Christophe Hurter
email: christophe.hurter@aviation-civile.gouv.fr

Audrey Girouard
email: audrey@cs.queensu.ca

Nathalie Riche

Catherine Plaisant
email: plaisant@cs.umd.edu

Active progress bars : facilitating the switch to temporary activities

Keywords: Progress bars, participatory design, frustration, task switching H5.m. Information interfaces and presentation (e.g., HCI): Miscellaneous

come

Introduction

Have you ever been bored waiting for a progress bar to come to its end? Wished you could do something else productive while waiting? Or actually start doing something else then forgot to come back? You are not alone: time is precious and users are impatient [3]. Many designs have been proposed to improve the progress bars themselves [START_REF] Harrison | Rethinking the progress bar[END_REF] [START_REF] Harrison | Faster progress bars: manipulating perceived duration with visual augmentations[END_REF], but little has been done to help users make better use of the waiting time. The mobile applications on smart phones have clearly demonstrat-ed that users can accomplish many short tasks in a just a few minutes-even seconds-which can be productive or entertaining. We believe progress bars can be augmented with simple functionalities facilitating a switch to a temporary activity, and a smooth return to the primary activity. The remarkable adaptability of computer users means that users develop their own strategies. They may stretch, head for the coffee pot, rest (Figure 1), or stay on the computer and manually switch to a temporary activity such as checking email or Facebook, or reviewing their to-do list. Sometime they may forget to return to the primary task or choose not to return, breaking the flow [START_REF] Csikszentmihalyi | Flow: The Psychology of Optimal Experience[END_REF].

The primary activity, its context and the duration of the wait may determine the type of temporary activities users engage in while the primary one is on hold. Context will also dictate the benefitor danger -of switching to another activity. Users trying to stay focused on a single task may not want to switch, while others may welcome assistance in switching between tasks. Our goal is to investigate user interface designs that would facilitate task switching when users welcome it. We hypothesized there would be significant user differences in the choice of temporary activity and the level of control needed, and hoped to identify key design elements that would make active progress bars more beneficial.

Previous work investigated the complex research topic of self-interruption [START_REF] Csikszentmihalyi | Flow: The Psychology of Optimal Experience[END_REF]. It must be noted that task switching comes at a cost [START_REF] Csikszentmihalyi | Flow: The Psychology of Optimal Experience[END_REF]. Researchers keep trying to better understand the effects of interruptions which are typically considered a significant nuisance (e.g. [START_REF] Czerwinski | A diary study of task switching and interruptions[END_REF]). Interruptions are usually described as something which is not under users' control (e.g. somebody walking in the office), while a progress bar is generally the result of users' actions and may be considered a break and not an interruption.

In this paper we propose the concept of Active Progress Bars to facilitate the switch to temporary activities. We first describe a taxonomy of waiting period contexts by discussing characteristics of primary and temporary activities, and waiting periods. We then report on 5 participatory design sessions, and a follow-up survey of 96 respondents. Finally we describe an early prototype of active progress bars, and report on initial use.

Characterizing Waiting and Progress Bars

To guide our exploration of active toolbars we first defined a set of dimensions describing waiting and progress bars. A simple taxonomy was defined, and then iteratively refined as we analyzed the data from our participatory design sessions.

We identified three main temporal components: (1) the primary activity the user is performing before the progress bar appears; (2) the waiting period during which the user is shown a progress bar; and (3) the temporary activity the user can perform while waiting. For each component, we identified multiple dimensions.

Primary Activity -Context

We identify three types of contexts: work, entertainment activity, and idle. We use context as a dimension and not the software currently used, as this might vary per user. For instance, checking emails might be considered work if done by a lawyer during a week day, but might be entertainment for a teen user. The third contextidle -refers to situations when the user was not performing any specific task before the start of the progress bar (such as when booting up a computer). Primary Activity -Stage When the waiting occurs, the primary activity can be in one of four stages: (1) at the start of the activity, when loading a program; (2) during the activity, when performing data transformation; (3) at the end of the activity, when logging off; and finally (4) in a peripheral activity, when downloading a file required later.

Waiting period -Control of initiation

The start of the waiting period can be under user control or not. Users decide when to compile their code or save a large document, but the waiting may also be triggered automatically during the primary activity, such as when a large document is saved automatically.

Waiting period -Duration predictability

The waiting period can be either determined or undetermined. Determined waiting periods correspond to situations where an algorithm can estimate the length of the wait. Often the duration of the wait cannot be determined. It is often expressed in the interface by a rotating wheel or hourglass instead of a progress bar.

Waiting -Duration

The duration of waiting period can vary significantly. To simplify, we distinguish four ranges: the waiting can be instant -less than 1 second of wait, short -less than 5 seconds, mediumbetween 5 and 15 seconds, or long -more than 15 seconds to wait.

In this paper, we will discuss short, medium and long waits because they can be perceived by the user and influence the decision about what to do during this period. Note that recent work on progress bars only focused on short or medium wait times (5s for [START_REF] Harrison | Rethinking the progress bar[END_REF] [START_REF] Harrison | Faster progress bars: manipulating perceived duration with visual augmentations[END_REF]).

Waiting -Notification

We observed three types of notification of the progress during the waiting period. It can be discrete (e.g. number of seconds, if the end of the wait time is emphasized), continuous (percentage done), binary (icon or message) or there can be no notification at all.

Temporary Activity -Context

The temporary activity has a context with the same characteristics of the primary activity (work, entertainment or idle). However, the context of the temporary activity does not need to be identical to that of the initial activity. One could watch a short fun video (entertainment) while installing a statistical package (work). The idle context of a temporary activity refers to the user stopping all computer activity, e.g. to stretch.

Temporary Activity -Interaction

This dimension captures the level of involvement of the user in the temporary activity. The activity can be passive: does not require the user to perform any action, e.g. watching an animation. Or, it can be active and require users to interact with the computer, e.g. tagging a photo, browsing the web or playing games.

Temporary Activity -Determination of completion

The temporary activity may be finite or not. Finite activities have an identifiable end, such as tagging a photo, updating one's status. Non-finite activities do not have an explicit end, e.g. web browsing or reading a newspaper. They may be best accomplished during an undetermined length of time. Finite tasks may allow smoother return to the primary activity.

Participatory Design Sessions

We conducted five participatory design sessions to better understand the experiences of users when they wait. We selected the groups (22 users total) through an iterative process. We started with two groups of computer scientists, based on the results of the preliminary questionnaire and the two first participatory design sessions, we reworked the taxonomy and identified professional groups more likely to shed light on the design space. Specifically, we had heard that downloading and editing pictures were common activities that required waiting; therefore we selected a group of photographers. We had also heard that people were interested in managing their time during the wait, so we worked with a group of managers. After starting with programmers we focused on groups with less extensive technological training.

