High-Speed Downlink Packet Access

High-Speed Downlink Packet Access (HSDPA) is an enhanced 3G (third generation) mobile telephony communications protocol in the High-Speed Packet Access (HSPA) family, also dubbed 3.5G, 3G+ or turbo 3G, 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 14.0 Megabit/s. Further speed increases are available with HSPA+, which provides speeds of up to 42 Mbit/s downlink and 84 Mbit/s with Release 9 of the 3GPP standards.^[1]

Technology

HS-DSCH

For HSDPA, a new transport layer channel, High-Speed Downlink Shared Channel (HS-DSCH), has been added to UMTS release 5 and further specification. It is implemented by introducing three new physical layer channels: 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.

Hybrid automatic repeat-request (HARQ)

Data is transmitted together with error correction bits. Minor errors can thus be corrected without retransmission; see forward error correction.

If retransmission is needed, the user device saves the packet and later combines it with retransmitted packet 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. Retransmitted packet may be either identical (chase combining) or different from the first transmission (incremental redundancy).

Since HARQ retransmissions are processed at the physical layer, their 12 ms round-trip time is much lower compared to higher layer retransmissions.

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 continually 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 "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.

Adaptive modulation and coding

The modulation scheme and coding are 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 and 64QAM can significantly increase data throughput rates. With 5 Code allocation, QPSK typically offers up to 1.8 Mbit/s peak data rates, while 16QAM offers up to 3.6 Mbit/s. Additional codes (e.g. 10, 15) can also be used to improve these data rates or extend the network capacity throughput significantly.

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 decreases latency and so the round trip time for applications.

In later 3GPP specification releases HSPA+ increases data rates further by adding 64QAM modulation, MIMO and Dual-Cell HSDPA operation, i.e. two 5 MHz carriers are used simultaneously.

User Equipment (UE) categories

HSDPA comprises various versions with different data speeds. In 2009 the most common devices are category 6 (3.6 Mbit/s) and category 8 (7.2 Mbit/s) with retail prices around 60 euros without subscription.

The following table is derived from table 5.1a of the release 9 version of 3GPP TS 25.306^[2] and shows maximum data rates of different device classes and by what combination of features they are achieved. The per-cell per-stream data rate is limited by the Maximum number of bits of an HS-DSCH transport block received within an HS-DSCH TTI and the Minimum inter-TTI interval. The TTI is 2 ms. So for example Cat 10 can decode 27952 bits/2 ms = 13.976 MBit/s (and not 14.4 MBit/s as often claimed incorrectly). Categories 1-4 and 11 have inter-TTI intervals of 2 or 3, which reduces the maximum data rate by that factor. Dual-Cell and MIMO each multiply the maximum data rate by 2, because multiple independent transport blocks are transmitted over different carriers or spatial streams, respectively. The data rates given in the table are rounded to one decimal point.

3GPP Release	Category	Max. number of HS-DSCH codes	Modulation[3]	MIMO, Dual-Cell	Code rate at max. data rate[4]	Max. data rate [Mbit/s][5]
Release 5	1	5	16-QAM		.76	1.2
Release 5	2	5	16-QAM		.76	1.2
Release 5	3	5	16-QAM		.76	1.8
Release 5	4	5	16-QAM		.76	1.8
Release 5	5	5	16-QAM		.76	3.6
Release 5	6	5	16-QAM		.76	3.6
Release 5	7	10	16-QAM		.75	7.2
Release 5	8	10	16-QAM		.76	7.2
Release 5	9	15	16-QAM		.70	10.1
Release 5	10	15	16-QAM		.97	14.0
Release 5	11	5	QPSK		.76	0.9
Release 5	12	5	QPSK		.76	1.8
Release 7	13	15	64-QAM		.82	17.6
Release 7	14	15	64-QAM		.98	21.1
Release 7	15	15	16-QAM	MIMO	.81	23.4
Release 7	16	15	16-QAM	MIMO	.97	28.0
Release 7	19	15	64-QAM	MIMO	.82	35.3
Release 7	20	15	64-QAM	MIMO	.98	42.2
Release 8	21	15	16-QAM	Dual-Cell	.81	23.4
Release 8	22	15	16-QAM	Dual-Cell	.97	28.0
Release 8	23	15	64-QAM	Dual-Cell	.82	35.3
Release 8	24	15	64-QAM	Dual-Cell	.98	42.2
Release 9	25	15	16-QAM	Dual-Cell + MIMO	.81	46.7
Release 9	26	15	16-QAM	Dual-Cell + MIMO	.97	55.9
Release 9	27	15	64-QAM	Dual-Cell + MIMO	.82	70.6
Release 9	28	15	64-QAM	Dual-Cell + MIMO	.98	84.4

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 14.0 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 3GPP release 7 and has been named HSPA Evolved. It can achieve data rates of up to 42 Mbit/s.^[1] It introduces 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.

