T-Mobile Demos "Coordinated Multi-Point Transmission" for LTE Networks

T-Mobile, Deutsche Telekom Laboratories, and the Fraunhofer Heinrich Hertz Institute (HHI) have demonstrated for the first time interference-free real-time downlink transmission simultaneously from two base stations to two mobile terminals in two overlapping mobile cells.

The system used for this is known as Coordinated Multi-Point Transmission (CoMP) and is considered a key technology for LTE networks.

"What sounds unbelievable at first will allow for higher capacity to cope with the dramatic rise in mobile Internet traffic in our mobile communications networks," says Klaus-Jürgen Krath, Senior Vice President Radio Networks Development in Deutsche Telekom AG's T-Mobile business area. "The engineers have shown that it is possible to eliminate interference in mobile terminals as soon as these are known to the nearby base stations."

If interference is likely, base stations can jointly preprocess the information for several users prior to transmission. As a result of the preprocessing, signals are constructively overlaid at the desired user terminal but are eliminated at the antennas of other users. A terminal in a CoMP network thus behaves as if it were in an isolated cell because it is no longer interrupted by the data traffic in neighboring cells. This process considerably improves reception especially in the overlapping regions of the cells, which is where interference is highest. Consequently, all available frequencies can be reused in each cell of a network without restriction and yet the same capacity is achieved as in an interference-free network.

"In our research we focus on future wireless networks in which data can be transmitted ten to a hundred times faster than today," comments Holger Boche, Director of the Fraunhofer Heinrich Hertz Institute (HHI), holder of the Chair for Mobile Communications at Berlin's Technical University, and winner of the prestigious Leibniz Prize in 2008.

Although CoMP concepts are deemed to have potential and are considered key technology in the current LTE-Advanced standardization, misgivings have been voiced about the complexity, growing pressure on the backhaul and the overhead, especially for the downlink from the base station to the terminals.

The proposed solution reduces the complexity as well as the data rate on the backhaul through the use of a concept with distributed signal processing. The terminals transmit information on the characteristics of their radio channels to the base stations of their cells. The cells in turn exchange information as well as the user data through a meshed high-speed backbone with low latency. With the data thus provided and global channel information, each base station can calculate the signals being sent independently of the other base stations and transmit them over its local antennas. The overhead can be cut by reducing the repeat cycle of the channel feedback. The remaining interference is reduced through an optimum combination of the two antenna signals on the mobile terminal.

The research work was sponsored by the German Federal Ministry of Education and Research (BMBF) as part of EASY-C, a joint German research project involving partners from the industry, small and medium-sized companies, research institutes, and universities.