In cellular networks, when a mobile moves from cell to cell and performs cell selection/reselection and handover, it has to measure the signal strength/quality of the neighbor cells. In LTE network, a UE measures two parameters on reference signal: RSRP (Reference Signal Received Power) and RSRQ (Reference Signal Received Quality).
In LTE network, a UE measures:
- RSSI – Received Signal Strength Indicator: The carrier RSSI (Receive Strength Signal Indicator) measures the average total received power observed only in OFDM symbols containing reference symbols for antenna port 0 (i.e., OFDM symbol 0 & 4 in a slot) in the measurement bandwidth over N resource blocks.
The total received power of the carrier RSSI includes the power from co-channel serving & non-serving cells, adjacent channel interference, thermal noise, etc. Total measured over 12-subcarriers including RS from Serving Cell, Traffic in the Serving Cell
- RSRP – Reference Signal Received Power: RSRP is a RSSI type of measurement, as follows there are some definition of it and some details as well.
It is the power of the LTE Reference Signals spread over the full bandwidth (RSSI) and narrow-band (RSRP).
A minimum of -20 dB SINR (of the S-Synch channel) is needed to detect RSRP/RSRQ
- RSRQ – Reference Signal Received Quality: Quality considering also RSSI and the number of used Resource Blocks (N) RSRQ = (N * RSRP) / RSSI measured over the same bandwidth. RSRQ is a C/I type of measurement and it indicates the quality of the received reference signal.
The RSRQ measurement provides additional information when RSRP is not sufficient to make a reliable handover or cell re-selection decision.
In the procedure of handover, the LTE specification provides the flexibility of using RSRP, RSRQ, or both.
Of course, it must to be measured over the same bandwidth:
- Narrow-band N = 62 Sub Carriers (6 Resource Blocks)
- Wide-band N = full bandwidth (up to 100 Resource Blocks / 20 MHz)
RSRP 3GPP Definition
Reference Signal Received Power (RSRP), is defined as the linear average over the power contributions (in [W]) of the resource elements (REs) that carry cell-specific reference signals within the considered measurement frequency bandwidth. For RSRP determination the cell-specific reference signals R0 according TS 36.211  shall be used. If the UE can reliably detect that R1 is available it may use R1 in addition to R0 to determine RSRP.
The reference point for the RSRP shall be the antenna connector of the UE.
If receiver diversity is in use by the UE, the reported value shall not be lower than the corresponding RSRP of any of the individual diversity branches.
RSRP measurement, normally expressed in dBm, is utilized mainly to make ranking among different candidate cells in accordance with their signal strength. Generally, the reference signals on the first antenna port are used to determine RSRP, however, the reference signals sent on the second port can also be used in addition to the RSs on the first port if UE can detect that they are being transmitted.
- Applicable for: RRC_IDLE intra-frequency, RRC_IDLE inter-frequency, RRC_CONNECTED intra-frequency, RRC_CONNECTED inter-frequency
- Note 1: The number of resource elements within the considered measurement frequency bandwidth and within the measurement period that are used by the UE to determine RSRP is left up to the UE implementation with the limitation that corresponding measurement accuracy requirements have to be fulfilled.
- Note 2: The power per resource element is determined from the energy received during the useful part of the symbol, excluding the CP.
In other words RSRP (Reference Signal Receive Power) is the average power of Resource Elements (RE) that carry cell specific Reference Signals (RS) over the entire bandwidth, so RSRP is only measured in the symbols carrying RS.
- RSRP is the average received power of a single RS resource element
- UE measures the power of multiple resource elements used to transfer the reference signal but then takes an average of them rather than summing them.
- The reporting range of RSRP is defined from -140 dBm to – 44 dBm with 1 dB resolution. The mapping of measured quantity is defined in the table – RSRP mapping 3GPP TS 36.133 V8.9.0 (2010-03) –
- 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.
Reference Signals recap: OFDMA Channel Estimation
In simple terms the Reference Signal (RS) is mapped to Resource Elements (RE). This mapping follows a specific pattern (see to below).
- So at any point in time the UE will measure all the REs that carry the RS and average the measurements to obtain an RSRP reading.
