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.
RCModelReviews.com, just the facts.