## 1 Introduction  
The rapid growth in wireless communication methods and devices is a major reason for spectrum scarcity. Cognitive radio (CR) is an encouraging solution to tackle such a problem and has received special attention in the research community. CR has the ability to dynamically permit secondary users (SU) to operate in those spectral domains that are not being used by the primary users (PU) at certain times and localities (interweave mode) [1; 2; 3]. Efficient techniques are needed at the transmitter side to control the shapes of the transmitted signal so that both SU and PU can share the same spectrum resources with minimum interference. Orthogonal frequency division multiplexing (OFDM) is the best candidate for the CR [4; 5; 6], with the ability to divide the available wideband channel into multiple narrow band orthogonal channels/subcarriers and to transmit those subcarriers in parallel. Some attributes of OFDM include spectral efficiency, multipath delay spread, robustness to channel fading, _etc._ On the other hand, due to the large sidelobes of the OFDM subcarriers, CR based on OFDM experiences high out-of-band (OOB) radiation that may result in considerable interference with the adjacent bands used by either PU or SU. To tackle the OOB radiation problem, various techniques are proposed in the literature that can be categorized into two groups: time domain techniques and frequency domain techniques. Time domain techniques include filtering [7], which suffers from high complexity and lack of guard interval, and windowing [8; 9], which expands the signal in time domain and results in intersymbol interference (ISI). Frequency domain techniques include adaptive symbol transition (AST) [10], which expands the signal by injecting additional samples within a specified power threshold between two transmission blocks to reduce the interference of these two blocks at definite frequencies, active interference cancellation (AIC) [11; 12; 13], where a few subcarriers lying at the border of the licensed user band are reserved for reducing the interference. These subcarriers, called cancellation subcarriers, are not used for data transmission, but to cancel the interference in a specific frequency range. In Cancellation carriers (CC) [14; 15; 16] method the authors propose the same technique as in AIC, but consider limiting the power to cancellation subcarriers. In both cases, finding the optimal complex values for cancellation carriers is a least squares (LS) optimization problem. In [17] the weights of cancellation carriers are calculated using Genetic Algorithm (GA) and Differential Evolution (DE). In [18] [PERSON], [PERSON], _et al._ propose a heuristic algorithm for calculating the weighting factors for the CCs with few computations. Active and null cancellation carriers (ANCC) [19] technique combine active and null cancellation carriers in the guard band. Subcarrier weighting (SW) [20] technique suppresses the sidelobes by weighting all the subcarriers with an optimal set of real-valued coefficients, advance subcarrier weightings (ASW) [21], in which [PERSON], [PERSON] and [PERSON] propose a heuristic approach for sidelobe suppression for OFDM systems using subcarrier weighting, multiple choice sequence (MCS) [22], where the sequence of symbols carried by subcarriers is mapped to an optimal sequence that yields lower sidelobe interference, Constellation expansion (CE) [23; 24; 25] transmits symbols from a higher order constellation set and chooses the sequence thatresults in the lowest possible sidelobe power level, Insertion of guard bands [26, 27] where the interference is alleviated by adaptively deactivating the nearby subcarriers, providing flexible guard bands between licensed and un-licensed users and spectral precoding schemes [28, 29, 30, 31, 32] to reduce the interference by designing special precoder matrices, _etc._  
In this paper, we propose a novel technique for the reduction of OOB radiation in OFDM-based CR by using a generalized sidelobe canceller (GSC) at the transmitter of OFDM.