- Channel estimation in LTE is based on reference signals (like CPICH functionality in WCDMA)
- Reference signals position in time domain is fixed (0 and 4 for Type 1 Frame) whereas in frequency domain it depends on the Cell ID
- In case more than one antenna is used (e.g. MIMO) the Resource elements allocated to reference signals on one antenna are DTX on the other antennas
- Reference signals are modulated to identify the cell to which they belong
RSSI (Received Signal Strength Indicator) is a parameter which provides information about total received wide-band power (measure in all symbols) including all interference and thermal noise. RSSI is not reported to e-NodeB by UE. It can simply be computed from RSRQ and RSRP that are, instead, reported by UE.
RSSI = wideband power = noise + serving cell power + interference power
So, without noise and interference, we have that 100% DL PRB activity: RSSI=12*N*RSRP
- RSRP is the received power of 1 RE (3GPP definition) average of power levels received across all Reference Signal symbols within the considered measurement frequency bandwidth
- RSSI is measured over the entire bandwidth
- N, number of RBs across the RSSI, is measured and depends on the BW
Based on the above, under full load and high SNR:
RSRP (dBm) = RSSI (dBm) – 10*log (12*N)
RSRQ 3GPP Definition
Reference Signal Received Quality (RSRQ) is defined as the ratio N×RSRP/(E-UTRA carrier RSSI), where N is the number of RB’s of the E-UTRA carrier RSSI measurement bandwidth. The measurements in the numerator and denominator shall be made over the same set of resource blocks.
- N is the number of Physical Resource Blocks (PRBs) over which the RSSI is measured, typically equal to system bandwidth
- RSSI is pure wide band power measurement, including intracell power, interference and noise (noise + serving cell power + interference power during RS symbol)
- The reporting range of RSRQ is defined from -3…-19.5dB
So we have that RSRQ depends on serving cell power and the number of Tx antennas
E-UTRA – RSSI (Carrier Received Signal Strength Indicator), comprises the linear average of the total received power (in [W]) observed only in OFDM symbols containing reference symbols for antenna port, in the measurement bandwidth, over N number of resource blocks by the UE from all sources, including co-channel serving and non-serving cells, adjacent channel interference, thermal noise etc.
The reference point for the RSRQ shall be the antenna connector of the UE. If receiver diversity is in use by the UE, the reported value shall not be lower than the corresponding RSRQ of any of the individual diversity branches.
- Applicable for: RRC_CONNECTED intra-frequency, RRC_CONNECTED inter-frequency
e.g. – Impact of serving cell power to RSRQ:
Example for noise limited case (no interference): If all resource elements are active and are transmitted with equal power, then
- RSRQ = N / 12N = -10.8 dB for 1Tx
- RSRQ = N / 20N = -13 dB for 2Tx taking DTX into account
(because RSRP is measured over 1 resource element and RSSI per resource block is measured over 12 resource elements).
Remember that RSSI is only measured at those symbol times during which RS REs are transmitted – We do not have to take into the count DTx!!!
So, when there is no traffic, and assuming only the reference symbols are transmitted (there are 2 of them within the same symbol of a resource block) from a single Tx antenna then the RSSI is generated by only the 2 reference symbols so the result becomes
- RSRQ = N / 2N = -3 dB for 1Tx
- RSRQ = -6dB for 2Tx
Update May 2016
Power Calculation Example
Lets try to calculate RSRP, RSSI and RSRQ for one very simple case of one resource block with 12 sub carriers and 0.5 ms in time domain. Let’s assume the power of reference symbols (shown by red square) and power of other symbols carrying other data channels (shown by blue square) is same i.e. 0.021 watt (take care anyway that also the Path Losses impacts on received power, not included in this RSRP calculation).
Since RSRP is linear average of downlink reference signal for given channel bandwidth therefore
RSRP = 10*log (0.021*1000) = 13.2 dBm
While RSSI is total received wide-band power. Therefore we have to add power of all 12 carriers in the given resource block
RSSI = 10*log(0.021*1000)+10*log(12) = 24 dBm
RSRQ is now simple ratio of RSRP to RSSI with N=1
RSRQ = 10*log(0.021/(12*0.021)) = -10.79 dB
dBm vs dB
dB is ratio between two power values while dBm is used to express an absolute value of power. So when we mention RSRP and RSSI we shall always use dBm since we are talking about absolute power values but we need to use dB with RSRQ since it is the ratio of RSRP to RSSI