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SDMA and hybrid SS techniques against near-far

Short answer

SDMA separates users by position using smart antennas that form narrow beams, reusing the same frequency and time in different directions. Hybrid DS/FHMA spreads data with a PN code then hops the center frequency, so a near user rarely shares a hop with a far user. TCDMA assigns each cell a code and each user a time slot, so only one user per cell transmits at a time, avoiding near-far.

What is space division multiple access? Explain any two hybrid spread spectrum multiple access techniques which minimize the effect of near-far effect.

Space division multiple access

Space Division Multiple Access (SDMA) separates users by their position in space. The base station uses directional or smart (adaptive) antennas that form narrow beams pointing at each user, so the same frequency and time slot can be reused for users in different directions within the same cell.

          beam 1 -> user A
 [BS]==== beam 2 -> user B   (same f, same t)
          beam 3 -> user C
  • Advantages: higher capacity, lower interference, less transmit power, larger range.
  • Ideal SDMA would give an infinitely narrow beam to each user; practical systems use sectoring or adaptive arrays.

Hybrid spread spectrum techniques that reduce the near-far effect

1. Hybrid DS/FHMA (direct sequence + frequency hopping)

  • The data is first spread with a PN code (DS). The centre frequency of the DS signal then hops pseudo-randomly over the wide band.
  • At any time a user occupies only a small part of the band. A strong nearby user is very unlikely to be on the same sub-band at the same moment, so it does not swamp the weak user. Hence the near-far problem is largely avoided.
 data->(x)PN code->(x)->[BPF]-> Tx
                    ^
          [Freq synthesizer]<-[hop PN gen]
  • Disadvantage: complex hardware; poor support for soft handoff because the base station must follow many hopping patterns.

2. Time Division CDMA (TCDMA)

  • Each cell is given a different spreading code (codes separate cells).
  • Inside a cell, users are separated in time: each user gets a time slot, so only one user transmits at a time in a cell.
  • Because there is never a near and a far user of the same cell received together, the near-far effect is avoided. On handoff, the user's code changes to that of the new cell.
 Cell 1 (code C1): |U1|U2|U3|U1|U2|U3|
 Cell 2 (code C2): |U4|U5|U6|U4|U5|U6|
  • Disadvantage: needs slot synchronisation; per-cell capacity limited by number of slots.

Other hybrids (FCDMA, TDFH) also exist, but DS/FHMA and TCDMA are the two usually cited for near-far protection.

✓ Answer checked by Claude Opus 5.5 (high effort)

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