Necessity for RACH optimization

The performance of Random Access performance is evaluated by its delay and success rate. The performance depends on following factors:

• Population under the cell coverage;
• Call arrival rate;
• Incoming handover rate;
• Whether the cell is at the edge of a tracking area;
• Traffic pattern, as it affects the DRX (Discontinuous Reception) and uplink synchronization states, and hence the need to use RACH.

These factors are affected by network configurations, such as antenna tilt, transmission power and handover threshold, and also by the load of network. If network configurations or load is changed, the performance of Random Access procedure may change greatly, which influences the performance of other procedures, such as call setup, data resuming and handover. Therefore the automatic optimization of RACH would be beneficial.

Possible RACH optimization algorithm

The configurations of RACH include:

• RACH physical resources
• RACH preamble allocation for different sets (dedicated, random-low and randomhigh)
• RACH persistence level and backoff control
• RACH transmission power control

Measurements are done in eNB, recording random access delay, random access success rate and random access load. The random access load can be indicated by the number of received preambles in a cell in a time interval. It is measured per preamble range (dedicated, random-low and random-high), and averaged over the PRACHs configured in a cell.

Thresholds are set separately for random access delay and success rate. If either of the thresholds is reached, RACH optimization is triggered. First, Random access load is analyzed to check if the random access is overload in any of the three preamble ranges. If one of them is overload, RACH preambles are reallocated among these three preamble ranges. If all of them are overload, more physical resources need to be reserved for RACH. If none of them is overload, other parameters need to be adjusted, such as increasing the transmission power step and distributing the backoff time in a wider range.