Maourrip nceived February 7;
1994;
revised;
July;
8;
1994. This work was arppared by !he US. Army Research Office under Grant DAAH04-93-G-02aI. The author is with the Depamnent of Electrical Engineering and Computer Sciences and tbe Elemxmics Research Laboratory;
University of Califomia;
BerLeley;
CA 94720 USA. IEEE Log Number 9405726;