Advances in Digital Front-End and Softw are RFProcessing:PartⅡ

Jun Fang

Fa-Long Luo

Mikko Valkama

Serioja Ovidiu Tatu

Tomohisa Wada

I n the first editorial of this two-part special issue,we pointed out that one of the biggest trends in wireless broadband,radar,sonar,and broadcasting technology is software RFprocessing and digital front-end[1].This trend encompasses signal processing algorithms and integrated circuit design and includes digital pre-distortion(DPD),conversions between digital and analog signals,digital up-conversion(DUC),digitaldown-conversion(DDC),DC offset,and I/Q imbalance calibration.Other important related topics are peak-to-average power ratio(PAPR)reduction,crest-factor reduction(CFR),pulse shaping,image rejection,digital mixing,delay/gain imbalance compensation,error correction,noise shaping,numerical-controlled oscillator(NCO),and various diversity methods.Digital techniques for RFprocessing have many advantages over traditional techniques in terms of power efficiency,cost and area reduction,flexibility,programmability,and reconfigurability.Digital RFprocessing is now regarded as key to enabling software-defined radio(SDR)that supports multistandard,multimode applications and fast time-to-market solutions[2],[3].

This special issue aims to stimulate and guide the development of new and improved systems for wireless communications and digital broadcasting.It is a timely and high-quality forum for scientists,engineers,technologists,broadcasters,manufacturers,software developers,and other professionals to engage in discussion.In this special issue of ZTE Communications published in English,we wish to show how the theory is translated into practicaltechnology for all the relevant standards.Readers are given ideal design methodologies to manage a rapidly increasing range of applications.

Step-by-step information for designing practical systems is included,and theory,principles,algorithms,standards,and implementation are all given comprehensive treatment.

The call-for-papers for this special issue attracted a good number of excellent submissions.After two-round reviews,twelve papers have been selected for publication in this special issue,which is organized into two parts and has been published in two consecutive issues in 2011.

The contents of part Iis divided into two groups.The first group consists of four papers,addressing different aspects of power amplification(PA)technologies.The second group,comprising two papers,is devoted to another interesting topic:architecture design and test of millimeter wave(60 GHz)wideband radio transceivers,which are mainly based on a six-port device and related technology.For more details on this part,refer to[1].Six papers have been selected for publication in part II,and these papers are organized again into two groups.The first group consists of three papers that focus on theory,design,and implementation of resampling,that is,up-conversion and down-conversion.The second group consists of three papers that describe non-uniform channelization of multiplexed frequency bands,selective subcarrier degradation-based crest-factor reduction,and an eigenfiltering-based antenna diversity scheme.

The first paper by Mehmood Awan,Yannick Le Moullec,Peter Koch,and Fred Harris describes efficient processing engines in the digital radio front-end.These engines,based on a polyphase channelizer,perform arbitrary sample-rate changes,frequency selection,and bandwidth control.The polyphase filter bank with five different resampling modes is used as a case study for embedded resampling in digital front-ends.These modes are(i)maximally decimated,(ii)under-decimated,(iii)over-decimated,and combined up-and down-sampling with(iv)single stride length and(v)multiple stride lengths.These modes can be used to obtain any required rational sampling rate change in an SDR front-end based on a polyphase channelizer.They can also be used for translation to and from arbitrary center frequencies that are unrelated to the output sample rates.In a future issue,the authors will report on their work analyzing hardware architecture of polyphase channelization.

The paper by Yue Zhang,Li-Ke Huang,Carsten Maple and Qing Xuan gives an overview of the concepts,theory,and design principles of digital down-conversion and up-conversion for SDR-based communication and broadcasting systems.After an introduction to essential concepts and principles,the paper discusses design issues in down-conversion,and super-heterodyne and direct-conversion architectures are presented.Details of the SDRdesign for up-conversion are described in this paper,and trade-offs in the design stages for filters,mixers,NCO,DAC,and signal processing are discussed in detail.

In the third paper of the first group,álvaro Palomo Navarro,RudiVilling,and Ronan Farrelldeal with the practical implementation of two non-uniform channelization techniques for multiplexed frequency bands.These techniques are based on generalized DFTfilter banks(GDFT-FB),which are a major processing task in the receivers of an SDRsystem.Filter bank-based schemes are efficient and flexible;however,the use of FIRfilters generally leads to high filter orders,and this is impractical.To overcome this problem,a multistage filtering structure is applied to the GDFT-FBto reduce the number of coefficients.In this approach,the number of filter coefficients and operations per input sample is less than that in the single-stage approach.These reductions make fixed-point implementation using existing FPGAplatforms more practical.

In the second group of papers,the first paper by R.Neil Braithwaite deals with crest-factor reduction.Selected data subcarriers are modified within an OFDM signal to reduce peaks in the time domain,and pilot and null subcarriers remain unchanged.In this approach,a set of peaks within an OFDM symbol interval are identified,and data subcarriers whose data element has a positive or negative correlation to the peak set are selected.For a subcarrier with an outer element and significant positive correlation,a bit error(reversal)is intentionally introduced in order to move the data element to the opposite side of the constellation.Outer elements on negatively-correlated subcarriers are increased in magnitude along the real or imaginary axis.Simulations described in this paper show that selecting the correct subcarriers for bit reversals and outward enhancements reduces the peak-to-average power ratio of the OFDM signal to a target value.At the same time,in-band degradation caused by bit error rate(BER)is limited.

In the paper by Fa-Long Luo,Ward Williams,and Bruce Gladstone a new antenna diversity system is proposed that uses eigenfiltering for OFDM-based wireless communication and digital broadcasting applications.Compared with the existing schemes,such as post-FFT,pre-FFT,and polyphase-based filter-banks,the proposed scheme performs optimally and has very low computational complexity.The adaptive algorithm for updating the eigenfiltering coefficients has the same complexity as the LMS algorithm.The proposed scheme offers a better compromise between performance,power consumption,and complexity in the real-time implementation of receivers in broadband communication and digital broadcasting systems.

The last paper in this special issue deals with analog-to-digital conversion and describes a histogram approach to error correction.The paper by Armin Jalili,J.Jacob Wikner,Sayed Masoud Sayediand Rasoul Dehghani describes a static calibration technique for flash analog-to-digital converters(ADC).The calibration technique is based on histogram test methods,and estimation of the equivalent errors in the flash ADC comparators is done in the digital domain,without any significant changes being made to the ADC comparators.In the trimming process,the reference voltages are adjusted to compensate for static errors.Behavioral-level simulations of a moderate-resolution 8-bit flash ADC show that,for typical errors,ADC performance is considerably improved by the proposed technique.Implementation of the proposed technique will be discussed in a future issue.

We would like to thank again all authors for their valuable contributions.We also express our sincere gratitude to all the reviewers for their timely and insightful comments on all submitted papers.It is hoped the content of this two-part special issue is informative and useful from various technologicaland implementation perspectives.