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Examining AT&Tís 3G-to-4G upgrade path

By picking its incumbent 3G vendors to build LTE, AT&T can take advantage of their upgradable equipment, but AT&Tís radio network head explains why turning a 3G base station into a 4G one isnít always the best option

When AT&T (NYSE:T) named Alcatel-Lucent (NYSE:ALU) and Ericsson (NASDAQ:ERIC) its 4G radio access vendors on Wednesday, it cited their incumbency in the 3G network as a critical advantage in deploying long-term evolution (LTE). The 3G base stations that Alcatel-Lucent and Ericsson deploy today can be upgraded to support LTE in the future, giving AT&T flexibility and even a capex advantage in its future 4G rollout. But just how upgradable is AT&Tís network?

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According to Gordon Mansfield, director of AT&Tís radio access domain, AT&T wonít be able to merely flip a switch in 2011 and have an instant LTE networkóthere is still extensive network deployment necessary to build the LTE network. But AT&T does have an advantage, Mansfield said, in its ability to re-use numerous elements of its high-speed packet access (HSPA) networks that will make its LTE deployment more seamless and less expensive than an operator starting from scratch.

ďThe biggest thing is that the equipment we buy today is upgradable in some way to LTE,Ē Mansfield said. ďIf I put LTE in at a different band I obviously need new radios, but the bottom line is on our base station equipment there are a lot of things that are completely re-usable.Ē

By implementing Alcatel-Lucent and Ericssonís next-generation software-defined radio equipment AT&Tís base stations are moving from centralized to distributed architecture. The easiest way to understand what that means is to think of previous generations as hardwired to a particular technology, supporting a specific radio standard at specific widths at specific frequencies. With a distributed architecture, the base station is no longer a purpose-built GSM or HSPA box, but a collection of generic baseband and radio resources, which can then be allocated to any technology or combination of technologies.

AT&T plans to build its LTE network at 700 MHz, and since its HSPA network uses the PCS and cellular bands it will need to install new radios for the lower frequency at each cell site. But AT&T doesnít necessarily have to install a new base station. Ericsson RBS 6000 and Alcatel-Lucentís Multi-Standard base station lines can allocate a portion of their baseband processing capabilities to an LTE carrier, while reserving the rest for HSPA.

ďIn general, when you talk about a distributed architecture youíre talking in terms of raw radio resources,Ē Mansfield said. ďAs long as we donít exceed what the baseband processor can support we can use any configuration of technologies.Ē

Under such a scenario, it would appear that AT&T is basically building its LTE network today. As part of its new 4G contract with Ericsson and ALU, all HSPA base stations deployed this year and onward will be able to support both 3G and LTE. But Mansfield said it would be a mistake to assume AT&T will simply slap some 700 MHz radios on its existing 3G cellsites in 2011 and have an instant LTE network. Just because all of its new base stations can be rescrambled for LTE, doesnít mean AT&T will use them that way. AT&T isnít turning off HSPA and replacing it with LTE. It will be using its HSPA capacity ahead of and concurrent with the LTE deployment, and in many cases HSPA will have spoken for all of the baseband capacity of many of its sites, Mansfield said.

ďIf I already have three carriers of HSPA on a base station, itís probably not a good candidate for an upgrade,Ē Mansfield said. He pointed out that LTE will have channel sizes as large as 10 MHz, double that of an HSPA carrier, meaning there would have to be a lot of extraneous baseband capacity in the box to support LTE, though he added AT&T might opt to install smaller 5 MHz LTE channels on a case by case basis. In urban environments, AT&T will likely need to deploy another baseband module as well as new radios alongside its HSPA kitóin effect install new base stations.

Supporting simultaneous LTE and HSPA makes the most sense in rural areas. Outside of the cities, AT&T has a lot of 3G sites that are only running one HSPA carrier, leaving plenty of untapped resources for LTE, Mansfield said. In those areas though, AT&T has to ensure that its backhaul connections are up to par as the overall capacity of the site will jump dramatically, from a maximum of 7.2 Mb/s to well over 50 Mb/s. In fact, itís those backhaul upgrades -- from T1s to fiber Ethernet connectionsóthat are the most concrete of AT&Tís 4G preparations today. Mansfield said the LTE and HSPA networks, whether running off the same base station or separately, will definitely share the same backhaul connections, meaning any upgrade AT&T makes to fiber backhaul today is one that will be unnecessary next year.

Thereís one other instance in which those upgrade capabilities could be highly useful to AT&T. As AT&T moves more of its data and voice traffic onto the LTE network (AT&T has aggressive plans for VoIP over 4G), it may start shutting down 3G and 2G channels in its most congested markets in order to take advantage of the greater spectral efficiencies of LTE. Since GSM and HSPA run at the cellular frequencies, AT&T wouldnít even have to install new radios to convert those carriers to LTE. If Alcatel-Lucent and Ericssonís claims about remote radio programming are true, it might literally require only a flip of the switch to turn those 2G/3G base stations into LTE sites.

The big question is how much of AT&Tís network is new enough to take advantage of these 4G migration capabilities. As part of their contract, all new ALU and Ericsson 3G installations must be LTE upgrade capable, but Mansfield said that doesnít mean anything deployed before 2010 is hard-wired for 3G. Mansfield wouldnít give any specifics about what percentage of its footprint was upgradable, saying AT&T was still doing a complete inventory of the different generations of base stations in its network. But he did say AT&T does have much more flexibility than the 2009 cut-off date would imply.

Ericssonís RBS 6000 is relatively new. It started going into networks in mid-2009. That likely means that most of the 3G network Ericsson built over AT&Tís PCS frequencies canít be upgraded, but more recent installations at the cellular frequencies are eligible. Alcatel-Lucent is a different story. It started its SDR program back in the 1990s and has been touting the software upgrade capabilities of its base stations for years. Alcatel-Lucent maintains that almost every GSM and UMTS base station itís deployed in the last 10 yearsósome 700,000 unitsócan support some kind of upgrade, though Alcatel-Lucentís vice president overseeing the AT&T radio contract Tim Krause acknowledged that some upgrades are easier than others. Alcatel-Lucent has gone through several generations of base stations, so older networks would require more site work and new equipment to make the move to LTE, while newer equipment such as its just released multi-carrier technology is completely software configurable.

If AT&T wanted to take the entire Alcatel-Lucent-built HSPA network and convert it into an LTE network itís possible, Krause said, but it would definitely require some work, and ultimately itís not what AT&T plans to do. AT&T wants to re-use what it can from its 3G deployment for 4G, but it isnít looking to replace one with the other, Krause said.

ďWhat AT&T is doing today is building a 3G network in such a way that when theyíre ready to pull the trigger on 4G a lot of the network is already built,Ē Krause said.

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© 2014 Penton Media Inc.

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