RSRP tells the UE how strong a cell is, but not how clean it is. RSRQ adds the second half of the picture by comparing the reference signal power with everything else the UE receives on the carrier. I'll start with the definition, then work out which values you can expect to see, and finish with how the UE reports the value.

• How is RSRQ defined ?
• Which RSRQ values should you expect ?
• How does the UE report RSRQ ?
• Reference

How is RSRQ defined ?

RSRQ is not a separate power measurement. The UE builds it from two measurements that it already makes, and the way the two are lined up decides what the ratio means.

RSRQ is defined as (N x RSRP)/RSSI in 3GPP 36.214, where N is the number of RBs over the measurement bandwidth.  

RSRQ = N × RSRP/(EUTRA carrier RSSI),

As you see from the definition of RSSI, RSSI contains all sorts of power including power from co-channel serving & non-serving cells, adjacent channel interference, thermal noise, etc. Therefore, (N x RSRP)/RSSI indicates "What is the portion of pure RS power over the whole E-UTRA power recived by the UE".

As you see, this is not the direct measurement, it is a kind of derived value from RSRP and RSSI. By deviding RSRP by RSSI, it could give some information about interference as well in addition to the strength of the wanted signal.

Since this is the ratio of two different power value, the unit of RSRQ is dB and the value would be always negative (because RSSI value will aways be larger than N x RSRP)

Why the factor N? RSRP is a power per resource element, while the E-UTRA carrier RSSI is a total power over N resource blocks. Multiplying RSRP by N puts the numerator on the same per resource block scale. 36.214 v19.0.0 clause 5.1.3 also requires that the numerator and the denominator are measured over the same set of resource blocks.

The other half of the definition is which OFDM symbols go into RSSI. By default, the UE measures RSSI only in the OFDM symbols that contain reference symbols for antenna port 0. If higher layers indicate all OFDM symbols, the UE measures RSSI over all OFDM symbols of the DL part of the measurement subframes. If higher layers indicate certain subframes, the UE measures RSSI over all OFDM symbols of those subframes. With discovery signal based measurements, RSSI comes from the configured discovery signal occasions.

As with RSRP, the reference point is the antenna connector of the UE. With receiver diversity, the reported RSRQ shall not be lower than the RSRQ of any individual branch.

• RSRQ = N x RSRP / RSSI : N is the number of RBs in the RSSI measurement bandwidth, and both parts use the same RBs.
• RSRQ is a derived value : the UE computes it from RSRP and the E-UTRA carrier RSSI.
• By default, RSSI comes only from the port 0 reference symbol OFDM symbols : higher layers can switch it to all OFDM symbols.
• Interference and load lower RSRQ : they raise RSSI without raising RSRP.

Which RSRQ values should you expect ?

The report table below stops at -3 dB, and a real cell rarely reaches even that. To see why, let's count resource elements in one resource block and one OFDM symbol that carries reference signals for antenna port 0.

Take the simplest case first. The cell has one antenna port, the UE sees only this cell, there is no noise, and every resource element that is transmitted has the same power P. In a symbol with port 0 reference signals, 2 of the 12 resource elements in the resource block carry the reference signal. If the cell carries no data, only those 2 resource elements have power. RSSI per resource block is then 2P and RSRP is P, so RSRQ is P / 2P, which is -3 dB. That is the top of the original table, RSRQ_34. If the cell is fully loaded, all 12 resource elements carry power P. RSSI becomes 12P, so RSRQ is 1/12, which is about -10.8 dB.

Now take two antenna ports under the same assumptions. Port 1 places its reference signals in 2 other resource elements of the same symbol, and each port stays silent where the other port sends its reference signal. With no data, the UE receives power in 4 resource elements, so RSRQ is P / 4P, about -6 dB. With full load, each of the 8 data resource elements carries power from both antennas, 2P each. RSSI is then 4P + 16P = 20P, and RSRQ is 1/20, about -13 dB.

So even a clean cell moves its RSRQ by 7 to 8 dB between no load and full load. Interference from neighbour cells and thermal noise then push the value lower still. Keep this in mind when you compare RSRQ values: a low RSRQ can mean a busy serving cell, not only a bad radio condition.

The ceiling changes when RSSI covers all OFDM symbols. The symbols without reference signals may carry little or no power in a lightly loaded cell, so the average RSSI drops and RSRQ can rise above -3 dB. That is one reason why the extended report range in the next section reaches +2.5 dB.

