Guardband Optimization for Cellular Systems Applying Raised Cosine Windowed OFDM

In this paper, we study the optimization of the guardbands between adjacent channels in OFDM-based cellular systems. It is well known that OFDM signals have strong sidelobes in spectrum, which lead to large necessary guardbands between adjacent channels to avoid severe adjacent channel interference. To reduce such guardbands and, thus, to increase the spectrum usage efficiency, we propose to apply raised cosine (RC) windowing of the time domain OFDM signals, which is a simple, efficient and effective method. In this context, an interesting tradeoff can be observed, i.e., a higher roll-off factor (ROF) of the RC-window is associated with higher signal overhead in time (in other words, longer OFDM symbol duration), but lower guardband overhead in frequency (due to better sidelobe suppression). We study this tradeoff and found out that the ROF and the guardband size can be jointly optimized, in the sense that the per-channel data rate can be maximized under adjacent channel interference constraints. Our mathematical analysis further shows that such joint optimization can be reduced to a single dimension optimization, i.e. the optimization of the ROF. Afterwards, this optimization procedure is illustrated and verified via numerical simulation, showing that large data rate increase can be achieved when using RC-windowing with an optimized ROF. Finally, practical analysis of the instantaneous per-OFDM symbol interference further verified the effectiveness of the proposed approach.