Opening up access to future wireless systems
The need to consider alternative communication systems is rising with the increasing number of licensed systems in the spectrum. Due to low power of transmitted signal, ultra-wideband (UWB) communications can operate as an unlicensed underlay system in the same frequency bands with licensed systems. Nevertheless, European and Japanese regulations require detect-and-avoid techniques in some common bands to protect the usage rights of licensed users. The EU-funded project 'Coexistence of ultra-wideband and licensed systems' (CUWLS) investigated use of UWB with primary systems. They explored licensed system 'detection' to decide whether the common band is in use or not. In addition, they investigated 'avoidance' issues in case the licensed system is active most of the time. The project sought to improve the UWB system ability to detect primary systems, assuming that primary systems are dependent. Instead of selecting the detection thresholds for each band individually as in conventional detection approaches, the bands were jointly processed. Maximum a posteriori decision variables were generated at the receiver, and bias terms were introduced to achieve trade-off between detection and false alarm probabilities. The gains were quantified in terms of the primary system interdependence and signal-to-noise ratio. Regarding the avoidance part, CUWLS investigated various pulse shapes and modified the UWB transceiver. Primary systems were modelled as orthogonal frequency-division multiplexing (OFDM) wideband systems, where the effects of bandwidth and number of subcarriers were investigated for various pulses. Higher-order linearly combined pulses compensated better for system performance. Furthermore, the UWB bit-error rate performance was improved with increasing OFDM bandwidth and the UWB system exhibited better performance in the presence of an OFDM system with a large number of subcarriers in the low signal-to-interference ratio region. These results were important as the modified transceiver can achieve reasonable system performance while complying with the European regulatory agency mandates. CUWLS addressed issues that are important for deploying wireless communication technologies in the future. Project results are therefore expected to have a positive impact.