In each session, users were asked to identify and describe three typical waiting situations before brainstorming on activities they may want to conduct to improve how they spend their waiting time. The brainstorming was situated in up to five predefined user cases, and their own identified situations.

The most common waiting situations reported by participants were loading software, installing/updating a program, loading webpage, downloading pictures from a camera, and booting a computer (Figure 2). In general, users reported a great diversity of situations, such as ripping and burning CDs, connecting to Wifi, syncing a phone, streaming web content, doing a backup, booking a flight, rendering video and logging off.

Participants in different groups mentioned the importance of avoiding waiting time as much as possible. This point was particularly stressed by the group of managers. The solutions proposed by this group included to schedule the booting up of their computer a few minutes before their arrival and scheduling soft-ware updates after their departure from work, as well as to rank the applications to be launched at startup to get working immediately.

Several participants suggested temporary activities that were in the same context as the primary activity. However, to avoid interfering with the primary activity, these temporary ones should remain passive (not require any action). One participant proposed the situation of booking a flight ticket, which lead to a large number of ideas. Most consisted of providing contextual information about the travel destination during the wait time, such as top tourists activities, weather, local headlines and events in the area (Figure 3).

In two different groups, several participants mentioned the fact that in many cases, they did not care about staying in context or in the flow. Indeed, they commented that these waiting periods could serve as breaks since they already used them as such when they knew the wait would be long. They explained that they could use their machine's booting up time to get coffee and greet colleagues, or a large software's loading time to browse the internet.

One of the predetermined user cases was the inclusion of some type of entertainment application in the progress bar. Overall very few participants responded favorably to this suggestion. The entertainment ideas mentioned were to show feeds from Facebook, news headlines, RSS feeds or the word of the day.

Survey

We surveyed 96 participants (47 females) on broader user preferences about temporary activities in different situations. Situations came from the discussions with our participants during the workshops. Most of them divided their activities in -at work‖ or -at home‖. While the terminology used may not be adequate (-at home‖ can be seen as the lunch time at work premises or -at work‖ can be seen as a working day out of the office), most of our participants use this simple classification to characterize their secondary activities and think about them. 47% of the participants mentioned staying idle for short waiting period (<5s), while 37% of the participants reported switching to a temporary activity. However, 65% answered that they switch to other activities for longer wait (> 15 s). Surprisingly only 50% preferred to do something else when the waiting period was more than 1 minute.

Active Progress Bar Design 65% of our participants indicated a willingness to use an active progress bar to help them switch to a temporary activity. They estimated that it was worth using the tool when the wait was over 36 seconds on average. 66% of the participants answered that the temporary activity should be customizable as a function of the application that makes you wait. 70% of the participants answered a preference for switch back automatically to their primary activity at the end of the waiting period. 36% of the participants indicated a desire to switch back to their activity on their own, by retrieving the appropriate window. However, they mentioned that they wanted to be notified that the wait was over. Most of the participants preferred a visual artifact that displayed the end of the waiting period.

Desired Activities when Waiting

In work settings, 60% of our participants answered that they would prefer to stay in context and perform a work related activity or manage their to-do list and calendar during a wait of more than 1m. During a medium waiting period of 15s, 40% of the participants would rather perform a passive activity such as getting information about the program waiting or about the weather or traffic. Only 15% of our participants mentioned that they would perform similar passive activities for short waiting periods. At home, 67% of our participants favored activities such as reading twitter or news feeds, in particular for longer waiting periods.

Active Toolbar Prototype

We developed an active progress bar prototype to investigate some of the ideas generated by participants. This software monitors the operating system (OS) and when it detects a waiting period proposes temporary activities that the user can customize (Figure 4). Since a common way to notify the user of a waiting period is to display a progress bar control that fills over time, our prototype monitors and evaluates all the system progress bars lasting longer than a second in the OS. Users can select the value in seconds of the minimum estimated waiting time that should trigger the active progress bar. User can choose if they want the temporary activity to start automatically when the wait is detected or be presented with a menu of activities.

Our Active Toolbar prototype proposes samples of secondary activity applications, combining existing applications with a normal progress bar. Entertainment activities include games such as Sudoku, a web browser with Facebook (Figure 5), and a photo viewer (Figure 6), while work activities include FreeWriter, which allows the user to manage a to-do list, or an RSS feed. Users can also choose any other application of their choice to be launched, but it would not include the integrated progress bar indicating the remaining time.

Conclusion

In this paper we introduced active progress bars, a new approach facilitating the switching to temporary activities when a primary activity requires users to wait. Our contributions are:

 a multifaceted study providing evidences that a subset of users see a benefit in switching to a temporary task while waiting and welcome the idea of getting assistance in switching tasks;

 we provided a taxonomy of waiting times;  we explored the design space of an active progress bar tool using four approaches (participatory design, survey, informal discussions, and prototype);  we demonstrated the feasibility of detecting the waiting with an active progress bar prototype.

In summary, we believe that with adequate user control, active progress bars have the potential to help users make better use of their time while waiting. We look forward to further development of the prototype and hearing users' feedback following its deployment.

Figure 1 :

 1 Figure 1: When asked to wait users are often frustrated. Could productive or relaxing activities be proposed?

Figure 2 :

 2 Figure 2: Paper prototypes for launching activities while downloading photos

Figure 4 :

 4 Figure 4: The Configuration window allows users to set their preferences about when to trigger the progress bar, what activity to trigger, and various parameters for those temporary activities such as which directory to use for the photo slideshow. A debugging window shows an event log.

Figure 5 :

 5 Figure 5: The user has configured the Active Toolbar to popup a browser window with Facebook. While waiting for a program to install (top right), he can login. Next time he needs to wait, he will see updates from his friends.

Figure 6 :

 6 Figure 6: The screenshot displays a large directory was being copied. At the bottom left appears the traditional progress bar produced by Windows Explorer. Active Toolbar window shows a slide show of photos from a favorite directory. Users can remain passive and enjoy the pictures for a few seconds, or be more active and rename files, a common way to annotate photos.


Fabien André
email: fabien.andre@enac.fr

Multiple views of control flows as a tool to improve programmer's understanding of interactive software

My work addresses the question of how to improve programming notations and environments used for developing interactive systems. I focus on representing the control flow in interactive programs in the hope that it will improve the programmer's ability to understand how their programs work. My thesis is that offering programmers the ability to create and manipulate multiple ad-hoc views of the control flow is the right way to go, where current programming languages and environment each choose one particular view.

Introduction

We are interested in GUI programmers, who build interactive software with high demands on behavior [START_REF] Myers | How designers design and program interactive behaviors[END_REF] and graphics [START_REF] Chatty | Revisiting visual interface programming: creating GUI tools for designers and programmers[END_REF]. One of their main difficulties is with external control flows: the execution order of instructions depends on users' actions or other external conditions. This conflicts with the imperative functional paradigm, and one needs workarounds such as callbacks: subprograms do not call each other but ask to be activated under certain conditions. As a result, one can not follow the execution flow straightly, and need to track all subscriptions (problem known as the "spaghetti of callbacks" [START_REF] Myers | Separating application code from toolkits: eliminating the spaghetti of call-backs[END_REF]).

This problem is particularly crucial when it comes to changing existing code. One needs to understand causal relationship before selecting the instructions that need editing: "What should I change in code to change this background color?", "What happens if I alter this line of code?".

In the next part, we will introduce a work scenario highlighting this problem. We will then review how others have addressed this problem, then present our angle and current results.

Work Scenario

Annette is coding an half-pie menu to navigate hierarchical data structure on the side of a tabletop system (as described in [START_REF] Hesselmann | Stacked Half-Pie menus: navigating nested menus on interactive tabletops[END_REF]). A colleague of hers has successfully implemented the menu with arrow button navigation. She is now adding a direct manipulation scrolling to her Menu. She notes a weird flickering in the motion of menu items.

The drag callback of the dragged MenuItem updates the MenuItem position, as well as the offset of the MenuLevel, other MenuItems are notified through an Observer (problem: spaghetti of callbacks). The problem comes from Menu Item embedding a "slow in /slow out" animation in position accessor, viz. setting the position of an Item launches an animation to reach new position (problem: machinery in seemingly innocuous code). This code was introduced for the step-by-step scrolling with buttons. The control flow interesting Annette is spread across two files, five functions, with inversions of control that make the control flow hard to follow.

State of the art

Studying programmer practice shows that they spend most of their time editing existing code, and thus trying to understand it [START_REF] Singer | An examination of software engineering work practices[END_REF]. A number of works have tackled program understanding in different communities: Programming Languages, Psychology of Programming, Software Engineering, Software Visualization.

A first class of tools promote a top-down approach to reduce information complexity1 . This started with structured programming (that aimed at "intellectual manageability" [START_REF] Wirth | On the composition of Well-Structured programs[END_REF]), and continued with Object-Oriented Programming and associated languages (Smalltalk, etc.) This approach is also at work in scene graph-based GUI toolkits. With this structured approach a limited set of object is manipulated at each level of structure in the program.

A second class of tool claim for a paradigm shift to empower the user by gathering instructions according to higher level relationship (increase locality). This class includes logical programming (Prolog, constraints in GUI Toolkits [START_REF] Myers | A new model for handling input[END_REF]), dataflow (Intuikit [START_REF] Chatty | Revisiting visual interface programming: creating GUI tools for designers and programmers[END_REF], ICon [START_REF] Dragicevic | Support for input adaptability in the ICON toolkit[END_REF]), or reactive programming (Squeak [START_REF] Cardelli | Squeak: a language for communicating with mice[END_REF], Esterel [START_REF] Fekete | Specification and Verification of Interactors: A Tour of Esterel[END_REF]). In the same way, some use existing models such as Statecharts [START_REF] Harel | On the development of reactive systems[END_REF][START_REF] Appert | SMCanvas: augmenter la boîte à outils Java Swing pour prototyper des techniques d'interaction avancées[END_REF] or Petri nets [START_REF] Palanque | Petri net based design of User-Driven interfaces using the interactive cooperative objects formalism[END_REF] to describe GUI behavior.

Alternatively some take into account that programs are composed of a set of concerns that can not all be reflected in the main decomposition. They try to increase locality of so-called cross-cutting concerns or control flows. This multidimensional structure may be achieved by a conceptual paradigm shift (Aspect-Oriented Programming [START_REF] Kiczales | Aspect-oriented programming[END_REF], Role Modeling [START_REF] Egil | System design by composing structures of interacting objects[END_REF]), other focus on presentation (CodeBubble [START_REF] Bragdon | Code bubbles[END_REF], CodeCanvas [START_REF] Deline | Code canvas[END_REF]) or static analysis of code (Concern Graph [START_REF] Martin | Concern graphs: finding and describing concerns using structural program dependencies[END_REF]). Ko worked on debugging tools and highlight the potential of providing causal relationship abstraction to help developers understand a program [START_REF] Andrew | Designing the whyline: a debugging interface for asking questions about program behavior[END_REF].

Ultimately, studies shows programmer organizing his work in task and sub-task and determining a working set of artifact concerned by his current task [START_REF] Andrew | Eliciting design requirements for maintenance-oriented IDEs: a detailed study of corrective and perfective maintenance tasks[END_REF].

Chosen Approach

Goal

My review of the state of the art suggests an evolution toward understanding the concept of locality as relative to a given task. At one precise moment, programmers are only interested in a few causality relations: "how moving a finger will scroll this lists elements?", "which components alter this variable value?", etc. This can be illustrated by the two scenarios below:

Version1: Annette ask the system to show link between finger coordinate and a circle coordinate. She notes that the path include a "smooth animation" dataflow boxes used to provide a "slow in / slow out" animation for the step by step version. This combined with sampling from tactile screen lead to flicker.

Version2: In a dataflow environment, Annette plugs the coordinates of fingers and circle on to a plotting view, then performs a drag and inspect the two curves. At a sufficient zoom level, she sees that sampling produce a common piecewise-constant function and the other curve is smoother following a "rotated atan" pattern. She identified this pattern as "slow-in / slow-out" and starts locating it to correct the bug.

In this context, my approach consists in trying to design programming tools that support relative locality, using design techniques from the user interface design community. Using scenarios such as the one above, I try to formulate the problem using a set of abstractions that capture the requirements, then I apply iterative design and evaluation methods.

Thesis: all we need is view

We studied other framework or guidelines for programming tools, such as Cognitive Dimensions of Notation [START_REF] Green | Cognitive dimensions of notations[END_REF], Cognitive Question in Software Visualization [START_REF] Petre | Cognitive questions in software visualisation[END_REF] or guidelines for errorpreventing programming system [START_REF] Ko | A framework and methodology for studying the causes of software errors in programming systems[END_REF]. We compared usability requirement of computing-oriented and interactive-oriented programming tools in [START_REF] Letondal | Usability requirements for interaction-oriented development tools[END_REF] to illustrate similarities and differences in requirement in these two families of systems. We concluded on the importance of locality that we define: "A local representation of a relationship is a representation in which a visual variable efficiently support perception of this relationship." Our definition of view is a variant of Reenskaug's definition in MVC [START_REF] Reenskaug | Models -views -controllers[END_REF]: "a view is a (visual) representation of its program. It would ordinarily highlight certain attributes of the program and suppress others. It is thus acting as a presentation filter". Representation may include dynamic data extracted from the program lifecycle: history, translations, execution machinery2 ; still in the idea of improving understanding of the program. Depending on the nature of data represented (static or dynamic) or representation (invertible [START_REF] Conversy | Improving usability of interactive graphics specification and implementation with picking views and inverse transformations[END_REF]), views may be editable and change underlying program.

