High-Order Code Shift Keying for Ambient Backscatter Communications

Ambient backscatter communication (AmBC) is a new advanced technology that utilizes ambient radio frequency signals to enable the communications of battery-free devices and has attracted much attention recently. Existing works develop the high-order modulation in signal domains including the time domain, the frequency domain and the space domain. However, the work in the code domain for AmBC remains missing. In this letter, we extend the high-order modulation to the code domain, i.e., high-order code shift keying (CSK). However, the detection that correlates the received signals with the candidate code cannot work due to the unknown source signals. To detect the active code index, the correlation energy detection (CED) is proposed in which the received signal energy instead of the amplitude is utilized. In this process, CED does not need the channel state information and source power knowledge. Simulation results show that the proposed CED can handle the complex source signal and achieve a desirable bit error rate (BER). The second-order CSK without the training sequences outperforms the on-off keying modulation with training sequences in terms of BER.