Packet Random Access in CDMA Radio Networks 1 (original) (raw)
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Enhancing the Performance of Random Access Networks with Random Packet CDMA and Joint Detection
EURASIP Journal on Advances in Signal Processing, 2009
Random packet CDMA (RP-CDMA) is a recently proposed random transmission scheme which has been designed from the beginning as a cross-layer method to overcome the restrictive nature of the Aloha protocol. Herein, we more precisely model its performance and investigate throughput and network stability. In contrast to previous works, we adopt the spread Aloha model for header transmission, and the performance of different joint detection methods for the payload data is investigated. Furthermore, we introduce performance measures for multiple access systems based on the diagonal elements of a modified multipacket reception matrix, and show that our measures describe the upper limit of the vector of stable arrival rates for a finite number of users. Finally, we simulate queue sizes and throughput characteristics of RP-CDMA with various receiver structures and compare them to spread Aloha.
Random Packet CDMA: Reducing Delay and Increasing Throughput of WLAN Systems
We revisit Random Packet Code Division Multiple Access (RP-CDMA), a recently proposed Physical/MAC layer scheme for wireless CDMA networks. We revise earlier results by adopting a more realistic Spread Aloha model for header transmission and packet sizes with distributions typical for Internet2 traffic. Thanks to timing recovery in the RP-CDMA header and greatly reduced packet collision probability, unlike Spread Aloha, RP-CDMA enables the use of multiuser receivers for data detection. We simulate the throughput characteristics of RP-CDMA with the matched filter, the decorrelator, the MMSE and partitioned spreading demodulation detection and compare performance to Spread Aloha in a base station centric network.
Two-stage detection of partitioned random CDMA
European Transactions on Telecommunications, 2008
Random Code Division Multiple Access (CDMA) with low complexity two-stage joint detection/decoding is considered. A sequence partitioning approach is used for modulation, where every spreading sequence is divided into M sections (partitions) which are interleaved prior to transmission. This setup, called partitioned CDMA, can be understood as a generalisation of (chip) interleave division multiple access (IDMA). An analysis of a low-complexity iterative cancellation receiver is presented for arbitrary received power distributions. It is shown that for equal rate and equal power users the asymptotic performance of partitioned CDMA is equal to the performance of CDMA with optimal a posteriori probability (APP) detection for system loads K/N < 1.49. Effects of asynchronous signal transmission are quantified for standard pulse shaping filters and it is shown that the signal-to-noise ratios achievable in an asynchronous system are improved with respect to fully synchronous transmission. The effect of unequal received powers is examined and considerable gains in performance are obtained by judicious choices of power distributions. For certain power distribution, partitioned CDMA with iterative detection can achieve arbitrary system loads, that is detection is no longer fundamentally interference limited. The practical near-far resistance of the proposed system is illustrated using an example of a receiver with a circular receive footprint and uniformly distributed transmitters (single cell system).
A wireless packet multiple access method exploiting joint detection
IEEE International Conference on Communications, 2003. ICC '03.
A new packet-based, multiple access scheme for connectionless, uncoordinated random access is proposed using code-division multiple access (CDMA) as the physical access method. The new method uses a novel packet format with a common header with identical spreading codes for all users and packets, and random spreading codes for the data portion. The receiver operates in two stages: header detection using a conventional spread spectrum receiver and data detection using a multiuser detector to allow for decoding of overlapping active packets. The headers are spread with a large enough processing gain to allow detection even in severe interference.It is shown that this system is detector capability limited and that it can significantly outperform conventional ALOHA systems whose performance is limited by the collision mechanism. This system also experiences a much smaller packet retransmission rate and better spectral efficiency than conventional or spread ALOHA.
A novel random wireless packet multiple access method using CDMA
IEEE Transactions on Wireless Communications, 2006
Random Packet CDMA, a novel packet-based multiple access scheme for connectionless, uncoordinated random channel access is proposed. Random Packet CDMA, or RP-CDMA, utilizes a novel packet format which consists of a short header and a data portion. Each header is spread with a unique spreading code which is identical for all users and packets, while the data portion of each packet is spread by a randomly chosen spreading sequence. The receiver operates in two stages: header detection and data detection. For header detection a conventional spread spectrum receiver is sufficient. Headers are spread with a large enough processing gain to allow detection even in severe interference. The data portion is decoded with a sophisticated receiver, such as a multiuser detector, which allows for successful decoding of overlapping active packets. It is shown that the RP-CDMA system is detector capability limited and that it can significantly outperform Spread ALOHA systems whose performance is limited by the channel collision mechanism. RP-CDMA also experiences a much smaller packet retransmission rate than conventional or Spread ALOHA, and provides better spectral efficiencies.
