- 2072 Magh · 6+2 marks
Spread spectrum multiple access; FDMA vs CDMA
Short answer
Spread spectrum multiple access includes FHMA, where carrier frequencies hop per a PN code, and CDMA (DS-SS), where data is spread by a high-rate PN code over a shared band. FDMA assigns separate frequencies to users, has hard capacity, and requires frequency planning, while CDMA lets all users share the same band with soft capacity, needs power control, and supports soft handoff and RAKE reception for multipath.
Explain the different types of spread spectrum multiple access techniques. Compare FDMA with CDMA.
Types of spread spectrum multiple access
Spread spectrum multiple access (SSMA) spreads each user's narrowband signal over a band much wider than needed, using a PN code. All users share the wide band; the code separates them. Processing gain G_p = (spread bandwidth)/(data rate) gives resistance to interference and multipath.
1. Frequency Hopped Multiple Access (FHMA)
- The carrier of each user hops pseudo-randomly over many narrow channels within the wide band.
- Slow FH: several symbols per hop; fast FH: several hops per symbol.
- Collisions (two users on the same frequency) are handled by error-correcting codes.
- Examples: Bluetooth, GSM slow hopping.
data->[FSK]->(x)->Tx synth driven by PN code
^ selects hop frequency
[synth]<-[PN]
2. Direct Sequence Multiple Access (DS-SS / CDMA)
- Data is multiplied by a high-rate PN code (chip rate >> bit rate), spreading it over the full band. All users transmit together on the same carrier.
- The receiver correlates with the wanted code; others remain as wideband noise.
- Needs power control (near-far problem). Examples: IS-95, WCDMA.
d(t)->(x)->(x)cos wct -> channel
^
c(t) PN code
3. Hybrid spread spectrum techniques
- FCDMA: band split into sub-bands, each running DS-CDMA.
- DS/FHMA: DS signal whose centre frequency hops; avoids near-far effect.
- TCDMA: different code per cell, time slots per user inside a cell.
- TDFH: new hop frequency each TDMA frame (GSM), gives fading diversity.
FDMA vs CDMA
| Point | FDMA | CDMA |
|---|---|---|
| Separation | Frequency | Unique code |
| Bandwidth per user | Narrow channel | Whole wide band |
| Frequency reuse | N = 4–12, needs planning | N = 1, no planning |
| Capacity | Hard limit (fixed channels) | Soft limit |
| Handoff | Hard | Soft |
| Power control | Not critical | Essential (near-far) |
| Multipath | Fading problem | RAKE receiver exploits it |
| Example | AMPS | IS-95 |
✓ Answer checked by Claude Opus 5.5 (high effort)
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