High-Speed Downlink Packet Access

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High-Speed Downlink Packet Access (HSDPA) is a 3G (third generation) mobile telephony communications protocol in the High-Speed Packet Access (HSPA) family, which allows networks based on Universal Mobile Telecommunications System (UMTS) to have higher data transfer speeds and capacity. Current HSDPA deployments support down-link speeds of 1.8, 3.6, 7.2 and 14.4 Mbit/s. Further speed increases are available with HSPA+, which provides speeds of up to 42 Mbit/s downlink.[1]

Contents

[edit] Technology

The High-Speed Downlink Shared Channel (HS-DSCH) lacks two basic features of other W-CDMA channels — variable spreading factor and fast power control. Instead, it delivers the improved downlink performance using adaptive modulation and coding (AMC), fast packet scheduling at the base station, and fast retransmissions from the base station, known as hybrid automatic repeat-request (HARQ).

[edit] Hybrid automatic repeat-request (HARQ)

HARQ uses incremental redundancy, where user data is transmitted multiple times using different codings. When a corrupted packet is received, the user device saves it and later combines it with the retransmissions, to recover the error-free packet as efficiently as possible. Even if the retransmitted packets are corrupted, their combination can yield an error-free packet.

[edit] Fast packet scheduling

The HS-DSCH downlink channel is shared between users using channel-dependent scheduling to make the best use of available radio conditions. Each user device periodically transmits an indication of the downlink signal quality, as often as 500 times per second. Using this information from all devices, the base station decides which users will be sent data on the next 2 ms frame and how much data should be sent for each user. More data can be sent to users which report high downlink signal quality.

The amount of the channelisation code tree, and thus network bandwidth, allocated to HSDPA users is determined by the network. The allocation is "semi-static" in that it can be modified while the network is operating, but not on a frame-by-frame basis. This allocation represents a trade-off between bandwidth allocated for HSDPA users, versus that for voice and non-HSDPA data users. The allocation is in units of channelisation codes for Spreading Factor 16, of which 16 exist and up to 15 can be allocated to HSDPA. When the base station decides which users will receive data on the next frame, it also decides which channelisation codes will be used for each user. This information is sent to the user devices over one or more HSDPA "scheduling channels"; these channels are not part of the HSDPA allocation previously mentioned, but are allocated separately. Thus, for a given 2 ms frame, data may be sent to a number of users simultaneously, using different channelisation codes. The maximum number of users to receive data on a given 2 ms frame is determined by the number of allocated channelisation codes. By contrast, in CDMA2000 1xEV-DO, data is sent to only one user at a time.

[edit] Adaptive modulation and coding

The modulation scheme and coding is changed on a per-user basis depending on signal quality and cell usage. The initial scheme is Quadrature phase-shift keying (QPSK), but in good radio conditions 16QAM modulation almost doubles data throughput rates. With 5 Code allocation, QPSK typically offers up to 1.8 Mbit/s peak data rates, while 16QAM up to 3.6. Additional codes (e.g. 10, 15) can also be used to improve these data rates or extend the network capacity throughput significantly. Theoretically, HSDPA can give throughput up to 14.4 Mbit/s.

[edit] Other improvements

HSDPA is part of the UMTS standards since release 5, which also accompanies an improvement on the uplink providing a new bearer of 384 kbit/s. The previous maximum bearer was 128 kbit/s.

As well as improving data rates, HSDPA also reduces latency and so the round trip time for applications.

Along with the HS-DSCH channel, three new physical channels are also introduced: HS-SCCH, HS-DPCCH and HS-PDSCH. The High Speed-Shared Control Channel (HS-SCCH) informs the user that data will be sent on the HS-DSCH 2 slots ahead. The Uplink High Speed-Dedicated Physical Control Channel (HS-DPCCH) carries acknowledgment information and current channel quality indicator (CQI) of the user. This value is then used by the base station to calculate how much data to send to the user devices on the next transmission. The High Speed-Physical Downlink Shared Channel (HS-PDSCH) is the channel mapped to the above HS-DSCH transport channel that carries actual user data.

[edit] HSDPA UE categories

HSDPA comprises various versions with different data speeds.

