LTE downlink Physical layer transport mechanism (3)-phich Channel

Before reading this article, it is recommended to take a look at the post "LTE-TDD HARQ (1)-upstream HARQ timing" to better understand the content of this article.

The main contents of this article include:

(1) What is the Phich channel and what is its role?

(2) How to uniquely identify a Phich channel

(3) What is the content of the Phich channel corresponding to the reg actual mapping?

(4) Location of the Phich channel

1. What is the Phich channel

PHICH Channel is the physical HARQ indication channel , the English full name is physical hybrid ARQ indicator channel, the role is ENB through the channel to the terminal feedback Uplink Pusch data response information ack or Nack. After the ENB is decoded to the upstream data block, it will be given a confirmation message (ACK or NACK) at the appropriate Phich channel, and the terminal will be at the specified time (see the Post "LTE-TDD HARQ (1)-uplink HARQ timing") at the specified location to decode Phich information.

Each Pusch transport block for each upstream sub-frame needs to correspond to a Phich channel . If there is a lot of Pusch data blocks, there are many Phich channels that need to be carried in a downstream sub-frame. If each Phich channel is mapped independently to a different re, it will occupy a lot of control areas, which is obviously inappropriate. So the protocol suggests that different PHICH channels can be mapped to the same location, and this position is called the PHICH Group (PHICH). In the same Phich group, different Phich channels are distinguished by different orthogonal sequences (orthogonal sequence). That is, all phich channels within a Phich group, corresponding physical resource mapping locations are the same, and the group is then differentiated by orthogonal sequences.

Multiple Phichs mapped to the same set of resource elements constitute a PHICH group, where Phichs within the same PHICH G Roup is separated through different orthogonal sequences.

Therefore, each PHICH channel can be uniquely identified using (N_group_phich,N_seq_phich) . WhereN_group_phich represents the group number of the current PHICH group, andN_seq_phich represents the orthogonal sequence index number within the group. The range of N_seq_phich is 0,1,..,7 (downstream normal CP) or 0,1,2,3 (Downlink extended CP), i.e., for normal CP, A Phich group can contain up to 8 different Phich channels, and for extended CP, a Phich group may contain up to 4 different Phich channels .

So how many Phich groups are allowed in a system bandwidth? The protocol uses N_group_phich to represent the number of groups in the PHICH group, so the N_group_phich range is 0,1, ...,n_group_phich-1. Let's talk about how to calculate the Phich number parameter.

for LTE-FDD formats , the number of groups in the PHICH group n_group_phich can be calculated from the following formula:

Where the downlink bandwidth n_dl_rb parameter is configured by RRC and sent to the terminal via the dl-bandwidth of the MIB message. The Ng parameter is also configured by RRC and sent to the terminal via Phich-config in the MIB message. Therefore, the Phich channel can be decoded only if the MIB information is decoded into the PBCH. The downstream CP type is normal or extended, and the terminal can be obtained by synchronizing the signal pss/sss (refer to the blog "LTE cell search-physical cell ID and synchronous signal PSS, SSS").

Thus, the terminal to decode the Phich channel, one of the prerequisites is to read the cell synchronization signal pss/sss, and decode the PBCH MIB message. The N_group_phich value for the Normal CP type is shown in the following table.

for LTE-TDD , the feedback of multiple upstream sub-frames appears in the same downstream sub-frame because of the different upper and lower sub-frame configurations, corresponding to the number of sub-frames of the upper and lower rows. For example, the upper and lower sub-frame configuration 0, the downlink 0# sub-frame and the AA sub-frame are required to feedback 2 upstream sub-frame Phich information (corresponding to 3#, 4# and 7#, 8# upstream sub-frame), and downlink # sub-frame and 6# sub-frame, all only need feedback 1 The Phich information for the upstream sub-frames (corresponding to the 7# and the * Uplink sub-frames, respectively), as shown in the following figure, which will be mentioned later. Therefore, the number of groups in the Phich group of the TDD format N_group_phich is different from the FDD format.

