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  Friday, 17 July 2009 
Deutsche Telekom Successfully Tests System for Increasing Capacity in Future Mobile Communications Networks PDF Print E-mail
Thursday, 09 July 2009
www.telekom.com

Deutsche Telekom’s R&D lab and the Fraunhofer Heinrich Hertz Institute have jointly demonstrated interference-free Coordinated Multi-Point Transmission (CoMP) at the LTE-Advanced downlink testbed. The CoMP is considered a key technology in the current standardisation of the future mobile communications standard LTE-Advanced. It guarantees a more efficient use of the radio spectrum and hence greater bandwidth.

T-Mobile, Deutsche Telekom Laboratories, and the Fraunhofer Heinrich Hertz Institute (HHI) 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) among experts and is considered a key technology for the future mobile communications standard "LTE-Advanced" (Long Term Evolution). By deploying the CoMP system that the researchers have now implemented in real time for the first time, better use can be made of the radio frequency spectrum. The future mobile communications networks implemented with this technology will afford subscribers greater bandwidth, e.g., for rapid mobile Internet access.

"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 research teams from Deutsche Telekom Laboratories and the HHI in Berlin and Darmstadt have risen to this challenge. Their 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.

Other technical details
The Berlin testbed uses two base stations and two terminals at 2.6 GHz with a bandwidth of 20 MHz for the downlink and uplink.Each base station has two transmission antennas and at each terminal there are two receiving antennas. While the transmission protocol is essentially based on the currently applicable LTE standard, a number of changes had to be made before the new concept could be implemented: the base stations are synchronized with the Global Positioning System (GPS). They use cell-specific reference signals that enable mobile terminals to guess the downlink channel to the base stations located in the vicinity. The channel information reported back is transmitted as normal data packages via the uplink. Without this feedback the mobile terminals could only reduce the interference in part by optimally combining the received signals of their multiple antennas, but would suffer data rate losses on account of the remaining interference. If the new signal preprocessing method is activated on the base stations, an increase in the data rate available can be observed on the two terminals, which is due to the suppression of the interference between the cells. This allows for a maximum of four data streams to be transmitted simultaneously in the two cells on identical frequencies without interfering with one another, which achieves much greater spectrum efficiency.

Deutsche Telekom Laboratories
Deutsche Telekom Laboratories are Deutsche Telekom's research and development institute. As an affiliated institute of the Berlin University of Technology (TU Berlin), Deutsche Telekom Laboratories facilitates close knowledge sharing between the scientific community and industry in order to research and develop new information and communication technologies. This results in innovative services, products, and solutions for customers of Deutsche Telekom.

 
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