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%.

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In this paper we consider the uplink of a multi-carrier code-division multiple-access (MC-CDMA) system employing combined equalization and we address the problem of channel estimation in the presence of residual timing offsets. In particular, we propose a method based on the maximum-likelihood (ML) principle that jointly estimates the pre-equalized channel response and timing offset of each user. The resulting scheme operates in the frequency-domain and exploits some training blocks (carrying known symbols) placed at the beginning of the uplink time-slot. Numerical results are given to highlight the effectiveness of the proposed method and to make comparisons with other existing solutions