Adaptive Nonlinear RF Cancellation for Improved Isolation in Simultaneous Transmit-Receive Systems

This paper proposes an active radio frequency (RF) cancellation solution to suppress the transmitter (TX) passband leakage signal in radio transceivers supporting simultaneous transmission and reception. The proposed technique is based on creating an opposite-phase baseband equivalent replica of the TX leakage signal in the transceiver digital front-end through adaptive nonlinear filtering of the known transmit data, to facilitate highly accurate cancellation under a nonlinear TX power amplifier (PA). The active RF cancellation is then accomplished by employing an auxiliary transmitter chain, to generate the actual RF cancellation signal, and combining it with the received signal at the receiver (RX) low noise amplifier (LNA) input. A closed-loop parameter learning approach, based on the decorrelation principle, is also developed to efficiently estimate the coefficients of the nonlinear cancellation filter in the presence of a nonlinear TX PA with memory, finite passive isolation, and a nonlinear RX LNA. The performance of the proposed cancellation technique is evaluated through comprehensive RF measurements adopting commercial LTE-Advanced transceiver hardware components. The results show that the proposed technique can provide an additional suppression of up to 54 dB for the TX passband leakage signal at the RX LNA input, even at considerably high transmit power levels and with wide transmission bandwidths. Such novel cancellation solution can therefore substantially improve the TX-RX isolation, hence reducing the requirements on passive isolation and RF component linearity, as well as increasing the efficiency and flexibility of the RF spectrum use in the emerging 5G radio networks.Comment: accepted to IEE

Performance of fractional delay estimation in joint estimation algorithm dedicated to digital Tx leakage compensation in FDD transceivers

International audienceThis paper deals with the performance of the fractional delay estimator in the joint complex amplitude / delay estimation algorithm dedicated to digital Tx leakage compensation in FDD transceivers. Such transceivers are affected from transmitter-receiver signal leakage. Combined with non linearity of components in the received path, it leads to a pollution in the baseband signal. The baseband polluting term depends on the equivalent Tx leakage channel, modeling leakages and the received path. We have proposed in [7, 8] a joint estimation of the complex gain and the fractional delay and derived asymptotic performance of the complex gain estimator, that showed the necessity of the fractional delay estimation. In this paper, we propose a comprehensive study of the fractional delay estimation algorithm and its analytic performance. The study is based on the analysis of the S-curve and loop noise variance of the timing error detector, from which an approximation of the asymptotic performance of the joint estimation algorithm is derived

Air-Induced Passive Intermodulation in FDD MIMO Systems : Algorithms and Measurements

In this article, we model and develop effective cancellation schemes for passive intermodulation (PIM) distortion induced by external objects in the vicinity of the transmitter—referred to as air-induced PIM—in frequency-division duplex (FDD) multiple-input–multiple-output (MIMO) systems. PIM is interference generated from the transmitted signals undergoing nonlinear transformation, which, in FDD systems, may cause desensitization of the receiver chain. First, we present a general model of the received air-induced PIM signal, with an arbitrary number of dual-carrier TX chains active and an arbitrary number of PIM sources causing interference, on all the intermodulation frequencies. Then, the derived model is used to develop a cancellation scheme based on a complete set of basis functions (BFs) in a rank-2 dual-carrier MIMO system with four active carriers. To alleviate the high complexity of the aforementioned scheme, we then propose a novel alternative cancellation scheme with much reduced complexity, leveraging on the physical modeling of the system, which is capable of handling any number of PIM sources in the system. RF measurement-based experimentations carried out with real-life equipment evidence excellent cancellation capabilities of the complete BF model, which can be retained with much reduced complexity with the proposed alternative technique.publishedVersionPeer reviewe

High Performance LNAs and Mixers for Direct Conversion Receivers in BiCMOS and CMOS Technologies

The trend in cellular chipset design today is to incorporate support for a larger number of frequency bands for each new chipset generation. If the chipset also supports receiver diversity two low noise amplifiers (LNAs) are required for each frequency band. This is however associated with an increase of off-chip components, i.e. matching components for the LNA inputs, as well as complex routing of the RF input signals. If balanced LNAs are implemented the routing complexity is further increased. The first presented work in this thesis is a novel multiband low noise single ended LNA and mixer architecture. The mixer has a novel feedback loop suppressing both second order distortion as well as DC-offset. The performance, verified by Monte Carlo simulations, is sufficient for a WCDMA application. The second presented work is a single ended multiband LNA with programmable integrated matching. The LNA is connected to an on-chip tunable balun generating differential RF signals for a differential mixer. The combination of the narrow band input matching and narrow band balun of the presented LNA is beneficial for suppressing third harmonic downconversion of a WLAN interferer. The single ended architecture has great advantages regarding PCB routing of the RF input signals but is on the other hand more sensitive to common mode interferers, e.g. ground, supply and substrate noise. An analysis of direct conversion receiver requirements is presented together with an overview of different LNA and mixer architectures in both BiCMOS and CMOS technology

