High Resolution Low Power Nonlinear Chirp Radar Pulse Compression using FPGA Y. VIDYULLATHA

Transcription

1 ISSN Vol.03,Issue.26 September-2014, Pages: High Resolution Low Power Nonlinear Chirp Radar Pulse Compression using FPGA Y. VIDYULLATHA 1 PG Scholar, CMRIT, Hyderabad, India, y.vidyu@gmail.com. Abstract: Pulse Compression Techniques are used in Radar systems to avail the benefits of large range detection capability of long duration pulse and high range resolution capability of short duration pulse with low power consumption. In this paper, details of these techniques are given which achieves high range resolution and low peak side lobe levels under various noise conditions, Doppler shift and multiple target environments. The three very important Radar Pulse Compression techniques called Poly phase, Bi-phase and linear frequency modulated (LFM) Codes are discussed in this paper considering their various parameters. Keywords: Radar, Pulse Compression (PC), Peak Side Lobe Level (PSL), Range Resolution, Side Lobes. I. INTRODUCTION In this paper, Pulse compression radar has an advantage compare to conventional high power short pulse radar that use special transmit waveform and process a technique of the received signal in pulse compression which offers higher resolution in range and increased transmit power simultaneously. Although many advantages of pulse compression, the drawback of this method is unignorable range side-lobe from processing to reduce this effect, pulse shaping in time and/or frequency domains should be applied. In time domain, since such performance is affected by linearity of components, practical application was rendered difficult in high power devices [2]. The other approach is to apply linear FM (LFM) and demodulation using a window function [3], but it is still disadvantageous in power efficiency due to filtering out a considerable quantity of power during applying weighting function. To avoid such drawbacks, direct nonlinear waveform generation on transmitter has been analyzed in several researches such as [4] [5], but practical implementation is rare, especially in wideband long duration waveform [6]. In this paper, the method of nonlinear waveform generation using delta frequency extraction, and the implementation using FPGA for high power, pulse compression radar are described. Also the experiment configuration and test validation using developed Ka-band solid state radar are presented. II.NEED OF PULSE COMPRESSION Pulse Compression is an important signal processing technique used in Radar Systems to reduce the peak power of a Radar pulse by increasing the length of the pulse, without sacrificing the range resolution associated with a shorter pulse. Fig 1 illustrates two pulses having same energy with different pulse width and peak power. To get the advantages of larger range detection ability of long pulse and better range resolution ability of short pulse, Pulse Compression techniques are used in Radar systems. Fig.1. Transmitter and receiver ultimate signals. The range resolution depends on the bandwidth of a pulse but not necessarily on the duration of the pulse. The relation is shown in eq. (1). (1) Where, c = speed of light, ρ = range resolution, τ = pulse duration, B= signal bandwidth. The Pulse Compression ratio (PCR) must be very high and is defined in eq. (2). The Block Diagram of a Pulse Compression Radar system is shown in Fig 2. In Pulse Compression techniques, a pulse having long duration and low peak power is modulated either in frequency or phase before transmission used to increase the bandwidth of a long duration pulse to get high range resolution having limited peak power and the received signal is passed through a filter to accumulate the energy in a short pulse. The Trans-receiver (TR) is a switching unit which helps to use the same antenna as the (2) 2014 SEMAR GROUPS TECHNICAL SOCIETY. All rights reserved.

