Which is really best: FHSS or DSSS?

TWO SOLUTIONS TO THE SAME PROBLEM

Dated: 24 May 2010

Have you noticed that most of the newer 2.4GHz RC systems coming onto the market now call themselves FHSS or some variant thereof (AFHSS, ACST, etc)?

Pretty much the only pure DSSS systems left on the market are now the original Spektrum DSM/DSM2, Corona, FlySky and Assan.

So why is everyone opting to go hopping and is this really the best option for creating the most robust, reliable link for 2.4GHZ RC operation?

When FHSS isn't really FHSS

Before proceeding, it should be pointed out that all of the so-called frequency-hopping spread spectrum systems currently available in the RC world are actually DSSS systems.

A much more accurate name for the systems sold by Futaba, Airtronics, Hitec, FrSky, XPS and others is "constantly agile DSSS".

You can see in the image above that the FHSS system covers almost the entire band but, looking at the bottom graph in that image, each individual channel shows the wide-based shape characteristic of a DSSS signal like the one below.

If we were being pedantic, our RC systems are not actually "true" FHSS systems at all, because instead of using pure frequency-hopping, they also use DSSS "spreading" techniques to ensure that they also create distributed a spread spectrum (DSSS) output. However, for the purposes of this article and to avoid confusion, I'll use the term FHSS for these systems.

This is important because a true FHSS system (without spreading on the individual frequencies used) is actually inferior in many ways to DSSS. The hybrid DSSS/FHSS technique our "FHSS" RC systems use actually delivers the benefits of both technologies, making it better than either system alone.

So which is best?

Under normal circumstances, in a relatively benign environment with say 6-10 transmitters all operating at the same time, some Bluetooth activity and maybe a WiFi system in the distance, both DSSS and FHSS will work perfectly adequately. This is proved by the number of flying-fields all over the world where, every weekend, groups of people gather and fly models on 2.4GHz without any problems at all.

At such gatherings you'll usually find a good mix of the different systems, all operating without interference, despite sharing the same 2.4GHz band.

But what happens when we start pushing the boundaries?

What about large events such as SEFF and Joe Nall, where there can be dozens or even hundreds of people all trying to use the 2.4GHz band together? Which system provides the most protection against lockouts or getting shot down?

Well all-else being equal (which it's often not, thanks to sub-standard power systems, and poor installations), the FHSS system should have an edge over the DSSS system when it comes to retaining control of your model.

As my own tests have shown, when a DSSS system like the Spektrum encounters strong interference on its two operating channels, it loses link and control is lost. Sometimes, if you're unfortunate enough to have your system select two very closely spaced parts of the band, your system can fail, even though the other 80% of the band is completely free of interference.

By comparison, all of the FHSS systems I tested showed their ability to continue working even after 80% of the band was saturated with interference and only a small portion was free.

So, in theory and in practice, a good FHSS system will keep working long after many 2-frequency DSSS systems have locked out.

However, that's not the whole story.

Because the FHSS systems are using much (or all) of the band, there are situations where they can be badly affected while a DSSS system is not affected at all.

This can occur if the band is heavily congested except for a small portion and the DSSS system chooses that portion on which to operate. It's signals may be allocated to the clean part of the band and thus provide faultless communications, while the FHSS one loses a good percentage of its data to noise because it's hopping through interference.

Experience shows however that this scenario is far less likely than the converse one, where the DSSS systems shut down while the FHSS ones continue to provide a measure of control

If you look at the anecdotal evidence from large meets such as SEFF and Joe Nall it becomes apparent that there were far more "issues" and radio-related crashes associated with the Spektrum DSSS equipment than with FHSS systems of other brands. Now this will be at least in part due to the popularity of the DSM2 system and hence -- the more DSM2 radios in use, the more they'll be represented in the crash statistics.

However, it's still very clear that DSSS systems are less tolerant of very noisy environments than FHSS systems and lockouts are more common with Spektrum/JR than with FASST and other hoppers.

I'm sure that Horizon would say "there were hundreds of DSM2 systems at these meetings yet only a very few experienced radio issues" -- to which I would ask in reply "the DSM2 flaw I discovered only occurs with about the same regularity - could these crashes have been related to that problem?"

The Bottom Line

It strikes me that, as the 2.4GHz band becomes more congested with RC fliers, the number of problems being experienced by DSSS fliers seems to be also increasing disproportionately to the number of those systems in use.

Theory makes it clear that a constantly agile system (FHSS) should be more resilient to noise on the band and this seems to be born-out in practice.

If you've got a DSSS system and had no issues then don't rush out and swap it for FHSS just because of the small theoretical and practical extra level of insurance it may offer.

However, if you're looking to buy a new RC system, think carefully about the benefits FHSS might offer before committing to a system (DSSS) that is quickly losing favor in the RC marketplace (even JR is switching to FHSS elsewhere in the world).

If you do opt for a DSSS system, try to choose one that uses at least three or more different frequencies. This will give you maximum use of the band and reduce the chances of encountering the kind of interference that forces your radio into failsafe.

And remember -- there's far more to reliable 2.4GHz operation than just choosing between DSSS and FHSS. You've also got to have very good batteries (A123s rule!) plus a good installation with receiver antenna(s) well clear of wiring and other metal parts.

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