In this paper, we formulate the SINR-constrained power control problem in the wideband CDMA cellular system as a game. Using the cooperative theory and Nash bargaining model, we compared it to the non-cooperative game scheme proposed in (S. Koskie and Z. Gajic, 2005). We show that the resulting operating point using Nash bargaining model is fair and Pareto optimal, however, the Nash equilibrium operating point obtained in the non-cooperative game scheme is not in general Pareto optimal.
This paper addresses the problem of (nonlinear) iterative multiuser detection (MUD) for synchronous CDMA. In the literature, there are several related methods for solving this problem. These methods involve a linear estimator combined with nonlinear detection of the transmitted symbols but generally do not apply per-symbol iteration. However, the convergence analysis of iterative schemes has not been addressed yet (to our knowledge). In this paper, we present a new algorithm for MUD and provide a convergence analysis for an approximation to it. We believe this approach yields an understanding not only of the behavior of our proposed scheme but also of other possible iterative algorithms for MUD.; http://www.elsevier.com/wps/find/journaldescription.cws_home/622818/description#description; Sylvie L. Perreau and Langford B. White
We analyze the use of the bipolar-equivalent coding method in a incoherent fiber optic time-encoded CDMA communication system in an attempt to emulate the behavior of a coherent bipolar system. Specifically, we consider two possible asynchronous versions of the system which we refer to as appended and interleaved. We then describe a unified theoretical analysis for these systems based on the average interference parameter and the signal-to-noise ratio. We then consider the performance of the appended and interleaved systems and compare them to the performance of similar bipolar systems using length-127 m-sequences and Gold codes. We find that interleaving performs best of the methods considered--performance is very similar to the bipolar system we were trying to emulate, except for a factor of two increase in bandwidth requirement.
Journal Paper; The mobile wireless channel affords inherent diversity to combat the effects of fading. Existing code division multiple access (CDMA) systems, by virtue of spread-spectrum signaling and RAKE reception, exploit only part of the channel diversity via multipath combination. Moreover, their performance degrades under fast fading commonly encountered in mobile scenarios. In this paper, we develop new signaling and reception techniques that maximally exploit channel diversity via joint multipath-Doppler processing. Our approach is based on a canonical representation of the wireless channel which leads to a time-frequency generalization of the RAKE receiver for diversity processing. Our signaling scheme facilitates joint multipath-Doppler diversity by spreading the symbol waveform beyond the inter-symbol duration to make the channel time-selective. A variety of detection schemes are developed to account for the inter-symbol interference (ISI) due to overlapping symbols. However, our results indicate that the effects of ISI are virtually negligible due to the excellent correlation properties of the pseudo-random codes. Performance analysis also shows that relatively small Doppler spreads can yield significant diversity gains. The inherently higher level of diversity achieved by time-selective signaling brings the fading channel closer to an additive white Gaussian noise channel...
Journal Paper; We address the problem of multiuser detection in fast fading multipath environments for DS-CDMA systems. In fast fading scenarios, temporal variations of the channel cause significant performance degradation even with the RAKE receiver. We use a previously introduced Time-Frequency (TF) RAKE receiver based on a canonical formulation of the channel and signals to simultaneously combat fading and multipath effects. This receiver uses the Doppler spread caused by rapid time-varying channel as another means of diversity. In dealing with multiaccess interference and as an attempt to avoid the prohibitive computational complexity of the optimum Maximum-Likelihood (ML) detector, we use the Expectation Maximization (EM) algorithm to derive an approximate ML detector. The new detector turns out to have an iterative structure very similar to the well-known multistage detector, but because of using the EM algorithm, it has better convergence properties than the multistage detector.
