6.2 HS-DSCH MAC architecture – UTRAN side

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

This subclause describes the modifications to the MAC model with respect to the Release ’99 model to support the features for HS-DSCH on the UTRAN side. Both MAC-hs and MAC-ehs are responsible for handling the data transmitted on the HS-DSCH. Furthermore they are responsible for the management of the physical resources allocated to HS-DSCH. Upper layers configure which of the two entities, MAC-hs or MAC-ehs, is to be applied to handle HS-DSCH functionality.

6.2.1 Overall architecture

New MAC functional entities, the MAC-hs and the MAC-ehs, are added to the MAC architecture of Release ’99. Both the MAC-hs and the MAC-ehs are located in the Node B. If an HS-DSCH is assigned to the UE the MAC-hs and MAC-ehs SDUs, i.e. MAC-d PDUs to be transmitted are transferred from MAC-c/sh to the MAC-hs or MAC-ehs via the Iub interface in case of Configuration with MAC-c/sh, or from the MAC-d via Iur/Iub in case of Configuration without MAC-c/sh.

In FDD and 1.28Mcps TDD, in HS-DSCH transmission for the UE in CELL_FACH, CELL_PCH and URA_PCH state, the HS-DSCH operation is as defined in clause 14 and 15 for FDD, 16 and 17 for 1.28Mcps TDD, and the same overall MAC architecture is used, with the addition that MAC-c/sh SDUs from CCCH and PCCH can be transferred to MAC-ehs.

Figure 6.2.1-1: UTRAN side overall MAC architecture, MAC-hs

Figure 6.2.1-1b: UTRAN side overall MAC architecture, MAC-ehs

The multiplexing chain for HS-DSCH on the UTRAN side is illustrated below:

Figure 6.2.1-2: UTRAN side of MAC multiplexing for MAC-hs

Figure 6.2.1-3: Multiplexing and protocol architecture for MAC-ehs.

NOTE: In figure 6.2.1-3 the MAC-ehs support multiplexing between different UEs similarly to MAC-hs.

6.2.2 Details of MAC-c/sh

The data for the HS-DSCH is subject to flow control between the serving and the drift RNC.

A new flow control function is included to support the data transfer between MAC-d and MAC-hs/MAC-ehs.

Figure 6.2.2-1: UTRAN side MAC architecture/MAC-c/sh details

In FDD and 1.28Mcps TDD, when operating in CELL_FACH, CELL_PCH and URA_PCH, HS-DSCH reception is as defined in clauses 14 and 15 for FDD, 16 and 17 for 1.28Mcps TDD, and the MAC-c/sh can transfer CCCH and PCCH PDUs to MAC-ehs and the MAC-c/sh does not include any MAC-c/sh protocol header for those logical channels. MAC-c adds the U-RNTI when required to the RLC PDU of the SRB#1.

6.2.3 Details of MAC-hs

MAC-hs receives configuration parameters from the RRC layer via the MAC-Control SAP. There shall be priority handling per MAC-d PDU in the MAC-hs. The MAC-hs is comprised of four different functional entities:

– Flow Control:
This is the companion flow control function to the flow control function in the MAC-c/sh in case of Configuration with MAC-c/sh and MAC-d in case of Configuration without MAC-c/sh. Both entities together provide a controlled data flow between the MAC-c/sh and the MAC-hs (Configuration with MAC-c/sh) or the MAC-d and MAC-hs (Configuration without MAC-c/sh) taking the transmission capabilities of the air interface into account in a dynamic manner. This function is intended to limit layer 2 signalling latency and reduce discarded and retransmitted data as a result of HS-DSCH congestion. Flow control is provided independently per priority class for each MAC-d flow.

– Scheduling/Priority Handling:
This function manages HS-DSCH resources between HARQ entities and data flows according to their priority class. Based on status reports from associated uplink signalling either new transmission or retransmission is determined when operating in CELL_DCH state. In FDD, When operating in CELL_FACH, CELL_PCH and URA_PCH state HS-DSCH reception as defined in clauses 14 and 15, the MAC-hs can perform retransmission without uplink signalling. Further it sets the priority class identifier and TSN for each new data block being serviced. To maintain proper transmission priority a new transmission can be initiated on a HARQ process at any time. The TSN is unique to each priority class within a HS-DSCH, and is incremented for each new data block. It is not permitted to schedule new transmissions, including retransmissions originating in the RLC layer, within the same TTI, along with retransmissions originating from the HARQ layer.

