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PN Offset Question

Discussion in 'GENERAL Wireless Discussion' started by ChaosThyre81, Apr 7, 2005.

  1. ChaosThyre81

    ChaosThyre81 Member
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    "CDMA uses something called Spread Spectrum, where it can use more than one PN offset at one time, completing your voice/data calls using up to 3(or 6 in some markets) different PN offsets. This has many advantages over GSM(in my opinion), but also some disadvantages, especially if the network is not set up correctly."
    Hope this helps.

    My Samsung SCH-A310 shows a PN Offset of 180 and changes between 172 and 180. According to what I read, CDMA can use 3 to 6 different PN offsets to complete a call. So, why doesnt it show all the PN offsets it's using as opposed to one? Also when my phone idles, is the PN Offset it's 'listening' to the paging channel? I'm unclear on this.

    Also, when the phone is first powered on and it 'listens' to the PN offets, does it select the strongest one then send out a signal to that tower letting it know it's online?

    Thanks!

    Mike.
     
  2. larry

    larry Sprint loyalist and former mod
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    Actually the Sprint Audiovox PM-8920 does show all of the PN's your phone is connected to (dominant plus the 3 in the neighbor list). But most brands only show the one you happen to be on without showing the neighbor list.

    Sometimes I believe that using more than one PN offset/tower at a time is a disadvantage for CDMA. It can cause missed calls as the network can become confused as to where you are if the neighbor list is too big or some weaker offsets are not dropping out at the right signal/noise level. In other words the network requires more planning and thought and there can be mistakes made when setting up neighbor lists and such.
     
  3. ChaosThyre81

    ChaosThyre81 Member
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    So if a phone is using 3 PN offsets and it only shows one, does it just show the dominant one? If it's using 3 PN Offsets, is the phone being controlled by 3 different control channels?

    I hear ya on the missed calls, my phone does that from time to time. It's annoying.

    Thanks,

    M.
     
  4. Andy

    Andy Diamond Senior Member
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    Using more than one PN offset can be a problem, but it also has many advantages like combing the signal of various sites to make it stronger.
    Here's a question I have. If I talk to someone driving in the car and my phone is switching PN offets(using 3 at the same time), what happens if one of the 3 that my phone is using/wants to use doesn't have enough T-1 lines available to process my call. Does that PN offset just drop out or will my whole call be dropped at this time?
     
  5. larry

    larry Sprint loyalist and former mod
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    Good question. There's a couple of guys on Howards that know all of this CDMA technical stuff but not sure if anyone here can give an answer. I would guess the PN would drop out of the list without the call dropping.
     
  6. Andy

    Andy Diamond Senior Member
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    That's what I would have guessed-if you ever find out the answer, though, please let me know.
     
  7. SNSE

    SNSE Senior Member
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    The network will not allow the phone to do soft-handoff into a cell/sector that is congested. It's not about T-1's in CDMA. It's about Channel Elements and Vocoders. There's not this 1:1 mapping like you think of with Amps or TDMA, or GSM for that matter for conversations. Back to your question, the phone, when it sees a candidate for soft handoff, will send a request for resources on the reverse access channel of the cell, and say "Hey, got a call going, need soft handoff resources". If there are resources available, the network adds that cell to the mix. You are actually talking to 3 cells AT THE SAME TIME, meaning, if one drops, you are still talking to 2 other cells, so your call does not fail.

    Hope that answers it for ya.
     
  8. Andy

    Andy Diamond Senior Member
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    Thanks, SNSE, that explains a lot. So am I understanding it correctly, that if I'm on a call and the phone sends out a 'page' asking the site if resources for a soft handoff are available, the cell takes into consideration that it's T-1 lines might be congested and will not let the phone use it if that's the case, correct?
     
  9. Andy

    Andy Diamond Senior Member
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    Another question I just came up with(sorry, lol).
    Sometimes I field test while standing still and on a call. The phone uses 1 sector and the Noise Level is quite good, but then, all of a sudden, the phone starts using two other sectors and the noise level skyrockets going up to around -20 or so and calls get more choppy. Why would the phone/network do that?
     
  10. larry

    larry Sprint loyalist and former mod
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    Andy,

    I've experienced the exact same thing. You would think by now that CDMA technology could have evolved so that the phone stays locked on the best sector with the lowest noise level instead of bouncing around.
     
