Question 6 of 7: Spread Spectrum Techniques

Question 6: Spread Spectrum Techniques (20 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

(1) Concept of, and reasons for, spread spectrum

Spread spectrum deliberately spreads a signal's energy over a bandwidth much wider than the minimum needed to carry its data, using a spreading code — a pseudo-random (PN) sequence — that is independent of the actual data being sent. To anyone without the code, the transmitted signal looks like wideband noise. At the receiver, correlating against the SAME code de-spreads the signal back down to its original narrow bandwidth (recovering the data) while simultaneously spreading OUT any narrowband interference that entered the channel, diluting its effect.

The main reasons for using spread spectrum are: (i) resistance to jamming and narrowband interference — an interferer can only ever occupy a small slice of the spread bandwidth, so its effect is diluted after despreading (the processing gain); (ii) low probability of interception/detection — transmitted power is spread thinly across a wide band and can sit below the noise floor per hertz, and only a receiver holding the correct code can recover the signal, which is also a security benefit; (iii) multiple access — assigning each of several users a different, near-orthogonal code lets them share the same frequency band at the same time (Code-Division Multiple Access); and (iv) resilience to multipath fading — because the signal occupies a wide bandwidth, a narrowband fade can only knock out a small part of the spread spectrum rather than the whole signal.

(2) Frequency-Hopping Spread Spectrum (FHSS)

In FHSS the carrier frequency itself is switched ("hopped") among a set of discrete frequencies in a pattern set by a pseudo-random (PN) code, at a rate governed by a chip clock. At the transmitter, the data first modulates a carrier using a conventional scheme (e.g. FSK); a mixer then translates that modulated signal up to whichever frequency the PN-code-driven frequency synthesizer currently selects, and a bandpass filter cleans the result before it reaches the antenna. The receiver must know — or independently generate, synchronized to the transmitter — the identical PN sequence and chip-clock timing, so that its own frequency synthesizer retunes through the SAME hop pattern in lock-step, letting its mixer translate the incoming signal back down for demodulation.

(3) Direct-Sequence Spread Spectrum (DSSS)

In DSSS each data bit is combined (XORed, for baseband binary data) with a much higher-rate pseudo-random chip sequence before modulation, so one data bit is represented by many "chips"; the resulting high-rate chip stream then modulates the carrier directly (e.g. BPSK) and is transmitted — this multiplication in time is what spreads the signal's energy over a wide bandwidth in frequency. At the receiver, the incoming signal is first demodulated, then correlated (multiplied and integrated, i.e. "despread") against the SAME chip sequence used at the transmitter; because the correct chip sequence multiplied by itself averages to a constant over each bit period, this integrate-and-dump step recovers the original narrowband data bit while any interference uncorrelated with that code is spread out and suppressed by the correlation.