Efficiency enhancement by reconfigurable matching networks in LINC transmitters

This paper proposes the use of a transmitter based on a linear amplification with nonlinear components (LINC) architecture, in which the reconfigurable matching networks (RMNs) are included. By varying the RMN active cell number, it is possible to change the load impedance at the power amplifier (PA) output, improving the amplifier drain efficiency and therefore the efficiency of the whole system. A long-term evolution (LTE) downlink signal with a bandwidth of 1.4 MHz and a peak-to-average power ratio (PAPR) of 11.48 dB is applied in order to carry out the experiments. Results show that the use of the RMNs in a LINC architecture improves the efficiency at all tested frequencies, especially at 927 MHz reaching an enhancement of 36.50%. Regarding the distortion, the adjacent channel leakage ratio (ACLR) values increase in all cases, with an improvement of 3.5 dB at 958 MHz. Finally, in terms of error vector magnitude (EVM), the proposed architecture offers a value of 1.96% at 927 MHz

Radio-Communications Architectures

Wireless communications, i.e. radio-communications, are widely used for our different daily needs. Examples are numerous and standard names like BLUETOOTH, WiFI, WiMAX, UMTS, GSM and, more recently, LTE are well-known [Baudoin et al. 2007]. General applications in the RFID or UWB contexts are the subject of many papers. This chapter presents radio-frequency (RF) communication systems architecture for mobile, wireless local area networks (WLAN) and connectivity terminals. An important aspect of today's applications is the data rate increase, especially in connectivity standards like WiFI and WiMAX, because the user demands high Quality of Service (QoS). To increase the data rate we tend to use wideband or multi-standard architecture. The concept of software radio includes a self-reconfigurable radio link and is described here on its RF aspects. The term multi-radio is preferred. This chapter focuses on the transmitter, yet some considerations about the receiver are given. An important aspect of the architecture is that a transceiver is built with respect to the radio-communications signals. We classify them in section 2 by differentiating Continuous Wave (CW) and Impulse Radio (IR) systems. Section 3 is the technical background one has to consider for actual applications. Section 4 summarizes state-of-the-art high data rate architectures and the latest research in multi-radio systems. In section 5, IR architectures for Ultra Wide Band (UWB) systems complete this overview; we will also underline the coexistence and compatibility challenges between CW and IR systems

Linearity of Outphasing Radio Transmitters

The outphasing transmitter is a promising technique, which can simultaneously achieve high linearity and power efficiency, thereby addressing the major design requirements of next generation transmitters. It employs highly non-linear power amplifier (PA) classes in a linear manner, in principle transmitting a distortion-free signal. Due to symmetric nature of the outphasing architecture, its linearity performance is constrained by any mismatches and non-linear effects encountered in the RF paths. This thesis analyzes the linearity performance of outphasing transmitters (in terms of ACLR specification) for LTE base station applications, under the non-linear effects and tolerances present in practical implementations. The system-level model, built in Matlab software, investigates the important non-linear effects present in outphasing transmitters, including gain and phase imbalance, IQ modulator mismatches, delay imbalance, and the non-linear effects of PAs and Chireix combiners. The path and delay mismatches result in only partial cancellation of the wideband quadrature signal, and thus create interference in both the in-band and out-of-band frequency regions. The misalignment in IQ modulators, such as gain/phase imbalance and carrier leakage, introduces amplitude and phase modulation in the outphased signals. The quadrature modulator mismatches, in conjunction with amplifier nonlinearity, result in spectral regrowth around the carrier frequency. The transmitter linearity performance is also affected by mismatches in the non-linear characteristics of the PAs. Realistic square-wave signals, exhibiting finite rise- and fall- time, also create spectral leakage for distinct rise/fall times in each outphasing branch. Furthermore, the Chireix combiner severely degrades the linearity of outphasing transmitters; it produces ACLR well below the specified limit for LTE base stations. This makes mandatory the compensation of Chireix combiner induced non-linearity in outphasing transmitters. The strict linearity requirements (for LTE downlink applications) present a small tolerance window for mismatches experienced in practical circuits. The relatively small tolerance margin indicates the need of linearization and compensation techniques in outphasing transmitters

