Mohamed Helaoui

2-D Digital Predistortion (2-D-DPD) Architecture for Concurrent Dual-Band Transmitters

This paper presents a novel 2-D digital-predistortion (2-D-DPD) technique that is applicable for linearization of concurrent dual-band transmitters. This technique uses a unique way for distortion compensation and linearization of dual-band transmitters by selecting, characterizing, and applying predistortion in each band separately. Compared to conventional linearization techniques, this 2-D-DPD method requires a lower sampling rate for digital-to-analog and analog-to-digital converters. The performance of the 2-D-DPD topology is evaluated using two modulated signals, Worldwide Interoperability for Microwave Access and wideband code-division multiple-access, separated in frequency by 100 MHz. The measurement results show an adjacent channel power ratio of less than -50 dBc and a normalized mean square error of less than -40 dB.

Design and Linearization of Concurrent Dual-Band Doherty Power Amplifier With Frequency-Dependent Power Ranges

A design methodology for a concurrent dual-band Doherty power amplifier (PA) with frequency-dependent backoff power ranges is presented in this paper. Based on a dual-band T-shaped network and a coupled line network, different dual-band components needed in Doherty PA topology, including a 3-dB branch-line coupler, an offset line, and a quarter-wavelength transformer, are developed. Two prototypes with balanced and imbalanced backoff power range modes are implemented to verify the feasibility. Continuous wave signal test results show that the proposed dual-band PA successfully achieves a power-added efficiency of 33% and 30% at the 6-dB backoff point from the saturated output power at 880 and 1960 MHz, respectively. To meet linearity requirements, the PA nonlinear behavior is characterized by using digital multitone signals, which categorize the distortions of a concurrent dual-band PA into intermodulation and cross-modulation. Finally, a 2-D digital predistortion technique is used to compensate for the nonlinearity of PA in dual bands. Two two-tone signals are applied to the dual bands for linearization, and the experimental results show that this technique achieves improvements of better than 19.1 and 24.6 dB for the intermodulation and cross-modulation in the dual bands, respectively.

A Novel Architecture of Delta-Sigma Modulator Enabling All-Digital Multiband Multistandard RF Transmitters Design

This paper proposes a new architecture of delta-sigma (DS) modulator suitable for RF digital transmitter design. This novel architecture considerably reduces the speed requirements of the digital signal processing block. The novelty lies in the implementation of a specific fully digital up-conversion in combination with a low-pass DS modulator to produce high-frequency digital-like signals, which can be used to drive highly efficient switching-mode power amplifiers. The proposed architecture is suitable for reconfigurable all-digital, multistandard and multiband wireless transmitters. The novel transmitter architecture has been validated using simulation and implemented on a field-programmable gate array development board for two different signals, code division multiple access and orthogonal frequency division multiplex.

Crossover Digital Predistorter for the Compensation of Crosstalk and Nonlinearity in MIMO Transmitters

This paper proposes a novel crossover digital predistorter (CO-DPD) model to compensate for crosstalk and nonlinearity in multiple-input multiple-output (MIMO) radio systems. Crosstalk can take place before or after the power amplifiers, designated herein as nonlinear and linear crosstalk, respectively. This paper demonstrates that, contrary to linear crosstalk, nonlinear crosstalk significantly affects the performance of the digital predistortion algorithm; and, it cannot be embedded and compensated for by the conventional channel matrix inversion algorithm at the receiver side of MIMO links. Based on a parametric study of system-level simulations and measurements, it was found that for a -20-dB nonlinear crosstalk level, the use of a memory multibranch polynomial predistorter, along with the channel matrix inversion algorithm, bounds the adjacent channel power ratio (ACPR) for a wideband code division multiple access (WCDMA) signal to -46 dBc and the error vector magnitude (EVM) for a world interoperability for microwave access (WiMAX) signal to -43 dB in MIMO links. The proposed CO-DPD was investigated and analyzed for the MIMO transmitter with N = 2, where N is the number of RF front-ends. Its performance was evaluated through measurements, the experimental results obtained show that, in the presence of -20-dB nonlinear crosstalk, the proposed CO-DPD improve the ACPR of the WCDMA signal by 13 dB to -56.81 dBc from those obtained using a conventional digital predistorter. The same improvement was observed in the EVM measurement of the WiMAX signal, where the EVM decreases from -21.22 dB for the conventional DPD to -49.71 dB for the proposed CO-DPD.

