P25 Phase 2
The main feature of P25 Phase 2 is that of improved radio spectrum efficiency. In a nutshell, the spectrum efficiency on a P25 trunked system’s traffic channels is doubled, allowing two voice and/or data circuits per 12.5kHz radio carrier.
P25 Phase 1 systems use Frequency Division Multiple Access (FDMA) for the air interface, and each channel occupies a 12.5kHz bandwidth for a single voice or data user. (The air interface is the radio transmission between the base station and terminals).
P25 phase 2 employs a Time Division Multiple Access (TDMA) air interface for P25 so that two voice or data transmissions can occupy the same bandwidth as one channel in a 12.5kHz system.
A P25 trunked system uses a control channel, where radio terminals will register and wait for call activity or request a call setup amongst other things.
A P25 Phase 2 trunked system uses a P25 Phase 1 FDMA control channel, so all radio terminals must register to the trunked system using FDMA. This is required for backward compatibility with P25 Phase 1 terminals. When a Phase 2 terminal initiates a call on the Phase 2 trunked system, a Phase 2 TDMA traffic channel is allocated for the voice or data call. When a Phase 1 terminal sets up call on a Phase 2 system, a Phase 1 FDMA channel will be allocated for the voice or data call.
When a Phase 2 terminal radio communicates directly with another Phase 2 terminal, it switches back to FDMA or Phase 1 mode.
Conventional (non-trunked) P25 systems such as voted systems use Phase 1 FDMA for the air interface, as there is no TIA definition for the operation of Phase 2 TDMA in conventional P25.
P25 Phase 2 is attractive where frequency spectrum is scarce or where more channels can be utilised from the same allocated spectrum on a trunked system. Phase 2 does not offer any significant additional features or functions when compared with Phase 1 from the radio user's perspective.