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In many textbooks it is stated that DSSS is more robust than FHSS. Especially in terms of interference and delay spread?

What is the exact reason. Lets say we multiply a given bit 1 with a larger bit sequence. How does this make the communication more immune against delay spread & interference?

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  • \$\begingroup\$ I asked this question to the clever guys at Cambridge Consultants (en.wikipedia.org/wiki/Cambridge_Consultants) in the 1990s and they reckoned, all things considered, that Frequency hopping would yield a marginally better SNR to DSSS. I was about to embark on a 915 MHz transceiver design that never got anywhere but we got CC to give us their best. I'm not saying this is the case nowadays because someone may have thought of some cunning plan in those years. \$\endgroup\$
    – Andy aka
    Commented Jul 24, 2014 at 17:03

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I think that claim, taken at face value [sound byte'd], is somewhat dubious. Such a claim depends on the application. I also disagree that one inherently more robust in terms of interference immunity. If you are operating in a fixed channel using DSSS and you have a loud interference source in the same channel (could be fixed frequency - but higher power than you) you will loose a higher % of your packets than if you were using a FHSS system that utilized the entire band. But if you are using an FHSS system in an area where a lot of your neighbors are also using FHSS and have the same general set of hop channels then you would be in trouble. I've run into both of these situations. The first was using 900MHz ISM DSSS radios in an area where a local internet provider was using louder licensed (somehow) radios in the same channel as me (they were also using DSSS) - that channel was relatively saturated/highly utilized. The latter case was when using a FHSS radio that only had a small set of channels that it actually hopped to - so it didn't utilize the entire band. If there were other broadcasters in the same part of the band it was in it would loose a higher percentage of packets. In particular - I was jamming myself (or rather some installations of the product I was supporting were installed close together and were jamming each-other).

A poor implementation of either can lead to jamming / excessive packet loss. Also, there is no reason why you can't use a combination of the two together to more uniformly utilize the spectrum. Essentially, utilize DSSS at each FH channel.

There are some obvious decisions you should make when designing a system. For example, if you want to operate in the 2.4GHz ISM band you should pick channels between the WIFI channels if you know your device may operate in an area with multiple wifi networks (e.g. avoid picking something right in the middle of channels 1, 6, and 11 - pick frequencies between channels 2-5, 7-10, 12-14).

This article actually says that DSSS systems do WORSE with multipath / delay spread in large areas. But in enclosed small areas they do alright. "We shall also conclude that for long distances, point-to-multipoint topologies in reflective environments such as cellular deployments in a city, DSSS has no chance to survive, leaving FHSS the absolute winner, based on its famous multipath resistance." http://sorin-schwartz.com/white_papers/fhvsds.pdf

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  • \$\begingroup\$ I echo the points in the answer. In some cases, one or the other may hold an advantage. All depends on circumstances. \$\endgroup\$ Commented Jul 26, 2014 at 19:57
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Direct-sequence spread spectrum (DSSS) works by "spreading" a narrowband information channel over a much wider segment of the frequency spectrum, using a specific pseudorandom code. The receiver uses the same code to "despread" the desired signal back down to a narrowband signal, and in the process, all other interfering narrowband signals in that segment of spectrum are themselves spread out. This "coding gain" is what allows the receiver to reliably recover the information, despite the slightly decreased SNR created by all of those spread-out interferers.

The coding gain of DSSS also makes it robust against random (wideband) noise sources, such as thermal noise in the environment, antenna and receiver as well as other wideband interferers, including DSSS transmissions using uncorrelated spreading codes. It is this feature that allows a GPS receiver to operate reliably, even though the received signal level at the Earh's surface is 10 to 20 dB below the thermal noise level!

Frequncy-hopping spread spectrum (FHSS) works by transmitting the narrowband signal in short bursts using a large number of different carrier frequencies within the selected segment of frequency spectrum. The signal is still fundamentally narrowband, however, which means that a narrowband interferer on any of its carrier frequencies will potentially wipe out the entire burst on that frequency. While it's possible to deal with such bursts of errors in a robust way, it requires that the information content of the signal be spread out among multiple bursts. This means that the receiver must collect those multiple bursts before it can reconstruct the original information stream, and this is what causes the overall latency to be greater.

In addition, FHSS does not have the advantage that DSSS has over wideband noise and interference. Its primary advantage is that it is relatively straightforward to convert narrowband communications gear to FHSS, which is what led to its early adoption.

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  • \$\begingroup\$ The coding gain is a property of the PRNG code. A heavy carrier might wipe out a particular frequency of a given chip but the next chip will be onto another frequency. In fact a FHSS system more uniformly spreads it's energy across the frequency band, whereas a DSSS will have a sinc^2 envelope. \$\endgroup\$ Commented Jul 24, 2014 at 22:42
  • \$\begingroup\$ @placeholder: You seem to be a bit confused about how the two systems work. The chips in DSSS don't have a "particular frequency" -- their energy is inherenetly broadband. And while FHSS may have a more uniform power spectrum when averaged over a long time, the instantaneous energy distribution (for the duration of a single hop) is still narrowband. \$\endgroup\$
    – Dave Tweed
    Commented Jul 25, 2014 at 3:36
  • \$\begingroup\$ YES, Thus the sinc envelope for SSDS therefore a broader spectrum. If you don't _average over a long time _ i.e. the PRNG repeat length, then you don't get process gain for FHSS. So talking about an individual frequency being wiped out shows a very limited understanding. I think what confused you is my use of the "chip", in a DSSS system it is the output of the PRNG to the modulator, a bit, in FHSS it refers to the state of the PRNG, all the bits. While imprecise, it is done to ensure consistent labelling. \$\endgroup\$ Commented Jul 25, 2014 at 4:27
  • \$\begingroup\$ There is nothing stopping you from using an arbitrarily large set of frequencies in your FHSS system and letting your PRNG pick from them (assume a perfect PRNG that picks numbers uniformly). But then the common issue that crops up is resynchronizing the devices when things DO fall apart or at T=0. That process either begins to take very long for the devices to "find" each other (slow hop / fast hop), or you have to have a fixed sync freq (or subset of freqs) which can its self be jammed. \$\endgroup\$
    – Nick
    Commented Jul 28, 2014 at 19:29

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