Radio Parameters in LTE – VoLTE:
The Parameters over LTE and VoLTE are completely similar, leaving very few additional parameters for supporting measurements for VoLTE, since VoLTE requires more keen metrics than other Packet Switching services used in LTE.
- RSRP: Reference Signal Received Power
The linear average power measured over the cell-specific reference signals. These reference signals are used for channel estimation and system acquisition in the downlink.
The Metric is used for measuring the Signal strength over an LTE network. The unit of measurement is [dBm]. And the RSRP ranging from -44dBm to -140 dBm.
- RSSI: Received Signal Strength Indicator
The total power that is seen by a receiver i.e., including the measured RSRP, the Interference at the UE and the thermal noise experienced together comprises the RSSI.
The unit of measurement is [dBm].
- RSRQ: Reference Signal Received Quality
The received quality of the signal with respect to the total power measured by the receiver is the received quality in LTE network. The unit of measurement is [dB]. The range from -3dB to -20dB.
RSRQ = (N*RSRP)/RSSI = RSRP – (RSSI)
RSRQ together with RSRP is a very useful metric to assess cell load. Especially, when RSRP is good and RSRQ is poor. The reason would be high load at the location and also probably be because of external interference.
Hence, for Inter frequency handovers RSRQ is useful.
Whereas Intra frequency handover RSRQ is not useful since differentiating RSSI within the same frequency is near to impossible hence RSRQ is also very difficult to differentiate between two sectors operating on the same carrier.
Handover KPIs have improved surprisingly when RSRQ has been used as the measurement.
UE for (Release 9 or higher), the UE can use RSRQ for cell-selection and cell-reselection in idle mode also.
- SINR: Signal to Interference plus Noise Ratio
SINR is the representation of the level of interference present at the UE.
The ratio of Signal to the total Interference and Noise measured will provide the level of signal above the interference and noise. The unit of measurement id [dB]. The range is from 0dB to 30dB.
SINR = RSRP/(RSSI-RSRP) = RSRP – (RSSI-RSRP)
VoLTE is very sensitive to SINR.
- MOS: Mean Opinion Score
MOS is the nearest representation of the voice quality over a VoLTE call. The range is given from o to 5.
The MOS depends on Latency and Jitter ie., the variable latency, mouth to ear delay.
The MOS decreased as the path loss increases.
The average time between the packet arrival is noted as Jitter, also known as the variable latency experienced within each components participating in a VoLTE call.
For the voice quality of the VoLTE to be at acceptable levels, the thresholds are given as below.
MOS at > 3.5
Jitter at < 100ms
Packet loss (FER) should be < 1%
Latency at 285ms, i.e., End to End. (at ENodeB level <= 40-80 ms)