Periodic and a-periodic on-off coded waveforms for non-coherent RADAR and LIDAR

Transcription

1 Periodic and a-periodic on-off coded waveforms for non-coherent RADAR and LIDAR Nadav Levanon Tel Aviv University, Israel With contributions from: Itzik Cohen, Tel Aviv univ.; Avi Zadok and Nadav Arbel, Bar-Ilan univ.; J. Mike Baden, GTRI Radar Symposium Ben-Gurion Univ., 25 January

2 With the knowledge accumulated in coherent radar, we enrich the branch of non-coherent radar. 2

3 Non-coherent pulse compression On-off keying transmitted signal +. Can utilize saturation amplifier, pulsed oscillator (e.g., magnetron) and especially laser +. Random phase of each sub-pulse is acceptable and even advantageous. Basic compression codes + : Bipolar a-periodic or periodic codes (Barker, Ipatov, Legendre, m-sequences ) Simple envelope detection in receiver +. Doppler tolerance + (but no Doppler information -). SNR loss due to non-coherency -. Sensitivity to multi-scatterer targets -. Non-coherent integration of many pulses (summing) is much simpler + than coherent integration (which requires FFT), but less efficient -. Low-sidelobe response requires mismatched processor. Additional SNR loss because of the mismatch -. 3

4 p =1 Magnitude detector p =2 Magnitude square detector BPF p b n b 2 Rectified output b 1 Output Receiver block diagram 4

5 PERIODIC NON-COHERENT WAVEFORMS 5

6 Periodic on-off waveforms (Example: Laser range finder) Classical approach: Time Of Flight (TOF); Amplitude (power) limited (A max ) A 0 t p T r time T i M T r, R CT r Target illumination time 1 max 2, unambiguous range R t p average, range resolution P A t T A t C 2R on target 2 2 p r p max P MA t T A t T T A t TC 4R p r p i r p i max Assumes the beam apperture on target is smaller than the target P A t TC 4R on target p i max Implies a conflict between power-ontarget and unambiguous range. Increasing A: Complicates hardware Increases probability of intercept Possible solution: Add more pulses within T r without reducing the unambiguous range. 6

7 Example of periodic on-off waveform: Barker 13!! S = R = 1 b b b 1 b 1, b=-2 Perfect Periodic Cross-Correlation (PPCC) 7

8 All unipolar Barker codes can exhibit PPCC (with the proper b in their reference) N b N = Code type m-seq & Legendre m-seq & Legendre Legendre m-sequences (shift register sequences) All unipolar m-sequences exhibit PPCC, with their corresponding bipolar reference (b = -1). m m-sequences are available at lengths N 2 1, m 2,3,4,... cw lidar, Applied Optics, Vol. 22, No. 9, 1 May 1983, pp Legendre sequences, N. Takeuchi, et. al. Random modulation All unipolar Legendre sequences exhibit PPCC, with their corresponding bipolar reference (b = -1). Legendre sequences are available at lengths P 4 k 1, k 1, 2,3, 4,... ; P is a prime Modified Legendre sequences All unipolar modified Legendre sequences exhibit PPCC, with their corresponding bipolar reference (b = -1). Modified Legendre sequences are available at lengths P 4k 3, k 2,3, 4,... ; P is a prime Ipatov sequences 8

9 function [s,r] = perfect_periodic_legendre_on_off( N ) % Generates a periodic coded signal using Legender and modified Legendre sequences % The signal exhibits perfect periodic crosscorrelation with its reference % N is any odd prime Nspt=sprintf('%g element Legendre on-off waveform ',N); if isprime(n)==0 disp('not a prime') return end s=ones(1,n); if rem((n+3)/4,1)==0 % modified Legendre s(mod((1:n-1).^2,n)+1)=0; r=s*2-1; s(1)=0; else % Legendre s(mod((1:n-1).^2,n)+1)=0; r=s*2-1; end d=abs(ifft(fft(s).*conj(fft(r)))); figure, plot(d, 'k') title(['periodic cross-correlation of ' Nspt ]); end 9

