ShiftFFT: An Efficient Approach to Mitigate Adjacent Channel Interference in OFDM Systems

Adjacent channel interference (ACI) in wireless systems is commonly mitigated through the use of filters and guard bands. Guard bands are not used for any transmissions and are therefore wasted spectrum. The use of sharp filters can reduce the size of required guard bands but they are costly and often not present in devices that are already deployed. We focus on OFDM wireless systems, which form the basis for almost all modern wireless networks, and propose a novel technique called shiftFFT that can be deployed at an OFDM receiver to mitigate the impact of out-of-band interference from legacy OFDM transmitters. shiftFFTexploits the presence of over-provisioned cyclic prefixes in most OFDM wireless standards to optimize the starting time of the FFT operation at the receiver, which can significantly mitigate ACI. The technique increases the cost of the modified receiver but it does not require any changes to the rest of the network. We implement shiftFFT, evaluate it in simulations and in a small experimental testbed, and we show that we can reduce the effects of the out-of-band interference by up to 30 dB.