FHSS versus DSSS, which is best?

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FHSS versus DSSS, which is best?

Postby RCModelReviews » Sun May 23, 2010 10:41 pm

I discuss the benefits of FHSS over DSSS in this article and invite your comments.

Would you consider a DSSS system when you buy a 2.4GHz radio or upgrade to a new system?

Will Spektrum be the only kid on the block still using DSSS soon?

Have you flown at large events with DSSS? Did you have any issues or see anyone else with problems that might be related to excessive noise on the 2.4GHz band?
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Re: FHSS versus DSSS, which is best?

Postby flying-things » Tue May 25, 2010 12:50 am

I think Spektrum's dsm2 days are numbered, there is more than enough evidence that 2 channels are becoming inadequate for 100% reliable operation. Not to mention other spektrum problems...

However, with most fhss systems using the entire band, there are problems on that route also. If you have a hundred people using fhss, the band would be extremely crowded and noisy, and jerky response and or loss of control is very possible. Now, if it were hitec's fhss, which chooses the most "peaceful" part of the band, and doesn't use all the channels, I think that would be better for many people flying at once.

I think if you just fly humbly at your local flying club, where the most is 4 people flying at once, there is no need to jump away from dsss. However, for huge models and competitions, you should really stick with fhss.
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Re: FHSS versus DSSS, which is best?

Postby RCModelReviews » Tue May 25, 2010 1:06 am

FHSS should provide us with plenty of capacity for the time being.

Remember that it's nearly impossible to fly (safely) when there are more than a few (maybe a dozen) models in the air at a time so, at the average club field, the chances of running into crippling levels of interference from other RC systems is low.

Also, since there is usually a reasonable distance between other interference sources (WiFi, wireless video senders, etc) and the average flying field, those aren't likely to be a problem either.

At a crowded field (where it's possible there may be 20 or more radios all turned on at the same time) I'd put my money with FHSS over DSSS, even if only because the FHSS systems will "degrade" more progressively, rather than simply just "stopping" because their one or two channels got stomped on. If you notice your FHSS system getting sluggish and slow to respond, it's time to land! With DSSS you may not get that warning.
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Re: FHSS versus DSSS, which is best?

Postby flying-things » Tue May 25, 2010 9:56 pm

RCModelReviews wrote:FHSS should provide us with plenty of capacity for the time being.

Yes, for the time being. When spektrum came out with the dx6, everybody thought it was amazing technology allowing new freedom at the flying field. Now, spektrum is becoming obsolete. 2 channels are not enough anymore, and even without dozens of radios turned on at once, there have still been problems. A radio that costs upward of $400 with rx's $40 a piece, should not just have technology that is "good enough" rather, it should last into the future.
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Re: FHSS versus DSSS, which is best?

Postby Engineer » Sat May 29, 2010 4:22 am

There seems to be a miss understanding about how 2.4 works.
A channel is not limited to just one system at a time. There can be many system on the same channel at a time. How many? A channel channel is 1 MHZ wide. If we were operating on 72mhz rules, and had separation of frequencies of 10kc there would be 100 separate frequencies to operate on.
In the testing done by Cal Orr there seems to be a limit of 16 systems before the range starts to degrade. Thus we could have 16 x 40 = 640 Spectrum DSSS radios happily operating at one time.
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Re: FHSS versus DSSS, which is best?

Postby RCModelReviews » Sat May 29, 2010 4:45 am

Engineer wrote:There seems to be a miss understanding about how 2.4 works.
A channel is not limited to just one system at a time. There can be many system on the same channel at a time. How many? A channel channel is 1 MHZ wide. If we were operating on 72mhz rules, and had separation of frequencies of 10kc there would be 100 separate frequencies to operate on.
In the testing done by Cal Orr there seems to be a limit of 16 systems before the range starts to degrade. Thus we could have 16 x 40 = 640 Spectrum DSSS radios happily operating at one time.

What you're referring to is temporal density.

Because 2.4GHz systems don't transmit all the time (like the old MHz systems did), you can "share" a single part of the band with other transmitters.

