Continuous Packet Connectivity - HS-SCCH Less Mode

This is a continuation of explanation of the features in CPC (see my earlier post for a general description on CPC).

The transmission in the downlink on the HSDPA channels involves transmitting the control information essential for decoding the data. The control information is transmitted on HS-SCCH (High Speed Shared Control Channel) and the data is transmitted on up to 15 HS-PDSCH (High Speed Physical Downlink Shared Channel). The decoding of data on HS-PDSCH channels is communicated by the UE to the network on HS-DPCCH channel in the uplink. Typical transmission using HS-SCCH, HS-PDSCH and HS-DPCCH is illustrated below:

The control information on HS-SCCH mainly consists of following for correctly demodulating and decoding the data:

• Modulation Scheme
• Number of Channelisation Codes
• Transport Block Size
• HARQ Identifier
• Redundancy and Constellation Version
• New Data Indicator
• UE Specific CRC

Modulation Scheme and the number of channelisation codes form part 1 of the control information and it is used by the UE to demodulate the signal. Once the signal is demodulated the remaining bits of the control information is used by the UE to decode the data. Since the HSDPA channels are shared by several UEs the network should indicate which UE the data is meant for. It will do that by coding the CRC with a UE specific identifier called H-RNTI, which is signalled by the network to the UE in a RRC signalling message.

To summarise, data transmission on HS-PDSCH channels entail transmitting control information on HS-SCCH i.e. in one sense HS-SCCH is an overhead for transmitting certain types of traffic such as low data rate traffic like VoIP and gaming over HSDPA.

To overcome this drawback 3GPP has introduced the concept of HS-SCCH less transmission for these kinds of low data rate traffic in Rel-7 specifications.

In HS-SCCH less mode operation, HS-SCCH is not transmitted for the initial transmission of data on HS-PDSCH. Therefore, UE will try to blindly decode the data on HS-PDSCH with predefined control information. If the UE is unable to blindly decode the initial transmission successfully then the data shall be retransmitted for a maximum of 2 times. During the retransmissions HS-SCCH is transmitted though. The HS-SCCH that is used for 1st and 2nd retransmission is called HS-SCCH Type 2. This is a new type of HS-SCCH that is defined for retransmitting data in HS-SCCH less mode operation.

Some of the benefits of HS-SCCH less mode operation are:

• Significant increase in VoIP capacity because of the reduction in overhead associated with HS-SCCH transmission
• Availability of HS-SCCH channel for other services because services like VoIP amongst others could be serviced without HS-SCCH
• The above two benefits translate into a large gain in the throughput available to the co-existing best effort's traffic at medium VoIP user loads
• More users can be code multiplexed in a single TTI for the delay sensitive traffic

Typical HS-SCCH Less Mode operation is illustrated is given below:

In HS-SCCH less mode operation, since HS-SCCH is not transmitted along with HS-PDSCH for the initial transmission the UE will use the following control information to demodulate and decode the data correctly.

• QPSK modulation is used. This is fixed for HS-SCCH less transmission.
  
• A maximum of 2 channelisation codes that are adjacent to each other is used. The 1st channelisation code (also called as code offset) is signalled to the UE in RRC Signalling Message. The 2nd channelisation code is derived by adding one to the code offset
  
• A maximum of 4 Transport Block Sizes are used. These TB Sizes are derived from the TB Size Indexes that are signalled to the UE in RRC signalling message. Note: see below for the difference in calculation of TB Size from TB Size Index in HS-SCCH less mode and non HS-SCCH less mode
  
• The redundancy and constellation version, also called as Xrv, to be used is fixed (pre-defined) to 0
  
• Since the initial transmission is with HS-SCCH less operation there is no need to indicate whether it is a new data or a retransmission
  
• 24 bit CRC (called as CRC attachment method 2) is coded in HS-PDSCH transmission with a UE specific identifier (H-RNTI)
  

For the initial HS-PDSCH transmission UE will not send NACK on HS-DPCCH if it is unable to decode the data. It will only ACK on HS-DPCCH if it is able to successfully decode the data. Because of the fixed timing relation between HS-PDSCH and HS-DPCCH i.e. a difference of 7.5 slots between the end of HS-PDSCH frame to the beginning of its corresponding HS-DPCCH frame (refer to section 7.7 of TS 25.211 v760) if the network does not detect an ACK for a HS-PDSCH transmission in HS-SCCH less mode then the network will think of it as a NACK and it will retransmit the data using HS-SCCH Type 2.

