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Effective line-of-sight range of CDMA?

Discussion in 'GENERAL Wireless Discussion' started by TKR, Feb 27, 2006.

  1. TKR

    TKR Senior Member
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    I am curious about the effective range a CDMA handset can reach a tower with. I assume the handset's limited power output is the limiting factor.

    I was on the Appalachian Trail this past weekend, and at a camping location at 4000 ft where at night you could clearly make out the "city" lights of the areas near the towns of Hiawassee and Helen, GA. I even could see the blinking lights of what I know are cell towers off in the distance near Hiawassee.

    However, my VI2300 had difficulty locking into a signal. It would occasionally pick up an unusable Sprint signal. Other times, it would vacilate between analog and CDMA 800 on two different Verizon SIDS (41 and 1129). I was able to make calls fairly reliably on analog, but only got one through once on CDMA, despite it showing a fairly strong signal (a couple of bars tyically, with dBs in the 80s or 90s at times). My brother's Cingular PDA had no difficulty with calls.

    So, my question is, why could I not make calls, and is CDMA limited even with clean line of sight?

    (now don't rag me for being on the AT and wanting to make calls - it was more a technical curiosity exercise!)
     
  2. Jay2TheRescue

    Jay2TheRescue Resident Spamslayer
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    I would think that even though you could see the cell towers and neighboring cities/towns below the panels on those towers are probably angled slightly downward to enhance coverage in those areas immediately surrounding the tower. The amount of signal that strays out of this range would most likely be weak and unusable.

    -Jay
     
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  3. TKR

    TKR Senior Member
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    I understand that antennas are designed/tuned for certain propogation patterns, and higher elvations are often left out of the design. (But I was getting a seemingly usable signal on the debug screen, I just could not connect very well.) That being the case, I've always wondered how it is that people can use their cell phones in flight, such as happened on 9/11/01.
     
  4. Jay2TheRescue

    Jay2TheRescue Resident Spamslayer
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    It is possible, and especially since the planes on 9/11 were being flown low-level in urban areas it is more likely to get signal. I also believe that some technologies operate better at high altitudes (I may be wrong on that) but I think GSM is more able to handle it for some reason. I've read about it somewhere, but I don't remember where now. That may explain why the Cingular phone was able to operate.

    -Jay
     
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  5. walkguru

    walkguru Wireless Guru
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    that just proves that gsm is better than cdma.



    or to put it another way, cingular is so much better than verison.:loony:
     
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  6. Andy

    Andy Diamond Senior Member
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    Maybe we can stay on the subject and not start another Cingular vs. Verizon fight my friend.
     
  7. Andy

    Andy Diamond Senior Member
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    I'm certainly no expert in this field but I will give it a start. In my honest opinion, this is one of the weaknesses of CDMA. If you're in a high altitute situation, your phone might very well show you a good signal in the debug screen as well as show plenty of bars on your phone. The problems comes in hand once you want to make a call( incoming voicemail notifications or even outgoing texts may work fine in these sitations) but when you want to make a call, your phone sees too many PN offsets and gets flooded and doesn't know which one to use. Basically, seeing too many PN offsets is bad for a CDMA phone. I hike a lot and have sometimes experienced this problem too. Interestingly I do not find myself having these problems hiking on the mountains facing the Salt Lake Valley where my phone can see countless offsets, but I have this problem in some high altitute rural areas where there are only a few offsets.
    GSM will work better in this situation because it doesn't use soft handoff the way CDMA does.
    I hope this is a good start and was accurate information. This was purely out of my head, but I'm sure one of the experts will be here shortly. :)
     
  8. Jay2TheRescue

    Jay2TheRescue Resident Spamslayer
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    I agree. This isn't a Verizon vs Cingular debate. We are discussing the technologies, and how they operate. Each has its own strengths & weaknesses. Last time WG was taken out behind the woodshed if you know what I mean...
     
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  9. TKR

    TKR Senior Member
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    Thanks for the input. I don't think there was a lot of pilot pollution in this case, as I had a fairly good Ec/Io ratio. Maybe it is pilot pullution is some cases, but in this one (sparse area of far north Georgia) I do not think that was the case. I suspect a simple range issue. Maybe not?
     
