Principles of Modern Radar - Volume 2 1891121537

70 CHAPTER 2 Advanced Pulse Compression Waveform Modulations2.5.7 SummaryAn SF waveform is designed to achieve a narrow instantaneous bandwidth on both transmitand receive to accommodate hardware limitations. Fortunately, a wide bandwidth signalmay be synthesized by the stepping pulses over frequency and coherently processing thereturns. The achieved range resolution is inversely proportional to the total frequencyextent over which the waveform is stepped. The waveform requires a long collectioninterval, which often limits its application; however, the waveform is used in some widebandinstrumentation radars and in some tactical systems. The frequency stepping introducesambiguities in range that must be addressed through judicious waveform parameterselection.2.6 QUADRIPHASE SIGNALS2.6.1 IntroductionBiphase-coded waveforms are formed by concatenating chips (or subpulses) and modulatingthe phase chip to chip. The phase of each chip is either 0 or 180 degrees. The spectrumof the resultant waveform retains many of the characteristics of the constituent subpulseincluding high spectral sidelobes. The sidebands of the spectrum are a source of EMIand have the potential to interfere with other RF systems operating nearby. Quadriphasewaveforms were developed by Taylor and Blinchikoff [27] for use in the AN/TPS-63 radarto reduce the spectral sidelobes of biphase-coded waveforms while retaining their timedomainsidelobe performance [28-30]. Quadriphase encoding is similar to the minimumshift keying (MSK) technique applied in some communication systems and may also beviewed as a modified version of frequency shift keying (FSK) [31].The relatively high spectral sidelobes, characteristic of phase-coded signals, are anartifact of the rectangular subpulses commonly used to synthesize the waveform. Anonrectangular-shaped subpulse (e.g., a Gaussian-shaped subpulse) may be used to suppressthe sidelobes, but the waveform’s envelope exhibits a time-varying amplituderesponse that is undesirable from an energy perspective. Taylor and Blinchikoff [27]developed a biphase-to-quadriphase (BTQ) transformation that produces a waveform withthe following properties:1. Spectral sidelobes lower than those achieved with a phase-coded waveform employingrectangular subpulses2. Time sidelobes near those achieved with a specified biphase-coded waveform3. A nearly constant amplitude response2.6.2 BTQ TransformationThe BTQ transformation may be applied to any biphase code. The first step is to select abiphase code that exhibits the desired range sidelobe response. Next, the biphase code istransformed into a four-phase (quadriphase) code:q n = j s(n−1) c n n = 1, 2,...,N (2.137)