High Speed Uplink Packet Access (HSUPA): A Tutorial

By Zahid Ghadialy

Last Updated: 26/03/2006

Introduction:

High Speed Uplink Packet Access (HSUPA) is a release 6 feature in 3GPP specifications and is part of HSPA (High Speed Packet Access) family. HSUPA is more often called as the Enhanced Uplink Dedicated Channel (E-DCH) by the technically aware people. The main aim of HSUPA is to increase the uplink data transfer speed in the UMTS environment and it offers data speeds of up to 5.8 Mbps in the uplink. HSUPA achieves its high performance through more efficient uplink scheduling in the base station and faster retransmission control.

Requirements:

HSUPA was designed based on the following requirements

  • The Enhanced Uplink feature shall aim at providing significant enhancements in terms of user experience (throughput and delay) and/or capacity. The coverage is an important aspect of the user experience and that it is desirable to allow an operator to provide for consistency of performance across the whole cell area.
  • The focus shall be on urban, sub-urban and rural deployment scenarios.
  • Full mobility shall be supported, i.e., mobility should be supported for high-speed cases also, but optimisation should be for low-speed to medium-speed scenarios.
  • The study shall investigate the possibilities to enhance the uplink performance on the dedicated transport channels in general, with priority to streaming, interactive and background services. Relevant QoS mechanisms shall allow the support of streaming, interactive and background PS services.
  • It is highly desirable to keep the Enhanced Uplink as simple as possible. New techniques or group of techniques shall therefore provide significant incremental gain for an acceptable complexity. The value added per feature/technique should be considered in the evaluation. It is also desirable to avoid unnecessary options in the specification of the feature.
  • The UE and network complexity shall be minimised for a given level of system performance.
  • The impact on current releases in terms of both protocol and hardware perspectives shall be taken into account.
  • It shall be possible to introduce the Enhanced Uplink feature in a network which has terminals from Release'99, Release 4 and Release 5. The Enhanced Uplink feature shall enable to achieve significant improvements in overall system performance when operated together with HSDPA. Emphasis shall be given on the potential impact the new feature may have on the downlink capacity. Likewise it shall be possible to deploy the Enhanced Uplink feature without any dependency on the deployment of the HSDPA feature. However, a terminal supporting the Enhanced Uplink feature must support HSDPA.

Abbreviations:

It would be important to remember following abbreviations before proceeding:

  • AG: Absolute Grant
  • E-AGCH: E-DCH Absolute Grant Channel
  • E-DCH: Enhanced Dedicated Channel
  • E-DPCCH: E-DCH Dedicated Physical Control Channel
  • E-DPDCH: E-DCH Dedicated Physical Data Channel
  • E-HICH: E-DCH Hybrid ARQ Indicator Channel
  • E-RGCH: E-DCH Relative Grant Channel
  • E-RNTI: E-DCH Radio Network Temporary Identifier
  • E-TFC: E-DCH Transport Format Combination
  • HARQ: Hybrid Automatic Repeat Request
  • HSDPA: High Speed Downlink Packet Access
  • RG: Relative Grant
  • RLS: Radio Link Set
  • RSN: Retransmission Sequence Number
  • SG: Serving Grant
  • TSN: Transmission Sequence Number

HSUPA General Features

  • Maximum transmission rate of 5.76Mbps
  • BPSK modulation
  • No adaptive modulation
  • Multicode transmission
  • Spreading Factor either 2 or 4
  • 10ms and 2ms TTI but initially only 10ms TTI to be used.
  • Hybrid ARQ (HARQ)
  • Fast Packet Scheduling in the uplink
  • Soft Handover supported

Protocol Architecture of E-DCH

The following modifications to the existing nodes are needed to support enhanced uplink DCH:

UE: A new MAC entity (MAC-es/MAC-e) is added in the UE below MAC-d. MAC- es/MAC-e in the UE handles HARQ retransmissions, scheduling and MAC-e multiplexing, E-DCH TFC selection.

Node B: A new MAC entity (MAC-e) is added in the Node B to handle HARQ retransmissions, scheduling and MAC-e demultiplexing.

