Exercise 6.6 Consider the downlink of the IS-856 system.
1. Suppose there are two users on the cell edge. Users are scheduled on a TDMA basis, with equal time for each user. The received SINR of each user is 0 dB when it is transmitted to. Find the rate that each user gets. The total bandwidth is 1.25 MHz and you can assume an AWGN channel and the use of capacity-achieving codes.
2. Now suppose there is an extra user which is near the base-station with a 20 dB SINR advantage over the other two users. Consider two ways to accommodate this user:
• Give a fraction of the time slots to this user and divide the rest equally among
the two cell edge users.
• Give a fraction of the power to this user and superimpose its signal on top
of the signals of both users. The two cell edge users are still scheduled on a TDMA basis with equal time, and the strong user uses a SIC decoder to extract its signal after decoding the other users’ signals at each time slot.
Since the two cell edge users have weak reception, it is important to maintain the best possible quality of service to them. So suppose the constraint is that we want each of them to have 95% of the rates they were getting before this strong user joined. Compare the performance that the strong user gets in the two schemes above.