Facing the variety of abstraction levels and concerns, my thesis is that in order to maintain intellectual manageability we need multiple views, one for each type of question that the programmer is addressing at a given time. The views can be made of text or graphics.

Thus we face the problem of providing representation and offering manipulation of the representation and of the program. We aim to provide a coherent set of views and handle to fit the programmer need to manage all the angles of his programs.

Method

I practice participatory design to target usable tools. I wrote down scenarios (as presented in 1.1) from interviews and observation of UI programmer. A part of these scenarios anticipate the arising need for dynamically configured multi-devices app (spreading for a while as proof-ofconcept). Part of the conceptual complexity induced by such systems is addressed by a toolkit my lab is designing: an interacting component model assembled in a scene graph embedding behavior [START_REF] Chatty | Supporting multidisciplinary software composition for interactive applications[END_REF]. We used scenarios to imagine and design program visualization (textual or graphical, alterable or passive); editor and translator features, or more conceptual ideas.

Current results

Based on users' wishes, I first explored traditional text views -syntax of programming languages -with direct mapping to a scene graph. I explored different (not exclusive) tracks: emphasis on terseness through tiny syntax and default arguments, binding using visual clues (indentation), dataflow constraints. r : R e c t a n g l e {

x : 1 0 y : 1 0 w: 2 0 h : 2 0 b e h a v i o r : S t a t e m a c h i n e { i d l e : S t a t e { T r a n s i t i o n { on : r . p r e s s ; t o : d r a g g i n g } } d r a g g i n g : S t a t e { Connector { i n : system . mouse . x out : r . x} Connector { i n : system . mouse . y out : r . y} T r a n s i t i o n { on : r . r e l e a s e ; t o : " i d l e "} } } } Fig. 1. Example of lightweight tree syntax. This program display a draggable rectangle.

I then worked on more pictorial views of scene graphs and developed a component tree view with an overlay representing dynamic transfer control across the tree. A concern with this tree-oriented views is the differentiation between hierarchy and control transfer relations. Hierarchy provide structure (abstraction and names) as well as control transfer between parent and descendent. Thus, when focusing on control transfer, the tree representation emphasizes a distinction where it is not needed.

Consequently I investigated graph representations of components to avoid this difference. This work is a straightforward application of the above mentioned concept of view. The whole program is presented as a big and dense graph. This graph can be filtered different way: abstraction of nodes in higher-level nodes, selection of specific node of interest, selection of transfer of control (edge) according to their nature (initialization, dataflow, network) 3 . Figure 2 shows an example of extracting dataflow control transfers from a control flow graph. Based on this, I am exploring the hypothesis that language syntaxes each privilege some types of arcs and hide the others.

Future work

We consider designing and implementing more complex pattern and transformation on the graph view. We are thinking of displaying profiling informations, as well as linking our view with existing models representation (Statecharts, ICon dataflow [START_REF] Dragicevic | Support for input adaptability in the ICON toolkit[END_REF]) either by dedicated views or through "morphing" on the graph. We are rather confident in the use of several notation to answer the varied task of programming.

Until now, I have evaluated my work informally using framework listed in 3. These frameworks relying on trade-offs between dimensions are useful at design-time but I would like to provide objective evaluation and comparison between my solutions and toward other tools. Thus I am seeking for qualitative (Bertin?) or quantitative criteria (LOC, what else ?) and wonder if experimental evaluation would be possible / profitable. Especially with the ultimate goal of comparing "whole programming systems".

Acknowledgements

I would like to thank my advisor Stéphane Chatty, as well as Stéphane Conversy and Christophe Hurter for their invaluable help, advice and encouragement writing this paper.

Fig. 2 .

 2 Fig. 2. Highlight of dataflow control transfer among the complete graph of interaction between entities.

italicized words are concepts from the framework sketched in[START_REF] Letondal | Usability requirements for interaction-oriented development tools[END_REF]

the running application is also a view of the program.

thus, the hierarchy tree is a particular case of this graph: a filtered view of the whole program showing only hierarchical relationships.


Jong-Hoon Hoon Won

Diana Fontanella

Matteo Paonni

Bernd Eissfeller

Andrea Emmanuele

Marco Luise

Axel Javier

Garcia Peña

Olivier Julien

Christophe Macabiau

Francesca Zanier

Marco Luise Axel

Gustavo Lopez

Jong Hoon

Receiver architecture for multicarrier-based GNSS signals

come

GTTI, Gruppo Telecomunicazioni Teoria dell'Inform azione. His main research interests lie in the area of wireless communications, with particular emphasis on CDMA/multicarrier signals and satellite communications and positioning.

Axel Garcia is a researcher/lecturer at the signal processing laboratory of ENAC (Ecole Nationale de l'Aviation Civile -French Civil Aviation University), Toulouse, France. His research interests are GNSS navigation message demodulation, optimization and design, GNSS receiver design and GNSS satellite payload. He received his double engineer degree in 2006 in digital communications from SUPAERO and ETSETB UPC, and his PhD in 2010 from the Department of Mathematics, Computer Science and Telecommunications of the INPT (Polytechnic National Institute of Toulouse), France.

Olivier Julien is a researcher/lecturer at the signal processing laboratory of ENAC (Ecole Nationale de l'Aviation Civile -French Civil Aviation University), Toulouse, France. His research interests are GNSS receiver design, GNSS multipath and interference mitigation and GNSS interoperability. He received his engineer degree in 2001 in digital communications from ENAC and his PhD in 2005 from the Department of Geomatics Engineering of the University of Calgary, Canada.

Christophe Macabiau graduated as an electronics engineer in 1992 from the ENAC in Toulouse, France. Since 1994, he has been working on the application of satellite navigation techniques to civil aviation. He received his Ph.D. in 1997 and has been in charge of the signal processing lab of the ENAC since 2000. His research now also applies to vehicular, pedestrian and space applications, and includes advanced GNSS signal processing techniques for acquisition, tracking, interference and multipath mitigation, GNSS integrity, as well as integrated GNSS inertial systems and indoor GNSS techniques.

Francesca Zanier received the M.E. (cum laude) in telecommunications engineering and the PhD degree in information engineering from the University of Pisa (Italy), in 2004 and 2009, respectively. During her PhD studies, she cooperated with the Radio Navigation working group at the European Space Research and Technology Center (ESTEC) of the European Space Agency (ESA), concerning aspects on signal design for future GNSS. Since 2009, she has been with the Radionavigation section at ESA/ESTEC, Noordwijk. Her research activities are mainly focused on digital communication theory, signal processing and satellite positioning systems, with special emphasis on time delay estimation and signal design for satellite positioning systems.

Gustavo Lopez Risueno has received the M. Sc. and the Ph.D. in telecommunications engineering from the Universidad Politécnica de Madrid, Spain. He has held several positions in academia, ESA and NC3A (NATO). Currently, he is a Radionavigation systems engineer at ESA/ESTEC, Noordwijk working mainly on signal processing for GNSS receivers and monitoring.

