Operations in high frequencies ranging between 10 and 66 GHz were initially specified in the earlier versions of the 802.16 standard for fixed access. With this specification, only line-of-sight (LOS) signal propagation, with unobstructed path from the transmitter to the receiver, is feasible. Though high-frequency operations have the advantage of less interference, however most wireless technologies prefer lower frequencies because RF signals penetrate structures much better at low frequencies, enabling non-LOS propagation techniques. In non-LOS or multipath propagation modes, the transmitted signals are scattered, reflected, and diffracted by objects in the propagation paths between the transmitter and the receiver as shown in Figure 1. Thus, the receiver receives multiple copies of the transmitted signal, each arriving with different amplitude and phase or delay. These multipath signals may combine destructively at the receiver resulting in severe signal fades. To accommodate services in non-LOS conditions in the WiMAX system, 802.16-2004 standard subsequently specifies operations at lower frequencies, between 2 and 11GHz. Single-carrier transmission, known as wirelessMAN-SC, as well as two multicarrier transmissions, wirelessMAN-OFDM (orthogonal frequency division multiplexing) and wirelessMAN-OFDMA (orthogonal frequency division multiple access) are also specified. The WiMAX system also specified a number of advanced PHY layer and antenna technologies, both fixed and adaptive, to combat the severe fading effect of the multipath propagation channel, to enhance system performance.