Linearization of Concurrent Dual-Band Power Amplifier Based on 2D-DPD Technique

This letter reports a two-dimensional digital predistortion technique to compensate for the nonlinearity of a concurrent dual band PA. Two different multi-carrier WCDMA signals with more than 1 GHz frequency spacing are concurrently employed to drive the PA and verify the linearization performance. Experimental results show that the proposed technique bounds the adjacent channel power ratio to lower than -46 dBc, which is more than 12 dB improvement for both bands concurrently.

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.

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

Power and efficiency enhancement of 3G multicarrier amplifiers using digital signal processing with experimental validation

This paper proposes a digital signal-processing-based approach suitable for the performance optimization of third-generation (3G) amplifiers in terms of spectrum and power. A peak-to-average power ratio (PAPR) reduction method, which is coding and modulation independent, based on peak clipping and digital filtering techniques, is proposed. Moreover, the multibranch memory polynomial pre-distorter identified with an optimized recursive least square technique was efficiently implemented in a digital signal processor. The cascade of the proposed PAPR reduction technique with the memory pre-distorter results in a substantial enhancement of the power amplifier (PA) output linear power and efficiency, while still meeting the 3G partnership project standard requirements. An experimental validation carried out on a 90-W laterally diffused metal-oxide-semiconductor PA, which was fed with a wide-band code-division multiple-access signal, led to a 4-dB rise in output mean linear power accompanied with 60% increase in its power-added efficiency.

On the RF/DSP design for efficiency of OFDM transmitters

In this paper, a system-level RF/digital signal processing (DSP) design approach of power-efficient orthogonal frequency-division multiplexing (OFDM) transmitters is proposed. A DSP-based low-IF architecture, which allows a significant enhancement of their power and spectrum efficiencies, is proposed. The cascade of the peak-to-average power ratio (PAPR) reduction technique, predistortion technique, and the in-phase and quadrature modulation led to impressive improvement in the power efficiency and effective linear output power of the OFDM transmitter. Measurement results carried out on an IEEE 802.11a transmitter designed and built for this experiment are presented in terms of error vector magnitude (EVM), adjacent channel leakage ratio, and power efficiency. The power stage of this transmitter uses a heterojunction bipolar InGaP transistor operating in a deeply class AB. The cascade of the PAPR reduction and baseband predistortion processing modules results in the reduction of the power backoff operation point by approximately 10 dB accompanied by a relative increase in the wireless local area network transmitter power efficiency by roughly 400% while meeting the emission mask spectrum and EVM levels demanded by the 802.11a standard.

A robust modeling and design approach for dynamically loaded and digitally linearized Doherty amplifiers

In this paper, an active load-pull-based large-signal modeling approach suitable for designing and optimizing Doherty amplifiers is proposed. The optimization of the drive dependant dynamic loads seen by both carrier and peaking amplifiers transistors was carried out using a large-signal load-pull-based behavior model built into CAD software. The latter was also used to optimize the biasing conditions of the peaking and carrier amplifier transistors. The simulation versus measurement results demonstrated the strong ability of the constructed model to predict the nonlinear behavior of 16 watts peak-envelope power designed Doherty amplifier in terms of power added efficiency, AM/AM, and AM/PM distortions. In addition, a complex digital predistortion technique was used to significantly enhance the linearity of the Doherty amplifier and to preserve an ACPR level better than 50 dBc under a WCDMA signal while attaining a power added efficiency of about 44% at the same time.