In our field test mode post, we refer to few acronyms such as RSRP, RSSI, RSRQ, SINR but did not go into many details to prevent going off-topic. This post elaborates on that terminology for greater understanding. What do the terms RSRP, RSSI, RSRQ, SINR mean when talking about cell phone signal strength or quality of calls and consistency or speed of data connectivity? Let us understand the meaning and concepts intrinsic to those terms.
When determining the signal level and quality in modern radio communication networks, there are a number of parameters that are measured and/or calculated. These measurements and calculations are done by special chipsets present in any User Equipment (UE) that uses these signals. Although some of these parameters may at first glance appear to be very similar, they each serve a different purpose and are used in different scenarios.
In cellular networks, for example, a mobile phone (UE) moving from cell to cell has to perform cell selection and handover. To do this, it has to measure the signal strength and quality of the neighboring cells.
There're many acronyms related to measuring wireless signal strength including the following:
- RSSI = Received Signal Strength Indicator.
- RSRP = Reference Signal Received Power.
- RSRQ = Reference Signal Received Quality.
- SINR = Signal to Interference plus Noise Ratio.
- SNIR = Signal to Noise plus Interference Ratio.
- SNR = Signal to Noise Ratio.
- ASU = Arbitrary Strength Unit.
- RS SINR or RSSNR = Signal to Noise Ratio.
However, in this article, we will dig into the following four main parameters and explain what they mean and how they are used. The four parameters are:
- RSRP. In dBm, LTE - The Reference Signal Received Power has a range of -45dbm (good) to -140dbm (bad) - see chart above for actual assessment. It is a variation of RSSI.
- RSSI. In dBm - The Receive Strength Signal Indicator measures the average total received power of the whole band. See chart above for value range from excellent to worst.
- RSRQ. In dB, LTE - The Reference Signal Received Quality has a range from -40dB (bad) to -3dB (good) - see chart above for actual assessment. This is the most important factor when trying to determine if devices will work or not. The reason is that even if there's signal power, the noise can negate the benefits and render it useless.
- SINR. In dB - The Signal to Noise Ratio is derived from the desired signal divided by undesired noise. It looks at signal as related to noise. See chart above for value range from excellent to worst.
- RSRP - Reference Signal Received Power.
RSRP is the LTE Reference Signals' power spread over the full narrowband and bandwidth.
To detect RSRP or RSRQ, a minimum of -20 dB SINR (of the S-Synch channel) is required.
RSRP is defined as the linear average over the power contributions (measured in Watts) of the resource elements that carry cell-specific reference signals within the frequency bandwidth that is considered for measurement.
To determine RSRP, the cell-specific reference signals R0 is used according to TS 36.211 . R1 may be used in addition to R0 to determine RSRP if UE is able to detect reliably that R1 is available.
RSRP's reference point is the antenna connector of UE.
An alternative definition would be that RSRQ is the ratio N × RSRP / (E-UTRA carrier RSSI), where N is the number of resource blocks of the E-UTRA carrier RSSI measurement's bandwidth. The measurements used for both the denominator and numerator are made over the same set of resource blocks.
If UE uses receiver diversity, the value reported should not be less than the corresponding RSRP of any of the individual diversity branches.
RSRP is used for RRC_IDLE intra-frequency, RRC_IDLE inter-frequency, RRC_CONNECTED intra-frequency and RRC_CONNECTED inter-frequency.
The number of resource elements within the measurement period that are used by UE to determine RSRP and within the considered measurement frequency bandwidth is left up to the UE's implementation with the limitation that the requirements for corresponding measurement accuracy have to be fulfilled.
Stated differently, RSRP is the average power of Resource Elements (RE) that carry cell specific Reference Signals (RS) over the entire bandwidth. This means that RSRP is only measured in the symbols that carry Reference Signals.
- RSRP is the average received power of a single RS resource element.
- UE measures the power of several resource elements that are used to transfer the reference signal, but then uses their average rather than their sum.
- Reporting range is between -44 dBm and -140 dBm with a 1 dB resolution.
RSRP does a good job of measuring signal power from a specific sector while excluding interference and noise from other sectors.
RSRP levels for usable signal typically range from about -120 dBm at the edge of LTE coverage to -75 dBm close to an LTE cell site.
