A direct microwave M-QAM adaptive transmitter for fixed wireless ATM networks

Abstract

Wireless ATM plays a key role in the realization of broadband wireless networks. The transmission of various classes of traffic and the provision of bandwidth on demand over a wireless channel poses a number of new technical challenges. This thesis addresses the design of a low cost adaptive transmitter for fixed wireless ATM/B-ISDN systems with emphasis on optimum use of wireless network resources. A new architecture for a direct microwave wireless ATM transmitter is proposed. The transmitter capacity adaptation is implemented by using an admission control metric and an M-QAM modulator. The two main components of the transmitter are: an M-QAM control unit and a direct microwave QAM modulator unit. The M-QAM control unit is used to select an optimum modulation level for the QAM modulator. The modulation level is adjusted based on the bandwidth demand, QoS requirements, and outage conditions of the wireless ATM link. The direct microwave QAM modulator unit transforms the broadband traffic to a modulated microwave signal that is suitable for transmission over a wireless network. The required bandwidth of the broadband traffic is estimated using an effective bandwidth metric. An analytical relation, called the capacity reduction factor, is derived to represent the performance degradation due to the wireless channel and channel fading in a B-ISDN network. Using the effective bandwidth metric and the capacity reduction factor, a QoS metric for the wireless broadband network is introduced. This metric is termed as, modified effective bandwidth. This metric is used to adapt the M-QAM modulator. Another significant contribution of this research work is a new architecture for the direct QAM modulator. This is based on use of PIN diode reflection attenuators. The PIN diodes operate in forward bias condition thereby overcoming the speed limitation problem due to charge storage. Using residue theory, analytical results to model the large signal forward bias operation of PIN diodes are presented. This theory also examines the transition time of a PIN diode with bias changes from a reverse bias to a forward bias. The direct microwave QAM modulator implementation using MIC and silicon MMIC technologies is examined. While a realization using MIC is simple and straightforward, a silicon MMIC realization offers a very cost effective solution. A system study was conducted to examine the operation of the adaptive direct microwave M-QAM modulator in the wireless channel with ATM traffic. The operation has been examined for different wireless channels and for various classes of traffic. The call acceptance and outage performance are compared with those for a fixed QAM modulator. The results show that the proposed system can be used for implementation of cost effective adaptive transmitters for broadband wireless applications.