Power control

Power control, broadly speaking, is the intelligent selection of transmitter power output in a communication system to achieve good performance within the system.^[1] The notion of "good performance" can depend on context and may include optimizing metrics such as link data rate, network capacity, outage probability, geographic coverage and range, and life of the network and network devices. Power control algorithms are used in many contexts, including cellular networks, sensor networks, wireless LANs, and DSL modems.

Benefits

Increasing transmit power on a communication link has numerous benefits:^[1]

• In general, for any particular set of channel conditions, a higher transmit power translates into a higher signal power at the receiver. Having a higher signal-to-noise ratio (SNR) at the receiver reduces the bit error rate of a digital communication link.
• A higher SNR can also allow a system that uses link adaptation to transmit at a higher data rate, resulting in a system with greater spectral efficiency.
• In a wireless fading channel, using higher transmit power provides more protection against a signal fade. In a cellular network, for example, this results in a lower dropped call probability.

Using a higher transmit power, however, has the following drawbacks:

• Overall power consumption in the transmitting device is higher. This is of particular concern in mobile devices, where battery life is reduced correspondingly.
• Interference to other users in the same frequency band is increased. In cellular spread-spectrum systems such as CDMA, where users share a single frequency and are only separated by different spreading codes, the number of users that a cell can support as well as the size of the cell are typically limited by the amount of interference present in the cell; increased interference therefore results in decreased cell capacity and size. Even in FDMA systems such as GSM where each user in a cell uses a different frequency, interference is still present between different cells and reduces the amount of frequency reuse the network can support. In wireline networks such as DSL, lines from many subscriber homes are often bundled together, and interference between signals on different lines manifests itself as crosstalk and reduces the achievable data rate to each home.

Typically, there is no simple answer to the problem of power control, and a good algorithm must strike a balance between the benefits and drawbacks associated with targeting a particular transmit power based on the performance criteria of most importance to the designer.

Transmit Power Control

Transmit Power Control is a technical mechanism used within some networking devices in order to prevent too much unwanted interference between different wireless networks (e.g. the owner's network and the neighbour's network).

The network devices supporting this feature include IEEE 802.11h Wireless LAN devices in the 5 GHz band compliant to the IEEE 802.11a. The idea of the mechanism is to automatically reduce the used transmission output power when other networks are within range. Reduced power means reduced interference problems and increased battery capacity. The power level of a single device can be reduced by 6 dB which should result in an accumulated power level reduction (the sum of radiated power of all devices currently transmitting) of at least 3 dB (which is half of the power).

UMTS

Because of the interference in the WCDMA system, power control plays a very important role in the quality control for the different services in the UMTS system. Power control is executed 1500 times per sec. whereas in GSM system it is ~2 times per sec.

See also

• Cellular network
• Cellular traffic
• IEEE 802.11h
• Radio resource management
• Spectral efficiency
• Wireless LAN

References

1. Guowang Miao, Jens Zander, Ki Won Sung, and Ben Slimane, Fundamentals of Mobile Data Networks, Cambridge University Press, ISBN 1107143217, 2016.