Radio communication system

A portable radio (2) is adapted for direct two-way peer-to-peer radio communication with one or more other portable radios (4, 6). Portable radio (2) may be a PMR radio, a Bluetooth radio, a 3G or 4G radio, or another radio communicating on an ad-hoc basis. The portable radio (2) comprises means (20) for entering into and awakening from a sleep mode, and means (20, 22, 24, 26) for transmitting synchronisation signals. The synchronisation signals are received by one or more other radios (4, 6), whereby the radios become synchronised. When synchronised, the portable radio (2) may vary the mark:space ratio of its receive period, and may vary the frequency with which it transmits synchronisation signals. The variation may be dependent on the rate at which the portable radio (2) receives synchronisation signals from other radios (4, 6).