In the development of wireless/cellular communication networks and systems a good coverage and support of higher data rate, are desired. At the same time, the cost aspect of building and maintaining the system is of great importance and is expected to become even more so in the future. The topology of existing wireless communication systems has traditionally been characterized by the cellular architecture with the fixed radio base stations and the mobile stations as the only transmitting and receiving entities in the networks typically involved in a communication session.
One way to introduce diversity in the received signal is to exploit the spatial diversity offered when multiple antennas are used at the transmitter with the possibility of using one or more antennas at the receiver. The use of multiple antennas offers significant diversity and multiplexing gains relative to single antenna systems. Multiple-Input Multiple-Output (MIMO) wireless systems can thus improve the link reliability and the spectral efficiency relative to Single-Input Single-Output (SISO) system. In wireless systems it is generally not desired to include more than one antenna in the mobile terminals, since they should be kept as small and light as possible.
Another method that offers macro-diversity is relaying. A relaying system is a conventional radio network that is complemented with relay nodes. The relay nodes communicate wirelessly with other network elements, such as a base station, another relay and/or a user terminal. In this way, relay nodes can be used to introduce diversity in a system where one or both of the communicating units only has one antenna. A cooperative relaying system is a relaying system where the information sent to an intended destination is conveyed through various routes and combined at the destination. Each route can consist of one or more hops utilizing the relay nodes. In addition, the destination may receive the direct signal from the source. Cooperative relaying system can be divided into numerous categories based on desired parameters.
P. A. Anghel, G. Leus, M. Kaveh, “Multi-User Space-Time Coding in Cooperative Networks,” in Proc. of IEEE ICASSP, Hong-Kong, Apr. 6-10, 2003, and P. A. Anghel, G. Leus, and M. Kaveh, “Distributed Space-Time Coding in Cooperative Networks,” In Proc. of the Nordic Signal Processing Symposium, Norway, Oct. 4-7, 2002 discuss relaying schemes based on transmit diversity.
The coherent transmission for cooperating transmitters has been investigated for a regenerative case and with identical transmit powers in Yung-Szu Tu and Gregory Pottie, “Coherent Cooperative Transmission from Multiple Adjacent Antennas To a Distant Stationary Antenna Through AWGN Channels”, in Proc. of IEEE VTC 2002, Birmingham, Ala., USA, May 6-9, 2002.
In the literature numerous relaying schemes have been compared to a single hop system in order to a give general recommendation for the suitable scenarios for the deployment of a multi-hop system.
Y. Fan and J. S. Thompson, “On the Outage Capacity of MIMO Multihop Networks”, IEEE Globecom 2005, St. Louis, USA, November 2005 discloses a routing algorithm where the decision is based on instantaneous channel state information (CSI). This method has limitations since in practice obtaining the instantaneous CSI is not realistic in an FDD system type.
V. Sreng, H. Yanikomeroglu, and D. D. Falconer, “Relayer Selection Strategies in Cellular Networks With Peer-to-Peer Relaying,” IEEE VTC Fall October 2003 discloses a routing algorithm based on the path-gain. A routing algorithm based on the path-gain is not optimal since the fast fading of radio channel is not taken into account in a proper way.