Two-stage detection of partitioned random CDMA (original) (raw)

Optimal Power/Rate Allocation and Code Selection for Iterative Joint Detection of Coded Random CDMA

IEEE Transactions on Information Theory, 2006

Iterative interference cancellation of coded 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 distribution achieves the AWGN channel Shannon bound as the numbers of classes becomes large. The optimal distribution of the size of these classes is shown to be uniform. If users are grouped into different rate classes with equal powers among equal rate users, the Shannon bound for AWGN channels can be achieved with an arbitrary distribution of the classes sizes, provided that the size of the largest rate class obeys the mild condition that its ratio of size to processing gain is much smaller than the inverse of the signal-to-noise ratio. The case of equal powers and equal rates among all users is addressed as a "worst case" scenario. It is argued that simple repetition codes provide for a larger achievable capacity than stronger codes. It is shown that this capacity monotonically increases as the rate of the code decreases. A density evolution analysis is used to show that the achievable rates exceed those of a minimum-mean square error filter applied to the uncoded signals. This lower bound is tight for small ratios of bit energy to noise power, and otherwise the iterative cancellation receiver provides an appreciably larger capacity. Relating to recent result from the application of statistical mechanics it is shown that the repetition-coded system with iterative cancellation achieves the performance of an equivalent optimal joint detector for uncoded transmission.

Iterative Multiuser Detection of Random CDMA Using Partitioned Spreading

2006

Iterative joint detection of random CDMA is considered in conjunction with error control coding, in particular using low-density parity-check (LDPC) codes. The spreading sequences are partitioned into sections, called "partitioned spreading (PS)", creating an artificial repetition code which interfaces the LDPC code with the multiple access channel. A suboptimal, but efficient cancelation processor is used at the multiple access processing node in the factor graph describing the overall system. This system is then analyzed using Gaussian density evolution and it is shown that partitioned spreading conditions the channels such that the efficiency of the code is increased substantially. A comparison between direct application of LDPC coding (without partition spreading) to a two- stage decoding procedure and a full iterative decoding schedule of PS-CDMA reveals that gains of over 100% in spectral efficiencies can be achieved with regular LDPC codes.

Optimal power allocation and code selection in iterative detection of random CDMA

International Zurich Seminar on Communications, 2004

Iterative joint detection of random CDMA is considered. It is shown that optimal power levels and optimal code choices can be found that achieve the capacity of this channel. Even few power levels can significantly improve the achievable capacity. Furthermore, it is shown that very low rate codes are optimal, but that practical codes, again, can achieve a large portion of the channel capacity.

Coded Random CDMA with Partitioned Spreading

2006

A Code Division Multiple Access (CDMA) system is considered, where a number of concurrent users, distinguished by random spreading waveforms, access the common additive white Gaussian noise (AWGN) channel. All users employ a regular low-density parity-check (LDPC) error control code (ECC). Additionally, a method, called Partitioned Spreading (PS) is used. Each user's spreading waveform is divided into M partitions which are interleaved and spread in time. Asymptotic performance of the proposed system is studied for two different decoding schedules. In the first schedule, called two-stage schedule, iterative multiple access (MA) detection and LDPC decoding are executed separately. The second schedule, named full decoding allows to exchange information between MA and LDPC sides. Supportable system loads are derived as functions of the users' signal-to-noise ratio (SNR). It is shown that significantly higher system loads can be achieved compared to the traditional LDPC coded random CDMA.

Coded asynchronous CDMA and its efficient detection

IEEE Transactions on Information Theory, 1998

In this paper, receiver design and performance analysis for coded asynchronous code-division multiple access (CDMA) systems is considered. The receiver front-end consists of the near-far resistant multiuser detector known as the projection receiver (PR). The PR performs multiple-access interference resolution and is followed by error-control decoding. The output of the projection receiver yields the appropriate metric (i.e., soft information) for decoding of the coded sequences. An expression for the metric is derived that allows the use of a standard sequence decoder (e.g., Viterbi algorithm, M-algorithm) for the error-control code. It is then shown that the metric computer has an elegant adaptive implementation based on an extension of the familiar recursive least squares (RLS) algorithm. The adaptive PR operates on a single sample per chip and achieves a performance virtually identical to the algebraic PR, but with significantly less complexity. The receiver performance is studied for CDMA systems with fixed and random spreading sequences, and theoretical performance degradations with regard to the single-user bound are derived. The near-far resistance of the PR is also proven, and demonstrated by simulation.