Further releases of the standard have introduced dual carrier operation, i.e. the simultaneous use of two 5 MHz carrier. By combining this with MIMO transmission, peak data rates of 84 Mbit/s can be reached under ideal signal conditions.

After HSPA Evolved, the roadmap leads to E-UTRA (Previously "HSOPA"), the technology specified 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.^[1]

Adoption

As of 28 August 2009 (2009 -08-28)^[update], 250 HSDPA networks have commercially launched mobile broadband services in 109 countries. 169 HSDPA networks support 3.6 Mbit/s peak downlink data throughput. A growing number are delivering 21 Mbit/s peak data downlink and 28 Mbit/s. Several others will have this capability by end 2009 and the first 42 Mbit/s network came online in Australia in February 2010. Telstra switches on 42 Mbit/s Next G, plans 84 Mbit/s through the implementation of HSPA+ Dual Carrier plus MIMO technology upgrade in 2011.^[6] This protocol is a relatively simple upgrade where UMTS is already deployed.^[1] First week in May 2010, Second-ranked Indonesian cellular operator Indosat launched the first DC-HSPA+ 42 Mbit/s network, beating Australia's Telstra, Singapore's StarHub and Hong Kong's CSL to stake its claim as the first operator in Asia-Pacific to offer theoretical download speeds of 42 Mbit/s via HSPA+.^[7][8]

CDMA2000-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. In July 2008, Bell Canada and Telus announced a joint plan to expand their current shared EVDO/CDMA network to include HSDPA.^[9] Bell Canada launched their joint network November 4, 2009, while Telus launched November 5, 2009.^[10] In January 2010, T-Mobile USA adopted HSDPA.^[11]

Marketing as mobile broadband

During 2007, an increasing number of telcos worldwide began selling HSDPA USB modems to provide 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. Some are marketed with connection speeds of "up to 7.2 Mbit/s",^[12] which is only attained under ideal conditions. As a result these services can be slower than expected, when in fringe coverage indoors.

See also

• 3GPP Long Term Evolution
• Cellular router
• High-Speed Uplink Packet Access
• High-Speed OFDM Packet Access
• List of device bandwidths
• List of HSDPA networks
• Multi-band
• UMTS
• UMTS frequency bands

References

1. ^ ^{a b c d} HSPA mobile broadband today
2. ^ 3GPP TS 25.306 v9.0.0 http://www.3gpp.org/ftp/Specs/html-info/25306.htm
3. ^ 16-QAM implies QPSK support, 64-QAM implies 16-QAM and QPSK support.
4. ^ The maximal code rate is not limited. A value close to 1 in this column indicates that the maximum data rate can be achieved only in ideal conditions. The device is therefore connected directly to the transmitter to demonstrate these data rates.
5. ^ The maximum data rates given in the table are physical layer data rates. Application layer data rate is approximately 85% of that, due to the inclusion of IP headers (overhead information) etc.
6. ^ Telstra switches on 42 Mbit/s Next G, plans 84 Mbit/s upgrade in 2011 | Comms Day http://www.commsday.com/commsday/?p=789
7. ^ Indosat first in Asia to launch 42 Mbit/s HSPA+ http://www.telecomasia.net/print/17244
8. ^ Indosat gears up for 4G and launches Asia's fastest network - Ericsson http://www.ericsson.com/news/142992
9. ^ "Telus, Bell Announce Switch from CDMA to HSDPA". http://www.iphonealley.com/news/telus-bell-announce-switch-from-cdma-to-hsdpa. 
10. ^ Marlow, Iain (3 November 2009). "Bell, Telus launch high-speed networks". Toronto Star. http://www.thestar.com/business/article/720096--bell-races-to-launch-high-speed-network-one-day-ahead-of-telus. 
11. ^ http://www.pcworld.com/businesscenter/article/185916/tmobile_usa_finishes_upgrade_to_hspa_72.html
12. ^ Vodafone UK 7.2 MBs service

Further reading

• Sauter, Martin (2006). Communication Systems for the Mobile Information Society. Chichester: John Wiley. ISBN 0470026766. 

External links

• GSM Association on HSPA
• Understand HSDPA's implementation challenges