CDMA (Code Division Multiple Access) is becoming a widely popular air interface for mobile telephony. The defining specification is IS-95, which mandates that every CDMA base station be 'timed' by a clock disciplined to GPS. The GPS clock is a critical component of the CDMA basestation. The most important outputs of these clocks are a precise frequency and the precise epoch. The frequency is usually 10 MHz or a multiple of 9.8304 MHz. The epoch is usually a pulse per second or a pulse every other second.
IS-95 requires that the frequency and epoch are synchronous (epoch usually derived by simply counting down the frequency), and that the epoch be no more than 7 microseconds in error relative to UTC time (sometimes GPS time is specified).
Most base station manufacturers also require that synchronization accuracy be maintained for up to 24 hours if the GPS signal is lost or if the GPS receiver malfunctions. This part of the requirement is known as "holdover". The holdover requirement is complicated by the fact that the base station temperature may vary over the range of 0°C to 50°C. Some base station manufacturers mandate an even larger temperature range such as 0°C to 70°C.
The combination of time and temperature make the holdover requirements very challenging, and only high quality quartz or Rubidium oscillators can achieve the 7 microsecond specification. Cesium clocks are not usually considered for this application due to the prohibitive cost. A high quality quartz oscillator matched with a Kalman filter algorithm seems to provide the best compromise between quality and reliability versus cost.