Radar waveform diversity has received considerable attention in recent years due to increasing spectral congestion and the burgeoning capabilities of digital waveform generation. The promise of waveform diversity is far greater utilization of available degrees of freedom to enhance sensing performance and to even facilitate new operating modes. This tutorial provides an overview of this very broad topic, from the basic principles upon which it is founded to the myriad different areas being explored in research for practical sensing applications.

Overview of radar waveform diversity

Radar pulse compression involves the extraction of an estimate of the range profile illuminated by a radar in the presence of noise. A problem inherent to pulse compression is the masking of small targets by large nearby targets due to the range sidelobes that result from standard matched filtering. This paper presents a new approach based upon a minimum mean-square error (MMSE) formulation in which the pulse compression filter for each individual range cell is adaptively estimated from the received signal in order to mitigate the masking interference resulting from matched filtering in the vicinity of large targets. The proposed method is compared with the standard matched filter and least-squares (LS) estimation and is shown to be superior over a variety of stressing scenarios.

Adaptive pulse compression via MMSE estimation

Polyphase radar codes promise enhanced performance and flexibility due to greater design freedom. While the search for better codes continues, the implementation issues of transmitter bandlimiting and nonlinear distortion have precluded their widespread use in high-power systems. This paper introduces a modified continuous phase modulation implementation that converts an arbitrary polyphase code into a nonlinear frequency-modulated waveform that is constant envelope and spectrally well contained. Experimental results assess the receive sampling and pulse compression effects.

Polyphase-coded FM (PCFM) radar waveforms, part I: implementation

In a first embodiment of the invention, a concatenated preamble code is formed by a kronecker product between two subcodes of the same or different lengths. The subcodes have favorable correlation properties and may be Barker codes, minimum peak sidelobe codes, Gold codes, Kasami codes, Boztas codes, or other codes. At the receiver a two-stage processor is used to detect the concatenated preamble code. The two-stage processor comprises a series of two filters, one of which is preferably a mismatched filter. In another aspect of the invention, a repeated codeword preamble is formed from a series of the same repeated short subcode. The short subcode may be an augmented or truncated odd-length code. At the receiver a single matched filter is preferably used to generate a series of spikes separated by the period of the subcode. An alert/confirm detector non-coherently adds together individual correlation spikes and reject false alarms. The alert function measures total correlation energy and adjusts detection thresholds based thereon, while the confirm function ensures that the proper number of correlation spikes exceed a detection threshold at the correct times. The preamble codes and their means for detection are preferably employed in a TDMA spread spectrum communication system, wherein a plurality of user stations communicate with a base station, one in each time slot (or virtual time slot).

Preamble code structure and detection method and apparatus

An apparatus for decoding a received spread spectrum signal having a data signal modulated with a pseudo-noise code and transmitted as an RF spread spectrum signal. A first reference register holds a first pseudo-noise signal, a second reference register holds a second pseudo-noise signal, and a receive-register holds a received spread spectrum signal. A first modulo adder adds each chip of the received spread spectrum signal with each respective chip of the first pseudo-noise signal, thereby generating a first plurality of chip-comparison signals. A first summer adds the first plurality of chip-comparison signals, generating a first correlation signal. The comparator compares the correlation signal to an upper-threshold level and a lower-threshold level, and respectively generates a first data-symbol signal or a second data-symbol signal.

Spread spectrum correlator

A description is given of a class of radar signals called quadriphase codes. The quadriphase code described is derived from a prototype biphase code by a biphase-to-quadriphase (BTQ) transformation. These codes feature subpulses with a half-cosine shape and a phase change between the adjacent subpulses restricted to either +90 degrees or -90 degrees . This feature results in the spectrum falloff of 12 dB/octave compared with the biphase code spectrum falloff of 6 dB/octave. After a discussion of biphase codes with low autocorrelation sidelobes and tabulating selected biphase codes, the authors show the synthesis of the quadriphase code from the prototype biphase code. Quadriphase-code characteristics, including the autocorrelation function, the compressed pulse shape, the spectral characteristics, the energy pulsewidth, the pulse compression ratio, the range sampling loss, the Doppler effect, and the tabulation of selected quadriphase-code properties are presented. Digital decoding of the quadriphase codes is discussed, including the linear decoder and the coded pulse anticlutter system (CPACS) decoder, one form of phase discrimination constant false alarm rate. >

Quadriphase code-a radar pulse compression signal with unique characteristics

Filters that reduce the sidelobes of the quadriphase-coded waveform are realized by applying the biphase-to-quadriphase transformation to the filter designs that reduce the sidelobes of the prototype biphase code. The mismatch loss is invariant under this transformation, but the resulting peak-to-sidelobe ratio (PSR) can decrease 1.5 dB maximum. An example illustrates the procedure for a compressor composed of a sidelobe reducing filter cascaded with the matched filter and a compressor realized as one mismatched filter.

Range sidelobe reduction for the quadriphase codes

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Quadriphase code-a radar pulse compression signal with unique characteristics

Range sidelobe reduction for the quadriphase codes