Ammar B. Kouki

On the linearity and efficiency of outphasing microwave amplifiers

A new approach for the analysis of the efficiency and linearity of Chireix-outphasing combiners is presented. The approach takes into account, in an explicit manner, the effect of impedance mismatch between the amplifiers and the lossless combining structure. It is shown that the impedance mismatch leads to new expressions for the output voltages from both branches of the amplifier. These expressions explain the origin of the lack of linearity reported in the literature for the Chireix architecture and lead to a new expression for the instantaneous efficiency of a Chireix combiner. Various simulations using a commercial simulator are performed and their results are compared to those predicted by the derived equations. A quasi-exact agreement between the simulator and derived equations is found for all simulations conducted, including voltage expressions, linearity analysis, and instantaneous and average efficiency calculations for a code-division multiple-access signal. The impact of combiner parameters on its linearity and efficiency is also studied.

Phase-only predistortion for LINC amplifiers with Chireix-outphasing combiners

Nonlinearities in linear amplification with nonlinear components (LINC) amplifiers using Chireix-outphasing combiners are studied and their sources are identified. A generalized analytical approach for finding phase-only predistortion functions to linearize these amplifiers is presented. Two distinct analytical solutions are found and are applied for phase-only linearization. The first solution consists of a static constant phase predistortion, which is introduced in one of the two RF amplifier branches. This constant phase is determined analytically as function of the value of the reactance of the stub used in the Chireix combiner. The second solution is a variable phase-distortion function, which is derived in explicit form. The two predistortion functions preserve constant envelope operation of the amplifiers. The impact of the two predistortion functions on the combiner efficiency is also investigated. It is found, through simulation with a code-division multiple-access signal and an experimental measurement with a 16 quadrature-amplifier-modulation signal that the two predistortion functions perfectly linearize the amplifier. It also shown analytically that, with perfect linearity, the efficiency of Chireix-outphasing amplifier is reduced and is, at best, equal to that of the LINC amplifier with a resistive combiner.

New compound upper bound on MIMO channel capacity

MIMO channel capacity may be severely degraded due to correlation between individual sub-channels of the matrix channel. Several models, which are limited to some specific scenarios, have been developed to date to account for this effect. In this letter, we derive a new upper bound on the mean (ergodic) MIMO capacity, which is not limited to a particular scenario and accounts for both transmit (Tx) and receive (Rx) end correlations in such a way that their impact can be estimated separately and compared. Thus, a conclusion can be made as to which end contributes more to capacity reduction. In general, the higher correlated end has a dominant effect on the capacity.

Direct parameter-extraction method for HBT small-signal model

An accurate and broadband method for the direct extraction of heterojunction bipolar transistor (HBT) small-signal model parameters is presented in this paper. This method differs from previous ones by extracting the equivalent-circuit parameters without using special test structures or global numerical optimization techniques. The main advantage of this method is that a unique and physically meaningful set of intrinsic parameters is extracted from the measured S-parameters for the whole frequency range of operation. The extraction procedure uses a set of closed-form expressions derived without any approximation. An equivalent circuit for the HBT under a forward-bias condition is proposed for extraction of access resistances and parasitic inductances. An experimental validation on a GaInP/GaAs HBT device with a 2/spl times/25 /spl mu/m emitter was carried out, and excellent results were obtained up to 30 GHz. The calculated data-fitting residual error for three different bias points over 1-30 GHz was less then 2%.

An analytical approach to LINC power combining efficiency estimation and optimization

A new approach for studying the combiner efficiency in a LINC amplifier based on analytical expressions of the instantaneous combiner efficiency and the signals probability distribution function (PDF) is proposed. The analytical expressions, derived for both a matched combiner and a Chirex outphasing combiner, are applied to compute the combiner efficiency of a LINC amplifier under CDMA and 64QAM signals. A direct approach to determining the optimal shunt reactance used in an outphasing combiner is also proposed and applied to both signals. Validation with a commercial simulator is given.