2 transmitter and the receiver. The Pulse Compression filter is usually a matched filter whose frequency response matches with the spectrum of the transmitted waveform. The filter performs a correlation between the transmitted and the received pulses. The received pulses with similar characteristics to the transmitted pulses are picked up by the matched filter whereas other received signals are ignored. Y. VIDYULLATHA Each sub pulse is assigned with a phase value ɸ i, where i = 1,2,...N. The received echo is passed through a filter to get a single output peak. The most popular phase coding is Biphase or binary coding. A Bi-phase Code consists of a sequence of +1 and -1. The phase of the transmitted waveform is 0 for +1 and 180 for -1. The Coded signal is discontinuous at the point of phase reversal. Fig.2.Block Diagram of Pulse Compression Radar system. A. Radar Signals In Radar systems a particular waveform is first determined for a given application and it is used to design the optimum detection system. The waveform should provide least amount of uncertainty or ambiguity when the reflected signal is used to extract the information about the range, the velocity and the number of true targets present in the environment. The different types of signals those are mostly used in Radar systems are discussed. These are Frequency modulated signal and Phase Coded signal. Linear frequency modulated (LFM) signals are used in most of the Radar systems to achieve wide operating bandwidth. In this case, the frequency increases (up chirp) or decreases (down chirp) linearly across the pulse. The instantaneous frequency is a linear function of time, and hence is called as linear frequency modulation. Fig 3 illustrates the instantaneous frequency of LFM waveform that sweeps from f 0 to f 1. Fig.3.The instantaneous frequency of the LFM waveform over time. The increase in bandwidth can also be achieved by Phase modulation in this case a long pulse width T p is divided into a number of sub pulses each of width t b as shown in Fig 4. Fig.4. Phase modulated waveform. The matched filter output of LFM signal has narrow main lobe width than that of Phase Coded signal and hence has better range resolution capability. However it is associated with side lobes which are unwanted in output from the filter and it is also evident from that the phase Coded signals are also associated with the side lobes. The PCR of phase Coded pulse is obtained in eq. (3). The modulated signals provide better range resolution as compared to un-modulated signals but the matched filter output of the modulated signals suffer from the side lobes. These side lobes may hide the small targets or may cause false target detection. The side lobe having largest amplitude is called Peak side lobe. The lower the peak side lobe level (PSL) the better is the Code. This can be defined as shown in eq. (4). III.PULSE COMPRESSION TECHNIQUES This Section briefly discusses the three Digital Pulse Compression techniques. They are LFM, Binary Phase Codes and Poly-phase Codes. The Pulse Compression goodness of a Code is determined based on its autocorrelation function since in the absence of noise; the output of the matched filter is proportional to the Code autocorrelation. Given the autocorrelation function of a certain Code, the main lobe width (compressed pulse width) and the side lobe levels are the two factors that are needed to be considered in order to evaluate the Code s Pulse Compression characteristics. In the case of Binary Phase Codes, a relatively long pulse of width τ is divided into N smaller pulses; each is of width Δτ = τ /N. Then, the phase of each sub-pulse is randomly chosen as either 0 or π radians relative to some CW reference signal. It is customary to characterize a sub-pulse that has 0 phase (amplitude of +1 Volt) as either 1 or +. One family of binary phase Codes that produce compressed waveforms with constant side lobe levels equal to unity is the Barker Code. For example, Fig 5 illustrates this concept for a Barker Code of length seven. (3) (4)

