Multiuser detection of time-hopping PPM UWB system in the presence of multipath fading

Ultra-wideband (UWB) impulse radio (IR) systems are currently being considered for several applications due to their attractive features that include low-power carrierless and ample multipath diversity. Among the various modulation and multiple-access schemes, time-hopping (TH) pulse position modulation (PPM) is a popular technique in application. Most past works rely on strict power control and perform single-user detection (matched filtering) on the desired signal. This paper aims to apply multiuser detection techniques in binary PPM (BPPM) UWB IR multiple-access systems. Moreover, we consider frequency-selective multipath fading channels to account for the wireless cellular environment. A class of linear multiuser detectors (LMDs) is applied to extract the information bits while eliminating multiuser interference (MUI) in the presence of multipath fading. Simulation results are provided to compare the performance of different LMDs.     

Differentially-Encoded Di-Symbol Time-Division Multiuser Impulse Radio in UWB Channel

Ultra-wideband (UWB) communication systems are expected to operate in a highly frequency-selective multipath fading environment. To exploit multipath diversity gains in a multiuser scenario, we developed a differentially-encoded, di-symbol time-division multiuser impulse radio (d²TD-IR) system with delay-sum autocorrelation receivers. In traditional time-division multiple access systems, each user transmits a single pulse during a symbol duration in a pre-assigned chip which is longer than maximum excess delay of the channel. However, due to the exponential decay property of UWB channel, we proposed the use of much shorter chip duration, which significantly increases the transmission rate. Because dense pulse transmission will induce multiuser interference, two time-hopping access sequences, which alternately encode the odd- and even-index symbols, are employed with delay-sum autocorrelation receivers to maximally suppress the interference. It was shown that when the chip duration is properly chosen, the proposed system outperforms the conventional time-hopping impulse radio system at high signal-to-noise ratio. This paper also proposed a method to estimate the optimal chip duration when only the average power decay profile of the UWB channel is known.     

Block precoding for MUI/ISI-resilient generalized multicarrier CDMAwith multirate capabilities

Potential increase in capacity along with the need to provide multimedia services and cope with multiuser interference (MUI) and intersymbol interference (ISI) arising due to wireless multipath propagation, motivate well multirate wideband code-division multiple-access (CDMA) systems. Unlike most existing continuous-time symbol-periodic and multipath-free studies, the present paper develops an all-digital block-precoded filter-bank framework capable of encompassing single- or multirate transceivers for asynchronous or quasi-synchronous CDMA transmissions through multipath channels. Thanks to symbol blocking and through appropriate design of user codes, the resulting generalized multicarrier (GMC) CDMA system not only subsumes known multicarrier CDMA variants, but also equips them with flexible multirate capabilities. It is computationally simple, and guarantees symbol recovery regardless of the (possibly unknown) FIR multipath channels in both downlink and uplink setups. Simulations corroborate that the novel GMC-CDMA system outperforms existing multirate alternatives in the presence of asynchronism and multipath, and illustrate the feasibility of recovering blindly multirate transmissions received through unknown frequency-selective channels in the uplink. The performance of GMC-CDMA system in UMTS channels is also simulated and compared with existing multirate schemes     

Combined multiuser detection and diversity reception for wirelessCDMA systems

Code division multiple-access (CDMA) techniques using interference cancellation are being explored for the capacity increase in third-generation universal mobile telecommunications systems. However, multipath fading is a major constraint on the performance of wireless CDMA systems, with multipath propagation worsening the effects of multiple-access interference, and fading on propagation paths leading to the near far problem. Multiuser detection, exploiting the knowledge of other users to cancel multiple-access interference, has the capability of eliminating the near far problem and providing a significant capacity increase in CDMA systems. On the other hand, diversity techniques effectively combat the fading effects of the channel. This paper investigates multiuser receivers that combine explicit antenna diversity, RAKE multipath diversity, and multipath decorrelating detection. Both coherent reception with maximal-ratio combining and differentially coherent reception with equal-gain combining are analyzed. The results demonstrate a significant increase in up-link capacity over the conventional RAKE receiver, at the expense of complexity. In the case of limited receiver complexity, where the number of correlators is less than the number of resolvable paths at the RAKE front-end, antenna diversity is shown to be effective in reducing residual multiple-access interference     

Large system performance of linear multiuser receivers in multipathfading channels

