COTS UE Connection Issues With SrsRAN GNB (BW > 20 MHz)

by Alex Johnson 56 views

Introduction

When deploying srsRAN, a common challenge arises when Commercial Off-The-Shelf User Equipments (COTS UEs) fail to connect to the gNodeB (gNB) when the bandwidth (BW) is set above 20 MHz. This issue can be particularly perplexing, as the UE often detects the Synchronization Signal Block (SSB) but does not proceed with the attachment process. This article delves into the potential causes and solutions for this problem, focusing on configurations and settings within the srsRAN environment. We will explore the experiences of users who have encountered this issue, such as those using Google Pixel 5 and OP 8 5G mmWave devices, and provide a comprehensive guide to troubleshooting and resolving these connectivity challenges. Understanding the intricacies of network configuration, UE compatibility, and the interplay between hardware and software components is crucial for a successful deployment.

Problem Description

The core issue revolves around the inability of COTS UEs to attach to an srsRAN gNB when the configured bandwidth exceeds 20 MHz. Users report that while the UE can detect the SSB transmitted by the gNB, the attachment procedure never initiates. This behavior is consistent across different UE models, indicating a potential configuration or compatibility issue within the srsRAN setup. The problem is significant because it limits the achievable data rates and network capacity, undermining the benefits of wider bandwidths. Specifically, the configuration file used for the gNB plays a vital role, and discrepancies in parameters such as the downlink ARFCN, band, channel bandwidth, and system information can lead to attachment failures. Thorough examination of these settings is essential for resolving the connection problem.

Initial Observations

The typical scenario involves a user configuring an srsRAN gNB with a bandwidth greater than 20 MHz. Despite proper setup and the UE’s ability to detect the SSB, the connection process stalls. Common observations include:

  • UE Detection of SSB: The UE successfully identifies the SSB, suggesting that the RF front-end and initial synchronization are functioning correctly.
  • Attachment Failure: The UE does not proceed with the attachment procedure, indicating a problem in the subsequent steps, such as system information acquisition or random access.
  • Configuration Consistency: The issue persists across different UE models, pointing towards a common factor within the gNB configuration or srsRAN software.

Specific User Experiences

Users have reported this issue with various COTS UEs, including the Google Pixel 5 and OnePlus 8 5G mmWave. These devices are known for their compatibility with a wide range of 5G networks, making the connection failure with srsRAN particularly noteworthy. The consistency of the problem across different devices suggests that the issue is not isolated to a specific UE implementation but is more likely related to the gNB configuration or the interaction between the UE and the srsRAN software.

Analyzing the gNB Configuration

The gNB configuration file is central to understanding and resolving the connection issue. Key parameters within the configuration determine the network’s behavior and compatibility with UEs. Let's dissect a sample configuration and highlight the critical aspects.

Sample gNB Configuration

cu_cp:
  amf:
    addr: 127.0.0.5
    bind_addr: 127.0.0.1
    supported_tracking_areas:
      - tac: 1
        plmn_list:
          - plmn: "00101"
            tai_slice_support_list:
              - sst: 1

ru_sdr:
  device_driver: uhd                                            # The RF driver name.
  device_args: type=x300, num_recv_frames=64,num_send_frames=64
  clock: external                                                        # Specify the clock source used by the RF.
  sync: external                                                         # Specify the sync source used by the RF.
  tx_gain: 31                                                   # Transmit gain of the RF might need to adjusted to the given situation.
  rx_gain: 28   

cell_cfg:
  dl_arfcn: 536020                                              
  band: 7  
  channel_bandwidth_MHz: 30
  common_scs: 15
  plmn: "00101"
  tac: 1

Key Configuration Parameters

  1. cu_cp Section: This section configures the Central Unit Control Plane (CU-CP), which handles control plane signaling. The amf parameters specify the address and bind address of the Access and Mobility Management Function (AMF), a core network element. The supported_tracking_areas parameter defines the Tracking Area Code (TAC) and Public Land Mobile Network (PLMN) supported by the gNB. Ensuring these parameters are correctly set and aligned with the core network configuration is crucial for successful attachment.

  2. ru_sdr Section: This section configures the Radio Unit Software Defined Radio (RU SDR) parameters. The device_driver specifies the RF driver, typically UHD for Ettus Research devices. The device_args parameter provides arguments for the driver, such as the device type and number of frames. The clock and sync parameters define the clock and synchronization sources, which should be set to external for optimal performance. The tx_gain and rx_gain parameters control the transmit and receive gains, which may need adjustment based on the RF environment.

  3. cell_cfg Section: This section configures the cell-specific parameters. The dl_arfcn (Downlink Absolute Radio Frequency Channel Number) and band parameters define the operating frequency. The channel_bandwidth_MHz parameter specifies the channel bandwidth, which is the primary focus of this issue. The common_scs parameter sets the common subcarrier spacing, and the plmn and tac parameters define the PLMN and TAC for the cell. These parameters must be correctly set to match the UE’s capabilities and network requirements. The channel_bandwidth_MHz parameter is of particular interest as it directly relates to the reported issue. A value of 30 MHz, as in the example, may be problematic if the UE or the network configuration does not fully support this bandwidth. It's essential to verify that the UE and the srsRAN configuration are aligned regarding the supported bandwidths.

Potential Causes and Solutions

Several factors can contribute to the failure of COTS UEs to connect to srsRAN gNBs with bandwidths exceeding 20 MHz. Addressing these potential issues systematically is essential for effective troubleshooting.

