Wideband slope of interference channels

Abstract

This dissertation studies the bandwidth-power tradeoff of K-user interference channels in the low-SNR regime. A system working in the low-SNR regime--a concept defined in [1], is characterized by very small R. The low-SNR regime performance of an interference channel was studied in[2]. It implies that interference indeed deteriorates system performance as long as R is non-zero. [1] indicates that the low-SNR regime performance is approached as SNR tends to zero, where SNR is the signal to noise ratio per second per Hz. This dissertation considers two cases in which the limit SNR ! 0 is achieved. One is the large bandwidth case, where the bandwidth B ! 1 while power P is fixed and finite. The other case is the small bandwidth case, where P ! 0 and while B is fixed and finite. The large bandwidth case is characterized by propagation delays considerably larger than the symbol duration. For K-user interference channel working under this case, this dissertation proposes an interference alignment scheme over time domain which achieves S0 = 1 2 with probability one, independent of the number of users K. It improves the best known performance achieved by TDMA by a factor of K/2. Outer bounds on S0 are also developed, which reveal that this interference alignment scheme is asymptotically optimal as K ! 1. In the small bandwidth case, propagation delay is assumed to be zero. Quite the contrary to the large bandwidth case, this dissertation shows that there exists a set of channel realizations with non-zero probability, for which the achievable S0 satisfies S0 < 1/K + , 8 > 0, therefore TDMA is almost optimal. For channel realizations not in this set, an interference alignment scheme based on the concept of circularly asymmetric signaling is proposed. For some channel realizations, it gives noticeably better performance comparing with existing achievable schemes.