you can search it in JSSCC
there are many papers about MUX and DEMUX before 2002(sorry , i havent found them in 2003's journals)
just now ,i have a printed paper about them abstracted from JSSCC,VOL 36,NO.6,JUNE,2001
[There are lots of papers in IEEE-MTT transations, make a search.
You can find good information in MYJ book("microwave Filters,impedance matching networks and coupling structures". Also look for books or papers by following authors: David Rhodes, Ian Hunter, Ralph Levy, Randy Rhea, Malherbe. It depends what kind of answer you look for.