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Multiple access, TCDMA and TDFH

Short answer

Multiple access allows many users to share scarce spectrum by dividing it in frequency, time, code, or space. TCDMA assigns each cell a different spreading code and uses time slots within a cell so only one user transmits at a time, avoiding near-far effect but needing time sync. TDFH combines TDMA with slow frequency hopping, where the subscriber changes carrier frequency at each new TDMA frame, gaining frequency diversity and averaging interference, as used in GSM.

Explain the terms multiple access, Time Division CDMA (TCDMA) and Time Division Frequency Hopping as related to wireless communication system.

Multiple access

Multiple access is the method that allows many mobile users to share a limited radio spectrum simultaneously. The common resource is divided in frequency (FDMA), time (TDMA), code (CDMA) or space (SDMA). Wireless systems often combine these in hybrid forms to get the advantages of each, for example GSM uses FDMA + TDMA, and hybrid spread spectrum schemes combine CDMA with TDMA or frequency hopping.

Time Division CDMA (TCDMA)

TCDMA (also called TDMA/CDMA) is a hybrid technique that combines code division between cells with time division inside a cell.

  • A different spreading code is assigned to each cell.
  • Within a cell, each user is given a time slot, so only one CDMA user transmits at a time in each cell.
  • When the user hands off to another cell, its spreading code is changed to the code of the new cell.
 Cell 1, code C1: |U1|U2|U3|U1|U2|U3| ...
 Cell 2, code C2: |U4|U5|U6|U4|U5|U6| ...
          time ------------------------->
  • Advantage: avoids the near-far effect, because a near and a far user of the same cell are never received at the same time. Code separation still lets neighbouring cells use the same frequency.
  • Disadvantage: needs time synchronisation; capacity within a cell is limited by the number of slots.

Time Division Frequency Hopping (TDFH)

TDFH combines TDMA with slow frequency hopping.

  • The subscriber hops to a new carrier frequency at the start of each new TDMA frame, following a hopping sequence set by the network.
  • Within one frame (burst) the frequency is constant; it changes from frame to frame.
 freq ^
  f3  |        [U1]
  f2  |[U1]                [U1]
  f1  |               ...
      +---frame1--frame2--frame3--> time
  • Advantages: frequency diversity – a user does not stay in a deep fade or on a heavily interfered channel for long; co-channel interference is averaged over many users, which helps in heavily loaded cells.
  • Disadvantage: needs a central hopping plan and frame synchronisation between base station and mobile.
  • Example: GSM uses TDFH (slow frequency hopping, about 217 hops/s, one hop per TDMA frame of 4.615 ms).

✓ Answer checked by Claude Opus 5.5 (high effort)

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