TD-SCDMA (Time Division- Synchronous CDMA) systems show performance degradation due to I/Q imbalance resulting from direct-conversion transmitters and receivers. In overall system analysis, it is important to consider both transmitter and receiver I/Q imbalances to account for low cost base stations (BS's) such as femtocells. Two approaches are considered in this paper. The first one is a purely time-domain approach based on widely linear minimum mean square error estimation (WL-MMSE). The second one is a frequency-domain approach with reduced computational complexity. Simulation studies are performed and comparison results presented.

In this paper, we analyze the performance of various receive antenna diversity schemes for use in combination with transmit diversity on the downlink of wideband code-division multiple-access third-generation systems. We consider open loop and closed loop versions of both maximal ratio combining and selection diversity, and study the impact of channel correlation on the performance of these schemes. We also present an analysis of the asymptotic performance of coded systems. The analytical results are compared with simulation results obtained in typical channels.

We analyze the performance of a fixed-point turbo decoder using two sliding window algorithms for the 3GPP standard. We show that the bit and frame error rate performance of the sliding window scheme strongly depends on the guard window size. Simulation results also indicate that small guard window sizes can significantly decrease the iteration gain for large frames.

We evaluate and compare several data detection schemes used in GSM systems. In particular, we compare the performance of decision feedback equalization (DFE) and nonlinear data directed estimation (NDDE) to that of maximum likelihood sequence estimation (MLSE). Establishing the performance of the basic NDDE detector is a first step in investigating the applicability of block transmission techniques to GSM systems. Our simulation results, obtained both for fading multipath channels and adjacent- and co-channel interference scenarios, suggest that the NDDE offers certain performance advantages over the DFE, and the performance of both detectors is comparable to that of the MLSE for the SNR region of interest in practical systems. Thus, they both represent viable alternatives to the MLSE detector.

Error probability of an adaptive multiuser diversity receiver is evaluated in terms of channel fading rate and the number of code-division multiple access users. Fading-induced performance loss, which leads to the error probability floor, is established for the proposed coherent combining scheme and compared to that of the differentially coherent receiver with equal-gain combining.

An adaptive multipath decorrelating multiuser receiver is considered for application in Rayleigh fading multipath channels with significant Doppler spread. Coherent diversity combining is performed using adaptively obtained channel estimates in a manner that minimizes the impact of estimation errors on data detection. The bit-error rate of the receiver is evaluated analytically, showing dependence on the fading rate of the channel and tracking capabilities of adaptive least mean square and recursive least square algorithms, in addition to the order of multipath diversity and the number of active code-division multiple-access users.

In this paper, we evaluate low complexity detectors for use in the GSM system. In particular, we compare the performance of decision feedback equalization (DFE) and nonlinear data directed estimation (NDDE) to that of the optimal maximum likelihood sequence estimation (MLSE). Establishing the performance of the basic NDDE detector is a first step in investigating the applicability of block transmission techniques to the GSM system. Simulation results, obtained both for fading multipath channels and adjacent- and co-channel interference scenarios, show that the NDDE performs just as well as the DFE, and the performance of both low complexity detectors is comparable to that of the MLSE for the SNR region of interest in practical systems.

We investigate a noncoherent detection technique based on a phase-locked loop (PLL) for wireless communication applications, with an emphasis on the Digital Enhanced Cordless Telephone (DECT) system. The performance of the PLL receiver in additive white Gaussian noise (AWGN) and interference-limited environments is simulated and compared to the performance of the limiter-discriminator detector (LD) and the differential detector (DD). The results indicate that a well-designed PLL receiver based on the selection of PLL parameters and post-detection filtering is a better candidate for the DECT than either LD or DD receiver.

In this paper, two probabilistic adaptive algorithms for jointly detecting active users in a DS-CDMA system are compared. The first one, which is based on the theory of hidden Markov models (HMM) is proposed within the CDMA scenario and compared with the previously developed Viterbi-based algorithm. Both techniques are completely blind in the sense that no knowledge of the signature sequences, channel state information or training sequences is required for any user. After convergence, an estimate of the signature of each user convolved with its physical channel impulse response (CIR), and estimated data sequences are provided. This CIR estimate can then be used to switch to any decision-directed (DD) adaptation scheme. Performance of the algorithms is verified with simulations as well as with experimental data from an underwater acoustics (UWA) environment. In both cases, performance is found to be highly satisfactory, showing the near-far resistance of the analyzed algorithms.

