Suppression of Mutual Interference in OFDM Based Overlay Systems

A promising appraoch for overcoming spectrum scarcity are overlay systems that share a frequency band with already existing licensed systems by using the spectral gaps left by the licensed systems. Due to its spectral efficiency and flexibility orthogonal frequency-division multiplexing (OFDM) is an appropriate modulation technique for overlay systems. To enable a successful co-existence, techniques for suppressing mutual interferences between the overlay and the licensed system are proposed

Suppression of Mutual Interference in OFDM Based Overlay Systems

A promising appraoch for overcoming spectrum scarcity are overlay systems that share a frequency band with already existing licensed systems by using the spectral gaps left by the licensed systems. Due to its spectral efficiency and flexibility orthogonal frequency-division multiplexing (OFDM) is an appropriate modulation technique for overlay systems. To enable a successful co-existence, techniques for suppressing mutual interferences between the overlay and the licensed system are proposed

Leakage Compensation in OFDM Overlay System for VHF Aircraft Radio

In order to increase spectral efficiency, an orthogonal frequency-division multiplexing (OFDM) based overlay system should be operated in the very high frequency (VHF) band in addition to the already existing aeronautical voice communication system. The overlay system is designed not to disturb the transmission of the VHF system, whereas the VHF system causes interference in the overlay system. This influence is even amplified by leakage effects which are a systematic problem of the discrete Fourier transform applied for demodulation at the receiver of the OFDM system. In this paper, we analyze the leakage effects occurring in the considered system and propose a compensation scheme mitigating the wideband deterioration of signal-to-interference-and-noise ratio (SINR) induced by leakage. Simulation results show, that with the proposed compensation method the influence of leakage effects can be reduced to the noise floor. Moreover, several simplifications of the compensation algorithm are proposed in order to reduce computational complexity

Mitigation of Dynamically Changing NBI in OFDM Based Overlay Systems

In overlay systems based on orthogonal frequency-division multiplexing (OFDM) narrow-band interference (NBI) from licensed systems transmitting in the same frequency band degrades system performance significantly. In this paper, the impact of NBI is either reduced by means of an optimized compensation matrix or by simply subtracting an estimated NBI signal from the received signal. The required estimation of the NBI signal is performed by means of a few observation subcarriers. Variations in the center frequency of the interferer are tracked by exploiting information from pilot OFDM symbols for channel estimation. Simulation results show that with both techniques system performance is improved significantly even in presence of strong and dynamically changing NBI

Mitigation of Pulsed Interference in OFDM Based Systems

In order to enhance spectral efficiency an orthogonal frequency-division multiplexing (OFDM) based system is operated in a frequency band already in use by a licensed system. When transmitting an OFDM signal in the spectral gaps left by the licensed system, the OFDM system is exposed to severe interference, in the considered example characterised by short but strong Gaussian shaped pulses. Well-known approaches for mitigating the impact of pulsed interference are pulse blanking or clipping. As these techniques have not been applied to OFDM systems with this type of interference so far, the optimal thresholds for both techniques are determined. Simulations in a realistic interference scenario in the L-band show, that with clipping or pulse blanking based on the optimal threshold the impact of interference can be reduced considerably. The remaining gap to the interference-free case can be partly explained by the fact that not only the interference signal but also the desired OFDM signal is affected by pulse blanking or clipping

Interference Mitigation for the Future Aeronautical Communication System in the L-Band

In this paper, an OFDM based inlay system is considered, which is operated in the spectral gap between two adjacent channels of an already existing system in the L-band. The strong impact of pulsed interference from the existing L-band systems is mitigated by means of pulse blanking and clipping. When applying these well-known approaches to OFDM systems, the two main issues to be solved are the optimisation of the threshold for pulse blanking or clipping and the detection of the interference pulses in the received signal. Simulation results with the optimal threshold show that interference can be mitigated considerably. However, the performance of the interference-free case is not reached since the desired OFDM signal is impaired by pulse blanking and clipping