Praise for Fundamentals of WiMAX

300 Chapter 8 • PHY Layer of WiMAXThe BS indicates to the MS, using the UL_MAP message if a UL sounding zone has beenallocated for a user in a given frame. On the receipt of such instructions, the MS sends a ULchannel-sounding signal in the allocated sounding zone. The subcarriers within the soundingzone are divided into nonoverlapping sounding frequency bands, with each band consisting of18 consecutive subcarriers. The BS can instruct the MS to perform channel sounding over all theallowed subcarriers or a subset thereof. For example, when 2,048 subcarriers are used, the maximumnumber of usable subcarriers is 1,728. Thus, the entire channel bandwidth can be dividedinto 1,728/18 = 96 sounding frequency bands. In order to enable DL channel estimation at theBS in mobile environments, the BS can also instruct the MS to perform periodic UL channelsounding.The channel-sounding option for closed-loop MIMO operation is the most bandwidthintensiveMIMO channel-feedback mechanism, but it provides the BS with the most accurateestimate of the DL channel, thus providing maximum capacity gain over open-loop modes.8.10 RangingIn IEEE 802.16e-2005, ranging is an uplink physical layer procedure that maintains the qualityand reliability of the radio-link communication between the BS and the MS. When it receivesthe ranging transmission from a MS, the BS processes the received signal to estimate variousradio-link parameters, such as channel impulse response, SINR, and time of arrival, whichallows the BS to indicate to the MS any adjustments in the transmit power level or the timingoffset that it might need relative to the BS. Initial and periodic ranging processes that allow theBS and the MS to perform time and power synchronization with respect to each other during theinitial network reentry and periodically, respectively are supported.The ranging procedure involves the transmission of a predermined sequence, known as theranging code, repeated over two OFDM symbols using the ranging channel, as shown inFigure 8.20. For the purposes of ranging, it is critical that no phase discontinuity 6 occur at theOFDM symbol boundaries, even without windowing, which is guaranteed by constructing theOFDM symbols in the manner shown in Figure 8.20. The first OFDM symbol of the rangingsubchannels is created like any normal OFDM symbol: performing an IFFT operation on theranging code and then appending, at the begining, a segment of length T g from the end. The secondOFDM symbol is created by performing an IFFT on the same ranging code and by thenappending, at the end, a segment of length T g from the beginning of the symbol.Creating the second OFDM symbol of the ranging subchannels in this manner guaranteesthat there is no phase discontinuity at the boundary between the two consecutive symbols. Sucha construction of the ranging code allows the BS to properly receive the requests from an un-6. During a ranging process, the BS determines the parameters of ranging by correlating the receivedsignal with an expected copy of the signal, which is known by the BS a priori. In order for the correlationprocess to work over the entire ranging signal, which spans multiple OFDM symbols, theremust be no discontinuity of the signal across OFDM symbols.