**1. Introduction**

This chapter will explore The PHY layers defined in IEEE 802.16d&e (2004/5 standards updates), OFDM/OFDMA and SOFDMA in the frequency domain, Symbol mapping and channel encoding, Link Adaptation - Channel coding and modulation schemes, Control mechanisms and antenna diversity and spatial multiplexing.

The discussions thereof shall thus be based on IEEE 802.16d, herewith referred to as *fixed*  WiMAX and IEEE 802.16e herein referred to as *mobile* WiMAX. The specific PHY technical descriptions are given in the WiMAX Forum -T21/3 documentations or the original IEEE 802.16d suite of standards [2, 3, 4].

The PHYsical Layer in Fig. 1 provides the air interface between the Base Station (BS) and the Subscriber Stations (SS/MS) in different frequency bands for the entire range of IEEE 802.16\* standards for single and multi-carrier bands with OFDM/OFDMA and SOFDM/SOFDMA. PHY) layer thus takes MAC PDUs at the PHY SAP and arranges them for transport over the air interface [4, 5].

Fig. 1. WirelessMAN Protocol Layers

IEEE 802.16 (2001) specified PHY in 10-66 GHz range. This was further updated in 2004 and 2005 to 802.16d and 802.16e to define the 2-11 GHz range with enhancements such as Scalable Orthogonal Frequency-Division Multiple Access (SOFDMA) to the original multiplexing technique for fixed WiMAX, Orthogonal Frequency-Division Multiplexing (OFDM). Further updates have since been done in 2009 and 2011 to define Fixed and Mobile Broadband, and Mobile WiMAX respectively. In between these updates are several mergers, superseded and withdrawn projects. This chapter will NOT give further discussions on 2009 and 2011 updates to the 802.16 standard which have introduced multi-hop relay features (2009) and enhanced mobility and data rate features (2011).

The IEEE802.16 suite of standards defines four PHY layers in the Licensed Band namely WirelessMAN SC, WirelessMAN SCa, WirelessMAN OFDM (IEEE 802.16-2004) and WirelessMAN OFDMA (IEEE 802.16-2004), with further modifications to a Scalable OFDMA with a further one in the Unlicensed Band, WirelessHUMAN as summarized in Table 1 [2, 3].


\* is for the High-speed Unlicensed band using license except frequencies in the 2-11 GHz band . P2P is Point to Point

PMP is Point to Multi-Point

Table 1. Brief of IEEE 802.16 PHY

A functional WiMAX PHY Layer is represented by Fig. 2 both in time-frequency domains and digital-analog domains . The figure is shown from the transmitter end which is by default what's defined and the receiver is mostly left to vendor discretion. The first stage has to deal with Forward Error Check (FEC), channel encoding, puncturing or repeating, interleaving, and symbol mapping.

The next functional stage is the construction of the OFDM symbol in the frequency domain by mapping data onto the appropriate sub-channels/ subcarriers and inserting pilot symbols into pilot subcarriers to enable the receiver to estimate and track the channel state information (CSI). Space/time encoding for transmit diversity is also implemented in this stage. The final stage involves the conversion of the OFDM symbol from the frequency domain to the time domain and eventually to an analog signal for transmission over the air interface [9, 4].