  11. MeatChicken

    MeatChicken Senior Member
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    A CDMA phone, once a call is placed, starts out on the strongest sector P/N, & then is downloaded a "neighbor list" of adjacent sites/sectors pn #s, that the phone is "allowed" to request a soft handoff with.
    The phone's built in rake receiver is always scanning for these neighbors, & if 1 or more of these p/n's signal strenth goes above a pre-determined level known to the phone, (called "T-add"), the handeset "requests" to the system that it becomes an "active" P/N. The system can approve or deny, & can be set to allow up to 6 active, although most systems are actually set to 3, 4 or 5.
    Likewise, if a currently used active P/N drops below a pre-determied level (T-Drop), It is dropped from the active set.
    If more p/n's than is allowed are above T-add, they become cadidates that can be added if the phone/system decides to delete a current active.
    As the phone adds/deletes sectors during a call, this causes the neighbor list to be updated to include "new" candidates, & remove old.
    A P/N not in the neighbor list, even if strong, is ignored & can't be used or requested.
    More of the call decisions are "handset based" rather than "Tower based" in a CDMA system. The phone tells the system what sectors it can "see" clearly, & which are above/below system level requirements, vs. The tower making these decisions for the phone. This is generally considered an advantage in weak signal areas with CDMA.
    When the phone is Idle, not in-call, there is no neighbor list, the phone's rake receiver simply scans & hangs onto the nearest p/n signal on it's assigned frequency, & may bounce p/n's slightly, but the "action" begins after the send button is pressed.
     
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  12. larry

    larry Sprint loyalist and former mod
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    Great post! Thanks.
     
  13. Andy

    Andy Diamond Senior Member
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    Awesome, that explains a lot, Thanks! CDMA is a very complex and complicated system as I see.
     
  14. larry

    larry Sprint loyalist and former mod
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    I still don't understand why the phones will sometimes prefer a weaker PN offset with a much higher EC/IO than one much stronger with a much lower EC/IO. This is where CDMA technology could use some updating and improvement.
     
  15. ChaosThyre81

    ChaosThyre81 Member
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    "When the phone is Idle, not in-call, there is no neighbor list, the phone's rake receiver simply scans & hangs onto the nearest p/n signal on it's assigned frequency, & may bounce p/n's slightly, but the "action" begins after the send button is pressed."

    And as for 'assigned frequency' I thought all CDMA phones ran on the same frequency?

    Out of curiousity, if the system pages a phone and there is no response, I'm under the impression that the system 'Flood Pages' the phone from multiple towers. But, if the phone hangs onto the nearest p/n signal on it's assigned frequency, then how can another tower page it? Maybe I'm unclear how the paging thing works? Can anyone clear this up? Thanks!

    M.
     
  16. MeatChicken

    MeatChicken Senior Member
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    The carrier's licencesed band (CDMA) is seperated into approx 1.25Mhz wide "chunks" or assigned CDMA frequencies. The Company decides how many "chunks" of spectrum it needs to deploy for CDMA in it's market.
    In some rural areas, There may still be only 1 channel (chunk) in use, in which case all phones using the system are on the same assigned frequency.
    In most larger systems, there are multiple channels up & running, & the system then "assingns" or "hashes" phones to a channel, based on the phones MIN & ESN, to spread the many users across the usable "frequency chunks"
    Each 1.25Mhz wide Freq channel can be thought of as it's own mini-CDMA system. Soft handoff is only possible within the same assigned channel, & yes, there could be (& are) thousands or more handsets each simultaneously assigned to the same frequency . If a carrier has deployed 8 CDMA frequency channel "chunks", then all users of that system are divided up between the 8.
    Phones tend to stay on their assigned channel during both calls & idle, but the system can do a hard handoff to another frequency channel if nessessary.
    As far as paging the phone, different systems handle that in different ways. Idle phones can be instructed to "register" with the system as the pass from one area of the system to another, so that the system knows what cluster of cells the phone is idle within, & therefore what area to page/not page. Some smaller systems may send a page signal out from all the cell sites, & wait for the phone to respond back.
     
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  17. Andy

    Andy Diamond Senior Member
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    Yes, I still wonder the same thing. My phone doesn't really hang out to a higher ec/io PN offset, but it tends to bounce to it, then the signal goes down from 5 bars to 1, and slowly comes back up.
     
