In conventional full-duplex radio communication systems, the transmitter (Tx) is active at the same time as the receiver (Rx). The isolation between the Tx and the Rx is ensured by duplex filters. However, an increasing number of long-term evolution (LTE) bands crave multiband operation. Therefore, a new front-end architecture, addressing the increasing number of LTE bands, as well as multiple standards, is presented. In such an architecture, the Tx and Rx chains are separated throughout the front-end. Addition of bands is solved by making the antennas and filters tunable. Banks of duplex filters are replaced by tunable filters and antennas, providing a duplexer function over the air between the Tx and the Rx. A hardware system has been designed and fabricated to demonstrate the performance of this front-end architecture. Measurements demonstrate how the architecture addresses inter-modulation and Rx desensitization due to the Tx signal. The filters and antennas demonstrate tunability across multiple bands. System validation is detailed for LTE band I. Frequency response, as well as linearity measurements of the complete Tx and Rx front-end chains, show that the system requirements are fulfilled.Index Terms-Low-noise amplifier (LNA), power amplifier (PA), tunable filters, tunable front-end, tunable microelectromechanical systems (MEMS) capacitors, tunable narrowband antennas.

The conventional full-duplex radio communication systems require that the radio transmitter (Tx) is active at the same time as the radio receiver (Rx). The Tx and the Rx are using separate dedicated frequency bands and the Tx-Rx isolation is ensured by duplex filters. However, increasing number of frequency bands crave for multiband and multimode operation, which either require agile duplexers or a bank of narrow-band filters with a switch. While practical agile duplexers are not available, a bank of narrow-band filters with a switch is bulky and incurs switching loss. This paper proposes an approach that separates the Tx and Rx chain throughout the front end (FE). The complexity of the FE is reduced dramatically by replacing the duplex filters with tunable filters and closely integrating the tunable antennas in the FE, providing filtering which can be used to lower requirements for the tunable filters. For this purpose, very small narrow-band antennas are designed, which can cover 1710-2170 MHz by using tunable capacitors. Simulations and measurements of the antenna concept are carried out in the proposed FE architecture, serving as a proof of concept.Index Terms-Capacitive loaded antenna, highantenna, inductive loaded antenna, MEMS, radio frequency (RF) front-end (FE), Rx antenna, transmitter-receiver isolation, tunability, Tx antenna.

No abstract

To address the antenna design challenges posed by many frequency bands, introduced with long-term evolution deployment, this study proposes the use of separate transmit (Tx) and receive (Rx) narrow-band antennas. In addition, a diversity Rx (Dx) antenna is needed for multiple-input multiple-output performance. Although the isolation between two antennas at low frequencies (700 MHz) is crucial for the successful implementation of 4G in handheld terminals, it becomes more challenging when considering isolation among three antennas (one Tx and two Rx antennas) at low frequencies. Hence, a method that improves the isolation between the ports of one Tx and two Rx antennas is presented here.

Tunable antennas are very promising for future generations of mobile communications, where antennas are required to cover a wide range of operating bands. This reported work was aimed at characterising the loss mechanism of tunable antennas. Tunable antennas typically exhibit a high quality factor (Q), which can lead to thermal loss due to the conductivity of the metal. The investigation shows that copper loss is non-negligible for high-Q values. In the proposed design, the copper loss is 2 dB, for a Q of 260 at 700 MHz.Introduction: With the band proliferation that followed the standardisation of the fourth generation (4G) of mobile communications, active antennas have been investigated to enhance the operating bandwidth of mobile phone antennas while keeping a low profile. Active antennas can reconfigure their resonance frequency using microelectromechanical systems [1], pin diodes [2] or varactors [3]. These active components will add a varying reactance to the impedance of the antenna, thus modifying its resonance frequency. A recent overview of the tuning techniques is given in [4]. Independently of the tuning technique, when the antenna is forced into resonance at a lower frequency than its natural frequency, its bandwidth decreases and its quality factor, Q, increases inversely proportionally [5]. As the Q of the antenna increases, its efficiency decreases due to higher currents in the equivalent series resistance of the tuner. For high-Q values, the loss due to the tuner alone cannot explain the measured total loss [6]. In the work reported in this Letter, the authors investigated the existence of a thermal loss in high-Q antennas, due to the conductivity of copper. For this investigation, the authors have designed a large patch antenna, naturally resonating at 700 MHz, as it is the lowest frequency to reach with 4G nowadays [7]. Different widths of the patch result in different antenna Q, while maintaining the resonance frequency. The measured radiation efficiencies are compared to determine the influence of Q on the thermal loss of antennas.