The agreement stipulates that for TDD, the number of PHICH group N_group_phich need to be multiplied by a factor Mion the basis of FDD, that is N_group_phich = Mi * N_group_phich. Mi Values are shown in the following table. As can be seen from the table, when the upper downlink sub-frame is configured to 0, and the downstream sub-frame is 0#, AA, the number of groups in the PHICH group needs to be increased by one times, while the other scenarios of the PHICH group N_group_phich, then the same value as the FDD.

The above analysis seems to be able to obtain such a conclusion:The terminal can decode the Phich channel only after decoding the ul/dl config parameter in the SIB1 to know the Mi value . In fact, since SIB1 is transmitted in Pdsch, Pdsch's resources need to be indicated by the pdcch of the control area, that is to say,decoding SIB1 is required for PDCCH instructions.. Later in the explanation of PDCCH, will introduce PDCCH calculation is need Phich information. This creates a problem with "chicken eggs" and "egg chickens": Calculating the location of the Phich requires the use of SIB1 's pdcch information ( ul/dlconfig parameter in Pdsch is required to determine Mi value ), and the Phich information is required to calculate the PDCCH channel . Therefore, in order to solve this problem,when decoding SIB1, the terminal does not need to know the value of mi (i.e., the terminal will try mi =0,1,2 in turn), and then obtains SIB1 corresponding PDCCH location through blind detection ..

Further analysis of the above table, for LTE-TDD, will deduce the following:

(1) The number of Phich channels doubles only when the upper and lower sub-frames are configured with 0 .

(2) not every downstream sub-frame can send the Phich channel . Mi = 0 of the downlink sub-frame, is no Phich channel.

(3) The collection of sub-frames that send the Phich channel and the set of sub-frames that send the DCI0 are consistent . For example, the upper and lower sub-frame configuration 1, only in 1, 4, 6, 9 of these four sub-frames to send phich information, and only in these four sub-frames, to send DCI0 information. For the DCI-related content, the post will be more elaborate later.

Location identification of the 2.PHICH channel

As mentioned earlier, a PHICH channel can be identified by (N_group_phich,N_seq_phich), whereN_group_phich represents the group number of the current PHICH group, N_seq_phich represents the orthogonal sequence index number within a group. These two parameters can be used to determine the respective values by the following formula:

which

(1)N_group_phich parameter: PHICH group number, the previous article has said the calculation method, need to distinguish between FDD and TDD two ways.

(2)I_phich parameter: When the upper and lower sub-frame is configured to 0, 3, 4th sub-frame uplink data of the response, in the No. 0 sub-frame of the PHICH channel sent, 8, 9th sub-frame uplink data, the PHICH channel of the 5th sub-frame is sent. This parameter is used to differentiate the case where a downstream sub-frame corresponds to two uplink subframe responses . If the ENB needs to give PHICH feedback for the number 3rd sub-frame at number No. 0, set the i_phich parameter to 0, and if the feedback is the reply of the 4th uplink subframe, set the i_phich parameter to 1. If you are not currently on the top-down sub-frame configuration 0, the i_phich parameter is fixed to 0.

(3)I_lowest_index_prb_ra parameter: Indicates the starting position of the upstream RB occupied by the Pusch transport block. For a terminal that is dynamically scheduled, the starting position of the RB for each upstream sub-frame may be different, so the value of this parameter is not fixed. The upstream RB allocated by the same terminal in the same sub-frame is continuous, and the upstream RB allocated between each terminal does not overlap, so this parameter is not the same for different terminals in the same sub-frame. is a simple one that is used to help understand.

(4)n_phich_sf parameter: This parameter is a coefficient related to modulation. The current behavior of the normal CP,n_phich_sf=4, the current behavior extended CP,n_phich_sf=2.

(5)N_dmrs parameter: This parameter equals the value of the "Cyclic shift for DM RS" field in DCI0 , the range is 0-7, the value is as follows (details about DCI0, later in the DCI format). When the actual code is implemented, it is determined by the ENB side of the algorithm.

