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Crookshanks

join:2008-02-04
Binghamton, NY

1 edit

it really has nothing to do with the aircraft....

.... it has everything to do with the design of the cellular network. From 35,000 feet your phone is going to "hear" dozens of base stations. Seven miles is nothing for a cell phone radio, particularly with a clear line of sight.

Two issues here:

1) Phones and the cellular network were not designed for the number of rapid hand offs that will occur at 500 knots. Recall that on 9/11 people were able to make 911 calls from the airplanes but they were all dropped fairly quickly. These drops were failed handoffs, not a lack of signal.

2) From altitude, your phone will hear dozens of base stations. Conversely, all of those base stations will hear your phone. This has the effect of lowing the signal to noise ratio in the cellular network and making it that much harder for clients with low signal levels to maintain a connection to the network. The cellular network has to be very carefully designed to minimize this problem, which is hard enough under the best of circumstances. Trying to manage this problem with mobiles at 35,000 feet that are heard by dozens of different base stations is impossible.

Google 'near far problem' for some very interesting reading on this subject. It truly is amazing just how much engineering goes into making the cellular network function as well as it does. Base stations have to be very carefully located so they don't interfere with one another, particularly in those neighboring areas where coverage overlaps exist. Every single mobile is commanded to adjust its transmission power dozens of times per second while in an active call/data session. OFDM (LTE) networks actually broadcast at different power levels for each subchannel to mitigate interference in fringe areas/allow for successful handoffs.

Ponder all of this the next time someone tells you that bandwidth problems can be solved "simply" by adding more base stations, or when one of your NIMBY neighbors complains about a new tower going up because "They could just use that one over there."

Voice/data sessions _could_ be made to work by having a femto/pico cell on the aircraft. Then the mobiles could transmit at a very low power level and would not cause interference to clients on the ground. Of course, I'm sure the airlines would charge a hefty premium for this service, and frankly I'm not sure I want the self-important asshat sitting next to me screaming into his phone for the whole flight. Data is more easily provided with wi-fi than a femto/pico cell, so not much reason for it there.



Omega
Displaced Ohioan
Premium
join:2002-07-30
Somerset, NJ

You missed the most important part. Allowing people to have voice conversations on their phone on a plane would be annoying as hell.
--
What smells like blue?


patcat88

join:2002-04-05
Jamaica, NY
kudos:1
reply to Crookshanks

said by Crookshanks:

2) From altitude, your phone will hear dozens of base stations. Conversely, all of those base stations will hear your phone. This has the effect of lowing the signal to noise ratio in the cellular network and making it that much harder for clients with low signal levels to maintain a connection to the network. The cellular network has to be very carefully designed to minimize this problem, which is hard enough under the best of circumstances. Trying to manage this problem with mobiles at 35,000 feet that are heard by dozens of different base stations is impossible.

Put 1 parabolic antenna at each tower pointing straight up into outer space, end of problem.

Crookshanks

join:2008-02-04
Binghamton, NY
reply to Omega

I covered that with the self-important asshat comment.


rradina

join:2000-08-08
Chesterfield, MO
reply to patcat88

Help me understand how that solves the problem. It might if the antenna and all aloft devices automatically use a different frequency. If not, how does a parabola stop the regular cell antennas from hearing airborne devices? Even if it was huge, in an attempt to block vertically transmitted signals from the tower's normal antennas, it wouldn't stop reflected signals.

If something like this is desired, it would be much more effective to use the same tower antennas and a different frequency when aloft. The greater range of airborne devices might mean fewer towers would need to be adapted and fewer hand offs. However, that would severely limit capacity and in heavily populated areas, there are a ton of planes with many thousands of passengers in the air.

Furthermore, unless the FCC frees up a ton of spectrum, no carrier is going to devote any such precious spectrum to airborne-only devices. They might if given a ridiculous amount of bandwidth at extremely high frequencies (5ghz, 10ghz) but at these frequencies, penetrating even a composite aircraft body would seriously reduce signal strength not to mention require more power. That would trash battery life. It might work if planes had built-in signal repeaters. However, we're talking about an industry that will eventually mag-stripe the lavatory, weigh the waste so they can charge by the ounce -- including the water you use to wash your hands -- and add a unit premium for solids! They aren't going to install free signal repeaters.


patcat88

join:2002-04-05
Jamaica, NY
kudos:1

2 edits

said by rradina:

Help me understand how that solves the problem. It might if the antenna and all aloft devices automatically use a different frequency. If not, how does a parabola stop the regular cell antennas from hearing airborne devices? Even if it was huge, in an attempt to block vertically transmitted signals from the tower's normal antennas, it wouldn't stop reflected signals.

