Gabor Varga

A highly linear broadband LNA for TV white spaces and Cognitive Radio applications

Communications in the TV White Spaces are gaining increasing interest due to the crowded frequency spectrum and rising demand for higher data rates. Especially the dynamic access by Cognitive Radios is under research, offering a more efficient usage of the spectrum. Due to the broadband nature of Cognitive Radio systems and the presence of numerous strong interfering signals, the expectations on the linearity of such systems are extremely hard. In this paper, an integrated CMOS broadband highly linear fully differential LNA is presented, which enables a receive path to fulfill the hard linearity requirements for broadband cognitive TV White Space applications. The proposed LNA has a bandwidth of 300MHz to 1 GHz and works without any external components. It utilizes Complementary Derivative Superposition to cancel third order intermodulation products and has an IIP3 of 19 dBm, CP1dB of -3dBm and Noise Figure of 2.6 dB within the TV White Space 470 MHz-790 MHz, while consuming 4.8mA from a 3.3V source.

A broadband RF converter empowering cognitive radio networks in the TV white space

Research in Cognitive Radio wireless networks has been experiencing increased interest in the last decade, as it could represent a considerable solution to todays crowded spectrum and inefficient spectrum usage. However, testing and verifying algorithms and hardware for such systems over-the-air pose difficulties in the already licensed telecommunication bands. Meanwhile, digital TV broadcasting has left vacant and unlicensed bandwidth among the allocated spectrum, the so called TV white space, which blends itself for research in the fields of cognitive radio applications, possibly leading to opportunistically offloading LTE traffic to those unused bands. In this paper the system design and implementation of a Radio Frequency Converter Platform is presented that facilitates cognitive radio research to be extended to the air interface using the TV white space without causing interference to licensed telecommunication operators. The converter uses a high-IF architecture and not only fulfills LTE specifications but also legacy standards. It can interface with any experimental or commercially available Software Defined Radio front end and any LTE modem or transceiver, contributing to the BMBF founded kogLTE project. [1] [2] Embedded in the system, the converter enables dynamic and cognitive use of available frequencies in the TV white space. Our presented converter platform therefore enables research in decongesting networks, operation at high network loads, investigating new cognitive radio algorithms and repurposing unused network resources.

Radio front-end enabling WLAN over white spaces cognitively

This paper describes a radio transceiver prototype based on TDD for TV white space frequencies, which can be used for WLAN based cognitive radio solutions. The prototype is designed out of off-the-shelf components and can be flexibly employed with any commercially available access points (AP) for sensing and accessing unused white spaces. This hardware with a WLAN device forms a high-IF architecture that provides high out-of-band linearity. The architecture of the prototype is briefly discussed outlining the constraints for the reception and transmission catering to 802.11b/g requirements. Performance conformance tests are discussed at length for both receiver and transmitter sections. It is shown that with the addition of this UHF converter, the performance of an AP is not degraded. The receiver sensitivity is found to be -90.7 dBm which is 8 dB better than that specified by the standard. The maximum transmitted power is 21 dBm with all the spurious requirements and EVM of the WLAN standard being satisfied.

System design of a high-IF to UHF converter enabling cognitive radio

It is known that cognitive radio (CR) is an attractive concept to overcome the prevailing spectrum scarcity and inefficient spectrum usage. Also, with their intermittent spectral vacancies, TV white spaces offer a faster deployment potential considered with respect to cognitive radios. In this paper, system design of a high intermediate frequency (IF) to UHF frequency converter for cognitive radios have been discussed and the performance of such a converter analyzed as a proof of concept. For the design of RF front-ends, CR brings forth huge architectural challenges such as in-band local oscillator harmonics, ultra low sensitivity and high linearity. As shown in this paper, a high-IF radio front-end can successfully tackle some of these challenges. Additionally, such a high-IF converter enables white space operation for existing commercial devices by acting as frequency converters. From detailed measurements, the capabilities in both physical layer and application layer performance of a high-IF front-end developed out of off-the-shelf components is explained and is shown to provide negligible degradation to the commercial device being connected to.