Category Max. number of
HS-DSCH codes		 Modulation Max. data rate
[Mbit/s]
1 5 QPSK and 16-QAM 1.2
2 5 QPSK and 16-QAM 1.2
3 5 QPSK and 16-QAM 1.8
4 5 QPSK and 16-QAM 1.8
5 5 QPSK and 16-QAM 3.6
6 5 QPSK and 16-QAM 3.6
7 10 QPSK and 16-QAM 7.3
8 10 QPSK and 16-QAM 7.3
9 15 QPSK and 16-QAM 10.2
10 15 QPSK and 16-QAM 14.4
11 5 QPSK only 0.9
12 5 QPSK only 1.8

[edit] Roadmap

The first phase of HSDPA has been specified in the 3rd Generation Partnership Project (3GPP) release 5. Phase one introduces new basic functions and is aimed to achieve peak data rates of 14.4 Mbit/s (see above). Newly introduced are the High Speed Downlink Shared Channels (HS-DSCH), the adaptive modulation QPSK and 16QAM and the High Speed Medium Access protocol (MAC-hs) in base station.

The second phase of HSDPA is specified in the upcoming 3GPP release 7 and has been named HSPA Evolved. It can achieve data rates of up to 42 Mbit/s.[1] It will introduce antenna array technologies such as beamforming and Multiple-input multiple-output communications (MIMO). Beam forming focuses the transmitted power of an antenna in a beam towards the user’s direction. MIMO uses multiple antennas at the sending and receiving side. Deployments are scheduled to begin in the second half of 2008.

After HSDPA the roadmap leads to HSOPA, a technology under development for specification in 3GPP Release 8. This project is called the Long Term Evolution initiative. The first release of LTE offers data rates of over 320 Mbit/s for downlink and over 170 Mbit/s for uplink using OFDMA modulation.

For details, see [1]

[edit] Adoption

See also: List of Deployed HSDPA networks (not updated)

As of May 25, 2007, 102 HSDPA networks have commercially launched mobile broadband services in 55 countries. Nearly 40 HSDPA networks support 3.6 Mbit/s peak downlink data throughput. A growing number are delivering 7.2 Mbit/s peak data downlink, leveraging new higher-speed devices coming into the market. One network has been declared as “14.4 Mbit/s (peak) ready” and several others will have this capability by end 2007. The first commercial HSUPA uplink network is launched, with several more set to follow in 2007.

This protocol is a relatively simple upgrade where UMTS is already deployed.[1]

CDMA-EVDO networks had the early lead on performance, and Japanese providers were highly successful benchmarks for it. But lately this seems to be changing in favour of HSDPA as an increasing number of providers worldwide are adopting it. In Australia, Telstra announced that its CDMA-EVDO network would be replaced with a HSDPA network (since named NextG), offering high speed internet, mobile television and traditional telephony and video calling. Rogers Wireless deployed HSDPA system 850/1900 in Canada on April 1, 2007. Singapore is currently the only country boasting nationwide HSDPA.MobileOne (M1) HSDPA Network, Singapore. Retrieved on 2008-05-24.

So far, 171 device models from 47 suppliers have been launched, comprising: 53 handsets, 35 notebooks, 30 datacards, 19 wireless routers, 15 modems, 11 embedded module, 2 wireless modules, 1 wireless residential gateway, 1 media player, 1 camera, 1 GPS handset, 1 convergence platform & 1 baseband processor. For details, see gsmworld.com/HSPA.

[edit] Marketing as mobile broadband

During 2007, an increasing number of telcos worldwide began selling HSDPA USB modems as mobile broadband connections. In addition, the popularity of HSDPA landline replacement boxes grew — providing HSDPA for data via Ethernet and WiFi, and ports for connecting traditional landline telephones. Marketed with connection speeds of "up to 7.2 Mbit/s",[2] which is only attained under ideal conditions. As a result these services can be slower than expected, especially when in fringe coverage indoors. However, signal strength can be greatly improved by using commercial solutions that can attach 3G external antennas.[3]

[edit] See also

[edit] References

  1. ^ a b c d HSPA mobile broadband today
  2. ^ http://online.vodafone.co.uk/dispatch/Portal/appmanager/vodafone/wrp?_nfpb=true&_pageLabel=template12&pageID=PPP_0026 Vodafone UK 7.2MBs service
  3. ^ http://www.poyntingdirect.co.za/ProductInfo.aspx?productid=ADPT-024 and http://www.poynting-europe.com/
  • Martin Sauter: Communication Systems for the Mobile Information Society, John Wiley, September 2006, ISBN 0-470-02676-6

[edit] External links


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