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In this paper we study the capacity of spectrum sharing cognitive radios (CR) with Maximal Ratio Combining (MRC) diversity at the secondary receiver under Rayleigh fading. The secondary user does not have perfect channel information of SUtx-PUrx link, where the estimation error is considered. Our mathematical analysis and numerical results show not only that with MRC combining diversity at the secondary receiver more capacity is achieved, but also that the estimation error could be compensated. For instance, we show that even the estimation error variance is large, for example σ2 = 0.8, to increase the degrees of MRC combining diversity, for instance L = 8, is able to achieve more capacity than in estimation error free case σ2 = 0.