!

This paper presents the receiver architecture analysis on FMT signals for navigation purposes, which are a multicarrier signaling technique based on OFDM for CDMA signals with SRRC pulses for the spectral efficiency. A new type of FMT signal processing scheme making use of the complex FMT replica to have a narrow correlation peak and thus to achieve precise ranging without performing the IFFT operation is proposed. The distinctive features of the proposed receiver architecture are discussed and preliminary considerations on the signal processing schemes, e.g. acquisition and tracking, for FMT signals are described.

"#!

Current Signals In Space (SIS) of Global Navigation Satellite Systems (GNSSs) has been designed mainly based on rectangular chip pulses. Such a choice has indeed many advantages in terms of simplicity of the generation scheme and the receiver architecture; however these signals are characterized by high out of band (OOB) emissions which may interfere with other services in neighbor bands. Therefore, bandwidth efficient modulation schemes by shaping of pulses or waveforms should be taken into account for the design of next generation GNSS signals [START_REF] Won | Proc. the 18th GNSS Workshop[END_REF].

GNSSs are very complex systems and a considerable trade off is necessary to fulfill the various requirements, such as maximizing the synchronization performance, providing the required data delivery rate, minimizing the overall signal acquisition time and so on. In particular the design of a navigation system has to take into account for communications and ranging aspects at the same time. The Code Division Multiple Access (CDMA) is a technique widely used in GNSS since it perfectly meets the needs of these two aspects, i.e. an easy multiple accessibility and data delivery for different users and an optimal ranging capability due to the wide spectral occupancy of the signals which corresponds to a high time resolution. Another good property of CDMA is its intrinsic robustness to interference and jamming.

The multicarrier (MC) modulation scheme widely used in the communication community could be another interesting technique. The principle of classical multicarrier transmission is to convert a serial high rate data stream onto multiple parallel low rate sub streams; thus each sub stream is modulated on another sub carrier, i.e. MC modulation transmits the data across multiple narrow band channels by dividing the bandwidth into many small orthogonal frequencies, effectively achievable using the FFT, where the multiple narrow band channel suffers only from flat fading. As a consequence, the symbol rate on each sub carrier is much less than the initial serial data symbol rate and the distortions introduced by the channel are attenuated; Inter Symbol Interference (ISI) is significantly decreased in MC signals with the effect of reducing the required complexity of the equalizer. Another good property of MC modulation is indeed the spectral efficiency which reflects in low OOB emissions. Furthermore, the MC transmission is a low complex technique to efficiently modulate multiple sub carriers by using digital signal processing based on IFFT operation. For all these reasons, MC techniques have been adopted in digital broadcast television and radio, and accepted as the next generation standard for wireless LAN systems [START_REF] Fazel | Multi Carrier and Spread Spectrum Systems[END_REF].

The MC modulation however has also some drawbacks such as difficulty in subcarrier synchronization and sensitivity to frequency offset. Another disadvantage is represented by its non constant envelope nature, which results from the fact that the composed signal is obtained as the sum of several subcarriers with overlapping power spectra; as a consequence the transmitter amplifier is forced to work in the nonlinear region [START_REF] Fazel | Multi Carrier and Spread Spectrum Systems[END_REF]. Despite these drawbacks, the multicarrier CDMA seems to be a promising technology and is expected to play a role in GNSS scenarios near future. In particular it is of interest in this paper the filtered multi tone (FMT), which is a MC technique based on Orthogonal Frequency Division Multiplexing (OFDM) for CDMA signals with Square Root Raised Cosine (SRRC) pulses [START_REF] Zanier | ! & , ![END_REF][START_REF] Emmanuele | Proc. ION GNSS 2011[END_REF].

In this paper the results of the receiver analysis on FMT signals conducted in the framework of ESA funded research project, so called Advanced Signal In Space Techniques (ADVISE), are presented. First the general aspects of multicarrier signaling techniques are reviewed. And two different types of FMT signal processing schemes, a conventional MC demodulation based receiver and a receiver scheme making use of the complex FMT replica, are presented. In particular this second scheme is of interest since it operates on the complex signal to provide accurate ranging information without performing the IFFT operation. Finally, the distinctive features of the proposed receiver architectures will be discussed and some preliminary considerations on the signal processing schemes, e.g., acquisition and tracking, for FMT signals will be given.

$ % #& ! &

In fact there are so many different configurations of OFDM technology that should be dealt in different ways.

In this section we review the MC signaling technique, e.g. OFDM, with a special attention on navigational purposes.

The navigation system is a combination of communication system and ranging system. For this, a CDMA technology is widely used due to its benefits stated before. Therefore, the core technology expected near future in this field is multicarrier CDMA. Originally, the multicarrier CDMA was proposed by Yee et al. for indoor wireless radio networks [START_REF] Yee | + ! '[END_REF], where they used a little bit different definition on multicarrier CDMA technology from that widely used in nowadays. An overview on different forms of multicarrier CDMA technologies was well summarized in [6 9] including its relationship with OFDM, a comparison of its performance with direct sequence CDMA, and some specific implementation problems.

Figure 1 is three distinctive multicarrier modulations with four sub channels ! = 4 in comparison with a single carrier direct sequence (DS) CDMA signal redrawn from [START_REF] Fazel | Multi Carrier and Spread Spectrum Systems[END_REF]. Note that the three dimensional time/frequency/power density representation is used to illustrate the principle of various schemes. Also note that for the sake of simplicity we do not consider the signal pulse shaping effect in this figure but this will be taken into account later for FMT signals. Figure 1 (c) shows the MC CDMA, also referred as OFDM CDMA. This is the combination of frequency domain spreading and multicarrier modulation. In this scheme, the multiplexing is carried out by applying different spreading codes in the code domain at the same time for the same bandwidth. The use of multicarrier leads to the decrease of the occupied bandwidth. The MC CDMA transmits a data symbol of a satellite simultaneously on several narrow sub channels by mapping the chips of a spread data symbol in frequency direction over several parallel sub channels.

