Review on codes for multiple access communications

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.

Multiple access interference (MAI) appears in Code Division Multiple Access (CDMA) systems when the communication channel is a multi-path channel and the spreading codes are not orthogonal. Orthogonality between spreading codes cannot be maintained at the CDMA receiver because the codes may be asynchronous due to channel delay and multi-path spread. The receiver cannot perfectly separate the different signals of the multiple access users, and the resultant MAI limits the capacity of CDMA systems. MAI is a function of the cross correlation property between used spreading codes. In this paper we focus on the cross correlation of the spreading codes, we propose a method to find spreading codes with minimum magnitude of cross correlation. Employing these codes will reduce the resultant MAI in the CDMA system; hence it will increase the system capacity. A great enhancement is shown by comparing found minimum cross correlation spreading codes (MCCSC) with Hadamard and Gold codes.

Code division multiple access (CDMA) based on spread spectrum technique has become one of the most important multiple access technologies in contemporary communication systems. The properties of spreading codes largely govern the performance of CDMA systems. Therefore, CDMA can insure interference free operation when code design methodology takes into account of major impairing factors such as intersymbol interference (ISI), asynchronous transmission and random signs in consecutive symbols. In this study, the authors propose a design scheme for CDMA signal reception with zero ISI in the multipath environment. Cyclic shift property of maximal length sequence (m‐sequence) is utilised in the design to avoid ISI. The proposed scheme exploits inherent diversity present in the multipath channel and can be seen as antenna diversity. This scheme avoids the use of complex detectors and the performance is compared by simulations with conventional CDMA system using Walsh–Hadamard codes and m‐sequence. The impact of the Rician factor on the end‐to‐end performance is also studied in detail and suitable receiver to be used has been suggested for different fading scenarios. Simulations results presented indicate that the proposed scheme provides superior performance over traditional systems.

In recent years, the joint decoding of combined source and channel coding have shown the capability of delivering performances exceeding that of separately decoded schemes. In this paper, a well-known joint source/channel coding scheme is investigated for possible use in future 4th Generation (4G) direct sequence spread spectrum multiple access (DS/SSMA) code division multiple access (CDMA) communication systems. Due to the availability of potentially large sets of sequences with good correlation characteristics, the DS/SSMA CDMA system considered in this paper employs complex spreading sequences (CSS). A multiuser CSS-based DS/SSMA CDMA communication system was implemented as test platform for the joint source/channel coding scheme. Several families of CSSs, including Zadoff-Chu (ZC), double sideband constant envelope linearly interpolated root-of-unity filtered general chirp-like (DSB CE-LI-RU Filtered GCL), analytical bandlimited complex (ABC) and quadriphase (QPH) sequences are investigated on this platform. Simulated bit error rate (BER) performances are presented for uncoded, separate source and channel coded and joint source/channel coded systems under realistic multipath fading channel conditions.

For high-speed code division multiple access (CDMA) systems, interchip interference that is caused by multipath propagation becomes severe and cannot be ignored. Together with the multiple access interference due to nonideal conditions in CDMA, they become major obstacles to overall system performance. In this paper, both batch and adaptive algorithms based on linear prediction are proposed for joint blind equalization and multiuser detection for asynchronous CDMA systems. It is shown that the new methods are insensitive to estimation error of propagation delay or chip timing. In addition, the adaptive algorithm is computationally efficient. Simulation results show that the proposed methods are also near-far resistant and compare favorably to many existing methods.

This paper carries comprehensive comparison of Interleave Division Multiple Access (IDMA) and Code Division Multiple Access (CDMA) systems in terms of performance (BER) and behavior to different number of users. Performance analysis of two systems whose transmitter and receiver structures are almost same are studied under Additive White Gaussian Noise (AWGN) and performance differences are illustrated by simulation results. Although Multiple Access Interference (MAI) and Intersymbol Interference (ISI) can affect the performance of the CDMA system, simulation results illustrates that IDMA system model achieves this type of problem using their interleavers. Results also show that large number of users can easily be adapted by the IDMA scheme much easier than CDMA scheme.

