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A review of practical predetection diversity techniques for mobile radio is presented. Theoretical carrier-to-noise distributions for selection combining, equal-gain combining, and maximal ratio combining in two-branch predection diversity systems are derived as a function of gain unbalance between branches. Experimental results are also presented from an equal-gain system, including predetection level statistics and post detection baseband average noise measurements as a function of gain unbalance. It was found that unbalances as high as 10 dB can be tolerated with equivalent input signal-to-noise degradations less than 3 dB for equal-gain and maximal ratio systems. Selection diversity degrades much faster than equal-gain or maximal ratio, but significant diversity improvement is given for unbalances less than 6 dB.