Power Approaches for Implantable Medical Devices

Implantable medical devices have been implemented to provide treatment and to assess in vivo physiological information in humans as well as animal models for medical diagnosis and prognosis, therapeutic applications and biological science studies. The advances of micro/nanotechnology dovetailed with novel biomaterials have further enhanced biocompatibility, sensitivity, longevity and reliability in newly-emerged low-cost and compact devices. Close-loop systems with both sensing and treatment functions have also been developed to provide point-of-care and personalized medicine. Nevertheless, one of the remaining challenges is whether power can be supplied sufficiently and continuously for the operation of the entire system. This issue is becoming more and more critical to the increasing need of power for wireless communication in implanted devices towards the future healthcare infrastructure, namely mobile health (m-Health). In this review paper, methodologies to transfer and harvest energy in implantable medical devices are introduced and discussed to highlight the uses and significances of various potential power sources.

Analyzing LINC Systems

In this article, an analysis of LINC PA efficiency and linearity is observed. The use of two highly efficient PAs operated with constant envelope signals in the LINC transmitter does not in and of itself guarantee that the overall efficiency of the LINC amplifier will be high. Its shown that the use of matched combiner gives good linearity but the efficiency is decreased when the peak-to-average of signal increases. The same isolation between the two amplifiers is what gives the high linearity at the output. To avoid the efficiency losses in the matched combiner, the lossless Chireix-outphasing combiner has been presented. The nonisolating combiner presents time-varying impedances to the output of RF amplifier of each branch as the phase difference between the branches increases, thus significantly improving the LINC average efficiency. Impedance mismatching between the amplifiers and outphasing combiner have been explicitly taken into account. Closed-form expressions for the voltages at the output of the combiner, the instantaneous impedance seen by each amplifier, as well as the instantaneous efficiencyof the combiner, have been developed and presented. It has been shown that the new voltage expressions are consistent with the nonlinearity behavior of the Chireix combiner/thus demonstrating that the combining structure is a source of nonlinearity. To validate this approach many types of signals are considered as CDMA WCDMA and OFDMA (QPSKA, 16-QAM, 64-QAM) signals. Other sources of distortion such as imbalances between two branches are presented and how to compensate this imbalance is proposed. In light of the results obtained, the use of the Chireixoutphasing combiner with saturated nonlinear amplifiers to reach the objective of both high-efficiency and high-linearity amplification does not seem possible. The potential use of phase predistortion to reach this objective remains to be investigated.

Experimental characterization of LINC outphasing combiners' efficiency and linearity

In this paper, an experimental characterization of the efficiency and linearity of resistive and outphasing combiners for linear amplification using non linear components (LINC) amplifiers is presented. The combining efficiency is measured for three combiners, i.e., one Wilkinson combiner and two Chireix combiners, with different electric stub lengths. The measured results are compared to theoretical predictions for the three cases. The nonlinear effects of Chireix combiners are highlighted and phase adjustment of one of the systems branches is proposed to linearize it. We show that linearization is feasible in this manner only at the expense of reduced efficiency.

A review of absorbing boundary conditions for two and three-dimensional electromagnetic scattering problems

The derivation of two- and three-dimensional absorbing boundary conditions (ABCs) for mesh truncation in the partial differential equation solution of electromagnetic scattering problems is briefly reviewed. The approximate 2-D ABC is compared with its exact counterpart and ways are suggested by which the ABCs can be improved. A direct application of the Wilcox expansion to the mesh truncation problem that bypasses the ABC altogether is discussed. The question of using a conformal (as opposed to cylindrical) outer boundary is addressed, and the ABC for this geometry is derived for 2-D scattering and guided wave problems. >

A New Mode-Multiplexing LINC Architecture to Boost the Efficiency of WiMAX Up-Link Transmitters

This paper proposes a new amplifier architecture based on the outphasing technique intended for the efficiency enhancement of linear amplification with nonlinear components (LINC) transmitters. The proposed mode-multiplexing linear amplification with nonlinear components (MM-LINC) scheme operates according to the LINC concept for input signal magnitude drive levels below a certain threshold and as a balanced amplifier beyond this threshold. The setting of this threshold level influences the performance of the amplifier in terms of average power-added efficiency and linearity. A 2-W up-link transmitter prototype for worldwide interoperability for microwave access (WiMAX) applications was designed using this new architecture and optimized for a WiMAX signal with an 11.3-dB peak-to-average power ratio. The experimental results revealed a significant increase in power-added efficiency, from 6% for a LINC transmitter to 21% for the MM-LINC amplifier, while maintaining an error vector magnitude value under 8%, which is compliant with the standard requirement