QAM256, QAM16, and BPSK are three mechanisms to modulate bits onto the electromagnetic waves. In QAM-based modulation mechanisms, the transmitter varies the amplitude and phase to create distinct states of a radio wave. In QAM16 for example, 4 levels of amplitude and 4 levels of phase shift allow 16 distinct states, therefore, allowing modulation of 4 bits. Similarly, using QAM256, one can modulate 8 bits of data per radio wave. BPSK is short for binary phase shift keying, which uses only 2 levels of phase shift and therefore modulates only 1 bit of data per radio wave. Although QAM256 allows a higher bit rate than the other two mechanisms, it is more vulnerable to noise and interference, as it is harder to differentiate amplitude and phase correctly at a finer granulation. This is why in the figure below one sees how the bit error rate (BER) of QAM256 is much higher than QAM16 and BPSK given the same level of signal to Noise ratio (SNR). 10-1 10-2 10-2 1 10-5 10-6 10% 10 30 40 20 SNR(dB) QAM256 (8 Mbps) QAM16 (4 Mbps) BPSK (1 Mbps) (a) If one wishes to keep the BER below 10 over a wireless link, which modulation mechanism should the link layer protocol use for SNR at 30dB, 20dB, and 10dB respectively? (6%) (b) Provided the background above, argue why WiFi transmission rate drops as one moves away from the WiFi AP. (4%)