Decision feedback equalization of the indoor radio channel

The measured multipath profiles from five different indoor areas are used for the performance analysis of a binary phase shift keying (BPSK) modem with a decision feedback equalizer (DFE). The performance from the measured multipath profiles is compared with the performance predictions based on a computer simulated channel model. Both average probability of error and probability of outage are calculated for a DFE with three fractionally spaced forward and three feedback taps. An equivalent delay power spectrum function, determined from the ensemble of the measured channel impulse responses, is defined. Using this function, analytical lower bounds on the average probability of error and the probability of outage of the BPSK/DFE modem with an infinite number of feedback taps and three forward taps are determined and compared with the results based on measured data and the computer generated channel impulse responses     

Decision feedback equalization

As real world communication channels are stressed with higher data rates, intersymbol interference (ISI) becomes a dominant limiting factor. One way to combat this effect that has recently received considerable attention is the use of a decision feedback equalizer (DFE) in the receiver. The action of the DFE is to feed back a weighted sum of past decision to cancel the ISI they cause in the present signaling interval. This paper summarizes the work in this area beginning with the linear equalizer. Three performance criteria have been used to derive optimum systems; 1) minimize the noise variance under a "zero forcing" (ZF) constraint i.e., insist that all intersymbol interference is cancelled, 2) minimize the mean-square error (MMSE) between the true sample and the observed signal just prior to the decision threshold, and 3) minimize the probability of error (Min Pe). The transmitter can be fixed and the receiver optimized or one can obtain the joint optimum transmitter and receiver. The number of past decisions used in the feedback equalization can be finite or infinite. The infinite case is easier to handle analytically. In addition to reviewing the work done in the area, we show that the linear equalizer is in fact a portion of the DFE receiver and that the processing done by the DFE is exactly equivalent to the general problem of linear prediction. Other similarities in the various system structures are also shown. The effect of error propagation due to incorrect decisions is discussed, and the coaxial cable channel is used as an example to demonstrate the improvement available using DFE.     

Channel reuse strategies for indoor infrared wirelesscommunications

We examine systems of fixed-channel reuse for base stations in an indoor infrared wireless communication system. The following techniques are compared: time-division multiple access (TDMA) using on-off keying (OOK) or pulse-position modulation (PPM); frequency-division multiple access (FDMA) using binary phase-shift keying (BPSK) or quadrature phase-shift keying (QPSK); code-division multiple access (CDMA) using OOK with direct-sequence spreading by m-sequences or optical orthogonal codes (OOCs). We define a parameter γ, which equals the signal-to-noise ratio (SNR) for unit optical path gain and is proportional to the square of the transmitted average optical power. Using measured pathloss data, it is found that in a system using hexagonal cells and a reuse factor of three, for cell radii above 3 m, TDMA with OOK or 2-PPM, and CDMA using OOCs all require approximately the same γ to achieve a worst-case bit-error rate (BER) of 10^-9 within a cell. Using TDMA with 4-PPM results in a 6-dB decrease in the required value of γ. CDMA using m-sequences requires an increase in γ of 5 dB over TDMA using OOK, and FDMA with BPSK requires an increase of 12 dB. For a given reuse factor N in the noise-limited regime, the required value of γ decreases in inverse proportion to N² for TDMA schemes and inversely with N for FDMA and CDMA schemes. For cell radii below 3 m, cochannel interference dominates the systems using TDMA, FDMA, and CDMA with an OOC, resulting in an irreducible BER above 10^-9 at cell radii below 1.5 m. Only CDMA with m-sequences does not develop an irreducible BER, making it the only choice for cell radii below 1.5 m     

Concatenated coding for DS/CDMA transmission in wirelesscommunications

We investigate the use of forward-error correction (FEC) as well as concatenated coding for reliable data transmission in asynchronous direct-sequence code-division multiple-access communications over frequency-selective Rayleigh fading channels. The FEC scheme combines antenna diversity with low complexity concatenated codes which consist of a Reed-Solomon outer code and a convolutional inner code. Under this concatenated coding scheme, we analyze the average bit-error rate performance and capacity tradeoffs between various system parameters under a fixed total bandwidth expansion and concatenated codes constraint requirements     