– HARQ:
One HARQ entity handles the hybrid ARQ functionality for one user. One HARQ entity is capable of supporting multiple instances (HARQ process) of stop and wait HARQ protocols. For FDD and 3.84 Mcps/7.68 Mcps TDD, there shall be one HARQ process per TTI , for 1.28 Mcps TDD, there shall be one HARQ process per HS-DSCH on each carrier per TTI.

In 1.28 Mcps TDD multi-frequency HS-DSCH cell:

– multiple HARQ processes are assigned for HS-DSCH operaton on every carrier for every user , namely HARQ sub-entity; only one HARQ process in HARQ sub-entity is allowed to receive HS-DSCH in one TTI for each carrier.

– Choice of 6bit or 9bit TSN is configured by upper layer signalling.

– TFRC selection:
Selection of an appropriate transport format and resource combination for the data to be transmitted on HS-DSCH.

Figure 6.2.3-1: UTRAN side MAC architecture/MAC-hs details

Figure 6.2.3-2: UTRAN side MAC architecture / MAC-hs details(1.28Mcps TDD multi-frequency HS-DSCH operation mode only)

6.2.4 Details of MAC-ehs

MAC-ehs receives configuration parameters from the RRC layer via the MAC-Control SAP. There shall be priority handling per MAC-ehs SDU in the MAC-ehs.

The MAC-ehs comprises of six different functional entities:

– Flow Control:
The flow control for MAC-ehs is identical to the flow control for MAC-hs.

– Scheduling/Priority Handling:
This function manages HS-DSCH resources between HARQ entities and data flows according to their priority class. In FDD, the scheduler determines for each TTI if single or dual stream transmission should be used. Based on status reports from associated uplink signalling either new transmission or retransmission is determined when operating in CELL_DCH state. In FDD, when operating in CELL_FACH, CELL_PCH and URA_PCH state HS-DSCH reception, the MAC-ehs can perform retransmission without uplink signalling. Further it sets the logical channel identifiers for each new reordering SDU and TSNs for each new reordering PDU being serviced. To maintain proper transmission priority a new transmission can be initiated on a HARQ process at any time. The TSN is unique to each priority class within a HS-DSCH. It is not permitted to schedule new transmissions, including retransmissions originating in the RLC layer, within the same TTI over the same HS-DSCH, along with retransmissions originating from the HARQ layer.

– HARQ:
One HARQ entity handles the hybrid ARQ functionality for one user per HS-DSCH transport channel. One HARQ entity is capable of supporting multiple instances (HARQ process) of stop and wait HARQ protocols. There shall be one HARQ entity per HS-DSCH, one HARQ process per TTI for single stream transmission and two HARQ processes per TTI for dual stream transmission.

In 1.28 Mcps TDD multi-frequency HS-DSCH cell:

– multiple HARQ processes are assigned for HS-DSCH operaton on every carrier for every user, namely HARQ sub-entity; only one HARQ process in HARQ sub-entity is allowed to receive HS-DSCH in one TTI for each carrier.

– Choice of 6bit or 9bit TSN is configured by upper layer signalling.

– TFRC selection:
The TFRC selection for MAC-ehs is identical to the TFRC selection of the MAC-hs.

– Priority Queue MUX:
This function determinates the number of octets to be included in a MAC-ehs PDU from each priority queue based on the scheduling decision and available TRFC for this function.

– Segmentation:
This function performs necessary segmentation of MAC-ehs SDUs.

Figure 6.2.4-1: UTRAN side MAC architecture/MAC-ehs details.

Figure 6.2.4-2: UTRAN side MAC architecture / MAC-ehs details (1.28Mcps TDD multi-frequency HS-DSCH operation mode only)