A MAC protocol supporting wireless video transmission over multi-code CDMA personal communication networks
Introduction
Recently, Code Division Multiple Access (CDMA) has become an attractive technique for medium access control in cellular networks and Personal Communication Networks (PCN). The CDMA scheme spreads a relatively low user data rate (e.g. 9.6 Kbps)over a large allocated bandwidth (e.g. 1.2288 MHz in Refs. 2, 15). The spreading employs a spread spectrum waveform, which is termed as a code. Previous research has shown that, comparing with its counterparts such as Time Division Multiple Access (TDMA) and Frequency Division Multiple Access (FDMA), CDMA enjoys advantageous features such as enhanced spectrum efficiency, simple frequency planning, soft handoff, graceful degradation, and tight security 8, 14. A second-generation system standard IS-95 2, 14 and a third-generation system testbed CODIT [1] have been proposed or built, which demonstrates the feasibility of applying CDMA technology to personal communications.
Meanwhile, integrated multimedia Personal Communication Networks have come to draw more and more attention. In such networks, a large variety of mobile hosts with different types of Quality of Service (QoS) requirements, such as Constant Bit Rate (CBR), Available Bit Rate (ABR), and Variable Bit Rate (VBR), have to be served. The accommodation should be not only efficient but also fair in a bandwidth-on-demand manner. Digital video is a major traffic component in VBR services. It has QoS requirements such as bursty bandwidth and real-time.
Traditional CDMA has its drawbacks in supporting multimedia services such as digital video. First, CDMA practically provides narrowband data rate services, with the basic rate carried by one CDMA code being only 8 or 16 Kbps. This leads to severe limitation in the use of CDMA in multimedia communications where a high data transmission rate is a must. Second, because CDMA is usually considered interference-limited in contrast to FDMA and TDMA, its data transmission is more prone to error when compared with that of its counterparts.
A Multi-Code CDMA (MC-COMA) scheme is proposed in Refs. 4, 5 as a flexible means to provide multi-rate services for Personal Communication Networks (PCN) which support multimedia applications. With the introduction of MC-CDMA, a mobile host can use more than one code at one time to achieve a transmission rate higher than the basic rate of the CDMA system. Based on MC-CDMA, Liu et al. 11, 12 proposed Distributed-Queueing Request Update Multiple Access (DQRUMA), which is an efficient demand access control protocol that can support multi-rate services in a MC-CDMA system. They have shown that their protocol provides close to ideal-access performance for mobile hosts with mixes of different traffic characteristics. However, they have not used their scheme to deal with a realistic multimedia PCN environment.
Based on the above trends and previous research, we propose in this paper a medium access control protocol for supporting multimedia PCN, borrowing some design features from Refs. 11, 12. We consider a realistic multimedia PCN environment, where different types of services are provided. Since the video traffic requires the highest network bandwidth and most stringent delay constraint, we choose video service as the focus of this research. The MAC protocol performs the video multiplexing using a scheduler in the base station. An admission control scheme is used to achieve good system resource utilisation. We derive the upperbound of the system performance measured by how many video mobile hosts can be supported in the system, and show that it is impossible to achieve this limit due to the traffic characteristics and QoS requirements of the video services. Many system parameters, such as the error-protection rate for the transmitted packets and the signal-to-interference ratio (SIR), are shown to have a considerable impact on the system performance. The simulation study presented is a useful evaluation of our access control protocol performance and an assessment of the impact of system and traffic parameters. These types of study are very important for third-generation multimedia CDMA PCNs, not only in the system design, but also in the capacity evaluation and bandwidth management.
The rest of the paper is organised as follows. In the next section we introduce the MC-CDMA approach and H.263 as our multimedia traffic carrier. A MAC protocol will be presented in the third section, specifically for supporting H.263 video transmission. In Section 4, we address system design issues together with the system analysis model. Simulation results are presented and explained in Section 5. Section 6concludes this paper.
Section snippets
Background
In this section, as background for our later discussion, we first provide an overview of the H.263 standard, which will be used as the carrier of multimedia traffic in our proposed CDMA multimedia PCNs. We then present previous research results in MC-CDMA which give feasible solutions to supporting multimedia traffic in CDMA wireless networks.
The protocol
In this section, we propose a protocol which uses MC-CDMA as the medium access control scheme to support a multimedia PCN which uses H.263 to transfer video traffic. We will first address the problems and then present the protocol.
System design issues and modeling
In this section, we consider some key system parameters. Some system design issues, such as, a video traffic model and performance measures are also investigated.
Simulated results
The purpose of this section is two folds: (1) to gain insight into how our proposed protocol works, and (2) to investigate what impacts the system parameters, such as the target SIR and the BCH code rate, will have on the CDMA system performance.
We considered a MC-CDMA PCN within a single cell. The spread spectrum processing gain of the system, G, was set to 18 dB, i.e. 63.1. The basic rate of the system, Rb, was 16 000 bps. All the mobile hosts were assumed to have the same maximum transmitter
Summary
In this paper, a multi-priority random access protocol for multimedia PCN has been proposed and studied using simulations. We exploited the features of Multi-code CDMA to maximize the usage of the system capacity. A random access control scheme was proposed for the mobile hosts to contend for CDMA codes. Simulation results were presented to show that the BCH code can be chosen carefully to achieve a very significant system performance improvement. We plan to conduct more intensive system
Acknowledgements
Research reported herein was supported in part by the US Army Research Office under contract no. DAAL03-92-G-0184, and by the Performance Analysis Lab of The Ohio State University. The views, opinions, and/or findings contained in this paper are those of the authors and should not be construed as an official Department of the Army position, policy or decision.
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