- RSSI – Received Signal Strength Indicator.
The carrier RSSI measures the average total received power observed, but only uses 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 non-serving cells and co-channel serving, adjacent channel interference, thermal noise, etc. The total is measured over 12-subcarriers, including RS from Serving Cell and Traffic in the Serving Cell.
RSSI is a parameter which provides information about total received wide-band power measure in all symbols, including all thermal and interference noise.
A UE does not report RSSI to eNodeB as it can be calculated from RSRQ and RSRP that are reported by UE.
RSSI = serving cell power + interference power + noise.
Without interference and noise, we will have 100% DL PRB activity, i.e. RSSI = 12 x N x RSRP.
- RSRP is the received power of 1 RE (3GPP definition) average of power levels received across all Reference Signal symbols within the selected measurement frequency bandwidth.
- RSSI is measured over the entire bandwidth.
- N: number of RBs across the RSSI measured and depends on the bandwidth.
Based on above, under full load and high:
RSRP (dBm) = RSSI (dBm) -10 x log (12 x N).
So we have:
RSRQ = RSRP / (RSSI/N).
- N = Number of PRBs (Physical Resource Blocks).
- RSSI = noise + serving cell power + interference power during RS symbol.
- RSRQ depends on serving cell power and the number of Tx antennas.
- RSRQ – Reference Signal Received Quality.
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.
RSRQ is defined as the ratio N × RSRP / (E-UTRA carrier RSSI), where N is the number of RBs 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.
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:
- Narrowband N = 62 Sub Carriers (6 Resource Blocks).
- Wideband N = full bandwidth (up to 100 Resource Blocks / 20 MHz).
E-UTRA Carrier Received Signal Strength Indicator (RSSI), comprises the linear average of the total received power (measured in Watt) observed only in OFDM symbols containing reference symbols for antenna port 0, in the measurement bandwidth, over N number of resource blocks by UE from all sources, including co-channel serving and non-serving cells, adjacent channel interference, thermal noise etc.
The reference point for the RSRQ is the antenna connector of UE. If receiver diversity is used by UE, the reported value will be less than the corresponding RSRQ of any of the individual diversity branches.
RSRQ is used for RRC_CONNECTED intra-frequency and RRC_CONNECTED inter-frequency.
RSRQ = N x RSRP / RSSI.
- N is the number of Physical Resource Blocks (PRBs) over which the RSSI is measured, typically equal to system bandwidth.
- RSSI is a pure wide band power measurement, including intracell power, interference and noise.
- The reporting range of RSRQ is from -3 dB to -19.5 dB.
- SINR – Signal to Interference plus Noise Ratio.
SINR is the reference value used in system simulation and can be defined as:
- Wide band SINR.
- SINR for a specific resource element, or for a specific subcarrier.
Although SINR is also a measure of signal quality, it is not defined in the 3GPP specs, but by UE vendor and it is therefore not reported to the network.
SINR is used by the LTE industry in general and by many operators, as it quantifies the relationship between Throughput and RF conditions better. LTE UEs normally use SINR to calculate the CQI (Channel Quality Indicator) that is reported to the network.
SINR as an indicator is commonly used for network quality. It should however be noted that SINR is not specified by 3GPP and UEs therefore do not report SINR to networks. SINR is however measured internally by most UEs and recorded to be used by test tools.
RF scanner manufacturers and UE chipsets have unfortunately implemented SINR measurement in many different ways which are not always easy to compare. While it may seem at first that defining SINR should be clear, this is not the case with LTE downlinks. This is due to different REs within a radio frame carrying different channels and physical signals, each of which measures different interference power depending on inter-cell radio frame synchronization.
In for example a frame-synchronized network, SINR estimations based on synchronization signals (PSS/SSS) result in a different SINR than a SINR estimation based on Reference Signals, since in the latter case, the frequency shift of the RS depends on the PCI plan.
SINR = S / (I + N) measured over the same bandwidth.
Why Signal to Noise plus Interference ratio (SINR) is not checked as much as RSRQ? SINR is a function of RSRQ, so it is somewhat redundant. RSRQ - Reference Signal Received "Quality" is an overarching main factor that already has Signal to Interference plus "Noise" Ratio figure incorporated or configured into it.