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pbcrazyboy
EVDO Fledgling
Joined: 04 May 2008
Posts: 25
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 Posted: Sat Jun 14, 2008 6:12 pm Post subject: Why does the signal always change? |
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| Why does the signal always change? Is it something to do with with the power output of our computer or the tower or does it happen to everyone? |
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Border Target
EVDO User
Joined: 16 Apr 2007
Posts: 26
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| Posted: Mon Jun 16, 2008 9:04 pm Post subject: |
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I'm going to throw some general info in here so hopefully it will make my answer clear.
A typical cell site is made up of multiple antennas that are pointed in different directions to help shape the signal. These are referred to as "sectors." Each sector has a unique identifier called a "PN" (psudeo noise or sometimes pilot number) A phone is said to "hand-off" when it is passed from one sector to another. Signal strength is strongest the closer to zero it is. Typically right under the site you see in the -55dBm range and once you reach ~-104dBm you are at the other limits and will likely drop.
EVDO speeds are greatest when the user is close to the site and in the center beam of that sector. In this situation there should be no hand-off occurring. If you moved directly away from the tower and remain in the main path of that sector you would normally see your speeds decrease as your signal level decreased. Eventually hitting the 1xrtt(Non-power vision) trigger of ~ -98dBm. Once your signal is beyond the trigger you will no longer be able to establish on EVDO.
Your device is always trying to find and use the strongest PN. Even while active the device constantly scans for a better signal from surrounding sites. When the device "talks" to the strongest site, it receives instructions on how to behave. One of the main instructions it receives is a neighbor list telling it which nearby PNs are "ok" to hand-off to. If the device finds a PN that is stronger then that of the current sector it switches to the new PN. While in normal voice CDMA this isn't a big deal because the call will establish on both sectors at the same time, EVDO is a different beast. It uses one and only one sector at a time. Handing back and forth between two sectors constantly will cause slow speeds.
The signal can fluctuate for several different reasons. Objects moving into the path between your device and the sector it's serving on. Heavy loading on a site can sometimes cause it to limit the amount of power its expending. Faulty hardware, radio transmitters, etc. Improperly set up neighbor list. I'm sure I could think of a few more but you get the picture.
The most common cause of your signal strength changing is due to a weak coverage area. Without a dominate PN you are prone to hand-offs. |
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Jim_in_VA
EVDO Junkie
Joined: 09 Apr 2007
Posts: 846
Location: On the Chesapeake Bay, Virginia
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| Posted: Tue Jun 17, 2008 5:13 am Post subject: |
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Good explanation Border Target, I couldn't have said it better.
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evdo-tips.com |
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Alex
Site Admin
Joined: 19 Sep 2006
Posts: 3074
Location: Dallas, TX
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| Posted: Tue Jun 17, 2008 7:06 am Post subject: |
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Border Target... that was awesome.
i give you advance warning that i will re-use and forget to give credit.
mostly because i'm aging badly, and will forget who the real source was, quickly.
Any chance this info is actually from some official/technical guide i can simply link to?
really. awesome. stuff. |
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n6gn
EVDO Junkie
Joined: 22 Aug 2006
Posts: 571
Location: Northern California
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| Posted: Tue Jun 17, 2008 10:35 am Post subject: |
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| Border Target wrote: |
EVDO speeds are greatest when the user is close to the site and in the center beam of that sector. In this situation there should be no hand-off occurring. If you moved directly away from the tower and remain in the main path of that sector you would normally see your speeds decrease as your signal level decreased. Eventually hitting the 1xrtt(Non-power vision) trigger of ~ -98dBm. Once your signal is beyond the trigger you will no longer be able to establish on EVDO.. |
Actually EVDO can "back off" to much slower speed than (full rate) 1xRTT - down to ~35 kbps or so, I don't have the precise number in front of me.
While most sites that have EVDO also have 1xRTT as well, the two are really different systems and on different RF carrier frequencies.
EVDO speed, as seen by the user is actually an average of an awful lot of things. For voice, handsets and bases are negotiating power levels hundreds of times per second. For EVDO, the full base power is run constantly. The data rate is related in a complex way to the received power level and the signal quality. Thus, what the user sees is a time average of the effects of a lot of little variations.
At the physical level, the signal is likely to be changing very rapidly for non-LOS signals. This is because of multipath signals; multiple routes the signal can take before adding together at the radio's antenna (as Border Target said above). Signal level can change 10's of dB for a fraction of a wavelength change in position - or in the position of the obstacles and clutter that are providing one or more of the paths. At PCS a wavelength is about 6" so even one inch change can make a huge difference under some conditions. Thus in windy conditions or when the handset is moving, it is easy for the signal (and power levels) to adjust hundreds of times every second. The data rates may also be rapidly changing so the combined rate is a statistical sort of thing and hard to predict exactly. For uplink, the handset typically runs out of power at about 200 milliwatts (+23 dBm) and when the radio path gets bad enough, if communications is to be maintained, the only alternative is to reduce the data rate, as for the downlink.
| Border Target wrote: |
The most common cause of your signal strength changing is due to a weak coverage area. Without a dominate PN you are prone to hand-offs. |
It's not so much the size of the signal but the degree of clutter and indirection in the paths. Weak signals occur because we don't have LOS to the base. If we did have LOS, even at the edge of a cell - usually about 19 miles away- we would have more than enough signal and the signal would be stable (as long as we kept the device's antenna well located, polarized vertically, etc).
