WiMAX/IEEE 802.16

This chapter gives an overview of WiMAX (Worldwide Interoperability for Microwave Access). A Wimax system consists of access service network (ASN) and internet-based connectivity service network (CSN). It specifies both a FDD mode when operating in paired spectrum, and a TDD mode for upaired spectrum. Multiple access capability is achieved by OFDMA, in particular different users are assigned different subcarriers. We first review the physical layer. Wimax uses OFDM (with cyclic prefix) with adaptive modulation and coding for both the uplink and the downlink. For error correction coding, convolutional codes and turbo codes are foreseen; further error correction is achieved by hybrid ARQ (HARQ) with chase combining or incremental redundancy. We then turn to the logical channels and physical channels (time/frequency resources). The time axis is divided into zones, frames, and subframes. In addition to the data bursts for the separate users, a frame contains preambles and control signaling (DL and UL-MAP, Frame Control Header FCH), and the ranging subchannel. Various schemes exist for subcarrier permutation, i.e., assigning data to the time/frequency resources (called subchannels and tiles, i.e., subcarriers or groups of subcarriers). Different types of permutations are defined for use in different zones: the PUSC (partial use of subcarriers), TUSC (tiled use of subcarriers), FUSC (full use of subcarriers), and AMC (Adaptive modulation and coding). We then describe multiple antenna techniques in Wimax. For space-time coding, Alamouti, antenna selection, antenna cycling, and/or beamforming is used. Spatial multiplexing is either open-loop, or closed-loop (precoding). We finally discuss link control, including establishing of a connection (including scanning, synchronization, initial ranging), scheduling and resource request, quality of service (QoS) control, power control, handover, and mobility support.