RSSI is the simplest of the LTE power measurements, because it does not care where the power comes from. That also makes it the easiest one to misread. LTE actually defines two RSSI measurements, and they measure different OFDM symbols for different purposes. I'll start with what RSSI collects, then separate the two definitions, and finish with the one that the UE reports on its own.

• What does RSSI measure ?
• Why does LTE define two kinds of RSSI ?
• How does the UE report RSSI ?
• Reference

What does RSSI measure ?

RSSI is the total power  UE observes across the whole band. This includes the main signal and co-channel non-serving cell signal, adjacent channel interference and even the thermal noise within the specified band. This is the power of non-demodulated signal, so UE can measure this power without any synchronization and demodulation.

Following is an example of one downlink radio frame. The red part is the resource elements in which reference signal is being transmitted. Blue and light blue part is for synchronization signal. Yellow part is for PDCCH. Green part is for MIB. Whitepart is PDSCH where user data is being transmitted. RSSI is the total power for all color and any possible noise/interference existing over all these area.

Figure 1. One downlink radio frame. RSSI adds up the power of every colour in the grid, plus noise and interference. So it rises with cell load even when the radio link stays the same.

• Time runs from left to right over one radio frame, and frequency runs from top to bottom.
• The red resource elements are the cell-specific reference signals. They are the only part that RSRP averages, while RSSI counts them together with everything else.
• The white area is PDSCH. When the cell carries more user data, more of this area carries power, and RSSI goes up.
• The blue, light blue and green columns carry the synchronization signals and the MIB, and they are present whatever the load is.

For practical calculation example, refer to RSRQ Calculator (YouTube)

Let's put a number on the difference between RSSI and RSRP. Take a 20 MHz carrier with 100 resource blocks, one antenna port and full load, so that all 1200 subcarriers in a symbol carry the same power as the reference signal. If RSRP is -90 dBm per resource element, RSSI over the 1200 subcarriers is -90 + 10 log₁₀(1200), which is about -59.2 dBm. The two numbers differ by about 31 dB, and neither of them is wrong. RSRP is a power per resource element, and RSSI is a power over the whole measurement bandwidth.

One point in the description above needs care. The standalone RSSI can be measured without demodulation. But the E-UTRA carrier RSSI that goes into RSRQ is measured only in specific OFDM symbols, so the UE needs the cell timing to pick them. The next section explains which symbols those are.

• RSSI counts all received power : serving cell, co-channel non-serving cells, adjacent channel interference and thermal noise.
• RSSI grows with load : more PDSCH means more power in the measured symbols.
• RSSI and RSRP differ by the bandwidth factor : with 100 RBs at full load and one port, the gap is about 31 dB.
• The RSSI inside RSRQ needs cell timing : it is measured in specific OFDM symbols of the cell.

Why does LTE define two kinds of RSSI ?

When a log or a paper says RSSI, it may mean either of two measurements in 36.214 v19.0.0. The first is the E-UTRA carrier RSSI, which exists only as the denominator of RSRQ. The second is a standalone RSSI that was added for operation in unlicensed spectrum, where the UE has to tell the network how busy a channel is.

The table below puts the two side by side.

Both measurements share three rules. Both include power from all sources, including co-channel serving and non-serving cells, adjacent channel interference and thermal noise. Both use the antenna connector of the UE as the reference point. And with receiver diversity, the reported value shall not be lower than the value of any individual branch.

The difference in symbols matters most. The E-UTRA carrier RSSI looks at symbols that always carry reference signals, so it tracks the load of the cells the UE hears. The standalone RSSI looks at whatever symbols the network configures, so the network can sample a channel at chosen times and see how often it is occupied.

• E-UTRA carrier RSSI is the denominator of RSRQ : it is never reported on its own.
• Standalone RSSI is a separate, configured measurement : it applies only in RRC_CONNECTED.
• The two use different OFDM symbols : port 0 reference symbols by default for the first, configured symbols for the second.
• Check which RSSI a log shows : a value from an RSRQ calculation and an rssi-Result are not the same measurement.

How does the UE report RSSI ?

Only the standalone RSSI has its own report. The network configures when the UE samples it, and the UE returns an average RSSI together with a channel occupancy. This was designed for carriers with frame structure 3, where the eNB needs to know how busy an unlicensed channel is before it uses it.

36.331 carries the configuration in rmtc-Config-r13 of MeasObjectEUTRA, and the result in MeasResultForRSSI-r13. RSSI-Range-r13 is the value type for both the result and the occupancy threshold.

Following is based on 36.331 v19.3.0 (Release 19)

RSSI-Range-r13 ::=                      INTEGER(0..76)

RMTC-Config-r13 ::= CHOICE {
    release                             NULL,
    setup                               SEQUENCE {
        rmtc-Period-r13                 ENUMERATED {ms40, ms80, ms160, ms320, ms640},
        rmtc-SubframeOffset-r13         INTEGER(0..639)                 OPTIONAL,       -- Need ON
        measDuration-r13                ENUMERATED {sym1, sym14, sym28, sym42, sym70},
        ...
    }
}

MeasResultForRSSI-r13 ::=           SEQUENCE {
    rssi-Result-r13                         RSSI-Range-r13,
    channelOccupancy-r13                    INTEGER (0..100),
    ...
}

• rmtc-Period-r13 is the RSSI measurement timing configuration, RMTC, periodicity for the frequency. The value ms40 means a 40 ms period, and so on.
• rmtc-SubframeOffset-r13 is the RMTC subframe offset, and it should be smaller than rmtc-Period. For inter-frequency measurements, the UE picks a random offset if the network does not configure one.
• measDuration-r13 is the number of consecutive symbols over which the physical layer reports RSSI samples. The value sym1 means one symbol, and sym14 means 14 symbols.
• rssi-Result-r13 is the average of the RSSI samples in the reportInterval.
• channelOccupancy-r13 is the rounded percentage of samples in the reportInterval that are above channelOccupancyThreshold-r13 of the reportConfig.

The integer in rssi-Result-r13 maps to dBm through 36.133 v19.5.0 Table 9.1.18.5.1-1. The reporting range is -100 dBm to -25 dBm with 1 dB resolution. RSSI_00 means RSSI < -100 dBm, RSSI_01 means -100 <= RSSI < -99 dBm, and the steps continue up to RSSI_76, which means -25 dBm <= RSSI. So a reported value n from 1 to 75 covers -101 + n <= RSSI < -100 + n dBm. The accuracy requirements that go with this table apply only for CA with frame structure 3.

• Only the standalone RSSI is reported by itself : the E-UTRA carrier RSSI stays inside RSRQ.
• The network sets the sampling with rmtc-Config-r13 : a period, an offset and a duration in symbols.
• The UE reports an average and an occupancy : rssi-Result-r13 and channelOccupancy-r13 in percent.
• RSSI-Range-r13 runs from 0 to 76 : 1 dB steps from below -100 dBm to -25 dBm and above.

Reference

• 36.214 v19.0.0 : E-UTRA Physical layer - Measurements. Clause 5.1.3 Reference Signal Received Quality, clause 5.1.24 Received Signal Strength Indicator.
• 36.133 v19.5.0 : E-UTRA Requirements for support of radio resource management. Clause 9.1.18.5.1 RSSI measurement report mapping.
• 36.331 v19.3.0 : E-UTRA RRC. RSSI-Range, MeasObjectEUTRA, MeasResults and clause 5.5.5 measurement reporting.
• For practical calculation example, refer to RSRQ Calculator (YouTube)