Research On Parameter Estimation And Adaptive Pilot Pattern In OFDM Channel

Pilot-aided channel estimation is an efficient method to obtain channel response in OFDMsystem. It is convenient to estimate channel response in pilot subcarriers, and obtain thechannel responses’ estimated values in data subcarriers by using interpolation. Inconventional OFDM system, pilot pattern is usually arranged to match the extreme ormoderate channel condition. When channel is changed, the original pilot pattern may not besuitable for the new channel state: too many pilots will bring unnecessary system overhead, ortoo few pilots will seriously decrease the channel estimation performance. In OFDM system,if receiver can real-timely track the variations of channel state information (CSI), which arefed back to transmitter to modify pilot number and pilot pattern, the efficiency ofcommunication system will be improved.Multipath delay and Doppler frequency shift are two key parameters that describe wirelesschannel’s selective variation in frequency and time domains, respectively. The main job ofthis dissertation is to study how to efficiently estimate the two parameters, in order to adjustpilot number and pilot pattern in the transmission system.Respectively, frequency-domain correlation method and time-domain correlation methodare investigated to estimate multipath delay. Simulation demonstrates that the two methodscan effectually recognize tap locations of multipath and then easily estimate delay of each tap.Algorithms are slightly influenced by noise and Doppler frequency shift, and have goodstabilities. In addition, time-domain correlation method has lower computational complexitythan frequency-domain correlation method. Without specific requirement for the pilotdistribution in frequency domain, the time-domain correlation method has greater generality.Based on the recognition of multipath tap locations, a phase difference method is proposedto estimate Doppler frequency shift of each tap. Algorithm can obtain precise result and beslightly subject to the influence of noise on the condition that channel has larger Dopplerfrequency shift. The computational process is simple.On the bases of multipath delay and Doppler frequency shift, this dissertation finallycalculate the channel’s coherent bandwidth and coherent time, which are used to design adaptive pilot intervals. Corresponding pilot patterns and channel estimating strategies areproposed for time invariant channel and time-varying channel, respectively.Simulation for dynamic adjustments of pilot pattern is carried out in OFDM system ofIEEE802.16protocol. Simulating results show that adaptive pilot pattern will greatly reducethe total pilot number (more than80%reduction) without obvious degradation of systemperformance (about1~2dB SNR loss) under time invariant channel. And as for time-varyingchannel, the original pilot pattern is so inappropriate that there is a bad bit error rate (BER);after adaptive pilot adjustment, BER performance is improved greatly and the total number ofpilot is considerably decreased (about32%reduction).