Carrier aggregation: How AT&T will use Qualcomm's MediaFLO spectrum to double LTE speeds

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AT&T Mobility (NYSE:T) hopes to double the speeds of its planned LTE network by essentially gluing together the 700 MHz spectrum it is working to purchase from Qualcomm (NASDAQ:QCOM) with its existing AWS, 1900 MHz or 850 MHz spectrum holdings. The carrier plans to use carrier aggregation technology--which is a key element in the specifications for LTE Advanced, WiMAX 2 and future iterations of HSPA+--to pull the trick off, and if it is successful the action could have a major impact on the future design of wireless networks.

Kris Rinne, senior vice president of architecture and planning with AT&T Mobility, said during FierceWireless' Path to 4G event March 22 in Orlando, Fla., that the operator is "very interested" in trialing carrier aggregation technology in the 2012 timeframe, with a goal of deploying it in 2014.

Qualcomm's MediaFLO spectrum appears to be the cornerstone of that strategy. AT&T announced late last year a $1.93 billion agreement to purchase the 700 MHz spectrum Qualcomm had used for its now-defunct MediaFLO mobile TV effort. Though the transaction still requires FCC approval (and a number of smaller operators are urging the FCC to reject the transaction) AT&T hopes to supplement capacity on its planned LTE network by taking MediaFLO spectrum and pairing it with spectrum in the Advanced Wireless Services (AWS) 1.7/2.1 MHz, 1900 MHz or 850 MHz bands. Interference problems preclude the operator from bonding Qualcomm's spectrum with AT&T's lower 700 MHz B- and C-block spectrum.

ATT Kris Rinne"Carrier aggregation is in the next release (of the LTE standard) and gives us the opportunity to take disparate spectrum to get a 2 x 10 spectrum configuration even if you have a split like 700 MHz and 850 MHz," explained Rinne. "MediaFLO spectrum is one-way spectrum, and we would use that aggregation capability to add that and use it from an asymmetrical standpoint."

The benefits are huge. By bonding non-contiguous spectrum into a single, wider channel, operators can address the asymmetry of data flows between downlink and uplink channels. The data traffic that is growing the fastest is video, and it is asymmetrical traffic. As such, carrier aggregation helps operators efficiently manage these downlink flows.

AT&T pushing ahead

If AT&T scores access to Qualcomm's spectrum, and if its attempt at carrier aggregation is successful, the move could give AT&T the upper hand in the battle for the fastest data speeds quite early in the LTE game. (AT&T has said it plans to launch LTE this year, and will cover around 70 million people with LTE by year-end.) For example, if 10 MHz of the Qualcomm spectrum is added to a 10 MHz downlink on the LTE network, the network could experience slightly more than a two-fold increase in the capacity available, Rinne pointed out in an FCC filing that the carrier submitted before it announced plans to acquire T-Mobile USA for $39 billion.

"As a result, AT&T will be able to handle a far larger number of customers using mobile broadband services," Rinne wrote in the filing. "Also, user data speeds will increase since peak data rates increase linearly with the bandwidth of the carriers deployed. Thus, where downlink capacity is doubled from 10 MHz to 20 MHz, customers may experience a doubling of peak speed."

The most likely scenario for AT&T is to bond the MediaFLO spectrum with AWS spectrum in the 1.7/2.1 GHz bands. As part of its plan to buy T-Mobile for $39 billion, AT&T said it will migrate T-Mobile subscribers off T-Mobile's HSPA+ 1.7/2.1GHz band, and use T-Mobile's 1.7/2.1GHz spectrum as well as AT&T's own unused 1.7/2.1GHz spectrum to launch LTE service. Of course, AT&T also will use its 700 MHz spectrum for LTE.

However, if AT&T were to pair its AWS spectrum with Qualcomm's MediaFLO spectrum, the result wouldn't provide nationwide coverage. The license area is scattered throughout the country, and the set-up would only work in markets where AT&T owned sufficient quantities of both.

In the FCC filing, Rinne said AT&T plans to move aggressively to integrate MediaFLO spectrum into its LTE network if the FCC approves the transaction and after the relevant 3GPP standards are released. While the LTE Advanced standard is expected to be released at the end of this year, the performance requirement specs for each individual carrier supplement downlink scenario need to be developed by the 3GPP. After the standards are released, vendors would need to build the necessary equipment for the MediaFLO spectrum blocks. As such, AT&T estimates it could deploy handsets with carrier aggregation technology as early as 2014.

The HSPA+ question

Dan Warren, senior director of technology with the GSMA, said carrier aggregation is most advantageous for LTE operators because the technology can be deployed in spectrum swathes as small as 1.4 MHz all the way to 20 MHz. However, HSPA+ operators will run into more difficulty in using carrier aggregation technology, given the requirement that HSPA+ must run in 10 MHz swathes of spectrum. As such, HSPA+ operators may find it more difficult to refarm more spectrum to bond wider channels.

Today, T-Mobile USA is deploying dual-carrier HSPA+ technology that effectively doubles the speed of HSPA+ 21 networks to theoretical speeds of 42 Mbps. The higher data speeds come about because T-Mobile is taking two 5-megahertz HSPA+ downlink carriers--each supporting theoretical speeds of 21 Mbps--and bonding them together.

For those operators that can access additional spectrum, HSPA+ technology shows a path to 168 Mbps using a multi-carrier aggregation by bonding two 20-megahertz HSPA+ downlink carriers. Australia's Telstra is aiming to move to HSPA+ at 84 Mbps this year. And the 3GPP is considering a move beyond HSPA+ 168 by looking at data speeds of 336 Mbps, and there's even talk of 650 Mbps, said Chris Pearson, president of 4G Americas.

"Whether 3GPP moves beyond HSPA+ 168 depends on operators and vendors," Pearson said. He said T-Mobile and Nokia Siemens Networks have been the ones aiming to push data speeds beyond 168 Mbps. But given AT&T's acquisition of T-Mobile and its plans for rolling out LTE, it may not want to make the investment given the fact that 2x2 MIMO is required to reach the 84 Mbps and higher speeds.

Implications for Europe, Verizon

Carrier aggregation technology has worldwide implications. Earlier this year at the Mobile World Congress, NSN demonstrated carrier aggregation technology by bonding LTE channels in non-contiguous spectrum. Specifically, it combined spectrum at 800 MHz and 2.6 GHz--two bands the Europe is auctioning for 4G technologies--by using the same base station but with two radios tuned to the two different spectrum bands.

And what of Verizon Wireless (NYSE:VZ)? The carrier currently offers commercial LTE service in more than 40 markets, and promises downlink speeds of between 5 and 12 Mbps.

Verizon Wireless has enough 700 MHz spectrum nationwide to launch 10 MHz carriers, although it has more spectrum in some bigger markets. The operator can rely on the FCC to bring more spectrum to market or begin refarming its existing CDMA spectrum to gain more frequencies. Verizon has also said it will use its AWS spectrum holdings to provide additional LTE capacity in select markets in the future.

During FierceWireless' Path to 4G Event, David Small, chief technology officer of Verizon Wireless, said the company is comfortable with its spectrum position. "There are a number of different things we can do to conserve the air link and make backhaul more efficient. We are looking at a number of things," he said. "We have the ability to offload the EV-DO network onto LTE if we need to."

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