The structure of HS-SCCH Type 2 is as given below (courtesy 3GPP TS 25.903), and it is used to transmit control information for the 1st and 2nd retransmissions:

• CCS (Channelisation Code Set) indicates the channelisation codes used for HS-PDSCH transmission. It can take values 1 or 2. 1 means channelisation code 1 and 2 means channelisation codes 1 and 2 are used for HS-PDSCH transmission
  
• 1 Bit (value 0) is used to indicate the QPSK modulation scheme
  
• Format ID together with CCS is used to indicate to UE that it is HS-SCCH Type 2
  
• TB format (2 bits) is used to indicate one out of 4 Transport Block Sizes configured by higher layers. This is one of the four values indicated to UE RRC in hs-scch-LessTFSI IE (see below in RRC ASN.1 IEs)
  
• Xrv = 3 and 4 for the 1st and 2nd retransmission respectively
  
• reTx ID (1 bit) is used to indicate if it is a 1st or 2nd retransmission
  
• Previous Tx Point (3 bits) is used to indicate the time of previous transmission in terms of offset from the current TTI
  
• 16 bit CRC (called as CRC attachment method 2) is coded with a UE specific identifier (H-RNTI)
  

In HS-SCCH less mode operation a UE shall be able to store 13 TTIs of data that could not be decoded. If the 1st retransmission using HS-SCCH Type2 could not be decoded then it will combine and store the 1st retransmission data with the initial HS-SCCH less transmission data in the cyclic buffer and this will be used again to decode the data associated with the 2nd retransmission. Therefore, "Previous Tx Point" of 3 bits given above is used to indicate the relative TTI position within the 13 TTIs of cyclic buffer to identify the data that needs to be used for combining and decoding the retransmission data.

HS-SCCH less mode operation can be used only with MAC-hs entity in MAC layer. It cannot be used with MAC-ehs entity. MAC-ehs entity which will be described in the future posts. Virtual IR Buffer size (HARQ memory size) of at least 4536 bits should be configured in L1 HARQ when HS-SCCH less mode is configured.

HS-SCCH less mode can only be configured if (Reference: section 8.5.35 of 25.331)

• UE is in Cell-DCH state
• No DCH transport channels are configured
• MIMO is not configured
• Virtual IR buffer size (or alternatively called as HARQ memory size) of at least 4536 bits is configured in HARQ

How to derive Transport Block Size from Transport Block Size Index in HS-SCCH less mode?

The Transport Block size indicated by Transport Block size index is derived by taking the TB Size value corresponding to the index given by Transport Block size Index in the 1st table in Annexure A TS 25.321 v7b0. For example: if value 45 is indicated in hs-scch-LessTFSI IE then it would indicate a TB Size of 662 bits.

Note: this is different to the way Transport Block size is calculated from the Transport Block size index in non HS-SCCH less mode.

RRC ASN.1 IEs

The following are the RRC ASN.1 IEs related to HS-SCCH Less Mode that will appear in Physical Channel information elements section of RRC downlink signalling messages like Radio Bearer Reconfiguration Messages:

HS-SCCH-LessInfo-r7 ::= SEQUENCE {

hs-scchLessOperation CHOICE {

continue NULL,

newOperation HS-SCCH-Less-NewOperation

}

}

HS-SCCH-Less-NewOperation ::= SEQUENCE {

hs-pdsch-CodeIndex INTEGER (1..15), -- specifies the starting code of HS-PDSCH

hs-scch-LessTFS HS-SCCH-LessTFSList

}

HS-SCCH-LessTFSList ::= SEQUENCE (SIZE (1..maxHS-SCCHLessTrBlk)) OF SEQUENCE {

hs-scch-LessTFSI INTEGER (1..90), -- indicates the TFRI associated with the Transport Block size

hs-scch-LessSecondCodeSupport BOOLEAN -- indicates the use of two consecutive HS-PDSCH codes when the TB size indicated by hs-scch-LessTFSI is used for HS-PDSCH transmission

}

References: TS 25.903, TS 25.212, TS 25.321 & TS 25.33