ALU breathes life into LTE cellular 'organism'

Though they form something called a ‘network,’ cellular systems have always been fairly isolated and autonomous creatures. Like movers handing off boxes from one pair of hands to the next, cells never carry their loads together. They serve their individual users and then pass them on to the next cell. But the next generation of wireless networks—the ones beyond what we call 4G today—will be much more cooperative.

Alcatel-Lucent’s (NYSE:ALU) research arm Bell Labs has begun developing smart antenna and cellular collaboration technologies that will allow multiple cells to share the burden of a single connection. Called coordinated multipoint transmission (or CoMP), the technology was on display last week in Berlin, where Deutsche Telekom (NYSE:DT) Laboratories, antenna maker Kathrein and Bell Labs demonstrated that a device on the edge of two long-term evolution (LTE) cells could receive transmissions from both towers rather than merely pass its signal from one to the other.

CoMP uses a technology that is now familiar in 4G and WiFi, multiple input-multiple output (MIMO) smart antennas. But MIMO systems today transmit their identical data streams over the same parallel paths from a single transmitter. Bell Labs, which invented the first MIMO systems in the 1990s, is following a variation of that approach, called Network MIMO. The key difference: rather than transmit those parallel signals from the same source, it sends them from different sources, allowing each stream to reinforce the other. A way to visualize network MIMO is to imagine a phone tethered to the network not by a single rope but by web of strands. Conceptually, the network ceases being a collection of individual cells and becomes an organism where every cell is working together to support all users simultaneously, said Hans-Peter Mayer, head of advanced wireless research and LTE projects at Alcatel-Lucent Bell Labs.

“An organism--that’s more or less the paradigm we are going for,” Mayer said. “If you couple all of these base stations together they act as a whole.”

The demo network in Berlin used only two cells as a proof-of-concept, but as network MIMO is incorporated into future technology such as LTE-Advanced, individual devices could be sending and receiving data to and from as many as six or seven towers simultaneously, Mayer said. The benefit of such an architecture isn’t to increase the overall capacity of the cell, but to increase the capacity at the edge of the cell, where signal strength most often degrades. Signals from neighboring towers reinforce the transmission rather than interfering with it, flattening out the data peaks and valleys of the network as customers move between cells, Mayer said.

“No one is unhappy with the network’s performance in the center of the cell,” Mayer said. “It is the edge of the cell that has the most problems. … We’re trying to bring more stability to the cell edge, more consistent throughput.”

Alcatel-Lucent is still putting the technology through the motions, so Mayer isn’t prepared to make any generalizations on how much the technology will increase capacity at the cell edge. But in the initial trials Mayer has seen upwards of 500% boost in uplink capacity at cell boundaries, which could have huge ramifications for the adoption of Web 2.0 and other upstream-heavy data applications.

For years, researchers in the industry have been exploring the concept of evolving the wireless network from one built on one-to-one links—from device to tower—to a network built on the one-to-many links—from a single device to multiple towers. Such a collaborative or organic topology is only the first step, though, said Prabakhar Chitrapu, principal engineer for Interdigital Communications (NASDAQ:IDCC), in an earlier interview on the future of the wireless network. Relay points in the network, and even other users’ devices, could all be used as nodes to create a web of reinforcing connections, which would allow the network to self-organize into whatever configuration would be most optimal for a device on the network at any given moment, Chitrapu said. By creating a collaborative architecture, the wireless access network begins to resemble the Internet, and the efficiencies of the Internet can be applied, Chitrapu said. The result could be a network that not only is consistent and ubiquitous, but one that can support tremendous amounts of capacity over the same radio resources of a network today, he said.

“By treating these two links as a single network, your collective network capacity will be greater than two times the individual link capacity,” he said. “You're really going from a one-dimensional line to a two-dimensional plane.”

Alcatel-Lucent’s technology isn’t going to such extremes. In fact, network MIMO and collaborative mobile communications is still limited by the processing capabilities of today’s technology, particularly in the handset. In order to coordinate their reinforcing transmissions the cell sites have to time themselves perfectly. If two signals intended to reinforce one another arrive at the device at separate times, the wind up interfering with another—the opposite effect of what network MIMO is supposed to accomplish.

While this presents few troubles on the uplink (the base stations can sort out mismatched signals easily), downlink timing is of vital importance, Mayer said. Each device has to determine its distance and relative relationship to multiple cell towers in real-time and then transmit that information to the base stations controlling those towers. The base stations then jointly calculate the optimal way to send those signals. In Bell Labs’ CoMP technology, one base station—usually the one with the strongest signal—takes the lead in that collaboration, ordering the other base stations to transmit according to its preferences, Mayer said. As the user moves from one primary cell to the next, the cell with the strongest signal takes over as the ‘master’ base station while the other revert to a support role.

The Berlin trial used two towers, a single device and a single base station—a simple scenario designed to show that network MIMO is possible. In a real network though, the scenarios become immensely complex: thousands of users moving among hundreds of cells, each establishing, maintaining and dropping connections between multiple base stations at any given moment. The number of individual connections individual base stations would manage would jump exponentially, each being a master base node to its normal complement of users while being a ‘slave’ node to hundreds of others.

The biggest inherent limitation to that architecture is rapid movement, something network MIMO can’t yet support. While CoMP can handle users moving slowly within and without cells, making the gradual adjustments necessary, a customer moving 60 mph on a highway would add a tremendous number of new variables to the device and network calculations, Mayer said. Fortunately the use cases for most high-bandwidth applications imply a user is stationary, he added.

“We don’t assume you implement this scheme if you’re moving at higher speeds,” Mayer said. The vast majority of PC card and other data device users are stationary when they access the Web or another data-heavy app, which greatly improves the network’s ability to adjust, he said.