1. Department of Electrical Engineering, Copperbelt University, Kitwe 10101, Zambia
2. Department of Mathematics, University of Dar-es-Salaam, Dar-es-Salaam 16101, Tanzania
3. Council for Scientific and Industrial Research, Pretoria 0083, South Africa
4. Department of Electrical and Electronic Engineering Science, University of Johannesburg, Johannesburg 2006, South Africa