Loss of phase coherence in Granlund pre-detection diversity combiners

The Granlund (1956) pre-detection diversity combiner is an effective implementation of a maximal ratio pre-detection diversity combiner. The Granlund combiner phase corrects and weights an arbitrary number of input signals before adding them at a common summing junction. The Granlund technique uses a tone developed by cross correlation of an input signal with the combined output as a local oscillator in each channel. If the input consists of a signal plus noise, the local oscillator signals develop an associated phase noise component. A system consisting of receiver, combiner, carrier tracking loop, and demodulator are considered. Measured bit error rate data on a specific configuration is presented. Through simulations, the phase noise inserted on a BPSK signal is shown to have an approximate K/f spectral density inside the arm filter bandwidth at low signal to noise ratio. The phase noise is demonstrated to be a function of arm filter bandwidth and pre-combiner signal to noise ratio.