Decisions on the BS working mode are made at the initial moment of each period t. While BS on-off switching can save energy, a switching action that is not compatible with the traffic load during a given period will result in a high call blocking probability. An appropriate switching decision should maximize the amount of saved energy during that period and, at the same time, achieve acceptable service quality such as in terms of call blocking probability. Also, it is desirable to minimize the frequency at which a BS changes its working mode from inactive to active in order to avoid the switching cost due to the additional energy consumption required for BS startup. From assumptions A1 and A2, the aggregate traffic arrivals to the cell are modeled by a Poisson process with mean rate λ_n= v_n+ v_n. The aggregate traffic arrival rate for each period is estimated using the data of traffic arrivals observed in previous days, as the traffic load in general follows a repeating pattern every day. The channel holding time in the cell is the minimum of the user cell dwell time and the call duration. From assumptions A3 and A4, the channel holding time is exponentially distributed with parameter μ_u= μ+ η. Hence, the call blocking probability can be calculated using the Erlang B loss model. The optimal BS on-off switching decision for a given period t can be obtained using the following optimization problem: