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High Speed Packet Access

Look, another entry in the endless, Sisyphean march of wireless network technologies. You seem to have stumbled upon High Speed Packet Access, or HSPA. Try to keep up.

• Part of a series on the Wireless network technologies Analog

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Mobile telecommunications •

• v • t • e

High Speed Packet Access (HSPA) isn't one thing; it’s a forced marriage of two mobile protocols: High Speed Downlink Packet Access (HSDPA) and High Speed Uplink Packet Access (HSUPA). Its entire reason for being was to drag existing 3G mobile networks, kicking and screaming, into an era where they were slightly less disappointing. It operated on the WCDMA protocols, essentially giving them a shot of adrenaline.

Of course, because progress is a relentless grind, an even shinier version called Evolved High Speed Packet Access (or HSPA+, for those who appreciate the illusion of forward momentum) was standardized by the 3GPP late in 2008. The world, ever eager for the next best thing, began adopting it around 2010. This newer standard made grand promises of bit rates rocketing as high as 337 Mbit/s on the downlink and a comparatively sluggish 34 Mbit/s on the uplink. Let’s be clear: these are theoretical speeds, the kind you achieve in a hermetically sealed lab with a tailwind and a prayer. In the real world, populated by things like walls, distance, and other people, these speeds were, and are, a fantasy.

Overview

The initial HSPA specifications were tasked with hoisting peak data rates to a then-impressive 14 Mbit/s for downloads and 5.76 Mbit/s for uploads. Beyond just raw speed, the protocols were designed to reduce latency—that awkward pause between when you click something and when the universe decides to respond. The result was a network that could, on a good day, provide up to five times more system capacity on the downlink and double the capacity on the uplink when compared to the original, creaking WCDMA protocol. It was a necessary, if not entirely revolutionary, patch on a system straining under humanity's burgeoning need to watch videos of cats on the bus.

High Speed Downlink Packet Access (HSDPA)

High Speed Downlink Packet Access (HSDPA) is an enhanced 3G (third-generation) mobile communications protocol nestled within the HSPA family. Marketers, in a fit of uninspired creativity, branded it as 3.5G or 3G+, as if adding a decimal point could fundamentally alter reality. Its purpose was to permit networks built on the Universal Mobile Telecommunications System (UMTS) to handle higher data speeds and greater capacity. A significant side effect of HSDPA was the reduction in latency, which in turn shortens the round-trip time for applications, making the internet feel slightly less like shouting into a canyon and waiting for the echo.

HSDPA made its debut in 3GPP Release 5. It arrived with an improved uplink that offered a new bearer of 384 kbit/s, a welcome upgrade from the previous maximum of a paltry 128 kbit/s. Naturally, the cycle continued. Evolved High Speed Packet Access (HSPA+), introduced in 3GPP Release 7, pushed the data rates even higher by throwing more complex technology at the problem: 64QAM modulation, MIMO (using multiple antennas to send and receive more data at once), and Dual-Carrier HSDPA operation. By the time 3GPP Release 11 came around, the theoretical maximum speeds had ballooned to an almost unbelievable 337.5 Mbit/s.

The initial phase of HSDPA, as specified in 3GPP Release 5, was focused on establishing new fundamental functions to hit that 14.0 Mbit/s peak data rate while significantly cutting latency. The improvements in speed and responsiveness were meant to lower the cost per bit for operators and bolster support for high-performance packet data applications—a polite way of saying "streaming video and larger downloads." HSDPA is built upon the concept of shared channel transmission. Its key architectural features include shared channel and multi-code transmission, the use of higher-order modulation, a short [Transmission Time Interval](/Transmission Time Interval) (TTI) of 2ms, fast link adaptation and scheduling, and a fast hybrid automatic repeat request (HARQ) system. The protocol also introduced new channels, namely the High Speed Downlink Shared Channels (HS-DSCH), and utilized quadrature phase-shift keying and 16-quadrature amplitude modulation. At the heart of it all, in the base stations, was the High Speed Medium Access protocol (MAC-hs).