A linear multiuser receiver for a particular user in a code-division multiple-access (CDMA) network gains potential benefits from knowledge of the channels of all users in the system. In fast multipath fading environments we cannot assume that the channel estimates are perfect and the inevitable channel estimation errors will limit this potential gain. We study the impact of channel estimation errors on the performance of linear multiuser receivers, as well as the channel estimation problem itself. Of particular interest are the scalability properties of the channel and data estimation algorithms: what happens to the performance as the system bandwidth and the number of users (and hence channels to estimate) grows? Our main results involve asymptotic expressions for the signal-to-interference ratio of linear multiuser receivers in the limit of large processing gain, with the number of users divided by the processing gain held constant. We employ a random model for the spreading sequences and the limiting signal-to-interference ratio expressions are independent of the actual signature sequences, depending only on the system loading and the channel statistics: background noise power, energy profile of resolvable multipaths, and channel coherence time. The effect of channel uncertainty on the performance of multiuser receivers is succinctly captured by the notion of effective interference     

A sliding window decorrelating receiver for multiuser DS-CDMAmobile radio networks

We detail the development of a near-far (NF) resistant sliding window decorrelating algorithm (SLWA) that overcomes the near-far problem (NFP) pertaining to the conventional linear correlation receiver (CLCR) and alleviates some of the limitations of the standard decorrelator. The SLWA architecture is extended to incorporate differentially coherent multipath (RAKE) diversity combining techniques and hence achieves simultaneous rejection of multiuser and multipath interference. This paper also presents a novel algorithm for updating the decorrelator coefficients using a fully parallel architecture. We present a mathematical model for the performance of a sliding window scheme, the main contribution of which is the analysis of finite sequence length multipath decorrelation under the practical limitation of incomplete RAKE combining. In addition to numerical results, we present results pertaining to capacity improvements achieved over the CLCR, derived from a simulation of a multiuser direct-sequence code-division multiple-access (DS-CDMA) mobile radio system     

Diversity and channel estimation using time-varying signals and time-frequency techniques

We propose the use of time-varying (TV) signaling in modulation schemes to provide multiuser detection and multipath diversity in TV wireless channels. Specifically, we design an orthogonal linear chirp modulation scheme that is based on assigning different users with optimally designed parameters in order to reduce multiple-access interference. We also derive conditions on the parameters of the modulation signals to achieve multipath diversity. Furthermore, we propose the use of TV pilot signals with nonlinear instantaneous frequency and matched time-frequency (TF) techniques to estimate fast-fading channels with unknown state information. The proposed algorithm simplifies to the estimation of the parameters of multiple linear chirps, which we perform using the modified matching pursuit decomposition. We compare our estimation method with the use of pilot signals with linear instantaneous frequency, which we implement using the reassigned spectrogram. The proposed modulation scheme is applied to a frequency-hopped code-division multiple-access system for which we demonstrate improved performance when compared with frequency-shift-keying (FSK) modulation due to the designed multipath diversity and low multiple-access interference. Our simulations also demonstrate the increased estimation performance when pilot signals with nonlinear structures are used instead of linear structured ones to estimate TV channel parameters.     

Multiuser detection in asynchronous CDMA frequency-selective fading channels

Multipath fading severely limits the performances of conventional code division multiple-access (CDMA) systems. Since every signal passes through an independent frequency-selective fading channel, even modest cross-correlations among signature sequences may induce severe near-far effects in a central multiuser receiver. This paper presents a systematic approach to the detection problem in CDMA frequency-selective fading channels and proposes a low complexity linear multiuser receiver, which eliminates fading induced near-far problem.We initially analyze an optimal multiuser detector, consisting of a bank of RAKE filters followed by a dynamic programming algorithm and evaluate its performance through error probability bounds. The concepts of error sequence decomposition and asymptotic multiuser efficiency, used to characterize the optimal receiver performance, are extended to multipath fading channels.The complexity of the optimal detector motivates the work on a near-far resistant, low complexity decorrelating multiuser detector, which exploits multipath diversity by using a multipath decorrelating filter followed by maximal-ratio combining. Analytic expressions for error probability and asymptotic multiuser efficiency of the suboptimal receiver are derived that include the effects of multipath fading, multiple-access interference and signature sequences correlation on the receiver's performance.The results indicate that multiuser detectors not only alleviate the near-far problem but approach single-user RAKE performance, while preserving the multipath diversity gain. In interference-limited scenarios multiuser receivers significantly outperform the RAKE receiver.This paper was presented in part at the Twenty-Sixth Annual Conference on Information Sciences and Systems, Princeton, NJ, March 1992 and MILCOM'92, San Diego, CA, October 1992. This work was performed while author was with the Department of Electrical and Computer Engineering, Northeastern University, Boston, USA.     

