3G/UMTS

 

 

 

Channel Mapping : Enhanced Cell FACH R7

 

Before Release 7, a UE in CELL_FACH received all its downlink data on FACH. Release 7 lets the UE receive HS-DSCH in CELL_FACH as well, and this is called enhanced CELL_FACH. The downlink in CELL_FACH is then no longer limited to the FACH on the SCCPCH. This page shows one example mapping, where every downlink logical channel goes to HS-DSCH.

Compare it with the Cell FACH R99/R5/R6 page for the FACH mapping, and with the Cell DCH R7 page for MAC-ehs in CELL_DCH.

What does the enhanced CELL_FACH mapping look like?

In enhanced CELL_FACH, HS-DSCH carries the common channels as well as the dedicated ones. So one transport channel now serves broadcast, paging, CCCH, the SRBs and user data. MAC-ehs has to keep all of them apart.

The diagram below reads from top to bottom. The top row lists eight logical channels with their rb-id and LCID. The CCCH shows LCID = SIB5, because the UE learns that value from system information. The logical channels feed three MAC-d flow boxes, the boxes feed three queues, and all queues end on HSDSCH/HSDPSCH.

UMTS enhanced CELL_FACH R7 logical channel to HS-DSCH mapping with MAC-d flows and queues

MAC-ehs carries every downlink logical channel of the UE on HS-DSCH. In this example, CCCH and SRB1 share Queue 0 with the broadcast and paging channels.

  • Queue 0 sits behind MAC Flow Id = 1 : it holds CCCH 0, DCCH 0, BCCH 1 and PCCH 0.
  • Queue 1 holds DCCH 1 to DCCH 3 : these are SRB2 to SRB4, with LCID 2 to 4.
  • Queue 2 holds DTCH 1 : the user plane bearer, with rb-id 8 and LCID 15.
  • Two boxes read MAC Flow Id = 2 : the DCCH box and the DTCH box carry the same flow id in the picture. Either one MAC-d flow feeds two queues here, or the second label should read 3. The picture alone does not settle which.

BCCH and PCCH are handled differently from the other channels in Queue 0. Their SDUs carry LCH-ID 1111 in the MAC-ehs header, and 25.321 applies no reordering to them. The UE identifies them by the H-RNTI on HS-SCCH instead. It uses the BCCH-specific H-RNTI from SIB5 for broadcast, and an H-RNTI reserved for paging for PCCH. So the Queue 0 box shows where these two channels arrive. It is not a reordering queue that they share with CCCH.

CCCH also looks different on HS-DSCH. On FACH, the TCTF field marked a CCCH PDU. On HS-DSCH, the CCCH SDU has no MAC-c header, and MAC-ehs finds it by its LCH-ID, like any other logical channel.

  • BCCH and PCCH use LCH-ID 1111 : the H-RNTI on HS-SCCH tells the UE which of the two it is.
  • No reordering for BCCH and PCCH : the TSN is set to zero and ignored.
  • CCCH gets a real LCH-ID : the TCTF of FACH is not used on HS-DSCH.

Where does the UE get the CCCH mapping?

A UE that sends RRC Connection Request has no dedicated configuration yet. It still has to know which LCH-ID and which queue will carry the RRC Connection Setup on HS-DSCH. That is why the diagram labels the CCCH LCID as SIB5.

SIB5 carries the IE HS-DSCH common system information in its Release 7 extension. The ccch-MappingInfo field gives the logical channel identity and the MAC-ehs queue of CCCH. The optional srb1-MappingInfo does the same for SRB1. The diagram puts DCCH 0, which is SRB1, in Queue 0 next to CCCH, the same kind of shared queue that srb1-MappingInfo can set up.

The same IE lists the common MAC-ehs reordering queues, the HS-SCCH codes, the HARQ settings and up to four common H-RNTIs. A UE without a dedicated H-RNTI uses one of the common H-RNTIs to receive CCCH on HS-DSCH. The bcchSpecific-H-RNTI marks the HS-SCCH that schedules BCCH. The tile below gives the FDD version of the IE and the types that set the numbers in the diagram.

Following is based on 25.331 v19.0.1 (Release 19)

HS-DSCH-CommonSystemInformation ::=     SEQUENCE {
    ccch-MappingInfo                        CommonRBMappingInfo,
    srb1-MappingInfo                        CommonRBMappingInfo                     OPTIONAL,
    common-MAC-ehs-ReorderingQueueList      Common-MAC-ehs-ReorderingQueueList,
    hs-scch-SystemInfo                      HS-SCCH-SystemInfo,
    harq-SystemInfo                         HARQ-Info,
    common-H-RNTI-information               SEQUENCE (SIZE (1..maxCommonHRNTI)) OF
                                                H-RNTI,
    bcchSpecific-H-RNTI                     H-RNTI
}

CommonRBMappingInfo ::=             SEQUENCE {
    logicalChannelIdentity              LogicalChannelIdentity,
    mac-ehs-QueueId                     MAC-ehs-QueueId
}

Common-MAC-ehs-ReorderingQueueList ::=  SEQUENCE (SIZE (1.. maxCommonQueueID)) OF
                                        Common-MAC-ehs-ReorderingQueue

Common-MAC-ehs-ReorderingQueue ::=  SEQUENCE {
    mac-ehs-QueueId                     MAC-ehs-QueueId,
    t1-ReleaseTimer                     T1-ReleaseTimer,
    reorderingResetTimer                Treset-ResetTimer                       OPTIONAL,
    mac-ehsWindowSize                   MAC-hs-WindowSize
}

LogicalChannelIdentity ::=          INTEGER (1..15)

MAC-ehs-QueueId ::=                 INTEGER (0..7)

The value ranges explain the numbers in the diagram. LogicalChannelIdentity runs from 1 to 15, the same range as the LCID labels. MAC-ehs-QueueId runs from 0 to 7, so Queue 0 is a valid queue number, while the Cell DCH pages start at Queue 1. The list of common queues is limited by maxCommonQueueID, which is 2, so SIB5 defines at most two common queues.

  • ccch-MappingInfo is mandatory : every cell with HS-DSCH common system information maps CCCH to HS-DSCH.
  • srb1-MappingInfo is optional : it gives SRB1 a common mapping as well.
  • Up to four common H-RNTIs : maxCommonHRNTI is 4.
  • At most two common queues : maxCommonQueueID is 2.

Reference

[1] 3GPP TS 25.301 v19.0.0 - clause 5.3.1.1.2, Mapping between logical channels and transport channels

[2] 3GPP TS 25.321 v19.0.0 - clause 9.1.4, MAC PDU for HS-DSCH, clauses 9.2.1.3 and 9.2.1.4, MAC header for PCCH and CCCH, and clause 9.2.2, MAC-ehs header parameters

[3] 3GPP TS 25.331 v19.0.1 - clause 10.3.6.36c, HS-DSCH common system information, and the ASN.1 of clause 11.3