The power amplifier is a module circuit in the RRU equipment, referred to as the PA module. Complete small signal amplification and power amplification in the TRX link. The transmitted output power signal is filtered by a filter to remove out-of-band spurious signals and then fed to the antenna unit and radiated to the space; at the same time, the small space signal received by the antenna unit is processed, and the low-noise amplification is sent to the subsequent circuit for processing. The PA module is located between the TRX circuit and the duplexer or filter. PA module includes multi-stage amplifier circuit, LNA circuit, forward and backward sampling circuit, coupler circuit, time division switch circuit, PA tube protection control circuit and time division control drive circuit. The transmitting circuit is designed with a multi-stage amplifier, which amplifies the small transmitted signal from TRX to the rated power, filters out the out-of-band spurs and radiates to the space through the antenna. At the same time, the output signal is sampled forward and backward through the coupler. DPD performs pre-distortion compensation on the downlink signal, corrects the distortion of the power amplifier, improves the transmission intermodulation, and realizes the monitoring of the matching status of the PA module. The receiving circuit performs low-noise amplification on the small signal from the antenna to achieve the best received signal-to-noise ratio.
The PA module mainly realizes the power amplification of the radio frequency signal, but it will be accompanied by signal distortion and heat loss during the power amplification process. With the development of communication technology, RRU complete machines have higher and higher requirements for PA distortion and efficiency. The PA module technically implements power back-off technology, negative feedback technology, feedforward technology and pre-distortion technology. Pre-distortion technology includes analog pre-distortion and digital pre-distortion. With the development of broadband digital modulation technologies, such as BPSK, QPSK, 16QAM, 64QAM, 256QAM, and even 1024QAM, the standards from 3G WCDMA/TD-SCDMA/CDMA2000, 4G LTE to 5G are all non-constant envelope signals. The peak-to-average ratio is high, and the PA module is required to meet the linear index of a large dynamic range.
However, the efficiency of the AB type PA that relies on linear fallback is too low to meet the RRU miniaturization and heat dissipation, as well as the increasing energy-saving requirements of operators. The digital predistortion technology composed of Doherty amplifier and DPD has become the mainstream. Figure 7-1 is a block diagram of the circuit principle of the digital predistortion PA module in TDD mode. It consists of radio frequency power amplifier circuit, LNA circuit, power signal detection circuit and power amplifier protection and control circuit. The RF power amplifier circuit is generally designed as multi-stage according to the gain requirements, and the common three stages are the excitation stage, the driver stage and the final amplifier. The circuit is also designed with attenuators (including temperature compensation attenuators), couplers and loops. Device.