The interference-limited spectral efficiency in the many- user regime was calculated for both CDMA and OFDM. Now suppose SIC is used instead of the conventional receiver in the CDMA system. In the context of SIC, the interference I0 in the target cb/I0 requirement refers to the interference from the uncan- celled users. Below you can always assume that interference cancellation is perfect.

1. Focus on a single cell first and assume a background noise power of N0. Is the system interference-limited under the SIC receiver? Was it interference-limited under the conventional CDMA receiver?

2. Suppose there are K users with user k at a distance rk from the base-station. Give an expression for the total transmit power saving (in dB) in using SIC with the optimal decoding order as compared to the conventional CDMA receiver (with an cb/I0 requirement of /3).

3. Give an expression for the power saving in the asymptotic regime with a large number of users and large bandwidth. The users are randomly located in the single cell as specified in Exercise 4.11. What is this value when /3 = 7 dB and the power decay is r -2 (i.e., a = 2)?

4. Now consider the two-cell system. Explain why in this case the system is interference-limited even when using SIC.

5. Nevertheless, SIC increases the interference-limited capacity because of the reduc- tion in transmit power, which translates into a reduction of out-of-cell interference. Give an expression for the asymptotic interference-limited spectral efficiency under SIC in terms of /3 and a. You can ignore the background noise and assume that users closer to the base-station are always decoded before the users further away.

6. For /3 = 7 dB and a = 2, compare the performance with the conventional CDMA system and the OFDM system.

7. Is the cancellation order in part 5 optimal? If not, find the optimal order and give an expression for the resulting asymptotic spectral efficiency.