室内多载波CDMA系统在深度衰落环境下的能量受控均衡策略

多载波CDMA(MC-CDMA)是将多载波调制OFDM与DS-CDMA结合起来的多址访问技术。如何从接收到的信号恢复出发送信号，涉及到频率分集接收和均衡。本文提出的能量受控均衡技术，在多用户的环境下能较好地恢复扩频码的正交性且避免信道深度衰落的影响，而且，实验表明该方法对于不同的信道状况均能很好适应。     

914 MHz path loss prediction models for indoor wirelesscommunications in multifloored buildings

Qualitative models are presented that predict the effects of walls, office partitions, floors, and building layout on the path loss at 914 MHz. The site-specific models have been developed based on the number of floors, partitions, and concrete walls between the transmitter and receiver, and provide simple prediction rules which relate signal strength to the log of distance. The standard deviation between measured and predicted path loss is 5.8 dB for the entire data set, and can be as small as 4 dB for specific areas within a building. Average floor attenuation factors, which describe the additional path loss (in decibels) caused by floors between transmitter and receiver, are found for as many as four floors in a typical office building. Path loss contour plots for measured data are presented. In addition, contour plots for the path loss prediction error indicate that the prediction models presented are accurate to within 6 dB for a majority of locations in a building     

Performance of adaptive equalization for indoor radiocommunications

Adaptive equalization is used to ensure that the outage probability is less than 10^-3 for a target bit error rate of 10^-4 in buildings with RMS delay spread of up to 100 ns. A time-division multiple-access system with four-level quadrature amplitude modulation point-to-point links strikes the right balance between flexibility and complexity. It is shown that such a system can support rates of at least 1 Mb/s     

Mitigating error propagation in decision-feedback equalization for multiuser CDMA

This letter presents a robust decision-feedback equalization design that mitigates the error-propagation problem for multiuser direct-sequence code-division multiple-access systems under multipath fading. Explicit constraints for signal energy preservation are imposed on the filter weight vector to monitor and maintain the quality of the hard decisions in the nonlinear feedback loop. Such a measure protects the desired signal power against the detrimental effect of erroneous past decisions, thus providing the leverage to curb error propagation.     

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.     

Blind equalization in antenna array CDMA systems

Multipath induced interchip-interference (ICI) alters waveforms of transmitted signals and presents a major obstacle to direct-sequence (DS) code-division-multiple-access (CDMA) communications. For systems with aperiodic pseudorandom (PN) spreading sequences, the primary way to counter fading is through employing RAKE receivers that enhance the signal-to-interference ratio (SIR) by combining multipath signals from the desired user. In this paper, we formulate a discrete-time model for antenna array CDMA systems and study the 2-D RAKE receiver problem by casting it into an optimum vector FIR equalizer design and estimation framework. A novel aspect of the present work is the full exploitation of the potential of 2-D RAKE receivers without requiring any detailed knowledge of the multipath channels     

Blind decision feedback equalization for terrestrial televisionreceivers

In December 1996 the Federal Communications Commission (FCC) adopted the Grand Alliance (GA) system as the digital television broadcasting standard for the United States ending a seven-year-long search for a fully digital television standard. MPEG-2 was chosen as the video compression standard, and trellis-coded 8-vestigial sideband (VSB) with a training sequence was chosen as the transmission standard. The laboratory tests that were performed on the final two competing systems, 8-VSB with training sequence and 32-quadrature amplitude modulation (QAM) with blind equalization, showed a need for blind equalization in dynamic channels that could not be adequately handled by an equalizer training on the training sequence alone. Hence, the final GA system recommended the use of blind equalization in the receiver. In this paper, we describe the U.S. digital television transmission standard as it pertains to the equalization problem, typical transmission channel characteristics and the need for blind equalization in terrestrial television receivers     

Interference suppression for space-time coded CDMA via decision-feedback equalization

A single-user receiver structure is proposed for space-time coded code-division multiple-access (CDMA) downlink in a multiuser frequency-selective channel. This structure is a two-dimensional (2-D) decision-feedback equalizer (2D-DFE) whose filters are optimized based on the MMSE criterion to mitigate noise, intersymbol interference (ISI), and multiuser interference (MUI) with a moderate complexity. By modeling the spreading codes of the interfering users as random sequences, system performance was evaluated using the Gaussian approximation. Two models for the desired user's spreading sequence have been considered and compared. Our numerical results show that in both cases the 2D-DFE exhibits significant performance improvement over the standard space-time coded RAKE, especially in interference-limited conditions. It is also observed that the gain obtained by using DFE in a MISO channel is less that in a SISO channel and this problem can be solved by providing diversity at the receiver.     