Adaptive transmitter optimization for blind and group-blind multiuser detection

The linear subspace-based blind and group-blind multiuser detectors recently developed represent a robust and efficient adaptive multiuser detection technique for code-division multiple-access (CDMA) systems. In this paper, we consider adaptive transmitter optimization strategies for CDMA systems operating in fading multipath environments in which these detectors are employed. We make use of more recent results on the analytical performance of these blind and group-blind receivers in the design and analysis of the transmitter optimization techniques. In particular, we develop a maximum-eigenvector-based method of optimizing spreading codes for given channel conditions and a utility-based power control algorithm for CDMA systems with blind or group-blind multiuser detection. We also design a receiver incorporating joint optimization of spreading codes and transmitter power by combining these algorithms in an iterative configuration. We will see that the utility-based power control algorithm allows us to efficiently set performance goals through utility functions for users in heterogeneous traffic environments and that spreading code optimization allows us to achieve these goals with lower transmit power. The signal processing algorithms presented here maintain the blind (or group-blind) nature of the receiver and are distributed, i.e., all power and spreading code adjustments can be made using only locally available information.     

Performance analysis of linear multisensor multiuser receivers forCDMA in fading channels

Bit-error probability (BEP) analysis for linear multiuser receivers with multiple sensors in frequency selective Rayleigh fading channels is presented. The analysis is applied to evaluate the BEP in antenna diversity reception and in a cellular CDMA system. Diversity and multiuser receivers are compared based on the examples. It is observed that adding new diversity antenna elements improves performance even if the correlation between the antenna elements is relatively large (up to 0.7). However, the large correlation values pose a significant reduction in the diversity gain in comparison to the zero correlation. It is also seen that the macroscopic diversity improves the performance of receivers significantly in cellular CDMA networks. When comparing diversity and multiuser receivers it is concluded that multiuser receivers are necessary to provide low BEPs. It is also highly beneficial to have at least two diversity antennas available, in particular, if there is no multipath diversity provided by the channel. The results also show that the reduction of intercell multiple-access interference yields a significant performance advantage in cellular networks. It is also demonstrated that the combination of spatial diversity and a multiuser receiver provides a significant receiver performance or system capacity gain in comparison to implementing only one of them     

A multiuser interference cancellation technique utilizingconvolutional codes and orthogonal multicarrier modulation for wirelessindoor communications

This paper suggests that multicarrier modulation reduces the complexity and the delay caused by the multiuser interference cancellation process utilizing convolutional codes. For spread spectrum multiple access, multiuser interference (interference due to signals from other users) limits the performance of the communication link. To remove this interference, a multiuser interference cancellation technique which utilizes orthogonal convolutional codes has been proposed for the uplink (mobiles to a base station) of the cellular code-division multiple-access (CDMA) systems. However, this technique requires large interleavers and huge memory, or artificial multipath diversity and a RAKE system to achieve sufficient coding gain if it is applied to wireless indoor communications and fading is slow compared to the data rate. To reduce the complexity of the canceller, multicarrier modulation is employed as it provides frequency diversity gain and coding gain without the interleavers or a RAKE system. This paper shows that multicarrier modulation reduces the complexity of the canceller and still provides sufficient coding gain in order to cancel the multiuser interference. A canceller with decoding in the initial decision and multicarrier modulation improves the capacity by a factor of 1.4 as compared with a canceller without decoding     

Iterative space-time processing for multiuser detection in multipath CDMA channels

