Ce chapitre résume la philosophie de la solution d’exploration architecturale proposée dans cette thèse, à savoir fournir les moyen à un concepteur de trouver un optimal global
en termes d’opérateurs communs pour un équipement SDR multi-standards. Dans le
chapitre 6, nous avons relevé l’intérêt de l’opérateur DMFFT pour la modulation OFDM et le codage canal, et dans le chapitre 8 celui du FRMFB pour la canalisation. Dans un graphe plus complet, nous montrons dans ce chapitre que cela peut changer les équilibres et remettre en cause les optimums trouvés localement. C’est tout l’intérêt de ce travail de thèse. Nous n’avons pas étudié de graphe plus complexe encore en raison du manque de chiffres d’implantation sur cibles. Il n’était pas non plus possible d’envisager développer des chaînes entières de communication (en VHDL par exemple), ce qui était en-dehors des objectifs de cette thèse qui devait rester concentrée sur la méthode de conception.
Conclusion générale et perspectives
Ce manuscrit a présenté mon travail de recherche effectué au sein de l’équipe SCEE de SUPELEC/IETR sur l’optimisation d’équipements SDR multi-standards en utilisant une approche théorique. Cette approche, qui est basée sur un formalisme de graphe, fait partie de la catégories des techniques de paramétrisation par opérateurs communs.
Nous avons commencé le manuscrit en situant notre problème de conception au sein du
domaine de la radio logicielle dans le chapitre 1. L’approche de conception de typevelcro
encore majoritairement utilisée ne sera bientôt plus viable. D’où la nécessité d’introduire de nouvelles méthodes d’optimisation de la conception, en particulier d’équipements multi-standards, qui vont se généraliser.
Partant de ce constat, le chapitre 2 détaille l’approche de conception par paramétri-sation et met en particulier en valeur l’approches dite par opérateurs communs. Cela consiste à étudier les opérations du système à plusieurs niveaux de granularité. La sélec-tion du niveau optimal permet de faire l’équilibre entre complexité et performance. Une représentation du problème sous la forme d’un graphe est retenue.
Dans le chapitre 3, les caractéristiques du graphe permettant de modéliser les traite-ments effectué par un équipement de radio communication sont définies.
Le chapitre 4 étudie les différentes possibilités en termes de paramètres de coût qu’il faut ajouter pour annoter le graphe, en fonction des composants cibles visés. La fonction de coût associée au graphe est aussi définie. Une interface graphique a été développée pour permettre de construire les graphes de l’étude.
Une étude de différentes techniques d’optimisation fait l’objet du chapitre 5 et les con-fronte à notre problème d’optimisation. Le choix d’un algorithme de recuit simulé y est expliqué.
Les chapitres 6, 7, 8 et 9 des exemples de problèmes de conception sont traités pour des problèmes restreints à un sous-graphe. Cela permet de valider l’approche pour le cas des opérateurs DMFFT et LFSR notamment. Enfin, un dernier exemple tend à mon-trer le problème de manière plus complète et comment l’approche de conception proposée s’applique à un cas de conception plus étendu, ce qui est sa raison d’être.
Les perspectives à la suite de ce travail sont très nombreuses. Tous les problèmes n’ont d’une part pas été résolus, et d’autre part de nombreux axes de recherches ont été identifiés pour de futures investigations. En voici les plus importants :
• La solution proposée vise à ré-utiliser des opérateurs communs. Par conséquent un
problème d’ordonnancement en découle automatiquement. Ceci a été volontaire-ment éludé dans nos travaux en raison de nombreux autres points à régler en prior-ité. Cependant, nous pouvons déjà y apporter cet éclairage. A un premier niveau, tant que les opérateurs peuvent répondre en temps-réel quel que soit le nombre d’opérations faisant appel à eux, le problème se ramène à un cas conventionnel. Soit
des solutions d’ordonnancement statique sont envisagées (par période temporelle cor-respondant à la durée de vie d’un standard), soit des solutions d’ordonnancement en temps-réel exécutée par un processus gestionnaire de l’équipement (par exemple un processeur ayant un RTOS). A un second niveau, si un opérateur est surchargé, une solution consiste à dupliquer l’opérateur, une autre à changer le jeu d’opérateurs sélectionné. Il faudrait que cela soit pris en compte dans la méthodologie, ce qui semble nécessiter au moins un autre travail de thèse.
