MIMO (multiple-input, multiple-output) technology uses multiple antennas operating on the same frequency, but with different data, to increase data rate without a corresponding increase in RF bandwidth. This technology is used in 801.11n wireless networks and is also used on LTE networks. One problem with MIMO is that until now, multiple antennas were needed to take advantage of it. This isn't a big problem for base stations, but it becomes a significant problem as portable wireless devices become thinner and smaller. Scientists at EPFL announced last week that they have developed a single antenna that is capable of transmitting the same data rate as a two antenna system.
Julien Perruisseau-Carrier's group at EPFL, working with researchers from Athens Information Technology developed an antenna capable of simultaneously transmitting two independent signals. The "Beamspace MIMO" technique permits a significant reduction in the number of physical antennas in a MIMO system. An EPFL news release provided little detail on how Beamspace MIMO works, explaining "Contrary to what one might think, an antenna does not emit signals in a homogeneous and circular manner. Instead, the repartition of the antenna's radiation is called 'radiation pattern.' Scientists have managed to decompose this pattern in two, allowing simultaneous emission of two distinct information flows."
Julien Perruisseau-Carrier said, "This work required us to look at both antennas, particularly reconfigurable ones, and radio coding. Although they are closely linked, these two disciplines are unfortunately often considered separately, which hinders this kind of innovative work." The EPFL release said an advantage of this technique is that "only one coding and amplifying device is required to treat the first information flow. The second signal is treated only upstream and then used to make the radiation pattern decomposable. The latter signal acts therefore both as an information carrier and as a mean to decompose the principal pattern." Julien Perruisseau-Carrier added, "This technique allows us to get rid of part of the usual hardware." He said the process is difficult and complex, noting "One of the biggest challenges is to ensure that each pattern is really independent, to allow an efficient treatment of the information."
The EPFL web page Joint antenna-coding techniques has a bit more information on the technique. Only one RF system is used to feed the antenna but the antenna pattern is modified as part of the transmission system. The web page explains it this way: "In the so-called beamspace MIMO approach, instead of driving different symbol streams to different active antenna elements as in traditional MIMO, independent data streams are mapped onto an orthogonal set of basis functions in the antenna far-field (see figure). This novel approach has the tremendous benefit of using a single RF chain and very compact antenna dimensions."
The results are detailed in the article ""MIMO Transmission using a Single RF Source: Theory and Antenna Design" in the IEEE Joint Special Issue on MIMO Technology published by IEEE Transactions on Microwave Theory Technology and the IEEE Transactions on Antennas and Propagation, volume 60, pages 654-664.