Therefore, the terminal needs to save the DMRs value when decoding the DCI0 for subsequent Phich decoding . is one that is used to help understand.

At this point, all the parameters required for the calculation (N_group_phich,N_seq_phich) can be obtained on the terminal and ENB side, and the ENB and terminals will be able to determine the Pusch position corresponding to the PHICH transmission block (N _group_phich,N_seq_phich).

The contents of the mappings in the 3.PHICH channel corresponding to Reg

Similar to the Pcfich channel,when the content of the Phich channel is mapped to the physical resource, it is also mapped in the REG unit . According to the foregoing, although the function of the Phich channel is to feed back the ACK response of the Pusch transmission block, this does not mean that the physical resource Reg transmits a bit of ACK (bit=1) or nack (bit=0) information, but needs to go through multiple physical layer processing processes.

(1)the 1 bit HARQ indicator (HI) information is channel encoded (channels coding), generating 3 bit of HI repeat information . This process, some of the information is also called 3 times times the repeated encoding process.

When the ENB needs to return ACK information, after the channel encoding, the resulting data is (1,1,1), and ENB need to return to the terminal nack information, after the channel encoding, the resulting data is (0,0,0).

(2) The hi information of 3bits is BPSK modulated to generate 3 modulation symbols in complex form . The specific BPSK modulation process is to convert each bit B (i) of the 3 HI bit information into a complex X=I+JQ, where the values of I and Q are shown in the table below.

(3) using a 4bits or 2bits long orthogonal sequence, the modulated 3 complex-value modulation symbols are spread spectrum and scrambling, generating 12 or 6 modulation symbols . The length of the orthogonal sequence code is related to the downstream CP type, as shown in the following table. For the lower behavior of the normal CP, after modulation and quadrature sequence spread spectrum and scrambling, 3 bit of hi information has been converted to 12 complex value modulation symbols. Each modulation symbol corresponds to a re, and each reg corresponds to 4 Re, so each Phich group requires 3 reg components . These complex modulation symbols are subsequently re-mapped to the actual Reg resource group after the layer mapping and pre-encoding process.

The above steps (2) and (3) can be illustrated with the following formula, where mbit=ms=3. For the downstream normal cp,msymb=3*4=12, for the downstream extended cp,msymb=3*2=6.

Multiple Phich channels within each Phich group need to pass through several physical layer processes described above, and eventually mapped to the same Reg resource, as follows.

Location of the 4.PHICH channel

Phich Group mapping to the rules of Reg, similar to the Pcfich channel mapping, but also need to obtain good frequency diversity, while avoiding interference with the neighboring cell. Therefore, in the frequency domain, each Phich group maps 3 Reg, their interval is approximately one-third of the downstream cell bandwidth . in the time domain, the number of OFDM symbols per PHICH group is related to the phich_duration parameters (carried in the MIB) given above, as shown in the following table.

As you can see, either LTE-FDD or LTE-TDD, if the Phich_duration=normal,phich group is mapped only to one of the OFDM symbols . As motioned.

If phich_duration=extended, then the Phich group can be mapped to 2 OFDM symbols or 3 OFDM symbols, depending on the situation. is a.

If the Phich group appears in the first OFDM symbol, the Pcfich is assigned a reg resource first, and then the Phich group is mapped to a reg that is not occupied by Pcfich. Of course, finally, the PDCCH is mapped to the remaining Reg (or to the CCE, the contents of CCE, and later in detail).

Reference documents:
(1) 3GPP TS 36.211 V9.1.0 (2010-03) physical Channels and modulation

(2) 3GPP TS 36.213 V9.3.0 (2010-09) Physical layer procedures

(3) 3GPP TS 36.212 V9.4.0 (2011-09) multiplexing and channel coding

(4) "4G lte/lte-advanced for Mobile Broadband"

(5) Http://dhagle.in/LTE

(6) http://www.sharetechnote.com/

LTE downlink Physical layer transport mechanism (3)-phich Channel