Kiwi's XPS test results are up.

[tadawson]

	Quote:
	And once one turns on to the same channel as you are using on XPS, your goose is cooked.

Not true in the majority of cases. Devices such as XPS in the ISM band do not transmit constantly, and listen for "clear space" on the channel before transmitting. Due to this, numerous devices can occupy a single channel at the same time with no degradation or interference. The oddball video system is the notable exception, but is a very distant minority in terms of number of devices in the band. So, no, XPS is **not** like 72 in that if another radio turns on and selects the same channel, **nothing** happens, other than they both work with XPS . . . quite unlike 72 . . .

So, I vote with the folks that make the statement that XPS is superior in that regard, for the reasons detailed above . . .

- Tim

[USN_POPS]

I do not claim to be an expert, and hope you guys won't beat up on me too bad. I thought I'd take a shot using the basic XPD analyzer and my laptop and see what happened. I just monitored a supposedly better system at the field today with my XPD. And the results were suprising.

The noise floor at the field before any system was turned on was pertty stable at -83 to -78DB. As soon as the other system was turned on, the noise floor began to jump around at about -63 to -48 DB and then after a few seconds, the 2 channels appeared at about -42 to -43 DB. Which was just barely above the noise floor, and occasionally one of the channels would drop into the noise floor. The signals hung pretty close to the -42 to -43 range during the entire time. Transmitter was about 6-8 ft from the XDP.

My XPS system 6-8 ft from the XDP holds one channel at about -25 to -28 DB depending on orientation. As I move the TX away from the XDP at that distance, it never goes below -38 DB, which is higher than the other system on the same analyzer, and always aove the noise floor.

At home with all the wireless gear, wireless phones etc, the noise floor runs at -68 to -83 DB, and the XPS system still holds from -38 DB in another room to -23 at a range of 4-5 ft.

This is the first time I tried this at the field, but I plan on doing some more testing. Not even sure it proves anything yet. But in the past a stronger signal under any circumstances is better than a weaker one.

[BZFrank]

	Quote: Originally Posted by tadawson
	Devices such as XPS in the ISM band do not transmit constantly, and listen for "clear space" on the channel before transmitting.

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

[ss40]

Just another happy XPS user stating my opinion. It works for me and has for a season. 2nd season about to start. As one who has been a victim on 72Mhz (proven and documented and witnessed, luckily minor incident) I would like to say that in my flying conditions and location any of the 2.4Ghz systems are far superior to the 72Mhz system and when I started RC about 8 years ago I could not understand why we were playing with pins and crystals as I had left that far behind me many years prior in full-size flying of sailplanes. So I welcome the new world and it is anything but more dangerous. At the current time 2.4Ghz has added a level of safety at the majority of sites that has never existed before.
I am a technogeek and retired computer nut who welcomes new tech when it works.
My .02, and in the interest of fairness here I expect no rebuttal to my simple honest opinion.
Thank you for listening/reading.

[XJet]

	Quote: Originally Posted by RichardCorby
	I do not claim to be an expert, and hope you guys won't beat up on me too bad. I thought I'd take a shot using the basic XPD analyzer and my laptop and see what happened. I just monitored a supposedly better system at the field today with my XPD. And the results were suprising. The noise floor at the field before any system was turned on was pertty stable at -83 to -78DB. As soon as the other system was turned on, the noise floor began to jump around at about -63 to -48 DB and then after a few seconds, the 2 channels appeared at about -42 to -43 DB. Which was just barely above the noise floor, and occasionally one of the channels would drop into the noise floor. The signals hung pretty close to the -42 to -43 range during the entire time. Transmitter was about 6-8 ft from the XDP. My XPS system 6-8 ft from the XDP holds one channel at about -25 to -28 DB depending on orientation. As I move the TX away from the XDP at that distance, it never goes below -38 DB, which is higher than the other system on the same analyzer, and always aove the noise floor. At home with all the wireless gear, wireless phones etc, the noise floor runs at -68 to -83 DB, and the XPS system still holds from -38 DB in another room to -23 at a range of 4-5 ft. This is the first time I tried this at the field, but I plan on doing some more testing. Not even sure it proves anything yet. But in the past a stronger signal under any circumstances is better than a weaker one.

What you're seeing is inaccurate reporting due to the limitations of the XPD.

The XPD is *not* the good basis for a spectrum analyzer.

The XBee modules have a 5MHz channel spacing with a 1.6MHz channel width.

Spektrum has a 1MHz channel spacing.

This means it's more than likely a Spektrum system will *not* be operating on a frequency the XPD is able to receive. What you're seeing is simply some vestigial sideband energy, not the primary signal transmitted by the Spektrum (that is lost *between* the XPD channels)

To understand a bit further -- Assume the Spektrum starts on channels 7 and 53 of its 1-79 channel arrangement.

As far as the XPD is concerned, these are not frequencies it is sensitive to -- since channel 7 is 7x1MHz from the start of the band right between channel 1 (+5MHz) and channel 2 (+10MHz) on the XPD's receiver.

Likewise channel 53 is +53MHz from the start of the band which is proabably right between channel 10 and channel 11 of the XPD receiver.

Of course all the XPS channels coincide directly with the frequencies the XPD *can* receive, so the XPS signal shows up as being *much* stronger.

To get an accurate indication of the relative powers involved you will need a *real* spectrum analyzer with a resolution of 1MHz or better (which is five-times that of the XPD).

[davebnz]

Kiwi,

great test, and interesting thank you.

I was concerned, being a user of the XPS modules on four heli's.

Does Jim's explanation solve this for you ?

http://www.rcgroups.com/forums/showthread.php?t=816984

Cheers

David

[KrisW]

	Quote: Originally Posted by davebnz
	Kiwi, great test, and interesting thank you. I was concerned, being a user of the XPS modules on four heli's. Does Jim's explanation solve this for you ? http://www.rcgroups.com/forums/showthread.php?t=816984 Cheers David

I read the first three pages of that thread. JD SOUNDS so good. . . . . . but the arrogant and condescending attitude that came through the words just blew me away. And refusal to show a video of frequency hopping just nails the coffin shut, IMHO.

It's very simple to recreate the hopping environment, and show it on a video. Takes an hour, and saves you all these hassles and questions. If people are complaining about lockouts and problems and lost aircraft, there IS a problem. As a responsible person, if I was in JD's position, I'd be doing a lot of explaining about how the hopping actually works, what triggers it, and be showing people what to expect and how to recreate the trigger for hopping. Ending the questions is as simple as providing the answer everyone wants. NOT doing it is destructive to the business, and kills your credibility and trustworthiness.

I read these posts and keep getting the "Used Car Salesman" Impression.

And THAT is the main problem. Keith Baker did the same deny deny deny at BME for the last year he owned it. . You can't "deny" things out of existence. . you need to prove that they are fixed and no longer a problem.

[Julez]

Richard Corby: Please consider this:
http://www.flyinggiants.com/forums/f...tml#post411759

[JKos]

> For example, my understanding is that in situations where interference is short in duration, the
> transmitter system will retransmit data until the receiver acknowledges receipt of uncorrupted data.
> This may happen many times before the next frame

The XPS tx module will transmit up to four times. So that is up to three retransmits if the original transmission is not successfully acknowledged.

On collision avoidance, as pointed out, it certainly does not appear the XPS system actually makes use of it. Collision avoidance is typically only used in network situations in which you have multiple devices which need to communicate on the same "pipe", if you will.


- John

[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?