Marcus Windisch

Dirty RF: a new paradigm

The implementation challenge for new low-cost low-power wireless modem transceivers has continuously been growing with increased modem performance, bandwidth, and carrier frequency. Up to now we have been designing transceivers in a way that we are able to keep the analog (RF) problem domain widely separated from the digital signal processing design. However, with todays deep sub-micron technology, analog impairments-dirt effects-are reaching a new problem level which requires a paradigm shift in the design of transceivers. Examples of these impairments are phase noise, non-linearities, IQ imbalance, ADC impairments, etc. In the world of dirty RF we assume to design digital signal processing such that we can cope with a new level of impairments, allowing lee-way in the requirements set on future RF sub-systems. This paper gives an overview of the topic and presents analytical evaluations of the performance losses due to RF impairments as well as algorithms that allow to live with imperfect RF by compensating the resulting error effects using digital baseband processing

Dirty RF: A New Paradigm

The implementation challenge for new low-cost low-power wireless modem transceivers has continuously been growing with increased modem performance, bandwidth, and carrier frequency. Up to now we have been designing transceivers in a way that we are able to keep the analog (RF) problem domain widely separated from the digital signal processing design. However, with today’s deep sub-micron technology, analog impairments – “dirt effects” – are reaching a new problem level which requires a paradigm shift in the design of transceivers. Examples of these impairments are phase noise, non-linearities, I/Q imbalance, ADC impairments, etc. In the world of “Dirty RF” we assume to design digital signal processing such that we can cope with a new level of impairments, allowing lee-way in the requirements set on future RF sub-systems. This paper gives an overview of the topic and presents analytical evaluations of the performance losses due to RF impairments as well as algorithms that allow to live with imperfect RF by compensating the resulting error effects using digital baseband processing.

Blind I/Q imbalance parameter estimation and compensation in low-IF receivers

The rejection of the image signal is a problem inherent to all receiver architectures. One of the benefits of the low-IF receiver is, that image rejection is realized by I/Q signal processing instead of a fixed analog filter, making it highly reconfigurable and cost-efficient. However, unavoidable imbalances between the I- and Q-branch lead to a limited image attenuation. In this paper a novel I/Q imbalance compensation scheme is presented, in which the unknown analog imbalance parameters are estimated digitally without the need for any calibration or training signal. Based on these estimates the interference by the image signal is effectively compensated, upgrading the vulnerable ordinary low-IF receiver to a powerful advanced receiver architecture, where the mean value of the image-to-signal ratio never exceeds a pre-defined maximum value, regardless of the image signal power.

Latency Critical IoT Applications in 5G: Perspective on the Design of Radio Interface and Network Architecture

Next generation mobile networks not only envision enhancing the traditional MBB use case but also aim to meet the requirements of new use cases, such as the IoT. This article focuses on latency critical IoT applications and analyzes their requirements. We discuss the design challenges and propose solutions for the radio interface and network architecture to fulfill these requirements, which mainly benefit from flexibility and service-centric approaches. The article also discusses new business opportunities through IoT connectivity enabled by future networks.

Adaptive I/Q imbalance compensation in low-IF transmitter architectures

Advanced transmitter architectures based on I/Q signal processing are highly attractive to applications demanding a reconfigurable setting of the transmission parameters, such as carrier frequency and bandwidth. However, unavoidable imbalances between the I- and the Q-branch in the analog part lead to an insufficient suppression of the image signal. A digital predistortion structure is presented to overcome this problem. We show that the adaptation of the predistortion parameters can be performed blindly during the regular transmission mode. With the proposed novel adaptation algorithm, no special training or calibration signal is needed. After 10 iterations, an image suppression of 60-100 dB is achievable, depending on the accuracy of the measured power estimates of the desired transmit signal and the unwanted image signal.

Performance Degradation due to I/Q Imbalance in Multi-Carrier Direct Conversion Receivers: A Theoretical Analysis

I/Q imbalance has been identified as one of the most serious concerns in the practical implementation of the direct conversion receiver architecture. In particular, at the reception of multi-carrier signals the achievable error rate is strictly limited by the I/Q imbalance. Knowledge about the quantitative link between the hardware parameters and the resulting error rate is essential for a reasonable design of the receiver front-end. In this paper a novel framework for the analytical computation of the symbol error probability in multi-carrier systems is presented. We consider an arbitrary M-ary QAM modulated multi-carrier signal, which is corrupted by both a noisy Rayleigh fading channel and receiver I/Q imbalance. The theoretical results are validated exemplarily for the IEEE 802.11a WLAN standard.

Performance analysis for blind I/Q imbalance compensation in low-IF receivers

A novel blind I/Q imbalance parameter estimation and compensation scheme for low-IF receivers, which was previously proposed by the authors, is analytically analyzed in this paper. As an outcome, we present an equation for calculating the expected power ratio of image and desired signal after compensation. The excellent performance of the novel compensation scheme, which was so far justified by simulations only, is now substantiated by a solid theory.

On the Impact of I/Q Imbalance in Multi-Carrier Systems for Different Channel Scenarios

I/Q imbalance has been identified as one of the most serious concerns in the practical implementation of the direct conversion receiver architecture. Facing the performance-degrading impact of the I/Q imbalance, in particular at the reception of multi-carrier signals, various digital estimation and compensation techniques have been proposed in the literature. Knowledge about the quantitative link between the I/Q imbalance parameters, with or without digital compensation, and the resulting error probability is essential for both a reasonable design of the receiver front-end and for the dimensioning of the digital compensation algorithms. The objective of this paper is to quantify the impact of the I/Q imbalance in terms of closed-form error probability formulas, providing the desired theoretical link between cause and effect of the I/Q imbalance.

Blind Estimation and Compensation of I/Q Imbalance in OFDM Receivers with Enhancements Through Kalman Filtering

I/Q imbalance has been identified as one of the key challenges in the practical implementation of the direct-conversion receiver architecture. This holds in particular at the reception of multi-carrier signals, such as OFDM. While allowing highly integrated and low-cost solutions, direct-conversionmay cause serious signal distortions due to an imbalanced I/Q down-conversion. This paper shows how blind, i.e. non reference-based, signal processing techniques can be applied for a digital estimation and compensation of I/Q imbalance. Further enhancements of the blind estimator can be achieved through the application of Kalman filtering, including tracking and compensation of time-varying and frequency-selective I/Q imbalance.

On the impact of analog-to-digital conversion on blind I/Q imbalance parameter estimation in low-IF receivers

The low-IF receiver is very attractive, because the need for an analog image rejection filter is avoided. Instead, the image rejection is realized by I/Q signal processing. However, unavoidable imbalances between the I- and Q-branch in the analog part of the receiver result in a limited image attenuation. In previous work a blind I/Q imbalance compensation scheme for low-IF receivers was presented and its performance was analyzed. However, the impact of the analog-to-digital conversion (ADC) was not taken into consideration. In particular, the quantization noise, which is unavoidable in practical implementations, has been neglected. In this paper this blind I/Q imbalance compensation scheme is analyzed regarding to its sensitivity to an imperfect ADC. It can be shown by means of analysis as well as simulation, that the compensation scheme is robust against ADC effects.