Troubleshooting UE Connection Failures: Immediate Assignment Issues in GSM

When a wireless device (User Equipment, or UE) wants to use a channel or other resources, it first sends a request. The network then assigns those resources to the requesting device. In an ideal scenario, this “channel request” is transmitted by the UE and received by the Base Transceiver Station (BTS), NodeB, or eNodeB, depending on the wireless system in use. The network responds to the request with an immediate assignment message or another appropriate message.

Let’s examine this process within the context of a GSM system.

Channel Request in GSM

A CHANNEL REQUEST is sent by a UE in the idle state when it needs a channel. This seemingly small, 1-byte message contains important information, including the reason for the connection request (e.g., responding to a paging message, an emergency call) and the preferred channel type.

Importantly, the CHANNEL REQUEST message doesn’t have a standard hexadecimal message type because it’s sent via a special “access burst” and doesn’t conform to the regular message format.

RACH (Random Access Channel) can be sent within a 51-multiframe, and the location depends on channel configuration. The MAX_RETRAN parameter, which controls how many times the UE can retransmit the CHANNEL REQUEST, is sent in the BCCH/SYSTEM INFORMATION MESSAGES 1-4. Valid values for MAX_RETRAN are 1, 2, 4, and 7.

Troubleshooting UE Connection Failure

If the UE doesn’t receive an IMMEDIATE ASSIGNMENT after sending multiple CHANNEL REQUESTs (RACH), a timer (T3126) expires. This leads to a connection failure with the GSM network. Here’s a checklist to analyze this issue, covering both the transmission and reception paths:

UE Transmission Issues

• RACH Scheduling: Verify that the CHANNEL REQUEST is scheduled on the correct slots within the 51-frame multiframe, according to the combined or non-combined channel type.
• Power Level: Ensure the power level for the access burst is adequate.
• RFIC Gain: Check for proper gain settings in the Radio Frequency Integrated Circuit (RFIC).
• Upper Layer Values: Confirm that the upper layer values being embedded into the access burst are as expected.
• Channel Conditions: Assess the channel conditions at the time the CHANNEL REQUEST is initiated. Poor signal strength or interference can prevent the request from being received.

Network Reception Issues

• Channel Request Reception: Determine whether the CHANNEL REQUEST is being received at the network side. Network logs (captured using equipment like Agilent 8960 or Anritsu 8480) are crucial for this. If the request isn’t being received, the problem likely lies with the UE transmission or the channel itself.
• Response Transmission: If the CHANNEL REQUEST is being received, check whether the network is transmitting a response (e.g., IMMEDIATE ASSIGNMENT in GSM).
• UE Reception of Response: If the network is sending a response, but the UE isn’t receiving it, the problem may be with the UE’s receive circuitry.

Additional Considerations

• Timer T3122: This timer is used during random access or dedicated channel assignment in packet transfer mode (PS mode) after the UE receives an IMMEDIATE ASSIGN REJECT message.
• C/I Ratio: Poor Carrier-to-Interference (C/I) ratio can often prevent channel requests from reaching the network.

LTE Considerations

In LTE, RACH is transmitted using the PRACH physical channel. If a similar issue arises in an LTE system, the troubleshooting checklists above can still be a helpful starting point.

RF and Wireless Resources

• WLAN 802.11ac
• 802.11ad
• WiMAX
• Zigbee
• Z-Wave
• GSM
• LTE
• UMTS
• Bluetooth
• UWB
• IoT
• Satellite
• Antenna
• RADAR