6 DCH Enhancements (FDD only)

25.3003GPPGeneral descriptionRelease 17Stage 2TSUniversal Terrestrial Radio Access Network (UTRAN)

DCH enhancements aims at improving the link efficiency and UE battery performance for voice calls compared to R99 DCH. DCH enhancements constitutes of the following sub-features:

– DL overhead optimization

– Enhanced rate matching and transport channel multiplexing

– DL Frame Early Termination (DL FET)

– Uplink DPCCH with DL FET ACK

– Uplink DPDCH dynamic 10ms transmission

DCH enhancements supports two modes (Basic and Full). The mode choice controls how the DL Frame Early Termination sub-feature operates, as described in 6.3. All other sub-features are active in both modes.

DCH enhancements is only applicable if the TTI of all DCH transport channels on both downlink and uplink is at least 20 ms.

If a UE is configured with both CS and PS mapped to the DCH transport channel (in uplink or downlink or both), then DCH enhancements may be configured only when PS has UL:0 and DL:0kbps RAB configuration (3GPP TR 25.993 [13]).

6.1 DL overhead optimization

This sub-feature introduces new DL DPCH slot format by removing the dedicated pilot bits from DL DPCCH and reusing them for DL DPDCH instead.

The R99 downlink physical channel (DPCH) consists of 0.66ms slots that contain 2 groups of data (DPDCH) symbols and 3 groups of control (DPCCH) symbols. The size of the groups is determined by the slot format. The control symbol groups are TPC – controlling uplink transmit power, TFCI – specifying the downlink packet type, and dedicated pilot – supporting channel estimation for DL power control and closed-loop transmit diversity. While the TFCI group may be empty in certain slot formats, the pilot and TPC are currently always non-empty. The dedicated pilot bits are used for estimation of DL SIR. With this sub-feature, new DL DPCH slot formats are introduced by removing the dedicated pilot bits and reusing the TPC bits instead for estimating the DL SIR. Correspondingly, the number of data symbols in a slot is increased leading to less control channel overhead on the downlink.

DL closed-loop transmit diversity is not supported when this sub-feature is configured.

6.2 Enhanced rate matching and transport channel multiplexing

The physical layer in R99 is designed to carry potentially a large variety of transport blocks with different sizes. The drawback for this design is the rate matching may not be efficient when some transport format combinations are not frequently used. For example, DCCH channel carries non-zero transport blocks not as often as voice DTCH channel. The enhanced rate matching and transport channel multiplexing sub-feature sets a zero rate matching attribute for DCCH, whenever DCCH channel does not carry a transport block together with DTCH channel. The DCCH bit fields are used to transmit DTCH transport channels instead. This potentially improves link efficiency due to less puncturing and better rate matching of the transport block with the available physical channel resources.

6.3 DL frame early termination (DL FET) and UL DPCCH with DL FET ACK

In a power-controlled system such as R99 DCH, inefficiencies in the power-control loop, such as limited granularity, delays and errors in the feedback, result in the presence of excess SINR at the receiver. This means that packets such as the voice packets which have a long (20ms) transmission time interval (TTI) can often be early-decoded, i.e, decoded prior to reception of all the data symbols in a TTI by running the channel decoder at multiple time instants during the TTI instead of only once at the end of the TTI. This is referred to as Frame Early Termination (FET). As described below, DCH enhancements introduces new mechanisms to R99 DCH in order to support DL FET.

A new design of UL DPCCH is introduced to support DL FET. With the new design, TFCI information is carried in the first 10 slots of each 20ms TTI for the uplink. Sending the TFCI information early in each 20ms TTI allows sending of DL FET ACK or NACK information using the TFCI bits in remaining UL DPCCH slots that do not carry TFCI.

Furthermore, there are two modes of operation introduced with support for DL FET in DCH enhancements as described below.

6.3.1 DL FET Full mode (Mode 1)

In the Full mode of operation:

– The UE acknowledges successful early decoding of a DL packet via a DL FET ACK on the newly designed UL DPCCH channel, which then allows the NodeB to stop transmission of the packet.

– AMR Class A, B, C transport channels are concatenated on the DL which further helps in early decoding of DL DPDCH.

6.3.2 DL FET Basic mode (Mode 0)

In the Basic mode of operation:

– DL FET is achieved by applying the DL BLER target at slot 14 (10ms) in each 20ms TTI duration. The NodeB may decide to stop transmission of the DL voice packet at slot 14 provided that the Uplink is in 10ms transmission mode (see sub-clause 6.4). The UE does not indicate successful decoding of the DL packet via the DL FET ACK or NACK field in UL DPCCH.

– AMR Class A, B, C transport channels are not concatenated on the downlink.

6.4 Uplink DPDCH dynamic 10ms transmission

The R99 DCH transport channels for a voice call are typically configured with 20ms TTI. However, the transport block sizes for a voice call could potentially be transmitted over a shorter duration. The sub-feature of uplink DPDCH dynamic 10ms transmission allows for dynamically selecting a shorter transmission time, i.e. 10ms, at the physical layer to transmit a voice packet on the uplink. The UE selects on whether to use a 10ms or 20ms transmission duration based on considerations such as the power headroom at the UE. The UE also discontinues the transmission of UL DPCCH for the remaining duration of the TTI when both UL transport block has been completed transmitted and DL has been successfully decoded early.

With 20ms TTI transmission at the physical layer, the pilot channel (UL DPCCH) is sent for the entire 20ms duration. This sub-feature potentially improves link efficiency due to reduction in UL DPCCH overhead as well as improves UE battery performance by allowing the UE to turn off its transceiver once the reception and transmission has been completed before the end of a 20ms TTI.