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.

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Electrical balance duplexing enables simultaneous transmit and receive from a single antenna, however the transmitto-receive isolation depends on the ability of the balancing algorithm to determine the correct balancing impedance. A novel balancing algorithm based on in-situ characterization of the duplexer self-interference channel is proposed. The algorithm requires no a-priori knowledge of the antenna impedance or hybrid junction characteristics, and automatically compensates for circuit imperfections. A novel balancing network implementation which uses active signal injection is also proposed. A hardware prototype implementing the proposed balancing algorithm and combining passive and active balancing techniques has achieved 81.5dB isolation over an 80MHz bandwidth.

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Abstract-In-band full-duplex relaying has been of recent interest as it can potentially double spectral efficiency and decrease latency, thus improving throughput to the end user. The bottleneck in enabling full-duplex operation is the selfinterference (SI) due to the relay's own transmission, which must be mitigated at the antenna, radio frequency and digital domains. In the case of compact back-to-back relays which are proposed for outdoor-to-indoor relaying, the SI comprises direct coupling and multipath components. This paper models the SI channel across 300 MHz bandwidth at 2.6 GHz in two indoor environments with a back-to-back relay antenna. The power delay profile of the SI channel is modelled as a single decaying exponential function with specular components represented by delta functions. The fading characteristics of each tap are modelled by a normal distribution based on the measurements. The proposed model can be used to generate a tapped-delay model of the SI channel between compact back-to-back antennas for use in link-level simulations and hardware in the loop emulation.

Abstract-This paper analyses the effect of phase noise (PN) on digital non-linear self-interference (SI) cancellation for low delay spread non-linear SI channels typical of small form factor in-band full-duplex (IBFD) radios in indoor environments. Use of a shared local oscillator (LO) between the transmit and receive radios is assumed, and it is shown that, in theory, un-cancelled SI due to PN can be reduced by optimising the relative delay between Rx LO path and SI channel. Simulations and measurements from hardware IBFD transceivers using realistic LO phase noise characteristics demonstrate that, for these devices/environments, in practice LO phase noise does not limit digital cancellation when using shared local oscillators.Index Terms-In-band full-duplex, shared local oscillator, phase noise, non-linearity.

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