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TDMA, CDMA and SDMA

Short answer

Multiple access is the technique for sharing limited radio spectrum among many users via frequency, time, code, or space divisions. TDMA allocates time slots, allowing burst transmission on one carrier, saving battery and simplifying handoff. CDMA spreads each user's data with a unique code over the same frequency, providing soft capacity but requiring power control. SDMA separates users by spatial position using directional or adaptive antennas, increasing capacity and reducing interference.

What is a multiple access technique? Explain TDMA, CDMA and SDMA.

Multiple access technique

Multiple access is the technique by which many users share a common, limited radio spectrum at the same time without causing serious interference to each other. The shared resource is divided in frequency (FDMA), time (TDMA), code (CDMA) or space (SDMA).

TDMA (Time Division Multiple Access)

TDMA divides one radio carrier into time slots; each user transmits only in its own slot, in bursts, using the full carrier bandwidth.

 |<-------------- one TDMA frame -------------->|
 |pre| slot1 | slot2 | slot3 | ... | slotN |
        U1      U2      U3            UN
 slot: |tail|data|training|data|tail|guard|
  • Each frame has preamble (sync/address), slots and guard times.
  • Transmission is discontinuous → battery saving and mobile-assisted handoff in idle slots.
  • No duplexer needed; high burst rate needs equalization; tight synchronisation needed.
  • Example: GSM (8 slots per 200 kHz carrier), IS-136 (3 users per 30 kHz).

CDMA (Code Division Multiple Access)

All users transmit on the same frequency at the same time; each user's data is multiplied by a unique PN spreading code with chip rate much higher than the bit rate.

 d1->(x)c1 --\
 d2->(x)c2 ---(+)--> channel --> (x)c1 --> d1
 d3->(x)c3 --/
  • Receiver correlates with the wanted code; others remain as noise (reduced by the processing gain).
  • Features: frequency reuse factor 1, soft capacity, soft handoff, RAKE receiver for multipath.
  • Problems: near-far effect (needs power control), self-jamming.
  • Example: IS-95 (1.25 MHz, 1.2288 Mcps), WCDMA.

SDMA (Space Division Multiple Access)

SDMA separates users by their spatial position. The base station uses directional or smart (adaptive) antennas that form a beam toward each user, so users in different directions can reuse the same frequency/time/code in the same cell.

           /--beam1--> user A
 [BS ant]--+--beam2--> user B
           \--beam3--> user C
  • Advantages: higher capacity, less co-channel interference, lower transmit power, wider coverage.
  • Simplest form: sectored antennas (120°); ideal form: adaptive arrays tracking each user.

✓ Answer checked by Claude Opus 5.5 (high effort)

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