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RSSI vs RSRP: A Brief LTE Signal Strength Primer

Posted by WiWavelength, in Author: Andrew J. Shepherd 16 July 2012 · 67,667 views

Sprint LTE RSSI RSRP
RSSI vs RSRP: A Brief LTE Signal Strength Primer

by Andrew J. Shepherd
Sprint 4G Rollout Updates

Monday, July 16, 2012 - 1:40 AM MDT


As Sprint LTE 1900 has become live and discoverable in numerous markets over this past weekend, some of our readers, especially those who are using Android 4.0 ICS based ROMs, have expressed concern at the seemingly low signal levels that they have encountered.  For example, see this screenshot from an HTC EVO 4G LTE (under Settings > About > Network):


Posted Image



Note the -102 dBm signal level.  If this were measuring CDMA1X or EV-DO, then, yes, -102 dBm would be nearing the margin of usable signal.  But -102 dBm is actually relatively healthy LTE signal level.  To understand why, we need to learn the differences between two types of signal measurement:  Received Signal Strength Indicator (RSSI) and Reference Signal Received Power (RSRP).

First, an LTE downlink is divided into subcarriers.  A 5 MHz bandwidth downlink, which is the configuration that Sprint is deploying, contains 300 subcarriers.  And of those subcarriers, one in three carry LTE reference signals.  In other words, of the 300 subcarriers, 100 transmit periodic reference signals.

To illustrate, I captured this power vs frequency sweep with a spectrum analyzer.  The LTE downlink graph comes from a Sprint site in the Kansas City area in late April, well before Sprint stopped blocking devices from live LTE sites.  So, the sector depicted here exhibits no data traffic; it is transmitting only the periodic reference signals on 100 subcarriers, which you can clearly count in the graph:

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Now, RSSI is the more traditional metric that has long been used to display signal strength for GSM, CDMA1X, etc., and it integrates all of the RF power within the channel passband.  In other words, for LTE, RSSI measurement bandwidth is all active subcarriers.  If we take the above RF sweep of a Sprint 5 MHz bandwidth downlink, RSSI measures the RF power effectively of what is highlighted in yellow:

Posted Image

RSRP, on the other hand, is an LTE specific metric that averages the RF power in all of the reference signals in the passband.  Remember those aforementioned and depicted 100 subcarriers that contain reference signals?  To calculate RSRP, the power in each one of those subcarriers is averaged.  As such, RSRP measurement bandwidth is the equivalent of only a single subcarrier.  And using our graph once more, RSRP measures the RF power effectively of what is highlighted in red:

Posted Image


Since the logarithmic ratio of 100 subcarriers to one subcarrier is 20 dB (e.g. 10 × log 100 = 20), RSSI tends to measure about 20 dB higher than does RSRP.  Or, to put it another way, RSRP measures about 20 dB lower than what we are accustomed to observing for a given signal level.  Thus, that superficially weak -102 dBm RSRP signal level that we saw previously would actually be roughly -82 dBm if it were converted to RSSI.


To conclude, here are a few takeaways about RSSI and RSRP as signal strength measurement techniques for LTE:

  • RSSI varies with LTE downlink bandwidth.  For example, even if all other factors were equal, VZW 10 MHz LTE bandwidth RSSI would measure 3 dB greater than would Sprint 5 MHz LTE bandwidth RSSI.  But that does not actually translate to stronger signal to the end user.
  • RSSI varies with LTE subcarrier activity -- the greater the data transfer activity, the higher the RSSI.  But, again, that does not actually translate to stronger signal to the end user.
  • RSRP does a better job of measuring signal power from a specific sector while potentially excluding noise and interference from other sectors.
  • RSRP levels for usable signal typically range from about -75 dBm close in to an LTE cell site to -120 dBm at the edge of LTE coverage.

Sources: 3GPP, author's graphs






Very nice. Thanks for the clarification. Very well written.
Thanks for breaking it down for us.
Thanks Andrew, you made it very easy to understand, thanks for the clarification. I look forward to future articles from you.
Damn, feel like my brain is about to explode..............Excellent write up AJ, this was really good.
For an interesting bit of supplemental info that I decided not to include in the article, VZW has thrown its support behind RSSI and has made effort to standardize its devices around that measurement. For example, see this document:

http://support.veriz...e.html?id=37249

The upside is that LTE RSSI is always greater than RSRP. For VZW 10 MHz bandwidth LTE, RSSI should be approximately 23 dB greater than RSRP. And those larger numbers may be artificially reassuring to end users.

