A wearable circulator-like circularly polarised antenna for full-duplex wireless body area network applications

A circulator-like high gain, unidirectional, dual circularly polarised (DCP) wearable antenna with a low specific absorption rate of 0.088 W/kg for 5.8 GHz industrial, scientific, and medical (ISM) band ($5.725-5.875$ GHz) full-duplex wireless body area network (WBAN) applications is proposed. The DCP full-duplexing is achieved by a closed-loop feedback structure between its orthogonal-ports with enhanced axial ratio (AR) bandwidth and good impedance matching. The antenna is backed with a $7\times 7$ electromagnetic bandgap array that allows for improved directionality, AR and isolation between the ports. The cross-polarisation level above 17 dB is attained in the broadside direction, in both the E-plane and the H-plane, indicating good polarisation discrimination. Since the total power at each respective port is maintained and not halved ($-3$ dB) with the measured port isolation of $˜28$ dB in real scenarios (with the potential evidence of $˜34$ dB) within the ISM band, that leads to a circulator-like antenna, meaning that a typically cascaded circulator/duplexer can be relieved and leave to the digital processing any self-interference challenge of full duplex systems through digital cancelation techniques, which alleviates the overall costs of RF hardware and eases integration. Even though dual linear polarization as well as DCP can be supported, the closed-loop feedback structure uses a port as Tx (RHCP) and the other one as Rx (LHCP) simultaneously with enhanced AR bandwidth. Without this structure, it would require a circulator/duplexer for DCP full-duplex operation and the associated added insertion losses and/or $\frac{1}{2}$ power (if duplexers) for operation, together with a polarisation misalignment problem that is undesired in wearable WBAN applications.