###### Abstract  
This paper presents a method to determine transmission power based on power amplifier (PA) operations in order to improve the energy efficiency (EE) of a large-scale (LS) Multiple Input Multiple Output (MIMO)-OFDM system, which is a multi-carrier multiple antenna system with a large amount of transmitter (TX) antennas. Regarding the EE improvement, we propose two kinds of PA operation schemes: increasing the effective TX power (ITXP) and reducing the PA power consumption (RPC) assuming that a reduction of peak-to-average power ratio is applied in the appropriate manner. Closed-form expressions of relative EE are derived for both schemes, and the relative EE of the ITXP scheme is shown to depend on the precoding method that is applied to reduce the inter-user interference, while that of the RPC scheme is independent of the precoding method. The relative EE difference between the ITXP and the RPC schemes is  
consumption in current BS [11]. Therefore, the operation of PAs can be very important to improve the EE of an LS-MIMO system. It is well-known that orthogonal frequency-division multiplexing (OFDM) is a very attractive modulation choice, and there is no doubt that it would be combined with LS-MIMO for efficient frequency usage. However, OFDM signal suffers from very high peak-to-average power ratio (PAPR) and current BS uses digital predistortion (DPD) technologies to compensate it [12]. Since DPD is an expensive device, it cannot be used for LS-MIMO due to the excessive amount of TX antennas, while there are numerous well-known PAPR reduction techniques and some can be successfully applied in LS-MIMO-OFDM to increase EE [13]. The LS-MIMO-OFDM can extend cell coverage due to its beamforming effect. However, the extension in coverage is not always beneficial in modern high data rate wireless communication systems because one BS needs to support more users under maintaining a high data transfer services in such a situation. In this paper, we show that both increasing the effective radiation power based on beamforming effect under maintaining the PA power consumption, and reducing the effective radiation power based on the degree of beamforming effect under reducing PA power consumption can increase EE significantly. In the literature, several works focus on the transmission power and the related EE of LS-MIMO [14; 15; 16; 17; 18]. An optimization technique with various system parameters was used to find the global optimum point of the EE of LS-MIMO in [14]. In [15], an energy-efficient resource allocation problem was investigated for the downlink LS-MIMO frequency-division duplexing (FDD) system under a correlated Rayleigh fading channel. In this work, the authors tried to maximize the EE by adjusting the training duration, training power, and data power, under the constraints of a total transmit energy and spectral efficiency requirement for a user, which simultaneously evaluates the impact of the training and data transmission phases simultaneously. In [16], the authors derived tractable expressions for achievable uplink rate in a large-antenna limit, which approximates the results holding for any finite number of antennas. The results of analysis produced the scaling law that the users' transmit power should satisfy, while maintaining a desirable quality of service. In [17], an optimal resource allocation scheme, which jointly selects the training duration, the training signal power, and the data signal power maximizing the sum spectral efficiency for a given total energy budget spent in a coherence interval, was introduced. A closed-form approximation of the TX power based on an optimization scheme was also introduced in [18]. Even though all the previous works provide rather elegant schemes to improve the EE of an LS-MIMO, however, there is little work focusing on operating PA and relating the operation with the transmission power of the LS-MIMO-OFDM. This kind of work is very important from a practical point of view because the power control is based on the PA operation.