Wikipedia:Reference desk/Archives/Science/2016 September 12

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September 12

Frequency Profile of Direct Sequence Spread Spectrum

I am no expert in this field, but I am puzzled by a statement made in the article on direct sequence spread spectrum (DSSS) in this section.

It is said that with DSSS, the envelope is roughly bell shaped (around the carrier) as opposed to the rectangular (uniform in some band) envelope of FHSS. It feels to me like this statement in the article is equivalent to saying that the envelope of a binary phase shift keyed (BPSK) bit sequence is bell shaped around the carrier, when my instinct tells me it would be rectangular. The statement is even made earlier in the same section that the signal resembles white noise, which seems to contradict the bell shape statement.

I did some software simulations, multiplying a constant frequency carrier with a random stream of values from the set {-1, 1} to simulate DSSS. After the appropriate band limiting, my simple experiement seems to confirm a rectangular spectrum, not a bell shaped one.

Am I misinterpreting the statement about the bell shape?

41.164.7.242 (talk) 09:30, 12 September 2016 (UTC) Eon

The spectrum of a Pseudorandom binary sequence has a flat spectrum similar to a true random sequence of bits, except for absence of very low frequencies since the PRBS being deterministic must eventually repeat itself.

In DSSS transmitters the Pseudo Noise (PN) code that modulates (is multiplied with) the message consists of short duration pulses that, if unfiltered, would have infinite bandwidth. Transmitting unfiltered square or rectangular Radio waves however is unacceptable since it would constitute a wideband radio jammer. Therefore legal DSSS (like all other digital signal) transmissions are restricted to an allocated radio bandwidth generally by an Analogue filter, typically by an LC circuit bandwidth filter that has a bell-shaped Passband characteristic. The significant feature is that the transmitter passband is still much wider than would be needed to transmit the unmodulated message, which for a synchronized DSSS receiver gives advantages of Process gain and high resistance to interference. AllBestFaith (talk) 13:41, 12 September 2016 (UTC)

Resolved

Great answer, thanks. I do think that the article is somewhat misleading in defining the characteristics of a typical implementation when discussing the fundamental principles without specifying so. In my fairly simple simulation I used a smoothed rectangular filter which was just wide enough to preserve the PN bit information. I imagine that this is pretty much the optimal approach. Of course, in practice one would also have to widen this filter to allow for the additional spread due to the baseband signal. Eonzo (talk) 14:17, 12 September 2016 (UTC) Eon

I agree that the article needs better clarification of the bandwidth issue and I have edited the text. AllBestFaith (talk) 19:20, 12 September 2016 (UTC)