RACH Part 4: AICH transmission timing

AICH transmission timing, either 0 or 1, is informed to UE by RNC in SIB5/5bis via BCH.

  • If AICH transmission timing is 0, the next PRACH preamble will be initiated at earliest 3 access slots later after previous failed(no acknowledggement or acknowledggement is NACK) PRACH preamble. And AICH is expected to arrive at UE after 1.5 access slots;
  • If AICH transmission timing is 1, the next PRACH preamble will be initiated at earliest 4 access slots later after previous failed(no acknowledggement or acknowledggement is NACK) PRACH preamble. And AICH is expected to arrive at UE after 2.5 access slots;

Below figures show the timing of the whole PRACH process when AICH transmission timing is 0. tAIF stands for the transmission time on the air interface. Whereas tproc is the processing time within Node B, including receiving/transmission time in RF and baseband processing time, etc.

Mapping

As you can see, since the duration between PRACH preamble and AICH is fixed as 1.5 access slots, tAIF impacts tproc a lot:

2 * tAIF + tproc + 4096 chips <= 1.5 access slots

For simplicity, let’s assume:

tAIF = dUE / Clight

in above formula, dUE is the distance between Node B and UE while Clight is the light speed, i.e. 3e8 meters per second.

In case the cell radius is 20 kilometers, tAIF is 0.0667 millisecond. Therefore we can calculate the maximum allowed tproc is 0.8 millisecond. And the more cell radius, the less tproc is allowed. So generally, the vendors will limit the maximum cell radius(for instance 60 km) when AICH transmission timing 0 is used by the operators.