YOu pick the indices and get what you want.
x= [1 2 -6 9 -7 8 -8 -5 2 4 -6 4 -4 -5 -7 6 10 2 3 7 -8 9 -8 -5 4 -6]
Xp=x(x>0) ; idxp = x>0 ;
Xn=x(x<0) ; idxn = x<0 ;
% after separated the signal is transmitted
fr=[length(Xp), Xp, Xn]
% then in the recevier Rx
rp=fr(2:fr(1)+1); % positive Rx
rn=fr(2 + fr(1):end); %negativ Rx
iwant = zeros(size(x)) ;
iwant(idxp) = rp ;
iwant(idxn) = rn ;
Note that, the above code can be optimized by using completely indices.