• With one port and no load, RSRQ is -3 dB at most : RSSI then holds only the 2 reference signal resource elements.
• With one port and full load, RSRQ is about -10.8 dB : RSSI holds all 12 resource elements.
• With two ports, the same cases give about -6 dB and -13 dB : the second port adds reference signal and data power to RSSI.
• Load alone moves RSRQ by several dB : interference and noise lower it further.
• Measuring RSSI over all OFDM symbols can push RSRQ above -3 dB : this needs the extended report range.

How does the UE report RSRQ ?

UE usually measures RSRP or RSRQ based on the direction (RRC message) from the network and report the value. When it report this value, it does not use the real RSRQ value. It sends a non-negative value ranging from 0 to 34 and each of these values are mapped to a specific range of real RSRQ value as shown in the following table from 36.133.

< 36.133 Table 9.1.7-1 : RSRQ measurement report mapping >

Figure 1. The original RSRQ report mapping. Each reported value covers 0.5 dB, and the two end values are open ranges.

• RSRQ_00 means RSRQ < -19.5 dB.
• RSRQ_01 to RSRQ_33 are 0.5 dB wide. A reported value n covers -20 + n/2 <= RSRQ < -19.5 + n/2 dB.
• RSRQ_34 means -3 dB <= RSRQ, which matches the -3 dB ceiling worked out in the section above.

Just to save a little bit of time for readers, I expanded the table to show all the possible RSRQ value and mapping absolute power.

The current 36.133, v19.5.0 clause 9.1.7, extends the table at both ends. The reporting range of RSRQ is now -34 dB to 2.5 dB with 0.5 dB resolution. At the low end, RSRQ_-30 means RSRQ < -34 dB, and RSRQ_-29 to RSRQ_-01 fill the range from -34 dB to -19.5 dB. At the high end, RSRQ_35 means -3 <= RSRQ < -2.5 dB, and the steps continue up to RSRQ_46, which means 2.5 dB <= RSRQ. These extended values apply only to a UE that supports the extended RSRQ range.

In the RRC message, 36.331 keeps the original field at 0 to 34 and adds a second type for the extended values. A separate IE tells the network how the RSRQ was measured.

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

RSRQ-Range ::=                     INTEGER(0..34)

RSRQ-Range-v1250 ::=               INTEGER(-30..46)

RSRQ-Range-r13 ::=                 INTEGER(-30..46)

RSRQ-Type-r12 ::=                  SEQUENCE {
    allSymbols-r12                      BOOLEAN,
    wideBand-r12                        BOOLEAN
}

• A field of type RSRQ-Range-v1250 is signalled only if the corresponding original field is set to 0 or 34. Only a UE that indicates extendedRSRQ-LowerRange-r12 or rsrq-OnAllSymbols-r12 may report it, and it may do so without explicit configuration. If the network receives it, the network uses it and ignores the original field.
• RSRQ-Range-r13 covers the original range and the extended range in one field. It may be signalled without the original field.
• RSRQ-Type-r12 says how the RSRQ was measured. The field allSymbols set to TRUE means all OFDM symbols, and the field wideBand set to TRUE means a wider bandwidth.
• The network asks for these modes with measRSRQ-OnAllSymbols-r12 in MeasConfig and widebandRSRQ-Meas-r11 in MeasObjectEUTRA.

• The UE reports an integer, not a dB value : 36.133 Table 9.1.7-1 maps each integer to a 0.5 dB range.
• The original range is -19.5 dB to -3 dB : RSRQ_00 and RSRQ_34 are the open ends.
• The current range is -34 dB to 2.5 dB : RSRQ_-30 to RSRQ_46, for UEs that support the extended range.
• An RSRQ value only makes sense with its measurement mode : all-symbol and wideband RSRQ are not directly comparable with the default mode.

Reference

[1] 3GPP - 36.214 Evolved Universal Terrestrial Radio Access (E-UTRA); Physical layer - Measurements  

[2] 3GPP - 36.133  Evolved Universal Terrestrial Radio Access (E-UTRA);Requirements for support of radio resource management

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

[4]  RSRP and RSRQ Measurement in LTE  

[5] 3GPP - 36.214 v19.0.0, clause 5.1.3 Reference Signal Received Quality - the current text read for this page

[6] 3GPP - 36.133 v19.5.0, clause 9.1.7 RSRQ Measurement Report Mapping

[7] 3GPP - 36.331 v19.3.0, RSRQ-Range and RSRQ-Type information elements, MeasConfig, MeasObjectEUTRA