An adaptive decorrelating multiuser detector is considered for application in Rayleigh fading multipath channels with significant fading rate (Doppler spread). A suboptimal receiver structure, in which decorrelation is decoupled from multipath recombining, allows efficient implementation of adaptive channel estimation, e.g. via LMS algorithm. The channel estimates are used to perform maximum ratio combining. Bit error rate is evaluated analytically as a function of fading statistics and receiver tracking parameters. Results demonstrate performance sensitivity to the choice of the LMS step size when the channel is modeled as a second-order AR process. Performance degradation increases with fading rate, leading to the saturation of the error probability at a level determined by the fading rate of the channel and the deviation of the step size from the optimal value. The receiver is near-far resistant regardless of the fading rate of the channel.

Analytical expressions for the error probability of linear multipath-decorrelating receivers with coherent and differentially coherent reception are derived. Both multi-user receivers have superior performance compared to the RAKE receiver and eliminate error probability floor caused by multiple-access interference on a code-division multiple access (CDMA) reverse link. Results also emphasize benefits of using coherent multi-user reception with maximal-ratio combining in frequency-selective fading channels.

Multiuser detection in code division multiple access systems usually requires either knowledge of the transmitted signature sequences and channel state information or use of a known training sequence for adaptation. We develop a scheme that can be employed for the joint adaptive blind multiuser identification and detection in asynchronous CDMA systems. This scheme relies on a multiuser Viterbi algorithm that incorporates an adaptive estimation of the overall channel impulse responses, given by the convolution of the signature sequences of the users and corresponding physical channels impulse responses. Once the overall channel responses are estimated, the blind multiuser detection algorithm performs like the maximum-likelihood sequence estimator. Results are provided to illustrate the convergence of the blind multiuser approach, near-far resistance and sensitivity to the algorithm initialization.

Proposes a suboptimal low-complexity multiuser receiver for synchronous CDMA frequency-selective Rayleigh fading channels. In contrast to the conventional RAKE receiver, which suffers from near-far effects due to channel fading, the proposed multiuser receiver is shown to alleviate the near-far problem while preserving multipath diversity gain. This is demonstrated by comparing the symbol error probability and asymptotic multiuser efficiency of the proposed multiuser detector and RAKE receiver.>

Presents a performance analysis of a decorrelating detector for differentially coherent transmission over a CDMA frequency-nonselective Rayleigh fading channel. While the correlation-type receiver is near-far limited in this channel, it is shown that the aforementioned multiuser detector eliminates the bit error rate floor and alleviates the near-far problem.>

The acoustic local area network (ALAN) is designed for multipoint-to-point telemetry between ocean bottom sensors and a surface receiver in deep water areas. The main obstacle for coherent communications in this channel is strong multiple-access interference from cochannel signals, which affects a common narrowband request channel. Throughput and packet delay in this network depends on the capability of the receiver to resolve collisions between request packets. In this paper we propose an adaptive multiuser receiver for joint parameter estimation and multiple-access interference cancellation. It extends previous work on adaptive multiuser detectors to the more realistic communication scenario by estimating signal parameters rather than assuming that they are known. The adaptive approach is different from the one used in a centralized multiuser receiver in that no knowledge of the signature sequences of any of the active users is required.>

The performance analysis of maximum likelihood sequence and decorrelating multiuser detectors is developed for the single-path Rayleigh fading channel. The received signal in this multipoint-to-point channel is additively composed of white Gaussian noise and independently faded waveforms from a finite number of asynchronous users. The optimum sequence detector consists of a bank of single-user matched filters followed by a dynamic programming algorithm, has a demodulation complexity per symbol which is exponential in the number of users, and requires the knowledge of the system as well of the fading parameters. The decorrelating detector has a complexity which is polynomial in the number of users, and does not require knowledge of the signal amplitudes, which are assumed to vary slowly relative to the symbol rate. The single-user error rates are presented for both detectors and compared to conventional detection and isolated transmission, and the notion of asymptotic multiuser efficiency is developed for this channel. In contrast to conventional detection; which suffers from near-far effects due to Rayleigh fading, it is shown that the aforementioned multiuser detectors exhibit performance which rivals that of isolated transmission for sufficiently small thermal noise power. The results of the analysis are illustrated with numerical examples and indicate that the near-far problem may be alleviated by a more sophisticated receiver design.>

The performance analysis of low-complexity decorrelating detectors is presented for both coherent and differentially coherent transmission over the multiple-access, frequency-nonselective Rayleigh fading channel. While it is known that correlation-type receivers are near-far limited in the case when a single fading path dominates, it is shown that the multiuser detectors eliminate the bit error rate floor, alleviate the near-far problem, and rival optimum demodulation for isolated transmission. The results apply to wideband and narrowband CDMA systems.>