Design and Development of a Hybrid TDMA/CDMA MAC Protocol for Multimedia Wireless Networks�

Abstract

A Wireless Local Area Network (WLAN) provides high bandwidth to users in a limited geographical area. This network faces certain challenges and constraints that are not imposed on their wired counterparts. These include frequency allocation, interference and reliability, security, power consumption, human safety, mobility, connection to wired LANs, service area, handoff and roaming, dynamic configuration, and throughput. The wireless medium relies heavily on the features of the MAC protocol, and the MAC protocol is the core of medium access control for WLANs. The available MAC protocols all have their own merits and demerits. In our research work, we propose a hybrid MAC protocol for WLAN. In the design, we combine the merits of TDMA and CDMA systems to improve the throughput of the WLAN in a picocellular environment. We use the reservation and polling methods of MAC protocols to handle both low and high data traffic from mobile users. We strictly follow the standards specified by IEEE 802.11 for WLANs to implement the designed MAC protocol. We simulate the hybrid TDMA/CDMA?based MAC protocols combined with RAP (Randomly Addressed Polling) for Wireless Local Area Networks. We develop closed?form mathematical expressions analytically for this protocol. We also study the power control aspects in this environment and derive closed?form mathematical expressions analytically for the power control technique. This hybrid protocol is capable of integrating different types of traffic (such as CBR, VBR, and ABR services) and complies with the requirements of next?generation systems. Lower traffic arrival is handled using Random Access, while higher traffic arrival is handled with Polling methods. This enables us to obtain higher throughput and low mean?delay performance compared to contention?reservation?based MAC schemes. The protocol offers the ability to integrate different types of services in a flexible way using multiple slots per frame, while CDMA allows multiple users to transmit simultaneously using distinct codes. RAP uses an efficient backoff algorithm to improve throughput at higher user?data arrival rates. The performance is evaluated in terms of throughput, delay, and rejection rate using computer simulation. A detailed simulation is carried out regarding the maximum number of users that each base station can support on a lossy channel. This work analyzes the desired user抯 signal quality in a single?cell CDMA (Code Division Multiple Access) system in the presence of MAI (Multiple Access Interference). Earlier power?control techniques were designed to ensure that all signals are received with equal power levels. Since these algorithms are designed with imperfect power control, the capacity of the system is reduced for a given BER (Bit?Error Rate). We propose an EPCM (Efficient Power Control Mechanism)?based system capacity model, designed for the reverse link (mobile to base station), considering path loss, log?normal shadowing, and Rayleigh fading. We simulate the following applications for further improvement in the performance of the designed MAC protocol: � Testing the protocol under different traffic conditions � Evaluating multimedia traffic performance under application?oriented QoS requirements � Buffer management and resource allocation � Call Admission Control (handoffs and new?user arrivals) � Adaptability to variable traffic patterns � Propagation characteristics in the wireless medium The proposed MAC protocol is simulated and analyzed using C++/MATLAB programming in an IBM/SUN?SOLARIS UNIX environment. The results are plotted using MATLAB software. All functions of the protocol are tested by both analysis and simulation: � Call Admission Control: Simulation and analysis in a multimedia wireless network show that throughput is high at low traffic and remains reasonably constant at high traffic. Simulation results support the analytical results. � Dynamic Channel Allocation: Analysis and simulation show high throughput at low traffic and stable throughput at high traffic. � Buffer Management: Simulation results show that packet loss can be minimized by adjusting the buffer threshold level under any traffic condition. � Handoff Performance: Simulation shows fewer blocked calls at low traffic, and blocked calls remain low and stable at high traffic. Thus, the proposed model aims to achieve high throughput, high spectral efficiency, low delay, moderate BER, and moderate blocking probability. We consider a picocell with a maximum of several users and study the power efficiency of combined channel coding and modulation with perfect?power?controlled CDMA systems. Thus, our simulation of the 搒oftware radio� has flexibility in choosing the proper channel coders dynamically depending on variations of the AWGN channel.