This article proposes a novel adaptive architecture for blind identification and compensation of frequency response mismatches in 4-channel time-interleaved analog-to-digital-converters (TI-ADCs) Detailed frequency response mismatch modeling is first carried out elaborating in detail the interleaving mismatch spurs characteristics Stemming from the established mirror-frequency crosstalk nature of the different mismatch spurs, the interleaving mismatch identification process is then carried out using complex second-order statistics based methods The developed learning algorithm performs the mismatch identification and learns the mismatch compensation filter parameters in a blind manner for almost the full digital bandwidth of the 4 TI-ADC system The proposed solution's efficiency and performance are verified and demonstrated using state-of-the-art RF-sampling TI-ADC hardware measurements with GHz range instantaneous bandwidth In addition to this, the relationship between a four-channel TI-ADC and an I/Q sampling 2-channel TI-ADC is explored and an interesting link between the two is established in this work

Frequency Response Mismatches in 4-channel Time-Interleaved ADCs: Analysis, Blind Identification, and Correction

In this paper, novel blind identification and compensation architectures for the frequency response mismatch of a two-channel time-interleaved analog-to-digital-converter (TI-ADC) are proposed. First, detailed modeling of the frequency response mismatch is carried out, establishing a direct connection and similarity to the well-known in-phase/quadrature mismatch problem. Stemming from this modeling, the proposed blind mismatch identification and compensation architectures are then developed building on complex statistical signal processing. Compared to the existing methods in the literature, the proposed solutions can identify and correct the frequency response mismatch in a fully blind manner for the full digital bandwidth (BW) of the ADC system, and are also applicable in sub-sampling TI-ADC devices and RF sampling. The efficiency of the proposed solutions is verified and demonstrated using comprehensive measurements of actual RF-sampling TI-ADC hardware with gigahertz-scale instantaneous BW.

Analysis, Blind Identification, and Correction of Frequency Response Mismatch in Two-Channel Time-Interleaved ADCs

This paper presents an all-digital transmitter solution particularly suited for Massive multiple-input and multiple-output (MIMO) systems for mobile communications. Massive MIMO is a key candidate to address the challenges of future mobile communication standards, especially to provide higher capacity in dense urban scenarios. While the required communication theory is elaborated to a great extend, the transceiver hardware complexity remains a potential economical show-stopper. This paper demonstrates that all-digital transmitters can be employed to reduce size, cost, and engineering effort of heavily parallelized transmit architectures. Therefore, today’s all-digital transmitter concepts are analyzed and improvements are suggested to increase performance and feasibility. The realized setups prove that the key specifications of mobile communication standards can be met utilizing dedicated integrated circuits or even by using off-the-shelf FPGAs and their high-speed interfaces. We show that we can generate 8 parallel 5 MHz LTE signals at 2.6 GHz out of a single FPGA with an ACPR of 48 dB with a coding efficiency of 50% using only binary waveforms.

An All-Digital, Single-Bit RF Transmitter for Massive MIMO

This paper presents a frequency-domain analysis of the noise and distortion terms produced by a digital RF modulator that uses pulse width modulation (PWM) for the amplitude modulation and a square wave as RF carrier. Insight in these terms is important as they limit the error vector magnitude (EVM) the modulator can achieve. For each of the terms, frequency-domain expressions are derived which are valid as long as the quantization noise is small and the digital PWM is sufficiently close to natural-sampling PWM. The dependency of the terms on the different system parameters is estimated, and the calculations are supported and complemented with simulation results. The presented analysis improves the understanding of the dominant noise and distortion sources, which can significantly speed up the design of PWM-based transmitters.

Frequency-Domain Analysis of Digital PWM-Based RF Modulators for Flexible Wireless Transmitters

We present a novel method for the estimation and correction of mismatch errors in time-interleaved analog-to-digital converters. The estimation of the mismatch errors requires a training signal, but it is efficient, accurate, and effective while keeping low the complexity of the associated algorithm, when compared with other works in the literature. The compensation strategy uses the estimated parameters for offset and gain mismatch error correction, and implements a low-complexity (similar to that of a finite-impulse response filter) Lagrange interpolation filter to correct errors due to timing mismatch, which depends on the dynamics of the input signal. The proposed compensation strategy achieves almost ideal behavior over a wide bandwidth, i.e., only 12% of oversampling is required for an almost complete cancelation of distortion. The method has been tested under severe mismatch conditions (up to 10% timing mismatch and 5% gain and offset mismatch), showing an improvement of over 6 bits in the effective number of bits and 50 dB in the spurious-free dynamic range. In addition, when compared with the available techniques, no limitation on the number of channel converters is introduced since the compensator effectively cancels the distortion even in the presence of large mismatches.

