse-amplitude-modulation signals in the presence of intersymbol interference and noise. A realizable approximation to the Bayes structure has been derived as the combination of a matched filter and a nonlinear recursive equalizer structure. The equalizer has been made adaptive using a new algorithm that defines and maintains the time frame of reference and is constrained so that the equalizer's parameters always move toward their optimum values. Computer simulations have been used to demonstrate the properties of the estimate feedback equalizer (EFE) and to compare its performance to that of presently known equalizers.     

12 GHz枝节匹配宽带微带均衡器

给出了一种匹配良好的宽带微带均衡器的设计。它由在枝节上加载了薄膜电阻的微带枝节谐振器构造而成。通过大量的试验和仿真研究了电阻值对谐振器的影响。当加载薄膜电阻在枝节谐振器上时，可以得到可调的品质因数。利用HFSS分析可以看到电阻加载的枝节谐振器不仅频率可调，而且衰减可调、品质因数可调。文中利用四个加载了不同阻值的枝节谐振器得到了一个12GHz带宽的微带均衡器，给出了结构图和特性曲线。结果表明这种加载电阻的谐振器结构很适合宽带微带均衡器设计。     

Frequency-Domain Bit-Flipping Equalizer for Wideband MIMO Channels

We propose a low-complexity equalizer whose performance approaches that of the optimal maximum-likelihood estimators in wideband multiple-input multiple-output (MIMO) channels. The proposed algorithm makes use of a bit-flipping refinement procedure preceded by a frequency-domain equalizer and is based on local-optima searching algorithms. Through performance evaluations, it is demonstrated that the proposed equalizer can perform well when a large number of diversity branches are available in severely dispersive fading channels.     

一种适用于严重衰减串行链路的3.3 Gbit/s模拟均衡器

介绍了一种用于严重损耗串行链路的连续时间线性均衡器(CTLE)。为了解决传统均衡器存在的过均衡和欠均衡问题,提出了一种低频增益线性可调的改进结构,实现了对不同衰减信道的增益补偿。该结构主要包括均衡滤波模块和直流失调消除模块。均衡滤波模块采用均衡单元串联的结构,提高了对信号高频成分的补偿能力。直流失调消除模块用来消除芯片制造过程中因失配而产生的直流偏移。电路采用TSMC 0.18μm CMOS工艺设计,总面积为1.2 mm×0.65 mm。测试结果表明,当速率为3.3 Gbit/s的数据通过损耗为18.8 dB的信道时,均衡器工作正常。在1.8 V的供电电压下,芯片整体功耗为124.2 mW。     

Linear Minimum Mean-Square Error Estimators Applied to Channel Equalization

This concise paper investigates various aspects of the application of unbiased linear minimum mean-square error (ULMMSE) estimators to the equalization of channels used for digital data transmission. One application is to the equalization of the channel to a response free from intersymbol interference. Previous results regarding this application are clarified and extended. The other application is to the equalization of the channel to a finite short memory response. This stems from an interest in reducing the complexity of the Viterbi algorithm. Various new results on the optimum ULMMSE equalizer and its performance are presented. In both applications, the optimum ULMMSE equalizer is a stable, realizable, finite dimensional recursive filter.     

Simple equalization of time-varying channels for OFDM

We present a block minimum mean-squared error (MMSE) equalizer for orthogonal frequency-division multiplexing (OFDM) systems over time-varying multipath channels. The equalization algorithm exploits the band structure of the frequency-domain channel matrix by means of a band LDL/sup H/ factorization. The complexity of the proposed algorithm is linear in the number of subcarriers and turns out to be smaller with respect to a serial MMSE equalizer characterized by a similar performance.     

Pulse templates for transmission over an ensemble of channels

A method is given for finding the envelope of a family of pulses whose shapes are determined by a finite number of parameters. In addition, it is assumed that the parameters are analytically constrained collectively. In the second part of the paper, a family of pulse shapes is determined that can be efficiently transmitted through a channel that is selected at random from an ensemble of channels. The pulse envelope theory of the first part of the paper is then applied using this family of pulses to determine the envelope of mean-square channel outputs of the family of pulses that provide relatively low loss through an ensemble of channels     

Spatial-temporal equalization for IS-136 TDMA systems with rapiddispersive fading and cochannel interference

