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LTE informal research in South Africa

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A good friend of mine, who's an Engineer in real life, just made a really good post and I wanted to share it here for further comment.

 

By now we have quite a few forumites actually using LTE and we've accumulated a number of real results for various users' signal parameters and achieved speeds.

 

I thought it would be interesting to scrape all this info into one place, see what it looks like on some scatter charts and perhaps try to draw some conclusions. The question : given knowledge of RSSI, RSRP and RSRQ, can we make any useful statements or predictions about likely performance ?

If nothing else, it gives an interesting overview of actual results so far.

 

First, some preliminaries:

 

Units & measurements

There are many good sources of info on deciBels, dBm, RSSI, RSRP and RSRQ.

For the sake of completeness, here's my précis version:

  • deciBels, dB and dBm : The amount of power reaching your antenna system is best measured on the logarithmic “Bel” scale, in deciBels relative to a received power of 1 milliWatt (dBm). On that scale, a level of -50dBm - which is a really strong signal - represents a received power of 10nW (10 nanoWatts). Every 3dB fall on the deciBel scale represents a halving of the signal power, and every 10dB fall means a factor 10 reduction. So, a level of -80dBm represents a power level of 10 picoWatts : 1000 times (30dB) lower than the -50dBm level.

    (Incidentally, this is why the apparently small difference between antennas with gains of 8dB and 11dB is actually quite significant : double the signal power.)

  • RSSI (Received Signal Strength Indication) is an indication of the “raw” signal power reaching your antenna system. This includes both the power in the signal you’re trying to utilise, as well as any other signals in the frequency band your receiver is tuned to. Some equipment manufacturers use an arbitrary scale to indicate RSSI, such as 0 to 100% or 0 to 255. Increasingly, however, manufacturers present RSSI in the “proper” unit - dBm as described above.
  • RSRP (Reference Signal Received Power) is an indication of the power in just the relevant parts (the Reference Signals) of the signal(s) you’re receiving. As such, it should provide a better measure of the true incident power of the signal you’re trying to utilise. Like RSSI, RSRP is best presented in dBm. Logically, the RSRP level can never exceed the RSSI level - in practise, it's significantly lower.
  • RSRQ (Reference Signal Received Quality) is derived from a quotient between RSRP and RSSI and measures the amount of “useful power” reaching your antenna, as a fraction of the total power reaching it. Higher values indicate that more of the total signal power you’re receiving is associated with the signal you’re trying to utilise. RSRQ is presented in deciBels.
  • RSSI-RSRP Because of how RSSI and RSRP are defined for LTE, the difference between RSSI and RSRP values (in deciBels) has a theoretical minimum value. This article explains how this minimum difference is around 20dB, and although it probably relates primarily to FDD systems (not TDD ones), the principle of a minimum difference probably applies to TDD as well. Practical measurements support this.

     

    Background

  • Presumably, only received signals in the same frequency band as that to which the receiver is tuned would increase your RSSI without producing an improvement in your own signal performance. This includes signals from adjacent cells on the same network, and other networks using the same frequency. At the moment, MTN, Vodacom and Neotel use FDD in the same LTE frequency band (1800MHz) whereas Telkom Mobile is alone at 2300MHz with TDD.
  • In the context of an LTE connection, “good performance” has a number of dimensions : stability, latency, downlink speed, uplink speed etc. For this exercise we will judge performance purely by downlink speed, which is of course a simplistic view. However, a connection that can consistently achieve a high downlink speed is likely to be good in other respects too.
  • For the moment, I've looked only at results achieved on Telkom Mobile LTE.
What do we expect to see on the scatter diagrams ?

  • A possible correlation between RSSI and speed : the higher the RSSI, the greater the possibility of high speed (neglecting the effect of spurious signals for now).
  • Perhaps a better correlation between RSRP and speed : the higher the RSRP, the greater the possibility of high connection speed.
  • A possible correlation between RSRQ and speed : a larger RSRQ would indicate that most of the signal power incident on your antenna system comes from the signal you’re trying to utilise, and not other irrelevant signals.
  • Mathematically, the deciBel difference between RSSI and RSRP is somewhat analogous to the quotient between those same two quantities, ie to RSRQ. Thus we might also expect to see the same correlation between RSSI-RSRP difference and speed, as we do between RSRQ and speed.
The scatter diagrams and source data are all contained inside the attached PDF document.