Figure 1 (d) shows the MC DS CDMA that is the multicarrier modulation with a number of sub channels based on the OFDM for the DS codes. Apart from the MC CDMA, this method maps the chips of a spread data symbol in the time direction over several multicarrier symbols. First, a serial to parallel converter is applied to convert the high rate data symbols into parallel low rate sub streams before spreading the data symbols on each sub channel with a satellite specific spreading code in time direction. This corresponds to direct sequence spreading on each sub channel. Note that FMT, the multicarrier modulation signal candidate in this paper, is a type of MC DS CDMA incorporated with the SRRC pulses for spreading codes in time domain. There are several different types of MC DS CDMA schemes with regard to the direct sequence spreading before or after the multicarrier modulation. The multicarrier signaling can be realized by using the low complex OFDM operation by using an efficient FFT operation [START_REF] Fazel | Multi Carrier and Spread Spectrum Systems[END_REF]. , have another degree of freedom in the number of bits involved in the serial to parallel conversion, which determines both the symbol duration and chip duration. This also determines the total number of subcarriers within the bandwidth. Moreover, the MC DS CDMA is known to provide additional degrees of freedom, such as the chip duration in time domain that is used in spreading the symbol in time domain, and the number of bits involved in the serial to parallel conversion that determines the symbol duration. These additional ones determine the spreading factor of each subcarrier signal and the frequency spacing between two adjacent subcarriers [8]. However, the use of these degrees of freedom should be limited in designing a navigation signal because of the trade off in navigation signal between the data rate for a better data delivery capacity and the code rate for a better ranging performance.

! " !" 5 .

As mentioned before, the MC DS CDMA signal is generated by: 1) converting high rate data symbols into low rate data symbols, i.e., ! sub streams by using a serial to parallel converter,

2) applying DS CDMA on each individual sub stream,

3) and finally applying ! sub carriers to the corresponding DS CDMA signals in each individual sub channel.

Thus, in MC DS CDMA, each data symbol is spread in bandwidth within its sub channel, but in contrast to MC CDMA or DS CDMA, not over the whole transmission bandwidth for ! > 1. Note that an MC DS CDMA system with one sub carrier is equivalent to a single carrier DS CDMA system [START_REF] Fazel | Multi Carrier and Spread Spectrum Systems[END_REF].

Figure 2 shows a brief schematic view on a general architecture of MC DS CDMA transmitter. The data symbol has a 1/ of data symbol rate. A sequence of ! complex valued data symbol for = 0,…,! 1, is serial to parallel converted onto ! sub streams, thereby becoming a 1/(! ) data symbol rate on each sub stream. In a single sub stream, a data symbol is spread with the satellite specific spreading PN code of length $. With multicarrier direct sequence spread spectrum, each data symbol is spread over $ multicarrier symbols, each of duration . Therefore, the data symbols are transmitted simultaneously on ! parallel subcarriers, each occupying a small fraction 1/! of the total available bandwidth. The complex valued sequence obtained after spreading is given by [START_REF] Fazel | Multi Carrier and Spread Spectrum Systems[END_REF] ∑

- = = 1 0 2 ) ( ) ( ! % & π , 0 ≤ < $ (1)
with

$ ! = = and ) 1 ( α + = (2)
where &( ) is MC DS CDMA signals, is data symbol at , ( ) is spreading PRN code, is sub carrier frequency of sub carrier [Hz], ! is the number of subcarriers, is symbol time interval [sec], is data symbol interval [sec], and $ is code length.

The satellite specific spreading PN code of length $ is given by

) ( ) ( 1 0 $ - = ∑ - = (3)
where ) ( is the pulse shape of the chips (i.e., the SRRC pulses in this paper), and the choice of α depends on the selected pulse shaping form ) ( . In case of OFDM where ) ( has a rectangular form, α is equal to 0 while we used 0.2 for FMT. Therefore, the MC DS CDMA system with broadband sub channels can be regarded to be ! classical broadband DS CDMA systems, and multiple access detection techniques known for DS CDMA systems can be applied in each data stream. .0 0

' ! (

) Under this assumption, the complex valued sequence for a single data symbol can be modified from (1) as

∑ - = = 1 0 2 ) ( ) ( ! % & π , 0 ≤ < $ (4)
where is a single data symbol such that = { 0 , 1 , … , Nc 1 } with 0 = 1 = … = ! 1 . This type of multicarrier signal can be regarded as the signal in ! parallel streams of data×chips with each one modulating a different subcarrier, i.e., the spreading (the multiplication of data to chips) were performed before the multicarrier modulation. The input spread symbols with the given ranging code chip rate 1/ are split into ! parallel stream with the lower late 1/! using the serial to parallel converter, where = ! is the symbol interval. At the output of the serial to parallel converter the spread symbols are partitioned into groups of size ! and each of them is modulated on the ! equal spaced subcarriers, after being filtered by the filter bank for the pulse shaping. In this case we can have the higher tracking performance by directly using the multicarrier code signal replica compared to the case before but with the loss of the available high data rate that is limited by the spreading factor.

! $ ! ! # , ,

The incoming signals, which include the noise effect throughout the channel, are demodulated and despread by multiplying the sub carrier and the replica code pulses at each sub channel followed by an integrator for the correlation. The low rate output of each channel is converted to the high rate data symbols by a parallel to serial converter.

As mentioned before FMT in this paper is a special case of MC DS CDMA systems when the subcarrier spacing is equal to 1/(! ) by the efficient use of SRRC pulses for pulse shaping of codes. This kind of MC DS CDMA system is also referred as multi tone CDMA (MT CDMA).

As seen in Figure 2 the data is serial to parallel converted and spread by a (shaped pulse) code sequence; in alternate way, the code sequence is applied first and then the resulting code and data product is converted by a serial to parallel converter. After then, the ! parallel signals are modulated by using OFDM (i.e., ! multicarriers) and summed up to transmit the consequent multicarrier modulated spread signal &( ). By allocating the multicarrier frequency for the th channel, we can control the type of signal (i.e., monomodal or bimodal), power level, and frequency band separation [START_REF] Emmanuele | Proc. ION GNSS 2011[END_REF]. As well known the multicarrier modulation as indicated in dotted box can be done effectively by using IFFT operation.

Note that the product of data and code sequence • ( ) here is a monomodal signal (not bimodal signal). Even though this, the use of multicarrier with a null power near center frequency makes it possible to have a split of spectrum to left and right, thereby being bimodal signals. Also note that the parallel ! sub channels carry the same data symbol. This is a special case of MC DS CDMA signals to provide a high accurate tracking result for the navigation purpose even though the loss in the data symbol rate. For this there can be two types of receiver processing scheme as follows.

-( 50 6 ," .

A receiver block diagram based on the OFDM demodulation in Figure 3 (a) is widely used in the MC DS CDMA receiver. The receiver gets the received signal ( ) which includes the channel and noise effect. For the sake of simplicity we assume ( ) = &( ) in absence of channel and noise effect. First, a multicarrier demodulator is applied to process the multiacrrier modulated spread signal &( ) as indicated in dotted box. Second, the despreading process is performed by multiplying the code sequence ( ). Note that a SRRC shaped pulse code generator with a given roll off factor in the FMT should be employed. Finally, we use a parallel to serial converter. The input to sub channels should be controlled in a proper way (e.g. acquisition/tracking).