• De nouveaux algorithmes d’optimisation seraient à investiguer. D’autant plus si le
problème étudié ici est combiné avec celui de l’ordonnancement évoqué ci-dessus. Notamment cela replacerait les algorithmes génétiques en position de prétendant.
• Un travail très important pour continuer ce travail est d’identifier de nouveaux
opéra-teurs communs qui soient existent déjà soit sont à inventer.
• Des graphes de plus en plus complets, c’est-à-dire incluant plus de fonctions d’un
même standard et plus de standard, sont à construire. Nous n’avons regardé que des sous-graphes de la partie couche physique jusqu’à présent. L’un des buts de cette approche est justement de pouvoir trouver des opérateurs communs à des traitements des différentes couches OSI afin d’effectuer de l’optimisation inter-couches au niveau des traitements.
• Il pourrait être aussi possible de reformuler la fonction de coût. Nous avons utilisé
une somme pondérée mais d’autre possibilités existent.
• Un problème en ce qui concerne les coûts a été identifié en particulier dans le cas
d’une conception hétérogène impliquant du matériel (de type FPGA ou ASIC par exemple) et du logiciel (de type processeur ou DSP par exemple). Cela requiert une analyse complémentaire. Résoudre ce problème serait très prometteur car la méthode proposée permettrait alors de’effectuer un partitionnement automatique matériel/logiciel.
• Une perspective à long terme est de fournir un outil industriel aux concepteurs
d’équipements SDR multi-standards qui leur permette d’optimiser leur conception et d’orienter leur choix en termes de granularité en fonction des intérêts de leur entreprise. En effet, suivant qu’ils font partie d’une société de micro-électronique ou celle d’un fabricant de processeur, le choix en terms de granularité peut varier considérablement.
Chacun de ces sujets mériterait une thèse au moins pour y répondre ou commencer à y répondre.
1 Background and context
Wireless signal processing technology is experiencing a period of radical change, for both handsets and basestation applications. There has been an enormous proliferation of stan-dards in broadcast television, radio and mobile communications in a short duration of time. At present we have two or three standards at maximum, in our mobile phones like Global System for Mobile Communication (GSM), Universal Mobile Telecommunication System (UMTS) and Bluetooth etc. In the near future we will have in our mobile phones, standards like Wireless Fidelity (WiFi), Worldwide Interoperability for Microwave Access (WiMAX) for Wireless Local Area Networks (WLAN), Galileo and Global Positioning System (GPS) for positioning, Digital Video Broadcasting-Handheld/Terrestrial (DVB-H/T) for digital video broadcast, Digital Radio Mondiale (DRM) for digital radio etc. to name a few. Further, the rate of technology innovation is also accelerating and predicting technological change. These standards form the basis for an ever-growing number of sophis-ticated consumer electronic devices, each with the potential to sell in very high volumes. A multi-standards system is the right solution to successfully communicate with differ-ent systems using differdiffer-ent air-interfaces. Multi-standard systems can work within one standard family e.g. UMTS Terrestrial Radio Access-Frequency Division Duplexing/Time Division Duplexing (UTRA-FDD/TDD) for UMTS, or across different networks e.g. Digi-tal Enhanced Cordless Telecommunications (DECT), GSM, UMTS, WLAN etc. The need for transparency, i.e., the ability of radios to operate with some, preferably all, of these standards in different geographical regions of the world is ever increasing.
In conventional multi-standards designs, these complex standards are implemented using dedicated architectures, which are optimized to reduce cost to the absolute minimum. Products developed using dedicated architectures are often difficult to upgrade in order to support changes in the standards or to add new features.
In the beginning of 90’s, the concept ofSoftware Radio (SR) emerged from demonstrations
in military research to become a cornerstone of the 3G strategy for affordable, ubiquitous
and global communications. In addition, the need for amulti-band system which supports
more than one frequency band used by a wireless standard (e.g., GSM 900, GSM 1800,
GSM 1900), and amulti-channel system that supports two or more independent
The SR technology is a way to design a sufficiently programmable and reconfigurable ar-chitecture able to support many different transmission standards on a common platform. The reconfigurability of a SR system can offer a range of benefits at different levels. A radio system implemented on a reconfigurable architecture can be upgraded to fix bugs or to add functionalities, and it can also support new standards as it is assumed that there
is sufficient flexibility in the architecture. Demand for a multi-services system, which
provides different services e.g. telephony, data, video streaming, GPS and soon handheld television (TV) broadcast, Information Technology Services (ITS) etc. is rising. With the development of short-range networks like Bluetooth and IEEE 802.11, it is now possible to enhance the services of a radio by leveraging other devices that provide complementary services. Software radio’s reconfiguration capability will enable to support an almost infi-nite variety of service capabilities in a system.
