International Journal of Advanced Technology & Engineering Research (IJATER)
3rd International e-Conference on Emerging Trends in Technology
PERFORMANCE COMPARISON OF DIFFERENT CODES USED IN RADAR 1
Arun Kumar Singh, 1Samarendra Nath Sur, 1Rabindranath Bera, 2Bansibadan Maji Dept. Of Electronics and Communication Engineering, Sikkim Manipal Institute Of technology, Sikkim 2 Dept. Of Electronics and Communication Engineering, NIT Durgapur, West Bengal
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Abstract Radar as the name says it is Radio Detection And Ranging. Earlier it was used only for defence purpose but now a days its become popular in commercial applications too. In defence, it was used for detection for enemy. In commercial application also its use is detection only. One of the emerging commercial application is vehicular application. RADAR is used in vehicular application for the purpose of collision avoidance, increase security and saving of mankind. The performance of RADAR highly depends on the use of input waveform Its the input waveform which decides the efficiency and effectiveness of RADAR. In this paper the author has tried to show the performance of different waveforms.
Introduction In telecommunication and radio communication, spreadspectrum technique is a method by which a signal generated with a particular bandwidth is deliberately spread in the frequency domain, resulting in a signal with a wider bandwidth. These techniques are used for a variety of reasons, including the establishment of secure communications, increasing resistance to natural interference, noise and jamming, to prevent detection, and to limit power flux density (e.g. in satellite downlinks). The same technology is copied in Radar and thus named as Spread spectrum Radar[1][3]. One of the main advantages of Spread Spectrum is to have a secure communication. In case of Radar also we need the detection and tracking of the target[2] to be secure i.e. it should not be hacked or jammed by other frequencies that may be intentional or unintentional. As discussed above, Radar using this spreading technique has quite large bandwidth which gives it another advantage of Range resolution of the target. More is the bandwidth less is the range resolution which can be proved as an extraordinary advantage in terms of vehicular application. The performance of Radar depends upon the proper choice of waveform[2] and hence the topic is justified as Waveform Diversity in Spread Spectrum Radar. Waveform diversity involves manipulating the degrees of ISSN No: 2250-3536
freedom of waveforms to enhance target detection, bit error rate and the efficacy of countermeasures. Degrees of freedom that can be varied include pulse repetition frequency, carrier frequency, coding, polarization, spatial characteristics, bandwidth, amplitude, pulse width, jitter, and frequency shaping. Traditionally, communication, sensing and countermeasures has been treated as separate technologies and the lack of interchange between this discipline is a major problem. The paucity of available spectrum and the objective of full spectrum dominance make it imperative to carry out this interchange. Waveform diversity is an enabling technology for achieving this goal. The choice of waveforms depends upon the range sidelobe reduction and Doppler extraction characteristics. Modern radars are increasingly being equipped with arbitrary waveform generators that enable simultaneous transmission of different waveforms from different polarimetric antennas, even on a pulse-to-pulse basis. The available design space encompasses spatial location, polarization, time, and frequency. The complexity of the design problem motivates synthesis of waveforms from components having smaller time-bandwidth product and complementary properties. A waveform is assembled by sequencing the components in time and/or stacking them in frequency in such a way that they have negligible overlap. With this approach, the waveform design problem splits into two simpler pieces: the design of components that complement each other, and the design of time-frequency combinations of these components with desirable properties. Another advantage of modularity is that the time-frequency combinations can be varied in time to enable adaptive control of the radar’s operation. Examples of this approach include pulse trains of orthogonal waveforms (separation in time) and what is often referred to as orthogonal frequency division multiplexing (OFDM) radar, where waveforms are separated in frequency. In case of Vehicular Radar, the required maximum sidelobe is to be lower than -30dB to prevent a side lobe of a strong echo from masking the main lobe of a weak echo. Using Barker Code, the peak side lobes for 11 bit and 13 bit barker codes are -20.83 dB and -22.83 dB respectively which is far from the required level. However the
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use of Polyphase codes can be proved as much advantageous over the barker code. In the coming section of this paper, the simulation results of different codes used in spread spectrum radar will be shown.
1.1 Polyphase code The codes that use any harmonically related phases based on a certain fundamental phase increment are called Polyphase codes and these codes are derived conceptually coherently detecting a frequency modulation pulse compression waveform with either a local oscillator at the band edge of the waveform (single side band detection) or at band center (double sideband detection) and by sampling the resultant inphase I and Q data at the Nyquist rate. The Nyquist rate in this case is once per cycle per second of the bandwidth of the waveform.
