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This paper analyzes selection diversity as an effective tool to combat channel impairments in relay-assisted cellular systems. We consider the downlink scenario where the base station equipped with N antennas transmits to the mobile station either directly, or indirectly via a relay station, according to the link with the strongest received signal-to-noise ratio (SNR). For this system, we analyze the performance of fixed gain amplify-and-forward relaying that does not require full channel-state-information (CSI) at the relay. We derive new exact closed-form expressions for the outage probability and symbol error rate (SER) based on new statistical properties of the end-to-end SNR. Furthermore, we quantify the asymptotic behavior of the outage probability and SER. We explicitly reveal the impact of multiple antennas with relay selection diversity, on the array gain and the diversity order. Our new asymptotic results prove that the maximum diversity order of N+1 is achieved.