- 2078 Chaitra · 4+4 marks
FDMA vs TDMA; operation of FHMA
Short answer
FDMA assigns each user a separate frequency band with continuous transmission; TDMA shares one carrier in time slots with burst transmission and requires tight synchronization. FHMA pseudo-randomly hops each user's carrier over a wide band, and the synchronized receiver de-hops to recover the signal, with coding handling occasional collisions.
Compare between FDMA and TDMA. Explain the operation of FHMA.
Comparison of FDMA and TDMA
| Point | FDMA | TDMA |
|---|---|---|
| Basic idea | Each user gets a separate frequency band | Users share one carrier in different time slots |
| Transmission | Continuous | Burst (discontinuous) |
| Bandwidth per channel | Narrow (e.g. 30 kHz AMPS) | Wider (e.g. 200 kHz GSM) |
| Synchronisation | Not needed | Tight time sync and guard times needed |
| Equalizer | Usually not needed (low ISI) | Needed (high bit rate, ISI) |
| Duplexer | Needed (FDD, Tx and Rx together) | Not needed (Tx and Rx in different slots) |
| Handoff | Hard, mobile cannot measure easily | Mobile-assisted handoff in idle slots |
| Overhead | Guard bands | Guard time, sync and training bits |
| Base station cost | One transceiver per user | One transceiver serves many users |
| Example | AMPS, NMT | GSM, IS-136 |
Operation of FHMA
Frequency Hopping Multiple Access is a digital multiple access method in which the carrier frequency of each user changes (hops) in a pseudo-random way inside a wide band. The instantaneous signal is narrowband, but over time it covers the whole wideband channel.
Transmitter
data -> [Modulator] -> (x) -> [BPF] -> antenna
(FSK) ^
[Freq. synthesizer]
^
[PN code generator]
Receiver
antenna -> (x) -> [IF filter] -> [Demod] -> data
^
[Freq. synthesizer] <- [PN gen] (synchronised)
Steps:
- Data modulates a carrier (usually BFSK or M-FSK).
- A PN generator selects a frequency from the synthesizer; the mixer moves the signal to that hop frequency.
- The hopping pattern is unique to each user, so two users rarely occupy the same frequency at the same time.
- The receiver uses the same PN sequence, synchronised, to "de-hop" the signal back to a fixed IF, then demodulates it.
- If two users hit the same frequency (a collision), errors occur; error-correcting codes and interleaving recover them.
- Slow FH: several symbols per hop (GSM, 217 hops/s). Fast FH: several hops per symbol.
- Advantages: immunity to narrowband interference and fading, and a much less severe near-far problem than in DS-CDMA. Example: Bluetooth (1600 hops/s over 79 channels).
✓ Answer checked by Claude Opus 5.5 (high effort)
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