5 Basic structure of HS-DSCH

25.3083GPPHigh Speed Downlink Packet Access (HSDPA)Overall descriptionRelease 17Stage 2TS

Tools: ARFCN - Frequency Conversion for 5G NR/LTE/UMTS/GSM

5.1 Protocol structure

The HS-DSCH functionality should be able to operate in an environment where certain cells are not updated with HS-DSCH functionality. The PDCP and MAC-d layers are unchanged from the Release ’99 and Release 4 architecture. In addition to the RLC layer from Release ’99, it is possible to use Release 7 RLC layer, which is modified to support flexible RLC PDU sizes for RLC AM, when MAC-ehs is configured.

RLC can operate in either AM or UM mode. RLC can operate in TM mode when BCCH or PCCH is mapped on HS-DSCH in FDD and 1.28Mcps TDD.

PDCP can be configured either to perform or not to perform header compression.

MAC-d is retained in the S-RNC. Transport channel type switching is therefore feasible.

The new functionalities of hybrid ARQ, segmentation (MAC-ehs only) and HS-DSCH scheduling are included in the MAC layer. In the UTRAN these functions are included in a new entities called MAC-hs and MAC-ehs located in Node B. Upper layers configure which of the two entities, MAC-hs or MAC-ehs, is to be applied to handle HS-DSCH functionality. The transport channel that the HS-DSCH functionality uses is called HS-DSCH (High Speed Downlink Shared Channel) and is controlled by the MAC-hs or MAC-ehs.

Two MAC protocol configurations are possible on the UTRAN side:

– Configuration with MAC-c/sh: In this case, the MAC-hs or MAC-ehs in Node B is located below MAC-c/sh in CRNC. MAC-c/sh shall provide functions to HS-DSCH identical to those provided for the DSCH in the Release ’99. The HS-DSCH FP (frame protocol) will handle the data transport from SRNC to CRNC (if the Iur interface is involved) and between CRNC and the Node B.

– Configuration without MAC-c/sh: In this case, the CRNC does not have any user plane function for the HS-DSCH. MAC-d in SRNC is located directly above MAC-hs or MAC-ehs in Node B, i.e. in the HS-DSCH user plane the SRNC is directly connected to the Node B, thus bypassing the CRNC.

Both configurations are transparent to both the UE and Node B. Figures 5.1-1 and 5.1-2 show the respective radio interface protocol architecture with termination points for the above two configurations.

The same architecture supports both FDD and TDD modes of operation, though some details of the associated signalling for HS-DSCH are different.

In FDD, CELL_FACH, CELL_PCH and URA_PCH state HS-DSCH reception is defined in clauses 14, and 15, a single Iub HS-DSCH FP connection can be shared by CCCH and MAC-d(s) of different UEs.

In 1.28Mcps TDD, CELL_FACH, CELL_PCH and URA_PCH state HS-DSCH reception is defined in clauses 16, and 17, a single Iub HS-DSCH FP connection can be shared by CCCH and MAC-d(s) of different UEs.

Figure 5.1-1: Protocol Architecture of HS-DSCH, Configuration with MAC-c/sh

Figure 5.1-2: Protocol Architecture of HS-DSCH, Configuration without MAC-c/sh

5.2 Basic physical structure

5.2.1 HS-DSCH Characteristics

The HS-DSCH transport channel has the following characteristics:

– An HS-DSCH transport channel is processed and decoded from one CCTrCH;

– For FDD and 3.84 Mcps/7.68 Mcps TDD, there is only one CCTrCH of HS-DSCH type per UE, for 1.28 Mcps TDD, there is only one CCTrCH of HS-DSCH type per carrier per UE;

– The CCTrCH can be mapped to one or several physical channels;

– There is only one HS-DSCH per CCTrCH;

– Existence in downlink only;

– Possibility to use beam forming;

– Possibility to use MIMO;

– Possibility to use MIMO mode with four transmit antennas;

– Possibility of applying link adaptation techniques other than power control;

– Possibility to be broadcast in the entire cell;

– For FDD, when operating in CELL_DCH state, is always associated with a DPCH or a F-DPCH (FDD only) and one or more shared physical control channels (HS-SCCHs);

– For TDD, is associated with one or more shared physical control channels (HS-SCCHs).

5.2.2 DL HS-DSCH Physical layer model

5.2.2.1 FDD Downlink Physical layer Model

Figure 5.2.2.1-1: Model of the UE’s Downlink physical layer – HS-PDSCH with associated DPCH or F-DPCH (FDD only) in CELL_DCH state. HS-PDSCH is transmitted from cell 1 in this figure

When operating in CELL_DCH state the basic downlink channel configuration consists of one or several HS-PDSCHs along with an associated DPCH or a F-DPCH (FDD only) combined with a number of separate shared physical control channels, HS-SCCHs.

