Signal-to-interference ratio (SIR) is a major limitation factor in cellular radio systems. Capacity, coverage and quality are general key drivers in deploying any wireless system and they are all a function of SIR. Expanding capacity through conventional methods such as deploying new cell sites or new equipment upgrading is expensive and not always effective as it will introduce more interference in the current network. With the emerging of smart antenna technology, some performance improvement over conventional cellular systems is expected. This paper studies the use of a switched beam smart antenna system in a mobile communication system for reducing cochannel interference. Simulation has been carried out for the GSM cellular system employing various antenna techniques such as omnidirectional, directional and switched beam smart antenna. SIR performances are analyzed and results are compared with those obtained when omnidirectional and directional antenna patterns are used in base stations. SIR improvement of at least 6 dB throughout the network is achieved when the switched beam smart antenna technique is applied in the system.