Seyed Aidin Bassam

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.

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.

Transmitter Architecture for CA: Carrier Aggregation in LTE-Advanced Systems

CA has already found its place in the deployment of 4G wireless networks. It successfully shows its value as an efficient way to increase the signal bandwidth within the available spectrum. This article is focused on demonstrating and analyzing the possible transmitter architecture solutions for CA. Further, the benefits and issues of using concurrent dual-band transmitter architecture are investigated. The recent progress on deploying concurrent dual-band PAs has been surveyed, where recent published literatures reported significant improvement on the concurrent dual-band PA design and achievable efficiency. The challenges present in the development of the linearization schemes for concurrent dual-band transmitters are addressed and possible solutions are studied.

Subsampling Feedback Loop Applicable to Concurrent Dual-Band Linearization Architecture

This paper demonstrates an energy-efficient and low-complexity subsampling receiver adopted in the feedback loop of the dual-band power amplifier (PA) linearization architecture. The challenges and issues on finding the valid subsampling frequencies in nonlinear system are discussed, and a systematic approach for finding valid subsampling frequencies is presented. The subsampling-based receiver is applied as a proper solution for the feedback loop of the dual-band linearization architecture. It is shown that the subsampling feedback loop reduces the complexity and the cost of the dual-band linearization architecture. The simulation and measurement results show the proper functionality of the presented technique and demonstrate a good linearization performance. The measurement results show that when using this method, more than 15-dB improvement in normalized mean squared error and more than 17-dB improvement in adjacent channel power ratio are achieved for a wideband class-AB PA, compared with the unlinearized method.

Channel-Selective Multi-Cell Digital Predistorter for Multi-Carrier Transmitters

This paper demonstrates a new channel-selective multi-cell processing predistortion technique that compensates for the nonlinearities of multi-carrier transmitters. The proposed technique uses independent processing cells to compensate for the intra-band and inter-band distortions of nonlinear transmitters. This frequency-selective feature of the proposed technique significantly reduces the minimum sampling rate requirements of analog-to-digital and digital-to-analog converters, which are a critical issue for conventional digital predistortion (DPD) techniques dealing with wideband signals. The proposed technique was evaluated with four-carrier (1001) and six-carrier (100001) WCDMA signals, using a nonlinear 10-Watt power amplifier. The performance of the proposed technique was compared with look-up table, multi-branch and recently proposed frequency-selective DPDs, in terms of adjacent-channel power ratios (ACPRs) and sampling rate requirements. The proposed technique improved the ACPR and the carrier-to-intermodulation power ratio (CIMPR) of the 1001 WCDMA signal by more than 13 dB and 10 dB, respectively.

Block-Wise Estimation of and Compensation for I/Q Imbalance in Direct-Conversion Transmitters

This correspondence proposes a new technique for accurate estimation and compensation for I/Q imbalance in modulators and demodulators used in direct-conversion radio transmitters. This technique uses the actual in-phase (I) and quadrature (Q) components of the modulated signals at the input and output of the transmitter-under-test for adaptive determination of the modulator and demodulator complex gain imbalances. The accuracy of the new algorithm is carefully assessed and compared to earlier works. The robustness of the algorithm to a number of common practical implementation issues (group delay, DC offset, propagation delay) are also discussed and demonstrated.

Experimental study of the effects of RF front-end imperfection on MIMO transmitter performance

In this paper, the effects of power amplifier (PA) nonlinearity, and radio frequency (RF) crosstalk on the performance of the MIMO (multiple in, multiple out) transmitter are investigated comprehensively. The performance of the MIMO transmitter is studied based on the measured error vector magnitude (EVM) at the transmitting antennas. The simulation and measurement results show that PA nonlinearity and RF crosstalk need to be considered in the design process. In particular, it was found that the crosstalk represents a critical design factor that, if ignored, accentuates the effects of the other sources of impairments on the transmitter side, such as PA nonlinear behavior.

Concurrent dual band digital predistortion using look up tables with variable depths

A concurrent dual band predistorter implementation using look up tables is presented in this paper. The power amplifier introduces the main source of nonlinearity in a transmitter chain; and the distortion is further enhanced when sending signals in multiple carrier frequencies. Using a dual band predistorter based on a memory polynomial model allows for the compensation of these nonlinearities by taking into account both signals. Converting the coefficients to look up tables reduces the number of arithmetic operations needed in a signal processor. Results are presented with the conventional dual band predistorter and the look up table predistorter in operating bands 880 MHz and 2140 MHz, showing the linearization capability and accuracy with the proposed implementation agrees with the conventional predistorter.