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LTE PHICH -Physical Hybrid ARQ Indicator Channel

This page describes LTE PHICH channel or Physical Hybrid ARQ Indicator Channel with respect to LTE system.

The PHICH is used to carry positive or negative acknowledgements for uplink data transferred on the PUSCH. They are referred as ACK/NACK. Each connection can carry a max. of 1 TB(Transport Block) per subframe on PUSCH. Hence a maximum of 1 PHICH ACK per subframe per connection is needed.

A PHICH ASK is identified by two parameters:
-PHICH Group
-PHICH Orthogonal sequence index within PHICH group

LTE PHICH

As shown in the figure, three REGs that support a PHICH group are evenly distributed across LTE system BW. This provides frequency diversity. As mentioned here PCFICH always appear and will occupy first symbol of each subframe. PCFICH occupy 4REGs irrespective of channel bandwidth.

The number of PHICH groups is a function of downlink channel bandwidth and PHICH group scaling factor. Both the downlink channel BW and PHICH group scaling factor are tranmitted within MIB and carried by LTE PBCH channel. The table-1 below mentions number of PHICH groups for various scaling factor,CPs and channel BWs.

1.4MHz 3MHz 5MHz 10MHz 15MHz 20MHz
PHICH Group Scaling Factor Normal CP
1/6 1 1 1 2 2 3
1/2 1 1 2 4 5 7
1 1 2 4 7 10 13
2 2 4 7 13 19 25
PHICH group scaling factor Extended CP
1/6 2 2 2 4 4 6
1/2 2 2 4 8 10 14
1 2 4 8 14 20 26
2 4 8 14 26 38 50

The no. of PHICH groups become double when using extended CP because no. of orthogonal sequences for each group became halved.

Normal CP has 8 orthogonal sequences per PHICH group
Extended CP has 4 orthogonal sequences per PHICH group

A UE selects its PHICH group and orthogonal sequence index as per following equation:

PHICH Group = ( Ilowest + nDMRS ) *mod( N group )

PHICH Sequence Index = ( [Ilowest / N group] + nDMRS ) * mod(Nseq)

Where,
Ilowest = Lowest physical RB index allocated to slot-1 of PUSCH
nDMRS = Demodulation RS cyclic shift signalled 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)

Each PHICH ACK ismodulated using BPSK modulation scheme and occupies
• 12 REs when using normal CP
• 6 REs when using extended CP

PHICH ACK belongs to same group will occupy same set of REs and are differentiated using orthogonal sequences. In extended CP case, PHICH groups are paired and each pair of groups occupy 12 REs.

The Set of 12 REs allocated to each PHICH group is divided into three quadruplets. MIB on PBCH indicates whether PHICH uses normal CP or extended CP. A normal CP means PHICH uses first OFDMA symbol belongs to a subframe. A extended CP means PHICH uses first 3 OFDMA symbols belong to a subframe.

LTE PHICH channel REFERENCE: Document 3GPP TS 36.211

LTE RELATED LINKS

Difference between SC-FDMA and OFDMA
LTE Cyclic Delay Diversity
LTE eNodeB Physical Layer Measurements
LTE EPC Network Inerfaces
LTE UE Physical Layer Measurements
LTE Bearer types
LTE UE Event Measurement Reporting
LTE RSRP vs RSRQ
LTE Signalling radio bearers
LTE QoS
LTE EPS mobility management states
LTE EPS connection management states
LTE UE cell selection
LTE UE PLMN selection
LTE UE Paging Procedure

RF and Wireless Terminologies


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