3 High Resolution Low Power Nonlinear Chirp Radar Pulse Compression using FPGA chirp signal and its autocorrelation function. Range side lobes are inherent part of the Pulse Compression mechanism and they occur due to abrupt rise in the signal spectrum. In place of LFM, Nonlinear LFM (NLFM) can be used for transmission which does not need any weighing at the receiver to overcome the mismatch loss but the NLFM signals are more affected by Doppler shift and are difficult to design than the LFM signals. Fig.5. Binary phase Code of length 7. In Bi-phase Codes the selection of random phase 0 or π is a difficult task. The phases are selected so that the matched filter output of the Code has lower side lobes. Barker Codes are the special type of binary Codes having side lobes of unity magnitude. Exhaustive computer based search reveals that the Barker Codes are available for the length of 2, 3, 4, 5, 7, 11 and 13 only. The Barker Codes along with their side lobe reduction or PSL values are given in Table 1. The Barker Code have maximum compression ratio 13 and highest PSL magnitude is 22.3 db. TABLE 1: Barker Codes If the pulse is allowed to take more than two values, it is known as a Poly phase Code. The phases of the Poly phase Code are chosen in such way that its ACF should have lower side lobes. The binary phase Codes have better range resolution as compared to LFM. Bi-phase Codes are easily generated and the correlates for these Codes are very simple. But the compressed output of Bi-phase Codes is associated with the high time range side lobes and these Codes are more prone to Doppler shift. The application of a Pulse Compression technique depends on how efficiently it reduces the range side lobes associated with the compressed waveforms. The number of Barker Codes available is very less and hence seriously suffers from security problem. Apart from Bi-phase Codes, Poly phase Codes and frequency modulated Codes are also used in Radar systems. PSL of Poly phase Codes are lower than that of the Bi-phase Codes. The frequency modulated and Poly phase Codes are more Doppler tolerant and have less range side lobes compared to Bi-phase Codes. Fig.6. A Typical chirp signal and its autocorrelation function. Phase Coded waveforms are more compatible for digital generation and compression. However the Poly phases Codes are sensitive to Doppler shift. To overcome this problem the Poly phase Codes are derived from the phase history of the frequency modulated pulses. The Codes such as Frank, P1 and P2 are derived from step approximation to LFM waveform. These Codes provide lower peak side lobes than that offered by the best Bi-phase Codes for a particular length. Two more Poly phase Codes, P3 and P4 are derived from the LFM signals. These Codes are more Doppler tolerant as compared to P1 and P2 Codes. When Doppler shift is zero these techniques substantially reduces the side lobes of the compressed pulse. Grating lobes are appeared in the ACF of Frank and P1 Code with increase in Doppler shift. The P3, P4 Code has better side lobe and Doppler shift characteristics. IV. SIMULATION RESULTS A. Modelsim Tool for Seeing the Simulation Flow of the Project 1. Open the Modelsim 6.6b simulator software to In most of the practical Radar systems LFM waveform is extensively used because it is more Doppler tolerant than phase Coded signals. A compressed LFM signal at the receiver will produce a series of side lobes surrounding the main lobe and the first side lobe occurs at a level of 13 db below the peak of the main lobe. Fig 6 represents a typical Fig.7.

4 2. Go to File Option and choose the correct Code path from the Change directory option Y. VIDYULLATHA 5. Then we should open script do files present in directory by Typing the pwd (present working directory) in the transcript window then it will show the path where the code available. Then it will show the path location. Fig Select the Project code path from the displayed window. And click Ok. It will go the source directory. Fig Then we should open script do files present in directory by Typing the dir * do in the transcript window. Fig Open the Top level code from the open option in the File tab. It opens the code of that module (compiling). Fig.12. Fig It opens the All Do files written in the project. Then the select the build do file and copy it the Transcript window with do as prefix (do build. do). Buitd do is a script file which do all the manual steps necessary for a project simulation that are compilation, simulation and timing and finally waveform.

5 High Resolution Low Power Nonlinear Chirp Radar Pulse Compression using FPGA B. Final simulation waveforms Fig After this type do build. do script file to run the total project in modelsim6.6b.it will run the project with compiling, simulating and run the wave form for particular time. Then it shows the wave forms which will consists of the required signals (waveforms) to generate non linear chirp waveform. Fig.14. Fig.15. Fig.16. The wave for which consists of Rst: The reset 1 means clear the garbage values, pervious values and start from initial position (fresh next rising edge of the clock). Clk_50mhz: The clock which is FPGA spartan3e clock of 50 MHz. for synchronization purpose. FM_ENABLE: The fm_enable which is high signal means 1. PRI_WORD_REG: This signal used for generate the radar signal with pulse repetition interval (PRI) which is in the format of hexadecimal that is X" "; ( in decimal). PW_REG_WORD: This signal used for generate the radar signal with pulse width (pw) which is in the format of hexadecimal that is X"000003E8"; ( in decimal). FREQUENCY_WORD: This signal is used for give the data value to the instantaneous frequency word that is in the format of hexadecimal that is X"199999"; -- to generate 0.1 frequency clock (1 MHz cosine at 10 MHz clock). LFM_slope_factor: This signal used for the nonlinear frequency slope factor which is in the format of hexadecimal that is X"0003FF"; LFM OR NLFM: This signal is high signal 1. WINDOW_FUNC_SEL: This signal is in the position of 00 means signal generation of non linear waveform in the domain only. RADAR_PULSE: This signal is radar pulse generation. Here the radar pulse is in 1 means signal as it is otherwise 0. Actually radar pulse consists of 1 and 0 similarly pulse also like digital values 0 and 1. Inst_frequency_word_LFM: This signal is given the value of same as frequency_word. Both signal value is same but here it is adding with LFM_slope_factor. So it is summing with lfm_slope_factor and frequency_word. RADAR_SIG_OUT: This signal is the output of the radar signal output which consists of pulses. PULSE_OUT: This consists of 0 and 1. Here the radar signal generates with 1 and 0, where 1 is there the output of radar is enable otherwise 0.

