Nadav Levanon

Performances of order statistics CFAR

A previous analysis of order-statistics constant-false-alarm-rate (OS-CFAR) radar receiving a single pulse from a Rayleigh fluctuating target in a Rayleigh background is extended to a Rayleigh-plus-dominant target. The analysis includes effects of a multitarget environment. A detailed comparison of OS-CFAR, cell-averaging (CA) CFAR, and censored CA-CFAR is provided for a Rayleigh target in the presence of strongly interfering targets. The false-alarm analysis of OS-CFAR is extended to the more general case of a Weibull background. The deterioration of the CFAR property of OS as the shape factor, C, of a Weibull probability density function changes from Rayleigh (C=2) to a longer-tailed one (C >

Multicarrier radar signal - pulse train and CW

A multicarrier complementary phase-coded (MCPC) radar signal employs N subcarriers simultaneously. The subcarriers are phase modulated by N different sequences that constitute a complementary set. The subcarriers are frequency-separated by the inverse of the duration of a phase element t/sub b/. An N/spl times/M MCPC pulse achieves pulse compression ratio of NM. When an N/spl times/M MCPC signal is used in a coherent train of N pulses (e.g., in order to allow Doppler processing), it is advantageous to use pulse diversity. Each pulse is constructed using a different cyclic frequency shift of the first pulse. Such pulse diversity eliminates recurrent lobes at multiples of the pulse repetition interval. The constant volume property of the ambiguity function is maintained by shifting volume from the recurrent lobes to the sidelobe pedestal strips. However, if the sequence ordering along frequencies in the N pulses are arranged to create a complementary set in each frequency as well as in each pulse, the sidelobe level around the main autocorrelation lobe is dramatically reduced. Preferred MCPC coding for CW mode (contiguous MCPC pulses) and their delay-Doppler response are also discussed.

Multifrequency radar signals

A multifrequency radar signal is considered. It employs M subcarriers simultaneously. The subcarriers are phase modulated by different sequences that constitute a complementary set. Such a set can be constructed, for example, from the M cyclic shifts of a perfect phase-coded sequence of length M (e.g., P4). The subcarriers are separated by the inverse of the duration of a phase element by yielding orthogonal frequency division multiplexing (OFDM), well known in communications. The signal exhibits a thumbtack ambiguity function with delay resolution of t/sub h//M. The power spectrum is relatively flat, with width of M/t/sub h/. The signal can be constructed by power combining M fixed-amplitude signals. The resulting signal, however, is of variable amplitude.

High-resolution long-reach distributed Brillouin sensing based on combined time-domain and correlation-domain analysis

A new scheme for distributed Brillouin sensing of strain and temperature in optical fibers is proposed, analyzed and demonstrated experimentally. The technique combines between time-domain and correlation-domain analysis. Both Brillouin pump and signal waves are repeatedly co-modulated by a relatively short, high-rate phase sequence, which introduces Brillouin interactions in a large number of discrete correlation peaks. In addition, the pump wave is also modulated by a single amplitude pulse, which leads to a temporal separation between the generation of different peaks. The Brillouin amplification of the signal wave at individual peak locations is resolved in the time domain. The technique provides the high spatial resolution and long range of unambiguous measurement offered by correlation-domain Brillouin analysis, together with reduced acquisition time through the simultaneous interrogation of a large number of resolution points. In addition, perfect Golomb codes are used in the phase modulation of the two waves instead of random sequences, in order to reduce noise due to residual, off-peak Brillouin interactions. The principle of the method is supported by extensive numerical simulations. Using the proposed scheme, the Brillouin gain spectrum is mapped experimentally along a 400 m-long fiber under test with a spatial resolution of 2 cm, or 20,000 resolution points, with only 127 scans per choice of frequency offset between pump and signal. Compared with corresponding phase-coded, Brillouin correlation domain analysis schemes with equal range and resolution, the acquisition time is reduced by a factor of over 150. A 5 cm-long hot spot, located towards the output end of the pump wave, is properly identified in the measurements. The method represents a significant advance towards practical high-resolution and long range Brillouin sensing systems.

