6.9 Physical hybrid ARQ indicator channel
36.2113GPPEvolved Universal Terrestrial Radio Access (E-UTRA)Physical channels and modulationRelease 17TS
The PHICH carries the hybrid-ARQ ACK/NACK. Multiple PHICHs mapped to the same set of resource elements constitute a PHICH group, where PHICHs within the same PHICH group are separated through different orthogonal sequences. A PHICH resource is identified by the index pair 
, where 
is the PHICH group number and 
is the orthogonal sequence index within the group.
For frame structure type 1 and type 3, the number of PHICH groups 
is constant in all subframes and given by
where 
is provided by higher layers. The index 
ranges from 
to 
.
For frame structure type 2, the number of PHICH groups may vary between subframes and is given by 
where 
is given by the expression above and 
is given by Table 6.9-1 with the uplink-downlink configuration provided by the higher-layer parameter subframeAssignment. The index 
in a subframe with non-zero PHICH resources ranges from 
to 
.
Table 6.9-1: The factor 
for frame structure type 2
|
Uplink-downlink |
Subframe number |
|||||||||
|
0 |
1 |
2 |
3 |
4 |
5 |
6 |
7 |
8 |
9 |
|
|
0 |
2 |
1 |
0 |
0 |
0 |
2 |
1 |
0 |
0 |
0 |
|
1 |
0 |
1 |
0 |
0 |
1 |
0 |
1 |
0 |
0 |
1 |
|
2 |
0 |
0 |
0 |
1 |
0 |
0 |
0 |
0 |
1 |
0 |
|
3 |
1 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
1 |
1 |
|
4 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
1 |
1 |
|
5 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
1 |
0 |
|
6 |
1 |
1 |
0 |
0 |
0 |
1 |
1 |
0 |
0 |
1 |
6.9.1 Modulation
The block of bits 
transmitted on one PHICH in one subframe shall be modulated as described in clause 7.1, resulting in a block of complex-valued modulation symbols
, where 
. Table 6.9.1-1 specifies the modulation mappings applicable for the physical hybrid ARQ indicator channel.
Table 6.9.1-1: PHICH modulation schemes.
|
Physical channel |
Modulation schemes |
|
PHICH |
BPSK |
The block of modulation symbols 
shall be symbol-wise multiplied with an orthogonal sequence and scrambled, resulting in a sequence of modulation symbols 
according to
where
and 
is a cell-specific scrambling sequence generated according to clause 7.2. The scrambling sequence generator shall be initialised with 
at the start of each subframe.
The sequence 
is given by Table 6.9.1-2 where the sequence index 
corresponds to the PHICH number within the PHICH group.
Table 6.9.1-2: Orthogonal sequences 
for PHICH
|
Sequence index |
Orthogonal sequence |
|
|
|
Normal cyclic prefix
|
Extended cyclic prefix
|
|
0 |
|
|
|
1 |
|
|
|
2 |
|
|
|
3 |
|
|
|
4 |
|
– |
|
5 |
|
– |
|
6 |
|
– |
|
7 |
|
– |
6.9.2 Resource group alignment, layer mapping and precoding
The block of symbols 
should be first aligned with resource element group size, resulting in a block of symbols 
, where 
for normal cyclic prefix; and 
for extended cyclic prefix.
For normal cyclic prefix, 
, for 
.
For extended cyclic prefix,
for 
.
The block of symbols 
shall be mapped to layers and precoded, resulting in a block of vectors 
, 
, where 
represents the signal for antenna port 
, 
and the number of cell-specific reference signals 
. The layer mapping and precoding operation depends on the cyclic prefix length and the number of antenna ports used for transmission of the PHICH. The PHICH shall be transmitted on the same set of antenna ports as the PBCH.
For transmission on a single antenna port, 
, layer mapping and precoding are defined by clauses 6.3.3.1 and 6.3.4.1, respectively, with 
.
For transmission on two antenna ports, 
, layer mapping and precoding are defined by clauses 6.3.3.3 and 6.3.4.3, respectively, with 
.
For transmission on four antenna ports, 
, layer mapping is defined by clause 6.3.3.3 with 
and precoding by
if 
for normal cyclic prefix, or 
for extended cyclic prefix, where 
is the PHICH group number and 
, and by
otherwise for 
.
6.9.3 Mapping to resource elements
The sequence 
for each of the PHICH groups is defined by
where the sum is over all PHICHs in the PHICH group and 
represents the symbol sequence from the 
:th PHICH in the PHICH group.
PHICH groups are mapped to PHICH mapping units.
For normal cyclic prefix, the mapping of PHICH group 
to PHICH mapping unit 
is defined by
where
,
and where 
is given by Table 6.9-1.
For extended cyclic prefix, the mapping of PHICH group 
and 
to PHICH mapping unit 
is defined by
where
and where 
is given by Table 6.9-1.
Let 
, 
denote symbol quadruplet 
for antenna port
. Mapping to resource elements is defined in terms of symbol quadruplets according to steps 1–10 below:
1) For each value of 
2) Let 
denote the number of resource element groups not assigned to PCFICH in OFDM symbol 
3) Number the resource-element groups not assigned to PCFICH in OFDM symbol 
from 0 to
, starting from the resource-element group with the lowest frequency-domain index.
4) Initialize 
(PHICH mapping unit number)
5) For each value of 
6) Symbol-quadruplet 
from PHICH mapping unit 
is mapped to the resource-element group represented by 
as defined in clause 6.2.4 where the indices 
and 
are given by steps 7 and 8 below:
7) The time-domain index 
is given by
8) Set the frequency-domain index 
to the resource-element group assigned the number 
in step 3 above, where 
is given by
in case of extended PHICH duration in MBSFN subframes, or extended PHICH duration in subframes 1 and 6 for frame structure type 2, or extended PHICH duration in subframe with the same duration as the DwPTS duration of a special subframe configuration in frame structure type 3 and by
otherwise.
9) Increase 
by 1.
10) Repeat from step 5 until all PHICH mapping units have been assigned.
The PHICH duration is configurable by higher layers according to Table 6.9.3-1.
The PHICH shall not be transmitted in MBSFN subframes with zero-size non-MBSFN region.
Table 6.9.3-1: PHICH duration in MBSFN and non-MBSFN subframes
|
PHICH duration |
Non-MBSFN subframes |
MBSFN subframes |
||
|
Subframes 1 and 6 in case of frame structure type 2 |
Subframe with the same duration as the DwPTS duration of a specifial subframe configuration in case of frame structure type 3 |
All other cases |
||
|
Normal |
1 |
1 |
1 |
1 |
|
Extended |
2 |
2 |
3 |
2 |