Kiwi's XPS test results are up.

[gareth.ky]

I don't fully understand how devices on this band can co-exist on the same channel. I know that cell phones use code devision multiplexing (CDMA). Your cell phone is on the same channel as several others, and they work just fine. So it is being done, I just don't know how its being done on 2.4.

I think this thread lacks a solid technical explanation of that. E.g. does a wifi access point on the same 'channel' as a 2.4 system actually cause a conflict if the two are some sane distance apart. I mean, in the real world, I don't fly standing on top of a wifi base station. Most of those are inside anyway. Its hard as it is to get reception in the next room. 2.4 Signals don't pass through anything substantial. If the two are say 1/4 mile apart will you have an issue? If the Rx is between you and the AP? If the AP is between you and the Rx?

[gareth.ky]

Ok I like it when Wikipedia can answer my questions:

See Spread Spectrum

Direct-sequence spread spectrum link
In telecommunications, direct-sequence spread spectrum (DSSS) is a modulation technique. As with other spread spectrum technologies, the transmitted signal takes up more bandwidth than the information signal that is being modulated. The name 'spread spectrum' comes from the fact that the carrier signals occur over the full bandwidth (spectrum) of a device's transmitting frequency.

Features
1. It phase-modulates a sine wave pseudorandomly with a continuous string of pseudonoise (PN) code symbols called "chips", each of which has a much shorter duration than an information bit. That is, each information bit is modulated by a sequence of much faster chips. Therefore, the chip rate is much higher than the information signal bit rate.
2. It uses a signal structure in which the sequence of chips produced by the transmitter is known a priori by the receiver. The receiver can then use the same PN sequence to counteract the effect of the PN sequence on the received signal in order to reconstruct the information signal.


So each bit sent is modulated by several PN or chip codes that shift the carrier frequency. Then each of these chips is decoded by the Rx. Chips that don't decode correctly can be discarded. Several devices can co-exist on the same channel because they don't have the came PN sequence. Even if they do have a chip collide its no crutial as others with the same bit of information will get through unharmed.

Frequency-hopping spread spectrum link
is a method of transmitting radio signals by rapidly switching a carrier among many frequency channels, using a pseudorandom sequence known to both transmitter and receiver.

A spread-spectrum transmission offers three main advantages over a fixed-frequency transmission:

1. Spread-spectrum signals are highly resistant to narrowband interference. The process of re-collecting a spread signal spreads out the interfering signal, causing it to recede into the background.
2. Spread-spectrum signals are difficult to intercept. A frequency-hop spread-spectrum signal simply sounds like an increase in the background noise to a narrowband receiver.
3. Spread-spectrum transmissions can share a frequency band with many types of conventional transmissions with minimal interference. The spread-spectrum signals add minimal noise to the narrow-frequency communications, and vice versa. As a result, bandwidth can be utilized more efficiently.

continues:
One of the challenges of frequency-hopping systems is to synchronize the transmitter and receiver. One approach is to have a guarantee that the transmitter will use all the channels in a fixed period of time. The receiver can then find the transmitter by picking a random channel and listening for valid data on that channel. The transmitter's data is identified by a special sequence of data that is unlikely to occur over the segment of data for this channel and the segment can have a checksum for integrity and further identification. The transmitter and receiver can use fixed tables of channel sequences so that once synchronized they can maintain communication by following the table. On each channel segment, the transmitter can send its current location in the table.

and:
In the US, FCC part 15 on unlicensed system in the 900MHz and 2.4GHz bands permits more power than non-spread spectrum systems. Both frequency hopping and direct sequence systems can transmit at 1 Watt. The limit is increased from 1 milliwatt to 1 watt or a thousand times increase. The FCC prescribes a minimum number of channels and a maximum dwell time for each channel.


This is the technique that Futaba is using. Its also the system that the US military uses for its field radios. This has distinct advantages when the signal band is saturated with lots of fixed frequency devices. Do note that each individual reception on a channel is still in the same way as Direct-sequence spread spectrum, with multiple chips per bit. So this is DSSS with Hopping added, not simply a narrow band carrier hopping around.


Sorry for the long post but I hope this is useful information. I am certainly more enlightened for having done a little reading.

[tadawson]

	Quote: Originally Posted by BZFrank
	The first statement is true but not the second. XPS does not seem to use the IEEE 802.15.4 CSMA/CA (p-Persistance) feature, it just sends its data frames out regardless what else happens on the same channel. Frank

Well, Maxstream (Digi?) appears to disagree with you on that . . . . .

