A Modified Word Serial Bit Parallel Algorithm for FPGA Implementation of Digital Ionospheric Radar

Abstract

Digital signal processing (DSP) has become an integral part of wireless communication systems and equivalent traditional analog systems can be developed with required fidelity at reasonable cost. In a previous work, a DSP technique is employed for radar upconversion using combinations of upsampling and narrow band FIR filtering. An efficient FPGA implementation of the DSP modulator is possible exploiting filter symmetry since symmetrical properties of word serial bits parallel (WSBP) FIR filters improve system throughput. However arithmetic processing rate for the WSBP symmetric models is higher than that of the non-symmetric models. In this paper we present a modified WSBP symmetrical algorithm to reduce the arithmetic processing for implementation of direct conversion ionospheric radar. In WSBP approach processing is performed in integers as block of bits. Matching and buffering criterion are used to reduce computations up to fifty percent. The algorithm can be extended to applications with similar characteristics particularly for system on chip (SOC) techniques. I INTRODUCTION SuperDARN (Super Dual Auroral Radar Network) is an international radar network, which evaluates the space weather system. Tasman International Geospace Environment Radar (TIGER) is one component of the SuperDARN network that analyzes the space weather in southern hemisphere. The existing radar architecture is based on analog components with limitations of analog imperfections. Currently we are investigating efficient ways to implement this radar in FPGA technology to introduce more flexibility and reconfigure ability. In this paper we present a modified WSBP algorithm to realize a digital counterpart of the current analog transmitter system. In section II, a direct conversion method is described using FIR filtering. The proposed transmitter can be implemented with different ways; one of such methods is WSBP which is discussed in section III. WSBP algorithm can be employed using predefined Look Up Table (LUT) or with memory based variables. Arithmetic processing required for conventional WSBP methods is described in section IV with an example. Improvements in the conventional WSBP algorithm are discussed in section V. The proposed digital modulator is realized using Visual HDL and its performance is discussed in section VI. In the last section conclusions are drawn. II DIRECT CONVERSION TRANSMISSION METHOD In the literature different methods are described for direct conversion RF modulation. Most of these techniques use some type of sine or cosine multiplication that involves power sensitive signal processing function; power consumption of such systems is considerably higher since multiplication is performed at high frequencies. Using multirate digital techniques this can be implemented as a simple logic (4). In this section we present an equivalent method of generating digital signals for existing analog TIGER transmitter. We propose digital equivalent of only beam generation system of the transmitter while power amplifier and other associated analog circuitry remain the same. The block diagram of the proposed method is shown in Figure 1. The input Gaussian pulse is resampled at higher clock rate and a high pass FIR filter is applied to acquire the required spectral zone. The time delay is introduced to steer the beam at a set azimuth angle. This digital signal is converted to the analog domain using a suitable DAC. An analog reconstruction filter (ARF) is applied to reduce the quantization noise. The output of ARF is sent to the existing power amplifier through transmitter/receiver switch.