The communication chains of different air interface standards, intended to be imple-mented on a common platform, have some common signal processing operations such as channel coding, modulation, equalization, etc. In order to exploit to a great advantage the commonalities among these communication tasks for different standards, one need firstly to identify these commonalities and secondly find the optimal way to implement a generic hardware platform with programmable modules. In this sense, a technique called
parametrisation was introduced. Parametrisation is a very promising technique that
con-sists of designing radio systems entities in a way which permits to take advantage of the programmable or at least reconfigurable capabilities of the underlying hardware of SR sys-tems. The key idea is to get an optimal sharing between hardware and software resources and find a best way to reuse some hardware and software modules that can be adapted just by a parameter change without affecting the system’s performances.
This thesis aims to contribute to the design of multi-standard SR equipment from a partic-ular angle compared to research efforts in general, carried out in SR community. Indeed, it does not propose to study how to adapt the current design tools for new challenges
pro-posed by the software defined radio (hardware/software co-design, high-level abstraction
tools, etc.) but how to re-organize the processing chain, in order to exploit new degrees of freedom in design. It fits into the context of SR and more precisely in the Common Op-erators’ (CO) technique for parametrisation. Further it explores in detail, two approaches
of CO’s technique; The Pragmatic Approach in general and The Theoretical Approach in
particular. CO technique can help reduce the cost, size and power consumption of SDR equipments, which are required to support a wide range of existing and future wireless technologies and services.
2 Thesis outline
The manuscript of this thesis is divided into three parts. Part-I consists of two chapters.
Chapter 1 provides an introduction to SR technology in general and outlines some of the limitations associated with it. This chapter further describes what software defined radio
(SDR) is, plus various commercial and military efforts put in this regard. It also looks into various transceiver architectures. It highlights the need/drivers for SR and different challenges in designing SR. In addition to this, it presents our point of view of tackling the design of multi-standards SDR equipment as a problem of granularity adjustment. The SR’s problematic, in this thesis is considered from the viewpoint of a research of com-monalities across several standards and in the standards themselves. In this context, an
important technique calledparametrisation that aims to optimize the resources use in the
SR system is presented in detail in chapter 2.
Chapter 2 is about the parametrisation techniques in SRs and in particular about CO’s
technique. According to the methodology of parametrisation, the common aspects of the different standards will become one common processing procedure, which could be
in-stalled in the device. This common procedure could be executed by a simplecall, thereby
resulting in a gain of both size and time with respect to the amount of code to be down-loaded or read (only parameters) in order to modify the radio behavior. From the design point of view, the parametrisation will optimize the co-design and will probably reduce the time to market by enabling incremental design. The parametrisation techniques can be divided into two categories 1) Common Function Technique and 2) CO’s Technique. CO’s
technique can be further divided into: 1)The Pragmatic Approach and 2)The Theoretical
Approach. It is explained how the theoretical approach to find the common operators
in the graph model of multi-standards SDR equipment can help the SDR designer, in finding global optimality. Various criteria for measuring this optimality could be memory size, downloading speed, reconfiguration speed, computation complexity etc. Chapter 1, 2 form the foundation for the development of solution to multi-standards design problem presented in Part-II of thesis.
Part-II consist of three chapters and these chapters explain different steps to solve the multi-standards SDR design problem using theoretical approach in the context of CO’s technique for parametrisation described in Part-I of this manuscript. These steps include 1) The definition of the graph 2) The definition of cost parameters and development of the cost function and 3) The optimization algorithms.
Chapter 3 explains how to model air interface standards as graphs. Starting from the fun-damentals of graph theory and after exploring the already present graphs it is concluded that for the multi-standards SDR design problem, the best choice is to use hypergraph. This is because of the dependencies needed for problem which were not available for other graph types. After this it describes the development of graph theoretic models for multi-standards SDR systems. Simplified versions of graphs for tri-multi-standards SDR equipments are presented. In addition, because of availability of extensive literature in the domain of network theory this chapter illustrate a way to recast the problem as network design problem. Later because of some difficulties it is decided not to convert the graph into a network design problem.
