Necessity of multi-zone sensing
One of drawbacks of backscatter communication is short communication range due to its principle where reflection of radio wave is used. A coverage area of single interrogator might be insufficient to apply MSMA for SHM of large structures, such as aircrafts and bridges. Such situations can be accommodated by combining coverage areas of multiple interrogators (multi-zone sensing).
Inter-zone interference
A problem to realize multi-zone sensing is existence of sensor nodes located at positions where coverage areas of multiple interrogators are overlapped. Such overlapped areas cannot be eliminated when large structures are covered without any gaps. A sensor node located in the overlapped area generates subcarriers against the all of carriers transmitted by interrogators covering the sensor node. Therefore, if these interrogators independently allocate subcarrier frequencies to sensor nodes in their coverage area, there is a possibility that the same subcarrier frequency is used over multiple interrogators, which results in interference. This is inter-zone interference.
To avoid inter-zone interference, a simple method is channel reservation, i.e. suppresses allocation of the same subcarrier frequency over interrogators whose coverage areas overlap. In addition to basic inter-subcarrier interference rejection of MSMA, development of interference rejection techniques for this inter-zone interference is also on-going.