  18. Andy

    Andy Diamond Senior Member
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    so during a call it is possible to hard hand off to a different channel in case it is needed, capacity wise? This means that, in some areas, a specific channel might not have enough capacity and my phone will hard hand off to another channel, correct? If I travel into an area where my normally assigned channel is busy/at capacity, will my phone then be forced to idle on a difference channel or will I just miss calls?
     
  19. MeatChicken

    MeatChicken Senior Member
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    Yes CDMA systems can, & will do what you describe.
    Here in NY/NJ, VZW has added voice channels at 1900Mhz to their 800Mhz SYs ID 22 sytem, but not all sites across the system have yet had 1900 installed.
    I have personally (more than once), initiated a call while on 1900Mhz PCS channel (chunk) 825, & then driven into an an area that has not yet had 1900 installed at the cell sites, & have watched my test sceens as the hard handoff to 800Mhz occured, & the call continued on an 800Mhz channel.
    I , of course, did this on purpose to "see" a CDMA hard handoff, which almost never happens under "normal" conditions.
     
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  20. Andy

    Andy Diamond Senior Member
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    Yeah-hard handoffs in CDMA rarely occur, I know that much ;)
    Say, have all/most capacity issues people were reporting in some areas of NY/NJ now disappeared with 1900 being added for voice/data usage?
     
  21. larry

    larry Sprint loyalist and former mod
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    Andy,

    Sounds like what I've observed. Problem is when an incoming call comes in during one of these bounces I have missed calls before. Also if I try to place a call while on the weaker offset with high EC/IO the call will take longer to connect and sometimes even has failed all together and I had to start the call over again. Also if I try to send a text message while landed on the weaker offset I have received "network not responding' messages. The difference between the EC/IO of the good and bad PN offsets that I land on is about 07 to 31!! I suspect that being in the hills is the reason behind this erratic behavior but still you would think that modern CDMA technology should be able to filter out this type of BS.
     
  22. Andy

    Andy Diamond Senior Member
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    Yes-you'd think that modern CDMA technology would be able to handle it.
    Same thing happens to me. When I'm on a more polluted PN Offset, calls take much longer to connect, sometimes in an area where there are many strong offsets my phone can also take its time to connect since I guess it doesn't know which offset to use as its primary.
    Sending SMS, same thing has happened to me as you described, it takes forever to send them sometimes as the phone bounces between offsets.
     
  23. ChaosThyre81

    ChaosThyre81 Member
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    What's the point of having a PRL if when the phone makes a call it is given a list of neighboring cells that it can handoff too? What's the point of the PRL?

    Also, is it possible for the handset to listen to two different channels from two different towers at the same time via rake receiver? Or if it hard handoffs to a different channel, can it only use that same channel with other towers using that channel?

    M.
     
  24. MeatChicken

    MeatChicken Senior Member
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    To answer your 2 questions;
    1 - The PRL has Nothing to do with individual towers or cell sites. It is simply a "Preferred ROAMING List, or a list of home, and "other cell companies" system ID's (NOT tower or sector info), that the fone can hunt/scan for, after it loses or can't find home service/system.
    (the PRL is telling the phone, for example - "If you lose VZW signal, you are allowed to search & lock onto only Alltel or Sprint in this market, not US cellular or Metro PCS") - nothing to do with neighbor or individual towers, just other "approved roaming companies" the phone can/can't use.
    The neighbor list is a "temporary" list of adjcent cell sectors within the current system, that is sent to the phone only while a call is in progress, so it knows what sites to scan & possibly request handoff with during that call.
    The neighbor list constantly changes, as the phone adds new sectors & loses others during a call, each new sector added will have some "new" neighbors the the others didn't, & old neighbors are dropped with the lost sector(s).
    2 - No, the rake receiver can't listen to 2 different freq channels. Once it hard handoffs to a different channel, it then can scan & utilize sector p/n's on that new channel, & it is no longer on the old & vice/versa.
    Generally CDMA cell sites within a system will all have the same channels, with some phones assigned to one vs another. This way fones can seamlessly soft handoff between all sites in the system. There are select "border" areas where a channel may no longer be in the upcomming cells, & this is where hard handoff would occur, but these areas are rare, & may not even exist in many CDMA systems.
     