Efficient Estimation and Correction of Mismatch Errors in Time-Interleaved ADCs

Burst-mode operation of power amplifiers (PAs) is a promising concept towards higher power efficiency in radio frequency (RF) transmitters. Such transmitters use pulse-width modulation (PWM) to create the driving signal for the PA, and a reconstruction filter after amplification to obtain the transmission signal. However, conventional digital pulse-width modulated signals contain a large amount of distortion that cannot be removed by the reconstruction filter in a satisfactory manner.

Aliasing-Free Digital Pulse-Width Modulation for Burst-Mode RF Transmitters

In this paper we review the progress in the design of low-complexity digital correction structures and algorithms for time-interleaved ADCs over the last five years We devise a discrete-time model, state the design problem, and finally derive the algorithms and structures In particular, we discuss efficient algorithms to design time-varying correction filters as well as iterative structures utilizing polynomial based filters Finally, we give an outlook to future research questions

A review on low-complexity structures and algorithms for the correction of mismatch errors in time-interleaved ADCs

Digital pulsewidth modulators are used to transform nonconstant amplitude signals into pulsed signals, such that the information lying in the signal amplitude is encoded in the widths of pulses. Because of the inherent aliasing distortion in digital pulsewidth-modulated signals, additional signal processing steps are required to make pulsewidth modulation (PWM) suitable for applications like digital audio amplification or burst-mode radio-frequency transmitters. These processing steps, however, entail an undesirable increase in computational effort. This brief presents a multiplierless implementation of a digital aliasing-free pulsewidth modulator using lookup tables, adders, and arithmetic shifts only. Mathematical equations of asymmetric double-edge PWM are given, as well as a modified aliasing-free version of this PWM technique that directly integrates the distortion-avoiding signal processing steps into the pulsewidth modulator. Based on these equations, a multiplierless implementation of the aliasing-free PWM (AF-PWM) is developed. Simulation results obtained with a Simulink fixed-point model show that the proposed modulator implementation provides a feasible solution for realizing AF-PWM with low computational effort.

Multiplierless Implementation of an Aliasing-Free Digital Pulsewidth Modulator

The burst-mode transmitter has been proposed as an efficiency enhancement technique for signals with high peak-to-average-power ratios (PAPRs), where appropriate modulation schemes such as pulse-width modulation (PWM) are used to generate a switching signal to drive the radio frequency (RF) power amplifier (PA). Ideally, a PWM signal is of infinite bandwidth, however, high-frequency spectral components are attenuated, e.g., by the bandlimitation of the PA matching network or the PWM circuit itself. Bandlimitation introduces ripples in the signal amplitude, which might reduce the PA efficiency in burst-mode operation. In this paper, we show that a bandlimited PWM signal does not necessarily degrade the overall transmitter efficiency because of the higher coding efficiency. The coding efficiency of bandlimited PWM signals, i.e., PWM signals with a finite number of harmonics, is derived analytically and verified by measurements. Additionally, from the measurements, we exemplarily determine the number of harmonics for bandlimited PWM signals with the best transmitter efficiency-linearity trade-off, and therefore demonstrate the excellence of bandlimited PWM for burst-mode transmitters in terms of transmitter efficiency as well as transmission signal quality.

Coding efficiency of bandlimited PWM based burst-mode RF transmitters

Among other goals, multilevel burst-mode radio frequency (RF) transmitters have to achieve two main objectives: high efficiency, and good transmission signal quality. Both of these goals depend, to a large extent, on the driving signal modulator, which can, for example, be a digital pulse-width modulator. However, conventional digital pulse-width modulation (PWM) contains aliasing distortion that prevents achieving satisfying transmission signal quality. In this paper, an aliasing-free digital multilevel PWM method is introduced that allows for achieving good transmission signal quality, while at the same time offering the potential of reaching a coding efficiency of 100%. The mathematical equations for aliasing-free digital PWM are extended in a way that makes them suitable for the use in multilevel burst-mode RF transmitters. It is then shown how the aliasing-free multilevel PWM method can be used to achieve 100% coding efficiency. Simulations demonstrate the performance capabilities of the proposed PWM method.

Katharina Hausmair

Papers

Aliasing-Free Digital Pulse-Width Modulation for Burst-Mode RF Transmitters

A review on low-complexity structures and algorithms for the correction of mismatch errors in time-interleaved ADCs

Multiplierless Implementation of an Aliasing-Free Digital Pulsewidth Modulator

Coding efficiency of bandlimited PWM based burst-mode RF transmitters

How to reach 100% coding efficiency in multilevel burst-mode RF transmitters