Space-time processing and multiuser detection are two promising techniques for combating multipath distortion and multiple-access interference in code division multiple access (CDMA) systems. To overcome the computational burden that rises very quickly with increasing numbers of users and receive antennas in applying such techniques, iterative implementations of several space-time multiuser detection algorithms are considered here. These algorithms include iterative linear space-time multiuser detection, Cholesky iterative decorrelating decision-feedback space-time multiuser detection, multistage interference canceling space-time multiuser detection, and expectation-maximization (EM)-based iterative space-time multiuser detection. A new space-time multiuser receiver structure that allows for efficient implementation of iterative processing is also introduced. Fully exploiting various types of diversity through joint space-time processing and multiuser detection brings substantial gain over single-receiver-antenna or single-user-based methods. It is shown that iterative implementation of linear and nonlinear space-time multiuser detection schemes discussed in this paper realizes this substantial gain and approaches the optimum performance with reasonable complexity. Among the iterative space-time multiuser receivers considered in this paper, the EM-based (SAGE) iterative space-time multiuser receiver introduced here achieves the best performance with excellent convergence properties.     

Iterative semiblind multiuser receivers for a space-time block-coded MC-CDMA uplink system

Iterative multiuser detection and space-time coding are two promising techniques to improve the capacity and performance of coded multiuser systems in wireless channels. In this paper, we present iterative multiuser detection schemes for a space-time block-coded multicarrier code-division multiple-access system with multiple transmit and receive antennas. We consider a more general case of an uplink system in the presence of both intra- and intercell interferences. We propose two types of iterative semiblind space-time receivers for such an uplink environment. The first is based on the minimum mean-square error criterion and the second is a hybrid scheme based on a combination of parallel interference cancellation and linear multiuser detection. These iterative receivers are derived, using a subspace approach, which utilizes known users' information for the computation of log-likelihood ratios (LLRs) while blindly suppressing the unknown interference. The LLRs are refined successively during the iterative process by using the extrinsic information available through decoding of all known users. A turbo code is used for channel coding. Simulation results in a frequency-selective Rayleigh-fading environment are presented to verify the performance of the proposed schemes.     

A New Class of Efficient Block-Iterative Interference Cancellation Techniques for Digital Communication Receivers

A new and efficient class of nonlinear receivers is introduced for digital communication systems. These iterated-decision receivers use optimized multipass algorithms to successively cancel interference from a block of received data and generate symbol decisions whose reliability increases monotonically with each iteration. Two variants of such receivers are discussed: the iterated-decision equalizer and the iterated-decision multiuser detector. Iterated-decision equalizers, designed to equalize intersymbol interference (ISI) channels, asymptotically achieve the performance of maximum-likelihood sequence detection (MLSD), but only have a computational complexity on the order of a linear equalizer (LE). Even more importantly, unlike the decision-feedback equalizer (DFE), iterated-decision equalizers can be readily used in conjunction with error-control coding. Similarly, iterated-decision multiuser detectors, designed to cancel multiple-access interference (MAI) in typical wireless environments, approach the performance of the optimum multiuser detector in uncoded systems with a computational complexity comparable to a decorrelating detector or a linear minimum mean-square error (MMSE) multiuser detector.     

Capacity of synchronous coded DS SFH and FFH spread-spectrummultiple-access for wireless local communications

The performance of synchronous spread-spectrum multiple-access (SSMA) communications based on direct-sequence (DS), slow frequency-hopped (SFH), and fast frequency-hopped (FFH) systems for wireless local communications of micro-cellular personal communications is analyzed. Using an indoor multipath fading channel model with clusters of arriving rays, we investigate multiuser DS systems with RAKE and diversity reception by selection combining (SC), multiuser SFH systems with equal-gain (EG) diversity reception, and multiuser FFH systems with correlated EG and self-normalization (SN) combining techniques. Reed-Solomon codes are considered to further improve the system performance. Given a fixed available bandwidth with narrow band interference (NBI), capacities and packet error rates are determined under various system configurations. Total capacities of hybrid frequency-division multiple-access (FDMA)/SSMA (DS and SFH) systems are compared with those of wide-band SSMA systems. For high data rate communications, wide-band DS-SSMA systems have larger capacities than hybrid FDMA/DS-SSMA systems. For low data rate communications, a capacity comparison between wide-band DS-SSMA and hybrid FDMA/DS-SSMA systems depends on fading statistics. Hybrid FDMA/SFH-SSMA systems have larger capacities than wide-band DS-SSMA systems, FFH-SSMA systems could not provide satisfactory performance due to correlation among hopping bands     

Lagrange/Vandermonde MUI eliminating user codes forquasi-synchronous CDMA in unknown multipath

