AdvancedTopic 89 of 101Signals, Control & Comms
Spread spectrum
Spreads a signal over wide bandwidth for robustness and security.
In depth
Spreading a signal over much larger bandwidth using a pseudo-random code gives anti-jamming, low probability of intercept, multipath resistance and multiple access (CDMA). DSSS multiplies by chips; FHSS hops frequencies. Processing gain G=B_ss/R_b. Used in GPS, Bluetooth, early 3G.
Key points
- PN sequences and Gold codes
- Despreading
- Near-far problem
ExampleGPS signals lie below the noise floor and are recovered by correlation.
What is Spread spectrum?
Spreads a signal over wide bandwidth for robustness and security.
Why is it important to understand spread spectrum?
Spread spectrum spreads a signal over a wide band, which improves resistance to interference and makes the signal harder to detect.
What is processing gain?
The improvement in SNR gained by spreading, equal to the ratio of the spread bandwidth to the data bandwidth.
What is frequency hopping?
Changing the carrier frequency in a known pattern, so interference affects only some hops.
How is Spread spectrum used in practice?
Typical applications include using GPS signals that resist interference from other transmitters, operating a Bluetooth link in a crowded 2.4 GHz band with frequency hopping and protecting a military link against jamming with direct sequence spreading.
What should I learn before Spread spectrum?
Study "OFDM" first (topic 88 of 101 in Signals, Control & Comms). It covers the groundwork this topic builds on.
What are the key points of Spread spectrum?
PN sequences and Gold codes. Despreading. Near-far problem.
Can you give an example of Spread spectrum?
GPS signals lie below the noise floor and are recovered by correlation.
What comes after Spread spectrum?
The next topic is "Antennas and propagation", which builds directly on this one.
How difficult is Spread spectrum?
It is classed as Advanced in Signals, Control & Comms (topic 89 of 101). It is an advanced topic. It assumes you are comfortable with the intermediate topics in Signals, Control & Comms.