Just to keep things interesting amidst the sea of text, here's a teaser:

 

yby7ehev.jpg

 

How to the read the scatter diagrams

  • In the first chart, the RSSI (in dBm) is plotted on the left axis versus the achieved downlink speed (in Mbps) on the horizontal axis. These are the green squares.
  • In the second chart, the RSRP (also in dBm) is plotted on the left axis versus the achieved downlink speed on the horizontal axis. These are the red squares.
  • In the third chart, the difference between RSSI and RSRP (RSSI-RSRP) is plotted on the left axis, versus the achieved downlink speed on the horizontal axis. These are the pink squares.
  • In looking at all 3 graphs, things get better/faster going from left to right, and from bottom to top. Signal and speed Nirvana is in the top right corner of each graph.
Practical value ranges

  • For RSSI :-85dBm (weak) to -45dBm (very strong).
  • For RSRP : -114dBm (weak) to -72dBm (very strong).
  • For RSSI-RSRP : 29 (bad) to 22 (good).
  • For downlink speed : 5Mbps (poor) to 90Mbps (excellent).
  • Note that 100Mbps is the theoretical maximum for Telkom’s current class of LTE equipment.
Cautionary notes

  • By now I think most of us realise that having good signal conditions is a prerequisite for, but not a guarantee of, high connection speed. The type of equipment at the base station, the type of connection negotiated, the capacity of the backhaul and the number of users connected, all play a role.
  • The signal conditions at the receiver could differ from one occasion to another. This is especially true when using built-in or indoor antennas; less so when using a stable, permanent external antenna system.
  • Even given that your signal conditions are good, the base-station’s “capacity to perform” could differ from one occasion to one another.
  • Practically, there are different ways to arrive at “good signal conditions”. If you live very close to a base-station, and the signal is good anyway, then using just your modem’s internal antennas may get you to a good operating point. Alternatively, if you live far away, you may find you need a high-gain directional external antenna system to get to the same operating point. The results presented are a mix of all situations : everything from internal antennas deployed right next to the base station; all the way up to high-gain directional antenna systems deployed further away.
Observations

  • On all three charts, you can notice the gradual slope from bottom left to top right : As signal conditions improve (from bottom to top), so the achievable speed (on average) increases (from left to right).
  • Downlink speeds seem to fall into at least two fairly distinct “groups”. Broadly speaking, the downlink speeds are either in the range of approx 4-40Mbps, or 50-80Mbps. There may be other sub-groupings. These groupings possibly reflect different base-station operating modes, different backhaul constraints, or different classes of LTE equipment, eg “Category 3" 100Mbps equipment versus some lower-speed class(es).
  • Installations achieving speeds above 20Mbps all have RSSI = -75dBm. However, having RSSI > -75dBm is no guarantee of getting high speed : there are cases with high RSSI values and poor speed.
  • Installations achieving speeds above 20Mbps have RSRP = -100dBm. Likewise, installations achieving >60Mbps have RSRP = -90dBm. But again, having RSRP above those levels is no guarantee. Likewise, there are also examples where a comparatively low RSRP (say, below -95dBm) still gets good speed > 50Mbps.
  • Installations achieving speeds above 50Mbps have RSSI-RSRP = 26dB.
  • Looking at the table of values : Above a value of about -8dB, RSRQ seems to provide almost no useful indication of likely link speed. Once RSSI & RSRP are above a very low minimum signal level, the RSRQ quickly seems to max out at -6dB. However we have seen other examples (not included in these results) where a very poor RSRQ (say, -9dB or lower) is associated with poor performance.
Possible conclusions?

(made very cautiously, bearing all the earlier disclaimers in mind):

  • If your antenna and signal conditions are such that:

    - Your RSSI is below -75dBm; or

    - Your RSRP is below -100dBm; or

    - Your RSRP is more than 27dB below your RSSI

    then ... you are probably not going to see more than about 20Mbps.

  • If all three parameters exceed the above criteria, then you have the possibility (not certainty) of achieving speeds anywhere up to about 80Mbps.
Many thanks to all the forum users who responded to my requests for their complete test results!
PDF attachment

 

PDF note: Tapatalk cannot parse links with the % sign in it. Please access from your browser

 

 

Source: MyBroadband forums

 

A friendly warning. The source forum has a lot of religious and other anti-social content so browse at your own risk.

 

Also see: An introduction to SA carriers

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An enjoyable read. We have posted some similar articles and posts in the past. But this does a good job of wrapping up the issue from another perspective.

 

Robert via Samsung Note 8.0 using Tapatalk Pro

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An enjoyable read. We have posted some similar articles and posts in the past. But this does a good job of wrapping up the issue from another perspective.

 

Robert, the lengthy quote includes an inline link to my RSRP article. It has gone around the world and back again. Ha ha!

 

AJ

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Robert, the lengthy quote includes an inline link to my RSRP article. It has gone around the world and back again. Ha ha!

 

AJ

I didn't follow the link. But that's excellent!

 

Robert via Samsung Note 8.0 using Tapatalk Pro

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Robert, the lengthy quote includes an inline link to my RSRP article. It has gone around the world and back again. Ha ha!

 

AJ

I dropped that link on my home forum a little while ago as a standard resource for reference. I think the quality of the article speaks for itself. ;)
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