Gregory E. Bottomley

A generalized RAKE receiver for interference suppression

Currently, a global third-generation cellular system based on code-division multiple-access (CDMA) is being developed with a wider bandwidth than existing second-generation systems. The wider bandwidth provides increased multipath resolution in a time-dispersive channel, leading to higher frequency-selectivity. A generalized RAKE receiver for interference suppression and multipath mitigation is proposed. The receiver exploits the fact that time dispersion significantly distorts the interference spectrum from each base station in the downlink of a wideband CDMA system. Compared to the conventional RAKE receiver, this generalized RAKE receiver may have more fingers and different combining weights. The weights are derived from a maximum likelihood formulation, modeling the intracell interference as colored Gaussian noise. This low-complexity detector is especially useful for systems with orthogonal downlink spreading codes, as orthogonality between own cell signals cannot be maintained in a frequency-selective channel. The performance of the proposed receiver is quantified via analysis and simulation for different dispersive channels, including Rayleigh fading channels. Gains on the order of 1-3.5 dB are achieved, depending on the dispersiveness of the channel, with only a modest increase in the number of fingers. For a wideband CDMA (WCDMA) system and a realistic mobile radio channel, this translates to capacity gains of the order of 100%.

Adaptive arrays and MLSE equalization

In digital cellular communication systems, receivers are designed to combat the problems of fading, time dispersion, and interference. Separately, these problems can be solved using antenna diversity, equalization, and adaptive array processing, respectively. Joint solutions have been proposed which combine adaptive array processing with either linear equalization (LE) or decision feedback equalization (DFE). However, LE is sensitive to spectral nulls and DFE has the problem of decision error feedback. In this paper, an algorithm is proposed which combines adaptive array processing with MLSE equalization to mitigate fading, time dispersion, and interference. The optimal metric for the MLSE structure is derived. The receiver reduces to well known forms for special cases. D-AMPS simulation results illustrate the potential performance of the proposed algorithm, including the case when the fading on the different antennas is correlated.

Channel estimation in narrowband wireless communication systems

Channel estimation is an integral part of standard adaptive receiver designs used in narrowband, digital wireless communication systems. In this tutorial paper, commonly used approaches to channel estimation are reviewed. Both time-invariant and time-varying channels are considered. For time-varying channels, both pilot symbol interpolation and data-directed channel tracking are considered. Applications include the Global System for Mobile communications, the Enhanced Data rates for Global Evolution system, and another Time-Division Multiple-Access system known as Telecommunications Industry Association/Electronics Industry Association/Interim Standard—136 (TIA/EIA/IS-136 or IS-136). Copyright

Unification of MLSE receivers and extension to time-varying channels

Forney (1972) and Ungerboeck (1974) have each developed maximum-likelihood sequence estimation (MLSE) receivers for intersymbol interference (ISI) channels. The Forney receiver uses a whitened matched filter, followed by a sequence estimation algorithm using the Euclidean distance metric. The Ungerboeck receiver uses a matched filter, followed by a sequence estimation algorithm using a modified metric. A unified development of both receivers is given, in which each receiver is derived from the other. By deriving the Ungerboeck receiver from the Forney receiver, we show that the whitening operation is cancelled in the Euclidean distance metric, leaving the modified metric. In addition, the Ungerboeck receiver is extended to the case of a time-varying known channel. When the channel is unknown, decision-directed channel estimation is assumed, which requires channel prediction to account for the decision delay. It is shown that the Ungerboeck receiver requires additional channel prediction, degrading performance due to prediction uncertainty. To solve this problem, two alternative receiver forms are developed which do not require additional prediction, though the computational complexity is increased. Performance and complexity of the receiver forms are compared for the IS-136 digital cellular time-division multiple-access (TDMA) standard.

Successive cancellation of adjacent channel signals in FDMA/TDMA digital mobile radio systems

Capacity and signal quality can be improved in mobile radio systems by designing digital receivers with interference mitigation capability. In this paper, coherent MLSE receivers based on successive cancellation of adjacent channel interference in FDMA/TDMA mobile radio systems are developed. The proposed technique requires estimation of impulse response of the desired and adjacent channels, these are obtained via a novel approach employing pulse shaping information and band extrapolation. The receiver is simulated for the GSM digital cellular system under Rayleigh and Rician fading. The results show substantial improvements in bit error rate in interference limited environments.

