In the 4G, there are multiple RF measurement quantities we must understand if we want to optimize the performance in this context. In this post we will review the most important variables and the impact they have in the performance and user experience.
1. RF measurement quantity: RSRP (Reference Signal Received Power)
Reference Signal Received Power is a measure of the received power level in an LTE cell network and it is the most basic and important measurement quantity in RF. In other words, RSRP is the average power of Resource Elements (RE) that carry cell specific Reference Signals (RS) over the entire bandwidth, so RSRP is only a measurement of the symbols carrying RS.
In the next picture we can see how UEs sees RS from cells of a 3-sector site where there is no RS-RS interference since PCI mod3 are different.
RSRP is the linear average of received power of RS resource elements. UE measures the rx power of multiple RS REs and takes average of them. The reporting range goes from -44 to -140 dBm with 1 dB resolution. The mapping of measurements from UE and the reported value is in the table below:
So, the UE measures the RSRP but it does not report the actual value to the network, it will report the “Reported Value” from previous table based on the value measured that fits within the “Measured quantity value”.
2. RF measurement quantity: RSSI (Received Signal Strength Indicator)
RSSI measures all power within the measurement bandwidth hence it includes thermal noise, interference and serving cell power. It is actually a basic 4G measurement quantity but not commonly used during RF optimization. A basic formula to understand this metric is:
RSSI = wideband power = noise + serving cell power + interference power
This measurement is over those OFDM symbols that contain RS for antenna port R0. It is important to remark that RSSI is internal to UE, not reported in uplink to eNB
Relation between RSSI and RSRP
Using the previous formula, RSSI = wideband power= noise + serving cell power + interference power, without noise and interference, 100% DL PRB activity:
RSSI=12*N*RSRP
Where:
- N is number of PRBs across the RSSI is measured and depends on the BW.
Based on the above, under 100% PRB utilization and high SINR:
RSRP (dBm)= RSSI (dBm) -10*log (12*N)
Following table shows some examples for different BW:
Now, let’s check a chart example fo the relation between RSSI and RSRP in a fully loaded 10MHz cell.
If we consider cell load, RSSI increases about 5 dB when PRB activity increases to 100% in a 10MHz cell, however RSRP is independent of cell load.
3. RF measurement quantity: RSRQ (Reference Signal Received Quality)
RSRQ is the ratio NxRSRP/(E-UTRA carrier RSSI), where N is the number of RBs of the EUTRA carrier RSSI measurement bandwidth. The measurements in the numerator and denominator shall be made over the same set of resource blocks.
EUTRA 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 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 the 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.
RSRQ = N x RSRP / RSSI
RSRQ reporting range is from -3 to -19.5 dB. Some devices may report values below -20 dB. Same as RSRP, UEs do not report the actual value but an approximate value depending on the range of the RF measurement quantity value.
Now, let’s review the theoretical calues in idle and fully loaded cell.
Idle cell
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 = -3 dB for 1Tx
- RSRQ = -6 dB for 2Tx
Fully loaded cell
If all resource elements are active and are transmitted with equal power then:
- RSRQ = -10.8 dB for 1Tx
- RSRQ = -13 dB for 2Tx
In practice, UEs seem to report RSRQ ~11dB for fully loaded cell, assuming no other-cell interference
Following chart shows RSRQ measurement for 2Tx cell when only the serving cell is on air.
4. RF measurement quantity: SINR / SNR / CINR
Usually SINR=SNR=CINR unless the receiver is able to separate interference from thermal noise. This is probably the most important RF measurement quantity within LTE network. SINR means Signal to Interference+Noise Ratio.
SINR = S / (I+N)
The actual measurement definition is different for every measurement device. Typically UEs measure SNR from RS. Exact measurement method uncertain, since UE chipset vendors don’t typically reveal technical data. Scanners measure SNR from PSS/SSS and/or RS.
Let’s check some SINR measurements using different devices.
SINR shares a relation with RSRQ.
One uncomfortable property of the RSRQ to SINR mapping is that it depends on the instantaneous serving cell subcarrier activity factor x, which is typically not known in live network measurements.
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.
Theoretical formula for the SINR and RSRQ relation:
- 2RE/RB equals to empty cell. Only Reference Signal power is considered from serving cell
- 12RE/RB equals to fully loaded serving cell. All resource elements are carrying data
- In practice, mapping from RSRQ to SINR seems difficult. Currently available measurement UEs and scanners report SINR directly
5. Additional measurements to consider
UE orientation can have an impact on throughput performance.
- MIMO throughput is very sensitive to UE orientation
- Eliminating power imbalance or channel correlation alone is not sufficient for good MIMO throughput. Both should be eliminated.
In MIMO scenarios, channel correlation is also very important to ensure high throughput. Let’s check following example, even with constant SINR of 25 dB, the high spatial correlation causes rank-1 transmission even at very high SINR (2nd stream throughput goes to almost zero)
Useful links:
