RACH Part 4: AICH transmission timing

21 Jun 2014   Category: Radioaccess

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. t_AIF stands for the transmission time on the air interface. Whereas t_proc is the processing time within Node B, including receiving/transmission time in RF and baseband processing time, etc.

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

2 * t_AIF + t_proc + 4096 chips <= 1.5 access slots

For simplicity, let’s assume:

t_AIF = d_UE / C_light

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

In case the cell radius is 20 kilometers, t_AIF is 0.0667 millisecond. Therefore we can calculate the maximum allowed t_proc is 0.8 millisecond. And the more cell radius, the less t_proc 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.