Research On Enhancing The Quality Of Transceiver With Pre-distortion

At 2:30 pm on January 7,2009,Ministry of Industry and Informationization grant three license plate of 3G for China Mobile,China Telecommunications and China Unicom.This action indicated that China has entered into the epoch of 3G.Compare with 2G,3G has higher speed for data,more luxuriant services to choose,and 3G also has good compatibility with 2G.With more and more need of services for wireless mobile communications in this era,it need the technology of wireless mobile communications not only has higher quality,but also has bigger content.For higher speed of communication,we need more advanced and more efficient technologies to hold.OFDM,Orthogonal Frequency Division Multiplexing,is one preponderant technology in the area of mobile communication,it has higher efficient in the channel and lower cost,it become more and more commercioganic. QAM,Quadrature Amplitude Modulation,is one high efficient mode of modulation, become more helpful for mobile communication.OFDM has higher efficient in the channel,but it also has higher PAPR,and the QAM demand that power amplifier has good linearity which is in the end of the transceiver.So,how to take care of efficiency and linearity,guaranteeing the efficiency and reducing the interfere from the near channel,is a problem which need to be solved.Non-linear PA possesses the AM/AM and AM/PM,that is the ultimate reason to engender the nonlinearity.It makes the PA descend the gain and produce the IMD.That will affect the quality of RF transceiver.In one word,the linearity of PA is very important for the precision of the transceiver,so we start the work of linearizing the PA.There are some available methods to use:Power back-off,feed back,feedforward and pre-distortion.Power back-off means that make the input power back off 6dB to 10dB from 1dB compression point,far from saturation point,improve the coefficient of the third Intermodulation.Commonly,the third Intermodulation improve 2dB as the input power reduce 1dB.Power back=off is a conventional method and is very easy to obtain.The flaw is that it reduces the efficiency of the PA as it reduces the interfere.This is adverse for mobile phone.Feed back means that make the output signals back off the input port,compare with the input signal,and reduce the interfere which comes from the nonlinearity of PA.but the gain of the PA is also degressive.Besides,there is time delay in the system,it is not very steady when the output signal is feed back to the input port.So the emphasis is consider the stability of the system.Feedforward means when the signals pass the PA,there are IMD in the system,then compare the signals with the input signals,get the compensate signals which is nonlinear. Transmit the nonlinear signals to the error power amplifier,plus the original signals,then we will got good results.The feedforward need two power amplifiers,time delay and phase correct model.So that is a complex system,the adjust for gain and phase must be matching exactly.The cost is too much of the hardware.Pre-distortion means before the signals pass the PA,the signal pass the pre-distortion which has the inverse character of the PA,get a compensate,then pass the PA,make the result equate the signals pass the liner PA.Our aim is find the pre-distortion which has the inverse character of the PA,enhance the system linearity.The character of nonlinearity of the PA will changes while the temperature,voltage and the character of element change.So we raise the adapting pre-distortion in the system.We rectify the parameter with the adapting arithmetic,make the pre-distortion can follow up the change of the character of the PA,so it can improve the precision of the correction,that is the excellence of this method from the others.LMS,means least-mean-square,is one of the Stochastic gradient algorithm,simpleness is the most specialty.Do not need calculate the correlative function,nor need calculate the inverse of the matrix.So LMS become one of the most popular algorithm.In this paper,we show how to enhance the linearity of the PA with LMS and the results. We adopt Rapp and Saleh models,use Matlab as the emulator,achieve the aim via.m document and Simulink models.The passage also show the adjustment of the LMS when we use it in the practice.We make use of the flexible character of the software,confining the error of judgment,expedite the process of adaptation,make the time delay shorter than 10nS, so it is fit for using in the mobile RF transceivers.At last of this passage,compare the FDD and TDD,which are the most popular duplex mode for mobile phone,design one style of TDD transceiver which adopts the LMS,because TDD is more efficient in the channel and more agile when communicate,operating frequency is 2.4GHz,the source is OFDM and the modulation is 64-QAM.We also show the results,the ACPR declines more than 25dB and the EVM is equal 14.125%,satisfying the demand of 3GPP,the result proves that the transceiver is fit for using in practice.It also simplifies the configuration of the transceiver,economize the resource.This passage is significant for enhancing the efficiency,quality of communication and low cost.It is propitious to miniaturize the system and debase the cost,so there are immense commercial value.