RF frequency converters for White Space devices

The spectrum scarcity caused by the exploding wireless data demand is seen to be combatable with the help of cognitive radio techniques. However, not only in the fields of algorithms and digital signal processing, research has to be done. To derive accurate channel models, to conduct field tests and to find realistic spectrum sensing scenarios, testing with air interfaces is inevitable for conducting cognitive radio research. As conventional software defined radios either do not possess the required wide frequency range, the required linearity or the out-of-band rejection to fulfill the specifications, alternative possibilities for air interfaces have to looked into. In this paper, two frequency converters for operation of commercial radio transceivers in the TV White Space are presented. While “whiteLAN” enables the use of standard wireless LAN equipment in the TV white space, “kogHF” enables cellular LTE devices to operate in the TV white space. Both converters are described and their performance is evaluated. The combination of the realized frequency converter and any software-defined radio or any standard transceiver then fulfills the corresponding specifications and spectrum masks while operating in the TV White Space.

Design for TV whitespace operational compliance for cognitive radio enabled highIF WLAN/LTE front-ends

Digitization of the TV UHF frequencies has resulted in sparsely occupied TV bands paving way for opportunistic access technologies to be a solution for the impending spectrum demand. Though the researches in digital signal processing for spectrum sensing is showing maturity, the concerns in RF front-ends still needs to be addressed. The document ETSI 301 598 [1] is the latest focusing the technical requirements for the TV white space (TVWS) devices but the pre-requisites for a compliant design is still lacking. This paper serves to integrate the system design aspects of a TVWS device with that of an existing cognitive radio (CR) enabled front-end to ensure compliance with the ETSI requirements. The complete metrics for a transmit front-end is derived with an focus on a WLAN/LTE secondary user. Step by step it is shown that highIF is the optimized solution for the TVWS cognitive radio with the measurements ensuring the compliance.

A proven MIMO capable TDD/FDD frequency converter enabling cognitive features for LTE transceivers

Due to the observed increasing scarcity of economically usable spectrum, it is becoming difficult to accommodate the data demand of countless mobile data terminals into existing bands. The TV White spaces, the scarcely used bands of former analog TV broadcasting, could be a viable option to alleviate this problem. The concept of Cognitive Radio must be employed though, as primary users within the TV White Spaces must not be disturbed.

Improving the linearity of wideband receiver systems by component IM3 phasor manipulation

A linearity improvement technique for receiver systems is presented and verified on a 130nm CMOS high-IF upconverter with 470 –790MHz input and 2.4–2.6 GHz output frequency range, enabling WLAN and LTE transceivers to be used as TV White Space Devices. The upconverter reaches a stable IIP3 of 15 dBm, NF of 8 dB and Gain of 7 dB. A linearized LNA and mixer are used as a composite architecture to combine low NF with, even though, high IIP3. Instead of further maximizing the linearity of the components, the overall performance is optimized on the system level by manipulation and complementary exploitation of the remaining third order intermodulation products of the components.

Effect of integrated polysilicon resistors on the linearity of composite circuits

This paper describes the linearity degradation effect in integrated circuits caused by the silicide contacts of polysilicon resistors. This secondary distorting effect starts to play a role when high system linearity is required or the primary nonlinear components like transconductances are already optimized for low distortion. Physical background is given with an emphasis on circuit design using RF pcells and the effect on the linearity figure-of-merit IIP3 of composite circuits is demonstrated. The influence on a practical highly linear broadband LNA employing Complementary Derivative Superposition for Cognitive Radios is shown and recommendations to reduce or to use the effect is presented.

Design of an integrated CMOS highly sensitive true RMS RF power detector for Cognitive Radio applications

A high dynamic range true RMS power detector is designed for broadband frequency agile applications like Cognitive Radio. The RF bandwidth of the Power Detector covers 0.5 GHz - 3 GHz with a dynamic range of 50 dB and a minimum detectable power of -50dBm (50Ω). Its architecture is differential employing a pseudo-differential squaring circuit and it has a high input impedance enabling it to be placed on high impedance as well as on 50Ω lines. Particularly the noise behaviour of the circuit is investigated due to the inherently strong low-frequency noise of the squaring stage. Preamplifier stages, automatic offset cancellation, calibration and oversampling are employed to reach high sensitivity. Logarithmic amplifier stages after the squaring unit care for a linear-in-dB conversion. The power detector has been designed in a commercial UMC 130nm CMOS technology in order to provide easier integration with other mixed-signal blocks and low-cost production.