Large-Signal Analysis and Characterization of a RF SOI-based Tunable Notch Antenna for LTE in TV White Space Frequency Spectrum

The demand for spectrum resources has increased dramatically with the explosion of wireless mobile devices and services. The Cognitive Radio (CR) associated with TV white space frequencies defined between 470 MHz to 790 MHz is one solution to excel the spectrum resources shortage and address LTE low bands. The antenna design to cover such a wide band is a twofold challenge. First Electromagnetic fundamental limits impose a tradeoff between antenna efficiency, miniaturization and its frequency band wideness, secondly to be implemented in portable device (around 10 cm typical length) the antenna miniaturization needs is to be strong such as in λ/6 (where λ is the wavelength at 500 MHz). Thus, frequency reconfigurable antennas are suitable candidates and can be easy integrated. In this paper a compact frequencyagile notch antenna for LTE low-band using TV white space frequencies is designed and fabricated. The 18 x 3 mm notch size (area dedicated for the antenna) is only λ/33 at 500 MHz. The antenna aperture tuning is provided by a SOI CMOS tunable capacitor. The novelty of this paper is to present not only classical metrics (gain, return loss...) but also the experimental characterization of parameters that are usually used for active RF components. The non-standard tunable capacitor RF non linearity analysis is carried on and analyzed. The simulated and experimental performances are presented and demonstrate antenna tuning operation from 800 MHz down to 500 MHz. The slot electrical near field distribution is also investigated to confirm acceptable electrical field within the tunable component. High linearity is validated with measured ACLR levels lower than -30 dBc up to 22 dBm input power in the considered frequency range.


Introduction
Due to the rapid demand for data over cellular networks, it has become difficult to cover all the traffic with current frequency bands.Thus, more frequency will be required to attend the increasing demand.However, it is becoming more and more challenging to allocate parts of spectrum for the data demand since most of the frequency bands which are suitable to mobile communications, are already assigned to the existing wireless systems.[1] presents a solution to lower the usage of these widely used bands.The CR principles enable the unlicensed users to dynamically locate the unused spectrum segments and to communicate via these unused spectrum segments.CR used with TV White Space (TVWS) is one of solutions to excel spectrum resources shortage.TVWS are frequencies available for unlicensed use at locations where the spectrum is not being used by licensed services, such as television broadcasting.This spectrum is located from 470 MHz to 790 MHz in Europe [2].The antennas usually used for this band are geometrically large to cover the entire frequency band.Several wideband antennas have been proposed in these bands [3].In [4], an asymmetric fork-like printed monopole antenna is presented for DVB-T application, which achieves a -10 dB bandwidth of 451-912 MHz but with a size of 247×35 mm 2 .In [5], an UHF wideband printed monopole antenna has been introduced with dimension of 120×240 mm 2 .Reconfigurable antennas, which are much more compact and have a low profile structures, are the most compelling for TVWS where instantaneous bandwidth is only a narrow part of the overall band.

The concept of Cognitive Radio (CR) proposed by
In this paper a tunable miniaturized notch antenna [6] working in the TVWS bands is designed using a tunable capacitor.Since transmit RF circuits generally operate at high power level, linearity is a critical parameter to prevent distortions or inter-channel interferences.Therefore, a large signal analysis and characterization of the proposed tunable antenna has been performed by realizing adjacent channel leakage Ratio (ACLR) measurements on the antenna prototype using a LTE signal.
In the following sections, a frequency-agile notch antenna using a SOI CMOS tunable capacitor and addressing TVWS from 510 MHz to 900 MHz is studied, implemented and measured.First, TC tunable capacitor specifications are provided.Secondly, design, analysis and antenna measurements (impedance and efficiency) are presented.Then, near field of the notch antenna is studied in order to investigate antenna performances.Finally, a linearity analysis is performed.

Tunable Capacitor Design
The Tunable Capacitor (TC) used in this study is a Silicon-On-Insulator (SOI) CMOS integrated circuit based on a network of binary-weighted switched capacitors [7].

Tunable Capacitor Characteristic
A 5-bit tunable capacitor has been designed under ADS simulation tool.The capacitance is digitally controlled through a SPI interface and it can be tuned from 1.3 pF to 7.1 pF at 1 GHz, Figure .2.

Figure. 2. Simulated Capacitance (@1 GHz) vs TC State in Shunt configuration
One of the most commonly emphasized electrical specifications for the tunable capacitor is the Quality Factor (Q), which is determined by the resistive (dissipative) losses of the component.Quality factor at 1 GHz versus tuning states is given in Figure .3. It can be observed that a minimum quality factor of 40 is achieved for the highest TC state, whereas a maximum Q of 103 is obtained for the lowest state.

Antenna Tuning
The tunable capacitor has been modeled under ADS and each state is imported into the EM simulator.To address TVWS, a limited set of capacitor states (from 5 to 31) will be considered.Frequencies from 470 MHz to 510 MHz are not addressed with actual devise.It is so easily to cover them, just to use another tunable capacitor with a higher tuning ratio or to connect a fixed capacitor in parallel configuration.
The measured antenna radiation patterns including the co polarization and the cross polarization in both (xz) and (xy) plane at 510 MHz and 770 MHz are presented in Figure .9. The gain is measured in the CEA-Leti anechoic chamber.The gain decreases, as expected, when the antenna is tuned to lower frequencies.At 770 MHz the notch length is λ/21 while at 510 MHz the notch is λ/32.6.
In fact, as the antenna is tuned away from its natural resonance, higher currents run on the miniaturized surface and the series resistance of the tuning component causes higher insertion loss.Thus, it is a trade-off between antenna size, radiation performances and ESR.The radiation efficiency is affected by both the antenna size miniaturization and ESR value.In the following section the impact of both effects are analyzed.

