A radio frequency (RF) modem shelf for use in a fixed wireless access network comprising, a plurality of base stations capable of bidirectional time division duplex (TDD) communication with wireless access devices disposed at subscriber premises. The shelf comprises: a) a first RF modem for communicating with wireless access devices using TDD frames, each TDD frame having an uplink for, receiving data and a downlink for transmitting data; and b) a modulation controller associated with the RF modem shelf for determining an optimum modulation configuration for each of the wireless access devices communicating with the first RF modem. The controller causes the first RF modem to transmit downlink data to a first wireless access device in a first data block having a first optimum modulation configuration and to transmit downlink data to a second wireless access device in a second data block having a different second optimum modulation configuration.