annel is used to generate a 5-bit CRC using generating polynomial G(x) = x5 x4 + x2 + 1. The 13 bits of information together with the 5 bits of CRC are encoded into 72 bits using the TBCC encoder. The 72 coded bits are QPSK modulated to generate 36 data symbols. The physical resources in the data portion of the three distributed BR tiles are used to transmit these data symbols. In order to support mixed-mode operation, a BR tile is defined as four contiguous sub-carriers over six OFDM symbols (to be consistent with UL-PUSC subchannelization). As shown in Figure 10-19, CHAPTER 10 The IEEE 802. 16m Physical Layer (Part II) Legacy Mode UL Tile Frequency FIGURE 10-19 Physical structure of 4 X 6 bandwidth request tile [2] the BR preamble is only transmitted over the 24 available sub-carriers. In this case, the preamble index is randomly selected in the range of 0 to 23. 10.3.6 Power Control Power control is a mechanism where the transmit power of the downlink or uplink control or traffic channels are adjusted at the base station (downlink power control) or at mobile stations (uplink power control) based on instructions from the base station such that with minimal impact on the reliability of the downlink/uplink transmissions and throughput, the interference among users and base stations are reduced. Therefore, power control can be considered as a link adaptation mechanism that is utilized for interference mitigation in cellular systems. While increasing the transmit power over a communication link has certain advantages, such as higher signal-to-noise ratio at the receiver, which reduces the bit error rate and allows a higher data rate resulting in greater spectral efficiency, as well as more protection against signal attenuation over fading channels. A higher transmit power; however, has several drawbacks including increased power consumption for the transmitting device, reduced MS battery life and increased interference to other users in the same frequency band. The following sections describe the power control algo