A Tunable RF Front-End With Narrowband Antennas for Mobile Devices

Abstract

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.

Tunable Frequency-Division Duplex RF Front End Using Electrical Balance and Active Cancellation

“…if using an LNA with integrated SI cancellation). This analysis also assumes Tx noise in the Rx band of -129 dBm/Hz at the PA output, and -153 dBm/Hz at the RFIC output, which is typical for LTE user equipment (UE) RF front-ends [4], [35]. When tuned to maximize isolation in the Rx band, we may assume the EBD provides around 25 dB of Tx-Rx isolation in the Tx band.…”

“…Thirdly, the instantaneous filter bandwidth requirement is relatively narrow, being at most 20 MHz (for both Tx-band bandpass filter or Rx-band notch filter implementations) which is a 2.5% relative bandwidth at 800 MHz, allowing a relatively selective tunable bandpass/bandstop filter to be applied to achieve a steeper roll off. Achieving adequate noise rejection from the AC path filter is therefore realistic using currently available tunable filtering technologies suitable for integration; for example the micro-electro-mechanical system (MEMS) based tunable filters published in [4] achieve roll-off from 5 dB IL to 25 dB rejection over a relative bandwidth of <5%, as required for this application.…”

“…An example RFFE implementation is shown in Fig. 2; using the MEMS filter implementation given in [4], this integrated front-end would require just two external high-Q surface mount inductors for the AC path filter.…”

“…Since all of the subsystems in this design have previously been implemented on-chip and have demonstrated the necessary power handling and linearity for cellular applications (e.g., EBDs in [16], [17], [21], active cancellation in [11]- [13], and MEMS tunable filters in [4]), it is clear that integrated circuit implementation of this design is possible. However, rather than fabricating a CMOS prototype to determine on-chip performance, the goal of this work is to experimentally validate the feasibility of the proposed design, prior to embarking on the design of a prototype chip (the function of which also requires system integration with baseband processing, adding substantial complexity to the task).…”

“…Therefore, for duplexing, a variety of novel approaches have been considered. In [4], separate narrowband tunable antennas are used for transmit (Tx) and receive (Rx), however the tuning range and size of the antennas is limiting, and covering all LTE bands may still result in an unacceptable component count. Duplexers based on self-interference cancellation (SIC) have received substantial interest, not only as a potential alternative to filters in FDD applications, but also enabling in-band full-duplex operation (IBFD), which can theoretically double link capacity by allowing simultaneous transmission and reception on the same frequency [5], [6].…”