Question

2) A wireless internet access card requires for reliable operation the BER not exceeding 10-3. An engineer designing this card wishes to maximize the data rate over the SNR range of 0 to 20 dB. Select a proper modulation format for cach SNR at this range. Hint: M-ary constellation, such as M-QAM is a good candidate, with M-2 1-2,3,4... , for which you have to find a threshold SNR Yh(M) such that Pe(SNR,M)s10-3 if SNR 2Yth(M) , where Pe (SNR,M) is the BER as a function of SNR and constellation size M, and threshold SNR Yth(M) is a function of constellation size as well. Having done this, for each value of the SNR the engineer will use such constellation size M that (i) maximizes the date rate, and (ii) respects the BER constraint Pe(SNR,M) s10-3. Such a scheme is called adaptive modulation (signal constellation size is adjusted according to the SNR). Find and plot the resulting data rate of this adaptive modulation and the maximum achievable data rate versus the SNR if the channel delay spread is 0.1μ. Comment on the difference between the two and propose a way to bridge the gap. Additionally, plot the BER of this adaptive modulation versus the SNR and sketch a block diagram of a modulator and demodulator for this adaptive system.

Answer 1