For network operators, the upgrade to HSDPA was often a relatively painless software update for their existing WCDMA networks, which is likely why it saw such widespread adoption. It's always easier to tweak the code than to build new towers. In a functioning HSDPA network, voice calls are typically prioritized over data transfer, a quaint reminder of a time when phones were primarily for talking to people.

User equipment categories

The following table, meticulously derived from table 5.1a of the 3GPP TS 25.306 Release 11 specification, outlines the maximum data rates for different classes of devices. It details the specific combination of features required to achieve these theoretical peaks. The per-cell, per-stream data rate is constrained by two factors: 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 a fixed 2 milliseconds.

This is why, for instance, a Category 10 device can decode 27,952 bits every 2 ms, which calculates to 13.976 Mbit/s, not the 14.4 Mbit/s that marketing departments so often and so incorrectly claimed. Some categories (1-4 and 11) have inter-TTI intervals of 2 or 3, which acts as a multiplier that reduces the maximum data rate accordingly. Technologies like Dual-Cell and MIMO 2x2 each double the maximum data rate, not by magic, but by transmitting multiple independent transport blocks over different carriers or spatial streams. The data rates presented here have been rounded to one decimal place, a small mercy.

HSDPA User Equipment (UE) categories

Category Release Max. number of HS-DSCH codes (per cell) Modulation [note 1] MIMO, Multi-Cell Code rate at max. data rate [note 2] Max. downlink speed (Mbit/s) [note 3]
1 5 5 16-QAM .76 1.2
2 5 5 16-QAM .76 1.2
3 5 5 16-QAM .76 1.8
4 5 5 16-QAM .76 1.8
5 5 5 16-QAM .76 3.6
6 5 5 16-QAM .76 3.6
7 5 10 16-QAM .75 7.2
8 5 10 16-QAM .76 7.2
9 5 15 16-QAM .70 10.1
10 5 15 16-QAM .97 14.0
11 5 5 QPSK .76 0.9
12 5 5 QPSK .76 1.8

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

Notes:

  1. ^ 16-QAM implies that QPSK is also supported. 64-QAM implies that both 16-QAM and QPSK are supported. It's a hierarchy of complexity.
  2. ^ The maximal code rate is not strictly limited. A value approaching 1 in this column is a red flag indicating that the maximum data rate is achievable only under the most pristine, laboratory-grade conditions. To demonstrate these speeds, the device is practically wired directly to the transmitter.
  3. ^ The maximum data rates listed are physical layer data rates. The speed you actually experience, the application layer data rate, is about 85% of that figure. The other 15% is consumed by the necessary evil of IP headers and other overhead information.

Adoption

By August 28, 2009, the world was dotted with 250 commercially launched HSDPA networks offering mobile broadband services across 109 countries. Of these, 169 networks supported a peak downlink of 3.6 Mbit/s, while a growing, ambitious handful were delivering a peak of 21 Mbit/s. [citation needed]

For a time, CDMA2000-EVDO networks held an early lead in performance, with providers in Japan serving as particularly successful benchmarks for that standard. However, the tide turned decisively in favor of HSDPA as an overwhelming number of providers worldwide chose it as their upgrade path.

In 2007, a cottage industry emerged around HSDPA USB modems, with telcos globally pushing these dongles to provide mobile broadband connections. The popularity of HSDPA landline replacement boxes also surged. These devices captured the HSDPA signal and distributed it via Ethernet and Wi-Fi, often including ports to connect traditional landline telephones. They were frequently marketed with alluring speeds of "up to 7.2 Mbit/s" under ideal conditions. The reality, however, was often much slower, especially when used in fringe coverage areas or indoors, where radio waves go to die.

