hi,do it this way:
1:convert the 232 signal to digital sigal use a chip such as MAX232,maybe you need a cpu to controll the other signals;
2:mudulate the signal with FM,fsk,msk or ther methods,input the mudulated signal to a rf transformer to sent out;
now many chips can do this easily.if your data rate is not too high,it will be very easy.try these site for useful infomations:
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and many company have chips familar,but you did not notice what you need.hope this can help you.