A family of codes for low-complexity quasi-synchronous code division multiple access (CDMA) systems is developed in order to eliminate multiuser interference (MUI) completely in the presence of unknown and even rapidly varying multipath. Judiciously designed precomputable symbol-periodic user codes, which we term Lagrange or Vandermonde, and the corresponding linear receivers offer a generalization of orthogonal frequency division multiplexing (OFDM), which are especially valuable when deep-fading, carrier frequency errors, and Doppler effects are present. The flexibility inherent to the designed transceivers is exploited to derive transmission strategies that cope with major impairments of wireless CDMA channels. The symbol-periodic code design is also generalized to include the class of aperiodic spreading and orthogonal multirate codes for variable bit rate users. Performance analysis and simulations results illustrate the advantages of the proposed scheme over competing alternatives     

Performance analysis for the improved linear multiuser detectors in BPSK-modulated DS-CDMA systems

Recently, a new class of linear multiuser receivers for direct-sequence code-division multiple-access (CDMA) systems employing binary phase-shift keying modulation has been introduced. Unlike classical decorrelating and minimum mean-square error linear multiuser detectors, the new receivers exploit the information contained in the pseudo-autocorrelation of the observables, and are, thus, capable of achieving much better performance. We present new results on the performance analysis of this class of new receivers. In particular, with reference to a CDMA system with deterministic spreading codes, we show that the new receivers outperform the classical ones in terms of both error probability and near-far resistance. With regard, instead, to CDMA systems with random spreading, we compute the average system near-far resistance, showing that the new receivers can accommodate twice the number of users accommodated by the classical linear multiuser receivers.     

Performance of CDMA random access systems with packet combining in fading channels

We analyze the system performance of code-division multiple-access (CDMA) random access systems with linear receivers and packet combing in multipath fading channels. Both slotted and unslotted CDMA systems with random spreading codes are considered. The analysis is based on large systems in which both the offered load and the processing gain tend to infinity but their ratio is fixed. It is relatively easy to characterize the traffic in such large systems, which enables us to derive the system throughput and average delay. From the analysis results, it is observed that multiuser detection and packet combining substantially improve the system performance.     

Linear space-time multiuser detection for multipath CDMA channels

We consider the problem of detecting synchronous code division multiple access (CDMA) signals in multipath channels that result in multiple access interference (MAI). It is well known that such challenging conditions may create severe near-far situations in which the standard techniques of combined power control and temporal single-user RAKE receivers provide poor performance. To address the shortcomings of the RAKE receiver, multiple antenna receivers combining space-time processing with multiuser detection have been proposed in the literature. Specifically, a space-time detector based on minimizing the mean-squared output between the data stream and the linear combiner output has shown great potential in achieving good near-far performance with much less complexity than the optimum space-time multiuser detector. Moreover, this space-time minimum mean-squared error (ST-MMSE) multiuser detector has the additional advantage of being well suited for adaptive implementation. We propose novel trained and blind adaptive algorithms based on stochastic gradient techniques, which are shown to approximate the ST-MMSE solution without requiring knowledge of the channel. We show that these linear space-time detectors can potentially provide significant capacity enhancements (up to one order of magnitude) over the conventional temporal single-user RAKE receiver     

Performance analysis of a wireless multirate direct-sequence CDMAusing fast Walsh transform and decorrelating detection

We analyze the performance of a flexible multirate scheme for direct-sequence code division multiple-access (CDMA) mobile radio systems. The proposed scheme uses a variable processing gain serial pseudonoise modulation as a multirate strategy. To reduce the interference effects, the CDMA system utilizes the coherent fast Walsh transform transmission technique. The proposed scheme can be used in the reverse link (mobile-to-base station) of the upcoming third-generation wide-band CDMA system (has the feature of coherent reverse link). We analyze the system performance with and without using a decorrelating multiuser detector. The uncoded bit-error probability (BEP) with and without decorrelating detection on a multipath fading channel is derived analytically. In addition, the validity of the analysis results is demonstrated by computer simulations using the IMT-2000 vehicular multipath channel model. In order to make sure that the proposed processing techniques do not distort the soft values at the demodulator output, the proposed multirate scheme is also simulated in case of using turbo codes. The turbo-coded BEP is calculated for different user data rates and different number of decoding iterations