Power control

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Power control, broadly speaking, is the intelligent selection of transmit power in a communication system to achieve good performance within the system. The notion of "good performance" can depend on context and may include optimizing metrics such as link data rate, network capacity, 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.

Increasing transmit power on a communication link has numerous benefits:

  • 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.
  • 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.

[edit] See also

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