Field measurements of an indoor high-speed QAM wireless systemusing decision feedback equalization and smart antenna array

This paper reports on field measurements of point-to-point indoor high-speed (10 Mbit/s to 30 Mbit/s at 5 Mbaud) wireless communications realized using a flexible multilevel quadrature amplitude modulation (M-QAM) testbed that features real-time equalization and smart antenna-array technology. The results from an extensive set of measurements, 59262 trials in all, performed without cochannel interference under various receiver configurations and wireless environments are presented and analyzed. The results underscore the dramatic potential for a system that optimally combines equalization and a smart antenna array. For example, using only 10 mW of transmit power, the system delivered 30 Mitts at an uncoded bit error rate (BER) of 10 ^-3 with 5% outage at a coverage radius of 20 in. For a lower data rate of 10 Mbitts, the coverage radius was increased to 32 in, the uncoded BER dropped below 10^-7, and the outage improved to 1%. The field measurements indicate that a 4-tap feedforward-filter decision-feedback equalizer with eight feedback-filter taps is sufficient to mitigate the intersymbol interference for typical indoor environments. They also show a significant gain when using a smart antenna array. For example, when transmitting between rooms at a 2% outage probability, the signal-to-noise ratio (SNR) improves by 8.3 dB when using two antennas instead of one antenna. Doubling the number of antennas to four provided an additional SNR improvement of 5.2 dB. The paper also presents simulation results that confirm the performance trends observed from the field measurements     

基于PR—QMF理论的判决反馈均衡器

根据完全重构正交镜像滤波器（PR-QMF）的理论,我们构造了一类正交变换并且用于交换域判决反馈均衡器（OFE）的设计。在此基础上我们进一步讨论了该类DFE的优化设计。通过与卡胡南一洛也夫变换（KLT）和离散余弦变换（DCT）比较得出这样的结论：经过最优化设计,本文提出的算法具有更快的收敛速度,同时,它还具有复杂程度低和设计灵活的优点。     

Reduced complexity decision feedback equalization for digitalsubscriber loops

Decision feedback equalizers (DFEs) are widely used in modern local network digital transmission systems to remove the intersymbol interference caused by slowly decaying pulse tails. A gradient descent algorithm for adapting a coefficient to model the slowly decaying portion of the tail is described. An equalization strategy is described that exploits prior knowledge of the nature of the subscriber loop channel, together with the new adaptation algorithm, to give reduced complexity DFE structures. The use of this algorithm in FIR and IIR equalizer structures is described. The use of this algorithm in FIR and IIR equalizers is quantitatively compared to a conventional DFE in terms of performance and implementation complexity. An analysis is presented describing the operation of the adaptation algorithm in the presence of noise. Simulation results illustrate the training of the algorithm and its stability in the presence of near-end crosstalk noise     

Efficient decision feedback equalization for sparse wireless channels

A new efficient decision feedback equalizer (DFE) appropriate for channels with long and sparse impulse response (IR) is proposed. Such channels are encountered in many high-speed wireless communications applications. It is shown that, in cases of sparse channels, the feedforward and feedback (FB) filters of the DFE have a particular structure, which can be exploited to derive efficient implementations of the DFE, provided that the time delays of the channel IR multipath components are known. This latter task is accomplished by a novel technique, which estimates the time delays based on the form of the channel input-output cross-correlation sequence in the frequency domain. A distinct feature of the resulting DFE is that the involved FB filter consists of a reduced number of active taps. As a result, it exhibits considerable computational savings, faster convergence, and improved tracking capabilities as compared with the conventional DFE. Note that faster convergence implies that a shorter training sequence is required. Moreover, the new algorithm has a simple form and its steady-state performance is almost identical to that of the conventional DFE.     

