The data rate is just under 1Mbps. We're using ~480Kbps of it, TDMA, IIRC.
It is (it's certainly not outside the dome, if that's what you mean). It's been tried in the ceiling, on the field, in the stands... We've tried directionals, omnis, amplified, padded, just about everything. One thing we haven't done is separating the transmit and receive antennas (so they can be padded differently). The solution, so far, is to abandon 2.4G, in favor or
900M, which works reasonably well (except for other system limitations - e.g. bandwidth).
...been tried. We can get dome time just about anytime we need it.
Drops, *frequently*.
Lock and negotiation takes longer than a single hop. I don't understand the second question.
We are double-sending data, once for each antenna. The radios take care of that overhead (something we lose at 900M; see above).
We do a dumb switch, once on each TDMA slot, currently. Actually, that would only help on the base. The mobile unit doesn't have any diversity.
Don't think that's going to help us, at least directly.
I am still trying to understand the RF-characteristics of the signal.
Is this direct TDMA like GSM mobile phone in which up to 8 handsets share a common 200 kHz RF channel and each handheld sending the data as a burst within the allocated time slot. This works well for GSM at
900 MHz and reasonably well at 1800 MHz, so with a significantly larger bandwidth it should work well on 2G4.
Or are those individual signal time multiplexed into a single baseband and the modulate a single spread spectrum "carrier".
How wide is the actual emission ? Asking in a different way, how many (non-overlapping) RF channels can be selected ?
For SS, the spread signal (chip rate) should be one or two orders of magnitude faster than the actual data.
A sports dome would be absolutely the last place I would use any ISM band for any professional communication :-).
Setting up your system before the event and everything seems to work OK and there are plenty of SNR etc.
Then the huge public is admitted into the dome, each carrying one or more ISM devices (Bluetooth, WLAN) etc. While with spread spectrum emissions, you do not have a discrete frequency channel free/occupied situation, but adding more and more spread spectrum devices into the same frequency band will gradually increase the background noise level and the SNR after despreading drops gradually, until the SNR drops too low and the communication fails.
In the dome, the RF emitted from each device is bouncing around and finally absorbed by some soft tissue (the spectators).
Sounds very much like NBFM (12.5/25 kHz BW) on 1.3 GHz (23 cm) that I used a few decades ago.
As the theory predicts, a simple ground reflection will create a comb filter like spectrum and for a specific narrow band signal channel, there are deep nulls at very close distance from each other. Moving just a meter and the signal drops several times. Stopping a car with a fixed antenna at traffic light would almost always cause stopping at the multipath null :-). The only thing that helped, is moving the antenna a few centimeters (spatial) or changing channel (frequency diversity).
At highway speeds, the dropouts were so frequent (and short) that it did not affect the readability of the speech (equivalent to interleaving and ECC in digital communication).
It still sounds that you are suffering from a narrow band signal with the RF field strength punctured by multipath nulls distributed close to each other all over your area of interest. Of course, since some data is lost, you must use quite a lot of error correction bits.
Since you are only using about half of the available capacity, why not allocate the rest for ECC bits ? Since the multipath nulls create burst errors, interleaving should be used to convert burst errors to random errors that can then be corrected by ECC.
The problem with interleaving is that it adds delay, which is annoying in two way conversation. As you said that the dropouts are frequent (assuming several each second), this also means that they are short, thus reducing the interleave delay needed.
Those might be your worst problem.
I am still not sure what the emission is like. Do you first lock into some direct sequence spread spectrum signal, then lock into TDMA frame and finally lock to the individual signal ?
Or are each handset sending individual spread spectrum sequence during short time slots in a TDMA way ?
In a typical moving multipath situation, the signal is 80-95 % above threshold and only 5-20 % below threshold. In an ideal receiver, the recovery would be immediate and at least more than 80 % of the time the signal would be good even with a single receiver. With diversity reception with receivers, this time would be much closer to 100 %.
However, if a receiver takes more than 50 % of the good signal time to relock, even two diversity receivers would not be enough to reach even close to 100 % service times.
That's how it works (base gets five slots, each mobile gets one). It works well except in certain locations, mostly domes.
No, you had the networking layer pretty much right, above. I don't have the modulation details in front of me (it's not our radio).
FHSS over 43 channels, IIRC.
We don't, obviously. It works extremely well in open stadiums (>95% market share ;-).
Nope, that's the worst-case situation.
Masses of people absorb a lot of energy. That actually helps.
Right, but enough gets to the receiver. Spectators actually help without the dome.
We don't have that option, if it did work. The radios aren't necessarily moving, though.
Not really. We do a double-send and throw away faulty packets (CRC checked) but there aren't any correction bits, per se. Everything is G927 compressed so that hides a lot of uglies.
Don't think we have access to the bits. I'll have to look more closely into that, though. If we had access to them I'm sure we'd be making use of them for more data.
Each slot is .5ms, ten slots (five transmit, five receive) per hop. The system latency is pushing the limit for intelligibility (80-100ms). There is a *lot* of echo-cancellation stuff going on to minimize these issues.
Pretty hard to eliminate. The radios have to know who they're talking to before they can respond appropriately. Then there is the data in flight...
Yes. At least I think that's the way the radios work. They all know ahead of time what the sequence is. This is negotiated once during system setup while they're wired together (actually, downloaded from the base).
I'll have to think about that (SWMBO is yelling to leave).
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