Improving RACH Success
Improving RACH Success
The dynamic range of a UE is defined by its minimum and maximum transmit power. On the other hand, the dynamic range of the random access procedure is defined by the product of the preamble step size (powerStep) and the max number of preambles (TransMax).
For example, if powerRampStep = 2 dB, and preambleRetransMax = 10, the preamble dynamic range will be equal to 20 dB. This is far less than the UE’s dynamic range. Therefore, it is imperative the initial preamble transmit power, defined by the open-loop power control equation, is accurate.
However, in very good RF conditions, the initial preamble transmit power could be underestimated. This can result in all of the preambles being transmitted without obtaining the power required to reach the eNodeB.
To mitigate this problem, possible parameter changes are:
1.Increase the step size of the Preamble (powerStep): While this option will increase the preamble dynamic range, it will also result in increased preamble overshoot. Preamble overshoot is defined as the amount of power the preamble exceeds the required transmit power. The average preamble overshoot is equal to half the step size, so increasing the step size would contribute to increased interference
2.Increase the number of steps (TransMax): This is the best option as it will increase the preamble dynamic range, while minimizing the potential for increased noise rise. Because the UE stops sending preambles when it receives an Random Access Response (RAR), it is not required to send all the preambles defined by TransMax. The UE will only use the increased number of preambles when RF conditions require. Under typical RF conditions, the max number of preambles defined by TransMax will never be reached.
The last option is to modify the target value used in calculating the initial preamble power. Although this parameter change will not increase the random access dynamic range, it can be used to offset it.
To increase the chances of accessing the system, a correct estimation of path losses is required. This measurement will be incorrect in zones of bad coverage or high downlink interference, resulting in incorrect values of Initial transmit power.