S-RNC: A new MAC entity (MAC-es) is added in the SRNC to provide in-sequence delivery (reordering) and to handle combining of data from different Node Bs in case of soft handover.

HSUPA Physical Layer categories

The following E-DCH UE categories are defined in the specifications:
HSUPA
category

Maximum
number of
HSUPA
codes
transmitted

Minimum
spreading
factor
Support
for 10 and 2 ms
HSUPA TTI
Maximum
number of
bits
transmitted
within
a 10 ms
HSUPA
TTI
Maximum
number of
bits
transmitted
within
a 2 ms
HSUPA
TTI
Maximum
Bit rate
Category 1 1 SF4 10 ms TTI only 7296 - 0.73 Mbps
Category 2 2 SF4 10 ms and 2 ms TTI 14592 2919 1.46 Mbps
Category 3 2 SF4 10 ms TTI only 14592 - 1.46 Mbps
Category 4 2 SF2 10 ms and 2 ms TTI 20000 5837 2.92 Mbps
Category 5 2 SF2 10 ms TTI only 20000 - 2.00 Mbps
Category 6 4 SF2 10 ms and 2 ms TTI 20000 11520 5.76 Mbps

New Channels

Dedicated transport channel

E-DCH - Enhanced Dedicated Channel: The Enhanced Dedicated Channel (E-DCH) is an uplink transport channel.

Uplink Dedicated Physical channels

E-DPCCH and E-DPDCH:

The E-DPDCH is used to carry the E-DCH transport channel. There may be zero, one, or several E-DPDCH on each radio link. The E-DPCCH is a physical channel used to transmit control information associated with the E-DCH. There is at most one E-DPCCH on each radio link.

E-DPDCH and E-DPCCH are always transmitted simultaneously, except for the case that E-DPDCH but not E-DPCCH is DTXed due to power scaling. E-DPCCH shall not be transmitted in a slot unless DPCCH is also transmitted in the same slot.

Figure above shows the E-DPDCH and E-DPCCH (sub)frame structure. Each radio frame is divided in 5 subframes, each of length 2 ms; the first subframe starts at the start of each radio frame and the 5th subframe ends at the end of each radio frame. The E-DPDCH slot formats, corresponding rates and number of bits are specified in Table A. The E-DPCCH slot format is listed in Table B.

Table A: E-DPDCH slot formats
Slot Format #i Channel Bit Rate (kbps) SF Bits/ Frame Bits/ Subframe Bits/Slot (Ndata)
0 15 256 150 30 10
1 30 128 300 60 20
2 60 64 600 120 40
3 120 32 1200 240 80
4 240 16 2400 480 160
5 480 8 4800 960 320
6 960 4 9600 1920 640
7 1920 2 19200 3840 1280

Table B: E-DPCCH slot formats
Slot Format #i Channel Bit Rate (kbps) SF Bits/ Frame Bits/ Subframe Bits/Slot (Ndata)
0 15 256 150 30 10

Downlink Dedicated Physical channels

E-DCH Relative Grant Channel (E-RGCH):

The E-DCH Relative Grant Channel (E-RGCH) is a fixed rate (SF=128) dedicated downlink physical channel carrying the uplink E-DCH relative grants. Figure above illustrates the structure of the E-RGCH. A relative grant is transmitted using 3, 12 or 15 consecutive slots and in each slot a sequence of 40 ternary values is transmitted. The 3 and 12 slot duration shall be used on an E-RGCH transmitted to UEs for which the cell transmitting the E-RGCH is in the E-DCH serving radio link set and for which the E-DCH TTI is respectively 2 and 10 ms. The 15 slot duration shall be used on an E-RGCH transmitted to UEs for which the cell transmitting the E-RGCH is not in the E-DCH serving radio link set.

E-DCH Hybrid ARQ Indicator Channel (E-HICH):

The E-DCH Hybrid ARQ Indicator Channel (E-HICH) is a fixed rate (SF=128) dedicated downlink physical channel carrying the uplink E-DCH hybrid ARQ acknowledgement indicator. Figure above (same as E-RGCH) illustrates the structure of the E-HICH. A hybrid ARQ acknowledgement indicator is transmitted using 3 or 12 consecutive slots and in each slot a sequence of 40 binary values is transmitted. The 3 and 12 slot duration shall be used for UEs which E-DCH TTI is set to respectively 2 ms and 10 ms.

