Manuscript received July 15;
1996;
revised November 21;
1996. This work was support by the National Science Council;
Taipei;
Taiwan;
R.O.C. under Contract NCS85-2215-E-002-009. The authors are with the Department of Electrical Engineering;
National Taiwan University;
10617;
R.O.C. Publisher Item Identifier S 0018-9480(97)02907-4;