  10. Andy

    Andy Diamond Senior Member
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    My guess is no; the phone was seeing too many offsets and couldn't decide which one to use. Anyone else have any input?
     
  11. jones

    jones Silver Senior Member
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    :browani: i can agree. :p
     
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  12. Bugwart

    Bugwart Bronze Senior Member
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    Regardless of the service, a handset can detect the signal from a tower when it is too far away for the handset to connect to that tower. Any handset transmits a weaker signal than the tower. The tower will use a higher gain antenna than the antenna in your handset.

    Since received power is proportional to the square of the distance from the transmitter, the tower will receive a very weak signal versus what the handset is receiving.

    Verizon still uses analog in rural areas. The more rural the location, the less likley that the signal will be CDMA.
     
  13. Andy

    Andy Diamond Senior Member
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    I'm not exactly sure as to what you are trying to say, Bugwart.

    If you are in a high altitute location, whether urban or non-urban, and your phone will not connect on CDMA, even though it seems to have a good signal, I have had my phone connect on AMPS in these situations without a problem.

    All of Verizon's transmission sites have CDMA digital, so I'm not sure if you were trying to say what I stated above.
     
  14. jones

    jones Silver Senior Member
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    could be, that's what some say here.
     
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  15. bobolito

    bobolito Diamond Senior Member
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    One possibility is that since there are more digital connections in the air, that increases the likelyhood of interference at longer distances, whereas analog channels will be clean and interference free. Remember that Analog necessarily has to use different frequency channels than CDMA even if they are on the same sector/tower. It all depends on the frequency allocation plan designed for the area. Normally, analog will allow for longer distances because channels are cleaner, meaning that nobody is using those frequencies.

    The same thing can happen in digital. If the frequency reuse plan on the network allows for your specific area to have a very clean (interference free) signal in your particular channel, you can go much longer distances than you can imagine. 9 out of 10 times the reason why a signal becomes useless (even if you have two bars) is because of interference from adjacent sites and this usually happens when you are too far from the site. This is especially common at high altitudes.

    As for the Cingular phone that was working fine, just refer to what I said previously. The channel they were using was probably just clean and didn't have trouble communicating with the tower.

    By the way, before someone makes the mistake of mentioning it, there is no such thing as one technology having better range than the other. We're talking about radiofrequencies here and the RF reach is the same regardless. So, this applies to CDMA as well as GSM. That being said, the variables that cause one signal to reach longer than other are different, not the technology. It's true that GSM has a theoretical usable limit of 22 miles from the nearest tower, but that doesn't mean the signal cannot go beyond this limit. The signal will continue traveling, but the slot timing will be too far off to allow communication. It has nothing to do with RF range.

    The most common usable range limiters are three:

    1) Interference, interference, interference: This is the most common range limiter in any network. If your phone displays 2 bars but you can't make calls, I bet you it is interference!

    2) Directional panels are not aimed your way: Sometimes this happens and it can mean the difference between having a signal reach your basement or not.

    3) Power output of the site: Cell sites have different configurations and are designed to cover a specific area. Outside that area, the carrier doesn't expect you can use your phone reliably. Power output, height and positioning of the site are the prime determinants of the coverage area of the site and each carrier has its own unique site configuration. This can determine why one carrier works in one area and another one doesn't even through the towers may be at the same distance.

    While not that common, other factors are, the quality of the phone antenna, inability of the site to receive a signal from the phone even though the phone receives a signal from the tower, and the audio codec error correction robustness. Believe it or not, if the codec has weak error correction, it is the first thing that kills your usable range when there's interference.

    Finally, for the trolls who attempt to hijack this thread, just remember how they were banned in the other forum for doing the same. It could happen again. You know who I am referring to.
     