Modeling and Digital Suppression of Passive Nonlinear Distortion in Simultaneous Transmit—Receive Systems

In frequency division duplexing (FDD) based simultaneous transmit-receive systems, nonlinear behavior of the active and passive RF components can cause nonlinear distortion products falling at the receiver band. Such distortion may also arise over-the-air, if there are for example metallic objects in close vicinity of the antenna system. In this work, we focus on the modeling and digital cancellation of such distortion products, especially in case of passive harmonic distortion of the transmit waveform landing at the receiver band. We provide behavioral modeling of the problem, while also use the models to derive corresponding digital distortion cancellers. Practical RF measurement based numerical results are provided, focusing on a timely dual-band cellular transceiver scenario covering 5G NR bands n3 (1.8 GHz) and n78 (3.5 GHz). The RF measurement results demonstrate accurate modeling and distortion cancellation in the considered example cases.Peer reviewe

Passiivinen intermodulaatio suuritehoisissa radiolähetin-vastaanottimissa

Passive intermodulation (PIM) is a phenomenon which occurs when at least two signals are fed into a nonlinear passive device or circuit. Sources for PIM can be divided into two groups, nonlinearities in metal junctions and nonlinear materials. The most common source for PIM is a loose or a bad metal connection. The problem is more in a base station side because PIM requires high powers and a base station can have high trans-mission (TX) power and receive (RX) power may be low. In addition, there are con-nectors in use at base stations and antennas with metallic junctions. Furthermore, a base station duplexer may have a high isolation between TX and RX port leading to a situa-tion where intermodulation (IM) products due to TX power amplifier are attenuated well and PIM which is generated after the TX band-pass filter becomes significant. PIM is significant in the carrier aggregation technology, which uses more than one component carrier. In carrier aggregation, component carriers can be allocated non-contiguously on one or more frequency bands. If the duplex spacing is narrow, high-power 3rd order PIM products may fall on RX frequency band and desensitize the transceiver’s own receiver. Digital IM cancellation is based on estimating how TX signals are modified at the path to the receiver by taking and processing samples of the received signal. Then the main idea is to regenerate replicas of IM products and subtract them from the received signal. The aim in this thesis is to demonstrate that PIM products, which are generated after the TX band-pass filter, can be reduced with digital cancellation. The duplexer that is used in measurements is a frequency band 1 base station duplexer which has 190 MHz duplex spacing. Because of that, lower than 7th order IM products are not in the RX frequency band. For a reproducible test setup, a nonlinear connection at the antenna port of the duplexer is emulated with a diode. The diode circuit generated high-power IM products already with +20 dBm TX power at the antenna port and with this power the TX filter completely attenuated the IM products due to the power amplifier. With the digital cancellation, the 7th order IM product was successfully attenuated by 6 dB to 14 dB depending on the TX power. These measurement results demonstrate that it is possible to reduce PIM interference with digital cancellation. However, in this thesis the duplex spacing was considerably wide and therefore the most high-power 3rd order IM products could not be measured. For future research it would be important to measure how digital cancellation works when the duplex spacing is narrow and PIM product power is higher but the order is lower