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 CDMA Systems Employing Multiuser Decorrelating Detector and Antenna Array
IEEE Transactions on Vehicular Technology, 2007
This paper presents analytical expressions to evaluate the capacity of code division multiple access (CDMA) cellular radio systems employing a joint antenna array and a multiuser decorrelating detector. The performance is evaluated for chipsynchronous and asynchronous baseband and bandpass CDMA systems on additive white Gaussian noise and flat-fading Rayleigh channels. Simulation results have shown that the analytical model developed to evaluate the performance of those systems is a very tight approximation. Index Terms-Antenna array, code division multiple access (CDMA), multiuser detectors (MUD). I. INTRODUCTION C ODE DIVISION multiple access (CDMA) techniques have been highly regarded for future generation wireless communication systems, not only because they have high bandwidth efficiency but also because they are suitable for handling multimedia and multirate services [1]. Multipath propagation and time variance present on mobile channels produce multiple access interference (MAI) and intersymbol interference (ISI) at the receiver. Conventional receivers based on the matched filter and the rake detector treat MAI and ISI as noise, which can degrade the system performance severely, mainly if the power of interfering users is high. To improve the performance of such receivers, multiuser detectors (MUDs) have proposed by [2], which cancel the interferences from other users, before the desired user symbol is estimated. Thus, MUDs eliminate the MAI, at the expense of an undesired increase in the noise power. The optimum MUD, e.g., the maximum likelihood sequence detector, has computational complexity that increases exponentially with the number of users, which makes the detection infeasible for practical CDMA systems. As a consequence, efforts have been devoted to finding suboptimal linear detectors to achieve a good tradeoff between complexity and performance. The decorrelating detector [2] belongs to the class of linear detectors, and it is optimal over the near-far-resistance criterium. The simplicity and good performance of the decorrelating detector makes it very attractive to next-generation mobile systems [3]. On the other hand, in most scattering channels, antenna diversity is a practical, effective, and widely applied technique to reduce the multipath fading effect. Therefore, CDMA capacity
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.
IEEE Journal on Selected Areas in Communications, 1996
In cases where machines having bursty data are equally likely to transmit to one another, code-division multipleaccess (CDMA) ALOHA which allows for an individual "virtual channel" for each receiving station may he a better multipleaccess protocol than simple ALOHA. With the use of "receiverbased code" multiple-access protocol, it is also possible for a station to listen to the channel of the intended receiver before transmission, and also abort transmission when it detects others transmitting on the same channel. This paper describes a model for a fully-connected, full duplex, and slotted CDMA ALOHA network where channel sensing and collision detection are used. The model is analyzed using a discrete time Markov chain and some numerical results are presented. For a system with a large number of users, where Markov analysis is impractical, equilibrium point analysis is used to predict the stability of the system, and estimate the throughput as well as the delay performance of the system when it is stable. Finally, a comparison is made with a simple channel sense multiple-access with collision detection (CSMA-CD) network, showing that a substantial improvement in the performance is achieved by the proposed network.
Throughput analysis of CDMA systems using multiuser receivers
IEEE Transactions on Communications, 2001
Throughput bounds are attained for random channel access multichannel code-division multiple-access (CDMA) systems and spread slotted Aloha systems employing multiuser receivers. It is shown that the normalized throughput of these two systems reaches 1.0 exponentially fast in the region r=K < 1, where r is the average number of simultaneous users in each channel in the random channel access multichannel CDMA system and the packet arrival rate in the spread slotted Aloha system, respectively, and K is the maximum number of users which the multiuser receiver can handle at the same time. Therefore, both of the random channel access multichannel CDMA system and the spread slotted Aloha system employing multiuser receivers can achieve perfect throughput while being stable in the region r=K = 1 0 , > 0. The maximum throughput of the random channel access multichannel CDMA systems is found as K 0 (1 0 (1=M))K log K 0 O(log K), where M is the number of channels in the system. The maximum throughput is reached when the average number of simultaneous users is r m = K 0 (1 0 (1=M))K log K + O(K= log K). The maximum throughput of the spread slotted Aloha systems is K 0 p K log K 0 O(log K). The maximum throughput is reached when the packet arrival of Poisson distribution has the arrival rate m = K 0 p K log K + O(K= log K).