OFDM Based Overlay Systems - Design Challenges and Solutions

With respect to the ever-growing demand for mobile communications on the one hand and spectral scarcity on the other hand it is necessary to increase spectral efficiency. In this paper, we focus on OFDM (Orthogonal Frequency-Division Multiplex) based overlay systems, which share a frequency band with already existing systems by adaptively filling the spectral holes. In order to enable such a co-existence, on the physical (PHY) layer promising methods for minimising mutual interference between the overlay system and the licensed systems are presented. The medium-access control (MAC) layer protocols are designed such as to be able to efficiently signal the available subcarriers as well as to handle the changing available bandwidth. A third issue is the joint optimisation of PHY and MAC parameters which is essential in order to optimally exploit the available spectrum with minimum signalling overhead.