Praise for Fundamentals of WiMAX

302 Chapter 8 • PHY Layer of WiMAXby the BS and conveyed over the control channels. During a specific ranging procedure, an MSrandomly chooses one of the PN sequences allowed by the BS. This ensures that even if two SSscollide during a ranging procedure they can be detected separately by the MS owing to the pseudorandomnature of the ranging codes. The chosen PN sequence is BPSK modulated and transmittedover the subchannels and OFDM symbols allocated for the ranging channel.8.11 Power ControlIn order to maintain the quality of the radio link between the MS and the BS and to control theoverall system interference, a power-control mechanism is supported for the uplink with bothinitial calibration and periodic adjustment procedure, without the loss of data. The BS uses theUL ranging channel transmissions from various MSs to estimate the initial and periodic adjustmentsfor power control. The BS uses dedicated MAC managements messages to indicate to theMS the necessary power-level adjustments. Basic requirements [6] of the power-control mechanismas follows.• Power control must be able to support power fluctuations at 30dB/s with depths of at least10dB.• The BS accounts for the effect of various burst profiles on the amplifier saturation whileissuing the power-control commands. This is important, since the peak-to-average ratio(PAR) depends on the burst profile, particularly the modulation.• The MS maintains the same transmitted power density, regardless of the number of activesubchannels assigned. Thus, when the number of allocated subchannels to a given MS isdecreased or increased, the transmit power level is proportionally decreased or increasedwithout additional power-control messages.In order to maintain a power-spectral density and SINR at the receiver consistent with themodulation and code rate in use, the BS can adjust the power level and/or the modulation andcode rate of the transmissions. In some situations, however, the MS can temporarily adjust itspower level and modulation and code rate without being instructed by the BS.The MS reports to the BS the maximum available power and the transmitted power that maybe used by the BS for optimal assignment of the burst profile and the subchannels for UL transmissions.The maximum available power reported for QPSK, 16 QAM and 64 QAM constellationsmust account for any required backoff owing to the PAR of these modulationconstellations.On the downlink, there is no explicit support provided for a closed-loop power control, andit is left up to the manufacturer to implement a power-control mechanism, if so desired, based onthe DL channel-quality feedback provided by the SS.