The dynamic spectrum allocation model consists of a primary base station (P), cognitive radio base station (S), the number of licensed users (A), the number of unlicensed users(B), channel bandwidth (W), and the channel available for communication(C). In Fig. 1, the network model consists of Primary users' Base Station (PU-BS) and Secondary Users Base Station (SU-BS) with primary users denoted as PU= {1, 2,....PUa}, and secondary users expressed as SU= {1, 2... SUb} which is connected with PU-BS and SU-BS respectively. C= {1, 2… C} represents a communication channel with a total bandwidth of W. Cognitive users have some knowledge regarding channel state conditions. Two factors must be considered when assigning channels: the first is to avoid interferences between Primary Users (PUs) and Secondary Users (SUs), and the second is to reduce interferences between SU and SU. Secondary users must have information about channel occupancy.
The Signal interference and noise ratio (SINR) for bth secondary users represented by sth cognitive base station occupying cth spectrum band is given by:
$$\text{S}\text{I}\text{N}\text{R}=\frac{{g}_{s,b}^{c}{P}_{s,b}^{c}}{{\sigma }^{2}+{i}_{b}^{c}}$$
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Here
\({g}_{s,b}^{c}\) is the channel gain between sth SU-BS and bth cognitive radio on cth spectrum band
\({P}_{s,b}^{c}\) is the transmission power of sth SU-BS assigned to bth secondary users on cth spectrum band
\({\sigma }^{2}\) represents noise power
\({i}_{b}^{c}\) Interference between the PUs and SUs
Interference between the PUs and SUs is given by:-
$${i}_{b}^{c}=\sum _{a=1}^{A}{P}_{p,a}^{c}{g}_{p,a}^{c}+ {\sum }_{i=1,i\ne b}^{b}{\sum }_{l\in U/\left\{u\right\}}{P}_{l,i}^{c}{g}_{l,i}^{c}$$
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Here
\({P}_{p,a}^{c}\) is denoted as transmitted power from pth PU-BS allocated to ath PUs on cth spectrum band.
\({g}_{p,a}^{c}\) channel gain between pth primary base station allocated to ath primary users on cth spectrum band.
\({P}_{l,i}^{c}{g}_{l,i}^{c}\) is represented as interference due to other cognitive users.
The capacity acquired by each secondary user associated with sth cognitive base station over c resources expressed as:-
$${C}_{b}=\sum _{c\in C}\frac{W}{c}{a}_{s,b}^{c}{\text{log}}_{2}(1+SINR)$$
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Here w represents bandwidth, the number of resources denoted by c, and \({a}_{s,b }^{c}\)represents the spectral band allocated to secondary users.
The total capacity obtained by all SUs per unit bandwidth, represented as a system's spectral efficiency (in bps/Hz), is formulated as an objective function 1.
It is denoted as
Function1:\(SE=\sum _{b\in B}\frac{{C}_{b}}{W}\) (4)
Energy efficiency (in bps/Hz/W) is expressed as the total system capacity obtained by all CRs per total system power, formulated as an objective function 2.
Function:\(EE=\sum _{b\in B}\frac{{C}_{b}}{{P}_{T}}\) (5)
The multi-objective function can be formulated as
F1:MaxSE (6)
F2:MaxEE (7)
Subject to:
$${ C}_{1}:-\sum _{c\in C}{a}_{s,b}^{c}=1 ,s\in S,b\in B$$
$${ C}_{2}:- {a}_{s,b}^{c}\in \left\{\text{0,1}\right\} c\in C$$
$${ C }_{3}:- {a}_{s,b}^{c}{\text{log}}_{2}(1+SINR) \ge {C}_{b}$$
The main objective of our work how to improve both the independent objectives by utilizing the unutilized spectrum band for dynamic spectrum allocation. Dynamic Spectrum Allocation (DSA) is also an energy-consuming operation that also reduces energy efficiency. So efficient dynamic spectrum allocation technique can be achieved by maximizing spectral and energy efficiency in CRNs.
The two independent objectives cannot improve the system's performance individually. The trade-off analysis between the two conflicting objectives plays a vital role in performance improvement. MOO problem optimizes. M no of objectives maximize simultaneously to achieve Pareto optimal solutions.
Maximize y = F(x)= [f1(x),f2(x)…fm(x)]T (8)
Subjected to gk(x) ≤ 0, k = 1, 2,…N
and x =[x1,x2……………xp]T
y is known as the objective vector and gk represents a constraint.
