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Near-far effect and a hybrid SS technique

Short answer

The near-far effect in CDMA occurs when a nearby mobile's strong signal drowns out a distant user's signal because all share the same band and are treated as noise. Time Division CDMA (TCDMA) mitigates it by giving each cell a unique spreading code and assigning different time slots to users in the same cell, so near and far users never transmit simultaneously, eliminating in-cell near-far interference.

Define near-far effect. Briefly describe any one hybrid spread spectrum multiple access technique which can mitigate the near-far problem.

Near-far effect

The near-far effect occurs in the CDMA uplink when a mobile close to the base station is received with much higher power than a mobile far away. Since all users share the same band and other users act as noise, the strong near signal drowns the weak far signal, which then cannot be despread correctly. It is controlled mainly by power control.

Hybrid technique: Time Division CDMA (TCDMA)

  • Each cell is given a different spreading code.
  • Inside a cell, users are assigned different time slots, so only one user per cell transmits at any time.
  • A near user and a far user of the same cell are therefore never received together, and the near-far problem is avoided. On handoff the user's code is changed to the new cell's code.
 Cell code C1: |U1|U2|U3|U1|U2|U3|

(Hybrid DS/FHMA is another option: a DS signal whose carrier hops, so near and far users rarely share the same frequency at the same time.)

✓ Answer checked by Claude Opus 5.5 (high effort)

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