A Study on the Efficiency Optimization of RF Power Amplifier for Base Station

MasterThe mobile communication systems have evolved at a very fast rate recently due to the advent of smart phones. Unlike the previous systems that only support voice and small size data, the recent wireless communication systems serve not only voice but also large amount of data, using complex modulation schemes such as OFDMA technology. As the wireless communication systems are evolved, the peak to average power ratio (PAPR) and bandwidth of the signals are rapidly increased. Therefore, the base station power amplifier (PA) is required to have higher capacity than ever before. Since the power amplifier consumes a large portion of the power in base station, the efficiency of power amplifier is a very important factor in the system. The power amplifier of the previous base stations have employed an analog pre-distortion technology and feed forward method, but the circuitry is very complex and the power amplifier has a low efficiency. Since 2007, the WiBro system has employed a Doherty technique and digital pre-distortion (DPD) error correction and the amplifier delivers a high efficiency and a good linearity, simultaneously. As the base station system has advanced dramatically based on the WiMax, the system demands a power amplifier with higher efficiency and higher power. As the long term evolution (LTE) system has been adopted since 2010, and the system requires even higher efficiency power amplifier. The mobile communication system also evolves to remote radio head (RRH) system from the rack type for a small size. But the cooling apparatus is not present in RRH and the heat generated by the power amplifier is cooled by applying a natural convection method. Therefore the efficiency of the PA becomes more important issue, and several techniques have been studied by many researchers. The efficiency is improved the efficiency by using asymmetric dividing technique and utilizing the harmonic controls at the load and source. The conventional Doherty power amplifier could deliver a higher efficiency by applying these techniques. In this thesis, the load impedance of carrier amplifier is optimized for the efficiency at 2Ropt instead of Ropt. As a result, the efficiency is improved by about 3% point. The chapter 2 shows the basic operation principle of Doherty PA and method for optimizing the efficiency in the proposed structure. The chapter 3 presents the design of the PA using the ADS simulation for source pull and load pull with the information of the PCB. When the load impedance of the carrier amplifier is changed from Ropt to 2Ropt, the efficiency improvement is confirmed. The Doherty amplifier is realized with the peaking PA. The chapter 4 verifies that the 2Ropt case has better efficiency than the Ropt case, and the Doherty power amplifier is implemented by combining the carrier amplifier and peaking amplifier. The designed Doherty power amplifier has a efficiency of 60.7% at an output power of 49.10dBm. It also delivers the efficiency of 60.13% and a gain of 9.59 dB at an output power of 43.60dBm. After applying DPD operation, the power amplifier achieves the efficiency of 57% and a gain of 9.6 dB at an output power of 43.22dBm for a LTE signal. The ACLR is improved from -28.22dB / -27.24 dB to -46.14dB / -46.82dB by the DPD technique, which is improved by 18 dB. The proposed power amplifier has better efficiency and linearity satisfying the system specification by the DPD technique. The proposed method reduces the heat generation of the power amplifier and the size of the whole system. The maintenance cost of the system is also decreased due to the lower power consumption