Time Domain Analysis of an Automatic Gain Control Weighted Diversity Combining System.
NAVAL MISSILE CENTER POINT MUGU CA
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The time domain performance of an automatic gain control AGC weighted diversity combiner is studied for deterministic radio frequency RF input fading envelopes. The non-linear differential equation relating the AGC voltage to the RF input fading envelope is derived. For given fading signals this differential equation is evaluated by computer for the AGC voltages at discrete points in time. This information is used to compute the combiner output signal-to-noise ratio SNR at these times. It is shown that the AGC weighted combiner is optimum maximal-ratio only for fade rates where the AGC loop can track the fading RF input signals perfectly. It is found that for certain fading signals a critical fade rate exists for a given AGC time constant for which the AGC weighted combiner reaches a condition of worst performance. At higher fade rates the performance approaches that of an equal-gain combiner for RF signals of equal average value. It is shown that by using both the amplitude-modulated AM an AGC voltages for weighting, a maximal-ratio combiner can be built which is not limited by the tracking rate of the AGC loop. Existing AGC weighted combiners can be modified by use of one external logarithmic amplifier per channel. Also, it is shown that an optimum diversity selector can be implemented by use of both the AM and AGC voltages. Author
- Electrical and Electronic Equipment
- Radio Communications