This paper addresses robust communication in multiple-input multiple-output (MIMO) environment in reactive jamming situation. Jamming signal has been a problem in wireless communication systems. Reactive jamming is the most efficiient and effective one since it is not easily detected. If the channel from jammer to receiver is known, interference cancellation techniques can be employed to cancel out the jamming signal. In most cases, unfortunately, it is difficult to estimate the jammer channel at the receiver side. Therefore a method for jamming cancellation without jammer channel knowledge is needed.
We propose iterative channel tracking and jamming signal cancellation methods to make communication possible when there is a reactive jamming signal. We considered uplink MIMO system with single-antenna jammer sending jamming signal to the receiver.
If initial signal channel is known at the receiver, it is possible to track jammer channel ratio and signal channel. Jammer channel ratio refers to the ratio among entries of channel from jammer to the receiver. Since jamming signal cannot be known at the receiver, it is an alternative method to track jamming channel ratio, which represents jammer direction.
If the signal channel and the transmitted signal are known, it is possible to the jammer channel ratio. On the other hand, if the jammer channel ratio and transmitted signal are known, signal channel can be estimated. Therefore it is possible to alternatively and iteratively estimate the signal channel and the jammer channel ratio.
After channel tracking and estimation, jamming signal cancellation is performed. Jammer channel ratio can be used to cancel out the jamming signal. In this paper we assumed the number of receiver antennas is twice the number of transmitter antennas. The received signals are paired. Each pair is scaled and subtracted from each other so that jamming signal is cancelled out. After jamming signal cancellation, least square algorithm is employed to decode the transmitted signal.
We compared proposed jamming signal cancellation method with conventional zero-forcing scheme. There was no performance degradation even if proposed method only uses jammer channel ratio. On the other hand, zero-forcing scheme requires perfect knowledge of jammer channel. Therefore, proposed scheme has contribution in that it requires less information but still provides same performance. Iterative channel tracking error degrades bit error rate (BER) performance. However, communication is still possible with the iterative channel tracking and jamming signal cancellation schemes. To compensate for the performance degradation, we shortened the length of transmission at a time. The BER performance of shorter sequence of symbols was better than that of longer sequence of symbols. Therefore, it can be inferred that frequent short burst of signal transmission is more advantageous than sporadic long burst of transmission.
Proposed method makes communication possible through iterative channel tracking and jamming signal cancellation even in the jammed scenario. It has been shown that this iterative and alternative
channel tracking method can be applied to MIMO systems.