1. PHICH is carried on 1st symbol of each sub-frame and located in same symbol as PCFICH
2. It will have values ACK/NACK 0/1
a) 0 is represented by 000 and 1 by 111 by MAC single repetition coding
b) Then multipication factor of 4/2 in case of Normal CP/Ext CP. It become
i. 0000 0000 0000
ii. 1111 1111 1111
c) Then these bits are BPSK modulated, scrambled and user specific wash sequence is applied.
d) Each RE can carry 1 symbol. So carry all the 12-bit PHICH info we need 12 RE’s or 3 REG.
3. 1 PHICH group will have 3REG's (4 X 3 RE’s )
a) 1 REG (group) = 4 RE’s
b) 12 subcarrier of PHICH is mapped to 1 PHICH group
c) 1 PHICH group can carry 8 users as there are 8 wash sequences available in LTE in normal CP and 4 users in Extended CP.
d) Suppose Ng =1 and BW = 50, the N-PHICH group = 7. No of simultaneous PHICH’s information that cane be sent wil be 8*7 = 56 ACk/NACK for different users . No of RE’s used will be 7*3*4 = 84RE’s. (7 PHICH groups x 3 REGs per PHICH group x 4 REs per REG)
4. Multiple PHICH can use same set of PHICH's. These PHICH is called as PHICH group.
5. Each PHICH in a PHICH group is mapped to each UE.
6. PHICH ACK/NACK belonging to same PHICH group always mapped to same RE’s, just thet are differentiated by orthogonal sequences. So 8-PHICH ACK/NACK can be carried on same PHICH group /same set of 12-RE’s
7. PHICH is always transmitted on the same antenna port as PBCH
a) Antenna -0 - generally
b) Antenna {0,1} - transmission diversity
c) Antenna {0,1,2,3,4} - when 4 Antenna port is used.
8. Due to PHICH channel the overhead increases. Although it decreases with increase with BW.
a) Number of PHICH channels depends upon the Ng - factor.
b) It is minimum when the Ng = 1/6.
c) It is maximum when the Ng = 2.
9. Each individually PHICH within a group is determined by
10. Number of PHICH groups is calculated by the below formula
11. Ng => PHICH Group Scaling Factor.
Details of number of RB’s and Ng is present in MIB phich-Config
12. Below is the table for number of PHICH group for given BW and Ng calculations
13. Particular PHICH group and Sequence Index is calculated by following formula
a) PHICH Group = ( Ilowest + nDMRS ) *mod( N group )
b) PHICH Sequence Index = ( [Ilowest / N group] + nDMRS ) * mod(Nseq)
c)
Ilowest = Lowest physical RB index allocated to slot-1 of PUSCH
nDMRS = Demodulation RS cyclic shift signaled within LTE DCI-0
N group = No. of PHICH groups
Nseq = No. of orthogonal sequences per PHICH group (Normal CP: 8 and Extended CP:4)
14. PHICH duration depends upon the PCFICH value.
15. PHICH Configuration
a) Normal PHICH duration, Normal CP - All the PHICH Quadruplets ( 3REG/12 RE’s) are on the OFDM Symbol 0 of each sub-frame
b) Extended PHICH duration , Normal CP - All the PHICH Quadruplets ( 3REG/12 RE’s) are distributed in the OFDM symbol 0,1,2 of each sub-frame
