Advances in Digital Front-End and Software RF Processing:Part I

One of the biggest technology trends in wireless broadband,radar,sonar,and broadcasting systems is software radio frequency processing and digital front-end.This trend encompasses a broad range of topics,from circuit design and signal processing to system integration.It includes digital up-conversion(DUC)and down-conversion(DDC),digital predistortion(DPD),DC-offset calibration,peak-to-average power ratio(PAPR)or crest factor reduction(CFR),pulse/noise shaping,delay and gain imbalance compensation,numerical controlled oscillator(NCO),and conversion between analog signals and digital signals.Digital techniques for RF processing have many advantages over traditional techniques in terms of power efficiency,cost and area reduction,flexibility,and programmability.Digital techniques allow software defined radio to be reconfigured to support multiple standards and multimode applications for fast time-to-market solutions.These advantages are becoming increasingly important in future wireless infrastructure development and deployment.Implementing digital processing and circuits in front-end is highly desirable but also highly challenging.Huge efforts are required from industry,research institutes,and regulatory authorities to bring the next generation of wireless communication systems to fruition.Extremely stringent requirements on bandwidth,power consumption,and performance need to be met for future systems.It is also necessary to say a few words about software RF terminology.Software RF might be a pretty new concept but still encompasses digital RF.It belongs in the software defined radio(SDR)category but goes further,extending the SDR frontier.Software RF means almost limitless flexibility and scalability in the way RF signal processing is done,and digital is necessary for software RF.Software RF will ultimately become an operational part of SDR in coming years.

This special issue aims to stimulate and guide the development of new and improved RF systems for wireless communication and digital broadcasting.It aims to be a timely and high-quality forum for scientists,engineers,technologists,broadcasters,manufacturers,software developers and other professionals to engage in discussion.The call-for-papers for this special issue attracted a good number of excellent submissions.After two-round reviews,fourteen papers have been selected for publication in this special issue,which is organized in two parts and will be published in two consecutive issues in 2011.

The contents of Part I is divided into two categories.The first consists of four papers addressing different aspects of power amplification technologies.The second is devoted to another interesting topic:architecture design and testing of millimeter wave(60 GHz)wideband radio transceivers,which are mainly based on six-port devices and related technology.

The first paper by R.Neil Braithwaite provides a cost-effective method of measuring residual nonlinearities in an adaptive digitally pre-distorted amplifier.This method involves selective sampling of the amplifier output,integrated over the input envelope range,to adapt a fourth-order polynomial predistorter with memory correction.Simulations show that a transmitter using the proposed method can meet the ACLR specification.Inverse modeling of the nonlinearity is proposed as a future extension that will reduce the cost of the system further.

The second paper by W.Hamdane,A.B.Kouki,and F.Gagnon proposes a novel two-branch amplification architecture that combines baseband signal decomposition with RF front-end optimization.The proposed system separates the filtered modulated signals into two components that are amplified independently then combined to regenerate an amplified version of the original signal.A branch using an efficient amplifier transmits a low-varying envelope signal that contains the main part of the information.The other branch is used to amplify the residual portion of the signal.The baseband decomposition and RF part’s parameters are optimized,and the optimal configuration is determined for the best power efficiency and linearity.

In the third paper,Suranjana Julius and Anh Dinh deal with implementation of a power amplifier(PA)linearizer for an ETSI-SDR OFDM transmitter.This paper presents an interesting case study on satellite digital radio application in L-band.An adaptive linearizer is designed and implemented on the same FPGA device.Digital predistortion is used to correct the undesired effects of the PA on the transmitted signal.

This thematic group ends with an excellent paper by Ruili Wu,Jerry Lopez,Yan Li,and Donald Y.C.Lie that provides an updated overview of design technologies for RF PAs.The authors outline several promising design techniques for highly efficient silicon-based RF PAs and their use in mobile broadband wireless communications.Four important aspects in PA design are addressed.The paper is an invaluable source of information on new wireless standard requirements,design methodologies,power amplification architectures,and integrations.

Wireless applications have emerged in the 59-64 GHz ISM band in recent years,and these have attracted increasing interest from the IT and entertainment industries.60 GHz wireless local area networks(WLAN)are suited to applications with a short range and very high data-rate,such as high-speed home and office wireless networking,high definition television(HDTV),and interactive HD gaming among others.The fifth paper in Part I by Nazih Khaddaj Mallat,Emilia Moldovan,Serioja O.Tatu,and Ke Wu describes how advanced system simulation can be used to analyze and validate compact,low-cost six-port transceivers for future wireless LANs operating at millimeter-wave frequencies.Frequency division multiplexing is used by introducing four QPSK channels in the wireless communication link.A data rate of about 4 Gbit/s in the 60-64 GHz unlicensed band can be reached.Both single carrier and multicarrier architectures are presented,and results are compared.The proposed wireless system is potentially an excellent and efficient candidate for millimeter-wave communication systems operating at quasi-optical data rates.

The last paper in Part I,by D.Hammou,E.Moldovan,and S.O.Tatu,is devoted to a new implementation of a millimeter-wave heterodyne receiver based on six-port technology.The six-port model is implemented in ADS software using S parameter measurements for advanced,realistic simulation systems of a short-range 60 GHz wireless link.The proposed mixer is compared with a conventional balanced millimeter-wave mixer,and some improvements are observed in conversion loss and I/Q phase stability over the LO and RF power range.Furthermore,the bit error rate analysis and error vector magnitude analysis show that the proposed architecture can be successfully used for wireless link transmission up to 10 meters.

We would like to thank 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 that the contents of this issue are informative and useful from various technology and implementation aspects.Please stay tuned for the second part of this special issue.