This paper proposes a new transmission technique for heterogeneous networks with massive MIMO wireless back-haul with the objective of minimizing the power consumption cost. We assume that transmissions occur during two phases. During the first phase, the multi-antenna small-cell base stations (SBSs) receive signals from their associated users and from the macro-cell base station (MBS) thanks to MIMO spatial multiplexing. In the second phase, the SBSs transmit signals to the MBS and to the users. We study the problem of minimizing the sum SBS transmit power under minimum-rate constraints required at the users. We solve the formulated problem by deriving analytically the optimal time splitting parameter and the allocated transmit powers. Compared with the well-known reverse time division duplex (RTDD) with bandwidth splitting, considered as a benchmark, simulations show that the proposed transmission technique allows the SBSs to reduce considerably the power consumption.