Friday, July 27, 2012

PAGING – PF and PO Calculation - LTE



Paging is indication to UE to establish NAS Procedure basically happens at two occasions
1)      Incoming Call Alert: Means Mobile Terminated Call.
2)      Broadcast Change : E-UTRAN notifies the UEs by means of a Paging message including a SystemInfoModification flag

Paging message will be associated with PF and PO at RE - Grid Level.
Where PF is associated with Corresponding SystemFrame and PO is associated with SubFrame where Paging Message is  Broadcasted.

Then how to Caliculate Paging Frame and Paging Occasion for particular UE:
In caliculation the below factors plays role
·         SFN – System Frame Number
·         DefaultPagingCycle is from SIB2
·         PagingDrx Cycle from S1-Paging Message
·         Ue Id : Received from S1-Paging Message
·         nB : From SIB2


Now Let us Take Example to caliculate Paging Occasion :

In SIB2:
                nB -  T / 4
                DefaultPagingCycle :  64
                From S1-Paging Message:
                PagingDRX : 128
                UeIndex from S1 Paging : UE_ID = 0x0115 = (0000 0001 0001 0101 ) after Removing 6 LSBs which makes (0000 0001 00 = 4)
                The above UeIndex is in general UeIdentity % 1024 which means, results only  occupies 10 Bits Only. So, above we Removed 6 LSbits.



T = Min (PagingDRXCycle, DefaultPagingDrxCycle) = Min ( 128,64) = 64
nB = T/4 = 64/4 = 16
N = min(T,nB) = Min(64, 16) =  16
UE_ID = 4 (From above Steps derived from S1 Paging Message)

SFN mod T= (T div N)*(UE_ID mod N)
RHS => (T Div N) * (UE_ID mod N) = ( 64 Div 16) * ( 4 % 16) = 4 * 4 = 16
LHS => SFN Mod T = SFN Mod 64 = 16
So,

PF values could be any thing  where SFN =  (64 * i ) + 16  (I = 0 to N But SFN <= 1024)
i.e Values PF can  be any of 80, 144, 208, 272, 336, 400, 464, 528, 592, 656, 720, 784, 848, 912, 976, 1040, 1104, 1168.

Now Comes To Paging Ocassion:
Ns: max(1,nB/T) = max (1, 16/64) = 1

i_s = floor(UE_ID/N) mod Ns
ð  i_s = floot(4 / 16) mod 1 = 0

From the below table when i_s == 0 and Ns = 1 => P0 = 9 
FDD:
Ns
PO when i_s=0
PO when i_s=1
PO when i_s=2
PO when i_s=3
1
9
N/A
N/A
N/A
2
4
9
N/A
N/A
4
0
4
5
9


So considering current SFN is 100 : for the above UE PF which is mentioned in the series is 144. i.e Paging message can go in SF=144 and SFN=9
Ref : 3GPP 36.304 UE Procedures in Idle Mode. Section 7.1 and 7.2

6 comments:

  1. very nice article.

    ReplyDelete
  2. Please let me know if my understanding is correct ....as per your example...UE will listen 144,9....148,9...152,9 ........................208,9 (bcz nB = T/4 means one out of every four radio frame)

    ReplyDelete
    Replies
    1. Hi,

      with respect to the above example UE wakes up on subframes whose (SFN MOD 164 is 16). So next time UE will wake up on subframe number number (144, 208, 272 ...) assuming current subframe number is 100

      Delete
  3. How does MME know what is the value of T (for caluclating ue index value for paging.), does it take the default value or is it based on any theory?

    ReplyDelete
  4. How can we calculate paging capcity for radio interface?

    ReplyDelete