Journal Paper; Next-generation wireless computing platforms will contain flexible communications capabilites. At Rice University, the Rice Everywhere NEtwork (RENE) project is investigating a multi-standard, multi-tier integration of W-CDMA cellular systems, high speed wireless LANs, and home wireless networks. There are many challenges in mapping these advanced communication algorithms to real-time hardware computing platforms. In this paper, we present current work on the development of a reconfigurable baseband physical layer containing DSP processors and FPGA accelerators. Our goal is the design of a multi-tier network interface card (mNIC) which is capable of exploiting eÂ±cient, low-power reconfiguration.
Conference paper; CDMA is becoming an increasingly popular multiplexing scheme in wireless communications and this has necessitated the development of efficient detection techniques. The exponential complexity of the optimal detector on one end and inferior performance of conventional single-user detctor at the other have led to the development of suboptimal multiuser detectors with lower complexity. Most of these detection techniques involve solution of a linear system. In their naive implementation this requires O(nÂ³) operations in the size of the matrix. This cost can be reduced if we move towards modern iterative techniques for solution of the system. However maximum benefit can be achieved if we fully exploit the structure of the system. In this paper we have proposed several methods of reducing the computational complexity utilizing the above ideas. We have also come up with algorighms which computationally can achieve the lower bound in complexity.
Conference paper; We describe an iterative detection and decoding scheme for the uplink in a convolutionally coded direct sequence code division multiple access (DS-CDMA) system. We consider maximum-a-posteriori (MAP) decoding and reduce the complexity through an iterative interference cancelation scheme combined with a suboptimal channel decoding algorithm. The MAP priors are updated at every iteration step. We investigate the performance of this low complexity scheme and observe that it is close to optimal.
Conference Paper; An iterative detection and decoding algorithm is explored for a convolutionally coded DS-CDMA system. The optimum decoding strategy is to consider the trellises of all the users simultaneously, but this has exponential complexity in the number of users. We use an iterative +multistage detection scheme combined with the MAP algorithm to reduce the complexity of the receiver without significant performance degradation. The simulation results show that our algorithm consistently outperforms other existing low complexity algorithms. We also provide a framework where the similarities and differences between various algorithms become transparent. Based on this framework, we make a quantitative comparison of the various schemes under investigation.
Journal Paper; Multiple access interference (MAI) is a significant limiting factor in the performance of direct sequence code division multiple access (DS-CDMA) systems. Various multiuser detection techniques have been developed to combat the effects of MAI. These detection techniques either assume the knowledge of all the users in the system (conventional) or assume the knowledge of the user of interest only (blind). Due to the limitations of the blind algorithms in the presence of a large number of interferers, there is a significant performance gap between these two classes of detectors. Additionally, in practice, the receiver could have only partial knowledge of the interference. In this paper, we develop a new class of detectors, partially blind multiuser detectors, that use information about a subset of interferers and bridge the performance gap between the blind and the conventional multiuser detectors.
Conference paper; In this paper an original Bayesian approach for blind detec-tion for Code Division Multiple Access (CDMA) Systems in presence of spatial diversity at the receiver is developed. In the noiseless context, the blind detection/identification problem relies on the canonical decomposition (also re-ferred as Parallel Factor analysis [Sidiropoulos, IEEE SP 00], PARAFAC. The author in [Bro,INCINC 96] pro-poses a suboptimal solution in least-squares sense. How-ever, poor performance are obtained in presence of high noise level. The recently emerged Markov chain Monte Carlo (MCMC) signal processing method provide a novel paradigm for tackling this problem. Simulation results are presented to demonstrate the effectiveness of this method.
Conference paper; In this paper, we consider linear MMSE equalization for wireless downlink transmission with multiple transmit and receive antennas in fast fading environment. We propose a new algorithm based on conjugate-gradient algorithm with enhanced channel estimation. In order to be robust to the channel variations, the channel coefficients are estimated by using a weighted sliding window. Two methods to determine optimal weights with respect to the Doppler frequency are proposed. The algorithm has been tested in fast fading environment (Vehicular A for a velocity for the mobile station of 120 km/h). We show by simulations that good performance are obtained in correlated fast fading environment with reasonable complexity. Moreover, this method outperforms approaches based on forgetting factor, basic sliding window and LMS.