1.1.1 Frank Code The Frank code is derived from a step approximation to a linear frequency modulation waveform using N frequency steps and N samples per frequency. Hence the length of Frank code is N2. The Frank coded waveform consists of a constant amplitude signal whose carrier frequency is modulated by the phases of the Frank code. The phase of the ith code element in the jth row of code group is computed as
ɸi,j =(2π/N) (i-1) (j-1) -----------------(i)
(a) Autocorrelation under zero Doppler shift (b) Autocorrelation under Doppler = 0.05
From the above figure it is evident that the Frank code has the largest phase increments from sample to sample in the center of the code. Hence, when the code is passed through a bandpass amplifier in a radar receiver, the code is attenuated more in the center of the waveform. This attenuation tends to increase the sidelobes of the Frank code ACF. Hence it is very intolerant to precompression bandlimiting. .
1.1.2 P3 Code The P3 code is conceptually derived by converting a linear frequency modulation waveform to baseband using a local oscillator on one end of the frequency sweep and sampling the inphase I and quadrature Q video at the Nyquist rate. Letting the waveform to be coherently detected have a pulse length and frequency.
f=fo+kt ---------------------- (ii) Where k is a constant, the bandwidth of the signal will be approximately B=kT ------------------------------ (iii) This bandwidth will support a compressed pulse length of approximately
tc=1/B----------------------- (iv) Assuming that the first sample of I and Q is taken at the leading edge of waveform,the phases of successive samples taken apart is
-----------(v) Thus the phase sequence of the P3 signal is given by ɸi= (π/N) (i-1)2 -------------------------------------------------(vi)
Figure 1 Frank Code for length 100 ISSN No: 2250-3536
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1.1.4 Complimentary (CCK)
Code
Keying
Complementary codes [4] have properties that are useful in radar and communications systems. The sum of autocorrelations of each of a Golay complementary code pair is a delta function. This property can be used for the complete removal of sidelobes from radar signals, by transmitting each code, match–filtering the returns and combining them.
Figure 2. P3 Code for length 100 (a) Autocorrelation under zero Doppler shift (b)Autocorrelation under doppler = 0.05
1.1.3 P4 Code The P4 Code is conceptually derived from the same waveform as the P3 Code. The phase sequence of the P4 signal is given by ɸi=(π/N)(i-1)(i-N-1)-----------------------------------(vii)
Figure 3. P4 Code for length 100 (a) Autocorrelation under zero Doppler shift (b)Autocorrelation under Doppler = 0.05.
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Fig 4. (a,b) autocorrelation of complementary codes. (c)sum of autocorrelations.
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Simulation Results
(a)
(d) Fig. 5. a) Frank Code simulation for 128 bits. (b) p3 Code simulation for 128 bits. c) p4 Code simulation for 128 bits. d) CCK for 128 bits
Summary and Conclusion Simulation results for different code of same bit length have been shown. Results shows that frank code has a PSL of 3dB only. To overcome this p3 code was designed and it gives the PSL of 10.3dB for same bit length.P4 code also gives the same PSL but has better performance in Doppler condition than that of p3. Simulation result of CCK is also shown which gives excellent PSL as shown in the graph. Thus using these codes, the performance of spread spectrum radar can be improved.
References (b)
(c)
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[1] “Simulation of spread spectrum radar using rake at the receiver end”- Kandar, Sarkar, and Bera, Progress In Electromagnetics Research Letters, Vol. 7, 35–45, 2009 [2]. “Waveform Diversity in Radar Signal Processing”- Robert Calderbank, Stephen D. Howard, and Bill Moran, ieee signal processing magazine [32] january 2009 1053-5888/09 [3] “An introduction to spread spectrum”- Charles e. Cook and Howard S. Marsh, 0163-6804/83/0300-000$801 .00 @ 1983 IEEE [4]. “Generation of complementary Codes and doubly co-operative Ternary sequences and their Comparitive study in ambiguity Domain”- K. Srihari Rao et al. / International Journal of Engineering Science and Technology (IJEST), ISSN : 0975-5462 Vol. 3 No. 3 March 2011.
FIRST A. AUTHOR received the B.tech. degree in electronics and communication Engineering from Sikkim Manipal Universityin the year 2009 and did his M.tech from the same university in 2012. Currently, He is an assistant Professor of Electronics and communication Engineering at Sikkim Manipal University. His teaching and research areas include communication such as digital communication, mobile communication. Author may be reached at
[email protected]
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