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    #24 MeatChicken, Apr 15, 2005
    Last edited: Apr 15, 2005
  25. ChaosThyre81

    ChaosThyre81 Member
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    Hey! Thanks for the reply. I appreciate your time typing that out for me. I am curious, do GSM phones get a list of neighboring towers when you initiate a call? Or is it different? If so, how?

    Also, in analog mode my Samsung SCH-A310 is on channel 319 but then varies each time I make a call. So, is channel 319 the paging channel? Or the strongest channel on the tower that my phone is picking up before it makes a call?

    Also, when on a call, does the signal strength indicate the control channel strength or the channel the phone is using? Obviously when it's idle it's monitoring the paging channel I assume?? Also, while on a call and say you get call waiting coming in, does the control channel tell the phone this info or is it sent thru with the information on the voice channel?

    Do CDMA phones send out info on many different frequencies and use frequency hopping? I'm thinking a sector sends out the info to all the phones using the same frequency using walsh coding? Know of a good book to read that starts with the basics on radio waves and cellular technologies and moves up into more complicated stuff?

    And when the phone is first turned on, what exactly happens? I take it that all phones share the same frequency on the reverse link?

    Thanks!

    Mike
     
  26. MeatChicken

    MeatChicken Senior Member
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    Check out this site:
    http://www.cdmaonline.com/members/dl/DLcdma02.htm
    There are over 100 Q&A's in audio/video format, & it should be a big help & goes into good detail.
    To briefly answer some of your above questions;
    Analog & GSM phones are assigned a voice channel when needed, & it may be different each time the phone is used, based on what's available & what sectors the phone is on. All handoffs are "hard" to another channel/freq in GSM/Analog.
    CDMA does not use multiple frequiencies & hopping. Its all walsh codes/PN offsets within the same freq assignment.
    All phones (within their system frq assignment), share the same forward & reverse link frequencies.
    The signal strength meter is a fairly good indication of usable signal, & will firgure strength based on more than 1 sites signal if needed, just as the phone can benefit from being in multple cell soft handoff mode.
    When a CDMA phone is first turned on, it needs to "aquire the system" / Pilot channel, once that occurs it registers & receives various messages from the pilot channel, that give the phone various needed parameters of the system, & tell the phone weather to stay put, or park on a different freq assignment within the system. :)
     
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  27. ChaosThyre81

    ChaosThyre81 Member
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    So when a GSM phone is on a call, it doesn't get a list of neighboring towers that it can handoff to? How does it know which tower to handoff to then? Or is it more tower based handoffs as opposed to handset based handoffs?

    Also, on CDMA when I place a call and my phone sends out a 'message' that it's attempting to make a call, do all towers within range 'listen' to this 'message' and then the system picks which tower(s) is the best candidate to take the call?

    So when the phone is idle does the signal strength meter indicate the strength of the paging channel? Or is it monitoring the strongest PN offset?

    Thanks for the link!

    Mike.
     
  28. MeatChicken

    MeatChicken Senior Member
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    I'm not sure about GSM, but they don't have "neighbor list", although I'm sure there is something similar, weather tower or handset based.
    I believe w/ CDMA your call initiates on the site the phone was currently locked onto while idle, & then can be put into handoff w/other sites after the neighbor list is sent to the phone. That website can explain this in detail.
    I'm not sure, but I don't think it is the paging channel that the meter is using for its' readings.
     
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  29. MeatChicken

    MeatChicken Senior Member
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    ... Just to add & be presise, the Neighbor list is sent over the paging channel, & Idle phones do receive it, but It isn't used for Idle handoffs the same way it is during a call. The idle phone simply scans the p/n pilots & parks on the strongest, there is no requests to the system for soft handoffs or active set.
    Also, the paging channel is still capable of sending messages to a phone, even if all the voice channel codes are "full to capacity" on that sector/freq, so when an incomming or outgoing call is initiated on a sector/freq that is "full" , the paging channel will send a message that sets up the call & sends the phone to another frequency channel to begin the call.
     
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  30. Andy

    Andy Diamond Senior Member
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    I was just going to ask that question ;)
    So, a CDMA phone will idle on a sector/channel that is at capacity, but if it receives a page for an incoming call it will switch to a channel and/or sector that has available space, correct? That's great technology, since GSM just stays parked on the full sector.
    Thank you so much for all you explanations!!!
     

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