RSRP, or Reference Signal Received Power, is a UE most important measurement in LTE (Long-Term Evolution) networks. It is a key performance indicator used by User Equipment (UE) to assess the quality of the downlink signal from an LTE cell. Understanding RSRP is essential for optimizing network performance and ensuring robust cellular coverage.
Below picture of LTE drive test to cover coverage of RSRP and its acceptable ranges.
RSRP Definition and Calculation.
RSRP represents the average power level of the LTE Downlink Reference Signals (RS) received by the UE. These reference signals are specific to each cell and are used by the UE for channel estimation, signal strength measurement, and handover decisions. The formula to calculate RSRP is:
RSRP (dBm) = RSSI (dBm) – 10 * log (12 * N)
- RSSI (Received Signal Strength Indicator): The total received power including all the interference and noise.
- N: The number of Resource Blocks (RBs) over which the RSSI is measured, which depends on the channel bandwidth.
Importance of RSRP.
RSRP is one of the most fundamental measurements performed by the UE’s physical layer. Its significance lies in the following aspects:
- Signal Strength: RSRP provides a direct measure of the power level of the reference signals, indicating the strength of the received signal from the LTE cell.
- Path Loss Calculation: Knowing the absolute RSRP allows the UE to calculate the path loss between the cell and itself, which is crucial for power control and network planning.
- Handover Decisions: RSRP is used in algorithms for making handover decisions, ensuring that the UE connects to the cell with the best signal quality.
- Network Optimization: By analyzing RSRP measurements from multiple UEs, network operators can optimize cell coverage and adjust network parameters to improve overall performance.
RSRP Measurement Range.
The typical range of RSRP values is from -44 dBm to -140 dBm:
- -44 dBm: Represents a very strong signal, usually close to the base station (eNodeB).
- -140 dBm: Indicates a very weak signal, typically at the edge of the cell coverage area.
RSRP vs. RSCP.
In 3G networks, a similar concept to RSRP is RSCP (Received Signal Code Power). While both measurements serve the same purpose of indicating signal strength, RSRP is specific to LTE networks and RSCP to UMTS (3G) networks.
Uses of RSRP
RSRP is used by the UE in both idle and connected states:
- Idle State: The UE uses RSRP to select the best cell to camp on, ensuring optimal signal quality for initiating calls or data sessions.
- Connected State: RSRP measurements help the UE maintain a robust connection by enabling dynamic adjustments to transmission power and facilitating smooth handovers between cells.
RSRP Measurement in LTE.
RSRP (Reference Signal Received Power) is used to measure the coverage of an LTE cell on the downlink (DL). The UE (User Equipment) sends RRC (Radio Resource Control) measurement reports that include RSRP values in a binned format. These values range from -140 to -44 dBm with a 1 dB resolution, and the bin mapping is provided in a specified table.
Purpose of RSRP
The main purpose of RSRP is to determine the best cell on the DL radio interface, helping to select this cell as the serving cell for initial random access or intra-LTE handover. The UE sends RRC measurement reports with RSRP results when a predefined event trigger criterion is met.
Correlation with QoS.
RSRP also correlates with user plane QoS (Quality of Service). For outdoor environments, RSRP measurements can be categorized into three ranges:
- RSRP > -75 dBm: Excellent QoS can be expected, provided that the cell’s bandwidth is not heavily contested.
- RSRP between -75 and -95 dBm: Slight QoS degradation is expected. Throughput may decline by 30–50% as RSRP drops from -75 to -95 dBm.
- RSRP < -95 dBm: QoS becomes unacceptable, with throughput potentially declining to zero at around -108 to -100 dBm, leading to call drops.
For in-house cells, acceptable QoS can be maintained at RSRP values as low as -115 dBm, similar to observations in the 3G UTRAN environment for CPICH RSCP. This is due to the limited footprint of in-house cells, which can handle worse radio conditions compared to outdoor environments.
Conclusion.
RSRP is a critical measurement in LTE networks, providing essential information about signal strength and quality. It plays a vital role in network optimization, handover decisions, and ensuring reliable connectivity for users. By understanding and leveraging RSRP, network operators can enhance the performance and coverage of LTE networks, delivering a better user experience.