Cooperative MIMO Wireless Networks

Multiple antennas at transmitter and receiver introduce spatial degrees of freedom into a wireless communication system. Space-time signal processing utilizes these degrees of freedom to boost link capacity and/or to enhance link reliability of multiple-input multiple-output (MIMO) communication systems. With spatial multiplexing one can increase the data rate without additional cost of bandwidth or power by transmitting data streams simultaneously over spatial sub-channels which are available in a rich scattering environment. Space-time codes are used to compensate the fading effects by utilizing the spatial diversity of the MIMO channel.

It is expected that future wireless broadband communication systems will operate beyond 5 GHz, for example Wireless Local Area Networks (WLANs) at 17 GHz (Hiperlan) or at 24/60 GHz (ISM bands). In higher frequency bands it is possible to accommodate a larger number of antennas in a given volume (''rich array'') because the array size and the decorrelation distance scales down with increasing frequency. Further on, the array gain of the system can compensate the path loss which is inversely proportional to the square of the frequency [WittRank04].

For zero-mean i.i.d. Gaussian channel coefficients the ergodic capacity of a MIMO channel with N transmit and M receive antennas scales linearly with min{M,N} compared to a corresponding single-input single-output (SISO) channel. However, there is a major obstacle in the practical exploitation of MIMO technology: the capacity gain depends strongly on the propagation environment and diminishes with increasing correlation of the channel coefficients. In higher frequency bands we expect an increase in correlation because the propagation channel becomes more and more line-of-sight (LOS) and we are confronted with a rich array -- poor scattering dilemma [WittRank04].

In [WittRank03] and [RankWitt04] it is shown that with cooperative two-hop relaying one can increase the rank and therefore the capacity of correlated (ill-conditioned) MIMO channels. The main idea is to use amplify-and-forward relays that act as active scatterers and assist the communication between source and destination: The relays receive in the first time slot the signal from the source and forward an amplified version to the destination node in the second time slot. This way of relaying leads to low-complexity relay transceivers and to lower power consumption since there is no signal processing for decoding procedures. The goal of cooperative relaying here is to increase the rank of the compound (two time slots) channel matrix and to shape the eigenvalue distribution such that the channel matrix becomes well-conditioned and the capacity of the MIMO channel improves.

The goal of this research project is to develop a solid foundation of MIMO wireless networks where the individual nodes can cooperate. We work on performance limits of cooperative MIMO wireless relay networks in terms of information theoretic capacities, outage probabilities and spectral efficiencies of practical receiver structures. Based on the theoretical limits we are interested in developing cooperative physical layer signaling schemes for wireless relay networks which can exploit space and time diversity in a way that achieves reliable transmission of information and increase the capacity between a source and a destination. We expect that the results to be obtained in the course of the proposed project will lead to important insights on the integration of MIMO technology into future wireless networks.

Publications

• On the Capacity of Relay-Assisted Wireless MIMO Channels
  B. Rankov and A. Wittneben, Signal Processing Advances in Wireless Communications, SPAWC 2004, July 2004, to appear.
• MIMO Signaling for Low Rank Channels
  A. Wittneben and B. Rankov, International Symposium on Electromagnetic Theory, URSI, May 2004, to appear, (invited paper).
• Space-Time Processing for Cooperative Relay Networks
  I. Hammerstroem, M. Kuhn, B. Rankov, and A. Wittneben, IEEE Vehicular Technology Conference, VTC Fall 2003, Oct. 2003.
• Distributed Antenna Arrays versus Cooperative Linear Relaying for Broadband Indoor MIMO Wireless
  A. Wittneben and B. Rankov, International Conference on Electromagnetics in Advanced Applications, ICEAA'03, Torino, Italy, Sept. 2003, (invited paper).
• Impact of Cooperative Relays on the Capacity of Rank-Deficient MIMO Channels
  A. Wittneben and B. Rankov, Proceedings of the 12th IST Summit on Mobile and Wireless Communications, Aveiro, Portugal, pp. 421-425, June 2003.