High Speed Packet Access HSPA
Published on Nov 23, 2015
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 (HS-DSCH).
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 necessary.
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 Physical Layer Operation Procedure
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 HS-DSCH.
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.
1) Holma, H., Toskala, A. WCDMA for UMTS. Radio access for third generation mobile communications. West Sussex: John Wiley & Sons, 2004.
2) Juha Karhonen, Introduction to 3G Mobile Communications, Artech House, 2003
4) EDGE, HSPA, LTE: Broadband Innovation, September 2008, 3G Americas, RYSAVY Research
5) David Maidment, Understanding HSDPA's Implementation Challenges, picoChip Designs, 2005 http://www.eetimes.com/design/embedded-internet-design/4009356/Understanding- HSDPA-s-Implementation-Challenges
6) Eiko Seidel, Standartization updates on HSPA Evolution, Nomor Research GmbH, Munich, Germany, 2009
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