Bridging the smart antenna gap

With the IEEE 802.16e standard finally in place, more than a dozen WiMAX vendors are looking for some way to differentiate their highly standardized products. And they've settled on the smart antenna as their weapon of choice.

The two main camps are centered on multiple input/multiple output (MIMO) technologies and beamforming technologies, though MIMO clearly has the most cachet. But one smaller vendor is asking why not do both?

Navini Networks is combining MIMO and beamforming in its adaptive antenna system arrays and base station. MIMO is traditionally regarded as the truly mobile technology, using multiple signal transmissions, which are received and interpreted separately by the customer premises equipment.

The multi-path nature of the technology lets the network take advantage of obstacles, which bounce signals around, allowing users to maintain connections while moving at high speeds through dense urban areas. Meanwhile beamforming uses multiple antennas to create a single, strong, focused signal aimed directly at the customer. That signal strength adds capacity and range to each individual connection, but it doesn't have the mobility advantages of MIMO.

Although other companies have talked about deploying dual-mode systems that switch between MIMO and beamforming modes as a user moves throughout the network, Navini is applying beamforming techniques to the individual MIMO signals, creating multiple beamformed signal paths. At first the technologies may seem incompatible. MIMO requires antennas to be separated by multiple wavelengths to prevent the individual signals from mashing together, while beamforming, by definition, wants those signals mashed, requiring tightly grouped antennas, said Al Dumdei, Navini director of sales engineering. Navini, however, has massaged its traditional beamforming antenna array to simulate the spatial diversity of MIMO array, he said, basically creating virtual space where none exists,

Its Smart WiMAX solution gives the capacity and range benefits of beamforming while keeping the mobility and density advantages of MIMO.

“We form beams every five milliseconds, which is the same size of one WiMAX frame, and we can form 80 of those beams simultaneously,” said Paul Sergeant, director of market strategy for Navini. “With MIMO-A, we can get the robustness we need for a mobile environment where conditions are changing rapidly.”

When Sergeant says MIMO-A, he's talking about MIMO's two implementations, one that uses the multiple paths to send the same data stream (MIMO-A), and the other using those different paths to send different data (MIMO-B), thus compounding the capacity of the MIMO system. MIMO-B can double capacity to the individual user (essentially giving the customer two beamforming connections) in a stationary environment. And Navini is supporting both. In fact, Navini is using a software-defined radio architecture to allow the networks to switch between the different beamformed MIMO and straight-up beamforming modes in real time, ensuring that the network always is in the most optimal configuration, Sergeant said.