Reference Signal Received Power (RSRP)

To perform UE Attach, cell selection, cell reselection, handover procedures RSRP measurements are needed. During measurements Reference Signal Received Power (RSRP) is calculated per each eNB (DL) signal available at a given location. So based on this information it's possible to choose an eNB with the best channel quality to serve an UE (please notice another aspects also can be taken into account while choosing a serving eNB, e.g. load balancing features).
RSRP (Reference Signal Received Power) is an average received power of a single Reference Signal resource element.
Reference Signal is a special signal placed at a predefined position (exact position depends on PCI - Physical Cell ID value).
RSRP calculation is based on energy received within useful part of OFDM symbol and doesn't include cyclic prefix energy.

RSRP unit is dBm. UE reports not RSRP value but an integer value between 0 and 97. Applied mapping is shown below.

Reported	Actual RSRP, dBm
0	RSRP < -140
1	-140 ≤ RSRP < -139
2	-139 ≤ RSRP < -138
3	-138 ≤ RSRP < -137
4	-137 ≤ RSRP < -136
...	...
n	n-141 ≤ RSRP < n-140
...	...
96	-45 ≤ RSRP < -44
97	-44 ≤ RSRP

Maximum RSRP value can be calculated based on maximum input UE power UE which is -25 dBm and minimal LTE bandwidth which is 1.4 MHz, i.e. 72 resource elements. Then -25 - 10*LOG(72) = -44 dBm. Absolute RSRP measurement accuracy is between ±6 and ±8 dB.

Reference Signal Received Quality (RSRQ)

RSRQ (Reference Signal Received Quality) is calculated using the following expression:

RSRQ = RSRP / (RSSI / N), where

RSRP is Reference Signal Received Power;
RSSI is Received Signal Strength Indicator, calculated as an average of the total power measured within OFDM symbols that contain Reference Symbols (when MIMO is not used, it is symbols 0 and 4);
N is a number of resource blocks over which RSSI is measured.

RSRQ units are dB. UE maps RSRQ value to an integer value between 0 and 34 and sends it to an eNB. RSRQ mapping is based on the following table.

Reported	Actual, dB
0	RSRQ < -19.5
1	-19.5 ≤ RSRQ < -19
2	-19 ≤ RSRQ < -18.5
3	-18.5 ≤ RSRQ < -18
4	-18 ≤ RSRQ < -17.5
5	-17.5 ≤ RSRQ < -17
6	-17 ≤ RSRQ < -16.5
7	-16.5 ≤ RSRQ < -16
8	-16 ≤ RSRQ < -15.5
9	-15.5 ≤ RSRQ < -15
10	-15 ≤ RSRQ < -14.5
11	-14.5 ≤ RSRQ < -14
12	-14 ≤ RSRQ < -13.5
13	-13.5 ≤ RSRQ < -13
14	-13 ≤ RSRQ < -12.5
15	-12.5 ≤ RSRQ < -12
16	-12 ≤ RSRQ < -11.5
17	-11.5 ≤ RSRQ < -11
18	-11 ≤ RSRQ < -10.5
19	-10.5 ≤ RSRQ < -10
20	-10 ≤ RSRQ < -9.5
21	-9.5 ≤ RSRQ < -9
22	-9 ≤ RSRQ < -8.5
23	-8.5 ≤ RSRQ < -8
24	-8 ≤ RSRQ < -7.5
25	-7.5 ≤ RSRQ < -7
26	-7 ≤ RSRQ < -6.5
27	-6.5 ≤ RSRQ < -6
28	-6 ≤ RSRQ < -5.5
29	-5.5 ≤ RSRQ < -5
30	-5 ≤ RSRQ < -4.5
31	-4.5 ≤ RSRQ < -4
32	-4 ≤ RSRQ < -3.5
33	-3.5 ≤ RSRQ < -3
34	-3 ≤ RSRQ

Maximum RSRQ value can be calculated based on an assumption that only Reference Signals are transmitted and there is no user data transmission. There are two Reference Signals per OFDM symbol so it leads to the following expression:

RSRQ = RSRP / (RSSI / N) = RSRP / (2 * RSRP * N / N) = 0.5 = -3 dB.

Measurement accuracy for RSRQ is defined by 3GPP as ± 2.5 - ± 3.5 dB.





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