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Cell Phones In The Air?
jumbledInTheHead writes "Are you ever annoyed when someone near you talks unnecessarily loud on their cell phone? Or even worse, when it is in a tight, enclosed space and you can't walk away? The problem is about to get worse the next time you take a flight; the FAA is considering removing the ban on cell phone use on airlines."
Actually, the problem is that it's really easy to get a signal, because you have a clear shot to a crap load of cells at the same time while you are up in the air.
As a private pilot, I always leave my cell phone on when I fly VFR. (So far I haven't flown into any mountains due to navigational system confusion.)
The only way that I can get my phone to work is to descend very low in a rural area. If I'm up more than about 2000' AGL, then the phone doesn't work. I figure that it gets confused because it probably sees a dozen towers with strong signals.
Commercial aircraft would probably have to install special equipment to receive the signal inside of the airplane and then connect to the phone network directly.
Ha, ha! Nobody ever says Italy.
> Actually, the problem is that it's really easy to get a signal,
> because you have a clear shot to a crap load of cells at the
> same time while you are up in the air.
This is the big problem. If they use the existing cellular infrastructure, this will greatly increase interference and make voice quality even worse than it is today for everybody.
For existing cell phone towers, any cell phone in the air will be likely above the antenna mid-line. Since most antennas have a down-tilt of around 3-degrees (so they point slightly towards the ground), any coverage above the mid-line will be from one of the normally minimized antenna nodes that point up.
Antenna manufacturers try to make these nodes small because it's just wasted energy. One would rather have that energy pointed to where the traffic is. So the cell phones that are in the air will have moderate to poor signal strength which will require the cell phone and the base station radio to transmit at their highest power settings. For the base station, that's not too bad, but for the cell phone, you're suddenly going to have this giant source of interference because the phone will be broadcasting at its full power setting from nearly two miles above the surface. Since voice quality is indirectly proportional to the level of the noise floor in an area, cell phones service will get even worse.
However, the article talks about how they'll mitigate this. Airlines will install very small, low power base stations called "Picocells" aboard the plane. Thus, the cell phone will communicate with this nearby base station, reducing power levels significantly and minimizing the interference effect.
However, this will do nothing to mitigate the annoyance of listening to the person next to you screaming on their cell phone because they can't hear their voice over the roar of the engine.
Insert simplistic political, ideological, or personal proselytization here.
Antenna manufacturers try to make these nodes small because it's just wasted energy. One would rather have that energy pointed to where the traffic is. So the cell phones that are in the air will have moderate to poor signal strength which will require the cell phone and the base station radio to transmit at their highest power settings. For the base station, that's not too bad, but for the cell phone, you're suddenly going to have this giant source of interference because the phone will be broadcasting at its full power setting from nearly two miles above the surface. Since voice quality is indirectly proportional to the level of the noise floor in an area, cell phones service will get even worse.
I'm not sure if this is really true. Currently, cell phones operate totally without any line of sight component in a multipath envirinment (ie, a Rayleigh signal model). An airplane would have line of sight to a base station, but would still have multipath (ie, a Rician signal model). Having a LOS component greatly improves things. I haven't see a a Smith chart for a cell tower antenna in a while, but while the main lobe is basically horizontal, there will be a side lobe pointing up. Also, multipath dominates the quality issues in mobile communications. AWGN plays a part, but fading is a bigger problem.
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This will have the additional safety benefit that the signal from the phones doesn't have to be at full power, since the distance to the link is only ten or so meters instead of over 10km at cruising altitude and so the chance or interference to onboard equipment is reduced.
> I'm not sure if this is really true. Currently, cell phones operate
> totally without any line of sight component in a multipath
> envirinment (ie, a Rayleigh signal model).
> I haven't see a a Smith chart for a cell tower antenna in a
> while, but while the main lobe is basically horizontal, there
> will be a side lobe pointing up.
You're correct in that multi-path governs cellular communications, particularly for spread spectrum systems like IS-95 CDMA. However, wouldn't the fact that the device is flying above the skyline with no objects for the signal to bounce off of minimize multi-path effects? It's almost free air space with no obstructions when you're pointed at an airplane.
The airplane would offer an unobstructed path to the cell site, which could help things, but you'd still have a problem with the main lobe (on the vertical axis) pointing away from the airplane. You'd have to rely on a lobe pointing up or even on the back lobe in the case that the antenna is really tilted (like in an urban environment).
They've got some Smith charts on Andrew's web site that indicate the problem. I pulled a pattern for what I think is a typical cellular sectored antenna 854DG90VTESX running at ~824 MHz. If the top lobe is pointed at the plane (and there's no multi-path), along the blue vertical axis you're going to get a node 20 dB down from the main lobe. Thus, the line of sight coverage from the base station will be much lower than one would expect, and both the base station and mobile station will have to increase their power levels to make up for the lack of coverage.
