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In orthogonal frequency-division multiplexing (OFDM) systems, a cyclic prefix (CP) is often added at the transmitter and discarded at the receiver. When the length of the CP exceeds the delay spread of the channel, a portion of the CP can be used to recover additional signal energy. In the past, Nyquist windowing techniques have been proposed to recover signal energy, thereby improving performance. In this paper, linear maximum-likelihood (ML) and minimum-mean-square-error (MMSE) approaches are developed, which further improve performance, particularly when the signal-to-noise ratio (SNR) is low. Simpler two-valued windowing (TVW) solutions are also provided, which generally employ one or two non-Nyquist windows. Index Terms--Cyclic prefix (CP), demodulation, maximum-likelihood (ML) detection, multicarrier transmission, orthogonal frequency-division multiplexing (OFDM).

Several digital cellular systems employ coherent Rake reception, wherein channel estimates are used to combine despread values. In this paper, a maximum likelihood (ML) approach to Rake combining that accounts for channel estimation error, including error correlation across despread values, is examined. Fading and noise correlation are also considered. The performance of the ML approach is compared to traditional Rake combining as well as approximate ML-based approaches. Results show that when the channel estimation error and noise are of the same order, ML-based approaches can provide gains on the order of 1 dB over traditional Rake reception. Index Terms--Channel estimation, code division multiple access (CDMA), coherent demodulation, demodulation, maximum likelihood detection, multipath channels, spread spectrum communication.

In mobile communication systems, downlink (forward link) system capacity is limited by the ability of mobile receivers to recover the desired signal in the presence of cochannel interference (CCI). Joint detection of the desired and cochannel signals is a useful approach to improving receiver performance, thus increasing system capacity. In this paper, we show that a practical single-antenna joint-detection receiver can provide significant gains in system capacity for the time-division multiple-access (TDMA) standard Telecommunications Industry Association/Electronic Industry Association/Interim Standard-136 (TIA/EIA/IS-136 or IS-136). For a sectorized system, joint detection provides a capacity gain of 47% in a typical urban environment. When used in conjunction with transmit beamforming, the synergy between the two approaches leads to a capacity gain of over 200%. In determining these gains, practical aspects of the IS-136 system are considered, namely, unsynchronizod networks, limited receiver complexity, and adaptability. A semiblind acquisition process, which uses the training sequence of the desired user only, is employed, because the desired and interfering base stations are not synchronized. The receiver complexity is controlled by processing only one sample per symbol period, even though it is shown that multiple samples per symbol period should ideally be used. Finally, because receiver performance may be limited by its own intersymbol interference instead of CCI, an adaptive joint-detection process is used which selects between joint demodulation and single-user equalization for each slot. Index Terms--Antenna arrays, cellular radio, cochannel interference (CCI), interference suppression, maximum-likelihood detection (MLD), multiuser channels, time-division multiple access (TDMA).

Forney and Ungerboeck 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. In this paper 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 canceled 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 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. Index Terms - Adaptive equalizers, equalizers, fading channels, maximum likelihood estimation, radio receivers, sequence estimation, time-varying channels.

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. This paper presents an analysis 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 (WH) coding as a means of spreading the information signal. For a length N binary code (N 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.

The capacity and signal quality of wireless communication systems can be improved by enhancing receiver performance in the presence of adjacent channel interference (ACI). In this paper, maximum-likelihood sequence estimation (MLSE) receivers based on joint demodulation and successive cancellation of adjacent channel signals are developed. The proposed receivers provide significant improvement in performance over a wide range of carrier-to-interference ratios. To provide coherent reception, novel channel estimation techniques are used, exploiting knowledge of the transmit and receive filter responses. Performance is evaluated and compared via simulation for the Global System for Mobile communications (GSM) digital cellular system. Index Terms--FDMA, interchannel interference, interference suppression, maximum likelihood sequence estimation, TDMA.

Position estimation (geolocation) of a direct-sequence, spread-spectrum radio emitter is particularly challenging because of low signal-to-noise ratio at the radio sensors. In this letter, we develop a maximum-likelihood approach to geolocation that utilizes knowledge of the spreading sequence. Simulation results are used to show that spreading sequence knowledge can provide significant gains in position accuracy. [ABSTRACT FROM AUTHOR]

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 combatted 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 [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. Index Terms - Antenna arrays, array signal processing, equalizers, fading channels, interference suppression, maximum likelihood sequence estimation, time-varying channels.

This article introduces the readers of the Journal "IEEE Tansactions on Vehicular Technology," to its new Editor-in-Chief Tan F. Wang. Wang had been serving as associate Editor for past several years. It further informs about the technological progress, the journal had made. For past 3 years, it's peer review process has migrated from a paper-based system to a web-based system and as a result, reviews are conducted by a diverse group of international experts. It also thanks all those who are involved with the publishing process of the journals.