Signature optimization for CDMA with limited feedback

We study the performance of joint signature-receiver optimization for direct-sequence code-division multiple access (DS-CDMA) with limited feedback. The receiver for a particular user selects the signature from a signature codebook, and relays the corresponding B index bits to the transmitter over a noiseless channel. We study the performance of a random vector quantization (RVQ) scheme in which the codebook entries are independent and isotropically distributed. Assuming the interfering signatures are independent, and have independent and identically distributed (i.i.d.) elements, we evaluate the received signal-to-interference plus noise ratio (SINR) in the large system limit as the number of users, processing gain, and feedback bits B all tend to infinity with fixed ratios. This SINR is evaluated for both the matched filter and linear minimum mean-squared error (MMSE) receivers. Furthermore, we show that this large system SINR is the maximum that can be achieved over any sequence of codebooks. Numerical results show that with the MMSE receiver, one feedback bit per signature coefficient achieves close to single-user performance. We also consider a less complex and suboptimal reduced-rank signature optimization scheme in which the user's signature is constrained to lie in a lower dimensional subspace. The optimal subspace coefficients are scalar-quantized and relayed to the transmitter. The large system performance of the quantized reduced-rank scheme can be approximated, and numerical results show that it performs in the vicinity of the RVQ bound. Finally, we extend our analysis to the scenario in which a subset of users optimize their signatures in the presence of random interference.     

Iterative equalization with adaptive soft feedback

In this letter, a novel equalization algorithm applying soft-decision feedback and designed for binary transmission is introduced. In contrast to conventional decision-feedback equalization (DFE), iterations are necessary, because a simple matched filter serves as feedforward filter, which collects signal energy, but creates noncausal intersymbol interference. The rule for generating soft decisions is adapted continuously to the current state of the algorithm. In most cases, standard DFE methods are clearly outperformed. For a class of certain channel impulse responses, performance of maximum-likelihood sequence estimation is attained, in principle. The high performance of the scheme is explained using results from neural network theory     

Joint coding and decision feedback equalization for broadbandwireless channels

This paper introduces a new approach for joint convolutional coding and decision feedback equalization (DPE). To minimize error propagation, the DFE uses a combination of soft decisions and delayed tentative decisions to cancel intersymbol interference (ISI). Soft decisions are obtained by passing the DFE output through a (soft) nonlinear device. This simple method is shown to perform almost as well as an optimum soft feedback approach on wireless channels with diversity. Tentative decisions from the Viterbi decoder are used to cancel ISI due to multipath with large delays, thus remedying the increasing effect of error propagation in channels with large delay spreads. We consider the use of this soft/delayed feedback DFE (S/D-DFE) technique in broadband wireless channels (with delay spreads up to several tens of the symbol period) typical in high-bitrate mobile data applications. Simulation results indicate that the proposed joint coding and S/D-DFE technique performs to within 1-2 dB [in required signal-to-noise ratio (SNR)] of an ideal coded DFE without error propagation. When combined with antenna diversity and a reduced-complexity DFE concept with adaptive feedforward tap assignment, it provides high packet throughput against Rayleigh fading, severe delay spreads, and high Doppler rates     

基于LSQR算法和加窗技术的判决反馈均衡器

在正交频分复用(OFDM)系统中,高速移动造成的多普勒效应破坏了子载波间的正交性并产生载波间干扰(ICI).为了消除ICI,确保快时变信道下的可靠通信,该文提出了一种低复杂度加窗LSQR(least square QR)判决反馈均衡器(Decision Feedback Equalization,DFE).借助加窗技术和时域LSQR迭代计算,该算法减少了残存的ICI,有效克服了"地板效应";此外,该算法还利用带状矩阵分解降低了计算复杂度.理论分析和仿真结果表明,在快时变信道下与已有的块状线性均衡(Block Linear Equalization,BLE)算法比较,该算法在复杂度相当的情况下可以进一步改善系统的误码率性能.