(Cooperative) - Linear Scalable Dispersion Codes

Future communication nodes will have heterogeneous capabilities in terms of signal processing power, number of transmit and receive antennas etc. Because of the demanded data rates the system bandwidth will increase. Orthogonal frequency division multiplexing (OFDM) seems to be a good choice for these systems breaking up the frequency selective fading channel into several flat fading channels. Due to the large bandwidth the channel exhibits large propagation delay differences. This introduces additional frequency diversity. The networks furthermore will have to cope with heterogeneous requirements to the quality-of-service (QoS).

This imposes some new requirements on future space-time coding schemes:

• ability to utilize offered diversity (spatial, temporal, and frequency)
• ability to adapt to a given number of TX and RX antennas, transmit diversity, spatial multiplexing, and joint transmit diversity with spatial multiplexing
• ability to work well in different fading environments (Rayleigh and Ricean)
• ability to utilize large system bandwidth (OFDM).

In this project a class of linear space-time block codes which meet these requirements are developed. These linear space-time block codes are highly flexible and adaptive, we refer to them as linear scalable dispersion (LSD) codes.

The LSD coding scheme consists of a linear time-variant inner code and a linear time-invariant outer code (see Fig.). We intentionally decouple the designs of each constituent code in order to achieve the required adaptivity. The inner code is optimized with respect to the variation of the instantaneous channel capacity over the block length of the outer code. Efficient inner coding matrices are given for pure TX diversity and joint spatial multiplexing and TX diversity. The linear outer code is optimized for diversity performance. It is concluded that the proposed concatenated linear high rate space-time block coding scheme is a promising candidate for heterogeneous future mobile communication systems.

The capacity of these systems can be increased dramatically by using multiple antennas at the transmitter and the receiver working in a rich scattering environment. But in a line-of-sight environment the MIMO channel matrix is rank deficient and therefore the capacity increase diminishes.
Using cooperative amplify-and-forward relay nodes it is possible to overcome this problem by increasing the rank of the MIMO channel (see project ''Cooperative MIMO Wireless Networks''). A special signaling scheme is necessary to achieve this increase of channel rank. Therefore the use of existing space-time algorithms is not straightforward. Recent results have shown that the proposed class of LSD space-time codes is able to exploit the offered capacity and achieves good performance results.