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4G/LTE - PHY Measurement

RSRP, RSRQ, RSSI, SINR Interplay

This page is not for the explaining on each of these terminologies. If you are not familiar with each of these words, please refer to followings pages first. This page is mainly for practical sense of these terms and even more on inter-relationship among them.

Followings are topics covered in this page.

Equations Correlating the parameters

Now let me summarize several equations (formula) to show the correlations among these parameters. It can be summarized as below. Reading and understanding these equations and take a look at the plot shown above and see if you can understand any further than before you read this section.

The most fundamental (basic) measurement is RSRP. As you see here or described in RSRP page, this is basically a power measurement for a single subcarrier. The value does not change with bandwidth or number of RBs currently assinged for PDSCH. So, this measurement would give you the lowest value comparing to other parameters. RSRP gives you an idea of the strength of the signal it gets from the network, but it is not clear indication of how good the signal quality is.

Next fundamental measurement is RSSI. As you see here or described in RSSI page, it is the total power integrated across the whole bandwidth, so the measured value can change and the number of RBs allocated to PDSCH. The power in RSSI contains not only signal power (desired signal power) but also interference from other cells and any internal/external noise. So high RSSI does not necessarily mean high signal power or good signal quality.

Is there any parameter that can give you an idea of signal quality rather than the signal strength ? This is what RSRQ can give you. Considering RSSI is 'Signal + Noise' or 'Singal + Noise + Interference', you would novice that RSRQ is very similar to the definition of SNR or SINR which are the most common indicators of signal quality. Since 'N x RSRP' is always smaller than RSSI (total received power), the RSRQ is always negative value in dB. The higher RSRQ is, the better the signal quality is if Bandwidth and number of RB allocation is same. However, you cannot get the absolute signal quality information from RSRQ only since the value would vary depending on Bandwidth and RB allocation. As you see in the RSRQ page, the maximum RSRQ value you can achieve is -3dB.

You can get more absolute criteria for signal quality. For this, SNR or SINR would be more accurate. But SNR, SINR is not defined in 3GPP. However, SNR or SINR is very common RF term and the correlation of SINR and RSRQ can be approximated (in dB) as below. SNR, SINR can go negative but in most cases it would be positive value. If you see over 20 dB SNR or SINR, you can say you have have almost ideal signal quality.

The ultimate goal for any communication is to send/receive the data with minimum error and the reason why we all care about the signal quality is to achieve as much data rate as possible. So, you would want to know the correlation between data throughput and the signal quality. That correlation can be expressed as follows. This is from one of the most famous rule in communication theory called Shannon's law.

Example 1 > : RSRP/RSRQ/SINR at relatively stationary environment

For the motivation on studying the relationships (interplay) of all these different indicator, let's take a look at a measurement done by a UE in the field. Following plot is from the data captured by a drive test tool Azenqos Drive Test tool (AZQ Android). I got the log captured by the tool and exported the data as csv file and then plot it on Microsoft Excel.

First, just take a quick look and find which trace (color) indicates which parameters.. do you see any correlations among these traces ?

To me, I notice several different patterns as marked below (you may see different pattern). I cannot explain exactly why I have this kind of multiple different patterns over time (I would need some additional information to explain the reason), but anyway I can see several different patterns.

In (A), the difference between RSRP, RSRQ, SINR Rx(0), SINR Rx(1) are stay almost same and each of these traces stay same as well. Also, SINR Rx(0) and SINR Rx(1) are almost same (meaning the signal quality measured at primary Rx Antenna and secondary Rx Antenna are almost the same).

In (B), the difference between RSRP, RSRQ, SINR Rx(0), SINR Rx(1) are stay almost same and each of these traces stay same as well. But all of these values drops at the beginning and increase around the end of this section. Why ? From the signaling message from the same log, this is the section where UE camps on LTE Cell. But is there any correlation between LTE attach and RSRP/RSRQ/SINR drops ? As far as I know of, there would be no correlation.

In (C), all of the measurement disappears all of the sudden. Why ? You may easily guess two possible reason for this. Call drop or UE switched to other RAN (e.g, WCDMA Cell or GSM Cell). From the signaling log, this is where UE handovered to WCDMA Cell.

In (D), you see all the measurement appears again and the pattern is pretty similar to (A) and (B). The only small difference is that we see a little bit wider difference between SINR Rx(0) and SINR Rx(1). It means .. somehow the signal quality coming into the primary Reciever antenna is getting worse than the signal qualilty measured at the secondary reciever antenna. Why ? I don't know the answer. we need to look into other informations from the log or in some cases we may need to ask the exact measurement condition.

In (E), here we have another disappearance of the signal. Probably this is also due to the handover to other technology like WCDMA.

In (F), here we have the signal back, but here you see even more differences between SINR Rx(0) and SINR Rx(1). Why ? We don't know just from this plot.

Do you have any clear explanation for all of the plots shown above ? and do you have clear answers to all of the questions ("Why ?") in the description above ?

Don't worry, this is not to let you be under test. Even I don't have answers to all the questions. Some of the questions cannot be answered without additional information. This is just to let you think of various signal quality parameters and inter-relationship among them. For now, if you have questions (not the answers) about this, it is good enough.

Example 2 > : RSRP/RSRQ at driving/handover situation

Following plot is from the data captured by a drive test tool Azenqos Drive Test tool (AZQ Android). I got the log captured by the tool and exported the data as csv file and then plot it on Microsoft Excel. The map displaying the path of the measurement shown at the bottom is the one automatically created by AZQ reporting tool.

You would see that RSRP changes pretty dynamically even along the pretty straight highway/open space. You see many changes PCI (Physical Cell ID) of serving cell, meaning the UE has gone through many Handovers.

The dots at lower parts of plot shows the RSRP of the neighbour cells. As you see, at most of the places where handover happens you would notice that there is a certain points where neighbour cell RSRP is higher than the serving cell RSRP, even though you would notice a couple of points where neigbour cell RSRP is greater than serving cell but handover does not happen.

Example 3 > : RSRP/RSRQ/RSSI/SINR at driving/handover situation

Not much of new things to explain.. just try to make your own story based on the descriptions in previous sections / examples.

Reference :

[1] SINR, RSRP, RSSI AND RSRQ MEASUREMENTS IN LONG TERM EVOLUTION NETWORKS

Farhana Afroz, (1) Ramprasad Subramanian, (1) Roshanak Heidary, (1) Kumbesan Sandrasegaran

and (2) Solaiman Ahmed

(1) Faculty of Engineering and Information Technology, University of Technology, Sydney, Australia

(2) Department of Electrical and Electronic Engineering, University of Dhaka, Bangladesh

International Journal of Wireless & Mobile Networks (IJWMN) Vol. 7, No. 4, August 2015

[2] An On Filed Measurement Example

[3] What is the relationship between RSRP and SINR?

[4] Calculation of SINR, having the RSRQ

[5] Understanding LTE Signal Strength Values