The High Speed Packet Access technology is the most widely used
mobile broadband technology in communication world. It was already built
in more than 3.8 billion connection with GSM family of technologies.
The HSPA technology is referred to both High Speed Downlink Packet
Access (3GPP Release 5) and to High Speed Uplink Packet Access (3GPP
Release 6). The Evolved HSPA technology or HSPA + is the evolution of
HSPA that extends operator’s investments before the next generation’s
technology 3GPP Long Term Evolution (LTE or 3GPP Release 8). The HSPA
technology is implemented on third generation (3G) UMTS/WCDMA network
and accepted as the leader in mobile data communication.
Using the HSDPA optimization on downlink is performed, whereas the
HSUPA technology applying Enhanced Dedicated Channel (E-DCH) sets some
improvements for the uplink performance optimization. The products that
support HSUPA became available in 2007 and the combination of both HSDPA
and HSUPA were called HSPA. Adopting these technologies the throughput,
latency and spectral efficiency were improved. Introducing HSPA
resulted to the increase of overall throughput approximately to 85 % on
the uplink and a rise more than 50 % in user throughput. The HSPA
downlink available rates are 1 to 4 Mbps and for the uplink are 500 kbps
to 2Mbps as of 1 quarter of 2009. The theoretical bit rates are 14Mbps
at the downlink and 5.8 Mbps at the uplink in a 5MHz channel. Besides,
the latency is notably reduced as well. In the improved network, the
latency is less than 50ms, and after the introduction of 2ms
Transmission Time Interval (TTI) latency is expected to be just 30ms.
High Speed Downlink Packet Access:
The main idea of HSDPA concept is a growth of packet access
throughput with methods known from Global System for Mobile
Communication (GSM)/ Enhanced Data Rates for Global Evolution (EDGE)
standards, involving link adaptation and fast physical layers (L1)
retransmission combining. The demand of arriving to possible memory
requirements and bringing control for link adaptation closer to the air
interface brought forward the High Speed Downlink Shared Channel
The functioning of HSDPA is done in a way that after calculating
the quality of every HSDPA user based for example on power control,
ACK/NACK ratio, and HSDPA specific user feedback at the Node-B, then
scheduling and link adaptation are immediately conducted depending on
the active scheduling algorithm and user prioritization scheme. Using
HSDPA the fundamental features of WCDMA like variable spreading factor
(SF) and fast power control are switched off and replaced by adaptive
modulation and coding (AMC), extensive multicode operation and a fast
and spectrally efficient retransmission strategy.
The power control dynamics in downlink is 20 dB, and at the uplink
it is 70 dB. Due to intra-cell interference (interference between users
on parallel code channels) and Node-B implementation some limitation
are appeared for the downlink dynamics. Consequently, a near to Node-B
user’s power is unable to be reduced maximally by the power control.
Moreover, the reduced power beyond 20 dB influences a little on the
capacity. With HSDPA, this property is handled by the link adaptation
function and AMC to choose a coding and modulation combination that
demands higher Ec/Io, which is available to the user near to Node-B.
This leads to the increase of customer throughput. Utilizing
simultaneously up to 15 multicodes in parallel, a large dynamic range of
the HSDPA link adaptation and maintenance of a good spectral efficiency
are enabled. Using more robust coding, fast Hybrid Automatic Repeat
Request (HARQ) and multicode operation makes the variable SF no more
In order to profit from the short term variations, the scheduling
decisions are performed in the Node-B, so the capacity allocations for
one user are done in a short time, in a friendly conditions. The
physical layer packet combining is that the terminal accumulates the
received data packets in soft memory and in the case of decoding
failure, the new transmission is combined with the old one before
channel decoding. The retransmission can be the same as the first
transmission or can be with different bits relatively to the channel
encoder output received during the last transmission. With addition
incremental strategy, a diversity gain and improving decoding efficiency
can be achieved.
The steps of the physical layer operation of the HSDPA:
The scheduler in the Node B estimates the conditions of the channel, the pended
data in the buffer, the expired time since the last session of the user and so on.
After defining TTI for the terminal, the HS-DSCH parameters are assigned.
In order to inform the terminal of the necessary parameters, the HS-SCCH two slots
are transmitted by the Node-B before the corresponding HS-DSCH TTI.
The given HS-SCCHs are monitored and after the decoding of the
Part1 from an HSSCCHdetermined for that terminal, the rest of the
HS-SCCH is decoded and terminalwill buffer the necessary codes from the
As soon as the HS-SCCH parameters are decoded from Part 2, the terminal can
define to which ARQ process the data belongs and the whether it is required the
combine of the data and that already in the soft buffer.
After the potentially combined data is decoded, the terminal sends ACK/NACK
indicator in the uplink direction.
If the transmission is performed in the same TTI the same HS-SCCH is used.
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5) David Maidment, Understanding HSDPA’s Implementation Challenges, picoChip Designs,
6) Eiko Seidel, Standartization updates on HSPA Evolution, Nomor Research GmbH, Munich,