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Orthogonal frequency division multiplexing (OFDM) is a favourable technology for dynamic spectrum access (DSA) due to the flexibility in spectrum shaping. In spite of that, high sidelobes of OFDM subcarriers bring in considerable interference to the nearby users, particularly in OFDM based cognitive radio (CR) networks, where the secondary users (SUs) are capable of accessing the spectrum opportunistically. In this paper, two new techniques for the suppression of high sidelobes are proposed. The proposed techniques composed of an optimization scheme are followed by generalized sidelobe canceller. The proposed techniques can be considered as a two-level suppression technique in the sense that in the first level the sidelobe is reduced by using cancellation carriers (CCs), whose amplitudes are determined using genetic algorithm (GA) and differential evolution (DE), while in the second level further reduction of sidelobe is achieved using generalized sidelobe canceller (GSC). Simulation results show the power spectral density (PSD) performance of the proposed techniques in comparison with already existing techniques, demonstrating that the proposed techniques minimize the out-of-band radiation (OOBR) significantly, thus qualifying for more effective spectrum sharing.

To tackle the problem of spectrum overcrowding, Mitola [

Some advantages of OFDM include spectral efficiency, resistivity against frequency selective fading, protection against interference, and simpler channel equalization. However, due to the high sidelobes of the OFDM subcarriers, CR based OFDM experience high out-of-band radiation (OOBR) that may interfere with either LUs or secondary users (SUs) in the contiguous bands. Different techniques including cancellation carriers (CCs) [

In this paper, we propose two new techniques: GSC combined with CC using GA and GSC combined with CC using DE. In these new techniques, the sidelobes are reduced into two steps. In the first step, CCs are added to the original OFDM signal, where the amplitudes of CCs have been adjusted using GA and DE [

The remainder of the paper is organized as follows. In Section

Consider an OFDM system having a total of

After taking the Fourier transformation of the signal given in (

The signal in (

Spectrum of transmitted OFDM signal.

In this section, we present our two new techniques for suppression of sidelobes. The proposed technique I includes a combination of GSC with CC using GA, while the proposed technique II includes a combination of GSC with CC using DE. In both of these proposed techniques, the sidelobe suppression is done in two steps.

The first step involves the suppression of sidelobes using CCs, whose amplitudes are calculated using GA in our first proposed methodology and using DE in our second proposed methodology. The concept of GA was first given by Holland [

Randomly generate a set of candidate solutions (i.e., set of chromosomes).

Compute the fitness of each candidate solution in the current population.

Select the parents in the order of their fitness.

Produce the offspring using crossover (single point crossover, multiple point crossover).

Offspring for the new generation is selected using one of three methods, that is, Elitism.

Replace generation and survival of fitness.

If no improvement is found in the new generation, then do mutation.

All the offspring will be the new population; the parents will die.

The concept of DE was first given by Stone and Price [

Randomly generate the initial population.

Compute the fitness of the initial population.

Select three different solutions randomly.

Create one offspring using the DE operators.

Do this a number of times equal to the population size.

For each member of the next generation,

if

As discussed earlier, the total number of available subcarriers is

Concept of cancellation carriers.

The concept behind our proposed fitness function is that weights of CCs are calculated using the average of all samples. These samples are taken as the centre of each sidelobe for reducing the computational complexity and reduction of memory usage. Suppose the total number of samples taken on both sides of the OFDM signal is

In the second step, the samples of the signal with optimized CCs given in (

Block diagram of GSC.

The lower portion of the GSC comprises blocking matrix

The output of GSC after passing vector

The weight vector

The useful implementation of LCMV is the division of

Now, consider the partition of

In this section, we consider five different spectrum sharing scenarios to check the efficiency and authenticity of our proposed techniques. We compare the performance of our proposed techniques with the current techniques with the help of computer simulations in terms of normalized power spectral density (PSD). As discussed above, DE is simple and straightforward to implement and has much better performance in terms of accuracy, convergence speed, computational complexity, and robustness as compared to the other EAs like GA and others. Therefore, the performance of proposed technique II is better as compared to the performance of proposed technique I in all five different spectrum sharing scenarios.

In this scenario, assume that CR detects a single vacant band divided into 16 OFDM subcarriers mapped with BPSK and is utilized by a single SU. The efficiency of our proposed techniques for this scenario is compared with the current techniques, including CC [

The PSD performance comparison between the proposed techniques and existing techniques, Scenario I.

The PSD performance comparison between the proposed techniques and existing techniques, Scenario I.

In this scenario, assume that CR detects four vacant bands, denoted by II, IV, VI, and VIII, all having equal bandwidths. The spacing between these bands denoted by I, III, V, VII, and IX is also considered as of equal bandwidths. SUs functioning in bands II, IV, VI, and VIII use 32 OFDM subcarriers, mapped with BPSK. The performances of the proposed techniques in terms of PSD with others, including ASW [

The PSD performance comparison between the proposed techniques and existing techniques, Scenario II.

The PSD performance comparison between the proposed techniques and existing techniques, Scenario II.

In this scenario, consider the notion that white holes detected by CR are given by II, IV, VI, and VIII, which have the same bandwidth. The spacing between white holes is given by I, III, V, VII, and IX, respectively, which have different bandwidths. An equal number of OFDM subcarriers (i.e., 32) are used by the SUs operating in these white holes, mapped with BPSK. The reliability and effectiveness of the proposed techniques are shown in Figures

The PSD performance comparison between the proposed techniques and existing techniques, Scenario III.

The PSD performance comparison between the proposed techniques and existing techniques, Scenario III.

In this scenario, the bandwidth of the spectral white holes detected by CR is considered as unequal represented as II, IV, VI, and VIII. The spacing between them is considered as equal. SUs operating in white hole II use 16 subcarriers, in IV use 32, in VI use 64, and in VIII use 128, each modulated with BPSK. Figures

The PSD performance comparison between the proposed techniques and existing techniques, Scenario IV.

The PSD performance comparison between the proposed techniques and existing techniques, Scenario IV.

In this scenario, four spectral white spaces are considered to be detected by CR, represented by II, IV, VI, and VIII, which have unequal bandwidth. Spacing between them is also considered to be unequal. SUs operating in these spectral white spaces use 16 subcarriers in II, 32 in IV, 64 in VI, and 128 in VIII, modulated with BPSK. The performances of our proposed techniques for that scenario compared with the current techniques including CC [

The PSD performance comparison between the proposed techniques and existing techniques, Scenario V.

The PSD performance comparison between the proposed techniques and existing techniques, Scenario V.

In this paper, the OOBR problem, one of the major issues in CR based OFDM, is discussed. To handle that problem, we propose two new techniques: the first one is the combination of CCs using GA with GSC and the second one is the combination of CCs using DE with GSC. The purpose of combining different techniques is to take advantage of the individual techniques for a better reduction of OOBR. The strength and reliability of the proposed techniques are shown via computer simulations in different types of spectrum sharing scenarios, which shows that the proposed techniques get far better reduction of OOBR as compared to the existing techniques and the overall performance of the proposed technique is better.

The authors declare that there are no competing interests regarding the publication of this paper.