When operating in CELL_FACH, CELL_PCH and URA_PCH state, as defined in clauses 14 and 15, the basic downlink channel configuration consists of one or several HS-PDSCHs along with a number of separate shared physical control channels, HS-SCCHs.

The set of shared physical control channels allocated to the UE at a given time is called an HS-SCCH set. The UTRAN may use more than one HS-SCCH set in one given cell. There is a fixed time offset between the start of the HS-SCCH information and the start of the corresponding HS-PDSCH subframe.

Figure 5.2.2.1-2: Model of the UE’s Downlink physical layer in CELL_FACH, CELL_PCH and URA_PCH state (FDD only)-

The UE is provided one HS-SCCH set on HS-PDSCH configuration/re-configuration via RRC signalling.

When operating in CELL_FACH CELL_PCH and URA_PCH state as defined in clauses 14 and 15, the UE obtains the HS-SCCH and HS-PDSCH configuration from system information broadcast.

The number of HS-SCCHs in a HS-SCCH set as seen from the UE’s point-of-view can range from a minimum of one HS-SCCH to a maximum of four HS-SCCHs. The UE shall monitor continuously all the HS-SCCHs in the allocated set.

A two-step signalling approach is used for indicating which UE has been scheduled and for signalling the necessary information required for the UE to decode the HS-PDSCHs.

For each HS-DSCH TTI, each Shared Control Channel (HS-SCCH) carries HS-DSCH-related downlink signalling for one UE. The following information is carried on the HS-SCCH:

– Transport Format and Resource Indicator (TFRI):
The TFRI includes information about the dynamic part of the HS-DSCH transport format, including transport block size. The HS-SCCH also includes information about the modulation scheme and the set of physical channels (channelisation codes) onto which HS-DSCH is mapped in the corresponding HS-DSCH TTI. If MIMO mode is configured, it also contains the number of transport blocks and the precoding weight information which informs the UE of which precoding weight that is applied to the primary transport block.

– Hybrid-ARQ-related Information (HARQ information):
This includes the HARQ protocol related information for the corresponding HS-DSCH TTI (subclause 7.1.2.1) and information about the redundancy version.

The HS-SCCH carries a UE identity (via a UE-specific CRC) that identifies the UE for which it is carrying the information necessary for decoding the HS-PDSCH(s).

The HS-PDSCH channelisation codes that are used in a given cell are not sent to the UE using RRC signalling. The HS-SCCH signals the set of HS-PDSCH channelisation codes which are allocated to a UE for a given TTI.

The first part of the HS-SCCH contains the channelisation code set, precoding weight information, number of transport blocks and the modulation scheme for the HS-DSCH allocation with the second part containing the transport block size and H-ARQ related information. One CRC is calculated over both parts and the UE id, and attached to the HS-SCCH information.

In case of HS-DSCH transmission to the same UE in consecutive HS-DSCH TTIs, the same HS-SCCH should be used for the corresponding associated downlink signalling.

When operating in CELL_DCH state the upper layer signalling on the DCCH can be mapped to the DCH mapped to the associated DPCH or the HS-DSCH.

5.2.2.2 TDD Downlink Physical layer model

Figure 5.2.2.2-1: Model of the UE’s physical layer (3.84 and 7.68 Mcps TDD)

Figure 5.2.2.2-2: Model of the UE’s physical layer in CELL_DCH state (1.28 Mcps TDD)

Figure 5.2.2.2-3: Model of the UE’s physical layer in CELL_DCH state (1.28Mcps TDD multi-frequency HS-DSCH operation mode only)

Figure 5.2.2.2-4: Model of the UE’s physical layer in CELL_FACH, CELL_PCH and URA_PCH state (1.28 Mcps TDD)

When operating in CELL_DCH state, the TDD overall downlink signalling structure is based on associated dedicated physical channels and shared physical control channels. The downlink signalling information for support of HS-DSCH is carried by the HS-SCCH.

For 1.28 Mcps TDD multi-frequency HS-DSCH operation mode, the associated downlink control channel and uplink control channel pair controlling the HS-DSCH transmission on the certain carrier shall be allocated on the same carrier. The downlink control channel carries the HS-DSCH operation related info and the uplink control channel carries the feedback info from the UE side.

When operating in CELL_FACH, CELL_PCH and URA_PCH state, as defined in subclauses 16 and 17, the basic downlink channel configuration consists of one or several HS-PDSCHs along with a number of shared physical control channels, HS-SCCHs, The UE obtains the HS-SCCH and HS-DSCH configuration from system information broadcast.

As in Release ’99, the associated dedicated physical channel can also be a fractionated channel for efficient resource usage with a corresponding repetition period in terms of TTIs. The UE is informed of an HS-DSCH allocation by means of a signalling message on an HS-SCCH.

