Suppose the FCC allocates 300 kHz channels for a high quality video conference service. A service provider decides that this service requires 720 kbps (not including overhead and error control) for each channel.
a. Specify a modulation format (other than OFDM) that can provide this service along with the symbol rate and signal constellation. Allow for at least 25% excess bandwidth.
b. Suppose that the target error rate required is 10^5. Estimate the Signal-to-Noise Ratio (SNR) required at the receiver by using the following procedure:
(i) Determine the SNR needed for BPSK from the error rate curves in Week4.ppt. It turns out that the probability of error for QPSK is the same as that for BPSK. (This is because the distance between nearest QPSK constellation points is the same as the distance between the BPSK points.)
(ii) Doubling the number of points in a PSK constellation with more than 2 points requires approximately 6 dB more transmit power to maintain the same error rate. (That is, 8-PSK requires about 6 dB more than QPSK. This is due to the decrease in distance between nearest neighbor points.) Doubling the number of points in a QAM (rectangular) constellation requires approximately 3 dB more transmit power. (That is, 16 QAM requires approximately 6 dB more than QPSK, 32 QAM requires approximately 3 dB more than 16 QAM, and so forth.)
c. For the SNR computed in part b., what is the maximum data rate that is achievable according to Shannon? What is the minimum SNR required to achieve the 720 kbps data rate? (The difference between this SNR and the SNR computed in part b represents the amount of power that can be saved by using sophisticated coding methods.)
d. Suppose that OFDM is used with 15 kHz subchannels. (Two subchannels are used as guard channels on either side of the band.) Specify an OFDM symbol rate and modulation format for each subchannel that can achieve the target rate.