Doppler spread estimation in digital mobile radio systems is described. An optimal approach (in the sense of maximum likelihood estimation) for Doppler spread estimation is derived. Suboptimal approaches, with trading off in performance and complexity, are developed from the optimal algorithm. In practical systems, the Doppler spread must be estimated in the presence of frequency offsets between the transmitter and the receiver. Hence, an algorithm that decouples Doppler estimation from automatic frequency compensation (AFC) is also presented. The proposed algorithms rely on channel estimation over the known fields of a TDMA burst. Such algorithms can be applied easily to systems with pilot symbols or a pilot channel.

In this paper, efficient and practical approaches for Doppler spread estimation in wireless mobile radio systems are described. A hypothesis-testing approach, given the channel autocorrelation estimate, is utilized for Doppler spread estimation. The channel autocorrelation is estimated slot-by-slot using the knowledge of the channel estimates over the known fields of a TDMA burst, and averaged over several slots to reduce the effect of noise. In practical systems, the Doppler spread must be estimated in the presence of frequency offsets between the transmitter and the receiver. Hence, an algorithm that decouples Doppler spread estimation from automatic frequency compensation (AFC) is also presented. In addition to the mean estimation error performance, the convergence and the tracking ability of the algorithms are evaluated via simulation using ANSI 136 Rev-B modulation and signal transmission format.

The capacity and signal quality of wireless communication systems can be improved dramatically by enhancing the receiver performance in the presence of adjacent channel interference (ACI). The sequence error rate is minimized through joint demodulation of all signals, but with high implementation complexity. Complexity can be greatly reduced through the use of successive cancellation, in which signals are detected and subtracted in descending order of signal power. However, successive cancellation performs poorly when signals have similar power levels. In this paper, an improved successive cancellation receiver for adjacent channel signals is proposed. The conventional detector is replaced with a simple form of joint demodulation. As a result, the receiver combines the benefits of joint demodulation, which works well when signals have similar power levels, with the benefits of successive cancellation, which works well when signals have dissimilar power levels. Performance results for the Global System for Mobile (GSM) communication digital cellular system show an order of magnitude reduction in bit-error-rate when adjacent channel signals are closely spaced and have comparable power levels.

The capacity and signal duality of wireless communications systems can be improved by enhancing receiver performance in the presence of adjacent channel interference (ACI). In this paper, an MLSE receiver for joint demodulation of adjacent channel signals is developed. This receiver provides significant improvement in performance over a wide range of carrier to interference ratios. A practical approach is taken, in which conventional down-conversion of each signal is performed followed by joint demodulation at baseband. To provide coherent reception, a novel form of joint channel estimation is used, exploiting knowledge of the transmit and receive filter responses. Receiver performance is evaluated via simulation for the GSM digital cellular system.

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.

Achievable spectral efficiency in land-mobile radio (LMR) systems is limited by adjacent-channel interference (ACI) specifications. Spectral efficiency can be improved by using digital modulation schemes. With linear modulation schemes, intermodulation products produced by non-linear power amplifiers lead to increased ACI. In this paper, a new family of linear modulation schemes called envelope-constrained modulation schemes is introduced to improve the spectral efficiency of LMR systems using quasilinear power amplifiers. These modulation schemes exhibit low envelope variations and thus ease the linearity requirements on the power amplifier. Also, power savings can be realized by incorporating coded modulation. An analytical evaluation of the performance is performed to illustrate the gain achievable using the new modulation schemes.

This paper provides a new closed form expression for calculating the probability that an MPSK signal will lie in a particular decision region when received over N independent and identically distributed Rayleigh fading channels corrupted by additive white Gaussian noise. This expression is applied to provide a unified method to derive the exact symbol error rate and bit error rate for MPSK signals considering N channel diversity reception. The N channel diversity reception techniques considered are maximal ratio combining (MRC) and selection combining. MRC with identical channels and dissimilar channels is considered. The results for MRC can be extended to provide an approximation for the error rates of MPSK under equal gain combining.>

We evaluate the down-link (base-to-mobile) capacity of a frequency-hopped TDMA system using a theoretical development of outage probability. Expressions for outage probability are provided considering voice activity, antenna diversity and Reed-Solomon coding. The system capacity is evaluated with and without power control. Down-link capacity is compared with up-link capacity. The effect of increasing system bandwidth on capacity is analyzed. Finally, the issue of calculating capacity under the assumption of connecting mobiles to the closest base station instead of the strongest base station is examined.>

Set partitioned trellis codes and multi-level block codes can be used to increase the information rate of the American digital cellular system (IS-54) from 1.22 bits/symbol to 1.75-2 bits/symbol. This increase in information rate can be used to enhance speech quality or increase system capacity. The bit error rate performance of several block coded modulation schemes is studied via simulation for flat and dispersive Rayleigh fading channels using the IS-54 base-mobile frame format. Coherent and differential detection, and equalization are studied considering finite interleaving and channel estimation via interpolation.>

The spectral efficiency in land mobile radio (LMR) systems is limited by stringent adjacent-channel interference protection ratio (ACIPR) specifications of at least 60 dB. High spectral efficiency can be achieved using digital modulation techniques. Linear modulation schemes exhibit high spectral efficiencies but require linear power amplifiers. Power amplifiers typically used in LMR are nonlinear which degrade the ACIPR for conventional linear modulation schemes. The paper presents simulated performance results of a trellis-coded linear modulation scheme that is designed to be less sensitive to the nonlinearity exhibited by the power amplifier. Simulation results indicate that an ACIPR of 60 dB can be achieved. An E/sub b//N/sub 0/ of 14 dB is required to achieve a bit error rate of 3% with 3 users per 25 kHz channel, with an average data rate of 9.6 kbits/second/user.>

In mobile communications, the receiver must mitigate severe signal impairments arising from multipath and Rayleigh fading. Multipath produces intersymbol interference (ISI), necessitating the use of an equalizer in the receiver. The performance of the equalizer can be significantly improved using antenna diversity, thereby allowing the use of lower C/I values. This C/I reduction offers potential for capacity enhancement. This paper describes the use of pre-detection antenna diversity in conjunction with a MLSE receiver for the U.S. Digital Cellular (USDC) system. It is shown that the receiver meets all the USDC (IS-54) performance specifications with a significant margin and has a computational complexity of less than 4 MIPS.>