It's the clutter, obstructions and combination of (weakened) signals from multiple paths adding and subtracting together that causes the extreme variations in signal strength. It's for this reason that location of the antenna (external or internal) is probably the single most important factor in realworld communication.
As mentioned above, these variations can happen *very* quickly, especially for a moving handset, and most reporting tools; RSSI indicators or speed tests, only average over many seconds.
n6gn |
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Border Target
EVDO User
Joined: 16 Apr 2007
Posts: 26
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| Posted: Tue Jun 17, 2008 2:46 pm Post subject: |
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| Alex wrote: | Border Target... that was awesome.
i give you advance warning that i will re-use and forget to give credit.
mostly because i'm aging badly, and will forget who the real source was, quickly.
Any chance this info is actually from some official/technical guide i can simply link to?
really. awesome. stuff. |
Feel free to re-use however you want. Sorry, nothing I can give out or give a link to. |
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pbcrazyboy
EVDO Fledgling
Joined: 04 May 2008
Posts: 25
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| Posted: Thu Jul 03, 2008 8:22 pm Post subject: |
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| So why would the strength of the pn change so much. Does the power output of the tower change from time to time? |
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Dellman
EVDO User
Joined: 24 Dec 2007
Posts: 44
Location: NW Ohio
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| Posted: Sun Oct 12, 2008 6:44 pm Post subject: |
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| This is rather interesting as I'm having problems with my SCM staying connected to EVDO. The signal bars fluctuate between really good (4-6 bars) to no bars and then to Rx1TT signals. It does this on and off all the time now. I havent been able to figure out what the deal is as I'm in a really good EVDO coverage area. |
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Craized
EVDO Newbie
Joined: 01 Nov 2007
Posts: 7
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| Posted: Wed Oct 15, 2008 12:16 pm Post subject: |
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| Dellman wrote: | | This is rather interesting as I'm having problems with my SCM staying connected to EVDO. The signal bars fluctuate between really good (4-6 bars) to no bars and then to Rx1TT signals. It does this on and off all the time now. I havent been able to figure out what the deal is as I'm in a really good EVDO coverage area. |
Same EXACT problem I'm having. It just started sometime last week, before that I always had around -90dBm RevA but now I'm constantly getting dropped to 1xrtt. When I'm disconnected it still shows -90dBm RevA but almost immediately after I connect it drops me to 1xrtt. (sometimes I can get RevA for a few minutes, but usually it just drops me right away)
I'm from Ohio too, but I'm on the east side... I wonder if something is wrong with Sprint's network in Ohio? |
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falsedragon
EVDO User
Joined: 13 Oct 2008
Posts: 45
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| Posted: Wed Oct 15, 2008 9:29 pm Post subject: |
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| Quote: | | Why does the signal always change? Is it something to do with with the power output of our computer or the tower or does it happen to everyone? |
The power does change all the time. The max power can be up to 20watts. But depends on the Cell's load, to how close to that max power it is. Read a little on Closed Loop Gain Control for further explanation.
But the real question is "Why does it always Change". I would say... Welcome to RF. Like N6 says. this could be a lot of reasons.
Now, the cell (or the phone) does not use RSSI as the measurement of quality. It uses EC/IO for that. Typically a PN becomes a candidate around -17 and will become active around -12. If i remember right, DO is a little stricter on this. Also, RSSI is not really a measurement of the channel power, but Pilot and Page. This tends to be quite lower. |
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TraderStavros
EVDO Newbie
Joined: 26 Sep 2008
Posts: 2
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| Posted: Wed Jan 21, 2009 1:12 pm Post subject: |
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Here's an interesting add to the conversation. I tether my HTC Mogul on the top floor of my office building in downtown Detroit. I get EVDO (most of the time) until I start ONE program and then it drops to 1xRTT most of the time. It is a trading chart program so there is a constant data stream but there is something about it that triggers it to go into 1xRTT. It's almost comical about how able I am to reproduce this that I can't just leave it to chance.
No real question there I suppose, unless someone knows of anyway that the signal would drop based on the port/protocol/data/etc that is being received. |
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Nighthawke70
EVDO Heavy User
Joined: 16 Aug 2007
Posts: 114
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| Posted: Wed Jan 28, 2009 12:44 pm Post subject: |
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| That explains plenty why my PX500 yoyos between 20 and 40% on a regular basis. Wish I could lock the little fusebox onto the 40% tower and leave it be. |
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