We discuss the system engineering features of an 11 GHz 140 Mbit/s system primarily in terms of the requirements to provide an overlay on the existing British Post Office microwave radio network. Significant influences on the system characteristics adopted are the relatively dense nature of the network, with a multiplicity of traffic nodes, and the digital transmission performance objectives. A description is given of the system, including some details of particular units and sub-systems, and results of measurements on a simulated hop. We describe a prototype system, whose installation is imminent, and give an account of the attributes to be evaluated. This prototype is a precursor to fully operational systems to be placed into service by early 1982.