Implementation and Evaluation of Synchronized SS-CDMA Using Wireless Two-Way Interferometry (Wi-Wi)

Synchronized spread spectrum code division multiple access (SS-CDMA) is very effective for increasing the capacity and reducing the interference with a rapid spread of Internet of things (IoT) devices. Since the synchronized SS-CDMA requires receiving timing synchronization, it is essential to realize transmission timing control of each node using space-time synchronization. In this paper, we investigate precise time synchronization between nodes using Wireless Two-Way Interferometry (Wi-Wi). The measurement results show that the standard deviation of offset ($\sigma$) was less than 30ns. Furthermore, we implement a synchronized SS-CDMA communication function on Universal Software Radio Peripheral (USRP). The bit error rate (BER) characteristics are evaluated with different offsets of initial timing synchronization of the Wi-Wi module. It is found that BER characteristics when two signals are transmitted simultaneously become smaller as the Wi-Wi offset becomes smaller. For an offset of around 12 ns, the degradation of BER compared to the case with 1 transmission signal is negligibly small. As a result, we reveal that simultaneous communication between two terminals is possible without large degradation.