M.Sc Thesis

M.Sc StudentKarasik Roy
SubjectRobust Uplink Communications with Variable Backhaul
DepartmentDepartment of Electrical and Computer Engineering
Supervisor PROF. Shlomo Shamai )Shitz(


In cellular network, or Radio Access Network, each base station serves only the mobile users in its proximity, that is, in the same cell. As the demand to increase mobile data rates continues, frequency-division or time-division between adjacent cells is sometimes not an option, causing inter-cell interference. In traditional noncooperative cellular systems, inter-cell interference is a major limitation. One strategy to overcome this limitation is Multi-Cell Processing, or Cloud Radio Access Network. In cloud radio access network, the base stations act as "soft" relays, i.e., the base stations do not decode the received signal, but relay it to a remote central decoder that performs joint decoding (cloud computing).

In most cellular networks, the state of the channel usually does not remain constant.

Even if the receivers can obtain channel state information by estimating the channel from the received signal, in delay-constrained applications, it is often unrealistic to assume transmit channel state information, since feedback from receivers to transmitters is not possible. In a cloud radio access system, this calls for robust transmission strategies both for the mobile users and the base stations.

In this work we consider a cellular system where two mobile users communicate with a central decoder via two base stations. Communication between the mobile users and the base stations takes place over a Gaussian interference channel with constant channel gains or quasi-static fading. Instead, the base stations are connected to the central decoder through orthogonal finite-capacity links, whose connectivity is subject to random fluctuations. There is only receive-side channel state information, and hence the mobile users are unaware of the channel state and of the backhaul connectivity state, while the base stations know the fading coefficients but are uncertain about the backhaul links' state. The base stations are oblivious to the mobile users' codebooks and employ compress-and-forward to relay information to the central decoder. Upper and lower bounds are derived on average achievable throughput with respect to the prior distribution of the fading coefficients and of the backhaul links' states. The lower bounds are obtained by proposing strategies that combine the broadcast coding approach and layered distributed compression techniques. The upper bound is obtained by assuming that all the nodes know the channel state. In both scenarios, with and without fading, the analysis and numerical results reveal the importance of broadcast coding and layered compression in opportunistically leveraging advantageous channel and backhaul conditions.