D.2 MOBILE PHONES
Modern mobile telephones are adaptive radio transmitters whose output power varies continuously according to network requirements. Unlike broadcast transmitters, which generally operate at a fixed power, mobile phones use closed-loop power control so that they transmit only the minimum power necessary to maintain a reliable connection with the serving base station. Consequently, the transmitted power depends on many factors, including the distance to the base station, the degree of obstruction by buildings or terrain, movement of the user, network loading, and the radio access technology in use (2G, 3G, 4G, or 5G). Maximum conducted transmitter powers are typically about 0.2 W (23 dBm) for most 4G and 5G handsets, although some frequency bands and device classes permit higher outputs approaching 1–2 W. In normal operation, however, the average transmitted power is usually much lower because of power control, burst transmission, discontinuous operation, and varying traffic demand.
When a mobile phone is switched on but not actively engaged in a voice call or data session, its RF emissions are generally very low. The handset periodically exchanges short signaling messages with the network to maintain registration, report its location, and receive paging messages for incoming calls or notifications. These transmissions are brief and infrequent, resulting in a very low average transmitted power. During an active voice call, video call, or data transfer, the duty cycle increases and the average transmitted power rises accordingly, although it still varies continuously as the network adjusts the handset's output to the minimum level required for satisfactory communication. Activities involving sustained data transfer, such as video streaming or uploading large files, often produce higher average transmitter powers than ordinary voice calls, particularly where radio coverage is poor.
Hands-free accessories can significantly alter the user’s RF exposure. A wired hands-free kit or earphones contain no RF transmitter and simply carry electrical audio signals between the phone and the user, allowing the handset to be kept well away from the head. Because RF field strength decreases rapidly with increasing distance from the transmitting antenna, even moving the phone from the ear to the hand or a nearby table substantially reduces exposure to the head. Bluetooth headsets do contain a radio transmitter, but they operate at very low powers, typically between approximately 1 and 10 mW for most devices, with some Class 1 equipment capable of transmitting up to 100 mW under specific operating conditions. In practice, the average transmitted power of Bluetooth headsets is normally only a few milliwatts or less because they also employ adaptive power control. Although a Bluetooth headset introduces a second RF source close to the head, its transmitted power is typically one or two orders of magnitude lower than that of the mobile phone itself, while the phone can simultaneously be positioned much farther from the user, resulting in a substantial reduction in overall exposure to RF energy.