clear;clc 
x=[ 0 2 4 6 8 10 12 14 16 18 20 ]; 
y=[ -0.01 0.045 0.12 0.2 0.33 0.52 0.67 0.95 1.20 1.45 1.78 ]; 
P1 = POLYFIT(x,y,1); 
P2 = POLYFIT(x,y,2); 
P3 = POLYFIT(x,y,3); 
y1=polyval(P1,x); 
y2=polyval(P2,x); 
y3=polyval(P3,x); 
plot(x,y,'-o',x,y1,'-*') 
xlabel('SNR/DB'),ylabel('BER  QPSK') 
legend('LS','LMMSE','2','3',2) 
axis([0 20 0 10^-4])