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Orthogonal frequency-division multiplexing (OFDM) systems exhibit significant out-of-band radiation caused by high sidelobes of the modulated subcarriers. Existing techniques for reducing this undesirable effect have several drawbacks as they waste the scarce spectral resources or expand the signal in time domain. In this paper, we propose a new technique which overcomes these problems. A few so-called cancellation carriers are inserted on the left and right hand side of the used OFDM spectrum. These special subcarriers are not employed for data transmission, but carry complex weighting factors which are determined such that the sidelobes of transmission signal and cancellation carriers cancel each other. Simulation results show that with the proposed method a significant sidelobe suppression is achieved with only a small loss in bit error rate performance. This loss is due to the fact that a certain amount of the transmission power has to be spent on the cancellation carriers and is not available for data transmission.

SUMMARYIn this paper, we consider the problem of out-of-band radiation in orthogonal frequency-division multiplexing (OFDM) systems caused by high sidelobes of the OFDM transmission signal. Suppression of high sidelobes in OFDM systems enables higher spectral efficiency and/or co-existence with legacy systems in the case of OFDM spectrum sharing systems. To reduce sidelobes, we propose a method termed multiplechoice sequences (MCS). It is based on the idea that transforming the original transmit sequence into a set of sequences and choosing that sequence out of the set with the lowest power in the sidelobes allows to reduce the out-of-band radiation. We describe the general principle of MCS and out of it we derive and compare several practical MCS algorithms. In addition, we shortly consider the combination of MCS sidelobe suppression method with existing sidelobe suppression methods.Numerical results show that with MCS approach OFDM sidelobes can be reduced significantly while requiring only a small amount of signalling information to be sent from transmitter to receiver. For example, in an OFDM overlay scenario sidelobes power is reduced by around 10 dB with a signalling overhead of only 14%.

This paper studies the impact of a femto-cell underlay deployment that shares radio frequency resources with urban macro-cells. Femto-cells promise substantial gains in spectral efficiency due to an enhanced reuse of radio resources. However, owing to their random and uncoordinated deployment, they potentially cause destructive interference to macro-cells and viceversa. In order to maintain reliable service of macro-cells, it is most important to mitigate destructive femto to macro-cell interference. In the downlink, this can be achieved by dynamic resource partitioning, in the way that femto base stations (BSs) are denied access to resources that are assigned to nearby macro mobile stations (MSs). By doing so, interference to the macro-cells is effectively controlled, at the expense of a modest degradation in femto-cell capacity. The necessary signalling is conveyed through the wired backbone, using the downlink high interference indicator (DL-HII).