Conference Paper; Using a fiber-based testbed, we experimentally verify that the advantageous correlation properties of bipolar spread-spectrum codes can be preserved in an optical channel using direct detection and all-optical encoder/decoders. The power spectrum of an erbium-doped superfluorescent fiber source is encoded, the codeword correlations are verified and rejection of multiple-access interference is demonstrated
Conference Paper; We compare the performance of different accessing schemes for communication over an additive white Gaussian noise multiple access channel with multipath fading. We assume that only the receiver can track the channel variations. We investigate time division, frequency division and code division multiple access schemes and consider the Shannon capacity as well as the probability of outage. Although orthogonal code division multiple access has the same total capacity with frequency and time division, it has better probability of outage. We also provide an extension of our comparison to a CDMA system employing non-orthogonal codes and multiuser detection.
Journal Paper; A code-division multiple-access (CDMA) communication system is studied where a trellis-based scheme is used for data encoding and modulation. The signature sequences (spreading codes), which are assigned to the direct-sequence spread-spectrum (DS/SS) modulator according to the encoding rule, are taken from a biorthogonal set. We derive the optimum detector by maximizing the likelihood ratio. In addition to the optimum multiuser detector with very high computational complexity, we present a multistage detector and a scheme based on a reduced tree search algorithm. The error probability is evaluated by deriving upper and lower bounds as well as by Monte Carlo simulations. We show that the optimum receiver is near-far resistant. The results from the numerical examples indicate that the suboptimum detectors are also capable of alleviating the near-far problem.
Journal Paper; In this paper, we present an efficient circulant approximation based MIMO equalizer
architecture for the CDMA downlink. This reduces the Direct-Matrix-Inverse (DMI) of size (NF x NF) with O((NF)³) complexity to some FFT operations with O(NF log2(F)) complexity and the inverse of some (N x N) sub-matrices. We then propose parallel and pipelined VLSI architectures with Hermitian optimization and reduced-state FFT for further complexity optimization. Generic VLSI architectures are derived for the (4 x 4) high-order receiver from partitioned (2 x 2) sub-matrices. This leads to more parallel VLSI design with 3x further complexity reduction. Comparative study with both the Conjugate-Gradient and DMI algorithms shows very promising performance/complexity tradeoff. VLSI design space in terms of area/time efficiency is explored extensively for layered parallelism and pipelining with a Catapult C High-Level-Synthesis methodology.
Journal Paper; In this paper, we explore the displacement structure in a Kalman equalizer for MIMO-CDMA
downlink. A streamlined MIMO Kalman equalizer architecture is proposed to extract the commonality in the data path by exploiting the displacement structure of the transition matrix and the block-Toeplitz structure of the channel matrix. Numerical matrix multiplications with O(F^3) complexity are eliminated by simple data loading process. Utilizing the block Toeplitz structure of the channel matrix, an FFT-based acceleration is proposed to avoid direct matrix multiplications in the time domain. Finally, an iterative Conjugate-Gradient based algorithm is proposed to avoid the inversion of the innovation correlation matrix in Kalman gain calculation. The proposed architecture not only reduces the numerical complexity to O(F log2 F) per chip, but also facilitates the parallel and pipelined VLSI implementation for real-time processing.
Conference Paper; In this paper, a streamlined MIMO Kalman equalizer
architecture is proposed to extract the commonality in the
data path by jointly considering the displacement structure of the
transition matrix and the block-Toeplitz structure of the channel
matrix. Finally, an iterative Conjugate-Gradient based algorithm
is proposed to avoid the inverse of the Hermitian symmetric
innovation correlation matrix in Kalman gain processor. The
proposed architecture not only reduces the numerical complexity
to O(F log F) per chip, but also facilitates the parallel and
pipelined VLSI implementation in real-time processing.