Chapter 4 looks for different cost parameters and types of the costs. Various costs param-eters are identified and explained. Based on the selected costs associated with different graph entities, a cost/objective function is formulated that is to be solved by
optimiza-tion techniques which are presented in chapter 5. The objective funcoptimiza-tion is formulated as multi-objective optimization problem (MOP) also known as multi-performance/vector optimization problem. Many real-world scientific and engineering MOPs are irregular and hence deterministic search techniques are not suitable for them. To solve these irregular problems, stochastic search and optimization approaches such as Simulated Annealing, Monte Carlo methods, Tabu search and Evolutionary Computation Algorithms were de-veloped as alternative approaches. A graphical user interface is being dede-veloped in Java, to facilitate drawing of graphs and annotating the cost parameters and costs to various entities of graphs.
Chapter 5 discusses the selected optimization techniques. In chapter 5 of this part, vari-ous optimization techniques available in literature with their pros and corns are explained. Based on this investigation, some techniques are selected that seem to be most appropriate to the problem at hand. A tool based on these techniques is developed in C++, that can be used to solve the problem. Also presented is a generic design example with the costs based on intuition. This example is solved by the selected techniques of optimizations implemented by tool. The objective is to find the best suited technique. Results proved
thatsimulated annealing technique is better than other selected techniques.
Part III consists of four chapters and these chapters are composed of design scenarios based on different common operators.
Chapter 6 describes the use of dual mode fast Fourier transform (FFT) operator as CO. This chapter investigates the frequency processing of Reed Solomon (RS) codes with the aim to insert the classical FFT operator, initially used for complex Fourier transform, in
the encoding and decoding processes of RS codes. To be able to exploit the whole FFT-C
structure, it explores finite field transforms having a highly composite length. The
can-didate transforms are the Fermat transforms defined over GF(Ft). These transforms led
us to revive a specific class of RS codes defined over GF(Ft). The redesign of the FFT-C
is elaborated in such a way to be able to provide two functionalities: complex Fourier transform and Fermat number transform (FNT). Conceptually, the design of such a dual mode operator implies the design of arithmetical operators capable to operate over the
two domains: CandGF(Ft). Proposed in this chapter, is a reconfigurable architecture for
the butterfly that constitutes the core of the dual mode operator. Based on the FFT-C
structural strategy, the architecture of the dual mode FFT (DMFFT) operator is designed. To evaluate the complexity and speed performances of this operator, DMFFT is imple-mented on ALTERA’s Field Programmable Gate Array (FPGA) devices. We illustrate our design approach through a simplified sub-design example. Results show that RBPE offers minimal cost when designing of multi-standard systems is considered. Compared to a Velcro FFT/FNT operator, the DMFFT presented an important gain in terms of Adaptive Lookup Table (ALUTs) and memory saving.
Chapter 7 explains the use of linear feedback shift register (LFSR) and its different variants as COs, that can be used to perform various tasks in an SDR equipment. Use of LFSR as a CO is one of the first implementation of CO’s technique on the area of techniques of parameterizations. Reconfigurable LFSR (R-LFSR) and Extended Reconfigurable LFSR
(ER-LFSR) which are derived from an improved classical LFSR structure, can be
con-figured to implement functions such as pseudo random sequence generation, scrambling,
convolutional coding, cyclic coding and RS coding/decoding. These developed LFSRs
re-place functions that can be derived from LFSR operations and are particularly suited for Orthogonal Frequency Division Multiplexing (OFDM) based air interface standards like IEEE 802.11, IEEE 802.16 and 3GPP LTE, etc. We explain our four developed common ar-chitectures to design 16 LFSR COs that can carry out several operations of multi-standard SDR transceivers. We also give the implementation details on Altera Cyclone II. Either a single or combination of these LFSR operators can be used to perform the functions as necessitated by the different air interfaces to be supported by SDR. Further we illustrate our design approach through a simplified sub-design and by running the optimization al-gorithms on subpart of the graph. We prove that different common operators are chosen depending upon the considered scenarios and they give efficient solutions as compared to
Velcro methods.
Chapter 8 presents and compares various channelization techniques in order to find which