RSRP and RSRQ Measurement in LTE

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 two parameters on reference signal:

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 and narrowband. 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 reselection decision. 

In the procedure of handover, the LTE specification provides the flexibility of using RSRP, RSRQ, or both. 

It must to be measured over the same bandwidth:

o Narrowband N = 62 Sub Carriers (6 Resource Blocks)o Wideband 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 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 [3] 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.

Or, even better, Reference Signal Received Quality 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.

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.

Applicable for: RRC_IDLE intra-frequency, RRC_IDLE inter-frequency, RRC_CONNECTED intra-frequency, RRC_CONNECTED inter-frequency

Note1: 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.

RSRP

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. 

o RSRP is the average received power of a single RS resource element.

o 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.

o Reporting range -44…-140 dBm

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.

RSRP mapping 3GPP TS 36.133 V8.9.0 (2010-03)

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.

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.

E-UTRA Carrier Received Signal Strength Indicator (RSSI), comprises the linear average of the total received power (in [W]) observed only in OFDM symbols containing reference symbols for antenna port 0, 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

RSRQ

In formula:

RSRQ = N x RSRP / RSSI

o N is the number of Physical Resource Blocks (PRBs) over which the RSSI is measured, typically equal to system bandwidth

o RSSI is pure wide band power measurement, including intracell power, interference and noise

o The reporting range of RSRQ is defined from -3…-19.5dB

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.

o Channel estimation in LTE is based on reference signals (like CPICH functionality in WCDMA)

o 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

o 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

o 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 eNodeB 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

Where:

o 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

o RSSI is measured over the entire bandwidtho 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)

So we have:

RSRQ = RSRP / (RSSI/N)

o N = Number of PRBs (Physical Resource Blocks)o RSSI = noise + serving cell power + interference power

during RS symbolo So we have that RSRQ depends on serving cell power and the

number of Tx antennas

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

SINR Definition

SINR is the reference value used in the system simulation and can be defined:

1. Wide band SINR2. SINR for a specific subcarriers (or for a specific resource

elements)

SINR = S/(I+N), all measured over the same bandwidth

SNR vs. RSRP

RSRP is measured for a single subcarrier, noisepower for 15KHz= -125.2dBm

1. Noise figure = 7 dB2. Temperature = 290 K

Assumption: RSRP doesn’t contain noise power

RSRQ to SINR

It is common sense that the performance of any wireless system has a direct relationship with the RF conditions at the time.

To aid with performance analysis then, it is tipical to define some ranges of RF measurements that correspond to some typical RF conditions one might find themselves in.

The table on the rigth represents a good classification of RF conditions Vs LTE KPIs.

The source of this table is a EUTRAN vendor and has been complied during the RF tuning process for a major US operator. Of course there are no rules as to how various RF conditions are classified, so different tables will exist but to a great extent you can expect them to align.

In this particular example, three measurement quantities are used

o RSRP (Reference Signal Received Power)o RSRQ (Reference Signal Received Quality)o SINR (Signal to Interference & Noise Ratio)

In this article we would like to highlight some of question about SINR & RSRQ relations.

SINR is defined as:

 

o S: indicates the power of measured usable signals. Reference signals (RS) and physical downlink shared channels (PDSCHs) are mainly involved

 

o I: indicates the average interference power - the power of measured signals or channel interference signals from other cells in the current system

 o  N: indicates background noise,

which is related to measurement bandwidths and receiver noise coefficients

All quantities are measured over the same bandwidth and normalized to one subcarrier bandwidth.

SINR is a measure of signal quality as well but it is not defined in the 3GPP specs but defined by the UE vendor.

It is not reported to the network. SINR is used a lot by operators, and the LTE industry in general, as it better quantifies the relationship between RF conditions and Throughput. UEs typically use SINR to calculate the CQI (Channel Quality Indicator) they report to the network.

It is a common practice to use Signal-toInterference Ratio (SINR) as an indicator for network quality. It should be however noted that 3GPP specifications do not define SINR and  therefore UE does not report SINR to the network. SINR is still internally measureed by most UEs and recorded by drive test tools.

Unfortunately UE chipset and RF scanner manufacturers have implemented SINR measurement in various different ways which in the authors’ field experience are not always easily comparable. While at first it may seem that defining SINR should be unambiguous, in case of LTE downlink this is not the case. This is because different REs within a radio frame carry different physical signals and channels each of which, in turn, see different interference power depending on inter-cell radio frame synchronization.

For example, in a frame-synchronized network, SINR estimation based on synchronization signals (PSS/SSS) results in different SINR than SINR estimation based on Reference Signals, since in the latter case the frequency shift of the RS depends on the PCI plan.

RSRQ is defined as

In what follows we show one way of converting RSRQ to SINR. 

In OFDM own cell interference is often assumed to be negligible and consequently I is due to other cell interference only.

RSSI is defined as:

where the subscript ’tot’ indicates that the power is measured over the 12NRE subcarriers of the measurement bandwidth.

The total serving cell received power depends on the number of transmitted subcarriers in the OFDM symbol carrying R0, and on the number of transmit antennas. 

We can model this impact using the per-antenna subcarrier activity factor x and set

The value of x = 1 indicates full load such that all subcarriers of one transmit antenna are transmitted for the OFDM symbol carrying R0

If only RS is transmitted (i.e., unloaded cell) the resulting subcarrier activity factors would be x = 1/6 and x = 1/3 for one and two transmit antennas, respectively.

When calculating x for two transmit antennas, one should take into account that REs overlapping with adjacent antenna RS transmission are muted, and therefore, for example, in a fully loaded 2Tx cell the scaling factor is x = 5/3 , instead of two.

It is assumed that all subcarriers have the same power, i.e., there is no power boosting for any channel. 

Since subcarrier interference plus noise power is

by combining the above equations we have the following relation of average subcarrier SINR and RSRQ (Click on Formula To Enlarge):

and finally: 

To conclude, here are a few takeaways about RSSI and RSRP as signal strength measurement techniques for LTE:

o RSSI varies with LTE downlink bandwidth.  For example, even if all other factors were equal, VZW 10 MHz LTE bandwidth RSSI would measure 3 dB greater than would Sprint 5 MHz LTE bandwidth RSSI.  But that does not actually translate to stronger signal to the end user.

o RSSI varies with LTE subcarrier activity -- the greater the data transfer activity, the higher the RSSI.  But, again, that does not actually translate to stronger signal to the end user.

o RSRP does a better job of measuring signal power from a specific sector while potentially excluding noise and interference from other sectors.

o 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.