1. Bandwidth Support and UE Capabilities

Explanation: One of the most common reasons for connection failures is the UE's limited support for wider bandwidths. While many modern UEs support a range of bandwidths, some devices may have limitations, especially in specific frequency bands or carrier aggregation configurations. It is crucial to ensure that the UE's capabilities align with the gNB's configuration. Bandwidth support varies among different UE models, and discrepancies can lead to attachment failures. Ensuring the UE's modem supports the configured bandwidth is a fundamental step in troubleshooting.

Solution:

  • Verify UE Specifications: Consult the UE's technical specifications to confirm the supported bandwidths and frequency bands. This information is typically available on the manufacturer's website or in the device's user manual.
  • Test with Different UEs: If possible, test the connection with multiple UE models to determine if the issue is specific to a particular device. This can help isolate whether the problem lies with the UE or the gNB configuration.
  • Adjust Bandwidth Configuration: If the UE does not support the configured bandwidth, reduce the channel_bandwidth_MHz parameter in the gNB configuration file to a supported value, such as 20 MHz or lower. This adjustment can often resolve connectivity issues related to bandwidth incompatibility.

2. Incorrect Frequency Band Configuration

Explanation: The frequency band configuration, defined by the band and dl_arfcn parameters in the gNB configuration, must be accurate and supported by both the gNB and the UE. An incorrect configuration can prevent the UE from properly synchronizing with the network and initiating the attachment process. Frequency band mismatches are a common cause of connectivity problems in cellular networks. Ensuring that the frequency band configured in the gNB matches the UE's supported bands is crucial for establishing a connection.

Solution:

  • Verify Frequency Band: Ensure that the configured frequency band (band) and downlink ARFCN (dl_arfcn) are correct for your region and spectrum allocation. Consult the 3GPP specifications or a frequency band chart to confirm the appropriate values.
  • Check UE Support: Verify that the UE supports the configured frequency band. This information can be found in the UE's technical specifications or through network scanning tools.
  • Adjust Configuration: If the frequency band is incorrect or not supported by the UE, modify the band and dl_arfcn parameters in the gNB configuration file to match a supported band. Restart the gNB after making these changes to apply the new configuration.

3. RF Front-End and SDR Issues

Explanation: Problems with the Radio Frequency (RF) front-end or the Software Defined Radio (SDR) can also lead to connection failures. This includes issues with the SDR driver, clock synchronization, and gain settings. RF front-end problems can manifest as the UE's inability to detect the SSB or establish a stable connection. Ensuring the SDR is properly configured and the RF environment is optimized is critical for reliable performance.

Solution:

  • Check SDR Driver: Ensure that the correct SDR driver is installed and configured properly. For Ettus Research devices, the UHD driver is commonly used. Verify that the driver is compatible with the srsRAN software and the SDR hardware.
  • Verify Clock and Sync: Confirm that the clock and synchronization sources are correctly configured. External clock and sync sources are generally recommended for optimal performance. Ensure that the SDR is properly synchronized with the external sources.
  • Adjust Gain Settings: Experiment with the transmit (tx_gain) and receive (rx_gain) gain settings in the ru_sdr section of the gNB configuration. Incorrect gain settings can lead to signal saturation or insufficient signal strength. Adjust these parameters incrementally to find the optimal values for your environment. Start with moderate values and adjust based on observed performance.

4. System Information Broadcast (SIB) Issues

Explanation: The System Information Broadcast (SIB) messages contain essential network parameters that the UE needs to attach to the network. If the SIB messages are not being broadcast correctly or contain incorrect information, the UE will fail to connect. SIB transmission problems can prevent the UE from acquiring necessary network information, leading to attachment failures. Ensuring the SIB messages are correctly configured and broadcast is a fundamental aspect of network setup.

Solution:

  • Verify SIB Configuration: Check the SIB configuration in the srsRAN software to ensure that all required parameters are correctly set. This includes parameters such as PLMN, TAC, cell identity, and supported features.
  • Monitor SIB Transmission: Use a spectrum analyzer or a UE logging tool to monitor the SIB transmission. Verify that the SIB messages are being broadcast at the correct intervals and with sufficient signal strength.
  • Review srsRAN Logs: Examine the srsRAN logs for any errors or warnings related to SIB transmission. This can provide valuable insights into potential configuration issues or software bugs.

5. Core Network Connectivity

Explanation: Issues with the core network connectivity, particularly the connection to the Access and Mobility Management Function (AMF), can also prevent UEs from attaching. If the gNB cannot communicate with the AMF, it cannot complete the attachment procedure. Core network issues can disrupt the communication between the gNB and the core network elements, preventing successful UE attachment. Ensuring proper connectivity and configuration of the core network elements is crucial for end-to-end network operation.

Solution:

  • Verify AMF Configuration: Check the amf section of the gNB configuration to ensure that the AMF address and bind address are correctly set. The addresses should match the actual configuration of the AMF in the core network.
  • Test Network Connectivity: Use network diagnostic tools, such as ping or traceroute, to verify that the gNB can communicate with the AMF. Ensure that there are no firewall rules or network configurations blocking the communication.
  • Check Core Network Status: Verify that the core network elements, including the AMF, are running and functioning correctly. Review the core network logs for any errors or warnings that may indicate a problem.

Conclusion

Troubleshooting COTS UE connection issues with srsRAN gNBs at bandwidths above 20 MHz requires a systematic approach. By verifying UE capabilities, frequency band configurations, RF front-end settings, SIB transmission, and core network connectivity, you can identify and resolve the underlying causes of these problems. Remember to consult the srsRAN documentation and community forums for additional support and insights. Successfully deploying srsRAN with COTS UEs hinges on meticulous configuration and a thorough understanding of the interplay between hardware, software, and network parameters. This article provided the common issues and solutions to solve the problem of COTS UE connection issues with srsRAN gNBs at bandwidths above 20 MHz. For further information, visit the srsRAN Project Documentation.