OFDM-Based Broadband Wireless Networks: Design and Optimization

Chapter 6

OFDMA Design Considerations

While Chapter 5 establishes the OFDMA optimality in theory, many practical issues must be addressed before OFDMA can be utilized in real applications. In this chapter, we discuss some PHY and MAC design aspects of practical OFDMA systems. In particular, we introduce a cross-layer concept that benefits the OFDMA system performance by taking terminal mobility into account in OFDMA traffic channel design. The idea of dynamically reconfigurable traffic channel has been an active topic in IEEE 802.16e standardization activities.

6.1 Cross-layer design introduction

Cross-layer design is the concept of joint optimization over multiple layers in system design. In traditional systems, multiple layers are decoupled and layers do not operate interactively. As a result, each layer is designed to cope with the worst case condition, leading to inefficient usage of spectrum and energy. The separation of multiple layers are problematic in practice when handling the integration of mixed services in modern wireless networks. In high-speed data networks, traffic is highly diverse with distinct QoS parameters; channel and environmental conditions may vary dramatically on a short time scale; even the user pattern presents high dynamics – some users are highly mobile, while others may be semi-stationary. These added dynamics require a set of adaptive protocols to cope with these variations. However, the traditional decoupled layer design cannot meet such requirements. ...