High Speed Uplink Packet Access (HSUPA)

High-Speed Uplink Packet Access (HSUPA) is the other half of the HSPA equation, a 3G mobile telephony protocol focused on the often-neglected uplink. Standardized in 3GPP Release 6, its mandate was to improve the uplink data rate to 5.76 Mbit/s, expand capacity, and reduce latency. Paired with other enhancements, this made new applications feasible, such as Voice over Internet Protocol (VoIP), uploading high-resolution pictures without aging a decade, and sending large email attachments.

HSUPA represented the second significant step in the UMTS evolutionary timeline. It has since been rendered largely obsolete by technologies with far higher transfer rates, such as LTE (offering 150 Mbit/s for downlink and 50 Mbit/s for uplink) and LTE Advanced (with maximum downlink rates exceeding a staggering 1 Gbit/s).

Technology

HSUPA introduced a new transport channel to WCDMA, the Enhanced Dedicated Channel (E-DCH). It mirrored many of the improvements seen in HSDPA, including multi-code transmission and a shorter transmission time interval that enabled faster link adaptation. It also incorporated fast scheduling and a fast hybrid automatic repeat request (HARQ) with incremental redundancy, which made retransmissions of lost data packets more efficient.

Like its downlink counterpart, HSUPA employs a "packet scheduler." However, it functions on a "request-grant" principle. The user equipment (UE) must first request permission to send data, and the scheduler at the base station decides when and how many devices will be granted that permission. A transmission request includes information about the state of the UE's transmission buffer, its queue, and its available power margin. A critical distinction from HSDPA, however, is that uplink transmissions are not orthogonal to each other, creating potential for interference.

Beyond this "scheduled" mode, the standards also permit a self-initiated transmission mode from the UEs, known as "non-scheduled." This mode is particularly useful for services like VoIP, where even the reduced TTI and the Node B based scheduler can't provide the consistently short delay and constant bandwidth required.

Each MAC-d flow (a QoS flow) is configured to use either scheduled or non-scheduled mode. The UE independently adjusts the data rate for each type of flow. The maximum data rate for a non-scheduled flow is set during call setup and is not frequently altered. The power consumed by the scheduled flows is dynamically managed by the Node B through absolute grant messages (which set an actual value) and relative grant messages (a simple up/down bit for fine-tuning).

At the physical layer, HSUPA brought forth several 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) – This carries the control information associated with the E-DCH Transport Channel.
  • E-DPDCH (E-DCH Dedicated Physical Data Channel) – This carries the actual data of the E-DCH Transport Channel.

User equipment categories

The following table illustrates the uplink speeds for the various categories of HSUPA. Notice the numbers are considerably less glamorous than their downlink equivalents.

HSUPA User Equipment (UE) categories

HSUPA
Category		 Release Max.
Uplink
Speed
(Mbit/s)		 Modulation
1 6 0.73 QPSK
2 6 1.46 QPSK
3 6 1.46 QPSK
4 6 2.93 QPSK
5 6 2.00 QPSK
6 6 5.76 QPSK

Additional UE categories were defined from 3GGP Release 7 onwards as part of Evolved HSPA (HSPA+) and can be found in Evolved HSUPA UE Categories.

Evolved High Speed Packet Access (HSPA+)

Main article: Evolved High Speed Packet Access

Evolved HSPA, also known by the aliases HSPA Evolution and HSPA+, is a wireless broadband standard defined in 3GPP release 7 of the WCDMA specification. It's not a revolution, but an extension of the existing HSPA definitions, ensuring it is backward compatible all the way to the original Release 99 WCDMA networks. Evolved HSPA delivers data rates between 42.2 and 56 Mbit/s in the downlink and 22 Mbit/s in the uplink (per 5 MHz carrier) by leveraging multiple input, multiple output (2x2 MIMO) technologies and higher order modulation (64 QAM). By using Dual Cell technology, these figures could theoretically be doubled.

Since 2011, HSPA+ has been widely deployed by WCDMA operators, with nearly 200 commitments documented. It served as a critical, final enhancement to the 3G infrastructure before the industry's wholesale shift to 4G LTE.

See also