Research On The RF Power Amplifiers For TDD Systems

With the rapid development of the wireless communication system, TD-LTE, the evolution ofTD-SCDMA which is the third generation with the independent intellectual property rights in ourcountry, has been the new standard of the fourth generation. The operation mode of both TD-SCDMA and TD-LTE, belonging to the time-division system, are time division duplex (TDD).The wireless frame of TD-SCDMA and TD-LTE is divided into different time slots, and the uplinkand downlink data are transmitted in the corresponding time slots. At the same time, the ratio of theuplink and the downlink time slot can be configured flexibly. In order to minimize the interferenceto the receive channel and reduce the power consumption, the power amplifier (PA), consumingthe most power in the base station, should only operate in the transmission time slots and power offduring the receiving time slots. Different from the PAs in a frequency division system, the PAs in atime-division system should operate on the basis of the uplink and downlink signal, so theswitching time is restricted strictly. In order to guarantee isolation of the receiving and transmissionchannel, the PAs must be in the stable state during the scheduled turn-on time and shut downcompletely during the turn-off time. Thus the characteristics of the switching control signal of PAsin the time-division system will be studied in this paper. The methods of improving the switchingcharacteristics will be explored. The bias circuit of PA is designed in this paper to meet therequirement of switching characteristics.According to the standard of3GPP R1-081181, the turn-on time of the switching controlsignal should be ahead of the arising time of the signal at least2us for a TD-SCDMA system,while the turn-off time of the switching control signal should be latter than the falling time of thesignal at least2us for the TD-SCDMA system. The turn-on time should be10-40us and the turn-off time should be15-20us for a TD-LTE system. On the basis of the requirements, a switchingcontrol circuit is proposed to be added in the grate bias circuit of the PA so as to control theoperation mode of the PA in a TDD system. The switching performance is improved by acontrollable discharging circuit. The tests result illustrated that the falling time of the switchingcontrol signal without the controllable discharging circuit is50us, while20ns with the controllabledischarging circuit, which can meet the requirements of the switching time for a TD-LTE system.The OFDM modulation scheme, leading to a high peak to average power ratio (PAPR), isused in the TD-LTE system. In order to improve the efficiency of the PA, the Doherty technique isutilized which can keep high efficiency in a region of6dB back off the peaking output power. Considering that3G should evolve into4G smoothly, the PAs should be designed in a dual modeof TDS/TDL for reducing the cost of construction and operation. Therefore the simulation of adual-band Doherty PA is studied in this paper.The input and output matching circuit, the offset line and λ/4line in a dual-band DohertyPA should behave dual-band characteristics. Hence the dual-band impedance transformer and thecomposited right/left-handed transmission line are utilized in this dual band PA. Therefore, onefrequency band of TD-SCDMA is from2010MHz to2025MHz, and the4G TDD tests use thecentral frequency of3.45GHz whose bandwidth is20MHz. The PA is designed to operate in thetwo bands. According to the simulation results, when the output power is6dB back-off the peakingoutput power at the frequency of3.45GHz and2.017GHz, the PAE of the dual-band Doherty PAare more than50%for the two bands. Especially, the PAE increases19%and24%compared witha balance PA respectively.