The capacity of CDMA (code-division multiple access) systems is limited by its other-user interference because all users in a cell share the same frequency band. To achieve maximum capacity, the 'near-far' problem needs to be dealt with. In practice, this is done through appropriate power control. Due to the interaction between the power control process, CDMA systems may become unstable if the desired SNR is not set appropriately. When CDMA systems are stable, the received power at the base station converges to the optimal power with a desired SNR (signal-to-noise ratio) setting. We analyze the associated power and the interference estimation in the discrete time domain and find the stability SNR upper bounds that can guarantee the system stability.

Code division multiple access (CDMA) is currently considered as one of the promising technologies that can significantly improve the efficiency of modern and future communication networks. In code division multiple access systems, users can share a dedicated space-frequency-time resource to simultaneously transmit their own traffic. To ensure the separation of individual user streams on the receiving side, each user is provided with his own code sequence embedded in the broadband signal transmitted by this user, the spectrum of which, as a rule, occupies the entire allocated frequency band. When centimeter-and- millimeter wave wireless channels are used as the transmission medium, such factors as fading, multipath, and Doppler scatter can significantly degrade the performance of a code division multiple access system. The standard solution in this situation is to combine code division multiple access with OFDM (orthogonal frequency division multiplexing) technology, known as multicarrier CDMA (MC-CDMA). To ensure the acceptable level of performance of multiple access systems under oversaturated conditions, many novel approaches based on the use of non-orthogonal multiple access (NOMA) have been proposed. The paper considers a variant of multiple access with non-orthogonal coding that is close in approach to sparse coding multiple access (SCMA). The proposed access procedure is based on dividing the allocated time-frequency resource into relatively small clusters and sharing each cluster with its own group of users, equipped with a non-orthogonal cluster code with the ability to change the loading factor. For the proposed class of cluster codes, a general encoding and decoding scheme is presented. Examples of specific cluster code constructions and numerical results are given that allow one to get a number of the parameters for tradeoffs between an increase of the loading factor in the system and additional energy loss.

Improvement of the capacity of the CDMA (code division multiple access) system is a major objective. The capacity of the system is mainly limited by the signal to noise ratio, where the noise comes from the channel background and from MAI (multiple access interference). Usually, the way to improve the capacity is to decrease the MAI; this is why orthogonal codes are adopted by the IS-95 standard. One problem is that the number of orthogonal codes is constrained by the dimensionality of the signal space. So in IS-95, there are only 64 orthogonal codes available when the number chips for each sequence is 64. To solve this problem, a non-orthogonal code called WBE (Welch bound equality) sequences (Massey and Mittelholzer 1991) is used in this paper The criterion to construct the code is to minimize the MAI. Two methods are used to construct the WBE sequences: from a linear cyclic code or from a Hadamard matrix. In order to improve the performance of the WBE sequence set, an iteration receiver (Sari et al. 1999) is applied. The conclusion obtained is that when the number of users is slightly greater than the signal space dimensionality, after the first iteration, the performance is only marginally worse than the orthogonal CDMA system.

One of the major advantages of a code division multiple access (CDMA) system is the flexibility in frequency assignment, which allows all the users to share the same frequency band and, possibly, to partially overlay the spectrum allocation of different services. However, before synchronism to the desired sequence is achieved, the presence of multiple access interference (MAI) is a major impairment which determines the performance of the code acquisition sub-system. It has been argued that the acquisition performance in the presence of MAI may pose limits to the capacity of a CDMA system which are more stringent than those coming from bit error probability specifications. We introduce a general definition for acquisition-based capacity of a CDMA system. This quantity is evaluated for an acquisition sub-system with non-coherent correlator/detector and straight serial search in the time uncertainty region. The effect of imperfect power control is also analysed, showing that the capacity may be substantially decreased when the residual standard deviation of the received power level is above 1 dB. >

In synchronous code division multiple access (CDMA) systems, both multiple access interference (MAI) and intersymbol interference (ISI) arise. MAI is present due to the CDMA format while ISI is present due to channel multipath. One approach to overcome these problems is to detect the entire package of data for all users at once. This implies working with matrices with dimensions proportional to the product of the data package length and the number of active users. In this paper, we present an alternative approach. This alternative consists of a type of multistage detector that overcomes the MAI and ISI problems; in addition, it has complexity proportional to the number of CDMA users, is computationally efficient, and is suitable for pipeline implementation which allows fast data processing. Simulations are compared to a derived single user bound for the average bit error probability and the results prove the validity of the proposed detection method.