Power Amplifiers for Next Generation Wireless Platforms

Class-E amplifier has the potential to deliver high efficiency required for the next generation wireless systems. In this journal, we discuss a novel load pull analysis technique to characterize the efficiency performance of Class-E amplifier in an outphasing power combining scheme. Class-E amplifier is not an ideal current or voltage source as is required for the traditional analysis of outphasing structures. It requires a phase modulated input signal and has a non-linear transfer characteristic which is a function of load impedance. Here we define an operating load locus based on the load pull analysis which can be used to predict the non-linear transfer function, efficiency, output power, input drive phase and many other factors associated with the outphasing class-E amplifier. This scheme could also be used to characterize any amplifier class in an outphasing structure. Finally modulation performance of Class-E amplifier using PWM technique is also presented

LINC based amplifier architectures for power efficient wireless transmitters

Wireless communication trends Performance measuring of a communication system Power amplifiers and transmitters Power efficiency enhancement techniques Design and Optimization of LINC transmitter for OFDM applications LINC concept LINC signal decomposition LINC efficiency and combiner technologies Design optimization of LINC system Mismatch (imbalance) effects Advanced LINC transmitter architectures The 2X1 LINC transmitter system The 2X2 LINC transmitter system Mismatch effects

Multi-Band Outphasing Power Amplifier Design for Mobile and Base Stations

New generations of wireless communication systems require linear efficient RF power amplifiers (PAs) for higher transmission data rates and longer battery life. On the contrary, conventional PAs are normally designed for peak efficiency under maximum output power (Pout). Thus, in power back-off, the overall efficiency degrades significantly and the average efficiency is much lower than the efficiency at maximum Pout. Chireix outphasing PA, also called LINC (Linear amplification using Non-linear Components), is one of the most promising techniques to improve the efficiency at power back-off. In this method, a variable envelope input signal is first decomposed into two constant-envelope phase-modulated signals and then amplified using two highly efficient non-linear PAs. The output signals are combined preferably in a loss-less power combiner to build the desired output signal. In this way, the PA exhibits high efficiency with good linearity. In this thesis, first we analyze a complex model of outphasing combiner considering its nonidealities such as reflection and loss in transmission lines (TL). Then we propose a compact model with analytical formula that is validated through several comparative tests using ADS and Spectre RF. Furthermore, we analyze the effect of reactive load in Chireix combiner with stubs (a parallel inductor and capacitor), while distinguishing between its capacitive and inductive parts. It is demonstrated that only the capacitive part of the reactive load degrades the performances. Based on this, a new architecture (Z LINC) is proposed where the power combiner is designed to provide a zero capacitive load to the PAs whatever the outphasing angle. The theory describing the operations of the system is developed and a 900 MHz classical LINC and Z-LINC PAs are designed and measured. In addition, a miniaturization technique is proposed which employs λ/8 or smaller TLs instead of conventional λ/4 TLs in outphasing power combiner. This technique is applied to implement a 900 MHz PA using LDMOS power transistors. Besides single-band PAs, dual-band PAs are more and more needed because of an increasing demand for wireless communication terminals to handle multi-band operation. In chapter 5, a new compact design approach for dual-band transmitters based on a reconfigurable outphasing combiner is proposed. The objective is to avoid the cumbersome implementations where several PAs and matching network are used in parallel. The technique is applied to design a dual band PA with a fully integrated power combiner in 90 nm CMOS technology. An inverter-based class D PA topology, particularly suitable for outphasing and multimode operations is presented. The TLs in the combiner, realized using a network of on-chip series inductors and parallel capacitors, are reconfigurable from λ/4 in 1800 MHz to λ/8 in 900 MHz. In order to maximize the efficiency, the on-chip inductors are implemented using high quality factor on chip slab inductors. The measured maximum Pout at 900/1800 MHz are 24.3 and 22.7 dBm with maximum efficiencies of 51% and 34% respectively

Digital Pre-distortion for Interference Reduction in Dynamic Spectrum Access Networks

