RC Protocols

There are two basic forms of spread spectrum technology: Direct Sequence (DSSS) and Frequency Hopping (FHSS). DSSS spreads a narrow band signal over a larger band by mixing it with pseudo random "noise", while FHSS hops around the whole band several times a second.

DSM2 and DSMX:

DSM2 is based on something called CDMA (Code Division Multiple Access). In simple term, this means two things: 1) The signal is spread out over a wider frequency band and 2) each transmitter/receiver pair uses its own coding scheme to "scramble" the signal. The coding schemes are designed in such a way that even if two stations are transmitting on the same frequency, the respective signals can still be isolated by the receivers, thanks to the coding schemes.

CDMA is great, because it's resilient to static, interference, and other transmitters transmitting on the same frequency. However, there's a limit to how many stations can transmit on the same frequency before the signal gets too diluted to be successfully isolated by the receiver.

Spektrum DSM2 uses DSSS only on two channels selected at start up (the original DSM used only one channel). DSM2 tends to be better at rejecting low-level wide band interference, while FHSS tends to be better at rejecting narrow band interference.

So DSM2 employs a simple scheme to try to avoid this. When starting up, the transmitter tries to find two free frequencies and starts transmitting. The receiver is scanning the frequencies for the unique signature of its paired transmitter. Once found, the receiver locks in and you have a working link. DSM2 uses two separate channels, so if one gets knocked out by interference, the other channel may still be usable. The problem is that the channel allocation happens on transmitter startup, so if both channels become unusable at some later point in the flight, you may still lose the link.

And that's why DSMX was designed. DSMX essentially uses the same encoding scheme as DSM2, but the frequency changes thousands of times per second according to a pseudo-random sequence negotiated between the transmitter and receiver. Even if some channels used become completely saturated and unusable, it would normally only result in cut-outs a couple of milliseconds, which would be too short to even notice (in theory).

JR DSSS

JR DSSS is a different design to Spektrum, Futaba, etc. Internally the RF front end is made by Texas Instruments. The others are from Cypress.

The Ti chipsets have the ability to be truly FHSS in the sense they can maintain a very high rate of channel switches while using all the available channels. Sensitivity is 105db as opposed to 96 for Cypress. A massive difference. The RX has 2 complete RF sections for diversity on separate identical PCB's. In principle, these boards could have been satellites like Spektrum since they use a 4-wire bus as does the telemetry units.

Direct Sequence Spectrum System is JR’s own 2.4GHz protocol. A later entrant into the 2.4GHz market, DSSS can be considered as the truly ‘state of the art’ method of providing radio control for model use. And since JR has not rushed its own 2.4GHz protocol into production they have been able to incorporate the very latest techniques for ‘Wideband Data Transmission’ into the next generation of JR radio control sets.

DSSS combines the best features of Frequency Hopping Spread Spectrum (FHSS) and Direct Sequence Spread Spectrum (DSSS) protocols. FHSS is great for avoiding interference and DSSS offers high response. JR DSSS combines both these attributes, having both frequency hopping and high response.

The JR DSSS system uses a greater than 3Mhz wide signal as compared to most others that utilize a much narrower (1Mhz or less) signal.

If we know anything about spread spectrum signals, a wider base signal (3Mhz vs 1Mhz) allows for a huge increase in noise and interference immunity performance and at the same time allowing for much faster data bandwidth/throughput.

The fact that DSSS is using quite a wide bandwidth signal is a huge advantage in 2.4GHz performance and it will provide superior performance over any other current 2.4GHz system that is currently available.

DSSS also offers ‘Two-Way Communication’ (telemetry) with receiver voltage monitoring as standard. Other types of sensor are due soon, with Temperature and RPM sensors being the next types to become available from JR in late June.

Probably the most important thing about JR DSSS is that it is 100% pure JR, with no involvement from any other outside company. This means that JR is back in full control of product development, so it can now concentrate on doing what it does best – making what they consider are the world’s best quality radio control systems.

FUTABA’S FHSS, S-FHSS, FAST, FASSTest

FHSS is a simple frequency hopping system. It will not scan the band at boot and has no error checking capabilities.

FASST is a hybrid FH/DSSS channel shifting system, it will scan the band at boot and has error checking and correction capabilities. FASST is what Futaba uses in commercial systems.

FASSTest is FASST + telemetry. FASST and FASSTest are not compatible. The base protocol is the same, but the addition of the telemetry data makes them incompatible.

Futaba's FASST, Spektrum DSMX, and JR's DSSS use a combination of DSSS and FHSS. A DSSS signal hops around the whole frequency band using FHSS. On paper, this give the best resistance to both narrow band and wide band interference. In the real world and our application, there is no hard evidence that DSSS+FHSS has an advantage over FHSS alone.

OTHER SYSTEMS

Most other manufacturer's protocols use FHSS - this includes Futaba S-FHSS/FASST, FrSky ACCST, Hitec AFHSS, Airtronics FHSS, and others. These are much more resilient to narrow band interference since they'll simply hop past the bad section.