Explaining HSUPA

 

By Phil Medd, Product Manager, Aeroflex Test Solutions, Wireless Division

 

Introduction

 

With 3GPP High Speed Downlink Packet Access (HSDPA) services becoming a reality, the corresponding enhancement to the uplink, logically termed High Speed Uplink Packet Access, or HSUPA, is now at an advanced stage in the standards development process. The 3GPP Release 6 core specifications defining the enhanced uplink were completed in May 2005, and work continues on the definition of the conformance tests that HSUPA enabled devices must be able to pass. This article provides an overview of the technology used, the impact on network operators and the benefit to end-users. New services based on the IP Multimedia Subsystem (IMS) are likely to take advantage of the higher data rates and reduced delays offered by HSDPA and HSUPA, and the longer term significance of this combination is discussed. The testing challenges posed by this new 3.5G technology are presented.

 

 

Overview

 

  • Higher data rates, lower latency, more efficient use of spectrum
  • New scheduling scheme located in Node B
  • HSUPA terminals must support HSDPA
  • Similar to HSDPA, but uses a dedicated, not shared, channel
  • Unlike HSDPA, does not use Adaptive Modulation
  • Six categories for HSUPA, twelve for HSDPA

 

As with HSDPA, the aim with HSUPA has been to increase capacity and throughput while reducing delay. A number of enhancements have been introduced to achieve this, including a new uplink dedicated transport channel (E-DCH), which works in conjunction with a new scheduling mechanism located in the Node B. As with HSDPA, HARQ (Hybrid Automatic Repeat Request) and an additional 2msec TTI (Transmission Time Interval) are used to enhance the uplink. Attention has been paid in the standards to limiting the complexity of the enhanced uplink, ensuring that practical implementations are possible.

 

 

Physical layer changes

 

 

HSUPA introduces just one enhanced uplink channel, the E-DCH, while retaining the R99 DCH. The attributes of the E-DCH transport channel are:

 

·        New physical channels: E-DPDCH and E-DPCCH. Pilot symbols recovered from the E-DPCCH are used by the receiver to assist the decoding of the E-DPDCH, which carries the E-DCH transport channel data.

·        The E-DPCCH also carries the E-TFCI (E-DCH transport format combination indicator), RSN (retransmission sequence number) and ‘happy' bit, which is used to indicate whether the UE is satisfied with the resources currently allocated.

  • A range of data rates of up to 5.76Mbps can be achieved by varying the Spreading Factor and the number of channelisation codes used. Rate 1/3 turbo coding is used.
  • Both 2ms and 10ms TTI available according to suit the requirement. A 2ms TTI allows for faster power control and transport block size adjustment.
  • One transport block is sent per TTI

 

Note that there is only one E-DCH per UE, and that the R99 DCH is limited to 64Kbps whenever the E-DCH is configured. Unlike HSDPA, HSUPA does not use adaptive modulation and coding.

 

To support the enhanced uplink, there are three new downlink signalling channels introduced:

 

E-HICH – E-DCH HARQ Indicator Channel. This downlink physical channel is used by the HARQ process to acknowledge E-DCH transmissions from the UE.

 

E-AGCH – E-DCH Absolute Grant Channel. This shared downlink physical channel is used to indicate to the UE how much data can be sent on the uplink, allowing it to determine the E-DCH TFC (Traffic Format Combination) and maximum allowed power.

 

E-RGCH – E-DCH Relative Grant Channel. This shared downlink physical channel is used to increase or decrease the uplink resources compared to the previously used value.

 

 

Error handling

To make the uplink resilient to signal errors, retransmission of faulty packets can be requested by the Node B using the HARQ protocol. This ‘stop and wait' mechanism relies on acknowledgements/negative acknowledgements being fed back to the UE on the new E-HICH channel. By being located in the Node B rather than RNC, this protocol can provide fast recovery of lost or corrupted packets.

 


New Layer 2 entities

 

Within the UE, a new layer 2 MAC-e/es entity has been introduced between MAC-d and the physical layer. The MAC-e/es handles HARQ retransmissions, scheduling, MAC-e multiplexing and E-DCH TFC selection.

 

Within the Node B, a new MAC-e entity is used to handle HARQ retransmissions, scheduling and MAC-es de-multiplexing/re-ordering and forwarding to MAC-d.

 

In the SRNC (Serving Radio Network Controller), a new MAC-es entity is used to ensure in-sequence packet delivery and to handle soft handover with other Node B's.

 

The E-DCH is intended to support multiple services simultaneously, so the UE multiplexes dedicated logical channels onto MAC-d flows, each of which may have different QoS characteristics. Different MAC-d flows may be multiplexed into the E-DCH in the same TTI. A maximum of 8 MAC-d flows can be multiplexed and mapped onto an E-DCH.

 

Since the E-DCH is intended for the transport of dedicated logical channels (DTCH, DCCH), no architectural changes are needed above Layer 2, keeping the protocol layers above MAC unchanged from R99/R5. This simplifies the implementation and limits the amount of re-testing required. The RLC Unacknowledged Mode (UM) and Acknowledged Mode (AM) are used to transfer data via the DTCH/DCCH, which are mapped to the E-DCH.

 


Terminal Categories

 

Terminals are grouped into six categories according to the number of E-DCH codes, Spreading Factor and TTI used, see Figure 1. This results in peak data rates ranging from 730Kbps to 5.76Mbps. 

