Feasibility study on application of impulse-UWB for control channel in Cognitive Radio networks

Cognitive Radio (CR) will enhance the efficiency in spectrum usage by re-using temporally unused licensed spectrum. A promising transmission scheme to utilize even very fragmented and fast changing spectrum is Non-Contiguous OFDM (NCOFDM). Unfortunately, NC-OFDM requires tight synchronization between sender and receiver, i.e. the receiver needs perfect up-to-date knowledge about the set of subcarriers being used by the NC-OFDM sender. Therefore, a reliable and always available Control Channel (CC) is crucial for signaling the spectrum allocation information. Although, underlay Impulse-Radio Ultra-WideBand (IR-UWB) meets the theoretical requirements for such a CC in CR networks, i.e. communication range of up to 1km on a sufficient high data rate, there is a lack of practical studies of IR-UWB in real world environments, especially with respect to co-existence with NC-OFDM as in the envisioned CR multi-technology station. In this paper we present results of measurements in our state-of-the-art IR-UWB testbed. We show that IR-UWB can reach the required communication range of a few hundreds of meters in unobstructed as well as slightly obstructed Lineof- Sight propagation only. Although, IR-UWB is a wideband technology we show that it is severely affected by narrowband interference from close proximity sources which is typically the case in the envisioned multi-technology stations. Such a mutual disturbance can be mitigated by increasing the spatial separation between both air interfaces, orthogonalization in time or using only those parts of the radio spectrum for NC-OFDM which are outside the main IR-UWB transmission mask.