Principles of Modern Radar - Volume 2 1891121537

17.5 2D-CCF Sidelobe Control 777Auto-Correlation Function (dB)0−10−20−30−40−50ACFACF + BWNACF + BWN + ASRFFIGURE 17-17ACF with sidelobescontrol filters for aDSSS WiFi signalregistration ofduration 0.25 s.−600 1 2 3Delay (ms)4 5The DSSS system provides a processing gain of at least 10 dB that is accomplished bychipping the baseband signal at 11 MHz with an 11-chip pseudo noise code. An 11-chipBarker sequence is used as the pseudo noise code sequence, c = [1, −1, 1, 1, −1, 1, 1, 1,−1, −1, −1], where the leftmost chip is output first in time. The first chip is aligned atthe start of a transmitted symbol so that each symbol duration is exactly 11 chips long(T sym = 11 · T chip = 1 μs). Consequently, the resulting signal ACF will be dominated bythe Barker code [16].The ACF obtained for a DSSS WiFi signal registration of duration 0.25 s is reportedin Figure 17-17, for the AP broadcasting beacon frames at reasonably regular intervals.Aiming at a local area surveillance application, a delay extent of 5 μs is consideredthat corresponds to a bistatic range extent of 1.5 km. Specifically, five almost identicalsidelobes appear within time delays of T sym = 1 μs (300 m) that are obviously due tothe exploited 11-chip Barker code. As expected, these sidelobes show a PSLR of about20log 10 (11) = 20.83 dB and a temporal separation equal to 2T chip . In addition, other strongsidelobes appear at delays multiple of T sym (k · 300 m). These sidelobe structures are dueto the cyclical repetition of the Barker code, and their level equals the average correlationamong consecutive symbols of the data frame. Thus, assuming a random transmission andreasonably large integration times, the corresponding PSLR may be significantly greaterthan that due to the Barker code sidelobes. However, such additional sidelobes structuresshould not be a priori neglected since their level is highly time varying and stronglydepends on the signal content. For example, the PSLR could be seriously degraded ifidentical pulses of limited duration are periodically transmitted, as in the case of thebeacon frame. In fact, even increasing the integration time, the actual cross-correlationamong consecutive symbols is lower bounded by the cross-correlation evaluated over thesingle pulse consisting of a limited number of symbols.Therefore, proper filters are presented in the following for the sidelobes level controldistinguishing between those deriving from the Barker code and those appearing atmultiples of T sym .Weighting Network for Barker Sidelobes Reduction (BWN)An effective weighting network for reducing the Barker sidelobes in the DSSS WiFi signalACF is presented in [18] along the lines proposed in [49]. Its design exploits the a priori