JPL's Wireless Communication Reference Website

Chapter: Analog and Digital Transmission

Power Control for Cellular Systems

Contributed by Louis Yun, U.C. Berkeley

What's It Used for?

Power control can substantially impact the capacity and perceived quality in cellular wireless systems. Regardless of the mode of multiple access -- be it frequency, time or code division -- power control is necessary to combat the intercell, or co-channel, interference that arises from frequency reuse.

Additionally, power control is employed to minimize the intra-cell interference in direct sequence code division multiple access (DS-CDMA) systems. On an ideal channel, it is possible to maintain code orthogonality among all users of the same cell. Unhappily, a wireless channel introduces multipath, so this perfect code orthogonality is destroyed and the user experiences intracell interference. Power control is particularly crucial on the DS-CDMA uplink: with out it, the signal of a mobile at the cell periphery would be drowned out by an interfering mobile situated close to the base station. Furthermore, on the DS-CDMA uplink the code waveforms of different users arrive at the receiver unsynchronized with one another. Unless the receiver performs code resynchronization, the interference is heightened, exacerbating this "near-far" problem and increasing the need for power control.

Intracell interference can affect TDMA and FDMA systems as well, in the form of adjacent channel interference; for example, practical bandpass filters cannot perfectly reject adjacent frequencies, matched filters may not always sample over the correct intervals, and multipath can cause intersymbol interference across different users. This need for power control in cellular systems to combat interference has been recognized and is the subject of much research (see references).

Current Approaches

Present day power-control algorithms attempt to counter interference via one of the following:
  • maintaining constant received signal power
    • uses negative feedback from receiver to transmitter to keep the received power at a constant level
    • PRO: very efficient in controlling intracell interference
    • CON: has limited effect on intercell interference
  • performing carrier-to-interference ratio (CIR) balancing.
    • the measure of quality here is the carrier-to-interference ratio
    • allocate power to each user such that all users experience the same CIR


Louis Yun covering power control algorithms in a short course.

Recent Advances

Recent advances in power control have been motivated by the next-generation PCS systems, in particular the vision of wireless multimedia, as for instance address in the Infopad research project. Multimedia adds a new dimension of complexity to system design, since different types of media - video, audio, data - have different reliability requirements.

Question: how do we support the diverse needs of heterogeneous traffic and still maintain high throughput in a wireless system?

New Ideas and Answers

References



JPL's Wireless Communication Reference Website Louis Yun (Author) and Jean-Paul M.G. Linnartz (Editor), 1993, 1995.