Design Articles

Not Just A Mathematical Symbol: A Deeper Look At The "+" Of HSPA+

The "+" brings the always-connected experience to par with a desktop web-browsing experience and enables a whole new world of personalized user features.

By Ziad Asghar, Operator and Strategy Manager, Chief Technology Office, Wireless Business Unit, Texas Instruments

We’re entering a new realm of mobile possibilities, driven by advanced semiconductor solutions and innovations across the market. The mobile user experience is reaching exciting heights today, and capabilities will only increase as features and data transfer rates advance. Users are no longer tethered to their desks -the handset brings consumer electronics functionality and data management to the palm of their hands, and inspires connections without boundaries.

Significant changes made recently to the 3rd Generation Partnership Project's (3GPP) high-speed packet access (HSPA) in the form of HSPA+ standard promise to drive HSPA's capabilities to a new level to support this mobile evolution. Handset updates and air interface enhancements work together to bring the "+" to fruition and greatly improve the user experience.

While the "+" brings many exciting advancements to the table, what exactly does it mean for handset requirements and overall cost? Additional antennas, multiple receive and transmit channels in the digital baseband (DBB), enhanced handset memory and more complex communication algorithms are just some requirements needed to drive HSPA+ to its full potential. Other features of the standard drastically alter handset cost, size and power consumption as well.

Let's take a look at the salient features of HSPA+ and inspect what the "+" in HSPA+ truly means for handsets and the overarching mobile experience.

figure 1
Figure 1: HSPA+ technologies enable faster data rates that bring browsing experiences to life on mobile handsets.


Foundational Enhancements: Speedier Data Rates, Greater Spectral Efficiency, More Efficient Power Use

PC-based DSL connections typically provide peak data rate transfers of six Mbps in the downlink and less than one Mbps on the uplink, and previous versions of HSPA provide peak rates of 14.4 Mbps in the downlink and 5.76 Mbps in the uplink. HSPA+, however, allows data rates of up to 43.2 Mbps on the downlink and 11.5 Mbps on the uplink. The "+" brings the web browsing experience to life on handsets through these dramatically increased data rates. It enables speedier multimedia transfers, higher-resolution video streaming and multi-user gaming, among other options.

More efficient power consumption with the "+" also improves talk and standby time, and reduces interference to increase network capacity. Features like continuous packet connectivity (CPC) and enhanced cell forward access channel (FACH) make this possible and help HSPA+ drive longer standby and talk time.

CPC reduces the control channel overhead, which is especially important in cases where a small amount of data is transferred periodically, such as in the case of mobile Web browsing.
CPC employs discontinuous transmission and reception (DTX/DRX), which also reduces the interference that handsets experience in the downlink and uplink.  Though DTX and DRX are present in previous versions of HSPA, advancements in HSPA+ enable handsets to enter power-saving modes more quickly and frequently, instead of requiring more time in full awake mode before entering DRX/DTX.

Another version of CPC, high-speed shared control channel (HS-SCCH)-less transmission, does not require HS-SCCH transmission for the first hybrid auto request query (HARQ) transmission. HS-SCCH channel carries control information for HSDPA, introduced in Release 5 in the HSPA standards family. This feature, particularly important with smaller average packet sizes, uses blind detection of the data format to lessen the downlink signaling requirements. This reduces the amount of control data received and transmitted, increases network capacity and extends battery life. Such enhancements make voice-over-Internet protocol (VoIP)-like applications more feasible, since the handset can switch to a low-power state during the gaps between data packets.

Enhanced CELL_FACH is a feature that enables HSPA capability in the uplink and downlink while the handset is in the CELL_FACH state. This feature allows users to get packet access without any interruptions, which leads to a significantly improved user experience, faster data session establishment and speedier downloads.  Since the total download time is often dominated by the session setup time, reducing this component leads to a much better user experience. 

Multicast/broadcast single frequency network (MBSFN) was also introduced as a part of HSPA+ advancements. MBSFN, a more enhanced version of multimedia broadcast multicast service (MBMS) introduced in previous iterations of HSPA, improves cell edge performance. MBSFN requires multiple base stations to transmit time synchronized data using the same scrambling code to provide significant signal-to-noise ratio (SNR) improvements, better spectral efficiency and higher data rates at the handset. 

Together, these improvements in data rates, power consumption and spectral efficiency drive an “always connected” experience that brings faster, richer consumer electronics features to the palm of your hand (Figure 1).

