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Multicarrier code-division multiple access (CDMA) with time and frequency spreading has been recently considered as a candidate for fourth-generation (4G) wireless systems. This signaling scheme simultaneously utilizes code spreading in the time and frequency domains to simultaneously improve frequency diversity and minimize multiuser access interference. As a result, it is capable of outperforming multicarrier CDMA systems that employ 1-D spreading. In this paper, a novel adaptive subcarrier allocation algorithm is developed for multicarrier CDMA with time and frequency spreading to improve the overall bit error rate (BER) performance for all spreading configurations. This algorithm assigns users to subcarrier groups that provide favorable fading characteristics while simultaneously reducing the amount of interference caused to other users. The proposed algorithm is shown to provide a performance improvement, ranging from 1.5 dB with 2 x 16 (time x frequency) and spreading to 7 dB with 16 x 2 (time x frequency) spreading. The algorithm is also shown to maintain or improve the BER floor for each spreading configuration. It is concluded that at higher and lower levels of [E.sub.b]/[N.sub.o], a higher frequency- and time-domain spreading should be, respectively, employed to improve BER performance. Furthermore, the [E.sub.b]/[N.sub.o] threshold level to switch between time and frequency spreading for the analyzed system is found to be 2.5 dB. Index Terms--Adaptive subcarrier allocation, multicarrier code-division multiple access (CDMA), synchronous reverse links, time- and frequency-domain spreading.

This paper is concerned with the effect of multipath fading on the performance of frequency hop (FH) time synchronization and the resultant bit-error rate (BER) performance. The expressions of the mean value and the variance of the estimated FH timing error and BER are derived for the case of multipath Rayleigh fading, which is characterized by a path-delay vector and a path-strength distribution vector. Somewhat surprisingly, our results show that in comparison with single-path fading, multipath fading does not degrade the mean and variance of the estimated timing error and BER performance, but rather improves them owing to multipath diversity. The significance of our results is to demonstrate quantitatively the robustness of FH in the presence of multipath fading as far as the establishment of hop timing is concerned. Furthermore, we identify the spectral estimator as a robust method for establishing FH timing. Index Terms - Bit-error rate, frequency hop, multipath fading, spread spectrum, time synchronization.

An improved decorrelating decision-feedback multiuser detector (IDDFD) is proposed, whose complexity is linear in the number of users, but performs closely to the maximum likelihood detector (MLD), whose complexity grows exponentially with the number of users. The IDDFD also relaxes the requirement of correct power ordering which is essential to successfully cancel multiuser interference (MUI) for the standard decorrelating decision-feedback detector (DDFD). Simulation results show that in a direct-sequence spread-spectrum multiple-access (DS-SSMA) system using Gold codes of length 31 an IDDFD with only twice the complexity of the DDFD can simultaneously service 31 users with an error rate of each user close to the interference-free case. The IDDFD is then extended and studied to user numbers larger than the processing gain, complex signature waveforms, and MPSK signaling.

In digital wireless access communications systems, a large number of radio ports are deployed to provide wide-area coverage. Achieving time synchronization among ports is beneficial to these systems, especially for systems providing access to the infrastructure networks such as local exchange networks. This leads to better cochannel interference management and easier control for automatic link transfer. This paper describes a practical 'over-the-air' algorithm which is autonomously performed by individual ports and hierarchically controlled by the ports having the most reliable timing. This process involves only small overhead for demodulating special timing bits transmitted by other ports and adjusting timing accordingly. Computer simulations based on a time division multiple access (TDMA) system with port spacing of 2000-3000 feet and 500 kb/s transmission rate are used as an example to evaluate possible impacts on wireless access.

In this paper, we propose a new frequency synchronization algorithm for orthogonal frequency division multiplexing (OFDM) systems requiring only one training symbol, based on a conventional method which requires two training symbols. While the timing synchronization is obtained by using the conventional method, the carrier frequency offset is efficiently estimated by the proposed method. It is shown that the proposed method not only reduces the number of the training symbols but also possesses better performance than the conventional method without increase in complexity. Index Terms--Frequency synchronization, orthogonal frequency division multiplexing (OFDM).

In this paper, a data-aided frequency and symbol synchronization scheme for M-QAM OFDM signals is suggested. At first, the phase discriminator (PD) and frequency discriminator (FD) for, respectively, the symbol and frequency synchronization loop are described. Second, the transfer function and design parameters of the loop filters are provided. The acquisition and tracking performance of the synchronization system are evaluated using simulation for a 4-PSK signal constellation. The cases of additive white Gaussian noise (AWGN) and frequency and time selective multipath Rayleigh channels are separately tested. Index Terms--Frequency synchronization, multipath channel, OFDM, symbol synchronization.

This paper presents a code synchronization scheme--the time-window control loop (TWCL)--for postdetection RAKE receiver systems that offers the path-diversity effect. Laboratory test results from a postdetection RAKE receiver system with TWCL under a frequency-selective fading environment are introduced. RAKE combining methods, a simple integral demodulation method within a time window, and a recursive least squares (RLS) demodulation method within the time window that uses the RLS algorithm to realize maximal ratio combining, are investigated. It is found that TWCL performance with delay paths (double-spike frequency-selective Rayleigh-fading model) surpasses the performance of both acquisition and tracking without delay paths (nonfrequency-selective Rayleigh-fading model) because of the path-diversity effect. The laboratory results show that TWCL provides good acquisition and tracking performance under the fading conditions experienced with maximum Doppler frequencies of 5 or 80 Hz. It is also shown that the signal energy per bit to noise-power spectral-density ratio ([E.sub.b]/[N.sub.0]) required to achieve the bit error rate (BER) of [10.sup.-3] under the double-spike frequency-selective Rayleigh-fading model is reduced by 10 dB compared to the nonfrequency-selective Rayleigh-fading model. The difference in path-diversity effect between the two RAKE combining methods is only 1 dB. From a structural viewpoint, the simple integral demodulation method supports miniaturization and high-speed data-transmission. Index Terms--CDMA, code synchronization, multipath fading, path diversity, RAKE receiver.

In some private mobile radio/public access mobile radio (PMR/PAMR) applications, there is a stringent need for high-coverage locations probabilities. A spectrally efficient approach in this case is the use of several radio transmitters operating in simulcast mode. There have been several analog mobile radio systems working in this way up to now, but less is known about the performance of digital trunked radio systems operating in simulcast mode. In this paper, predicted digital Trans-European Trunk RAdio (TETRA) system performance results operating in quasi-synchronous mode are presented. These results were obtained by simulation of such a system under a wide range of operational conditions. A comparison is also presented with the European analog standard MPT 1327 currently in operation. It has been concluded that quasi-synchronous techniques well known in analog PMR/PAMR can also be successfully used in digital PMR/PAMR applications. Index Terms - MPT 1327, overlap coverage areas, simulcast, TETRA.