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Near-far problem in CDMA

Short answer

The near-far problem occurs when a nearby mobile's signal overwhelms a far mobile's signal at the base station because codes are not perfectly orthogonal. It is solved by fast closed-loop power control, where the base station commands mobiles to adjust transmit power so all signals arrive at equal strength.

Explain the "near-far problem" in CDMA. How can it be solved?

Near-far problem

In CDMA all users transmit in the same band at the same time, separated only by codes. The base station receives the sum of all signals. A mobile near the base station arrives with high power, while a mobile far away arrives weak. Since codes are not perfectly orthogonal, the strong signal's residual correlation produces interference that is larger than the weak signal itself, so the far user's signal cannot be decoded and capacity falls.

 Mobile A (near) ~~~~ strong ~~~~> [ BS ]
 Mobile B (far)  ~~ weak ~~~~~~~~>
 BS sees: A >> B -> B is drowned out

Solution

  • Power control (the main solution): the base station measures the received power of each mobile and commands it to raise or lower its transmit power so that all signals arrive at about the same level. In IS-95 this is done about 800 times per second (closed-loop), plus open-loop control by the mobile.
  • Using codes with low cross-correlation and multiuser detection (interference cancellation) also helps.

✓ Answer checked by Claude Sonnet 5.5

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