This paper presents the fault diagnosis mechanism for a proactive maintenance scheme for wireless systems. Its objective is to reduce the high operational costs encountered in the wireless industry by decreasing maintenance costs and system downtime. The fault diagnosis mechanism is based on the symbol frequency. An analytical method to calculate the symbol frequency is presented. The on-line monitoring system, based on the aforementioned fault diagnosis mechanism, is used to identify performance degradation, as well as its possible sources, so as to ensure that maintenance occurs only when necessary. Â© 2008 IEEE.