Praise for Fundamentals of WiMAX

186 Chapter 5 • Multiple-Antenna TechniquesTrainingSignalNo SignalFrequencyf 1f 2TrainingSignal 1f 3TXNo Signal TX TrainingSignalTXf 4f 1TXTrainingSignal 2Orthogonalf 2f 3f 4T 1 T 2TimeData Pilot Null(a) Independent Pattern (b) Scattered pattern (c) Orthogonal patternFigure 5.17 Three different patterns for transmitting training signals in MIMO-OFDMOFDM Packet (time domain)PreambleUser Data1 OFDM Symbol 3 OFDM SymbolsFrequencyPreamble-basedFrequencyPilot-based2D (Time-Frequency) Interpolation1D Frequency Interpolation1D Time InterpolationTimeTraining SymbolTimeData SymbolFigure 5.18 Training symbol structure of preamble-based and pilot-based channel estimationmethods5.7.2 Time versus Frequency-Domain Channel EstimationMIMO-OFDM channels can be estimated in either the time or the frequency domain. Thereceived time-domain signal can be directly used to estimate the channel impulse response; frequency-domainchannel estimation is performed using the received signal after processing itwith the FFT. Here, we review both the time- and the frequency-domain channel-estimationmethods, assuming that each channel is clear of interference from the other transmit antennas,which can be ensured by using the pilot designs described previously. Thus, the antenna indicesi and j are neglected in this section, and these techniques are directly applicable to singleantennaOFDM systems as well.