Introduction:
HSDPA is a major Release 5 feature. Some of the enhancements will be done in
Release 6. HSDPA is designed to support data rates upto 10.8 Mbps and at the
same time it will co-exist with R99 in the same frequency band of 5MHz. As
could be observed this data rate is much higher than the Broadband speeds
people generally use in their homes (512Kbps download). HSDPA will be able to
satisfy the most demanding Multimedia applications.
The peak rate offered by HSDPA is
about 10Mbps in 5MHz channel. One important thing to note is that the more
important thing here is not the peak rate but the throughput capacity which
increases significantly. This leads to more users being able to use high data
rates on a single carrier.
Whats New in HSDPA:
3 Slot TTI (2ms)
CQI: Channel Quality Indication
AMC: Adaptive Modulation and
Coding
Constellation Re-Arranging (16
QAM)
H-ARQ: Hybrid Automatic Repeat
Request
N-channel SAW: N-Channel Stop and
Wait Protocol
New MAC (High Speed) instance
New Stuff for HSDPA:
The implementation of HSDPA will require changes mainly in Physical layer.
Layer 2 will be affected as well because new high speed MAC entity will be required.
The impact to other layers will be minimum. The following sections will throw
some light on some of the new features required for HSDPA.
HSDPA Physical and Transport
Channels:
To support HSDPA new Physical channels have been defined:
HS-PDSCH or High Speed Physical Downlink Shared Channel: This is a downlink channel which is both time and
code multiplexed. The channelization codes have a fixed spreading factor, SF
= 16. Multi-code transmissions are allowed that translates to UE being
assigned multiple channelization codes in the same TTI, depending on the UE
capability. The same scrambling code sequence is applied to all the channelization
codes that form the single HS-DSCH CCTrCH. If there are multiple UE's then
they may be assigned channelisation codes in the same TTI (multiplexing of
multiple UE's in the code domain).
HS-DPCCH or High Speed Dedicated Physical Control Channel: This
is an uplink channel that carries the Acknowledgements of the packet received
on HS-PDSCH and also the CQI (Channel Quality Indication). THE CQI estimates
have to be transmitted by the UE every 2.0 ms frame. This information is very
important as it ensures reliability and impacts power capacity.
HS-SCCH or High Speed Shared Control CHannel:
The HS-SCCH is a
fixed rate (60 kbps, SF=128) downlink physical channel used to carry downlink
signaling related to HS-DSCH transmission. This provides timing and coding
information thus allowing the UE to listen to the HS-DSCH at the correct time
and using the correct codes to allow successful decoding of UE data.
The main features of the physical
channel are as follows:
Fixed Spreading Factor of 16 for
HS-DSCH
QPSK and 16 QAM Modulation
Static TTI Length of 3 Time Slots
= 2ms
Fixed CRC of 24 bits
Error Correction using 1/3 Turbo
Coding
To support HSDPA the following new Transport channels have been defined:
HS-DSCH or High Speed Downlink
Shared channel:
The High
Speed Downlink Shared Channel is a downlink transport channel shared by
several UEs. The HS-DSCH is associated with one downlink DPCH, and one or
several Shared Control Channels (HS-SCCH). The HS-DSCH is transmitted over
the entire cell or over only part of the cell using e.g. beam-forming
antennas.
Adaptive Modulation and Coding
(AMC):
HSPDA standard ensures that
highest possible data rate is acheived for all users regardless of whether
they are close to the base station or far off. This is done using ACM. For
HS-DSCH, the transport format, including the modulation scheme and code rate,
can be selected based on the downlink channel quality. The selection of
transport format is done by the MAC-HS located in Node B and is based on
channel quality feedback reported by the UE. The spreading factor cannot
change but the coding rate can change between 1/4 and 3/4. The higher coding
rate reduces the number of errors. Also the standards support multicodes.
This means that upto 15 codes can be allocated to a
UE.
Hybrid Automatic Repeat Request
(HARQ):
In case of ARQ, the receiving
system on receipt of data checks the CRC. If the CRC is the same as that
received in the message ACK is sent back to the sender. In case if CRC does
not match then NACK is sent back and the packet discarded. In case of HARQ,
this method of CRC checking is improved based on the following two things.
Chase Combining: In this when an error is detected in CRC, NACK is sent back
but the packet is not discarded. It is stored. In case the re-transmitted
packet is again erroneous then the previous and current packet is combined in
an attempt to recover from errors. Each time the packet is resent, the same
scheme is applied. Eventually the error will be either resolved or maximum
number of retries is reached. In that case higher layer protocols will deal
with the error.
Incremental Redundancy (IR):IR is similar to Chase combining but the redundant
information that was not transmitted earlier is also included to improve the
chances of reception without errors or with enough errors removed so as to
allow combining with the previously stored packet and resolve the errors.
Fast Cell Site Selection (FCSS):
When the UE moves between the
cells, it is possible that it would be served by different cells. Hence the
UE will construct a list of Active Set (the term Active Set is incorrect and
the term that will be used eventually is "Eligible Set") Cells that
it can use at any one time. The mobile will indicate on HS-DPCCH as to which
one is the best cell for DL transmission. The serving cell then decides the
modulation and coding scheme to be used for the mobile and in addition may
code multiplex multiple mobiles within that HSDPA frame. To Simplify this
procedure, it is further subdivided into Intra-Node B FCS and Inter-NodeB
FCS.
REFERENCES:
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