In this paper, we investigate an energy-efficient coordinated multiple point (CoMP) transmission strategy for downlink heterogeneous cellular networks. We combine CoMP joint processing (CoMP-JP) and coordinated beamforming (CoMP-CB), two special cases of CoMP, in a time division manner to improve both energy efficiency (EE) and spectral efficiency (SE). We formulate the problem as minimizing the total transmit power consumed by both the macro- and pico-base stations (BSs) under the constraints on the data rate requirements from the macro- and pico-users, and on the maximum transmit powers of the macro- and pico-BSs. Both the transmit time and the transmit powers allocated to the CoMP-JP and CoMP-CB transmissions are optimized. Simulation results show that the hybrid CoMP-JP and CoMP-CB strategy provides a larger capacity region than the CoMP-JP-only or CoMP-CB-only transmission. The time proportion of the CoMP-JP in the hybrid strategy decreases with the data rate requirement of the macro-user and increases with the maximum transmit power of the pico-BS and the average channel gain from the macro-BS to the macro-user. Increasing the transmit power of the pico-BS can improve the EE in the high SE region of the macro-user.
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