Introduction

Devices in a 5G Network need to maintain constant connectivity with the network for reachability and reliability purposes. There are three layers to how devices maintain the best possible connection with the network.

• Intra-frequency and Equal Priority Inter-frequency Reselection
• Higher Priority Inter-frequency Reselection
• Lower Priority Inter-frequency and Inter-RAT Reselection

Cell reselection is conducted by the device, with the current serving cell providing the reselection “rules” for the device to adhere to. It should also be noted that cell reselection occurs only while the device is in RRC IDLE mode. If the device was in RRC CONNECTED mode, then the network would dictate the devices cell reselection based on device measurement reports, this procedure is also known as a handover.

Measurement Triggers

Just because a device is camped on a cell in idle mode does not mean it constantly searches for other cells to reselect. The serving cell provides the device thresholds that dictate when specific measurements should occur. The threshold for intra-frequency measurements is configured in System Information Block 2 (SIB2) as s-IntraSearchP.

s-IntraSearchP is added to the value of q-RxLevMin to define the intra-frequency measurement threshold. The catch here is that each signaled value is multiplied by 2.

• -59 (-118) + 31 (62) = -56 dBm RSRP

This means that as long as the device measures the serving cell at a weaker value than -56, it will measure intra-frequency neighbors. The device would need to measure the serving cell at a very high value (-55 or better) to not be measuring for other cells.

Intra-frequency neighbors (SIB3)

The Intra-frequency neighbor list is broadcast on System Information Block 3 (SIB3) and is not required to be sent by the network to allow the device to measure intra-frequency neighbors. The presence of SIB3 allows the network to provide biases and offsets for specific PCIs that would otherwise not be taken into consideration during the neighbor ranking process.

• physCellId
  • Lists the intra-frequency neighbor PCI
• q-OffsetCell
  • (-24 to 24)
  • Higher values = lower neighbor ranking
    • Neighbor RSRP (-74) – q-OffsetCell (10) = -84 RSRP
  • Lower values = higher neighbor ranking
    • Neighbor RSRP (-74) – q-OffsetCell (-4) = -70 RSRP
• q-RxLevMinOffsetCell
  • (1dB to 8dB) actual value is multiplied by 2
  • Added to the corresponding neighbor PCIs q-RxLevMin value
    • Neighbor q-RxLevMin (-118) + q-RxLevMinOffsetCell (2) = -116
• q-QualMinOffsetCell
  • (1dB to 8dB)
  • Added to the corresponding neighbor PCIs q-QualMin value (if configured)

Cell Ranking

When a device measures neighbor cells for reselection, it must rank the neighbor cells against the current serving cell. Just because a neighbor cell measures higher than the serving cell does not mean the neighbor will rank higher than the serving cell.

When the device ranks the serving cell, it takes the current measured RSRP value of the serving cell and adds the q-Hyst value defined by the serving cell in SIB2. This value ranges from 0dB to 24 dB and makes the serving cell more attractive. This value helps prevent devices from frequently reselecting when serving and neighbor cells are measured at similar values.

• Serving Cell RSRP (-80) + q-Hyst (4) = -76 RSRP Serving Cell

When the device ranks the neighbor cell, it takes the current measured RSRP value of the neighbor cell and subtracts the q-OffsetCell from SIB3.

• Neighbor Cell RSRP (-80) – q-OffsetCell (2) = -82 RSRP Neighbor Cell

If the serving cell is not broadcasting SIB3, then the neighbor ranking is based on the neighbor cell’s RSRP measurement.

Scenario 1 – The device measures the Serving Cell and Neighbor Cell at the same value.

• Neighbor Cell RSRP -80 RSRP
• Serving Cell RSRP (-80) + q-Hyst (4) -76 RSRP
  • Serving cell ranks higher

Scenario 2 – The device measures the Neighbor Cell higher than the Serving Cell.

• Neighbor Cell RSRP (-77)
• Serving Cell RSRP (-80) + q-Hyst (4) -76 RSRP
  • Serving cell ranks higher

Scenario 3 – The device measures the Neighbor Cell higher than the Serving Cell.

• Neighbor Cell RSRP (-72)
• Serving Cell RSRP (-80) + q-Hyst (4) -76 RSRP
  • Serving cell ranks higher

Reselection

The device has been measuring intra-frequency neighbors and ranked them against its current serving cell. When the device ranks a neighbor cell higher than the serving cell, it must meet the time requirement set by the serving cell. The time requirement t-RelectionNR defined in SIB2, indicates the time in seconds, and ranges from 0 to 7 seconds.

In this example, the neighbor cell must rank higher than the serving cell consistently for two seconds before the device reselects to the highest ranked neighbor.

Conclusion

Other parameters not configured in this article’s examples may be considered for intra-frequency cell reselection. Device speed state, multiple SSB averaging and measurement thresholds, and more may be configured by the network. The parameters and concepts explained in this article are a basic over view of how devices determine when to measure, rank, and reselect intra-frequency neighbors.

There will be follow on articles in this series about the other types of cell reselection that were mentioned in the article introduction.

• Equal Priority Inter-frequency Reselection
• Higher Priority Inter-frequency Reselection
• Lower Priority Inter-frequency and Inter-RAT Reselection