In theory, if you timed it just right, a transmitter which each transmitted for just 5% of the time could co-exist with 19 other transmitters of the same type -- so long as no two tried transmitting simultaneously.

In the computer networking world the same thing happens. After all, how else could you get a dozen or so computers all sharing the same WiFi connection?

These systems use what's called carrier sense multiple access (CSMA) which means that before any transmitter sends a signal, it listens to make sure that the channel is free and not already in use. If it hears another transmitter using the band, it waits for a random length of time (a few mS) and then listens again.

Obviously, if you're dealing with non-realtime or soft-realtime data like a file-transfer between two computers then this isn't a problem. However, with RC model planes there would be a problem.

Imagine if your model is on the back-side of a loop, heading towards the ground and you need to give it up elevator at exactly the right time to avoid hitting the ground. The last thing you want/need is a transmitter that listens to the part of the band it's using and courteously waits for it to be free before sending your "up elevator" command to the model.

So RC systems generally don't use CSMA to control access to the band.

This means that we can never really achieve the theoretical maximum number of users on a single part of the band.

So, in the case of the Spektrum, it has only a very limited ability to share the same part of the band with other Spektrum RC systems. The more Spektrum radios that try to share the same part of the band, the more "collisions" occur because one or more radios transmit at the same time.

Because the Spektrum radios use the same spreading code, two Spektrums transmitting on the same frequency at the same time *will* interfere with each other. Unlike an FM radio, this interference won't produce a glitch -- it will simply result in a loss of the data. Lose enough data and the receiver goes into lockout/failsafe mode.

As a result, although "in theory" you might be able to have as many as 16 Spektrum systems all coexisting on the same part of the band, it simply doesn't work that way in practice.

I'm unaware of Cal Orr's comments but I suspect he's based them on the way that CSMA networks work -- and RC isn't a CSMA network -- it doesn't look before it transmits so even two Spektrum radios on the same channel may produce lockouts, if they try to send too many packets of data at exactly the same time.

Interestingly enough, a Spektrum and a Corona (or other-brand DSSS system) are far *less* likely to interfere with each other when using the same part of the band. This is because they use different spreading codes and therefore can (to some extent) actually both transmit at the same time without losing too much data.

Spread Spectrum is a different beast to FM and there are many details that need to be considered.

However, bearing all this in mind, in an RC context, FHSS (or to be more accurate - constantly agile DSSS) does offer more resilience and a far more progressive degradation when trying to deal with very noisy environments.
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Re: FHSS versus DSSS, which is best?

Postby Engineer » Sat May 29, 2010 6:31 am

In theory there could be a great deal more than 16 systems on one channel. Cal Orr's test seem to indicate the limit is 16 for Spectrum units. What he did was take 40 radios and start turning on 2 at a time and noted when the range he obtained started to degrade. If they checked for a clear channel and chose a clear channel they should all not degrade in range since there are 80 channels available. Obviously what happened is they all chose the same channel.
I agree SS is different than FM. I just made the comment to illustrate there is a lot of room in one channel.
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Re: FHSS versus DSSS, which is best?

Postby RCModelReviews » Sat May 29, 2010 8:59 am

The Spektrum does check for a clear channel before it allocates its operating frequencies so I don't see how he got more than 2 systems on a channel. In fact, if he had 40 radios, then they should all have chosen different channels (since Spektrum radios choose two out of 80 channels).

His methodology sounds suspect to me, as do his results.

Did he use a spectrum analyzer to determine whether the radios had indeed all used the same channel? I suspect he was making some rather nonsensical assumptions.
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Re: FHSS versus DSSS, which is best?

Postby Engineer » Sat May 29, 2010 3:03 pm

Here is a link to some of his tests. Other tests are published in the AMA magazine.

http://www.spektrumrc.com/Content/Im...SPMarticle.pdf
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Re: FHSS versus DSSS, which is best?

Postby RCModelReviews » Sat May 29, 2010 9:27 pm

Link says "not found"
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