 

Figure 1: HSUPA Terminal Categories

 

New Scheduling scheme

 

A key improvement is the introduction of a more efficient scheduling mechanism, allowing better use to be made of the available spectrum and power budget. Both scheduled and non-scheduled transmissions are specified in the standards. The scheduled grant system is controlled by the Node B, and uses QoS-related information from the SRNC as well as requests from the UE to determine the amount of resource to be made available to each UE. The resource level is allocated using either an Absolute Grant, which defines an absolute resource limit, or a Relative Grant. The Relative Grant complements the Absolute Grant and can increase or reduce the resource level compared to the previously used value. The Relative Grant can be sent either by the Serving or non-Serving Node B where more than one radio link set is being used. Scheduling Grants can be sent up to once per TTI, providing a rapid resource control process.

 

Both Absolute and Relative Grants can be sent to individual, multiple or all UEs in a cell. The grants are addressed using the E-RNTI (E-DCH Radio Network Temporary Identifier) for the UE or group of UEs.

 

In addition, the non-scheduled grant scheme permits the UE to send E-DCH data at any time, up to a specified maximum number of bits, reducing the signalling overhead and minimising the associated delay. Non-scheduled grants are only allowed when the SRCN signals to the UE that this mode of operation is permissible.

 

The result of the changes made with HSDPA and HSUPA, collectively referred to as HSPA, is improved spectral efficiency, with a 50% improvement over 3GPP WCDMA R99 technology being achieved.

 


Applications

 

  • Data-cards for laptops likely to be early mass-market, providing mobile broadband
  • IP Multimedia Sub-system (IMS) to be the basis for future services

 

Data-cards for laptops, rather than handsets, are likely to form the early mass-market, providing mobile broadband services. The intention is to provide a consistent user experience across urban, suburban and rural areas, with comparable performance to current fixed broadband links. Multiple applications are likely to be used in this situation – e-mail and chat clients, browsers accessing multimedia content etc. In the future, applications for services specifically created for mobile users are likely to emerge, for example ones that are location-dependent. Many of these services will be based on the IP Multimedia Sub-system (IMS), another 3GPP Release 6 feature.

 

More on IMS

 

  • Highly significant
  • Step on the road to an all-IP network
  • Eventually leading to mobile VoIP
  • Easier for network operators to add services based on IMS

 

IMS will make it easier and cheaper for network operators to offer a range of advanced services built on common IP-based core functions. These core functions will handle such tasks as charging, security and routing as well as interoperability and roaming. Without IMS it would be necessary to include these functions in each server type, whereas with IMS they can be re-used, allowing for the rapid creation and delivery of new services. This in turn will reduce the capital and operational expenditure required. IMS has already been selected by 3GPP as the underlying protocol for PoC (Push to talk over Cellular). IMS is based on the IETF's SIP (Session Initiation Protocol), already in wide use especially in Voice over IP networks.

 

By providing an all-IP environment, application developers will find it easier to adapt their products for the mobile network. The use of IMS in mobile packet data networks is a highly significant step, since it further promotes the convergence with fixed communication networks. For IMS to be effective, an efficient, high-speed packet data network is essential, hence the importance of HSPA.

 

Network roll-out

 

  • Software only upgrade to the infrastructure, but new terminals needed
  • Initial capability unlikely to offer maximum data rates
  • Likely timescales for launch of first services is end of 2006

 

Network operators are constantly seeking ways to increase the proportion of revenue from packet data services. In parallel with this strategy, HSPA will lower the cost per bit of providing packet data services, improving margins.

 

Compatibility with R99/R4/R5 networks has been carefully considered in the development of the R6 specification. This will allow network operators to introduce the new capability without impacting current users. Current WCDMA network infrastructure can typically be upgraded to support HSUPA with a software update. The care taken in the definition of the standard has minimised the complexity in both UE and network to achieve an improved level of system performance.

 

New terminals will be required to support HSUPA. Note that HSUPA devices must also support HSDPA. It is unlikely that initial devices will support the maximum data rates in both downlink and uplink. A likely combination offered will be Cat 6 downlink (3.6Mbps) with Cat 2 uplink (1.45Mbps)

 


Testing challenges

 

  • Higher baseband load since multiple cells will need to be simulated
  • Simultaneous HSDPA + HSUPA

 

 

 

Powerful test equipment will be required to take into account the higher baseband-processing load for multiple cell simulation and simultaneous HSDPA and HSUPA. Aeroflex's 6401 AIME handset test platform emulates a number of HSDPA cells on different carriers with arbitrary numbers of physical channels on each cell. Due to it's scaleable architecture, the 6401 AIME can be software upgraded to support the testing of HSUPA-capable chipsets and handsets during both the development and conformance test phases.

 


Conclusion

 

  • Potentially big impact for users, provided network operators can make attractive offerings
  • With the new high-speed packet access services, 3.5G will have arrived.
  • Due to the nature of the step from 3G to 3.5G being a network software enhancement, there will not be the large investment recovery necessary as with 3G.

 

HSUPA will deliver significant improvements to both users and network operators by enhancing data throughput and capacity. Due to the nature of the step from 3G to 3.5G being a network software enhancement, there will not be the large investment recovery necessary as with 3G. A new generation of IP-based services will be able to take advantage of the enhanced mobile networks, bringing about increased convergence with fixed and WiFi-based networks. The improved efficiency of HSPA will allow network operators to make better use of licensed spectrum, while the open architecture of the IMS will simplify the deployment of a range of services aimed at tempting users to make more use of the enhanced packet-switched networks.

 


Find out more

 

 

Find out more about this new feature in the 3GPP standards, available from www.3GPP.org. Document 3GPP TS 25.309 FDD Enhanced Uplink; Overall Description; Stage 2 (Release 6) is a good place to start.