  Multicell cooperation has recently attracted tremendous attention because of
its ability to eliminate intercell interference and increase spectral
efficiency. However, the enormous amount of information being exchanged,
including channel state information and user data, over backhaul links may
deteriorate the network performance in a realistic system. This paper adopts a
backhaul cost metric that considers the number of active directional
cooperation links, which gives a first order measurement of the backhaul
loading required in asymmetric Multiple-Input Multiple-Output (MIMO)
cooperation. We focus on a downlink scenario for multi-antenna base stations
and single-antenna mobile stations. The design problem is minimizing the number
of active directional cooperation links and jointly optimizing the beamforming
vectors among the cooperative BSs subject to
signal-to-interference-and-noise-ratio (SINR) constraints at the mobile
station. This problem is non-convex and solving it requires combinatorial
search. A practical algorithm based on smooth approximation and semidefinite
relaxation is proposed to solve the combinatorial problem efficiently. We show
that semidefinite relaxation is tight with probability 1 in our algorithm and
stationary convergence is guaranteed. Simulation results show the saving of
backhaul cost and power consumption is notable compared with several baseline
schemes and its effectiveness is demonstrated.