10 On-off signal based on Legendre 19 >>[s,r]=perfect_periodic_legendre_on_off(19); >> disp([s' r']) On-off signal based on modified Legendre 17 >>[s,r]=perfect_periodic_legendre_on_off(17); >> disp([s' r'])

11 Example of periodic on-off waveform: m-sequence 15 S = R = b b b b 1 b b b 1, b=-1 Perfect Periodic Cross-Correlation (PPCC) 11

12 Interference between two targets (simulations: m-sequence 127, Legendre 131) In CW waveforms the returns from two targets always coincide, even if the delay difference is large. The on-off non-coherent case is especially sensitive to the resulting interference. The simulation included different random phase of each sub-pulse out of the 650,000 subpulses. (code-length * number of periods processed * number of averages 650,000 ). No additive noise. 12

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15 The modulo-2 sum of an m-sequence and another phase (i.e. time-delayed version) of the same sequence yields yet a third phase of the sequence. 15

16 18 averages using the same code 18 averages of using 18 different codes 16

17 Performances with noise (simulation), Ipatov 121 Section of signal ( 60 elements), 5 samples per code element Periodic Cross-correlation output. The reference contained 3 periods. 17

18 Laser range finder experiment setup outside the Music Department at Bar-Ilan campus. 18

19 Experimental results with a laser range finder (4003 element on-off Legendre signal) R 0 = 273m R 1.2m R R 1 =R.139m R 3 =R m R 2 =R 0 Optical transmitted power = 22.5dBm. Sub-pulse width = 1ns dr=15cm Output after averaging 1000 consecutive measurements. Target illumination time = 0.2s (= ) Unambiguous range = 600m Target (white paper) relative forward power reflectivity =

20 [s r'] In each period, (N+1)/2 envelope detected samples (all positive), are multiplied by 1 and (N-1)/2 samples are multiplied by -1. Unique noise behavior 20

21 Sadogursky, A. and Levanon, N. Performances of Marcum s (S+N)-N integration scheme with fluctuating targets, IEEE Trans. AES, Vol. 50, No.1, Jan. 2014, pp

22 Noise only. Correlation with {1,0} reference. Noise only. Correlation with {1,-1} reference. Field trial at Tel Baruch coast, with a modified FURUNO magnetron marine radar (magneton project with Elisra. Project manager Erez Ben-Yaacov). 22

23 Extending the un-ambiguous range of a simple magnetron radar by a factor of 3 Transmitted non-coherent pulse train: (every 3 rd pulse is blocked) Reference pulse train: Legendre 3 and its reference Clutter and targets will replicate after 3 PRIs, rather than after 1 PRI. PRI Pulse fluctuations can slightly distort the perfect periodic crosscorrelation. 23

24 Field trial near the port of Ashdod, with a modified FURUNO magnetron marine radar (magneton project with Elisra. Project manager Erez Ben-Yaacov). 24

25 Ship target at a range of 13.1 km Correlation with {1,0} reference Near-clutter replicating at a range of 12km (corresponding to the PRI) 25

26 Ship target at a range of 13.1 km Correlation with {1,-1} reference 26

27 A-PERIODIC NON-COHERENT WAVEFORMS Example: Compressing a single long pulse by internal on-off coding 27

28 Trans Unipolar Barker 13: Manchester Encoded Unipolar Barker Ref 28

29 Unipolar Barker 13: The mismatched filter (MMF) used, is a sequence of length 65 (=5*13) optimized for minimum integrated sidelobes (SL), with higher weight given to the minimization of the near SL. The MMF can get any value, both positive and negative. A-periodic cross-correlations between a unipolar Barker 13 and a mismatched reference 29

30 Reduced duty cycle of the sub-pulses (1) Better range resolution (2) Higher bandwidth (3) Lower energy in the compressed pulse 30

31 ON-OFF SIGNAL - USE IN OPTICAL MASKS Fig. 1. Flow cytometry setup implementing spatial modulated emission. 31

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33 Thank you 33