Signature sequence selection in a CDMA system with orthogonal coding

In code-division multiple-access (CDMA) systems, recent attention has focused on the use of orthogonal coding to provide spreading. Each signal is coded with the same orthogonal or biorthogonal code, followed by a modulo-2 addition of a unique signature sequence. The set of signature sequences used determines how much signals interfere with each other at a receiver, thus determining the performance of the system. An analysis is presented to determine the properties of an optimal set of signature sequences for such a system. Using a Kerdock code, a set of signature sequences is presented which optimizes performance in a direct sequence CDMA system with (a) synchronous transmission, (b) no multipath time dispersion, and (c) orthogonal or biorthogonal Walsh-Hadamard coding as a means of spreading the information signal. For a length-N-binary code (where N is an even power of two), the set contains N/2 signature sequences. Approaches are discussed for the cases when N is an odd power of two and when more sequences are needed. >

Generalized RAKE receivers for MIMO systems

Recently, a variety of multiple-input multiple-output (MIMO) antenna techniques have been proposed for inclusion in the WCDMA standard. We develop two novel, multi-antenna receiver structures for MIMO operating in frequency-selective fading environments that offer a significant reduction in complexity compared to other MIMO equalizer structures. Both receivers are extensions of the generalized RAKE receiver (GRAKE) for single antenna systems; which receiver to use depends on the degree of dispersion in the channel. However, the two receivers together provide an attractive adaptive solution whereby the receivers are switched as the degree of dispersion changes. Moreover, it is shown that the combined solution is able to achieve performance close to the matched filter bound in a wide variety of frequency-selective fading environments.

Delay tracking for direct sequence spread spectrum systems in multipath fading channels

This paper considers the problem of multipath delay tracking in direct sequence spread spectrum systems operating in multipath fading channels. Due to the pulse shaping of the PN code chips, tracking multipath delays is a challenging problem, for which we introduce three novel techniques. In the first technique, maximum likelihood estimation, we search all possible combinations of delays and select the set of delays that minimize a metric derived from the error between the received signal and an estimated signal based on these postulated delays. To reduce complexity, we introduce the ordered maximum likelihood technique, in which the above mentioned metric is minimized iteratively assuming that the channel has one path, then two paths, etc. At each iteration, the delay estimates derived from previous iterations are fixed. Therefore, in each iteration only one delay estimate is produced. Another technique presented in this paper is envelope tracking with subtraction. In this technique, we select peaks of the correlation function between the received direct sequence spread spectrum signal and the local replica. After selecting each peak, the contribution due to the corresponding channel path is subtracted from the correlation function.

Interference cancellation with an array processing MLSE receiver

the capacity and transmission quality in cellular systems can be improved by using receivers that mitigate cochannel interference and multipath propagation. In this paper, a class of multichannel maximum likelihood sequence estimation (MLSE) receivers is developed for this purpose. Interference mitigation is accomplished via adaptive antenna arrays, while multipath propagation is combated via MLSE. Practical considerations are included, such as fixed front-end filtering, sampling, and estimation of parameters from received samples. Maximal ratio combining, conventional array processing and metric combining (MC) are shown to be special cases of the proposed receivers. Performance is evaluated for /spl pi//4-shift DQPSK, using the parameters and transmission format of the time-division multiple-access (TDMA)-based IS-136 (D-AMPS) digital cellular standard. Semi-analytical performance predictions are developed which confirm simulation trends. The results show that these receivers can operate at significantly lower carrier-to-interferer (C/I) levels than conventional MC receivers.

Adaptive array processing MLSE receivers for TDMA digital cellular/PCS communications

Array processing is a promising approach for improving quality, coverage, and capacity in digital cellular communication systems. By combining array processing with maximum likelihood sequence estimation (MLSE), intersymbol interference (ISI) introduced by multipath propagation can be mitigated as well. Novel symbol-spaced and fractionally spaced adaptive array processing MLSE receivers are developed for both diversity and phased array antenna configurations. The practical issues of synchronization and channel estimation are addressed. A novel approach to automatic frequency error correction (AFC) is proposed and is shown to be critical when cancelling cochannel interference. Performance is evaluated for the reverse link of the IS-136 TDMA-based digital cellular system. Substantial improvements are obtained over conventional antenna configurations for receiver sensitivity (2.5-4 dB) and over traditional antenna combining when cochannel interference is present (0.5-25 dB).