This paper studies detecting matrices in random dense and sparse Code Division Multiple Access (CDMA) systems. Detecting matrices were originally introduced in the coin weighing problem. Such matrices can be used in CDMA systems in over-loaded scheme where the number of users is greater than the number of chips. We drive some conditions in the large system limit for binary and bipolar random CDMA systems to ensure that any random matrix is a detecting matrix. Furthermore, we extend our results to sparse random ternary matrices that have been using in the sparse CDMA literature. Finally, a construction method for the sparse detecting matrices is introduced.

By calibrating the inter-system bias (ISB), an inter-system model can be achieved to improve the performance of multi-global navigation satellite system (GNSS) positioning. A number of studies were carried out to compose inter-system models among code division multiple access (CDMA) systems. However, due to the frequency division multiple access (FDMA) technique, GLONASS phase and code observations suffer from inter-frequency phase bias (IFPB) and inter-frequency code bias (IFCB) respectively, which make it difficult to obtain the ISB among GLONASS and CDMA systems. In addition, most of the current inter-system models are unable to handle mixed frequencies. A new single difference-based uniform inter-system model suitable for all of the CDMA and FDMA frequency combinations is proposed in this study. We first utilize a particle filter approach to extract the IFPB rate. Afterwards, the multi-ISB parameters can be estimated by the proposed model simultaneously. Four short baselines containing the signals of GLONASS, GPS, BDS3, BDS2, Galileo, and QZSS are selected to validate the model. The quantitative results indicate that the IFPB rate can be extracted precisely by a particle filter within ten epochs. The inter-system phase bias valuations are sufficiently stable on a daily scale and have standard deviation near 0.01 cycles. After de-weighting the GLONASS code observations to reduce the influence of IFCB, the inter-system code bias is also stable. Furthermore, imposing the stable ISB on the multi-GNSS inter-system model can effectively improve the single-frequency real-time kinematic positioning accuracy and reliability for severely obstructed situations with only a small number of satellites observed.

Chaotic sequences have good correlation properties and they can be used as address sequences in Spread Spectrum Communication. Chaotic functions are highly sensitive to initial condition and exhibit non-linear behavior. In Chaotic spread spectrum communication systems, different user may be assigned different sequences generated with different initial conditions. Set of binary sequences with good pair wise cross- correlation property finds applications in Direct Sequence- Code Division Multiple Access (DS-CDMA) systems. Security in Direct Sequence-Code Division Multiple Access systems is relatively limited. To enhance security in Direct Sequence- Code Division Multiple Access systems, chaotic sequences are generated. In this paper we analyze the generation of chaotic binary sequences and study of auto correlation and cross correlation properties.

The use of multiple antennas at the transmitter or receiver provides a powerful means, of increasing the capacity of wireless systems. The goal of this paper is to illustrate ways in which analogies with multiuser code division multiple access (CDMA) systems may be used to provide design insights for space-time communication systems, for both single and multiple users. Three examples are considered: (a) noncoherent space-time codes for transmit antenna diversity against Rayleigh fading, constructed using an analogy with a CDMA downlink; (b) optimal multiuser transmit beamforming on a cellular downlink, where the algorithm for computing beamforming weights is obtained by transforming the problem into a CDMA uplink with MMSE interference suppression and power control; (c) design of high bandwidth efficiency systems exploiting both transmit and receive antenna arrays, using analogies with CDMA systems with random and slowly time-varying spreading waveforms.