Principles of Modern Radar - Volume 2 1891121537

408 CHAPTER 9 Adaptive Digital Beamformingreceivers are effectively seeing the world through a much smaller antenna that has less directivity.This means that receivers for DBF require higher dynamic range, better linearity,and better filtering because the interference has not been attenuated by the low sidelobesof a fully formed beam. To be able to cancel that jamming later on in signal processing,the receivers have to be able to adequately capture the usually weak signals of interest inthe presence of strong unwanted interference without producing distortion in either. Formore information on nonlinear distortion in DBF arrays and mitigation approaches see[12,13].The distributed receivers also need to be kept in strict time alignment and matchedto each other in terms of their frequency responses. The former is necessary to preventgain loss and signal distortion in the beam output, and the latter is necessary to providegood jammer cancellation over the bandwidth. The mixers and the ADC converters in thereceive chain require local oscillator (LO) signals and clocks, respectively, to perform theirfunctions. Any misalignment of the LO and clock signals in the distributed receivers willresult in a loss of signal coherence from channel to channel. Therefore, the network fordistributing LO and clock signals to the receivers needs to be well calibrated and stable,which is an engineering challenge.Receivers have a certain probability of failure that impacts the overall system availability.The increased number of receivers for DBF implies that the probability that anyone of those receivers will fail will be higher. Availability is defined as the probability thata system or subsystem is operational at a given time. For simplicity, consider the impactof receiver failures on the system availability while neglecting all other contributors. Ifthe availability of a single receiver channel is A 1 , then system availability for n receiverchannels is given byA n = (A 1 ) n (9.6)Since the single-channel availability is by definition less than 1, the system availabilitybecomes much smaller as n becomes large. This assumes, however, that all the channels ina DBF system need to be available for the system to be considered available. That wouldbe true for a monopulse radar (n = 3) where the loss of even a difference channel meansthat the radar can no longer provide angle estimates in one dimension. In a DBF system,each receiver channel represents a much smaller portion of the total aperture. For example,in a 16-channel DBF system the loss of 1/16 of the array due to a receiver failure resultsin less than 0.3 dB loss in gain. The higher the number of channels, the more gracefullyperformance degrades as channels fail. The availability for an N-channel DBF system thatcan tolerate one failed receiver is given byA N:N−1 = A N 1 + NAN−1 1 (1 − A 1 ) (9.7)Figure 9-5 shows a plot of the system availability as a function of the single-channel availabilityfor 3-channel monopulse, 16-channel DBF, 16-channel DBF with at least 15 channelsavailable, and 16-channel DBF with at least 14 channels available. As expected, the16-channel DBF availability is significantly reduced compared with the monopulse, but ifthe loss of one or two channels can be tolerated, then 16-channel DBF can provide higheravailability than a monopulse system though at a degraded level of performance. Unfortunately,as the number of receiver channels increases, more and more receiver channelfailures need to be tolerated for DBF to maintain availability equivalent to a 3-channelmonopulse system. As a result, large-scale DBF is likely to require receivers with higher