CDMA Near Far Problem

- an overview of the near far problem encountered in CDMA and how it is overcome.

One of the problems encountered with CDMA is known as the "Near Far" problem. This CDMA near far problem is a key element in CDMA and as a result close control of the power within CDMA handsets is required.

The provision of a satisfactory solution of the CDMA near far problem was a key element in enabling CDMA to become a viable technology for providing a multiple access scheme for users within cellular and other radio based communications systems.

CDMA near far problem basics

The CDMA near far problem arises because handsets may be anywhere within the particular cell boundaries. Some handsets will be close to the base station, whereas others will be much further away.

In a free space scenario signals decay according a an inverse square law - in other words double the distance and the strength falls away to a quarter.

Signal   =   k   x   1   /   d2

k = a constant
d = distance

In cellular applications this situation may be worse. The effects of objects and other obstructions in the signal propagation path mean that in reality a signal decays at a greater rate than the simple inverse square law. It is somewhere between a law that follows a curve of an inverse of the distance to the power three or four. Many system planners may use a law of around 1 / d 3.4.

The result of this is that signals within a cell will have a huge variation in signal strengths. However for CDMA to operate correctly, the receiver must be able to receive all the required signals within the same channel bandwidth and it must be able to decode them.

For the receiver to be able to decode all the signals in the channel, they should ideally all be at the same signal strength - giving the CDMA near far problem.

CDMA near far problem solution

The CDMA near far problem is a serious problem, and requires an effective means of overcoming the problem for CDMA to operate correctly.

The schemes used to overcome the CDMA near far problem utilise fast and accurate power control systems.

Drawbacks caused by the CDMA near far problem

While the power control schemes that are adopted by the different cellular telecommunications technologies work very well and allow the CDMA systems to operate over a wide area, there are penalties for using them:

  • Reduced data capacity:   The power control mechanism requires data to be sent in both directions across the radio interface. This utilises data capacity that could be otherwise used for carrying revenue earning data.
  • High power handset power consumption at cell edges:   In order to be able to maintain the required signal level at the base station when the handset is close to the edge of the cell, it will be required to transmit at a high power level. This will reduce battery life. Other cellular systems might not require such high signal levels at the base station and may be able to conserve battery power as a result.

The CDMA near far problem is resolved in systems such as cdmaOne, CDMA2000 and W-CDMA by using sophisticated power control schemes to ensure that the power levels at the base station fall within a given band. Although there are some penalties to be paid for these schemes to overcome the CDMA near far problem, they operate well and enable significant gains to be made by using CDMA over previous technologies.

By Ian Poole

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