The bloom is off the 3G rose. With several major carriers having outlined their 3G migration plans, it's no longer noteworthy who has announced, but instead, who hasn't. How quickly we have been spoiled by the promise of more bandwidth and more intriguing applications. GSM carriers and vendors know it, too. Having already talked up and talked out UMTS, they are now hyping up the next big thing: HSDPA.

High-speed downlink packet access technology (HSDPA), initially standardized almost six years ago, is now a resident in more carrier labs than bad coffee and polyester. Last month, Cingular Wireless and Lucent Technologies announced the completion of successful HSDPA data calls on the UMTS trial network they have deployed in Atlanta. A few weeks earlier, U.K. mobile carrier O2 and its vendor, Nortel Networks, finished up similar tests.

Both carrier/vendor teams said they saw sustained data rate of more than 3 Mb/s, though as HSDPA launches commercially over the next few years, users can expect average data rates of between 400 kb/s and 800 kb/s. Still, that will provide plenty of bandwidth for applications such as mobile video that are just beginning to stoke the imaginations of carriers.

“When the network gets loaded, it will be an experience very much like DSL,” said John Leonard, vice president responsible for mobility and offer management at Lucent Technologies. “Cingular is targeting a user experience of about 400 kb/s to 700 kb/s.”

However, the trial calls that carriers and vendors have been conducting have been intended to gauge more than just bandwidth.

“With the Cingular tests, the main things we were able to prove were the performance of the technology and the capability of the platform to be reliable,” Leonard said.

One of the platform advantages Lucent was able to demonstrate was a measurable improvement over past versions of the HSDPA standard in roundtrip delay — the total amount of time from a request for content from the handset to the delivery of that content.

“Release 5 of the standard [the HSDPA standard approved by the 3rd Generation Partnership Project in 1999] calls for a minimum roundtrip delay of 120 milliseconds, but in the Cingular tests, we were able to demonstrate it at 80 milliseconds,” Leonard said. “It was important to show that in a real, live environment.”

“We've been able to demonstrate the stability of the technology,” added Alan Pritchard, vice president of marketing for UMTS and GSM at Nortel. In addition to typical bandwidth performance and connectivity tests, Pritchard said the tests with O2 also included a phase in which noise intentionally was brought into the carrier channel to gauge the effect it had on a data transmission being able to keep some of its integrity.

“We injected noise into the radio path to show what performance looks like as the radio path deteriorates,” Pritchard said. “This is something that carriers need to know as they launch HSDPA and especially as they working with new kinds of applications.”

Among other high-profile tests of HSDPA, Ericsson conducted live test calls at last fall's Telecom Asia conference in Shanghai, China. Also, at Nortel's worldwide user group conference, the company had an HSDPA link running live throughout the show and demonstrated the technology's facility with video by running one of the Harry Potter movies over the radio link.

The high-profile test call successes recently announced by these companies are not just an effort to pump their own reputations, they say. The current deployment of CDMA 1X EV-DO technology by Verizon Wireless, Sprint and other CDMA carriers is causing GSM-based operators to take a serious look at a technology with greater bandwidth capabilities than EV-DO. While EV-DO is capable of data rates up to about 2.4 Mb/s, many companies working with HSDPA technology believe that it can be applied in a commercial environment at up to 10 Mb/s — and possibly higher.

HSDPA, which wasn't much talked about in the years since its standardization — while carriers focused on incremental network upgrades — came into the spotlight during the 3GSM World Congress one year ago this month, and also weeks later at the CTIA's Wireless 2004 trade show. At these two events, vendors were coming off of their own lab work and were able to demonstrate HSDPA to live audiences.

“We've come a long way in the last year since those demonstrations,” said Leonard. “We've gone beyond prototypes and initial requirements to producing hardware and software enabled with HSDPA technology.”

Technically speaking, HSDPA derives its performance capabilities from a series of downlink transmission enhancements. These include adaptive rate modulation, a fast scheduling capability and fast retransmission, an innovation that encourages better overall service quality by creating redundancy.

“A HSDPA upgrade is similar to how the EDGE upgrades have worked,” Merji Khorchidian, head of technology at Nokia Networks. “You're enhancing the air interface, but you are still using the same 5 MHz per spectrum. You are using the same spectrum, but more efficiently, through rate modulation and packet transmission scheduling.”

Adaptive rate modulation encourages an environment where service quality remains consistent regardless of how far away users are from the nearest base station. The coding rate used depends on the quality of each specific radio link. Low coding would be used on strong radio links — users closer to the base station — and higher coding would be used on weaker links.

Fast scheduling is a process that is centralized in the base station, and again, varies based on the quality of the radio link between the base station and the user handset, as well as several other factors. It's made possible by using short frame lengths very close to the air interface.

Fast retransmission allows the mobile handset in an HSDPA network environment to request retransmission in a situation where link errors and other kinds of interference have occurred. This happens very quickly because the handset requests the retransmission from the base station, rather than the radio network controller, which is more typical in UMTS networks. Also, the retransmission is assembled by using the packets from the previous transmission that were unaffected by the interference, thereby avoiding the likelihood that similar errors will occur during the retransmission.

Another enhancement inherent to HSDPA is the development of a shared data channel, which is better for the transmission of bursty traffic than the individual dedicated data channels in UMTS networks (see figure). This shared data channel has several individual dedicated channels, or users, multiplexed onto it by time, allowing for the more efficient use of the radio network infrastructure.

“HSDPA increases the number of users you can put on the network, and that is one of the key things that goes beyond the data rate,” Lucent's Leonard said. “Overall, spectrum is one of the most important assets a carrier can have, but it is a limited resource. What HSDPA allows is a better use of that, and the ability provides users with a more consistent experience.”

