calculate array gain

Can you show me how to calculate gain of antenna array ?

details: An antenna array for BTS (Base Transceiver Station)
assume that: I have an microstrip antenna array; 4 elements. fr = 900MHz (GSM). Trace is bakelite εr = 4.8 , Dielectric is copper. d = λ\2.

Try to help me because i don't have money to buy book....I'm a student

Very thanks for help me: dungdn@gmail.com

Added after 3 hours 46 minutes:

I used Ansoft Ensemble to simulate a microstrip antenna array.
Power Gain = ?

try
http://www.ce-mag.com/archive/02/Spring/fogelle2.html
http://www.bbwexchange.com/turnkey/p...nk_budgets.asp

PL