Thursday, September 27th, 2012 01:19 pm GMT -7 Thursday, September 27th, 2012 01:19 pm GMT -7Thursday, September 27th, 2012 01:19 pm GMT -7
RC radio control receivers

JR’s new DMSS 2.4 GHz RC radio system made me take a closer look at the different spread spectrum signal encoding schemes.




Spread Spectrum Radio Signals

Our old FM 72 MHz RC transmitters operate with a channel spacing of 20 kHz. These channels were defined about 40 years ago. That is a very long time ago in terms of technology. High quality radio hardware technology has dropped dramatically in price since then. It would not surprise me at all if a modern RC radio system could be created today for a reasonable cost with channel spacings of only 1 kHz.

Why then are the new spread spectrum radio systems required by law to operate with a minimum channel spacing of 1 MHz? That is about a thousand times wider than really needed. That has to do with the nature of how they work. It is also where the name of the technology comes from. You see, the signal is spread out over a larger portion of the radio frequency spectrum than strictly necessary.

This is done to make the signal more resistant to radio noise and interference. The signal takes up more space than it really needs, allowing for redundancy in the data transmission. If the interference is only affecting a narrow range of frequencies, then spread spectrum is very robust.

Frequency Hopping

But what if the interference is wide enough in frequency to blanket an entire channel? Could a radio system switch to another channel if it detects interference in the first one? What if it is only the receiver that is being affected by the interference? In that case, the transmitter would have no idea there is interference. It would not know of the need to switch frequencies.

It is a tricky problem, because the time when switching frequencies is most needed is exactly when the transmitter and receiver cannot communicate reliably with one another. How would the receiver know the transmitter has switched to another channel? What if the new channel also has interference?

The frequency or channel hopping schemes circumvent all of these issues by switching to a new channel on a regular schedule. That way, even if the receiver loses the signal from the transmitter, it will know when and to what channel the transmitter will be switching to next. Clever, isn’t it?

For this scheme to work, the channel switching has to be done many times per second. Otherwise interference on one channel would disrupt communications for too long. That is why they typically hop every millisecond or so.

The transmitter tells the receiver what are the channels it will be hopping to and in what order. This is done during the initial signal locking phase. If the receiver loses power momentarily, all it has to do is listen on one of those channels and wait for the transmitter to hop onto it. Then the receiver knows it is synchronized and will begin hopping in unison with the transmitter once again.

Radio Equipment Regulations

There is an interesting wrinkle to this whole story that you may not know about. Although each country gets to define how its radio frequency spectrum is used, the laws for spread spectrum operation are very similar across countries. The standards document ETSI EN 300 328 covers the use of the 2.4 GHz band in european countries (link below).

These regulations generally state that radio transmitters operating in the 2.4 GHz band that do not employ frequency hopping schemes are limited to a maximum output power of 10 mW. That is not much power. Our old FM transmitters generally produced about 100 mW of transmitting power. But if the system employs frequency hopping, then the maximum allowed power output jumps to 100 mW.

Spread Spectrum Radio Systems in Practice

Up until now, RC spread spectrum radio system manufacturers have relied on one of the two schemes described above to make their systems resistant to interference.

Radio systems from JR/Spektrum (DSM), Assan, Corona, iMax, and XPS rely on wide signals to get their interference protection.

On the other hand, the systems from Futaba (FASST), Airtronics, Hitec, FrSky, FlySky V2 and most others use frequency hopping as their silver bullet.

Whoa! Given what I just said about non-hopping systems, does that mean that Spektrum DSM radio systems are limited to only 10 mW?

Direct Sequence Signal Encoding

The answer is no, because a radio system qualifies as hopping even if it does not switch frequencies while it is operating. As long as it selects from a group of frequencies before it starts transmitting, that is good enough.

That is exactly how the non-hopping RC radio systems work. They are called direct sequence spread spectrum (DSSS) systems. They select from the available channels when they are first turned on. Most select two channels. Then they transmit over those channels, using a complex encoding of their signal that mixes in pseudo-random noise to make it very robust. Frequency hopping systems normally do not employ this very sophisticated type of encoding.

Pros and Cons

So which type of system is better, frequency hopping or direct sequence? Well, they both have their advantages and disadvantages.

The direct sequence systems get to pick and choose which channels they are going to use when they are first turned on. If they have any intelligence to them, they are going to pick two channels that are well apart from each other and free of interference. Under most circumstances, it is extremely rare for both channels to be blocked at the same time.

Because these systems do not hop all over the place, it is a lot easier for the receiver to reacquire the signal from the transmitter after a power loss.

Hopping systems, on the other hand, are very robust. They make use of a large portion of the available radio spectrum, giving them excellent resistance to interference. However, a receiver recovering from a power brownout will take a little longer to resync with the transmitter. Also, because of the more complex hopping logic involved, hopping systems are likely to cost a little more to manufacture.

New JR DMSS System

So where does JR’s new DMSS technology fit into all of this? What are some of the characteristics of other systems on the market, like DSMX? Stay tuned!

ETSI EN 300 328 European Wideband Usage Regulations (pdf)

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