High Speed Packet Access

HSPA sign shown in notification bar on an Android-based (Samsung) smartphone.

High Speed Packet Access (HSPA)[1] is an amalgamation of two mobile protocols, High Speed Downlink Packet Access (HSDPA) and High Speed Uplink Packet Access (HSUPA), that extends and improves the performance of existing 3G mobile telecommunication networks utilizing the WCDMA protocols. A further improved 3GPP standard, Evolved High Speed Packet Access (also known as HSPA+), was released late in 2008 with subsequent worldwide adoption beginning in 2010. The newer standard allows bit-rates to reach as high as 337 Mbit/s in the downlink and 34 Mbit/s in the uplink. However, these speeds are rarely achieved in practice.[2]

Overview

The first HSPA specifications supported increased peak data rates of up to 14 Mbit/s in the downlink and 5.76 Mbit/s in the uplink. It also reduced latency and provided up to five times more system capacity in the downlink and up to twice as much system capacity in the uplink compared with original WCDMA protocol.

High Speed Downlink Packet Access (HSDPA)

High Speed Downlink Packet Access (HSDPA) is an enhanced 3G (third-generation) mobile 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 speeds and capacity. HSDPA has been introduced with 3GPP 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.HSPA+ introduced in 3GPP Release 7 further increases data rates by adding 64QAM modulation, MIMO and Dual-Cell HSDPA operation, i.e. two 5 MHz carriers are used simultaneously. Even higher speeds of up to 337.5 Mbit/s are possible with Release 11 of the 3GPP standards.[3]

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 with significantly reduced latency. The improvement in speed and latency reduces the cost per bit and enhances support for high-performance packet data applications. HSDPA is based on shared channel transmission and its key features are shared channel and multi-code transmission, higher-order modulation, short transmission time interval (TTI), fast link adaptation and scheduling along with fast hybrid automatic repeat request (HARQ). Further new features 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 upgrade to HSDPA is often just a software update for WCDMA networks. In general voice calls are usually prioritized over data transfer.

Technology

High Speed Downlink Shared Channel

For HSDPA, a new transport layer channel, High-Speed Downlink Shared Channel (HS-DSCH), has been added to 3GPP 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 to which the above HS-DSCH transport channel is mapped 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 in 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 the HS-DSCH. When the base station decides which users will receive data in the next frame, it also decides which channelisation codes will be used for each user. This information is sent to the user on one of up to 4 HS-SCCHs, which are not part of the HS-DSCH 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.

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.

User Equipment (UE) categories

The following table is derived from table 5.1a of the release 11 of 3GPP TS 25.306[4] 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 2x2 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.

Further UE categories were defined from 3GGP Release 7 onwards as Evolved HSPA (HSPA+) and are listed in Evolved HSDPA UE Categories.

Notes:
  1. ↑ 16-QAM implies QPSK support, 64-QAM implies 16-QAM and QPSK support.
  2. ↑ 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.
  3. ↑ 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.

Adoption

GPRS-speed in a HSDPA plan

As of 28 August 2009, 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.

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.

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",[5] which is only attained under ideal conditions. As a result, these services can be slower than expected, when in fringe coverage indoors.

High Speed Uplink Packet Access (HSUPA)

High-Speed Uplink Packet Access (HSUPA) is a 3G mobile telephony protocol in the HSPA family. This technology was the second major step in the UMTS evolution process. It was specified and standardized in 3GPP Release 6 to improve the uplink data rate to 5.76 Mbit/s, extending the capacity, and reducing latency. Together with additional improvements which are detaileld below this creates opportunities for a number of new applications including VoIP, uploading pictures and sending large e-mail messages.

In the meanwhile HSUPA has been superseeded by newer technologies further advancing transfer rates. LTE provides up to 300 Mbit/s for downlink and 75 Mbit/s for uplink. Its evolution LTE Advanced supports maximum downlink rates of over 1 Gbit/s.

Technology

Enhanced Uplink adds a new transport channel to WCDMA, called the Enhanced Dedicated Channel (E-DCH). Further it features several improvements similar to those of HSDPA, including multi-code transmission, shorter Transmission Time Interval (TTI) enabling faster link adaptation, fast scheduling and fast Hybrid Automatic Repeat Request (HARQ) with incremental redundancy making retransmissions more effective. Similarly to HSDPA, HSUPA uses a packet scheduler, but it operates on a request-grant principle where the UEs request a permission to send data and the scheduler decides when and how many UEs will be allowed to do so. A request for transmission contains data about the state of the transmission buffer and the queue at the UE and its available power margin. However, unlike HSDPA, uplink transmissions are not orthogonal to each other.

In addition to this scheduled mode of transmission the standards also allows a self-initiated transmission mode from the UEs, denoted non-scheduled. The non-scheduled mode can, for example, be used for VoIP services for which even the reduced TTI and the Node B based scheduler will not be able to provide the very short delay time and constant bandwidth required.

Each MAC-d flow (i.e. QoS flow) is configured to use either scheduled or non-scheduled modes; the UE adjusts the data rate for scheduled and non-scheduled flows independently. The maximum data rate of each non-scheduled flow is configured at call setup, and typically not changed frequently. The power used by the scheduled flows is controlled dynamically by the Node B through absolute grant (consisting of an actual value) and relative grant (consisting of a single up/down bit) messages.

At the Physical Layer, HSUPA introduces new channels E-AGCH (Absolute Grant Channel), E-RGCH (Relative Grant Channel), F-DPCH (Fractional-DPCH), E-HICH (E-DCH Hybrid ARQ Indicator Channel), E-DPCCH (E-DCH Dedicated Physical Control Channel) and E-DPDCH (E-DCH Dedicated Physical Data Channel).

E-DPDCH is used to carry the E-DCH Transport Channel; and E-DPCCH is used to carry the control information associated with the E-DCH.

User Equipment (UE) Categories

The following table shows uplink speeds for the different categories of HSUPA.

Further UE categories were defined from 3GGP Release 7 onwards as Evolved HSPA (HSPA+) and are listed in Evolved HSUPA UE Categories.

Evolved High Speed Packet Access (HSPA+)

Main article: Evolved HSPA

Evolved HSPA (also known as HSPA Evolution, HSPA+) is a wireless broadband standard defined in 3GPP release 7 of the WCDMA specification. It provides extensions to the existing HSPA definitions and is therefore backward-compatible all the way to the original Release 99 WCDMA network releases. Evolved HSPA provides data rates up to 168 Mbit/s in the downlink and 22 Mbit/s in the uplink (per 5 MHz carrier) with multiple input, multiple output (2x2 MIMO) technologies and higher order modulation (64 QAM). With Dual Cell technology, these can be doubled.

Since 2011, HSPA+ has been very widely deployed amongst WCDMA operators with nearly 200 commitments.[6]

See also

Wikimedia Commons has media related to High-Speed Downlink Packet Access.

References

Bibliography

External links

Wikimedia Commons has media related to High Speed Packet Access.
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