  We study the relay station (RS) sleep control mechanism targeting on reducing
energy consumption while improving users' quality of service (QoS) in green
relay-assisted cellular networks, where the base station (BS) is powered by
grid power and the RSs are powered by renewable energy. By adopting green RSs,
the grid power consumption of the BS is greatly reduced. But due to the
uncertainty and stochastic characteristics of the renewable energy, power
supply for RSs is not always sufficient. Thus the harvested energy needs to be
scheduled appropriately to cater to the dynamic traffic so as to minimize the
energy saving in the long term. An optimization problem is formulated to find
the optimal sleep ratio of RSs to match the time variation of energy harvesting
and traffic arrival. To fully use the renewable energy, green-RS-first
principle is adopted in the user association process. The optimal RS sleeping
policy is obtained through dynamic programming (DP) approach, which divides the
original optimization problem into per-stage subproblems. A reduced DP
algorithm and a greedy algorithm are further proposed to greatly reduce the
computation complexity. By simulations, the reduced DP algorithm outperforms
the greedy algorithm in achieving satisfactory energy saving and QoS
performance.
