Intersymbol Interference Cancellation Research Articles

Joint multi‐user detection and intersymbol interference cancellation for WCDMA satellite UMTS

AbstractIn this work we present two different intersymbol interference (ISI) cancellation systems, namely the commutation signaling (CS) and orthogonal code‐division transmit diversity (O‐CDTD) in combination with satellite path diversity, which leads to a high diversity order. The ISI is due to the use of satellite path diversity (in order to introduce multipath diversity artificially). This work combines the proposed ISI cancellation systems with subtractive multi‐user detectors—a variation of the conventional parallel interference cancellation (PIC) and successive interference cancellation (SIC) for the downlink channel of S‐UMTS. The blind detection is also considered without knowledge of spreading sequences or the channel state information (CSI) of the interfering users with a noise whitening matched filter (NWMF) that combats the MAI through the whitening of the interfering power spectrum, also with and without the ISI cancellation systems—CS and O‐CDTD. It is shown that the proposed PIC is a very promising detector to be used for the downlink of S‐UMTS, especially when combined with CS or O‐CDTD, the NWMF being efficient only when we consider jointly any of the proposed ISI cancellation systems for low number of interfering users. Copyright © 2003 John Wiley & Sons, Ltd.

Minimum redundancy for ISI free FIR filterbank transceivers

There has been great interest in the design of filterbank transceivers. Usually, with proper time domain equalization, the channel is modeled as an FIR filter. It is known that for FIR channels, the introduction of certain redundancy allows the receiver to cancel intersymbol interference (ISI) completely, and channel equalization is performed implicitly using FIR transceivers. This scheme allows us to trade bandwidth for ISI cancellation. In this paper, we derive the minimum redundancy required for the existence of FIR transceivers for a given channel. We see that the minimum redundancy is directly related to the zeros of the channel and to the Smith form of an appropriately defined channel matrix.

Simple space-time coded SS-CDMA systems capable of perfect MUI/ISI elimination

In this letter we propose a very simple strategy for combining space-time block coding (STBC) with code division multiple access (CDMA) to be used in single-user links or in broadcasting over time-dispersive channels. The proposed system provides the desired space diversity gain and is capable of perfect cancellation of multiuser interference (MUI) and intersymbol interference (ISI), using a very simple receiver structure. The advantage is obtained by simply incorporating cyclic prefixes in the user codes. We provide a theoretical analysis, verified via simulation results, and a comparison between CDMA and OFDMA systems incorporating STBC.

A class of block-iterative equalizers for intersymbol interference channels: fixed channel results

A new and efficient class of nonlinear equalizers is developed for intersymbol interference (ISI) channels. These -"iterated-decision equalizers" use an optimized multipass algorithm to successively cancel ISI from a block of received data and generate symbol decisions whose reliability increases monotonically with each iteration. These equalizers have an effective complexity comparable to the decision-feedback equalizer (DFE), yet asymptotically achieve the performance of maximum-likelihood sequence detection (MLSD). We show that, because their structure allows cancellation of both precursor and postcursor ISI, iterated-decision equalizers outperform the minimum mean-square error DFE by 2.507 dB on severe ISI channels even with uncoded systems. Moreover, unlike the DFE, iterated-decision equalizers can be readily used in conjunction with error-control coding, making them attractive for a wealth of applications.

Blind ISI and MAI cancellation based on periodically time-varying transmitted power

For multiple transmitters, with over-sampling and multiple antennas at the receiver, an algorithm is presented to achieve blind mitigation of both intersymbol interference (ISI) and multiple access interference (MAI) by selectively periodically varying the transmitted power of each user. In addition, the permutation ambiguity of many source separation algorithms is overcome.

On optimal ambiguity resistant precoders in ISI/multipath cancellation

Ambiguity resistant (AR) precoding has recently been proposed in intersymbol interference (ISI) and multipath cancellations, where the ISI/multipath channel may have frequency-selective fading characteristics and its knowledge is not necessarily known. With the AR precoding, no diversity is necessary at the receiver. In the precoding, the AR property for a precoder plays an important rule. In this paper, more families and properties of AR precoders are presented and characterized. In particular, all systematic AR precoders are characterized. More importantly, we introduce the concepts of precoder distance and optimal precoders, and characterize and construct all optimal systematic AR precoders, when additive channel random noise is concerned. A necessary and sufficient condition for an AR precoder to be optimal is given, which is easy to check. With the optimal precoders, numerical simulations are presented to show the improved performance over the known AR precoders in ISI cancellation applications.

Adaptive detection in asynchronous code-division multiple-access system in multipath fading channels

In code-division multiple-access systems transmitting data over time-varying multipath channels, both intersymbol interference (ISI) and multiple-access interference (MAI) arise. In this paper, we address interference suppression, multipath diversity and processing gain protection for multiuser detection with less noise enhancement by using a parallel cancelling scheme. The proposed detector consists of a RAKE filter, forward filter, and feedback filter with different functions for each filter. The RAKE filter increases the signal-to-noise ratio by taking the advantage of multipath and code diversities. The forward filter is proposed, in combination with the feedback filter, to remove the effects of MAI and ISI by parallel cancellation. In order to avoid performance deterioration due to unreliable initial estimation in the parallel cancellation, a cost function with proper weighting is introduced to improve the performance of the proposed detector. In the proposed design method, a recursive least square algorithm is employed to update the tap-coefficients of all filters for MAI and ISI cancellation. Finally, the performance of the proposed detector is analyzed and compared with other detectors.

