Performance evaluation of a generalized hybrid TDMA/CDMA protocol for wireless multimedia with QoS adaptations

Abstract

As multimedia and high transmission of continuous media, such as live audio and video, with high quality services become more popular on wireless networks, the various traffic requiring different qualities of service (QoS) will co-exist. To this end, a request time division multiple access (TDMA)/code division multiple access (CDMA) protocol for supporting multimedia traffic in wireless networks, where CDMA is laid over TDMA has been proposed recently. In this paper, we wish to extend this scheme by introducing several QoS to the end-user, and present a generalized performance analysis of a request TDMA/CDMA QoS-based protocol. Quality of service factors includes customer retrial rates due to both user impatience and system timeouts.

Our proposed analytical model allows us to understand how the end-to-end QoS guarantee can be obtained by analyzing the QoS requirements at every stage of the message delivery, from arrival to contention and transmission. In our studies, we have investigated several priority queuing disciplines: First In First Out, priority, and weighted fair queueing schemes, and we have investigated how customer impatience and system timeouts can affect the QoS provided by these priority scheduling schemes. Finally, we have applied the stretch-optimal algorithm in our protocol to improve further the high quality of QoS the mobile users may obtain.

Introduction

In recent years, wireless multimedia and Internet services have rapidly evolved to meet a variety of user demands. As a result, looking at the network layer in isolation may not always be sufficient to guarantee qualities of service (QoS) requirements. Researchers must look at the QoS requirements at all the different medium access control (MAC) protocol levels, since various traffic requires different methods of QoS [2].

Several MAC protocols have been proposed for supporting multimedia traffic in wireless networks. In this paper, we focus upon a hybrid protocol, where we layer code division multiple access (CDMA) over time division multiple access (TDMA) [3]. In this protocol, a time frame has two types of slots: data slots and control slots. The idea behind this scheme is to take advantage of both TDMA and CDMA features. Some of the advantages of TDMA include the need for fewer number of radio channels, no variable bandwidth needed, no duplexer needed, and lower power consumption is observed [12]. CDMA was selected mainly because many users can be accommodated on a single channel at the same time. CDMA provides higher security compared to other transmission schemes. To realize this higher security level, CDMA transmits the message signal over a wider bandwidth than that of the original signal. Furthermore, CDMA can accommodate 8–10 times more users than the AMPS system and 4–5 times more users than a GSM system [12].

In Ref. [3], the authors present a performance analysis of their hybrid TDMA/CDMA protocol for supporting multimedia traffic in wireless networks. The reported results are quite encouraging even though they did not incorporate many factors that are crucial to guarantee the QoS requirements needed to support a variety of user demands as well as the multimedia traffic. Some QoS parameters that are of interest include: bandwidth, delay, jitter bound, bit rate, temporal and spatial synchronization, and multiple priority and weight levels. We can classify the QoS requirements in many different ways. One way to categorize it is to be either of preferable quality or acceptable quality [11]. One may also use three guarantee levels: hard or deterministic in which the QoS is fully satisfied, soft or statistic in which the QoS is satisfied within a certain probability, p, and finally a best effort, which is not really a guarantee and might be hard to achieve [11]. Another way to categorize QoS is based on bit rates allocated to a user, i.e. constant, variable, and available bit rates [11]. In this paper, we focus on the QoS requirements that deal with the following parameters: customer retrials, user-initiated timeouts, and system generated timeouts. In this paper, we intend to extend the model developed in Ref. [3], to study the implementation of QoS requirements at different MAC protocol levels. We wish to generalize the TDMA/CDMA analytical model by taking into account several QoS factors, such as call blocking (i.e. whether it should be initiated by the customer or the system), customer retrials, timeout during the contention process, and the following packet class-based priority policies: First-in First-out (FIFO), Weighted Fair Queuing (WFQ), Priority Queueing (PQ). Although priority queues with respect to wireless multimedia have been investigated in Refs. [9], [10], to the best of our knowledge, none have been studied for a hybrid CDMA–TDMA protocol. In this paper, we will show how customer impatience and system timeouts can affect the scheduling schemes and how they affect the QoS received by the users. Finally, we will show how the stretch-optimal algorithm can be applied to our protocol and see what the resulting QoS we obtain.

The remainder of this paper is organized as follows: In Section 2, we review the hybrid TDMA/CDMA hybrid protocol in Ref. [3]. In Section 3, we present our generalized analytical model of the request TDMA/CDMA protocol. Section 4 discusses various scheduling disciplines to meet the QoS requirements. Section 5 discusses the impact of customer impatience and system timeouts on the scheduling schemes and the resulting QoS the user receives. Section 6 applies the stretch-optimal scheduling algorithm to our protocol and sees how the QoS is affected. Section 7 presents our conclusions.