DPCH

In UMTS (3G) cellular systems, a DPCH is a dedicated physical channel that carries traffic and control signals for a single mobile device. Unlike shared channels that multiplex many users, a DPCH reserves a specific code and timeslot for one subscriber for the duration of a call or data session. This isolation allows the base station to maintain tight control over power, timing, and quality of service without interference from other users on the same frequency.

A DPCH consists of two data streams: the downlink DPCH (from base station to mobile) carries speech, data, and high-layer signalling, while the uplink DPCH (from mobile to base station) carries user data and measurement reports. Both use spreading codes assigned from the UMTS code tree, with code lengths typically 256 chips per slot, and operate on the same carrier frequency as shared channels. The physical layer repeats every 10 milliseconds (one frame), divided into 15 timeslots of 2560 chips each.

Power control and the DPCH lifetime

The DPCH cannot exist before a call setup; the base station allocates codes and timeslots only when a mobile requests service or receives an incoming call. Once allocated, the downlink DPCH power is adjusted every timeslot (roughly every 667 microseconds) based on measurements reported by the mobile, compensating for fading and distance. The uplink power is similarly adjusted, though with longer feedback delays. This closed-loop power control is essential because UMTS systems are interference-limited; excessive uplink power from any mobile crushes other users' signals, while too little power causes call drops or low data rates.

A DPCH is deallocated as soon as the call ends or after a period of inactivity. In idle mode, the mobile listens to the common pilot channel and shared control channels instead. During handover between cells, the target base station assigns a new DPCH code and timeslot before the old one is released, ensuring continuity.

The DPCH differs fundamentally from the later LTE paradigm, which distributes user data across shared resource blocks and relies on packet scheduling rather than permanent channel assignment. In UMTS, however, the DPCH remains the fundamental unit of user separation and the point where power control, synchronisation, and rate matching are anchored. Understanding DPCH behaviour is critical for troubleshooting call quality, capacity bottlenecks, and interference problems in legacy 3G networks.