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    #15 bobolito, Mar 1, 2006
    Last edited: Mar 1, 2006
  16. TKR

    TKR Senior Member
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    Bobolito - you referred to codec error correction. I am not familiar with this. Is it an algorithem that takes an imperfect data stream (say it's corrupted due to interference or whatever reason) and attempts to rectify it by synthesizing what it thinks the missing/corrupted data elements in the stream most liklely were?

    I know the NTSB uses something perhaps similar for older flight recorders that use physical magnetic tapes for the data record. There are synch "words" in the data stream and occasionaly due to physical flight forces or other events like electrical interrupts, momentary data is "lost" due to loss of synch. The algorithem they use attempts to intelligently rebuild the data despite the lost synch. Same concept?
     
  17. bobolito

    bobolito Diamond Senior Member
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    Not exclusively. I remember some older codecs used that idea and they never did a good job. The voice ended sounding "robotized" as you would expect.

    In this case, what happens is, as with most error correction methods, data is transmitted with redundant data that allows for reconstruction of the lost data if packets are lost. In other words, alongside with the stream, there's an additional portion of the bandwidth that is allocated to this redundant data. If the decoder doesn't need it, it is simply discarded.

    With AMR, the amount of data redundancy versus the actual compressed audio stream is constantly traded off depending on network conditions. The idea is, you have a fixed bandwidth channel and within this channel you can either transmit more audio stream and less error correction data, or you can transmit more error correction data and less audio stream at the expense of audio quality. The worse the RF conditions, the more redundancy the transmission carries. However, this reduces the effective audio bitrate which causes degradation in audio quality. But still, that's much better than having unintelligible garbled audio, right? ;)

    AMR does such an excellent job at this, that I've had crystal clear calls at -110dBm, something that was impossible with the old EFR GSM codec.
     
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  18. TKR

    TKR Senior Member
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    Interesting. Thanks for the explanation!

    I guess that is kind of like XM broadcasts (duplicate streams for the purpose of keeping a signal due to going under bridges, etc), only much more compressed and real time since it is a live give and take conversation, not a delayed broadcast. (XM's signal as played on the receiver is about 14 seconds behind normal live broadcast of the same channels, such a radio disney) I don't know, maybe that is a bad analogy, but I got what you were saying anyway.
     
  19. bobolito

    bobolito Diamond Senior Member
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    CDMA (and all CDMA-based technologies) have an inherent advantage in this area as they can use signal from more than one tower to recover lost packets from the main link. Let's just clarify that some people confuse this idea with the perception that CDMA can take signals from different towers and somehow add them to give you stronger reception. Please, let's not fall for that mistake. Picking up two CDMA towers gives you redundancy and a more robust error-free connection, not a stronger signal.

    Most digital TV and satellite broadcast standards where multiplexing is used, employ some sort of CDMA-like method. This includes DVB and DSS for satellite networks and DOCSIS for digital cable TV (HFC) networks which can use S-CDMA although in some systems it can use TDMA. Also, your digital cordless phone probably uses some sort of DSS method to communicate with the base and let's not forget WiFi which uses a 20Mhz wide spread spectrum channel very similar in nature to CDMA.

    With all those land based repeaters, I wouldn't be surprised both XM and Sirius use a CDMA-like method. It would be impossible to do something like that on a TDMA based system. However, let's not forget that even though it is considered a live broadcast, digital broadcasting needs some sort of buffering which increases the encode/decode delay. DirecTV (which uses DSS) and Dish Network (which uses DVB) both carry a 4 - 5 second delay due to this and even terrestrial digital over the air TV broadcasting (SDTV and HDTV) have about the same amount of delay. HD FM Radio will be the same way.

    With all these advances, still the closest to real-time live TV is still analog broadcasting! ;)
     
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    #19 bobolito, Mar 2, 2006
    Last edited: Mar 2, 2006
  20. Jay2TheRescue

    Jay2TheRescue Resident Spamslayer
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    This beast really exists? I heard them talking about it on a local station the other day and I thought it was a joke.

    -Jay
     
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  21. TKR

    TKR Senior Member
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    As I understand it, because of signal compression, digital radio, such a XM and Sirius, do not approach the sound fidelity that CAN be achieved by a well tuned analog FM HIFi broadcast, or of course the level achieved from a traditional CD. But it does sound good, and of course sattelite gives you national coverage without static and other challenges of FM. I suppose that is the concept behind needing "HD" radio.
     
