Leo Laughlin

Optimum Single Antenna Full Duplex Using Hybrid Junctions

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.

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

Transceiver architectures utilizing various self-interference suppression techniques have enabled simultaneous transmission and reception at the same frequency. This full-duplex wireless offers the potential for a doubling of spectral efficiency; however, the requirement for high transmit-to-receive isolation presents formidable challenges for the designers of full duplex transceivers. Electrical balance in hybrid junctions has been shown to provide high transmit- to-receive isolation over significant bandwidths. Electrical balance duplexers require just one antenna, and can be implemented on-chip, making this an attractive technology for small form factor devices. However, the transmit-to-receive isolation is sensitive to antenna impedance variation in both the frequency domain and time domain, limiting the isolation bandwidth and requiring dynamic adaptation. Various contributions concerning the implementation and performance of electrical balance duplexers are reviewed and compared, and novel measurements and simulations are presented. Results demonstrate the degradation in duplexer isolation due to imperfect system adaptation in user interaction scenarios, and requirements for the duplexer adaptation system are discussed.

Passive and Active Electrical Balance Duplexers

Electrical balance duplexing enables simultaneous transmit and receive from a single antenna; however, the transmit-to-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 that uses active signal injection is also proposed. A hardware prototype implementing the proposed balancing algorithm and combining passive and active balancing techniques has achieved 81.5-dB isolation over an 80-MHz bandwidth.

Performance variation in electrical balance duplexers due to user interaction

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.

A Widely Tunable Full Duplex Transceiver Combining Electrical Balance Isolation and Active Analog Cancellation

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.

Electrical balance isolation for flexible duplexing in 5G mobile devices

Exploiting new technologies will be vital in meeting the demanding requirements of 5G radio access. Flexible duplexing architectures will be a key enabling technology in 5G handsets, virtualising the radio spectrum, reducing cost, and increasing link capacity through full duplex radio communication. The tunability, low cost and small form factor of Electrical Balance (EB) duplexing technology makes it an attractive choice for flexible duplexing in small form factor devices. Circuit simulations incorporating measured antenna data have been used to investigate the performance of a dual notch EB duplexer in a full duplex mode at 1900MHz. Results show that, in general, the isolation bandwidth of a dual notch EB duplexer increases with the notch separation frequency, and that at bandwidths below 75MHz, dual notch balancing achieves transmit to receive isolation within 3dB of optimal balancing. The simulated duplexer provides a 30dB isolation bandwidth of up to 200MHz. Therefore, in addition to providing tunable frequency division duplexing functionality, EB duplexing technology can also serve as a component of a broadband full duplex transceiver, thus providing a fully integrated flexible duplexing solution.

Electrical Balance Duplexer Field Trials in High-Speed Rail Scenarios

Electrical balance duplexers (EBDs) present a potential alternative to the fixed-frequency duplexing filters used for frequency division duplexing in cellular handset radio frequency front ends. However, the transmit-to-receive (Tx–Rx) isolation can be affected by interaction between the antenna and the environment, and therefore, the EBDs balancing impedance must adaptively track time-domain antenna impedance variation. A rail scenario presents a potentially demanding use case for an EBD, as fast moving trains in the vicinity of the antenna may cause dynamically changing reflections, which can be received as self-interference. In this paper, measured dynamic antenna reflection coefficients at 745 and 1900 MHz from train mounted antennas are included in the EBD circuit simulations in order to investigate the resulting variation in Tx–Rx isolation, and determine requirements for balancing impedance adaptation. This paper also presents the results from rail-based field trials of a hardware prototype EBD, which implements real-time antenna impedance tracking. Results show that the rail scenario does result in variation in Tx–Rx isolation, but that rebalancing the EBD at the intervals of 5 ms was sufficient to maintain >50 dB isolation for ~95% of the time.

Flexible duplex transceivers for 5G and beyond wireless access

There is now significant interest in the use of signal cancellation based architectures in an attempt to replace frequency domain filtering within the duplexing function for next generation wireless transceivers. This approach can potentially eliminate the need for band specific filters, which inhibits global operation of LTE handsets due to the diverse band plan. Further, using such methods in-band full-duplex communications is also possible, thus potentially doubling the spectrum efficiency when compared with conventional techniques.