3( 50 6 , 0

An alternative way of receiver processing can be implemented especially for correlation function of FMT signals (Figure 3 (b)). A relevant replica shaped pulse code sequence ) ( & generated by the signal generation method same as in Figure 2 except for the use of data bits was used in the correlation. Under the assumption on the same data symbol for all sub channels, the correlation output of this produces the data symbols. Figure 4 shows a comparison of these two receiver architectures in terms of received signal and correlation output properties. This is an example for an option for FMT with a special care to achieve a lower peak of secondary lobes of the correlation function for each component but with decreasing of performance in terms of Gabor bandwidth and Spectral Separation Coefficient (SSC) [START_REF] Emmanuele | Proc. ION GNSS 2011[END_REF]. The figure in left shows the PSDs of incoming FMT signals at the end of antenna with a spectrum split into monomodal and bimodal components where the actual signal power is not taken into account here. Even though the PSD of incoming signal is spread over wideband, the signal of each sub carrier component at each sub channel is a narrowband SRRC signal as depicted in black dotted line. The bandwidth of SRRC basic function equals the original code rate divided by the total number of subcarriers that was used in multicarrier modulation. Therefore, the consequent code rate of each sub carrier should be divided by the total number of sub carriers.

The receiver receives the signal occupying the intended signal band, where the PSD of the received signal can be split depending on the configuration of the monomodal and bimodal signal components. The direct use of FMT replica signal correlates the incoming signals with the local replica FMT signals that were generated by the FMT signal generator, i.e., the OFDM modulation to the SRRC pulse code sequence with assuming = 1 as in Figure 2. This is the type B receiver. Each signal components, such as monomodal and bimodal FMT signals, can be generated and processed separately.

Compared to this, in the OFDM demodulation based method, the OFDM demodulated signals (i.e., a parallel signal) are correlated with the replica SRRC shaped pulse codes in a bank of sub channel correlators (type A receiver). The frequency offset of each sub channel of multicarrier signal components is wipe off by the OFDM demodulator by using FFT operation with a proper frequency offset. The resulting output of the OFDM demodulator at each sub channel should be same to the SRRC shaped pulse spread spectrum code sequence with a narrow bandwidth, depending on the number of subcarriers used in a given FMT option. Consequently, the correlation function of each sub channel should be same to that of SRRC signals if it is expressed in chips. Note that we have sub channels for sub carriers. Thus, we have correlation functions with the same shape and bandwidth at different frequency offsets. It is obvious that the DLL noise jitter at each sub channel for this case is worse than the previous one because of the multiple narrower bandwidth sub channels. One of the easy ways to reduce the DLL noise jitter in case of multiple sub channels is to combine the output of all sub channels to produce a mean value of tracking errors but the consequent noise jitter is worse than the wideband single carrier case.

In addition, as well known, the performance of multicarrier synchronization tracking loops depends strongly on the RF oscillator phase noise characteristics. The combination of multicarrier modulation and CDMA in this way is seen as a very promising technique for the development of high capacity wireless communications. However, this scheme is very sensitive to the signal distortion generated by the imperfect frequency down conversion at the receiver due to the local oscillator phase noise and frequency offset. Moreover, the signal dynamics caused by the movement of a receiver will introduce the Doppler frequency shift that will pose additional probability to impair the reference oscillator performance. The vibration induced oscillator jitter caused by the vibration of receiver mounted on the navigational body will cause a worse environment than the normal communication system working in static mode. Therefore, the impact of both the carrier frequency offset and the phase noise on the system performance should be severe. Note that the navigation receiver does not only require the data delivery but also the precise ranging information that is highly affected by the performance of both code and carrier trackings. The performance loss of an MC DS CDMA system is mainly due to the presence of nonlinear RF power amplifier, phase noise, and frequency offset of the RF oscillator, chip synchronization timing errors, and fading. Each impairment produces loss of orthogonality between subcarriers, thereby increasing the ICI. .

∫ d ) ( & ∫ d ∫ d OFDM Demodulator !" #! $%& ' ()) * " !+ , -+ ! . "! / "-* + ()) * "!+ ,-+ ! 0 1 ()) * "!+ ,-+ ! 0*!1 & "
So far we have at least two methods of MC DS CDMA schemes in communication field [START_REF] Chen | Performance of Coded Multi Carrier DS CDMA Systems in Multi Path Fading Channels[END_REF].

• Method I -the serial data stream is first spread by a PN sequence, and then converted into ! parallel chip sequences with each one modulating a different carrier; the number of carrier required is !, which equals to the number of chips per data symbol; this type of system offers both the robustness of orthogonal modulation and the flexibility of the DS CDMA scheme. • Method II -the serial data stream is first converted into parallel data sub streams, and each sub stream is then spread by a PN sequence and transmitted over one of sub channels as narrow band DS waveforms; the number of carriers required is , which is usually much less than !; while maintaining the overall data rate unchanged, this scheme lowers the chip rate in each sub channel so that a longer chip time makes it easier to quasi synchronize the transmissions; the multiple access interference can be effectively reduced in this type of system by applying orthogonal spreading sequences.

In this paper we supposed to transmit the same data for all the ! subcarriers, for $= /! symbol times. Therefore, we defined an additional type of MC DS CDMA scheme.

• Method III the serial data stream is first spread by a PN sequence, and then converted into ! parallel streams of data×chips with each one modulating a different subcarrier; if is the spreading factor, we suppose to use =$×! chips of the PN sequence to transmit a single data; in this way the spreading is on frequency (! subcarriers) and in time ( /! symbol times); in this study we assumed is greater than !) On another hand, there are mainly three synchronization tasks in the signal processing of a receiver. They are 1) timing recovery, 2) carrier frequency recovery, and 3) carrier phase recovery.

In case of type A receiver, in particular for the OFDM based signals, the received signal should be processed first by means of the FFT operation at the beginning of digital signal processing to wipe off the sub carriers. Large numbers of FFT points resulting in long OFDM symbol durations make the system more sensitivity to the time variance of the channel, e.g. Doppler effect, and more vulnerable to the oscillator phase noise. On the other hand, a large symbol duration increases the spectral efficiency due to decrease of the guard interval loss. For any OFDM realization a trade off between the number of FFT points, the sensitivity to the Doppler and phase noise effects, and the loss due to the guard interval has to be found. This is different from the normal single carrier CDMA signal. The time positioning of the FFT window including the adjustment of the sampling rate is important. The time positioning of the FFT window determines the start point of parallel to serial converted symbol sequence, leading to a correct order of data symbol sequence. For this in case of an OFDM receiver realization, a coarse symbol timing estimator and/or a path time delay estimator can be employed in a FFT window controller [8].

In case of the type B receiver for the method III the three synchronization tasks are carried out in a similar way to the currently widely used BPSK or QPSK receiver except for the use of multicarrier modulated SRRC shaped pulses for the replica signal. However, it does not require the FFT operation at the beginning of digital signal processing. Therefore, there is no need to estimate the time positioning of the FFT window. The start point of symbols (or data bits) is acquired after the code and carrier locks by testing logics based on a priori information on the bits (i.e. preamble, available frame identifiers, fixed reserved bits, parity check, etc.) as similar to the bit synchronization process of current receivers. Also a normal acquisition process can be employed. Thus, we consider in this paper that the MC DS CDMA technology was used only for the spectrum efficiency not for the fully use of OFDM technology for the data transmission.

