For the code division multiple access (CDMA) downlink channel, we investigate the decision feedback equalizer (DFE) at chip-level to effectively suppress multiple-access interference (MAI) when the spreading sequences are orthogonal. The structure of the receiver with the chip-level DFE has been investigated and the minimum mean-square error solution has been derived. Due to the inherent structure of the chip-level DFE, some iterative techniques with hard and soft decisions have been proposed. It is shown that the proposed receivers with the chip-level DFE can provide satisfactory performance. In comparison with the adaptive chip-level linear equalizer, the number of users can be doubled by using adaptive chip-level DFE at a bit-error rate of 10/sup -3/. Throughout the paper, we assume that scrambled orthogonal codes are used for spreading sequences.; Jinho Choi, Seong Rag Kim and Cheng-Chew Lim
Journal Paper; Channel estimation techniques for code-division multiple access (CDMA) systems need to combat multiple access interference (MAI) effectively. Most existing estimation techniques are designed for CDMA systems with short repetitive spreading codes. However, current and next generation wireless systems use long spreading codes whose period is much larger than the symbol duration. In this paper, we derive the maximum likelihood channel estimate for long code CDMA systems over multipath channels using training sequences and approximate it using an iterative alogrithm to reduce the computational complexity in each symbol duration. The iterative channel estimate is also shown to be asymptotically unbiased. The effectiveness of the iterative channel estimator is demonstrated in terms of squared error in estimation as well as the bit error rate performance of a multistage detector based on the channel estimates. The effect of error in decision feedback from the multistage detector (used in the absence of training sequences) is also shown to be neglible for reasonable feedback error rates using simulations. The proposed iterative channel estimation technique is also extended to track slowly varying multipath fading channels using decision feedback. Thus...
Conference Paper; Channel estimation techniques for code-division multiple access (CDMA) systems need to combat multiple access interference (MAI) effectively. Most existing estimation techniques are designed for CDMA systems with short repetitive spreading codes. However, current and next generation wireless systems use long spreading codes whose period is much larger than the symbol duration. In this paper, we derive the maximum likelihood channel estimate for long code CDMA systems over multipath channels using training sequences and approximate it using an iterative algorithm to reduce the computational complexity in each processing window. The asymptotic convergence of the mean of the iterative estimate to the actual channel is also shown. The effectiveness of the iterative channel estimator is demonstrated in terms of squared error in estimation as well as the bit error rate performance of a multistage detector based on the channel estimates. Finally, the proposed iterative channel estimation technique is extended to track slowly varying multipath fading channels using decision feedback. Thus, an MAI resistant multiuser channel estimate with reasonable computational complexity is derived for long code CDMA systems over multipath fading channels.
Journal Paper; Maximum likelihood detection of superimposed signals in code-division multiple access (CDMA) communication systems has a computational complexity that is exponential in the number of users, and its implementation is practically prohibitive even for a moderate number of users. Applying the expectation maximization algorithm to this problem, we decompose the multiuser detection problem into a series of single-user problems, and thus present an iterative computationally efficient algorithm for detection of superimposed signals in synchronous direct-sequence CDMA communication systems. The resulting structure includes the well-known multistage detector as one of its special cases. With a proper choice of its parameters, the new detector can achieve the advantages of both the multistage and conventional detector and have good performance for both strong and weak users.
Journal Paper; Multiple access interference (MAI) significantly limits the performance of code-division multiple access (CDMA) systems. Existing multiuser receivers combat MAI using the knowledge of either all (conventional) or none (blind) of the interfering users. However, in practice, partial knowledge of the interference is often available at the receiver. For example, the base station of a cellular communication system typically has information about the interfering users within its cell only (about 60\% of the total interference). Therefore, conventional multiuser receivers cannot be used. We show the degradation in performance resulting from treating the unknown (out-of-cell) interference as noise or using the blind receiver. We then develop a new "partially blind receiver" for joint synchronization and detection that combines a nonlinear interference cancellation technique for the in-cell interferers with the linear blind minimum mean squares error (MMSE) technique for the out-of-cell users. The new receiver significantly outperforms the blind receiver and the receiver that ignores the out-of-cell interference. Additionally, due to the nonlinear interference cancellation technique, the proposed receiver can even outperform conventional linear multiuser receivers with the knowledge of all the users.