Fractional Dedicated Physical Channel (F-DPCH):

The F-DPCH carries control information generated at layer 1 (TPC commands). It is a special case of downlink DPCCH. Figure above shows the frame structure of the F-DPCH. Each frame of length 10ms is split into 15 slots, each of length Tslot = 2560 chips, corresponding to one power-control period. There are 2 bits/slot.

Common downlink physical channels

E-DCH Absolute Grant Channel (E-AGCH):

The E-DCH Absolute Grant Channel (E-AGCH) is a fixed rate (30 kbps, SF=256) downlink physical channel carrying the uplink E-DCH absolute grant. Figure above illustrates the frame and sub-frame structure of the E-AGCH. An E-DCH absolute grant shall be transmitted over one E-AGCH sub-frame or one E-AGCH frame. The transmission over one E-AGCH sub-frame and over one E-AGCH frame shall be used for UEs for which E-DCH TTI is set to respectively 2 ms and 10 ms.

HARQ protocol

General Principle

The HARQ protocol has the following characteristics:

  • Stop and wait HARQ is used;
  • The HARQ is based on synchronous downlink ACK/NACKs;
  • The HARQ is based on synchronous retransmissions in the uplink:
    • The number of processes depends on the TTI: 8 processes for the 2ms TTI and 4 processes for the 10ms TTI. For both scheduled and non-scheduled transmission for a given UE, it is possible to restrict the transmission to specific processes for the 2ms E-DCH TTI;
    • There will be an upper limit to the number of retransmissions. The UE decides on a maximum number of transmissions for a MAC-e PDU based on the maximum number of transmissions attribute (see subclause 11.1.1), according to the following principles:
      • The UE selects the highest 'maximum number of transmissions' among all the considered HARQ profiles associated to the MAC-d flows in the MAC-e PDU.
    • Pre-emption will not be supported by E-DCH (ongoing re-transmissions will not be pre-empted by higher priority data for a particular process);
    • In case of TTI reconfiguration, the MAC-e HARQ processes are flushed and no special mechanism is defined to lower SDU losses.
  • Intra Node B macro-diversity and Inter Node B macro-diversity should be supported for the E-DCH with HARQ;
  • Incremental redundancy shall be supported by the specifications with Chase combining as a subcase:
    • The first transmission shall be self decodable;
    • The UTRAN configures the UE to either use the same incremental redundancy version (RV) for all transmissions, or to set the RV according to set of rules based on E-TFC, Retransmission Sequence Number (RSN) and the transmission timing;
    • There shall be no need, from the H-ARQ operation point of view, to reconfigure the Node B from upper layers when moving in or out of soft handover situations.

Error handling

The most frequent error cases to be handled are the following:

  • NACK is detected as an ACK: the UE starts afresh with new data in the HARQ process. The previously transmitted data block is discarded in the UE and lost. Retransmission is left up to higher layers;
  • ACK is detected as a NACK: if the UE retransmits the data block, the NW will re-send an ACK to the UE. If in this case the transmitter at the UE sends the RSN set to zero, the receiver at the NW will continue to process the data block as in the normal case;
  • Error cases have been identified regarding the HARQ operation during soft handover:
    • In case the HARQ control information transmitted on the E-DPCCH could not be detected RSN_max times in a row for one HARQ process, a soft buffer corruption might occur. Each HARQ process uses RSN and the transmission time (CFN, sub-frame) elapsed since storing data in the associated soft buffer in order to flush the soft buffer and to avoid a wrong combining of data blocks.
    • Duplication of data blocks may occur at the RNC during soft handover. The reordering protocol needs to handle the detected duplications of data blocks.

Signaling examples

E-DCH Establishment with TTI Reconfiguration

This scenario shows an example of E-DCH configuration. Also TTI reconfiguration is shown in the same scenario. It is assumed that in this example DCH was established before.