  22. bobolito

    bobolito Diamond Senior Member
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    Jay, yes, HD Radio is a reality at least here in NYC. We have several stations already broadcasting in HD. I read the same project will be implemented for AM radio stations. For the first time AM will sound better than FM! ;)

    But I find the term HD innacurate for this type of radio. HD Radio is nothing more than digital broadcasting over the FM band. Nothing else.

    TKR, that is absolutely true about Sirius. I find their fidelity to be inferior to that of a well tuned Analog FM station, but it's not a big difference. However, I find XM to sound superior to FM stations and I think it approaches CD quality, but not quite there. I'm sure it has to do with their streaming rate. If Sirius used a higher streaming rate, they could sound just like a CD. But I guess their efforts to save bandwidth (to carry more stations per transponder) cuts down on their streaming rate. With Sirius, you can actually hear a very faint gargling that normally occurs on highly compressed digital audio.
     
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  23. John Sprung

    John Sprung New Member

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    > Most digital TV and satellite broadcast standards where multiplexing is used, employ some sort of CDMA-like method. This includes DVB and DSS ....

    The European DVB standard uses COFDM -- Coded Orthogonal Frequency Division Multiplexing. That's probably sort of CDMA-like. Here in the USA, the ATSC digital TV system uses 8 level Vestigial Side Band, AKA 8-VSB, decidedly non-CDMA-like.

    Delay is a big mess in digital TV. Often, the video and audio get sent thru separate paths, like to super a graphic or something. Thats how you can watch a TV news guy live on camera with a microphone in his hand, and he's talking out of sync.

    Back to the OP's question, so far nobody has considered (unless I missed it) that he's far from the city that's covered by cells, so the situation violates the basic premise of cellular architecture.

    As I understand it, each cell is supposed to have at most six neighbors, and those neighbors must use different frequencies than the first cell. Therefore, seven (7) sets of frequencies are used. The layout is kinda like old fashioned bathroom floor tile, a bunch of little hexagons.

    If you go outside the area that's covered with the hexagon tile cells, it's possible that your phone is getting signals from non-adjacent cells, and you might be seeing two sites that are on the same set of frequencies. They would probably interfere with each other and confuse the poor phone.

    Even if the non-adjacent cells are on different frequencies, the hand-off protocols wouldn't know what to do. Both signals being weak, the system would probably try to do a hand-off, and drop the call instead.

    Of course, I'm very new to this stuff. Please let me know if I got this wrong.



    -- J.S.
     
  24. bobolito

    bobolito Diamond Senior Member
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    That sounds pretty much correct.
     
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  25. Adkraus1

    Adkraus1 New Member

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    Verizon told me that the signal from their towers would be good for up to 8 miles.
     
  26. Andy

    Andy Diamond Senior Member
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    Depending on the location of the tower and the surroundings it may be more than that. I know some areas in the Utah West desert where the signal comes from far more than 8 miles away.
     
  27. Gamer03

    Gamer03 Technology Aficionado
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    When we used to have our old Sprint phones, and we went for vacation out west (the state at the time where the following happened was Colorado), I received a signal from a tower that was approximately 10 miles away (I was able to make calls with no problem).

    My uncle uses his Verizon phone when he is out on Lake Michigan on his boat (he can call us and we can call him when he is approximately 9 miles off shore).
     
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  28. John Sprung

    John Sprung New Member

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    Water makes a really nice ground plane, and wave patterns on the surface of the water can do some unusual things. When the wind and waves are just right, TV can jump around between England, Ireland, and France.



    -- J.S.
     
  29. maximus

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    I've routinely picked up a usable CDMA signal from 10-12 miles away. Pretty sure this was in my basement.
     
  30. Andy

    Andy Diamond Senior Member
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    I was out in the boonies today. Used a signal from a cellsite 30+ miles away without dropping the call/dropouts.
     

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