An improved quad diversity combining technique for effectively combating multipath, antenna gain and shadowing

Mobile operation suffers from platform agility, low antenna gain patterns, multipath fading and platform shadowing. Prime candidates for quad diversity (QD) are mobile platforms requiring hemispherical-coverage reception. QD combines signals from four antennas to improve link margin and provide spatial diversity. Low UHF satellite communications (SATCOM) link margin drives the mobile user to employ QD for combating multipath fading and low antenna gain, especially for high-latitude operation. QD is also needed to counteract the roll, pitch and yaw of aircraft and superstructure shadowing of naval vessels. The performance of a new combining scheme based on signal-to-noise ratio (SNR) is favorably compared with previously reported Granlund (1956) combiners. The SNR combiner has been implemented in a TMS320C6201 digital signal processor (DSP) embedded in the Joint Tactical Terminal (JTT). DSP algorithms are employed which allow the integration of carrier recovery with the combining algorithm, thereby enhancing rapid burst acquisition probability, reducing overall implementation loss and providing robust tracking during signal fades. Comparisons are made to the classical "ratio-squarer" combining method based on tolerance to mismatched receivers, computational complexity, signal dynamic range, bit error rate (BER) and acquisition probability. Because combining is based on SNR weighting, the JTT combiner performance is independent of the matching receiver gain or cable loss in each channel. No dynamic system calibration is required, thereby eliminating periodic field calibration and costly factory gain matching.