Its a random variable. You can generate it like this: 

X_dB = sigma*randn(1)

Every time you call this function, you will have a random value generated.

HTH

thank you

Can you give me the mathematical expression that gives me that exact amount, Please?

Salam alikom brother, 

For more acurate modelisation you must refere to this book :

https://books.google.dz/books?id=N-mdDXvs0kAC&printsec=frontcover&dq=wireless+channel+modeling&hl=fr&sa=X&ved=0ahUKEwjyj_6ym6TVAhXG5iYKHUFEAQUQ6AEIJjAC#v=onepage&q=wireless%20channel%20modeling&f=false

I can share with you this book and matlab program... Just send me your e-mail. Good luck 

Dear Tahereh,

welcome,

If you read the paper to near its end you will find data given about sigma: PLEASE I COPIED THE NEXT PARAGRAPH FROM THE PAPER :In this work, path loss of each node is calculated from its distance

to sink with 2.4 GHz frequency. We use path loss coefficient

of 3.38 and the value of standard deviation sigma is set at 4.1 (refer Section

2 of Section 3).

From the paper in the link:http://www.ee.columbia.edu/~roger/guest_lecture_wireless_mobile.pdf

you can get a value of possible loss due to fading: f(P)= 1/ (sigma sqroot of 2 pi) exp -(P-Pr)^2/2 sigma^2. where Pr is the average power.

assigning a value for f between 0 and 1 one gets p-pr which is equal to the fading margin.

Best wishes