A new QoS-aware TDMA/FDD MAC protocol with multi-beam directional antennas

Abstract

With the ever-increasing demand for wireless real-time services and continuing emergence of new multimedia applications especially for mobile users, it is necessary for the network to support various levels of quality of service (QoS) while maximizing the utilization of scarce and expensive wireless channel resources. Considering this fact, a new TDMA/FDD MAC protocol integrating a novel QoS management algorithm and multi-beam Directional Antennas (DAs) to efficiently exploit wireless resources has been developed and presented in this paper. It supports all ATM CBR, VBR, ABR and UBR service classes by adopting a well-managed dynamic guarantee-based QoS scheduling algorithm. The work mainly aims at increasing the wireless system throughput as well as improving the call-blocking ratios and end-to-end delays for real-time applications. This seamless communication enables both handling real-time multimedia traffics in a fair manner and granting call requests on the basis of the connection types. The system has been developed, modeled and simulated using OPNET Modeler. The simulation results show that the QoS-aware TDMA/FDD MAC with multi-beam DAs has substantially increased the system throughput and that the call-blocking ratio has been reduced from 86% to 18%, when the proposed MAC with 8-Beam antennas is employed instead of the regular MAC.

Introduction

Recently, the importance of wireless/mobile data communication has been increased, and the use of DAs has gained much importance to enhance the network capacity, together with the ever-increasing developments in both high performance wireless computers and QoS supported real-time multimedia applications. In addition, there has been a remarkable interest in developing new MAC (Medium Access Control) protocols for wireless networks equipped with smart antennas generally known as DAs to efficiently utilize the limited bandwidth.

Smart antennas (SAs) are one of the most promising technologies that enable a higher capacity in wireless networks by effectively reducing multi-path and co-channel interference. This is achieved by focusing the radiation pattern only in the desired direction. SAs employ a set of radiating elements arranged in the form of an array. The signals from these elements are combined to form a steerable or switchable beam pattern that follows the desired user/users. In a smart antenna system the arrays by themselves are not smart; it is the digital signal processing that makes them smart. The process of combining the signals and the focusing the radiation pattern in a particular direction is often referred to as digital beamforming [1], [2].

The main goal of using DAs is to maximize the performance of the wireless networks by increasing capacity, range and Signal-to-Interference-Plus-Noise Ratio (SINR) and reducing end-to-end delay (latency). DAs provide an increased radiated power due to focusing the transmitter power on one and/or desired direction. This improves promptly the range of the transmitter. On the other hand, the directivity of the antenna allows the wireless station to cancel interfering signals arriving concurrently at the receiver from other directions [1], [2], [3]. Here the antennas work reciprocal, resulting in also increased power at the receiver.

Resource allocation and QoS management are of high importance in efficiently utilizing wireless system resources. Mainly focusing on the traffic at the MAC layer, several scheduling algorithms providing QoS supported communications have been researched actively [4], [5], [6]. However, the exploitation of the “space” is not capably considered in these works, resulting in not only worse call-blocking ratios but also not allowing the system to achieve higher throughputs. Concerning these shortcomings, many researchers have focused on deploying DAs in MACs [7], [8], [9], [10]. Despite improving the performance of a slotted-ALOHA network with multiple beam adaptive arrays, QoS-aware communication was not concentrated in [7]. Similarly, [8] exploited the idea of SDMA; nevertheless, it did not have the projection to provide the multimedia applications of wireless terminals with the required level of QoS support. On the other hand, [9], [10] presented two different approaches to maintain asymmetric traffics, however all ATM service classes (i.e., CBR, VBR, ABR and UBR) for broadband applications were not envisaged.

In this work presented, a new QoS-aware MAC protocol based on TDMA/FDD (Multiple Access/Time Division Multiple Access/Frequency Division Duplexing), which allows QoS guarantees for multimedia application traffics (with ATM CBR, VBR, ABR and UBR service support) and utilizes DAs, has been designed, simulated and compared to a regular TDMA/FDD MAC counterpart utilizing omni-directional antennas. In the proposed MAC model, WTs (Wireless Terminals) use omni-directional antennas and communicate through a BS (Base Station) which is equipped with multi-beam DAs containing beamforming modules for both receiving and transmitting, each of which is capable of directing a beam at a predefined area in the space. The system model uses FDD for dublexing technique and employs a DSAT (Dynamic Slot Allocation Table) at the BS that is especially utilized for determining to which WT the slots will be assigned in the uplink (WT to BS) direction. By means of the DSAT in the BS, a new frame structure is constructed. This approach ensures an increased wireless system capacity, meanwhile reducing call blocking ratio and improving end-to-end delay performance with a multimedia traffic differentiating approach.

The paper is organized as follows. Section 2 presents a brief explanation of directional antenna systems and essentials of wireless ATM architecture facilitated especially for developing the new QoS algorithms. Section 3 points out previous researches on wireless MAC methods utilizing DAs. Overall properties and design stages of the proposed TDMA/FDD MAC protocol with related QoS algorithms and DAs are explained in detail in Section 4. Section 5 includes an example network scenario employing the proposed MAC technique, which has been modeled and simulated under varying and different type of traffic loads and increasing number of users, with OPNET Modeler including the Radio Module. The simulation results obtained from the new developed QoS-aware MAC models with different number of DAs are compared to those of their regular counterpart with omni-directional antennas under the same network conditions. And, the last section summarizes the proposed method with final remarks.