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; A multiuser communication system is considered where K users share a channel with multipath propagation by using code division for multiple access. Data modulation is carried out by binary phase shift keying and direct sequence spread spectrum signaling. The micro-cellular communication media is modeled as a frequency selective fading channel with multipath propagation. The multipath diversity of the received signals from the K users is exploited by a bank of K RAKE correlators. Algorithms based on the maximum likelihood rule have been developed for estimating the complex channel coefficients as well as for detection of the desired data packets from the sufficient statistics provided by the RAKE correlators. The performance of the resulting multiuser detector is evaluated analytically and via Monte Carlo simulations. The results indicate that the estimator of the channel coefficients has a variance close to the Cramer-Rao lower bound, and that the proposed multiuser detector is capable of eliminating the near-far effect as well as processing the signals propagated through multiple paths.
Elec 599 Project Report; The main goal of this project is to design a space-time code system for wideband code division multiple access (Wideband CDMA). In this work we tried to make the essential foundations for this goal. There are two key questions. First is the tradeoff between coding and spreading in a wideband channel. Since the bandwidth expansion is achievable both from coding and from spreading the signal in a code division multiple access (CDMA) system. And second is the type of spreading. Since in DS-SS system the signal power is spread in a wide bandwidth, in limit, the capacity approaches zero. We also looked at the tradeoffs between direct sequence and frequency hopped spread spectrum in this work.
PhD Thesis; Subspace-based algorithms are a class of algorithms for estimation problems in array signal processing and more recently near-far resistant Code Division Multiple Access (CDMA) acquisition problems. Subspace-based algorithms are based on estimating signal or noise subspaces from the received data vectors and then performing some form of optimization to estimate the desired parameters. This thesis presents two new parallel algorithms applicable in the estimation of signal or noise subspaces from the received data vectors. The first algorithm is a pipelined SVD algorithm which allows pipelining of multiple independent singular value decomposition (SVD) problems on a single processor array. The resultant algorithm uses the exibility provided by the Jacobi algorithm by defining a new parallel ordering to result in a simple uniform array in which all communication including the initial load and the final unload operations are pipelined. The second algorithm described in this thesis is a sliding window SVD updating algorithm where the signal or noise subspace is updated whenever a new observation vector is received by applying a fixed-length window over the data. An important result shown in this thesis is that an important property ofdowndating problems...
Conference Paper; In this paper, we evaluate several implementation issues in the application of subspace based methods to tracking channel parameters in code division multiple access (CDMA) communication systems, in the presence of multipath fading. We focus on the behavior of singular value decomposition (SVD) based schemes while tracking the time variations in the signal subspace, due to fading. We also evaluate the application of several techniques to reduce the complexity of the computationally expensive SVD procedure, to the channel estimation problem.
Journal Paper; In this paper, we evaluate several issues in the application of subspace based methods to tracking channel parameters in Code Division Multiple Access (CDMA) communication systems, in the presence of multipath fading. We focus on two aspects of the problem - the performance of Singular Value Decomposition (SVD) based schemes while tracking the time variations in the signal subspace, due to fading, and, the performance trades involved in using several complexity reducing approximate schemes. The performance benefits to be obtained from application of subspace based methods to channel estimation has been well studied. The aim of this work is to lay the groundwork for real time implementation of this computationally complex problem.
Conference Paper; The problem addressed in this paper is the estimation of the chan-nel parameters in a Code Division Multiple Access (CDMA) communication system, in the presence of multipath effects and multiple sensors at the base station receiver. The algorithm presented in this paper solves the problem by estimating a composite channel impulse response of each user, which can be directly used in the detection process to appropriately modify the spreading code of the user. In addition, the algorithm combines the benefit of spatial processing in the form of an antenna array at the receiver to gain an increase in performance of the system.