Near field Measurement
To visualize the electrical field around the notch, it was measured at several frequencies at a distance of 2 mm above the PCB with an optical probe [9] and with a 1 mm spatial sampling in the x and y directions around the slot area.The Vector Network Analyzer (VNA) is used to measure the received power of the probe's output.Figure .10 shows the measurement setup.The polarized probe is oriented to measure a given E field component.The input power is fixed for -10 dBm.

Figure. 10. Demonstrator setup
According to Babinet's principle [10] which relates the radiated fields and impedance of a slot antenna to that of its dual antenna.The dual of a slot is a dipole antenna, the conductive material and the air were interchanged.Thus, E-field and H-field are interchanged.Since the dipolefields are well known, the slot fields are also and its Efield is mainly within X.

Notch length
The field is maximum at the slot edge and especially at the slot open end.In measurement, high field is clearly located on the PCB edges which presents a difference from simulation.This difference is explained by the lower spatial resolution of the measuring probe and also the spatial sampling which is not accurate especially away from the slot area (10 mm spatial sampling) than in the simulation mesh.To visualize clearly the electrical field level in the slot, Figure .12 presents the total E level along the slot (Y slot range is between 0 to 50 mm) at 560 MHz for two positions in the notch x1 and x2, x1 corresponds to 26 mm and x2 is 27 mm.In this section, the focus will not be on differences between simulation and measurement but on the field distribution on the antenna.y=0 mm corresponds to the slot open end.A 2 mm from the open end, where the TC is implemented, an electrical field peak is observed in both measurements and simulations.The field decreases significantly from the open end to the slot short end of about 95%.At the PCB edge (y around 50 mm) the field re-increases (lesser than level in the notch) which demonstrates that PCB edges contribute to the antenna radiation.The same E-field distribution profile is observed in simulation and measurement.

Analysis and characterization of tunable antenna linearity
One critical specification for digital communication systems is the Adjacent Channel Leakage Ratio (ACLR) corresponding to signal distortion leaking in neighboring channels.Leakage power influences the system capacity as it interferes with the reception in adjacent channels.Therefore it must be rigorously controlled to guarantee correct communication for all subscribers in a network.ACLR is the ratio of the power in the adjacent channels to the power in the transmit channel.ACLR limits are imposed by the 3GPP standard for the whole system, however up to now there are no specific constraints for the antenna.Compared to the typical two ports antenna characterization, antenna linearity characterization has to be carried over the air (OTA).The tunable antenna respect the standard specification up to 22 dBm input power which corresponds to the LTE UE transmit power as specified in 3GPP TS36.101 (UE core specification) [11].

Conclusion
In this paper, a miniature tunable notch antenna is proposed to address LTE low bands in TVWS spectrum.
The notch antenna incorporates a SOI CMOS tunable capacitor at its open-end to operate in the different communication bands ranging from 510 MHz to 900 MHz.Measured results demonstrate trade-off between compact size, limited instantaneous operating bandwidth and radiation efficiency.The notch near field distribution has been investigated in order to predict the E-field level for a given input power which is a major limitation in the TC design.The RF linearity characterization of the tunable antenna has been performed through ACLR measurements on the antenna prototype and obtained results demonstrate that the proposed tunable antenna respects the standard specifications.Due to the dynamic frequency allocation within TVWS, fast transition times between frequency bands are expected to satisfy high quality communications, around 5.5 μs.At last, notice the proposed antenna solution is not restricted to LTE but is also compatible with other waveforms.

Rx Tx
Meas.Horn AUT

Figure. 7
gives a comparison between simulation and measurement of the antenna response for a fixed capacitor connected at the end of the stub (Cstub = 1 pF) and different settings of the tunable capacitor.Simulated resonance frequency can be tuned from 550 MHz up to 960 MHz.The achievable bandwidth at -6 dB reflection coefficient ranges from 12 MHz to 70 MHz (Figure.8)when operating frequency increases.The measured frequency response of the notch antenna for the different tunable capacitor settings ranges between 510 MHz and 900 MHz.A mean error of 8% is observed between the simulated and measured resonance frequency.This difference can be explained by some inaccuracies in the TC model.

Figure. 13 .
Figure. 13.Measured E-field (dB) along the notch at 500 MHz, 560 MHz and 600 MHz ACLR measurements are made using a spectrum analyzer and the required test signals are built using a signal generator.In the following setup, a Vector Signal Generator with an internal baseband generator is connected to the antenna in transmission to allow generation of a LTE signal and the received signal from the measurement (Horn) antenna is connected to a Spectrum Analyzer (Figure.14).For LTE, depending on the considered signal bandwidth, the adjacent channels are located at ±1.4 MHz, ±3 MHz, ±5 MHz, ±10 MHz, ±15 MHz and ±20 MHz offsets[11].EAI Endorsed Transactions on Cognitive Communications 02 2017 -05 2017 | Volume 3 | Issue 11 | e2

Table 1 .
ACLR is measured for different TC states and frequencies using an LTE 16 QAM 10 MHz uplink signal.TableIsummarizes the linearity performance of the tunable antenna.Measured power at the input of the tunable antenna for ACLR=-30dBc 4.2.ACLR Measurement Results