mit power in the direction of the user, thus enabling higher receive SINR at the AT. Space division multiple access (SDMA) further increases sector capacity by enabling simultaneous transmissions to spatially separated users using the same sets of sub-carriers. Space Time Transmit Diversity (STTD) can also be used where multiple transformed versions of a data are transmitted across a number of antennas and to exploit the various received versions of the data to improve the reliability of data-transfer. Thus, beamforming in combination with MIMO, STTD, and SDMA provides improved user data rates in both high and lower SINR regions [4,5]. In STTD mode, the number of effective antennas used can be two or four. In the four antenna case, there are two modes of STTD that are supported by the access terminal. When the number of effective antennas used is four, the STTD mode may be used in conjunction with antenna selection. 6.5.2.2 Service Robustness at Cell Edges The 802.20 allows the AT to independently select the strongest DL and UL sectors, through the use of independent active set management for these data links. This property ensures the AT receives the strongest link performance available and minimizes other sector interference for terminals at the sector edge. Additionally, the Wideband mode enables fractional frequency reuse (FFR) to improve cell edge performance by partitioning available carriers into several sets. Thus the system can put users at the cell edge into reuse with respect to the strongest interfering sectors, gaining an orthogonality advantage by properly utilizing the sets of sub-carriers. Lastly, the uplink power control feature enables the access network to tightly manage inter-sector interference. This feature is made possible through AN broadcasting other sector interference data on a regular schedule. The UL serving the AN makes assignment decisions based on certain parameters (QoS level, buffer size, etc). Thus, communications between the AN and the cell edges take place with the minimum req