Tham khảo tài liệu 'wimax, new developments 2011 part 5', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | The Application of p-Law Companding to Mobile WiMax 101 a QPSK Ped B c 16QAM Ped B e 64QAM Ped B f 64QAM Ped B Equalised Power Fig. 16. QPSK 16QAM and 64QAM Ped B BER probability curves as a function of p for situations of companded and equalised power companded WiMax 102 WIMAX New Developments The significant observations regarding mobility are as follows. The BER depreciates significantly for Veh A and Ped B channels as p increases for both companded and equalised power companded situations. It can also be seen that for each value of p as the SNR increases the BER flattens off to an asymptotic optimum value. This asymptotic value decreases with increasing p and also with increased data modulation on the subcarriers . the performance is best for QPSK deteriorates for 16QAM and further deteriorates for 64QAM. Thus a general conclusion is that increased companding will always degrade the performance of WiMax systems for larger SNR in the mobile channels considered. It may also be noted that for very small values of p the BER performance in the asymptotic region is comparable to the asymptotic value associated with standard WiMax. The main reason for the depreciated BER performance is clearly a combination of the companding profile the modulation and also the affects of the channel. Interestingly for the direct companding situations there is a marginal improvement in BER rate over WiMax at lower SNR values across a range of p values. An improvement in BER with companding is expected due to the inherent increased average power provided through the companding process itself. However the BER is still poor over the regions where the improvement over WiMax occurs. For the Veh A scenarios in Figure 15 a b and c the value of p which optimises the BER varies over the lower SNR range under consideration. The optimised p values over the lower SNR range is also nearly independent of the modulation employed. For example for QPSK 16QAM and 64QAM for SNR 4dB the curve for p 3 .