3.1 MOALO Algorithm Employed for Spectrum Allocation In CRN
SOO techniques are mostly used for finding one of the best solutions. MOO techniques are mostly used for finding more than one solution and MOO has multiple trade-off solutions. These techniques can generate multiple optimal solutions. Pareto solutions are the optimal solution for a given optimization problem. The multi-objective optimization problems are solved by MOALO using roulette wheel mechanisms. The significant characteristics of the algorithm are explained as the random walk of ants, entrapment in an antlion pit, constructing a pit, Sliding ants towards antlions, catching prey, and reconstructing the pits and elitism.ALO has some distinct features like convergence speed and diverse Pareto optimal front
Step 1:-
The algorithm starts with the random values
Y(t)=[0,cumsum((2k(t1))-1),cumsum((2k(t2))-1)… cumsum((2k(tn))-1)] (9)
Cumsum is used for finding the cumulative sum, maximum iteration, and the no of steps represented by n and t.
$$\text{k}\left(\text{t}\right)=\left\{\begin{array}{c} 1 random number>0.5 \\ 0 random number \le 0.5 \end{array}\right.$$
The fitness values of every ant are calculated using objective Functions in each iteration
To normalize the ant's random walk, the following equation is used.
$${\text{Y}}_{\text{i}}^{\text{t}}=\frac{\left[{\text{Y}}_{\text{i}}^{\text{t}}-{v}_{\text{i}}^{\text{t}}\right]\times \left[{\text{e}}_{\text{i}}^{\text{t}}-{\text{q}}_{\text{i}}^{\text{t}}\right]}{\left[{\text{u}}_{\text{i}}^{\text{t}}-{\text{v}}_{\text{i}}^{\text{t}}\right]}$$
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Here \({q}_{i}^{t}\) and \({\text{e}}_{\text{i}}^{\text{t}}\) represent the minimum and maximum of ith variable at tth iteration. The variables\({\text{v}}_{\text{i}}^{\text{t}}\) and \({\text{u}}_{\text{i}}^{\text{t}}\) represent the minimum and maximum random walks, respectively.
Step 2:-
ALO mimics the trapping of ants in antlion pits by altering the random walks in their vicinity.
\({\text{q}}_{\text{i}}^{\text{t}}={\text{a}\text{n}\text{t}\text{l}\text{i}\text{o}\text{n}\text{s}}_{\text{j}}^{\text{t}}+\) qt (11)
\({\text{e}}_{\text{i}}^{\text{t}}={\text{a}\text{n}\text{t}\text{l}\text{i}\text{o}\text{n}\text{s}}_{\text{j}}^{\text{t}}+\) et (12)
qt represents the minimum of all variables at ith iteration and et indicate the maximum of all the variable at ith iteration.\({\text{A}\text{n}\text{t}\text{l}\text{i}\text{o}\text{n}\text{s}}_{\text{j}}^{\text{t}}\)represents the position of the selected antlion at ith iteration.
Step 3:-
To improve their chances of survival, larger antlions dig deeper pits. Stronger antlions attract more ants using the roulette wheel mechanism.
Step 4:-
The boundaries of the random walk are decreased to mimic the sliding ants toward antlions.
$${q}^{t}=\frac{{q}^{t}}{I}$$
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$${e}^{t}=\frac{{e}^{t}}{I}$$
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The final step of the algorithm is catching the ants and reconstructing the pits.
Step 5:-
The last operator in ALO is elitism. Fitter antlion formed during optimization is stored in an archive. Random walks on antlions gravitate towards selected ant lions and the elite antlion.
$${\text{a}\text{n}\text{t}\text{l}\text{i}\text{o}\text{n}\text{s}}_{\text{i}}^{\text{t}}=\frac{{R}_{A}^{t}+{R}_{E}^{t}}{2}$$
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3.2 SPECTRUM ALLOCATION USING THE PROPOSED MOALO ALGORITHM
The spectrum band \({a}_{s, b}^{c}\) =1 when it is allocated to SUs and 0 otherwise. The spectrum band is allotted to SUs sequentially. Non-dominated Pareto solutions are obtained. The Pareto solutions are ensured with ranks using a roulette-wheel mechanism. The proposed algorithm used for the improvement of performance metrics such as SE and EE, which is given in Table 1.
Table 1
MOALO Algorithm for Spectrum Allocation
Algorithm 1: | MOALO optimization algorithm used for finding a trade-off between the two independent objectives(SE and EE) |
Step 1: Locate A no of primary users and B no of secondary users using channel C for communication having bandwidth W. Step 2: Calculation of Signal interference and noise ratio for bth secondary users of sth cognitive base station occupying cth spectrum band. Step 3: Define the objective function for optimal spectrum allocation (SE (objective 1) and EE (objective 2)). Step 4: Initialization of MOALO parameters (max_iter, no of population, archive Max Size, and number of objectives) Step 5: Initialization of ant positions and select the elite from the archive Step 6: Random walk around the antlion and its normalization Step 7: Fitness function evaluation Step 8: To find a Pareto front between two objectives, each solution compares with the rest of the solutions in the populations. Either a set of solutions dominates each solution or each solution dominates the domination count. Step 9: Update the archives Step 10: Ranking process(First non-dominated Pareto front considered as a Rank 1) Step 11:Delete some solutions using the Roulette wheel mechanism and from the archive to store new solutions Step 12:Pareto optimal solutions achieved between two objectives |
The flow chart of spectrum allocation using the proposed MOALO algorithm is presented in Fig. 2.In this paper we have also employed MOGOA algorithm for efficient allocation of spectrum. However, its flowchart is not provided due to space limitations.