HSPA - Mobile Broadband Today

Intro
Today’s mobile communication systems have been enhanced recently to more efficiently support packet
switched services. In UMTS HSDPA and E-DCH have been specified in downlink and uplink respectively.
By now UMTS is a well-established technology with manifold networks running globally and competitive
terminals on the market. A significant shift from traditional circuit-switched, often constant bit-rate services
to IP packet switched services is expected in the near future. UMTS Release 99, based on dedicated
resource allocation per user, is not well suited for IP packet data traffic. Therefore High Speed Packet
Downlink Access (HSDPA) and Enhanced Dedicated Channel (E-DCH) have been introduced as new
features of UMTS for Downlink and Uplink in UMTS Release 5 and Release 6, respectively. This technology
called High Speed Packet Access (HSPA) claims significant enhancements in end-to-end service provisioning
for IP based services. This introduces these future technology enhancements and assesses the
potential gains for future applications and in term user perception.
In addition to the paradigm change from using dedicated resources to making use of shared radio resources,
the main technology changes introduced are:
• Fast Node B scheduling with adaptive coding and modulation (only downlink) to exploit the varying
radio channel and interference variations and accommodate bursty IP traffic,
• Node B based Hybrid ARQ to reduce retransmission round trip times and add robustness to the
system by allowing soft combining of retransmissions,
• Reduced transmission time interval (TTI) for latency reduction and to support fast scheduler decisions
and quick HARQ retransmissions.
These added functionalities have been specified in several new MAC sub-layers and modifications of the
physical layer as is depicted


In general retransmissions are now performed directly between Node B and the User equipment (UE).
This reduces latency and saves resources on the Iub interface. The distributed scheduling performed by
RNC and Node B requires an additional scheduling buffer in the Node B as well as having an additional
flow control on the Iub interface. Furthermore the Node B needs to be made aware of certain QoS parameter
to ensure that the data transmission complies with the traffic requirements. Nevertheless HSDPA
and HSUPA can be implemented in the standard 5 MHz carrier of UMTS networks and can co-exist with
the existing 3GPP Release 99 networks. In the following sections the principles of HSDPA and E-DCH
are explained in more detail.

HIGH SPEED DOWNLINK PACKET ACCESS

In downlink a new entity called MAC-hs contains the new HSDPA functionality as seen in Figure 1. Instead
of a fixed code allocation with fast power control, the code and power resource is now shared
amongst all active HSDPA users. For this purpose a new transport channel, the High Speed Downlink
Shared Channel (HS-DSCH), has been defined that supports adaptive coding and modulation, whereby
every 2ms the transmission format can change dynamically.
In good radio channel condition 16QAM modulation can be used instead of QPSK and the rate 1/3 turbo
code may be punctured down to enable higher data rates. Depending on the UE capabilities up to 15
codes with a fixed spreading factor of 16 can be received if all codes are allocated to a single UE. Since
power control is replaced by rate control with adaptive coding and modulation the maximum data rate as
received by the user directly depends on the channel and interference conditions as well as the user position
in the cell.
The Node B scheduler must take care that fairness is maintained. The dynamic resource allocation by the
scheduler (per 2ms TTI) is signalled to the users on a new downlink control channel called High Speed
Signalling Control Channel (HS-SCCH). The following information is carried on the HS-SCCH:
• UE Identity (UE ID) via a UE specific CRC which allows addressing specific UEs on the shared
control channel.
• Transport Format and Resource Indicator (TFRI) which identifies the scheduled resource and its
transmission format.
• Hybrid-ARQ-related information to identify redundancy versions for the combining process.
Each user can monitor up to 4 HS-SCCHs. For the support of channel based scheduling and HARQ the
following feedback signalling is transmitted on the High Speed Dedicated Physical Control Channel
(HS-DPCCH) in the uplink:
• Channel Quality Information (CQI) to inform the scheduler about the instantaneous channel
condition.
• HARQ ACK/NACK information to let the sender know the outcome of the decoding process
and to request retransmissions.
Figure 2 depicts the data and signalling flow during HSDPA transmission. Based on the UE channel quality
report the Node B scheduler sends data on the shared downlink channel to the user. The UE will then
reply with an ACK or NACK message based on the outcome of the decoding. Note that the standard
does not specify scheduling and resource allocation which leaves significant freedom to Node-B implementations