An Overview of Code-Spread CDMA

Code-division multiple-access (CDMA) has gained a lot of attention recently when the third generation mobile communication systems are developed. In this paper we discuss how using low-rate channel coding instead of direct sequence spreading can more efficiently use the available spectrum. This leads to large capacity improvements.

Finite memory-length linear multiuser detection for asynchronous CDMA communications

IEEE Transactions on Communications, 1997

Decorrelating, linear, minimum mean-squared error (LMMSE), and noise-whitening multiuser detectors for code-division multiple-access systems (CDMA) are ideally infinite memory-length (referred to as IIR) detectors. To obtain practical detectors, which have low implementation complexity and are suitable for CDMA systems with time-variant system parameters (e.g., the number of users, the delays of users, and the signature waveforms), linear finite-memory-length (referred to as FIR) multiuser detectors are studied in this paper. They are obtained by truncating the IIR detectors or by finding optimal FIR detectors. The signature waveforms are not restricted to be timeinvariant (periodic over symbol interval). Thus, linear multiuser detection is generalized to systems with spreading sequences longer than the symbol interval. Conditions for the stability of the truncated detectors are discussed. Stable truncated detectors are shown to be near-far resistant if the received powers are upper bounded, and if the memory length is large enough (but finite). Numerical examples demonstrate that moderate memory lengths are sufficient to obtain the performance of the IIR detectors even with a severe near-far problem.

On the Performance of Partitioned-Spreading CDMA with Multistage Demodulation

2006 40th Annual Conference on Information Sciences and Systems, 2006

The performance of partition-spreading CDMA (PS-CDMA), a recently proposed multiple-access system, is investigated. It is shown via variance evolution that the asymptotic channel throughput of PS-CDMA improves monotonically as the number of partitions increases. The fundamental limit of the PS-CDMA system is calculated by evaluating the spectral efficiency of the layered channel that is generated by multistage PS-CDMA demodulation. Capacity-approaching codes of high rate are suggested in the encoding for each PS-CDMA user, which justifies a low-complexity two-stage decoding scheme for PS-CDMA.

On Joint Decoding and Random CDMA Demodulation

We consider a CDMA system on an additive white Gaussian noise (AWGN) channel with K concurrent users. Each user transmits equal-rate data and employs an LDPC error control code (ECC). Each user's bits are modulated with a random spreading waveform. These spreading waveforms are furthermore partitioned in M sections which are in turn interleaved and spread in time. This creates a rate 1/M repetition code which interfaces the LDPC-ECC with the CDMA channel. This method, called partitioned spreading, significantly improves convergence properties and maximum system load in conjunction with iterative (turbo) detection. Different decoding iteration schedules, viz. a low-complexity two-stage schedule which separates CDMA detection and LDPC decoding into two successive processes, and a full iteration schedule (full decoding) which invokes all message exchanges in a parallel decoding approach are presented and analyzed. We show that, under certain power and rate conditions, two-stage decoding achieves virtually identical performance as full decoding.

Increasing the capacity of CDMA using hybrid spreading sequences and iterative multistage detection

We introduce a code-division multiple access (CDMA) concept which allows one to accommodate a higher number of users than the spreading factor N. The idea is to assign orthogonal spreading sequences to the first N users and pseudo-noise (PN) spreading sequences to all additional users. The proposed technique can thus accommodate N users without any mutual interference and some additional at the expense of some signal-to-noise ratio (SNR) penalty. This represents a significant capacity increase with respect to presently available multiple access techniques. When the number of users is larger than N, detection is performed in two separate steps, one for the set of users with orthogonal spreading sequences and one for the set of users with PN sequences. Furthermore, the process is iterated two or more times to obtain more reliable receiver decisions