  1. The SRNC decides there is a need for a establishing E-DCH for a UE and prepares the RNSAP message Radio Link Reconfiguration Prepare which is transmitted to the CRNC.
    Parameters: DCHs to Delete IE, E-DPCH Information (TFCS, TTI), Serving E-DCH RL ID, E-DCH FDD Information.
  2. The CRNC requests the E-DCH Node B to perform a synchronised radio link reconfiguration using the NBAP message Radio Link Reconfiguration Prepare, for the E-DCH radio link.
    Parameters: DCHs to Delete IE, Servine E-DCH RL ID, E-DCH FDD Information.
  3. The E-DCH Node B returns a NBAP message Radio Link Reconfiguration Ready.
    Parameters: DCH Information Response , E-DCH FDD Information Response.
  4. The CRNC returns the RNSAP message Radio Link Reconfiguration Ready to the SRNC.
    Parameters: DCH Information Response, E-DCH FDD Information Response.
  5. The CRNC initiates set-up of a new Iub Data Transport Bearers using ALCAP protocol. This request contains the AAL2 Binding Identity to bind the Iub Data Transport Bearer to the E-DCH.
  6. The SRNC initiates set-up of a new Iur Data Transport bearer using ALCAP protocol. This request contains the AAL2 Binding Identity to bind the Iur Data Transport Bearer to the E-DCH.
  7. The SRNC proceeds by transmitting the RNSAP message Radio Link Reconfiguration Commit to the CRNC.
    Parameters: SRNC selected activation time in the form of a CFN.
  8. The CRNC transmits the NBAP message Radio Link Reconfiguration Commit to the E-DCH Node B including the activation time.
    Parameters: CRNC selected activation time in the form of a CFN.
  9. The SRNC also transmits a RRC message Radio Bearer Reconfiguration to the UE.
    Parameters: activation time, E-DCH Info and E-RNTI.
  10. The UE returns a RRC message Radio Bearer Reconfiguration Complete to the SRNC.
  11. The CRNC initiates release of the old Iub Data Transport bearer (DCH) using ALCAP protocol.
  12. The SRNC initiates release of the old Iur Data Transport bearer (DCH) using ALCAP protocol.
  13. The SRNC decides there is a need for a TTI reconfiguration and prepares the RNSAP message Radio Link Reconfiguration Prepare which is transmitted to the CRNC.
    Parameters: E-DPCH Information (TTI).
  14. The CRNC requests the E-DCH Node B to perform a synchronised radio link reconfiguration using the NBAP message Radio Link Reconfiguration Prepare, for the E-DCH radio link
  15. The E-DCH Node B returns a NBAP message Radio Link Reconfiguration Ready.
    Parameters: E-DCH FDD Information Response.
  16. The CRNC returns the RNSAP message Radio Link Reconfiguration Ready to the SRNC.
    Parameters: E-DCH FDD Information Response.
  17. The SRNC proceeds by transmitting the RNSAP message Radio Link Reconfiguration Commit to the CRNC.
    Parameters: SRNC selected activation time in the form of a CFN.
  18. The CRNC transmits the NBAP message Radio Link Reconfiguration Commit to the E-DCH Node B including the activation time.
    Parameters: CRNC selected activation time in the form of a CFN.
  19. The SRNC also transmits a RRC message Radio Bearer Reconfiguration to the UE.
    Parameters: activation time, E-DCH Info and E-RNTI.
  20. The UE returns a RRC message Radio Bearer Reconfiguration Complete to the SRNC.




REFERENCES:

[1] 3GPP TS 25.309 - FDD Enhanced Uplink; Overall description; Stage 2; Release 6

[2] 3GPP TR 25.896 - Feasibility Study for Enhanced Uplink for UTRA FDD; Release 6

[3] 3GPP TS 25.211 - Physical channels and mapping of transport channels onto physical channels; Release 6

[4] 3GPP TS 25.306 - UE Radio Access capabilities; Release 6

[5] WCDMA for UMTS - Harri Holma

[6] HSUPA - NEC Mobile Solutions

[7] Nokia HSPA Whitepaper

[8] Expert's Forum--Wireless telecom and 3G technology prospects - Huawei

[9] HSPA: High Speed Wireless Broadband - UMTS Forum

















































































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