Saw-Less radio receivers in CMOS

Smartphones play an essential role in our daily life. Connected to the internet, we can easily keep in touch with family and friends, even if far away, while ever more apps serve us in numerous ways. To support all of this, higher data rates are needed for ever more wireless users, leading to a very crowded radio frequency spectrum. To achieve high spectrum efficiency while reducing unwanted interference, high-quality band-pass filters are needed. Piezo-electrical Surface Acoustic Wave (SAW) filters are conventionally used for this purpose, but such filters need a dedicated design for each new band, are relatively bulky and also costly compared to integrated circuit chips. Instead, we would like to integrate the filters as part of the entire wireless transceiver with digital smartphone hardware on CMOS chips. The research described in this thesis targets this goal. It has recently been shown that N-path filters based on passive switched-RC circuits can realize high-quality band-select filters on CMOS chips, where the center frequency of the filter is widely tunable by the switching-frequency. As CMOS downscaling following Moore’s law brings us lower clock-switching power, lower switch on-resistance and more compact metal-to-metal capacitors, N-path filters look promising. This thesis targets SAW-less wireless receiver design, exploiting N-path filters. As SAW-filters are extremely linear and selective, it is very challenging to approximate this performance with CMOS N-path filters. The research in this thesis proposes and explores several techniques for extending the linearity and enhancing the selectivity of N-path switched-RC filters and mixers, and explores their application in CMOS receiver chip designs. First the state-of-the-art in N-path filters and mixer-first receivers is reviewed. The requirements on the main receiver path are examined in case SAW-filters are removed or replaced by wideband circulators. The feasibility of a SAW-less Frequency Division Duplex (FDD) radio receiver is explored, targeting extreme linearity and compression Irequirements. A bottom-plate mixing technique with switch sharing is proposed. It improves linearity by keeping both the gate-source and gate-drain voltage swing of the MOSFET-switches rather constant, while halving the switch resistance to reduce voltage swings. A new N-path switch-RC filter stage with floating capacitors and bottom-plate mixer-switches is proposed to achieve very high linearity and a second-order voltage-domain RF-bandpass filter around the LO frequency. Extra out-of-band (OOB) rejection is implemented combined with V-I conversion and zero-IF frequency down-conversion in a second cross-coupled switch-RC N-path stage. It offers a low-ohmic high-linearity current path for out-of-band interferers. A prototype chip fabricated in a 28 nm CMOS technology achieves an in-band IIP3 of +10 dBm , IIP2 of +42 dBm, out-of-band IIP3 of +44 dBm, IIP2 of +90 dBm and blocker 1-dB gain-compression point of +13 dBm for a blocker frequency offset of 80 MHz. At this offset frequency, the measured desensitization is only 0.6 dB for a 0-dBm blocker, and 3.5 dB for a 10-dBm blocker at 0.7 GHz operating frequency (i.e. 6 and 9 dB blocker noise figure). The chip consumes 38-96 mW for operating frequencies of 0.1-2 GHz and occupies an active area of 0.49 mm2. Next, targeting to cover all frequency bands up to 6 GHz and achieving a noise figure lower than 3 dB, a mixer-first receiver with enhanced selectivity and high dynamic range is proposed. Capacitive negative feedback across the baseband amplifier serves as a blocker bypassing path, while an extra capacitive positive feedback path offers further blocker rejection. This combination of feedback paths synthesizes a complex pole pair at the input of the baseband amplifier, which is up-converted to the RF port to obtain steeper RF-bandpass filter roll-off than the conventional up-converted real pole and reduced distortion. This thesis explains the circuit principle and analyzes receiver performance. A prototype chip fabricated in 45 nm Partially Depleted Silicon on Insulator (PDSOI) technology achieves high linearity (in-band IIP3 of +3 dBm, IIP2 of +56 dBm, out-of-band IIP3 = +39 dBm, IIP2 = +88 dB) combined with sub-3 dB noise figure. Desensitization due to a 0-dBm blocker is only 2.2 dB at 1.4 GHz operating frequency. IIFinally, to demonstrate the performance of the implemented blocker-tolerant receiver chip designs, a test setup with a real mobile phone is built to verify the sensitivity of the receiver chip for different practical blocking scenarios

Air-Induced PIM Cancellation in FDD MIMO Transceivers

In this letter, air-induced passive intermodulation (PIM) modeling and cancellation schemes are presented in a multiple-input multiple-output (MIMO) frequency-division duplexing (FDD) transceiver context. PIM is distortion generated by nonlinear passive devices either within the transmitter chain or outside the transceiver system, as is the case in air-induced PIM. In FDD systems, the PIM products may lie on the receiver band, thus possibly desensitizing the receiver chain. Unlike previous PIM cancellation works, we consider a challenging rank-2 dual-carrier MIMO transceiver scenario, with two spatially multiplexed signals per component carrier (CC). Stemming from the PIM modeling, we first present a cancellation method based on a complete set of identified basis functions (BFs). Additionally, to relax the processing complexity, we propose an alternative canceller solution with a reduced number of BFs, inspired by the problem modeling. RF measurements conducted with real-life equipment indicate favorable PIM suppression levels of up to 20 dB using both introduced techniques.publishedVersionPeer reviewe