Conference Paper; We focus on the design of multiuser receiver structures for Code Division Multiple Access (CDMA) communication systems, in the presence of multipath effects and multiple sensors at the base station receiver. The joint channel estimation and detection approach presented in this paper uses a miximum likelihood approach to estimate an effective spreading code  vector of each user. We design our channel estimation algorithm to estimate this vector in a computationally efficient manner, instead of focusing on estimation of the individual channel parameters, such as path delays, attenuation factors, and direction of arrival. We present a framework that allows the use of the estimated effective spreading code in the detection process  and facilitates the development of a computationally efficient solution to the combined problem of channel estimation and multi-shot detection in a scenario involving multiple users, multiple paths, and multiple sensors.
Journal Paper; We focus on the design of multiuser receiver structures for Code Division Multiple Access (CDMA) communication systems, in the presence of multipath elects and multiple sensors at the base station receiver. We present a exible and extensible framework that allows the use of an estimated elective spreading code from the channel estimation phase, in the multiuser detection process. The effective spreading code captures all the channel parameters such as path delays, attenuation factors, and directions of arrival. Hence estimation of this one composite vector removes the necessity of estimating each individual parameter, thus reducing computational complexity. Our results also show that this approach leads to better performance for multiuser detection, especially when the channel consists of a number of low energy paths in addition to a few discrete strong paths.
Conference Paper; This paper describes a testbed which simulates a third generation Code Division Multiple Access (CDMA) wireless link. The designer of next generation wideband CDMA systems has the choice of a wide array of advanced signal processing algorithms and a variety of operating environments. The Rice testbed is a tool to evaluate these design options and trade-offs in different system scenarios. The backbone of this testbed is a wireless CDMA multiuser link built using Simulink and Matlab. An efficient method of system modeling is used to accelerate the simulations. The testbed also uses a method for rapid prototyping of algorithms and DSP-based simulation acceleration for new algorithm complexity and performance evaluation.
Journal Paper; We consider the estimation of channel parameters for code-division multiple access (CDMA) communication systems operating over channels with either single or multiple propagation paths. The multiuser channel estimation problem is decomposed into a series of single user problems through a subspace-based approach. By exploiting the eigenstructure of the received signal's sample correlation matrix, the observation space can be partitioned into a signal subspace and a noise subspace without prior knowledge of the unknown parameters. The channel estimate is formed by projecting a given user's spreading waveform into the estimated noise subspace and then either minimizing the likelihood or minimizing the Euclidean norm of this projection. Both of these approaches yield algorithms which are near-far resistant and do not require a preamble.; Texas Advanced Technology Program; National Aeronautics and Space Administration
The combination of forward-error control (FEC) coding with code-division multiple access (CDMA) using random spreading sequences is considered. Such systems can be viewed as serially concatenated, and iterative (turbo) decoding principles can be applied.
Iterative interference cancellation of coded code-division multiple access (CDMA) using random spreading with linear cancellation is analyzed. If users are grouped into power classes and Shannon bound approaching codes are used, a geometric power distribu
Massive levels of integration following Moore's Law ushered in a paradigm shift in the way on-chip interconnections were designed. With higher and higher number of cores on the same die traditional bus based interconnections are no longer a scalable communication infrastructure. On-chip networks were proposed enabled a scalable plug-and-play mechanism for interconnecting hundreds of cores on the same chip. Wired interconnects between the cores in a traditional Network-on-Chip (NoC) system, becomes a bottleneck with increase in the number of cores thereby increasing the latency and energy to transmit signals over them. Hence, there has been many alternative emerging interconnect technologies proposed, namely, 3D, photonic and multi-band RF interconnects. Although they provide better connectivity, higher speed and higher bandwidth compared to wired interconnects; they also face challenges with heat dissipation and manufacturing difficulties. On-chip wireless interconnects is one other alternative proposed which doesn't need physical interconnection layout as data travels over the wireless medium. They are integrated into a hybrid NOC architecture consisting of both wired and wireless links, which provides higher bandwidth, lower latency...