Current research topics

Coding for wireless transmission

Under this topic we focus on code design for wireless communication systems, in particular for multiantenna ("MIMO") systems. Multiple antennas at the transmitter in a wireless system can be used to boost the data rate and/or improve the link reliability. In this area we pioneered the idea of combining orthogonal transmit diversity with partial channel knowledge at the transmitter, and we published one of the first textbooks on space-time coding for multiantenna systems.

• Utilization of channel state information at the transmitter
• Power control
• Code design for partial channel state information at the receiver
• Interference cancellation at the transmitter
• Modulation in known interference

Design of receivers for wireless communications

We design receiver structures for systems with strong outer coding. Specifically we study channel estimation and decision metrics for soft decoding. This problem is important and fundamental, because in practice the performance of systems with strong coding is often limited by the quality of the soft information provided to the decoder. Under this topic we also design optimum receivers for systems where the structure of the underlying transmission model is only partially known-for example, for interference suppression in environments where the number of interferers is unknown. Finally, we have in a related activity studied different receiver structures for optimal detection in the complete absense of channel state information at the receiver.

• Interference suppression
• Sparse channels
• Estimation in sparse regression models
• Generalized non-coherent receivers

Relaying and cooperative transmission

Cooperative transmission is a way of improving performance (specifically, reducing outage) in wireless networks by sending information from a source to a destination node via a relay. Our work has considered various aspects of radio resource allocation for relayed transmission, and the design of spectrally efficient transmission schemes. We are also interested in the field due to our related activities in wireless sensor networks. In particular, we are presently looking at different cooperation strategies to achieve improved energy-efficiency to reduce battery-drain.

• Spectrally efficient transmission
• Radio resource allocation
• Novel relaying protocols

Integrated coding and control

Here we study the highly interdisciplinary topic of designing closed-loop estimation and control algorithms, taking lossy communication due to a a noisy and/or bandwidth-limited feedback link into consideration. Specifically we study the problem of optimal encoder-decoder-controller design for unstable linear systems. This work is performed in collaboration with the Automatic Control laboratory at KTH EE.

• Combined source-channel coding and control

Source and channel coding for packet networks

In the area of source-channel coding, we pioneered the use of soft reliability information in source decoding, and we have contributed to vector quantization, source coding theory, channel robust image coding, and joint source-channel coding. Our current work focuses on multimedia transport over packet-switched networks with wireless access, with a focus on error-robust image and video coding.

• Multiple description coding
• Error-robust image coding