The paper discusses the implementation of a narrowband return channel via a terrestrial UHF transmission from a user set top box (STB) back to the broadcaster in order to provide interactive TV. The design must be capable of overcoming the severe variation in traffic requirements between dense urban environments and rural areas. The solution, which was developed as part of the ACTS INTERACT project, is a system based on a synchronous FDMA (SFDMA) modulation scheme and allocates the transmission into frequency and time slots. This modulation scheme has been shown to offer improved performance and capacity in the presence of high user density and background noise, whilst restricting the broadcast to a 1 MHz bandwidth within the UHF broadcast band. A demonstration system has been developed in order to verify the technical principles and practicality of the final operational system and this system is also described.
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