Phone sees stronger sky facing antenna, not 90 degree weak edge of ground aimed sector antennas. Phone locks onto sky facing channel/psuedochannel (CDMA) antenna, reverse link power control lowers the phone's transmit power, end of problem. All cellphone networks have interference, the question is how much. The sky facing antennas do frequency reuse just like ground aimed ones.

»en.wikipedia.org/wiki/Frequency_···cy_reuse

The problem isn't new and its been solved years ago. Take a ferry or drive a car over a long causeway/viaduct in a heavily populated metro area, with skyscrapers. Your phone will see 100s of towers. Does your phone drop or loose service while on the ferry or driving on the suspension bridge? NO.

The BSS can figure out when a phone is seeing too many towers, and steer all the phones in the bay (water) to a particular channel/tower that is reserved for open water bay phones. A phone can not switch towers without the tower's/network's permission.

»en.wikipedia.org/wiki/Radio_reso···nagement

rradina

join:2000-08-08
Chesterfield, MO

If this works, why haven't they done this? Every aircraft has several (possibly half the phones) accidentally or purposely left on during flight. Theoretically, these phones are transmitting at maximum power trying to find cell towers. Wouldn't that unnecessarily cause a lot of interference for ground-based traffic? Wouldn't the industry want to solve it the way you suggested?


Crookshanks

join:2008-02-04
Binghamton, NY

2 edits
reply to patcat88

said by patcat88:

Take a ferry or drive a car over a long causeway/viaduct in a heavily populated metro area, with skyscrapers. Your phone will see 100s of towers.

Urban areas have very few macro cells that provide long range coverage. Micro and pico cells are far more common. The macro cells are used to service fast moving (e.g., phones in cars, on the train, etc.) or more distant mobiles, while the rest are directed to micro/pico cells that serve a much smaller footprint. In your scenario the phone would not hear more than a handful of towers, as it would quickly move out of range of the micro and pico cells.

said by patcat88:

A phone can not switch towers without the tower's/network's permission.

Cellular networks are divided into paging areas. Your mobile does not notify the network when it switches to a different base station within the same paging area, it only tells the network when it moves into a new paging area. The cellular network has no idea which specific base station an idle mobile is listening to, and it may well be listening to more than one at a time in a CDMA network. Asking for permission for every single base station hand off while idle would consume a lot of power.

The network can command a mobile to switch channels or base stations, but the mobile does not have to request permission to move between base stations while idle. An idle mobile doesn't even have its receiver turned on a majority of the time, it wakes up every so often to check for pages, then goes back to sleep. This is another power saving feature, and one of the reasons why LTE can be such a battery hog -- getting low latency requires keeping the receiver on more than older technologies.

Crookshanks

join:2008-02-04
Binghamton, NY
reply to rradina

Phones don't transmit while looking for towers, they only transmit during initial registration and when moving between paging areas. Short bursts of communications don't have much of an impact on the overall signal-to-noise ratio of the network. The problem only becomes apparent when the phone is transmitting with a full duty cycle, e.g., while in a call, or engaged in a large data upload.

As for why they don't do it the way he wants, because it would be a foolish waste of money? Retrofitting every single base station in the country and sacrificing precious spectrum to service mobiles on airlines makes no sense. Pico/femtocells on the aircraft would be the way to solve this "problem", if the industry and airlines were so inclined.


Rekrul

join:2007-04-21
Milford, CT
reply to Omega

said by Omega:

You missed the most important part. Allowing people to have voice conversations on their phone on a plane would be annoying as hell.

How would that be any more annoying than passengers talking to each other?


TSprky

@verizon.net

said by Rekrul:

said by Omega:

You missed the most important part. Allowing people to have voice conversations on their phone on a plane would be annoying as hell.

How would that be any more annoying than passengers talking to each other?

Because people talking on the phone use their "phone voice".


ka9q

@ka9q.net
reply to Omega

Annoying, perhaps. But why do we require the legal weight of the United States government to solve it? Isn't that killing a fly with a battleship? What ever happened to our ability to work out minor interpersonal differences like the adults we supposedly are? Is it really so hard to say "your phone calls are bothering me, please keep them brief"?

Be careful what you wish for. Someday, one of those people who are screaming the loudest for a continued ban on phone conversations will have a personal emergency that can only be dealt with via a phone call from an airplane.