EMI Suppression Technique for ISM-Band WLANs Using Multicode Transmission and EMI Observation Channel

It is well known that the electromagnetic interference (EMI) radiated from industrial, scientific and medical (ISM) apparatus seriously degrades the performance of wireless communication systems. In this paper, an ISM-band wireless local area network (WLAN) which employs a direct sequencespread spectrum (DS-SS) system is designed to be sufficiently robust and reliable to operate in the presence of this EMI. To satisfy this goal, a technique to suppress EMI in the DS-SS system using a multicode transmission and an EMI observation channel is proposed. In the study, the EMI radiated from switching-type microwave ovens (MWO interference) which are ISM apparatus is concerned, and for a tractable investigation, a statistical model to represent MWO interference based on experimental measurement results is employed. As well known that MWO interference exhibits a bursty impulsive characteristic in time, a technique to transmit multiple long spreading codes (multicode transmission) is introduced for the DS-SS system to overcome the burstness of the interference. Moreover, inspired by the broadband in frequency of MWO interference, a technique to estimate the channel MWO interference by means of observing its levels in an adjacent channel is proposed, and this technique is applied to construct a multicode DS-SS receiver which can suppress the channel MWO interference. An evaluation of the bit error rate performance of the proposed multicode DS-SS system is conducted by computer simulation, and the numerical results demonstrate that the proposed DS-SS system can operate effectively even in the presence of MWO interference. key words: DS-SS, multicode transmission, MWO interference,