Periodic ambiguity function of CW signals with perfect periodic autocorrelation

A periodic ambiguity function (PAF) is discussed which describes the response of a correlation receiver to a CW signal modulated by a periodic waveform, when the reference signal in the receiver is constructed from an integral number N, of periods T, of the transmitted signal. The PAF is a generalization of the periodic autocorrelation function, to the case of non-zero Doppler shift. It is shown that the PAF of N periods is obtained by multiplying the PAF of a single period by the universal function sin(N pi nu T)/N sin( pi nu T), where nu is the Doppler shift, to phase-modulated signals which exhibit perfect periodic autocorrelation when there is no Doppler shift. The PAF of these signals exhibits universal cuts along the delay and Doppler axes. These cuts are functions only of t, N and the number M, the modulation bits in one period. >

ac Electrical Breakdown in Thin Silicon Oxide Films

Silicon oxide capacitors were produced by vapor deposition in vacuum on glass slides. On the application of ac voltages in the frequency range 10 to 50 000 cps, single‐hole breakdown, propagating breakdown, and maximum voltage breakdown events were observed. The processes in these breakdown events and the destruction were analogous to those occurring on dc. However, the peak value of the ac maximum dielectric strength, Fam, was found to be 10%–20% larger than the dc maximum dielectric strength Fdm; this is in contrast with the much larger dc strength found for thick insulations. Good agreement was found between observations and between calculated values of Fam and Fam/Fdm. Fam of the samples tested ranged from 1.75 to 6.9 MV/cm. Above 1−10 kc/sec, Fam decreased quasilinearly with frequency due to dielectric loss. At frequencies <10 cps, Fam decreased, probably due to temperature ripple. A small decrease in Fam is also caused by large series resistors.

Noncoherent pulse compression

Pulse compression can be performed in noncoherent radars by using coded on-off keying (OOK). We show how any bipolar pulse-compression code (e.g., Barker) can be modified into unipolar OOK through Manchester coding. The resulted transmitted signal is a burst of dense subpulses, with pulse position modulation. In the receiver, the envelope-detected signal is aperiodically cross-correlated with a mismatched bipolar reference signal, yielding noncoherent integration with a low-sidelobe response. The concept can be used in simple radars where Doppler information is not required, in direct-detection laser radars and in ultrawideband (UWB) radars. Examples are given with bursts of 13 and 70 subpulses. Detection probabilities dependence on SNR is studied and compared with coherent processing.

Mitigating Range Ambiguity in High PRF Radar using Inter-Pulse Binary Coding

The paper proposes a way to increase the energy within a coherent processing interval (CPI) using more pulses instead of longer pulses. Long coded pulses result in masking targets at close range and poor Doppler tolerance. Increasing the number of pulses implies high pulse repetition frequency (PRF), which suffers from range ambiguity and target folding. These drawbacks of a high PRF can be mitigated by inter-pulse coding. The approach suggested here should be attractive for close and mid range applications of radar, ground penetrating radar, ultrasound imaging, and more.

Detection loss due to interfering targets in ordered statistics CFAR

The ordered-statistics (OS) constant false-alarm rate (CFAR) is relatively immune to the presence of interfering targets among the reference cells used to determine the average background. OS CFAR performance in a multitarget environment was previously studied by simulation. The author obtains analytic expressions for the added detection loss, assuming strong interfering targets. The real target is assumed to be a Rayleigh fluctuating target. Numerical examples are included. >

Noncoherent Radar Pulse Compression Based on Complementary Sequences

Noncoherent pulse compression (NCPC), suggested recently, uses on-off keying (OOK) signals, obtained from Manchester coding a binary sequence with favorable a-periodic autocorrelation. This paper investigates the use of binary complementary pairs as a basis for NCPC. It shows that a pair of Manchester coded, N-element binary complementary sequences will yield a peak sidelobe (PSL) ratio of 1/(2N).