Mikko Heino

Recent advances in antenna design and interference cancellation algorithms for in-band full duplex relays

In-band full-duplex relays transmit and receive simultaneously at the same center frequency, hence offering enhanced spectral efficiency for relay deployment. In order to deploy such full-duplex relays, it is necessary to efficiently mitigate the inherent self-interference stemming from the strong transmit signal coupling to the sensitive receive chain. In this article, we present novel state-of-the-art antenna solutions as well as digital self-interference cancellation algorithms for compact MIMO full-duplex relays, specifically targeted for reduced-cost deployments in local area networks. The presented antenna design builds on resonant wavetraps and is shown to provide passive isolations on the order of 60-70 dB. We also discuss and present advanced digital cancellation solutions, beyond classical linear processing, specifically tailored against nonlinear distortion of the power amplifier when operating close to saturation. Measured results from a complete demonstrator system, integrating antennas, RF cancellation, and nonlinear digital cancellation, are also presented, evidencing close to 100 dB of overall self-interference suppression. The reported results indicate that building and deploying compact full-duplex MIMO relays is already technologically feasible.

Compact Inband Full-Duplex Relays with beyond 100 dB Self-Interference Suppression: Enabling Techniques and Field Measurements

In this communication, the self-interference (SI) channel and the novel enabling techniques for a compact inband full-duplex relay are described and characterized in different operating environments. The full-duplex operation is based on a novel antenna design that uses wavetraps to provide passive isolation of up to 70 dB between the transmit and receive antenna ports. The passive isolation is complemented with novel active RF and digital cancellation stages that further suppress the residual SI to the receiver noise floor. Measurement results of a complete prototype implementation show that the proposed design can achieve an overall SI cancellation performance of over 100 dB even with an ambitious instantaneous bandwidth of 80 MHz. Similar results are obtained both in an anechoic chamber as well as in realistic multipath indoor environments.

Design of Wavetraps for Isolation Improvement in Compact In-Band Full-Duplex Relay Antennas

In full-duplex (FDX) multiple-input and multiple-output (MIMO) systems, the self-interference between each transmitting and receiving antenna, caused by finite isolation between antennas, limits the obtainable signal-to-interference ratio of the system and thus the total capacity. In this paper, the design of planar wavetraps for isolation improvement between multiple antennas is introduced, with an emphasis on back-to-back relay structures. General guidelines for the design of planar wavetraps are deduced which could be applied also in isolation improvement of other antenna structures. A prototype back-to-back relay antenna with two dual-polarized patches for FDX MIMO is presented with the frequency of operation of 2.6 GHz. The prototype antenna is manufactured and the simulated results are validated with measurements. The minimum isolation among each antenna pair on the opposite sides of the relay is improved by 20 dB across a 18-MHz bandwidth, while 15-dB improvement can be obtained across 167-MHz bandwidth, yielding total isolation levels of 70 and 65 dB, respectively. The radiation pattern of the relay antenna is measured with and without wavetraps to show that the wavetraps do not have significant effect on the main lobe radiation properties.

RFID-Based Book Finder [Education Corner]

Locating items rapidly and accurately has become a crucial part of our modern society and industry. Accurate locating not only saves time and money but also reduces waste, as products do not get lost along supply chains. One promising indoor tracking method is provided by radio-frequency identification (RFID) technology. The main benefit of RFID technology is the ability to inventory items simultaneously and rapidly without the requirement of line of sight to the target. Because RFID tags are attached to various objects and are used in different environments, RFID engineers are required to design RFID tags that operate reliably within varying environments and medium materials. In this article, an RFID system is introduced to locate library books at the Aalto University School of Electrical Engineering. In the suggested approach, the books placed outside the main library are equipped with an ultrahigh frequency (UHF) RFID tag and can be located using a hand-held RFID reader device (Figure 1). In addition to locating the books, the system keeps the book inventory up to date.

Feasibility of multi-probe over-the-air antenna test methods for frequencies above 6 GHz

This paper discusses the feasibility of two multi-probe over-the-air antenna test methods at frequencies higher than 6 GHz. The two methods considered in this paper are prefaded signals synthesis (PFS) and plane wave synthesis (PWS) methods. Both methods suffer from huge challenges of the large number of dual-polarized probes required in emulating realistic standing waves in the antenna test zone, because physically small antennas become electrically large at the higher frequencies. However, the PFS method can take advantage of the nature of radio propagation channels at the higher frequencies, i.e., the sparsity of multipath channels and their angular selectivity, in order to reduce the number of active probes and hence signal feeds from a fading emulator. In contrast, the PWS method always needs signal feeds to all the probes and hence the implementation complexity is not reduced even under sparse multipath channels. The required number of probes is derived for the two methods using heuristic theoretical analyses and through practical antenna examples on a mobile-phone-sized ground plane at 5, 15, 30 and 60 GHz.

T-shaped decoupling network for wideband isolation improvement between two strongly coupled antennas

A compact T-shaped network for decreasing the port-to-port coupling between two strongly coupled antennas is investigated. The proposed circuit comprises of a pair of transmission lines followed by the T-shaped lumped element network connected between the two feeds of the antenna. Finally, a simple L-section matching circuit is added to each port to retune the input impedance matching. The proposed network gives a dual-resonant decoupling performance achieving isolation levels of 20 dB across 250 MHz bandwidth at 2.4GHz. The length of the transmission line is designed to introduce a trade-off between the decoupling level and bandwidth. This T-shaped network does not significantly decrease the antenna radiation efficiency. The experimental results coincide with the simulations. The decoupled closely spaced antennas could be used for Multiple-input Multiple-Output (MIMO) applications.

Double loop matching technique for robust UHF RFID tag antennas

This paper presents a double resonance UHF RFID antenna suitable for most capacitive chips. The antenna incorporates two inductive loops that facilitate broadband matching. The paper studies how different dimensions in the antenna affect the input impedance. It is shown that the input impedance can be tuned with three structural parameters. The wide impedance bandwidth of the antenna improves the antenna performance when the environment varies making it more robust.

Antenna decoupling structure for a single-frequency full-duplex planar dipole array

Single-frequency full-duplex wireless communication (SFD) can increase the efficiency of bandwidth utilization. SFD can be achieved by reducing the level of the self-interference between the transmitter (TX) and receiver (RX) to the RX noise floor. In this paper, a full-duplex planar antenna array with high isolation between omnidirectional dipole antennas on the horizontal plane is introduced. The high isolation between the RX and TX antenna is achieved with a decoupling structure between the antennas. The isolation is simulated and measured to be more than 50 dB for a bandwidth of 11 % and the decoupling structure is verified not to alter the radiation patterns of the dipole antennas.