An analog DFE for disk drives using a mixed-signal integrator

An analog adaptive decision-feedback equalizer (DFE) is described. The DFE cancels intersymbol interference using four feedback taps, and a fifth tap cancels dc offset. The coefficient for each tap is adapted using a small mixed-signal integrator. The DFE dissipates 220 mW at a data rate of 150 Mb/s. The active area is 1.8 mm/sup 2/ in a 1-/spl mu/m CMOS process.

Column-anchored zeroforcing blind equalization for multiuser wireless FIR channels

We propose a direct blind zeroforcing approach to cancel intersymbol interference (ISI) in multiple user finite impulse response (FIR) channels. By selectively anchoring columns of the channel convolution matrix, we present two column-anchored zeroforcing equalizers (CAZE), one without output delay and one with a chosen delay. Unlike many known blind identification algorithms, these equalizers do not need an accurate estimate of the channel orders. Exploiting second-order statistics (SOS) of the received signals, they can retain preselected d columns in the channel convolution matrix (d is the number of users) and force the remaining columns to zero. CAZE can effectively equalize single-input-multiple-output (SIMO) systems and can reduce dynamic multiple-input-multiple-output (MIMO) systems into a memoryless signal mixing system for source separation. Simulation results show that the CAZE is not only effective for blind equalization of linear quadrature amplitude modulation (QAM) systems, but it is also applicable to the nonlinear GMSK modulation in the popular wireless GSM systems when computational cost severely limits the use of nonlinear methods such as the Viterbi algorithm.

Blind Source Separation, ISI Cancellation and Carrier Phase Recovery in SDMA Systems for Mobile Communications

Current rates of demand for radio cellular services will soon reach the saturation point of the actual cellular infrastructures. Thus, spectral bandwidth-saving multiple access schemes are necessary to expand the actual networks‘ capacity. In the Spatial Division Multiple Access (SDMA) approach, users within the same cell share the same time/frequency channel and the base station receiver exploits the spatial dimension of the radio resource to remove crosstalk. In this paper, we present a blind multi-user equalization technique for SDMA architectures which performs the separation of digital sources, in the presence of intersymbol interference (ISI) and phase carrier drifts. We take advantage of the sources‘ joint finite alphabet and statistical properties, and use only second-order statistics in order to produce valid channel estimates based on short data packets. Computer simulations using the TU-GSM multipath channel model for radio communications in urban environments demonstrate the potential of our technique.

Residual ISI cancellation for OFDM with applications to HDTV broadcasting

An iterative technique is developed for orthogonal frequency division multiplexing (OFDM) systems to mitigate the residual intersymbol interference (ISI) that exceeds the length of the guard interval. The technique, called residual ISI cancellation (RISIC), uses a combination of tail cancellation and cyclic restoration and is shown to offer large performance improvements. The effects of imperfect channel estimation are also considered. The RISIC algorithm is applied to a typical terrestrial high-definition television (HDTV) broadcasting system that uses a concatenated coding scheme for error control. Results show that the RISIC algorithm can effectively mitigate residual ISI on static or slowly fading ISI channels.

Joint coding and decision feedback equalization for broadband wireless 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.

New precoding for intersymbol interference cancellation using nonmaximally decimated multirate filterbanks with ideal FIR equalizers

In this paper, we propose a new precoding method for intersymbol interference (ISI) cancellation by using non-maximally decimated multirate filterbanks. Unlike the existing preceding methods, such as the TH and trellis precodings, the new preceding (i) may be independent of the ISI channel; (ii) is linear and does not have to implement any modulo operation; (iii) gives the ideal FIR equalization at the receiver for any FIR ISI channel including spectral-null channels; (iv) expands the transmission bandwidth in a minimum amount. The precoding is built on non-maximally decimated multirate filterbanks. Based on multirate filterbank theory, we present a necessary and sufficient condition on an FIR ISI transfer function in terms of its zero set such that there is a linear FIR N/spl times/M precoder so that an ideal FIR equalizer exists, where the integers K and N are arbitrarily fixed. The condition is easy to check. As a consequence of the condition, for any given FIR ISI transfer function (not identically 0), there always exist such linear FIR precoders. Moreover, for almost all given FIR ISI transfer functions, there exist linear FIR precoders with size N/spl times/(N-1), i,e., the bandwidth is expanded by 1/N. In addition to the conditions on the ISI transfer functions, a method for the design of the linear FIR precoders and the ideal FIR equalizers is also given. Numerical examples are presented to illustrate the theory.

