Receive antenna shift keying for time reversal wireless communications

Wireless communication systems based on Time Reversal (TR) and a Single Tap Receiver (STR) are optimum at low Signal-to-Noise Ratio (SNR), while keeping low complexity at the receiver. Thus, they are interesting for low order Phase and Amplitude Modulations, such as Binary Phase Shift Keying (BPSK) or Quadrature Phase Shift Keying (QPSK). However, at higher Signal to Noise Ratio, Inter-Symbol Interference prevents from using higher order modulations such as 16 Quadrature Amplitude Modulation (16QAM) or 64QAM. This paper presents a new spatial modulation technique called Received Antenna Shift Keying (RASK), enabling the use of higher order modulation with TR and STR. Information is not conveyed by Phase and Amplitude Modulation (PAM). Instead, at the transmitter side, Time Reversal pre-filtering technique is used to focus a signal with constant amplitude and phase onto a single receive antenna among several receive antennas. Every symbol duration, the transmitter updates the target receive antenna. The index of the antenna is simply equal to the integer value coded by the binary sequence of information that has to be transmitted during the considered symbol. At the receiver side, each antenna has a Single Tap Receiver. The output of the Single Tap Receivers are used by a simple detector to determine the target receive antenna among all antennas. The detected antenna index is then converted into a binary sequence. If N is the number of receive antennas, a N-RASK system transmits Log₂(N) bits per symbol. The performance of 16RASK is compared to 16QAM, with Time Reversal, and 1 bit Time Reversal, using simulations modeling inter-symbol and un-perfect spatial focusing. The study shows that 16RASK outperforms 16QAM, under strong multipath or when the number of transmit antennas is high.