In-Band Full Duplex (IBFD) transceivers can potentially provide significant increases in spectral efficiency by allowing simultaneous transmit and receive on the same frequency at the same time. However any such gains are conditional on effective cancellation of the self-interference. The performance of a self-interference cancellation transceiver depends not only on the system design, but also on implementation specific parameters of the transceiver and cancellation circuitry. Thus, realistic evaluation of cancellation performance can only be performed using hardware prototypes. This paper provides a detailed overview of a highly flexible and reconfigurable self-interference cancellation testbed, which combines active cancellation with either separate Tx and Rx antennas, or an Electrical Balance Duplexer, along with digital baseband cancellation. The system can facilitate investigation on self-interference cancellation performance across a range of physical layer configurations, and provide a platform for the rapid prototyping of digital baseband algorithms within an operational self-interference cancelling transceiver.

Electrical balance duplexers can provide high transmit-to-receive isolation whilst facilitating transmission and reception from a single antenna, can be implemented on-chip, and are widely tunable, making this an attractive technology for implementing full duplex architectures in small form factor devices. This paper presents measurements from a novel hardware prototype full-duplex transceiver architecture combining electrical balance and active analog cancellation. The prototype duplexer achieves >80dB transmit-to-receive isolation over a 20MHz bandwidth at both 890MHz and 1890MHz, exceeding the performance of antenna separation architectures where the antenna isolation is limited to the levels achievable in hand held devices.

A key issue impacting the transmit to receive (TX-RX) isolation in Electrical Balance (EB) Duplexers is dynamic matching of the antenna impedance by the balancing network. Circuit simulations including embedded antenna measurements have been used to investigate the variation in TX-RX isolation due to interaction with the user and the local environment. Simulated results at 847MHz and 1960MHz show user interaction causes over 25dB of variation in the mean isolation over a 20MHz bandwidth. However, in reflective indoor environments, user interaction on average improves TX-RX isolation. This observed performance improvement is a result of reduced antenna radiation efficiency, which makes reflections from objects at longer ranges less significant and thereby reduces frequency domain variation in the antenna reflection coefficient.

This paper investigates electrical balance (EB) in hybrid junctions as a method of achieving transmitter-receiver 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 that include embedded antenna measurements show a mean isolation of 62 dB over a 20-MHz bandwidth at 1.9 GHz 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 39 dB of attenuation in the radio echo for a highly reflective indoor environment at 1.9 GHz and that the mean isolation of an EB duplexer is reduced by 7 dB in this environment. A full duplex architecture exploiting EB is proposed.