RACH congestion/access slots

[vishal]

Hi,

Can it be possible to have RACH congestion when more than 15 users try
to set up RRC congestion ?

I think there are 15 acess slots on RACH, and it works on slotted ALOHA
manner.

[wirelessman]

Of Course yes..

[vishal]

Then how it works ? Can we have more sigantures or access slots to
prevent congestion ?

> > manner.- Hide quoted text -- Show quoted text -

[siat ling]

sorry, i haven't find out the answer yet.

 

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[venkat-iitm]

Hi All,

Here is my answer for RACH congestion question.

if we see the RACH preamble structure and transmission. In a 20ms frame
interval it has 15 slots of 2slots each, with a transmission of 4096
chips in the 2slot (5120 chips).

The transmission is as follows:

1) UE transmitts the preamble in a TTI preconfigured by the higher
layers and waits for an ack from the n/w.
2) if it gets an ack it proceeds with the message transmission, else
3) if it gets an nack there is a back ot time whoose time is configured
by higher layers and waits for that time to try again, else
4) if it did not get a reply for max number of persitency checks with a
ramping in power of preamble part, it waits for a minimum of 10ms and
tries again.

Now, if we the preamble struct, it is a combination of scrambling code
and a signature.

there are 16 PRACH preamble scrambling codes in a cell (in relation to
the primary scrambling code of the cell)and 16 signatures (hadamard
codes).

So, with this combinations of 15 slots, 16 preamble codes specific to a
cell and 16 signatures, there will not be any congestion for RACH in
the n/w.

I think I am to the point, i any mistakes or queries, please get me
back. For more clarification you can refer to 25.213, 25.211 and 25.321
specs

Regards,
Venkat

[vishal]

Thanks Venkat.

But how slotted Aloha works ? What happens when 16 UEs try to set up
RRC connection ?

Rgds
Vishal

[vishal]

Hi All,

Thanks Venkat.

But what happens when multiple UEs use the same TTI to initiate the
call. I know it wokrs as slotted ALOHA. What if 16 UEs try to set up
RRC connection at the same time ?

Rgs
Vishal