From the XBeePro documentation:

	Quote:
	CCA (Clear Channel Assessment) Prior to transmitting a packet, a CCA (Clear Channel Assessment) is performed on the channel to determine if the channel is available for transmission. The detected energy on the channel is compared with the CA (Clear Channel Assessment) parameter value. If the detected energy exceeds the CA parameter value, the packet is not transmitted. Also, a delay is inserted before a transmission takes place. This delay is settable using the RN (Backoff Exponent) parameter. If RN is set to “0”, then there is no delay before the first CCA is performed. The RN parameter value is the equivalent of the “minBE” parameter in the 802.15.4 specification. The transmit sequence follows the 802.15.4 specification.

One of the primary features of the wireless stuff is that is does **not** blindly transmit and step on itself - that's how multiple devices can exist on the same channel cleanly!

- Tim

[Prof. Dr. YoMan]

You can disable CA on the Maxstream modules if you want.

[Simpleton]

I think X-Jet determined the XPS had disabled that feature with their custom firmware. Don't want to speak for X-Jet, I'm sure he'll clear up any confusion.

[Julez]

	Quote:
	I think X-Jet determined the XPS had disabled that feature with their custom firmware.

This appears to be the only logic solution. After all, one does not want one's system to stop transmitting, when interference is encountered, does one?

[JKos]

> One of the primary features of the wireless stuff is that is does
> **not** blindly transmit and step on itself

This is simply not true. There is no FCC requirement for collision avoidance, i.e. listen before transmit. This is a widely held myth that seems difficult to squash.

> - that's how multiple devices can exist on the same channel cleanly!

No, it's called spread spectrum. In a network situation in which multiple devices are attempting to share the same "pipe", if you will, collision avoidance makes sense. It is even used in wired networks like Ethernet. But for our application, it does not make sense nor are any of our RC 2.4 GHz products using it.

- John

[Four Stroker]

Julez

Have you read the article in RC Elektronik "2.4 GHz und die Geheimnisse einer neuen Technologie" ?

While we are myth busting:

There is absolutely no requirement to listen to a channel before transmitting on it in the 2.4GHz ISM band. That is, there is no collision avoidance requirement whatsoever.

Download for free and read part 15 section 247

http://www.fcc.gov/oet/info/rules/

If you can't find it in part 15, it's not a requirement (unlicensed equipment).

[tadawson]

	Quote: Originally Posted by Julez
	This appears to be the only logic solution. After all, one does not want one's system to stop transmitting, when interference is encountered, does one?

Actually, no, it isn't logical at all . . . if the channel is busy, transmitting anyhow will be useless. Waiting for a free "time slot" is far more logical, since it is a very small wait-time involved. Allowing free for all transmission, with all the subsequent retries will simply load the channel more, *decreasing* the chance of a good packet getting through . . .

- Tim

[gareth.ky]

"Spread Spectrum" means smearing your signal across the channel your transmitting on. If you'll read the post above or the wikipedia article you will see how several devices on the same "channel" can co-exist.

The big reason that systems like the XPS and Spektrum look for an empty channel is swamping. You don't want to place two emitters on the same channel in the same physical place.

People imagine that its some sort of time devision multiplexing. Each device transmits in the clear on its own time slice. Its actually far more clever and robust than that.

Its actually more like this. The device is transmitting on a continuous range of frequencies (a channel). Its sending a sine wave. That wave is modulated or distorted by a known value. Positive distortion indicates a 1 and negative distortion indicated a 0. This is called a chip. The radios send several chips for each bit.

For each chip the radio knows that it should be receiving on one of two possible very precise frequencies. like 2.4508 or 2.45016. Other chips being sent by other devices on the same channel wont land on those two frequencies and they can be ignored.

Seeing neither or both of those frequencies lit up causes the chip to be discarded. The send several chips per bit. So you might only need 2 good chips out of 8 to know you got a 1.

It's pretty slick stuff.

[Flyfast1]

	Quote: Originally Posted by JKos
	> One of the primary features of the wireless stuff is that is does > **not** blindly transmit and step on itself This is simply not true. There is no FCC requirement for collision avoidance, i.e. listen before transmit. This is a widely held myth that seems difficult to squash. - John

John,

While you're at it, would you please quash the myth about "lockout"!

-Ed B.

[JKos]

Ed,
I'm really not sure what you mean.

- John