On the use of tentative decisions to cancel intersymbol interference and nonlinear distortion (with application to magnetic recording channels)

We develop a theory of the error rate of receivers that use tentative decisions for adaptive cancellation of intersymbol interference (ISI). We formulate precise conditions under which tentative decisions can be effectively used to cancel linear or nonlinear ISI, and verify the predictions of the theory by Monte Carlo simulations on four typical channels. In the case of magnetic recording channels, we show that the conditions for effective cancellation are satisfied by the precursor interference generated by inductive read heads, resulting in an improvement of the noise margin of 2 dB at the output of a symbol-by-symbol detector or 1 dB at the output of a Viterbi decoder at a bit error rate of 10/sup -9/. This technique has been incorporated in an experimental VLSI partial response maximum-likelihood (PRML) receiver, and the results confirmed by laboratory measurements. We also show that no improvement is obtained when this technique is used to compensate nonlinear distortion, which is another common impairment of magnetic recording channels, and we establish fundamental limits for the improvement achievable by any nonlinear equalization technique in magnetic recording.

Error Rate Improvement of PRML System with ISI Canceller for (1,7) RLL Code

A method to improve the error rate performance of PRML systems with an Inter-symbol Interference (ISI) canceller having a tentative sequence detector is presented. The new system adopts a combination of a minimum phase noise whitening matched filter and a Viterbi detector with a path feedback scheme as a tentative detector. The result obtained by computer simulation shows that at a bit-error rate of 10-4, the proposed systems improves the SNR by about 6.3 dB compared with the conventional EPR4ML system without an ISI canceller.

An equalizer design for nonminimum phase channel via two-block H∞ optimization technique

A two-block H ∞ optimization technique is introduced to treat the single-channel equalization problem in the frequency domain and used to realize an optimal, causal and stable equalizer. An iterative design algorithm is presented for minimum-phase as well as nonminimum-phase channel systems. This algorithm is able to achieve intersymbol interference (ISI) cancellation and noise attenuation from the viewpoint of H ∞ optimization and is robust for the uncertainty of second-order statistics of inputs and external noises. When the communication system suffers some uncertainties from random parameter variations, an equivalent robust equalization problem from the two-block H ∞ optimization is also treated. So the proposed optimal equalizer is also found robust while the channel characteristics and the second-order statistics of inputs and external noises are uncertain. For the purpose of the illustration, one numerical example and simulation results are also presented.

A 35 Mb/s mixed-signal decision-feedback equalizer for disk drives in 2-μm CMOS

A 35 Mb/s mixed-signal adaptive decision-feedback equalizer (DFE) has been implemented in a 2-/spl mu/m CMOS technology. The DFE has four feedback taps for cancelling intersymbol interference (ISI) and one tap for cancelling dc offset. The ISI is cancelled using fully differential analog circuits. Coefficient adaptation is digital, and two adaptation rates are available. The DFE occupies 24 mm/sup 2/ and dissipates 165 mW.

High-speed optical receiver with soft decision ISI cancellation

Due to the stimulating success in developing optical amplifiers, intersymbol interference (ISI) will become the domi- nant limit on both bit rate and transmission distance in long-haul optical fiber communication systems. Here we present a receiver with Soft Decision IS1 Cancellation (SDIC) to deal with IS1 in high-speed lightwave systems. The analytical and simulation results show that the transmission distance can be doubled with SDIC. In addition, the receiver with SDIC allows the IS1 cancellation circuit operating with processing and propagation delay much longer than 1-b period. Therefore, it is very promising in very high bit rate optical systems.

Adaptive nonlinear cancellation for high-speed fiber-optic systems

The authors propose techniques for adaptive nonlinear cancellation of intersymbol interference (ISI) in the electrical signal at the receiver in Gb/s lightwave systems and describe several demonstrations of these techniques. Techniques for adjustable nonlinear cancellations are discussed and demonstrations of these techniques using commercially available integrated circuits (ICs) at data rates as high as 1.7 Gb/s are described. Techniques for automatic adjustment are discussed, and a demonstration of adaptive nonlinear cancellation at 450 Mb/s is described. The authors discuss how these techniques can be integrated onto the detector IC for operations at 2.5 Gb/s and higher data rates. These techniques allow a single IC detector with adaptive nonlinear cancellation to be used in long-haul and undersea lightwave systems to optimize the detector threshold and compensate for the ISI.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Analysis of predistortion, equalization, and ISI cancellation techniques in digital radio systems with nonlinear transmit amplifiers

An analysis made of the performance of predistortion, equalization, and intersymbol interference (ISI) cancellation techniques in compensating for the transmit amplifier nonlinearity in digital microwave radio systems. The study is carried out using the 64 QAM and 256 QAM signal formats and two values of the roll-off factor in the Nyquist pulse shaping. The simulated compensation techniques include three types of predistortion, two ISI cancellers, and several nonlinear equalizers with or without decision feedback. A basic result is that decision-feedback equalizers do not offer any significant advantage over nonrecursive equalizers. It is also shown that ISI cancelers with a memoryless equalizer as first-stage decision device do not perform any better than nonlinear equalizers of similar complexity. Another contribution an improved fifth-order analog signal predistortion technique is analyzed. The gain that can be achieved using a modified 256 QAM signal constellation that is more robust to nonlinear distortion is quantified. >