Optimum Single Antenna Full Duplex Using Hybrid Junctions

Abstract

Abstract-This paper investigates electrical balance (EB) in hybrid junctions as a method of achieving transmitter-receiver (TX-RX) isolation in single antenna full duplex wireless systems. A novel technique for maximizing isolation in EB duplexers is presented, and we show that the maximum achievable isolation is proportional to the variance of the antenna reflection coefficient with respect to frequency. Consequently, antenna characteristics can have a significant detrimental impact on the isolation bandwidth. Simulations which include embedded antenna measurements show a mean isolation of 62dB over a 20MHz bandwidth at 1.9GHz, but relatively poor performance at wider bandwidths. Furthermore, the operational environment can have a significant impact on isolation performance. We present a novel method of characterizing radio reflections being returned to a single antenna. Results show as little as 39dB of attenuation in the radio echo for a highly reflective indoor environment at 1.9GHz, and that the mean isolation of an EB duplexer is reduced by 7dB in this environment. A full duplex architecture exploiting electrical balance is proposed.

Electrical balance duplexing for small form factor realization of in-band full duplex

“…Fig. 4 compares these results to simulations reported in [6] which incorporate a measured antenna reflection coefficient (and therefore model the real antenna perfectly), and to novel measurements from a discreet hardware prototype EB duplexer including antenna presented here.…”

“…Analog and digital cancellation techniques are key enabling technologies for IBFD systems [4], whilst antenna based isolation techniques provide passive isolation by exploiting signal directionality at the antenna port [5], [6], or by using separate antennas for transmitting and receiving [3], [7]. Antenna separation architectures utilise separate transmit and receive antennas, and can exploit directional antennas and shielding to improve isolation [7].…”

“…This property of "Electrical Balance" can be exploited to isolate transmitter and receiver circuitry, but allow use of a shared propagation medium. The wireless duplexing application [6], [10], [11] is entirely analogous to the telephone duplexer, but substitutes the microphone, earpiece, and telephone line with the transmitter, receiver, and antenna respectively, as shown in In a wireless electrical balance duplexer the transmit current, supplied by the Power Amplifier (PA), enters the hybrid at the center tap of the primary winding (as shown in Fig. 1.b) and is split between two paths flowing in opposite directions, with one component flowing to the antenna (and being transmitted), and the other to the balancing impedance.…”

“…Furthermore, since the selfinterference signal is spread across the transmit band, maximising the isolation at the carrier frequency is not necessarily useful if this results in poor isolation in other parts of the transmit band. Instead, the goal is to maximize the receiver SNR, which is achieved by minimizing the mean Tx-Rx gain taken across the whole Tx band [6]. This optimum balancing occurs when the balancing impedance is chosen to minimize the mean square error between the balancing reflection coefficient and the antenna reflection coefficient: MMSE balancing.…”

“…This optimum balancing occurs when the balancing impedance is chosen to minimize the mean square error between the balancing reflection coefficient and the antenna reflection coefficient: MMSE balancing. It can be shown [6] that for simple balancing networks, such as the parallel RC circuits used in [11] and [10], that this maximized isolation is proportional to the variance of the antenna impedance with respect to frequency.…”