Insert simplistic political, ideological, or personal proselytization here.
The FAA has no business trying to keep people from being rude. Why should the FAA regulate rudeness. Should the FTC be concerned with people talking loudly or on cellphones in theaters? It should be up to the airline if phones are not to be used.
Example: Excuse me sir, if you keep up the loud obnoxious cellphone talking, we'll be forced to tazer you and sit you between the three body odor offenders in row F.
The FAA should only be concerned if there's a valid problem with equipment interferrance. Which I would hope not, because then we'd be forced to use those $4.50/min credit card phones on the seat-backs.
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As a more "socially acceptable" alternative, you could just use your cell phone (even if you're not calling for real) and start shouting at it "yeah, the flight is nice but there's an idiot next to me that just won't shut up, I was trying to sleep and all he does is to yell at his cell phone as if everyone cared about his boring life..." and the like... :)
I am a student pilot, and nowhere in the FAR does it say anything about cellphones. This rule is from the FCC, not the FAA. The closest the FAA has is FAR 91.21 which gives part 91 carriers the right to decide who can use what aboard their aircraft.
Again, give credit where credit is due: This is the FCC's rule, not the FAA's.
Take a gander at this "report" that suggests that calls made above 8000 ft. are impossible from an airplane, and that calls below that are unlikely...
So... is the conclusion drawn from this report wrong?
You're correct in that multi-path governs cellular communications, particularly for spread spectrum systems like IS-95 CDMA. However, wouldn't the fact that the device is flying above the skyline with no objects for the signal to bounce off of minimize multi-path effects? It's almost free air space with no obstructions when you're pointed at an airplane.
The only real way to be sure is to take field measurements, but I am pretty sure that this would be a Rician model (line of sight with multipath) with a fairly big Doppler shift. My reasoning is that since the mainlobe of the antenna is horizontal or pointing downward, it will pick up building and ground reflections. Since they are doing picocell and retransmitting, then can precompensate for the Doppler, though, by measurements on pilot channel. This topic has come up several times on comp.dsp. The general consensus was that the LOS component really helps things more than you think.
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Those who say its easy to use a cell phone on a plane have obviously never tried it.
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I thought the reason you couldn't use a phone on the plane actually had more to do with FCC regulations than FAA ones.
Cell phones work by assigning a particular set of frequencies to a particular geographic area, and then reusing those frequencies further away where there is no chance for interference (phones that use spread spectrum work more or less the same way, only the frequency separation is more dynamic). When you take a phone operating within such an arrangement and suddenly raise its altitude a few thousand feet, it can suddenly be present in many, many cells. This causes interference in every cell where the phone is not actually communicating with that cell's tower.
I have heard of plans to put micro-cells aboard planes. Such micro-cells would instruct the phones to use low enough power that this wouldn't happen. THAT is a much different scenario, but I wonder how many different modulation types (and therefore customer populations) will be able to be handled by such a scheme. Those who aren't covered by a cell in the plane should not be using their phones for the technical reasons described above.
As for whether people can talk on a phone or not, I fail to see the distinction between talking on a phone and talking to a person next to you. I've seen drivers distracted by their fellow passengers with equal frequency to drivers distracted on a phone. I've seen loud, obnoxious boors talking way too loud to people 3 feet away with equal frequency to the same boors shouting into a phone. What's the difference? Rudeness is the same whether technology is involved or not.
I went through providers over the years. I don't pick them by the ability to get a signal on the plane, mind you, but it never hurts to do a little test:
1. Verizon CDMA-800 kinda works when the plane's still low, and you might get a signal at cruse altitude every now and then. That phone also had analog roaming, and it picked up analog signal sometimes.
2. TDMA works quite well.
3. GSM-1800 doesn't work AT ALL.
BTW, if any of you don't understand how come the FAA worries about something as insignificant as a ell phone disrupting avionics, you can do a little experiment - put your cell next to a car radio and call it. I'm not sure how likely you'd be to have the same effect with CDMA and analog, but GSM phones time-multiplex the channel, and most radios would pick up the multiplex frequency, which would come out of your speakers as rather unplesant loud hum, akin to a modem sound.
Having trouble with the fellow next to you talking too loudly? A cell phone user cussing out their employee? Here's a nutty thought: Ask them politely to keep their voice down. Now, I know that sounds a little wacky to those of you who would rather hang back silently judging, but let me tell you I have tried it many times successfully. It requires a little known ability called courtesy, but with practice, you may be able to acquire this skill, too. Good luck, and welcome to the wonderful world of talking to strangers!