CDMA characteristics, advantages and limitations

CDMA

Code Division Multiple Access (CDMA) is a spread-spectrum multiple access technique in which all users transmit simultaneously on the same frequency band, and each user is separated by a unique spreading code (PN or Walsh). The narrowband data is multiplied by a high-rate code, spreading it over a wide band; the receiver correlates with the same code to recover the user's data, while other users' signals remain spread and appear as low-level noise.

Characteristics

• All users share the full bandwidth at all times; no time or frequency division.
• Users separated by codes with low cross-correlation; message signal bandwidth ≪ transmitted bandwidth (processing gain G = W/R).
• Interference-limited, soft capacity: adding a user only raises the noise floor slightly.
• Power control is essential to overcome the near–far problem.
• Multipath is exploited using the RAKE receiver.
• Soft handoff between cells is possible.
• Frequency reuse factor of 1.
• Example: IS-95 (1.25 MHz, 1.2288 Mcps), WCDMA (5 MHz, 3.84 Mcps).

Advantages

1. Higher capacity than FDMA/TDMA, with soft (graceful) capacity limit.
2. Universal frequency reuse – no frequency planning.
3. Soft handoff reduces call drops.
4. Multipath diversity through RAKE receivers; resistance to fading.
5. Voice activity and sectorization directly raise capacity.
6. Inherent privacy and resistance to narrowband interference/jamming.
7. Lower average transmit power, longer battery life.

Limitations

1. Near–far problem – strong nearby signals mask weak distant ones; needs accurate, fast power control.
2. Self-jamming / multiple access interference – codes are not perfectly orthogonal (especially with asynchronous users and multipath), so each user adds interference.
3. Complex receivers: code acquisition, synchronisation, RAKE, power control.
4. Cell breathing – cell coverage shrinks as load increases.
5. Needs wide contiguous bandwidth.
6. Performance degrades as the number of users grows.