FCC Band Plan Forum (Part 2): Duplex Gaps, Asymmetric Bands, Global Harmonization

At a forum earlier this week in Washington, D.C., the Federal Communications Commission (FCC) gathered experts from mobile device manufacturers, wireless carriers and industry organizations to discuss different options regarding band-plan designs for upcoming auctions of wireless spectrum. While the options vary, it seems likely that spectrum will be made available in smaller blocks than previously thought.

Large Duplex Gaps?

Historically, using Frequency-Division Duplexing (FDD) technology, the FCC found large blocks of spectrum, and split them between uplinks and downlinks, with a small duplex gap between. For Time-Division Duplexing (TDD) technology, the uplinks and downlinks are in the same band, but with guard bands at either end.

“The days of finding 100-to-200 contiguous megahertz of spectrum are probably behind us,” said Chris Helzer, electronics engineer at the FCC’s Wireless Telecommunications Bureau. “We’re more likely to find 30 megahertz here, 10 megahertz here and 50 megahertz there.”

One band-plan idea is to increase the size of the duplex gap. “How far can we separate the uplink and downlink? Can we take any two chunks we find and make them a pair? What are the limitations?” asked Helzer.

Increasing the duplex gap could cause challenges. Bill Aberth, CTO at Motorola Mobility, noted that large gaps in uplink/downlink frequency result in large differences in uplink/downlink effective cell sizes. He suggested the FCC consider TDD allocations instead of pairing widely disparate frequencies.

Asymmetric FDD

Another LTE trend for which the FCC must plan is that mobile broadband is dominated by downlink traffic. When one looks at the data imbalance, downlink traffic is “six to 13 times” greater than uplink traffic, said Al Jette, head of the North American Industry Environment for Nokia Siemens Network. Exceptions are special events like the Super Bowl, where video uploads can increase uplink traffic.

“For new bands, allocating more downlink than uplink spectrum should be considered,” posited Jette. Added Tom Sawanobori, vice president/Technology at Verizon, “Paired and supplemental downlink options should be made available.”

A related issue is that as spectrum comes online, it might not be available nationwide. “In the downlink, it’s critical to clear it nationwide,” explained Helzer, because devices need to work across the country. In the uplink, however, the band might not need to go coast to coast:  “Base stations generally don’t move. They would only need to support the regional band.”

Does Size Matter?

Participants in the FCC’s forum also discussed possible sizes of new spectrum blocks. Using multiples of 5-megahertz blocks is the prevailing wisdom because standards support that. Other sizes theoretically are possible, explained Peter Gaal, principal engineer at Qualcomm, but performance requirements haven’t been defined, making it difficult to build devices or components.

Doug Hyslop, representing the Rural Cellular Association (RCA), noted that 5 megahertz x 5-megahertz blocks would be beneficial to RCA’s members, and this was corroborated by Dan Wilson, principal engineer for T-Mobile, who said, “We’d like to see a minimum of 5×5 paired blocks; 5×5 and 10×10, less than that wouldn’t make sense.”

According to Iyad Tarazi, vice president/Network Development and Engineering at Sprint, it would be useful to extend the existing blocks of spectrum: “Large, new, clean blocks are optimal, but unlikely. The next preferred option is to grow block sizes adjacent to today’s allocation.”

Global Harmonization

Participants in the FCC’s forum also stressed the need for global harmonization, and having a spectrum road map will help with global planning. “If the rules are known in advance, we can work with carriers around the world to harmonize strategies,” said Sprint’s Tarazi.

Greater harmonization can yield economies of scale as devices and chips sets can operate on a global scale, and Tarazi urged the FCC to “find a big enough block of spectrum, and announce our intent to the world. If we come in at the last minute, with a small block, it’s hard to rally the world. We have to come in with a leadership position.”

— Jennifer Whalen