The characteristics of PHICH are:
- The PHICH
carries the hybrid-ARQ ACK/NAK
- Multiple PHICHs
mapped to the same set of resource elements constitute a PHICH group
- PHICHs within
the same PHICH group are separated through different orthogonal
sequences.
- Modulation used
is BPSK
PHICH
channel is like using CDMA over OFDM, where a UE is assigned a PHICH
group(identifying the resource elements carrying the PHICH) and a orthogonal
sequence(just like a CDMA code, which allows PHICH to be multiplexed over as
many UEs as are the orthogonal codes within the group). For Normal CP SF=4
codes are used and for Extended CP, SF=2 codea are used.
Number of PHICH groups (for
Normal CP) = Ceiling [Ng*(num-dl-RB/8)]
where Ng = {1/6, 1/2,1,2} as defined
by higher layers
num-dl-RB = Number of Resource
Blocks in DL
Therefore
for 20 MHz (num-DL-RB = 100) if Ng = 2 then
number of PHICH groups = ceiling (2*(100/8)) = 25
The
orthogonal codes used for Normal CP are SF=4 codes and after BPSK
modulation and using IQ multiplexing,
num of users in each PHICH group = 8
Therefore the number of UEs that can be supported on PHICH on 20
MHz and Normal CP = 25*8 = 200