Spectral Shaping Method for OFDM Combining Time-shifted Active Interference Cancellation and Adaptive Symbol Transition

Out-of-band emission (OOBE) is a major concern in orthogonal frequency division multiplexing (OFDM) based systems, as it requires large guard bands to avoid interfering other channels or systems. Spectral shaping techniques combining active interference cancellation (AIC) and adaptive symbol transition (AST) strategies, herein shortened as C-AIC-AST, are effective to reduce it at the cost of a data rate penalty and a modest implementation complexity compared to precoding and filtering techniques. This paper presents a spectral shaping method that innovates previous C-AIC-AST proposals by expanding both the AIC and AST terms to preceding and subsequent OFDM symbols. The time-expanded terms are orthogonal to the current symbol, which makes them transparent to the receiver. The additional degrees of freedom given by these supplementary OOBE reduction terms can be exploited in different ways. In pulse-shaped OFDM systems, it allows attaining the same OOBE level than existing C-AIC-AST techniques but avoiding the data rate loss of the latter. In systems with rectangular OFDM signals, it can notably reduce the OOBE with a much smaller reduction in the effective cyclic prefix length compared to currently employed methods. The efficiency of the proposed method in achieving both goals with reduced implementation cost is demonstrated through results obtained in actual wired and wireless systems.