In this paper, we investigate spatial-temporal equalization for IS-136 time-division multiple-access (TDMA) cellular/PCS systems to suppress intersymbol interference and cochannel interference and improve communication quality. This research emphasizes channels with large Doppler frequency (up to 184 Hz), delay dispersion under one symbol duration, and strong cochannel interference. We first present the structure of the optimum spatial-temporal decision-feedback equalizer (DFE) and linear equalizer and derive closed-form expressions for the equalizer parameters and mean-square error (MSE) for the case of known channel parameters. Since the channel can change within an IS-136 time slot, the spatial-temporal equalizer requires parameter tracking techniques. Therefore, we present three parameter tracking algorithms: the diagonal loading minimum MSE algorithm, which uses diagonal loading to improve tracking ability, the two-stage tracking algorithm, which uses diagonal loading in combination with a reduced complexity architecture, and the simplified two-stage tracking algorithm, which further reduces complexity to one M×M and one 3×3 matrix inversion for weight calculation with M antennas. For a four-antenna system, the simplified two-stage tracking algorithm can attain a 10^-2 bit error rate (BER) when the channel delay spread is half of the symbol duration and the signal-to-interference ratio (SIR) of the system is as low as 5 dB, making it a computationally feasible technique to enhance system performance for IS-136 TDMA systems     

Blind equalization for short burst wireless communications

In this paper, we propose a dual mode blind equalizer based on the constant modulus algorithm (CMA). The blind equalizer is devised for short burst transmission formats used in many current wireless TDMA systems as well as future wireless packet data systems. Blind equalization is useful for such short burst formats, since the overhead associated with training can be significant when only a small number of bits are transmitted at a time. The proposed equalizer overcomes the common problems associated with classic blind algorithms, i.e., slow convergence and ill-convergence, which are detrimental to applying blind equalization to short burst formats. Thus, it can eliminate the overhead associated with training sequences. Also, the blind equalizer is extended to a two branch diversity combining blind equalizer. A new initialization for fractionally spaced CMA equalizers is introduced. This greatly improves the symbol timing recovery performance of fractionally spaced CMA equalizers with or without diversity, when applied to short bursts. Through simulations with quasi-static or time-varying frequency selective wireless channels, the performance of the proposed equalizer is compared to selection diversity and conventional equalizers with training sequences. The results indicate that its performance is far superior to that of selection diversity alone and comparable to the performance of equalizers with short training sequences. Thus, training overhead can be removed with no performance degradation for fast time-varying channels, and with slight performance degradation for static channels     

Iterative MAP Equalization and Decoding in Wireless Mobile Coded OFDM

Orthogonal frequency division multiplexing (OFDM) system suffers extra performance degradation in fast fading channels due to intercarrier interference (ICI). Combining frequency domain equalization and bit-interleaved coded modulation (BICM), the iterative receiver is able to harvest both temporal and frequency diversity. Realizing that ICI channels are intrinsically ISI channels, this paper proposes a soft-in soft-out (SISO) maximum a posteriori (MAP) equalizer by extending Ungerboeck's maximum likelihood sequence estimator (MLSE) formulation to ICI channels. The SISO MAP equalizer employs BCJR algorithm and computes the bit log-likelihood ratios (LLR) for the entire received sequence by efficiently constructing a trellis that takes into account of the ICI channel structure. A reduced state (RS) formulation of the SISO MAP equalizer which provides good performance/complexity tradeoff is also described. Utilizing the fact that ICI energy is clustered in adjacent subcarriers, frequency domain equalization is made localized. This paper further proposes two computational efficient linear minimum mean square error (LMMSE) based equalization methods: recursive q-tap SIC-LMMSE equalizer and recursive Sliding-Window (SW) SIC-LMMSE equalizer respectively. Simulations results demonstrate that the iterative SISO RS-MAP equalizer achieves the performance of no ICI with normalized Doppler frequency f_dT_s up to 20.46% in realistic mobile WiMAX environment.     

Adaptive equalization for high-speed indoor wireless data communication

The performance of a 4-QAM indoor wireless data communication system with adaptive equalizer is investigated. The effectiveness of using linear and decision-feedback equalizer for Rayleigh and Rician frequency-selective indoor channels is evaluated, and contrasted to the performance of a 4-QAM modem without equalizer. The effects of some important channel and system parameters (multipath spreads up to 200 ns, data rates up to 25 Mbit/s, signaling pulse rolloff factor between 0.5 and 1.0, and additive, white Gaussian noise) on the indoor communication system performance are examined and presented in the paper. The indoor propagation measurements, carried out in a research laboratory, provided data to be used for BER performance assessments of the system with and without equalizer. The performance results based on computer generated channels are then compared with those obtained for measured channel impulse responses.     

超宽带1～8GHz可调均衡器的设计与实现

幅度均衡器对提高宽带接收机动态范围、改善接收机的多信号适应能力具有不可替代的重要作用。针对超宽带的设计需求,根据集总元件与微带的等效模型,介绍了采用集总元件与微带结构相结合的混合电路形式实现超宽带均衡器的设计方法。通过ADS电路仿真手段,考虑电路版图参数的影响因素,优化、微调了设计参数,得到均衡量可调的元件数值,并以实测数据验证了混合电路模型的正确性,设计方法适合工程应用。