- 2074 Magh · 8 marks
Spread spectrum multiple access variants
Short answer
SSMA spreads a narrowband message over a much wider bandwidth using PN sequences; processing gain gives immunity to interference and jamming. Variants include FHMA (Bluetooth, GSM), DS-CDMA (IS-95, WCDMA, GPS), and hybrids such as FCDMA, DS/FHMA, TCDMA, and TDFH. Applications cover cellular, WLAN 802.11b, Bluetooth, and satellite navigation.
Describe spread spectrum multiple access, SSMA variants and application with suitable diagram.
Spread spectrum multiple access (SSMA)
SSMA uses signals whose transmission bandwidth is several orders of magnitude larger than the minimum required bandwidth. A pseudo-noise (PN) sequence spreads a narrowband message into a wideband, noise-like signal. Many users share the same wide band at the same time; each is separated by its own PN code or hopping pattern. The ratio of spread bandwidth to data rate is the processing gain, which gives immunity to interference, jamming and multipath.
There are two main variants: FHMA and DS-SS (CDMA), plus hybrid forms.
1. Frequency Hopped Multiple Access (FHMA)
- Carrier frequency of each user hops pseudo-randomly over the wide band; at any time the signal is narrowband.
- Slow FH: several symbols per hop; fast FH: several hops per symbol.
data->[FSK mod]->(x)->[BPF]-> channel
^
[synth]<-[PN generator]
- Applications: Bluetooth, GSM slow frequency hopping, military radios.
2. Direct Sequence CDMA (DS-CDMA)
- Data bits are multiplied directly by a high-rate PN code (chips); all users use the same carrier at the same time.
- The receiver correlates with the wanted user's code; other users appear as noise.
d(t)->(x)->(x)-> channel -> (x)->(x)->[∫]-> d
^ ^ ^ ^
c(t) carrier carrier c(t)
- Needs power control (near-far problem); uses RAKE receiver.
- Applications: IS-95 (cdmaOne), CDMA2000, WCDMA (3G UMTS), GPS.
3. Hybrid spread spectrum techniques
| Hybrid | Idea | Benefit |
|---|---|---|
| FCDMA (FDMA/CDMA) | Band split into sub-bands, each a DS-CDMA system | Uses non-contiguous spectrum |
| DS/FHMA | DS signal whose centre frequency hops | Avoids near-far effect |
| TCDMA | Codes per cell, time slots per user | Only one user per cell at a time |
| TDFH | Hop to new frequency each TDMA frame | Diversity against fading (GSM) |
Advantages and applications of SSMA
- No frequency planning; universal reuse (CDMA).
- Soft capacity and soft handoff.
- Multipath diversity via RAKE receiver.
- Security and anti-jam capability (origin in military use).
- Used in cellular (IS-95, 3G), WLAN 802.11b (DSSS), Bluetooth (FHSS), satellite navigation (GPS).
✓ Answer checked by Claude Opus 5.5 (high effort)
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