Idea and abstract of MSMA
Structural Health Monitoring (SHM) requires perfectly-synchronized sensor data from multiple sensors. Noel et.al. introduced a set of quantitative requirements: 100~1000Hz sampling rate, 70 or more sensor nodes, and synchronization error less than 120us. Multi-Subcarrier Multiple Access (MSMA) answers to these requirements by realizing frequency-efficient simultaneous streaming from multiple backscatter sensor nodes preserving their simple structure.
Existing applications of backscatter communication uses TDMA-based method to retrieve information from multiple nodes, i.e. single node sends at certain time. On the other hand, SHM requires simultaneous streaming of sensor data from multiple sensor nodes. Subcarrier backscatter can be generated at any frequency by changing subcarrier frequency, i.e. changing frequency of the RF switch, so it seems that FDMA can be realized by allocating subcarrier frequency to each node without duplication. However, subcarrier generation by changing 2 state of RF switch results in occurrence of harmonics at higher frequencies which interfere other nodes generating subcarriers at those frequencies.
Allocating subcarrier frequencies to avoid interfered frequency is a simple solution. Common FDMA systems introduce suppression of harmonics at the node side. On the other hand, we have developed a technique to reject interfering harmonics by signal processing at the interrogator side. By combining sensor nodes, whose functionalities are the same to those of commercially-available sensor-enabled RF tags, and interrogators implementing this interference rejection technique, MSMA can realize sensor data collection for SHM with battery-free sensor nodes.
Principle of inter-subcarrier interference rejection
Subcarrier is generated by changing 2 state of the RF switch by subcarrier frequency. In other words, power of backscatter is changed to 2 state, i.e. rectangle wave of subcarrier frequency is multiplied on continuous wave from the interrogator, to generate subcarrier.
Fourier series expansion of this backscatter signal shows that the signal contains not only signal component at subcarrier frequency but also harmonics at its odd-multiple frequency (x3, x5, x7, …) whose power is multiplicative inverses (x1/3, x1/5, x1/7, …). This interfering/interfered relationship can be written with a lower-triangular matrix. Therefore, original signal backscattered from each node can be obtained by iterative calculation of harmonics replica from received signal and subtraction of calculated replica from interfered subcarrier signals. This is a basic principle of inter-subcarrier interference rejection of MSMA.
