5.3 WiMAX PHYs for Point-to-Multipoint FDD or TDD Operation
To communicate with stationary subscribers in a point-to-multipoint network, the 802.16 standard describes two basic options in the WirelessMAN-OFDM/HUMAN profiles.
For license exempt bands, time division duplex (TDD) is used. This means that the uplink and downlink directions between the base station and a subscriber use the same frequency band. Uplink and downlink are time multiplexed in a similar way as described in Chapter 6 for WLAN systems. The advantage of using a single frequency band for both directions is a flexible partitioning of the available bandwidth for the uplink and downlink directions. For applications like web surfing, the amount of data sent from the network to the subscriber is much higher than in the other direction. For such applications, more transmission time is assigned in the downlink direction than in the uplink direction. Disadvantages of TDD are that devices cannot send and receive simultaneously and that a device has to switch between transmit and receive states. As some time is required to switch between transmitting and receiving, some bandwidth is wasted during the required gap between the times allocated for sending and the times allocated for receiving.
Depending on national regulations, operators can also use licensed spectrum for their network. This will be the rule rather than the exception, as the operation in license-free bands is only allowed with minimal transmit power, usually ...