- 2075 Chaitra · 8 marks
Digital vs analog spread spectrum
Short answer
Digital spread spectrum uses a binary pseudo-noise code to spread digital data, as in DSSS (802.11b) and FHSS (Bluetooth). Analog spread spectrum spreads an analog message using continuous methods like chirp in radar or wideband FM. Digital techniques provide higher processing gain and easier multiple access.
Differentiate between digital and analog spread spectrum techniques using examples.
Spread spectrum spreads the signal bandwidth far beyond the minimum needed, using a spreading code independent of the data. The techniques can be grouped by the kind of signal and the kind of spreading used.
Digital spread spectrum
The information is digital (bits) and is spread by a digital code (chips) or digital hopping of the carrier. Examples:
- DSSS: each bit is XOR-ed with a chip sequence, e.g. 802.11b with an 11-chip Barker code.
- FHSS with FSK data: Bluetooth hops over 79 channels.
Analog spread spectrum
The information is analog (voice, video) or the spreading is done by a continuous analog function. Examples are frequency modulation with a wide deviation (wideband FM), chirp (linear frequency sweep) spread spectrum used in radar, and frequency hopping of an analog (FM) voice carrier.
Strictly, wideband FM spreads the spectrum in step with the message and not with an independent code, so it fits the definition of spread spectrum only loosely. The code-based DSSS and FHSS are the true spread spectrum systems.
| Point | Digital SS | Analog SS |
|---|---|---|
| Information | Binary data | Analog message |
| Spreading code | Binary PN sequence | Wideband FM / chirp / analog sweep |
| Processing | Easy with digital circuits | Needs analog circuits |
| Accuracy / stability | High | Lower, drifts |
| Multiple access | Easy (CDMA) | Difficult |
| Interference rejection | Good (processing gain) | Moderate |
| Examples | DSSS (802.11b, CDMA), FHSS (Bluetooth) | Chirp radar, wideband FM |
Digital spread spectrum is used in almost all modern systems because codes are easy to generate and synchronise.
✓ Answer checked by Claude Sonnet 5.5
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