Digital front end for base-station RF
Creators: name: Gopi, Deepak
Publisher: name: No Publisher Supplied
Year: 2011

description: The digital front-end (DFE) is the most critical stage in a wireless base-station. The DFE along with the analog to digital converter (ADC) is responsible for bridging the analog RF and IF processing on one side and the digital baseband processing on the other side. The most important reason for replacing analog with digital signal processing is the ability to softly reconfigure the channels in the base station RF in real time, thus allowing for the implementation of various signal conditioning, compensation and mitigation channel non-linear responses. Once tested, these algorithms can be implemented on a proprietary CMOS vector processor and commercial FPGA hardware platforms. In this thesis, we attempt to minimize the design efforts and lower the cost involved in the usage of analog electronics by using sophisticated digital signal processing (DSP) for restoring and enhancing the quality of the wireless channels. This thesis presents a versatile Digital Front-End architecture, which has been simulated using MATLAB/Simulink. The architecture includes the design of robust Digital Up-Conversion (DUC) blocks in the transmit downlink and Digital Down-Conversion (DDC) blocks present in the receiver uplink paths in a wireless base station RF. Crest factor reduction (CFR) schemes help reduce the Peak to Average Power Ratio (PAPR)of the signal entering the base-station and have been implemented widely for code division multiple access (CDMA) and Long Term Evolution (LTE) systems, this is important because if the signal with the high PAPR is allowed to pass through the power amplifier(PA) it will result in the amplifier operating in its nonlinear region creating non-linear distortions in amplitude and phase, and the only other way to avoid this is to back off the signal to the linear region of the amplifier thus reducing its efficiency. The selection and design of the DUC and DDC filters has been compared and optimized to match to the spectral mask requirements mentioned in the 3GPP standards. Crest factor reduction has also been studied in detail and a computationally efficient algorithm for meeting the desired PAPR in accordance with the 3GPP standards will be presented. By using the CFR algorithm, the PAPR of the LTE signal was reduced from 10.8 dB to 7 dB and from 10.5 dB to 8 dB for a WCDMA signal. Finally, we implement Digital Predistortion (DPD) which is a method by which one first stimulates a non-linear power amplifier (PA) with baseband samples and then observes the result of that stimulus at its output. Without this process we will need to use a power amplifier with a higher input power rating which needs to be backed off to operate in its linear region thus reducing the efficiency of the PA used and increasing its cost. The process involves the use of a digital predistorter which creates an expanding nonlinearity which when used in cascade with the PA nullifies the compressing nonlinear characteristics of the PA thus enabling its use in its linear region up to its saturating point. A Look-Up Table (LUT) type Adaptive Digital Pre-Distortion (ADPD) is presented; here we developed an algorithm where the output signal of the PA is used as a reference signal. This reference signal is then used to update the coefficients of the LUT, so that the non-linear responses of the PA will not the affect the amplified signals. In addition, we investigated methods such as the nonlinear auto-regressive moving average (NARMA) and the memory polynomial models. In the latter, the predistorter parameters are calculated from the output signal obtained from the PA through the adaptive functions obtained using the memory polynomial. From these parameters, the predistorted signal is reconstructed and fed to the input of the PA. By using the DPD algorithm the nonlinear distortions of the PA came down by 60 dB when a WCDMA signal was used and by around 40 dB when LTE signal was used. As the PA is the heart of the base-station RF, we show that the main function of the DFE is to ensure a PA linearized output with a high efficiency.