6 Y. VIDYULLATHA Delay_pulse_out: This signal is used for delay one clock pulse or same as pulse_out. NLFM_RST: This signal is used o reset all signals to initial position. ADDRA: This is the coefficient of the non linear waveform of window function named as Blackman Harris window function. Here the data_rom is used for address locations. It will locate the address and each address consists of coefficient of non linear waveform. inst_frequency_word_nlfm: This signal used for the frequency word of the non linear waveform coefficients. Here summation of frequency_word and NLFM_rom_coef_out. inst_frequency_word: This signal same as above signal. Cosine_carier: This signal which is generated by dds ip core as cosine signal. Fig.19. This is the final output of non linear chirp waveform which is generated by radar signal. Here the non linear waveform with respect to frequency and time domain in possible. Fig.17. Fig.18. This signal is inst_frequency_word is then see the output of radar output. Fig.20. V. CONCLUSION In this paper, the merits and demerits of different Pulse Compression techniques called LFM, Bi-phase and Poly phase Codes are known taking into consideration the important parameters like the main lobe width, range resolution and PSL. All the results of the simulations illustrate that Poly phase Codes have lowest Peak Side lobe Level (PSL) compared to Bi-phase Codes and LFM Codes, Bi-phase Codes and Poly phase Codes have better range resolution compared to LFM and the main lobe width is wider for LFM but the number of range side lobes are less compared to the other Codes. So Poly phase Codes are preferable in Radar Pulse Compression due to better range resolution and lowest PSL.

7 High Resolution Low Power Nonlinear Chirp Radar Pulse Compression using FPGA VI. REFERENCES [1] Hoon-Lee, Jaw-Wook Jung, Yong-Hoon Kim, A Design of Phase Nonlinear Chirp Waveform using FPGA for Pulse Compression Radar,vol.03,Issueno.06,March [2] Merrill I. Skolnik, Radar Handbook, 3rd edition, McGraw Hill, [3]B. Edde, Radar Principles, Technologies, Applications, Prentice Hall, [4] Chris Allen, Radar Signal Processing available online at (callen@eecs.ku.edu) and people.eecs. ku.edu/~callen/ 725/EECS725.htm, [5] Vijaya Chandran Ramasami, Principle of the Pulse Compression Radar, RSL, Univ of Kansas, [6] Chris Allen, Radar Pulse Compression, available online at Lectures, [7] Ajit Kumar Sahoo, Development of Radar Pulse Compression Techniques Using Computational Intelligence Tools, thesis for degree of PhD with National Institute of Technology Rourkela available at ac.in/3027/1/thesis.pdf, [8] Thomas Higgins, Dimensionality Aspects of Adaptive Radar Pulse Compression, Masters of Science thesis submitted to the Dept of Electrical Engineering & Computer Science & the Faculty of the Graduate School of the University of Kansus,2007. [9] Bassem R. Mahafza, Radar system analysis & Design using MATLAB, CRC Press, [10] M. A. Richards, Fundamentals of Radar Signal Processing, McGraw-Hill, New York, NY, USA, [11] Vijay Ramya K, A. K. Sahoo, G. Panda, A New Pulse Compression Technique for Poly phase Codes in Radar Signals, International Symposium on Devices MEMS, Intelligent Systems & Communication (ISDMISC) 2011 Proceedings published by International Journal of Computer Applications (IJCA), Vol. 2, Issue 4, pp.15-17, [12] Iulian-Constantin Vizitiu, Side lobes Reduction Using Synthesis of Some NLFM Laws, International Journal of Antennas and Propagation, Hindawi Publishing Corporation, Vol. 49, , 2013.