HSDPA is indeed a technology of potentially dramatic bandwidth capabilities and efficiencies. However, for a variety of reasons, its real potential will be achieved on a commercial basis over a somewhat gradual period of time. The HSDPA standard, as written, is intended to allow a theoretical peak bandwidth performance of about 14.4 Mb/s (see figure on page 30). The first of the standard being implemented commercially will be capable of network bandwidth of just more than 11 Mb/s. However, the mobile handsets being prepared for first-generation HSDPA launches are capable of realizing about 3.6 Mb/s, or roughly a little more than 25% of the theoretical peak bandwidth. This will be the state of the technology as carriers such as Cingular and O2 are launching their first HSDPA services later this year and next year.

The main reason for the bandwidth limitation is that Type 3 mobile handsets, the first generation to carry HSDPA technology, are capable of nothing more than 3.6 Mb/s. The second phase of the standard, to be commercially implemented in 2006 and later, will come closer to the maximum rate of about 14.4 Mb/s.

“The limitation will not be on the network side,” Leonard said. “It will be on the handset side.” Yet, Leonard also admitted, “That peak rate (14.4 Mb/s) doesn't have any applications yet.”

And, it is still important to remember that in the first phase of HSDPA's commercial life, most users in a loaded network will neither need nor have bandwidth beyond about 700 kb/s or 800 kb/s.

“We tend to focus on the average user rates,” said Nokia's Khorchidian. “That's what you want to know if you are looking at this from a carrier business model perspective.”

Still, HSDPA is primed for a culture increasingly hooked on having broadband access available wherever possible. In an age when many people are installing wireless LANs in their homes, corporations are making indoor wireless coverage a key part of their IT plans and visitors to coffee houses, burger joints, shopping malls and parks can expect wireless coverage, a 3.5G technology like HSDPA fits right in. It also can help fill in the gaps.

“A lot of people have DSL and a wireless LAN at home now,” Lucent's Leonard said. “But, they'll want ubiquity, and research shows they'll be willing to pay for it.”

In some cases, mobile broadband might become the less-costly alternative to other broadband options. Even in hotels, where Wi-Fi coverage is increasingly prevalent, but often fee-based, using an HSDPA-enhanced mobile phone could be much cheaper.

But, such enormous amounts of bandwidth also have serious implications for the rest of the public network that lays beyond the link between the base station and the handset. For instance, rapidly increasing transmission rate capabilities will increase the pressure for carriers to introduce ever-better backhaul technologies, said Nortel's Pritchard.

“Backhaul starts to get pretty scary with this level of bandwidth, and backhaul is really part of the overall service experience, so carriers will really have to look at doing backhaul upgrades,” he said.

Aside from creating a need for more state-of-the-art handsets and backhaul solutions, the evolution to HSDPA won't be overly traumatic. In most cases, it will be undertaken as rapid follow-up to carriers' UMTS upgrade — that's how Cingular plans to approach it next year, after it has introduced UMTS in several markets. It was previously thought that spectrum shortages might burden the transition to UMTS and HSDPA, but both technologies will use the same spectrum, and carrier consolidation is ensuring that network operators like Cingular, which acquired AT&T Wireless last year, have all of the spectrum they could possibly need.

For carriers such as Cingular and O2, both of which will have already deployed UMTS base stations by the time they are ready to begin their HSDPA upgrades, the move to HSDPA will happen through an installation of new software. Though this sounds like a simple enough process, it also requires all of the testing and certification that carriers normally do before new network and upgrade launches.

The downlink improvements introduced through HSDPA will come at a time when our concept of wireless applications is evolving rapidly. 3G is not just about mobile Internet access anymore but also about a variety of other content types, including video. However, the newest version of the standard will extend more bandwidth to the uplink as well

The uplink enhancements will be part of UMTS Release 6, which is currently under development at 3GPP. Submissions to the standard working group have been frozen, and vendors said they expect the standard to be finalized by the end of this year. The uplink technology goes by various names, depending on whom you ask. It is most often called, understandably high-speed uplink packet access (HSUPA), but also has been referred to as enhanced uplink data channel (EUDCH) and enhanced uplink packet access (EUPA).

“Everything's progressing nicely,” said Leonard. “They just haven't been able to agree on the acronym yet.”

HSUPA, or whatever it ends up being called, offers great promise for applications such as video sharing and other types of peer-to-peer services. The upstream bandwidth capability with basic UMTS technology is about 200 kb/s, but with the emergence of HSUPA, the uplink channel bandwidth will grow 4 Mb/s. That makes video calling and mobile video conferences a mainstream reality, as well as sharing of video and music clips, online games and other types of content.

“With HSUPA, you've really got a full 3G technology, with bandwidth on the uplink side that compares what you have on the downlink and allows you to do just about an application you can think of,” said Nortel's Pritchard.

Ultimately, air interface technologies like HSDPA and HSUPA need to be put in a broader network perspective. While the radio access network is evolving to new generations of technologies, the core of the network is doing the same. The move to IP Multimedia Subsystems (IMS) in network cores is likely to speed the transition to HSDPA and HSUPA and create a network environment in which the full potential of broadband applications can be realized across many more markets of mobile users. As Lucent's Leonard said, IMS makes it easier to converge technologies, so that solutions such as UMTS, HSDPA and HSUPA, which like EV-DO, have been seen as potentially too expensive for the consumer market, can be brought to many more people.

“We have always looked at 3G as something of a natural technology for the enterprise market, which is where 3G bandwidth can really be applied most immediately, and the cost of it validated by business users,” Leonard said. “With HSDPA, you've got a lot of network efficiencies and things that will bring down the cost and make it possible to apply to a broader market. This is how we can bring 3G to consumers.”