But there is a price to pay -- the signal strength simply gets skewed toward larger numbers. Note how the VZW document states that "RSSI should be greater than -58 dBm" and that "-96 dBm indicates no signal." The former seems surprisingly high as standard signal level, the latter likewise for no signal level.

However, subtract 23 dB from each measurement to convert roughly to RSRP and get -81 dBm and -119 dBm, respectively, both of which are largely in line with observed performance from Sprint LTE so far.

AJ
Nerd Alert!!

seriously though - this is another reason I love this site....the attention to detail is ridiculous. Instead of "my signal sucks" and everybody either piling on or replying "mine is great"....we get a look inside WHY people might think that (and how in this case they're wrong).
Take us to school Prof wavelength!! Thank you good sir, that was an excellent rundown that even a moron such as myself can understand. now to get it to the masses of other morons... you mind if I transmit that bit of RF awesomeness to other interwebs?
Wow great in-depth article.. Too bad I'm at work and I can't read it completely.

...you mind if I transmit that bit of RF awesomeness to other interwebs?


If you think that the article is informative enough to share elsewhere, we would be honored.

Thanks...

AJ
Wow! I had to reread this a few times, but I understand it better. Thanks for breaking it down for us!
Which Spectran unit are you using?
Nevermind, Blew up your screen grab!

Nevermind, Blew up your screen grab!


Yes, it is a Spectran HF-6065 V4. It has its quirks, but it serves my relatively simple purposes of observing deployed/fallow spectrum, identifying airlink types, and measuring occupied bandwidth quite well. The USB connection and freely downloadable spectrum analyzer software round out a nice overall package.

Let me know if you have any questions about it...

AJ
Things like this make this site awesome; thx for the lesson teach.
Awesome information! This site just gets better and better...
I just logged on to this site to see if there was any info on this. BAM here it is front page and very well written. Thanks!
Is there any better range on an LTE link versus EVDO?
Does this now mean I have to try to take the Extra Class exam now?

Is there any better range on an LTE link versus EVDO?


No, EV-DO tends to be the more robust airlink. So, all other factors being equal, LTE will likely have somewhat less range than will post Network Vision EV-DO. However, LTE coverage should be similar to pre Network Vision EV-DO coverage.

AJ

Yes, it is a Spectran HF-6065 V4. It has its quirks, but it serves my relatively simple purposes of observing deployed/fallow spectrum, identifying airlink types, and measuring occupied bandwidth quite well. The USB connection and freely downloadable spectrum analyzer software round out a nice overall package.Let me know if you have any questions about it...AJ

For surveying LTE/PCS signals do you find that you need one of the external amps for the Spectran? Thanks!

For surveying LTE/PCS signals do you find that you need one of the external amps for the Spectran? Thanks!


Are you referring to the optional preamp? I do not have it and, for my purposes, do not feel that I need it. I typically sweep individual sites/sectors, so I set up within a quarter of a mile of each site. At that distance, signal levels are well above the stated -135 dBm/Hz floor of my Spectran unit. Plus, I am not particularly concerned with amplitude accuracy. I primarily want to examine spectrum utilization. Thus, if I can see occupied bandwidth, that fulfills my purposes.

Let me know if you have any other questions...

AJ

Looks like someone decided to rip off your article word-for-word: http://communication...srp-in-lte.html

AJ, could you go into more detail about RSRQ? It seems like everyone around S4GRU uses RSRP and I wanted to know why. Is it because that RSRP is closer (in terms of dBms) to RSSI?

AJ, could you go into more detail about RSRQ? It seems like everyone around S4GRU uses RSRP and I wanted to know why. Is it because that RSRP is closer (in terms of dBms) to RSSI?


Yes. RSRQ is related to RSRP, but RSRQ is Reference Signal Received Quality. And in many ways, RSRQ is to Ec/Io as RSRP is to RSSI.

AJ

Yes. RSRQ is related to RSRP, but RSRQ is Reference Signal Received Quality. And in many ways, RSRQ is to Ec/Io as RSRP is to RSSI. AJ

Thanks, I found this, but wasn't sure how accurate it was.

 

http://www.sharetech...k_LTE_RSRQ.html