Research On Doppler Diversity Reception Technology For OFDM Communications

With the excellent ability of high spectrum efficiency and stronganti-multipath fading, OFDM (Orthogonal Frequency DivisionMultiplexing) technology is considered to be the mainstream technology inthe B3G system. However, in the mobile communication environment, theDoppler effection will destroy the orthogonality between varioussub-carriers in the OFDM system, which will result in serious Inter-CarrierInterference (ICI), and reduce system performance significantly. This paperhas studied the OFDM Doppler diversity reception technology to overcomethe effection of Doppler and improve the BER performance. The maincontents include:The classical OFDM Doppler diversity reception technology has beenresearched. The basic idea is that firstly the receiver receives signals whichhave been through a different frequency shift, and produces diversitybranch signals. Each branch signal is FFT demodulated. After demodulatedand matched filtering, the signals are combined to receive sentence.Experimental results show that after merging the three branch in which the frequency offsets are0,0.75T,-0.75T (1T is the sub-carrier spacing), theperformance of the receiver is better than that without diversity.The paper puts forward an improved OFDM Doppler diversityreception technology. In the classic diversity reception technique, thecondition to select offset values is that the interference between twobranches, in which one frequency offset is0and the other isf x, must beunrelated. In this improved receiver technology, the condition is that eachinterference of two of the three branches must be unrelated. The resultsshow that when the branch offsets are0,0,0.8T,-0.8T rather than0,0.75T,-0.75T in the classic technique, it can satisfy this condition.Thereby, each branch of the deviation values in improved receivertechnology can be0,0.8T,-0.8T. Experimental results show that theimproved diversity reception technology can get better bit error rateperformance than the classic one.The application of differential phase modulation technique in OFDMreceiver has been studied. Both of the classic and improved Dopplerdiversity reception techniques are all required to know the channelfrequency response function, which requires channel estimation, so thereceiver structure is complicated, what’s more, the receiver needs a large amount of calculation. In this paper, by introducing the differential Dopplerphase modulation in the OFDM receiver, the receiver can recover theoriginal data without channel estimation, and the implement is simple, thecomputational complexity is low. However, the program’s error rateperformance is lower than the classic diversity reception technology andimproved diversity reception technology, besides, it can not getperformance gains by diversity.