A scheme of the superimposing additional data signal in the time domain for orthogonal frequency division multiplexing (OFDM) system is proposed. The proposed scheme has a tradeoff between the degree of freedom for data transmission and inter-carrier interference (ICI), which provides the flexibility of data rate decision when the finite number of modulation and coding levels are available for the given channel condition. A performance analysis of the bit error rate (BER) confirms this tradeoff. In simulation on the practical environment which experiences multipath fading and error of channel estimation, the results show that much improvement of spectral efficiency has been achieved while keeping as nearly good bit error rate as the conventional OFDM. Moreover, the single carrier transmission of the superimposed additional data in the time domain also gives an opportunity of boosting the signal power up to the peak to average power ratio (PAPR) margin of the OFDM system.

In this letter, we present a closed-form bound of the average bit error rate (BER) performance for the multi-hop amplify-and-forward (AF) relaying systems with fixed gain in Rayleigh fading channel. The proposed bound is derived from the probability density function (PDF) of the overall multi-hop relay channel under the assumption of asymptotic high signal-to-noise ratio (SNR) at every intermediate relays. When intermediate relays actually operate at finite SNR, the proposed BER bound becomes looser as the SNR of the last hop increases. In order to reflect the effect of all the noise variances of relay links to the BER evaluation, a hop-indexed recursive BER approximation is proposed, in which the proposed bound of BER under asymptotic high SNR is used. The simulation results manifest that the proposed hop-indexed recursive BER approximation can not only guarantee accuracy regardless of the SNR of the last hop but also provide higher accuracy than the previous works.