  Relays in cellular systems are interference limited. The highest end-to-end
sum rates are achieved when the relays are jointly optimized with the transmit
strategy. Unfortunately, interference couples the links together making joint
optimization challenging. Further, the end-to-end multi-hop performance is
sensitive to rate mismatch, when some links have a dominant first link while
others have a dominant second link. This paper proposes an algorithm for
designing the linear transmit precoders at the transmitters and relays of the
relay interference broadcast channel, a generic model for relay-based cellular
systems, to maximize the end-to-end sum-rates. First, the relays are designed
to maximize the second-hop sum-rates. Next, approximate end-to-end rates that
depend on the time-sharing fraction and the second-hop rates are used to
formulate a sum-utility maximization problem for designing the transmitters.
This problem is solved by iteratively minimizing the weighted sum of mean
square errors. Finally, the norms of the transmit precoders at the transmitters
are adjusted to eliminate rate mismatch. The proposed algorithm allows for
distributed implementation and has fast convergence. Numerical results show
that the proposed algorithm outperforms a reasonable application of single-hop
interference management strategies separately on two hops.