More Substantial Advancements: MIMO And HOM

The majority of the changes discussed thus far address the DBB in the handset, enhance control and timing requirements, and add additional silicon area to the phone design. Given the progress in fabrication technology - where newer technology nodes are available every 18 months or so - these features provide significant advantages with a relatively small increase in cost, and are hence more likely to be implemented earlier. There are additional features, however, that take more time to evolve and impact handset costs more substantially.

One such feature that has a larger, more long-term impact on handset costs is multiple-input multiple-output (MIMO). MIMO (Figure 2) encompasses three different flavors:

  • Spatial multiplexing: In spatial multiplexing, each antenna carries different data to increase throughput, either through a single user or multiple users. Spatial multiplexing requires extremely efficient channel conditions.
  • Transmit diversity: Transmit diversity is the scenario where the same data is sent through each antenna stream to improve SNR at the receiver by combining the diverse streams.
  • Beamforming: Beamforming is the case where the antenna pattern is changed to optimize communication to the intended receiver.

MIMO requires channel rank to be transmitted back to the base station on the uplink to determine the number of streams the channel can support at any given time along with precoding control Information (PCI) since 3GPP implements MIMO with precoding. This adds to the signaling overhead.

figure 2
Figure 2: MIMO Configurations

While MIMO introduces significantly higher data rates and other “pluses,” there are challenges to successful implementation. For operators, MIMO means better spectral efficiency and support for new user applications, which drives data revenue. However, manufacturers must subsidize handsets at a higher level because of cost increases associated with upgrades. Since MIMO requires multiple antennas, multiple receive and transmit chains and more complex algorithms in the DBB, the handset bill of material (BOM) increases a fair amount. In addition, antennas must be well spaced with the spacing inversely proportional to the frequency for each MIMO channel to be uncorrelated to gain maximum benefit from this technique, which can be difficult for the form factor of a handset.

In addition to MIMO technology, higher order modulation (HOM) support is included in HSPA+ by including 64 quadrature amplitude modulation (QAM) in the downlink and 16-QAM in the uplink. Peak achievable rates heighten as technologies move to higher-order modulations, since each transmitted symbol translates to a greater number of bits.  For example, moving from 16-QAM to 64-QAM on the downlink means a single symbol translates into six (64-QAM) bits instead of four bits (16-QAM). This adds two additional bits, showing that we can transmit 50 percent more data using the same bandwidth, which allows for much more spectral efficiency.  Similarly, for a single receive and transmit antenna or single-input single-output (SISO) case with 64-QAM modulation, a peak rate of 21.6 Mbps can be achieved; but, introducing MIMO with 64-QAM doubles that peak rate to 43.2 Mbps.

Parallel to MIMO advantages, this increase in data rates with HOM support does not come without effort. HOM requires better channel conditions and tighter requirements for handsets' radio frequency (RF) sections. As technology moves to higher order modulations, it packs the signal constellation with more and more symbols, enhancing the need for advanced RF components that accurately receive and interpret symbols.

The challenges posed by MIMO and HOM are being met by advancements in RF technology and greater integration in the form of system-on-chip (SOC) solutions, which pack all major components of a phone into a single chip.  These two important factors bring forth the benefits of MIMO and HOM, and help keep handset costs low.

In addition to the features mentioned above, 3GPP is also considering the integration of more advanced, "type-3i" receivers that support interference cancellation.  In the past 3GPP specified rake-based (type-1) receivers, equalizer-based receivers (type-2) and equalizer based receivers that support receive diversity (type-3). When properly implemented, the addition of type-3i receivers increases the potential to provide significant cell-edge performance improvement.

Exciting Opportunities Ahead

As industry experts solve data rate challenges and the always-connected experience continues to unfold, applications support unique mobile advancements across the globe in all sectors, including business, education, government, medical and personal. Mobile devices will allow for remote patient monitoring and record keeping. They will enable long distance learning and expand the classroom boundaries beyond villages and homes. With GPS and cameras, mobile phones will become the center of the social networking universe, the key to location-based services and the real-time tool for user-generated media.

The possibilities are both limitless and exciting, and driven greatly by the offerings that HSPA+ adds to the handset. The "+" brings the always-connected experience to par with a desktop web-browsing experience and enables a whole new world of personalized user features. With faster data rates, proficient power features and better spectral efficiency, this is more than just a mathematical symbol. The "+" of HSPA+ is the addition that makes handsets more ubiquitous, functionality-packed, personal and indispensable than ever before.

Texas Instruments Inc.
Dallas, TX
(800) 336-5236

This article first appeared in the October/November issue of Portable Design. Reprinted with permission.

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