1. Institute for Communication, Systems, University of Surrey, Guildford GU2 7XH, UK
2. Department of Engineering Science, University of Oxford, Parks Road, Oxford OX1 3PJ, UK
3. Network Rail, The Quadrant:MK, Elder Gate, Milton Keynes MK9 1ER, UK
4. Institute for Digital Communications, University of Edinburgh, Edinburgh EH9 3FG, UK