For 3.84 Mcps TDD and 7.68Mcps TDD, the UE shall be allocated a set of up to four HS-SCCHs, and shall monitor all of these HS-SCCHs continuously. In any given TTI, a maximum of one of these HS-SCCHs may be addressed to the UE. In the case that a UE detects a message for it on a specific HS-SCCH, then it may restrict its monitoring of HS-SCCHs to only that HS-SCCH in the next TTI.

For 1.28 Mcps TDD, the UE shall be allocated a set of up to four HS-SCCHs per carrier, and shall monitor all of these HS-SCCHs continuously. In any given TTI, a maximum of one of these HS-SCCHs may be addressed to the UE on each carrier. In the case that a UE detects a message for it on a specific HS-SCCH on the certain carrier then it may restrict its monitoring of HS-SCCHs to only that HS-SCCH in the next TTI on this carrier.

5.2.3 UL Physical layer model

Figure 5.2.3-1: Model of the UE’s Uplink physical layer in CELL_DCH state

Figure 5.2.3-2: Model of the UE’s Uplink physical layer in CELL_DCH state (1.28 Mcps TDD multi-frequency HS-DSCH operation mode only)

In FDD, when operating in CELL_DCH state the uplink signalling uses an additional DPCCH with SF=256 that is code multiplexed with the existing dedicated uplink physical channels. The HS-DSCH related uplink signalling consists of H-ARQ acknowledgement and channel quality indicator.

In FDD, when operating in CELL_FACH, CELL_PCH and URA_PCH state HS-DSCH reception is as defined in clause 14 and the UE uses the common E-DCH for uplink transmission if the UE and cell support it, otherwise it uses the RACH for uplink transmission. The transmission of HS-DPCCH for HS-DSCH related ACK/NACK and CQI signalling is only supported when the UE is using a common E-DCH resource, is transmitting DTCH/DCCH data, has successfully resolved collision, and has been so configured by the network. Otherwise the transmission of HS-DPCCH is not supported.

In TDD, when operating in CELL_DCH state, the UE shall use a shared uplink resource (the HS-SICH) for transmitting ACK/NACK and CQI information. The relation between the HS-SCCH in DL and the HS-SICH in UL is pre-defined and is not signalled dynamically on the HS-SCCH.

In 1.28Mcps TDD, when operating in CELL_FACH, CELL_PCH and URA_PCH state, HS-DSCH reception is defined in subclause 16 and 17. UE uses E-DCH for uplink transmission. UE in CELL_FACH state with dedicated H-RNTI can send ACK/NACK and CQI signalling on related HS-SICH.

5.2.4 HS-DSCH physical-layer structure in the code domain

5.2.4.1 FDD

HS-DSCH relies on channelisation codes at a fixed spreading factor, SF=16. A UE may be assigned multiple channelisation codes in the same TTI, depending on its UE capability. Furthermore, multiplexing of multiple UEs in the code domain within a HS-DSCH TTI is allowed.

5.2.4.2 TDD

HS-DSCH relies on one or more channelisation codes with either SF=16 or SF=1, but not both simultaneously. Transmission on one or more timeslots is also allowed. Furthermore, a combination of code multiplexing and time multiplexing by timeslot within a HS-DSCH TTI is allowed, but the same set of channelisation codes must be used in all timeslots allocated to the HS-DSCH. The HS-DSCH TTI is not allowed to cross the frame (3.84 Mcps TDD) or the sub-frame (1.28 Mcps TDD with HS-DSCH not including TS0) boundary. For 1.28Mcps TDD with HS-DSCH including TS0, the HS-DSCH TTI is allowed to cross the sub-frame.

5.3 Transport channel attributes

The following is a list of HS-DSCH transport channel attributes:

1. Transport block size – dynamic for first transmission. An identical transport block size shall be applied for any retransmission. In TDD, there shall be no support for blind transport format detection. If FDD, the blind transport format detection is supported in HS-SCCH less operation as defined in subclause 12.1.

2. Transport block set size. The transport block set contains only one transport block for single stream transmission and two transport blocks for dual stream transmission, three transport blocks for three streams transmission and four transport blocks for four streams transmission.

3. Transmission Time Interval (TTI). For FDD the HS-DSCH TTI is fixed and equal to 2ms. The HS-DSCH TTI for 3.84 Mcps TDD is 10 ms. For 1.28 Mcps TDD a fixed 5 ms TTI shall apply.

4. Coding parameters:

– Type of error protection: turbo code rate 1/3.

5. Modulation – dynamic for first transmission and retransmission. Support for QPSK is mandatory in the UE whereas support for 16QAM and 64 QAM depends on the UE capability.

6. Redundancy version – dynamic.

7. CRC size – fixed size of 24 bits. There is one CRC per TB, i.e. one CRC per TTI for single stream transmission, two CRCs per TTI for dual stream transmission, three CRCs per TTI for three streams transmission and four CRCs per TTI for four streams transmission.