Principles of Modern Radar - Volume 2 1891121537

520 CHAPTER 11 Space-Time Coding for Active Antenna SystemsTABLE 11-1 A large variety of space-time codings, which can be implemented either infast-time (intrapulse) or in slow-time (from pulse to pulse)Code per transmitterCode perdirectionFrequency Code TimeFrequency No Circulating code Circulating pulse(regular circulation)CodeRIAS: 1 frequency/transmitter, + randominitial phaseTime Regular array, 1frequency/transmitterPseudo-random codeIntrapulse scanningCirculating pulse (nonregular circulation)No3. Designing the sub-arrays in a non undersampled way, at least in one dimension – forinstance per column or per row.This simple analysis shows that the architecture of the array is not independent of thetime waveform to be transmitted. Space-time coding involves the joint design of antennaillumination and time waveforms.11.4.3 Coding strategyThe issue of space-time waveform selection is illustrated in Table 11.1, where differentsolutions are listed with their specific attributes. The codings are classified into threecategories, by analogy with the communication field (Frequency Division, Code Division,and Time Division): frequency coding, phase coding, and time coding. Furthermore, eachcode can be conveniently described either in the space-time domain (a time code for eachtransmitter), or in the angle-time domain (a time code for each direction).It must be emphasized that the most generic code (pseudo-random code) encompassesa wide variety of specific codes optimized for different desirable properties: low sidelobesin angle, or in range, or in a specific sub-domains of the range-angle domain.This elementary representation of the wide diversity of space-time waveforms shouldalso be combined with the antenna architecture (undersampled sub-arrays / correctly sampledsub-arrays / full active antenna) to be used as a guide for radar systems architecturedesign.Furthermore, combinations of slow-time and fast-time codings could also be considered,for example fast-time coding in azimuth and slow-time coding in elevation, forshorter bursts at higher elevation angles.11.5 WIDEBAND MTI [12], [4]An essential limitation for standard radars comes from pulsed radar range-Doppler ambiguityrelation, which states that the ambiguous velocity V a and the ambiguous range D a arerelated by: D a xV a = λ × c/4. That relation means that many ambiguities, either in rangeor velocity (or both), have to be dealt with, which in turn implies the transmission of successivepulse trains with different repetition frequencies, requiring more time to be spent