Given the ever increasing reliance of today’s society on ubiquitous wireless access, the paradigm of dynamic spectrum access (DSA) as been proposed and implemented for utilizing the limited wireless spectrum more efficiently. Orthogonal frequency division multiplexing (OFDM) is growing in popularity for adoption into wireless services employing DSA frame- work, due to its high bandwidth efficiency and resiliency to multipath fading. While these advantages have been proven for many wireless applications, including LTE-Advanced and numerous IEEE wireless standards, one potential drawback of OFDM or its non-contiguous variant, NC-OFDM, is that it exhibits high peak-to-average power ratios (PAPR), which can induce in-band and out-of-band (OOB) distortions when the peaks of the waveform enter the compression region of the transmitter power amplifier (PA). Such OOB emissions can interfere with existing neighboring transmissions, and thereby severely deteriorate the reliability of the DSA network. A performance-enhancing digital pre-distortion (DPD) technique compensating for PA and in-phase/quadrature (I/Q) modulator distortions is proposed in this dissertation. Al- though substantial research efforts into designing DPD schemes have already been presented in the open literature, there still exists numerous opportunities to further improve upon the performance of OOB suppression for NC-OFDM transmission in the presence of RF front-end impairments. A set of orthogonal polynomial basis functions is proposed in this dissertation together with a simplified joint DPD structure. A performance analysis is presented to show that the OOB emissions is reduced to approximately 50 dBc with proposed algorithms employed during NC-OFDM transmission. Furthermore, a novel and intuitive DPD solution that can minimize the power regrowth at any pre-specified frequency in the spurious domain is proposed in this dissertation. Conventional DPD methods have been proven to be able to effectively reduce the OOB emissions that fall on top of adjacent channels. However more spectral emissions in more distant frequency ranges are generated by employing such DPD solutions, which are potentially in violation of the spurious emission limit. At the same time, the emissions in adjacent channel must be kept under the OOB limit. To the best of the author’s knowledge, there has not been extensive research conducted on this topic. Mathematical derivation procedures of the proposed algorithm are provided for both memoryless nonlinear model and memory-based nonlinear model. Simulation results show that the proposed method is able to provide a good balance of OOB emissions and emissions in the far out spurious domain, by reducing the spurious emissions by 4-5 dB while maintaining the adjacent channel leakage ratio (ACLR) improvement by at least 10 dB, comparing to the PA output spectrum without any DPD

Highly efficient RF power amplifier for wireless LAN applications

EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Integrated Filters and Couplers for Next Generation Wireless Tranceivers

The main focus of this thesis is to investigate the critical nonlinear distortion issues affecting RF/Microwave components such as power amplifiers (PA) and develop new and improved solutions that will improve efficiency and linearity of next generation RF/Microwave mobile wireless communication systems. This research involves evaluating the nonlinear distortions in PA for different analog and digital signals which have been a major concern. The second harmonic injection technique is explored and used to effectively suppress nonlinear distortions. This method consists of simultaneously feeding back the second harmonics at the output of the power amplifier (PA) into the input of the PA. Simulated and measured results show improved linearity results. However, for increasing frequency bandwidth, the suppression abilities reduced which is a limitation for 4G LTE and 5G networks that require larger bandwidth (above 5 MHz). This thesis explores creative ways to deal with this major drawback. The injection technique was modified with the aid of a well-designed band-stop filter. The compact narrowband notch filter designed was able to suppress nonlinear distortions very effectively when used before the PA. The notch filter is also integrated in the injection technique for LTE carrier aggregation (CA) with multiple carriers and significant improvement in nonlinear distortion performance was observed. This thesis also considers maximizing efficiency alongside with improved linearity performance. To improve on the efficiency performance of the PA, the balanced PA configuration was investigated. However, another major challenge was that the couplers used in this configuration are very large in size at the desired operating frequency. In this thesis, this problem was solved by designing a compact branch line coupler. The novel coupler was simulated, fabricated and measured with performance comparable to its conventional equivalent and the coupler achieved substantial size reduction over others. The coupler is implemented in the balanced PA configuration giving improved input and output matching abilities. The proposed balanced PA is also implemented in 4G LTE and 5G wireless transmitters. This thesis provides simulation and measured results for all balanced PA cases with substantial efficiency and linearity improvements observed even for higher bandwidths (above 5 MHz). Additionally, the coupler is successfully integrated with rectifiers for improved energy harvesting performance and gave improved RF-dc conversion efficienc