How Base Station (gNB) know the UE Pathloss?

Certainly! In 5G NR (New Radio), pathloss information is crucial for the gNB (gNodeB) to make decisions about power control, beamforming, and link adaptation. This information allows the network to adjust the transmit power for efficient communication. The UE (User Equipment) indirectly provides this information through uplink reference signals, which the gNB uses to estimate pathloss.

In 5G NR, the gNB (gNodeB) estimates the UE's pathloss using various uplink reference signals that the UE transmits. The gNB doesn't receive pathloss information directly from the UE; rather, it calculates the pathloss by comparing the power of the signals sent by the UE with the known transmit power of the UE.

Here’s a detailed breakdown of how the gNB estimates the UE’s pathloss:

1. Sounding Reference Signal (SRS)

Purpose: The SRS is the primary signal used for uplink channel sounding, including pathloss estimation.

How it Works:

• The UE transmits the SRS at a known power level.
• The gNB measures the received power of the SRS and compares it with the known transmit power of the UE.
• Using this difference (between the transmitted and received power), the gNB calculates the pathloss.

Frequency-Domain Coverage: SRS is typically transmitted over a wider frequency range, allowing the gNB to estimate pathloss over various frequency bands.

Channel Used:

• The Physical Uplink Control Channel (PUCCH) can carry the SRS when the UE sends control information like CSI.
• The Physical Uplink Shared Channel (PUSCH) is used when the UE is transmitting data along with the SRS, typically when the UE is scheduled for data transmission.

2. Uplink Reference Signals

• In addition to SRS, other uplink reference signals like Demodulation Reference Signals (DM-RS), which are transmitted along with uplink data on the Physical Uplink Shared Channel (PUSCH), can also be used to estimate pathloss.

How it Works:

• When the UE sends uplink data, it also transmits DM-RS for channel estimation and demodulation.
• The gNB can measure the power of DM-RS, calculate the pathloss, and adjust the link accordingly.

3. Pathloss Estimation Formula

Pathloss is typically estimated using the following formula:

. . . . . . . . . . . . Pathloss=P**transmit−P**received . . . . . . . . . . . .

Where:

• **P**transmit is the transmit power of the UE (which is known or can be derived based on power control settings).
• **P**received is the power of the signal received by the gNB (measured at the gNB).

4. Power Control Mechanisms

The UE uses a power control formula that allows the gNB to instruct the UE on how much power to transmit. For this, the gNB needs to continuously estimate the pathloss to maintain the link’s quality.

Open-Loop Power Control:

• Initially, the UE calculates its transmit power based on estimated pathloss and a predefined set of parameters provided by the gNB (like reference signal power and cell-specific power control parameters).

Closed-Loop Power Control:

• During communication, the gNB fine-tunes the UE’s transmit power using pathloss measurements from uplink signals like SRS or DM-RS. The gNB sends power control commands to the UE to adjust its power based on the current pathloss.

5. Uplink Control Information (UCI)

• Although UCI (like Channel State Information - CSI) reports provide information on channel conditions, the UE does not directly report pathloss. Instead, it sends CQI (Channel Quality Indicator), PMI (Precoding Matrix Indicator), and RI (Rank Indicator), which reflect the downlink channel conditions. However, the gNB primarily relies on uplink reference signals (e.g., SRS) for uplink pathloss estimation.

Example Process:

1. SRS Transmission: The UE transmits the SRS at a specific power level.
2. Signal Reception: The gNB receives the SRS and measures its power.
3. Pathloss Calculation: The gNB compares the measured power with the known transmit power of the SRS. Using the difference between these two values, the gNB calculates the pathloss.
4. Power Control Adjustment: Based on this pathloss estimation, the gNB may adjust the UE’s transmission power using uplink power control commands to maintain an optimal communication link.

Conclusion:

The gNB estimates the UE’s pathloss primarily by using the SRS and other uplink reference signals, such as DM-RS. By measuring the power of these signals and comparing them to the UE’s known transmit power, the gNB calculates the pathloss and adjusts uplink transmission settings accordingly to ensure efficient communication and optimal resource usage.