Figure 5 shows a detailed block diagram of the signal tracking loop of the receiver type B for the method III. The received signals are directly correlated with the replica FMT signals that were locally created by the method similar to Figure 2 (a) with = 1 for all sub channels. The output of the FMT signal generator at prompt and early/late arms includes not only the SRRC shaped pulse code sequence but also the multicarrier modulation function block, multiplexed into a serial sequence.

Detailed block diagrams on the signal tracking loop in case of the receiver type B for the methods I and II are shown in Figure 6 for separate tracking loop for individual sub channels, and in Figure 7 for combined tracking loop. Each individual sub channel has its own tracking loop with a slower code rate (i.e., narrower bandwidth), and the carrier tracking can be performed for each sub carrier. In case of combined tracking, the code and carrier discriminator outputs of each sub channel are combined to create a common code and carrier tracking error, and then applied to code and carrier loop closure. In particular for the carrier tracking, the estimated carrier Doppler is applied to FFT block where the estimated Doppler is used to create the reference carrier signal in each sub channel with relevant offsets. The output in inphase prompt arm is used to demodulate the data bits after the P/S converter.

The signal in Method III can be processed by using either the receiver types A or B whereas only the receiver type A can be applied to the methods I and II. The second solution is ideal for a pilot component due to its periodicity. This means that since the values of the transmitted symbols are the same, the tails of the SRRC shaped bits of the previous and posteriori shaped PRN code sequences appear with the same sign inside the main body of the current PRN local replica. Therefore, the generated local replica is equal to the received signal and the equivalent effect of a matched filter is found (best SNR).

The situation for a data component is different. In this case, the tail bits of the previous and posteriori shaped PRN code sequences do not have to have the same sign as the current shaped PRN code sequence. Therefore, the second solution could degrade the final correlation result with respect to first solution depending on the values of the received symbol ( ). In any case, the degradation introduced by the limitation of the integration time and the truncation of the local replica with respect to the ideal case of a matched filter is negligible. For example, assuming a PRN code of 1023 chips, a SRRC shaping with a duration limited to 60 times (chip time) and a data transmission of 1,+1, 1 (worst scenario), the SNR degradations of options 1 and 2 with respect to the matched filter are 0.0006 and 0.0017 dB respectively.

! !&#

In this paper the distinctive features of several receiver architectures of the FMT technology for GNSS uses have been evaluated. The proposed receiver architecture was for a new type of FMT signal processing scheme which is based on the assumption on the same data for all the ! subcarriers for $ symbol times. The proposed receiver architecture directly uses locally generated complex FMT signals in correlation. Thus, it has a narrow correlation peak and achieves precise ranging without performing the IFFT operation. This fact means the FMT technology in this paper was used in the transmitter to suppress OOB emission power. Also high level block diagram of three types of receiver implementation was described.

!< %& "

Figure 1 (

 1 Figure 1 (a) shows the single carrier DS CDMA signal generation. For signal spreading, pseudorandom noise (PN) code is made up from a number of chips for mixing the data with the code. The size of a chip determines the frequency spreading. Different user information is multiplexed by the use of different PN codes. The drawback is that synchronization becomes more difficult and requires a time consuming search technique or a pilot signals for aided acquisition.

Figure 1 (

 1 Figure 1 (b) shows the multicarrier modulation with ! = 4 sub channels based on the OFDM without the use of DS codes. The channel can be considered as time invariant during one OFDM symbol and fading per sub channel can be considered as flat. This lead to the fact that the OFDM symbol duration should be smaller than the coherent time of the channel and the sub carrier spacing should be smaller than the coherent bandwidth of the channel.

  In comparison with the single carrier DS CDMA (Figure1(a)), both multicarrier systems, such as MC CDMA (Figure 1 (c)) and MC DS CDMA (Figure 1 (d))

2 .

 2 and at the end of the PRN code are eliminated. The tails of the SRRC shaped chips which are located at the beginning and at the end of the PRN code and which are outside the PRN code sequence duration are folded inside the main body of the current PRN local replica.

  This paper is based upon the work done in the framework of the European Space Agency (ESA) funded study ADVISE, Advanced Signal In Space Techniques. The content of the present article reflects solely the authors' view and by no means represents the official ESA view or the one from ESA Galileo Project Office.

In 2004 he worked with Korea Aerospace Research Institute (KARI) on the high accuracy nationwide DGPS in Korea. Since August 2005 he has worked with the Institute of Space Technology and Space Applications (former the Institute of Geodesy and Navigation), University FAF Munich, Germany. He was nominated as Head of GNSS Laboratory in 2011 at the same institute, and involved in lectures on advanced receiver technology at Technical University of Munich since 2009. He has worked as a project engineer in many industrial/governmental funded research projects since 1995, and published many technical papers in the fields of GNSS signal, receiver, navigation, and target tracking systems. Diana Fontanella received her M.Sc. in Telecommunications Engineering from Politecnico di Milano, Italy (2009). She is research associate at the Institute of Space Technology and Space Applications (former Institute of Geodesy and Navigation) at the University of the Federal Armed Forces (UFAF) Munich. She is currently involved in ESA and EC/GSA projects with focus on GNSS signal design, GNSS radio frequency compatibility, and GNSS performance assessment. Matteo Paonni is research associate at the Institute of Space Technology and Space Applications (former the Institute of Geodesy and Navigation) at the University of the Federal Armed Forces Munich. He received his M.S. in Electrical Engineering from the University of Perugia, Italy, and is PhD candidate at the Faculty of Aerospace Engineering of the University FAF Munich. He is actually involved in several European projects with focus on GNSS as project engineer as well as project manager. His main topics of interest are GNSS signal structure, GNSS compatibility and interoperability and GNSS performance assessment. He is involved in the Galileo program, in which he actively supports the European Commission through the Compatibility, Signal and Interoperability Working Group (CSI WG), formerly Galileo Signal Task Force. Bernd Eissfeller is Full Professor of Navigation and Director of the Institute of Space Technology and Space Applications (former the . Till the end of 1993 he worked in industry as a project manager on the development of GPS/INS navigation systems. He received the Habilitation (venia legendi) Emmanuele received the B.E. and the M.E.

The generation of the local replica will vary depending on the type of receiver and on the chosen MC DS CDMA method. For the sake of simplicity, only the local replica of method II is described in this paper for both types of receivers, A and B. Besides, the local replica generation process of method II can be extrapolated to the local replica generation process of method III for the B type of receivers. In this paper, the local replica does not include the cosine and sine terms generated at the receiver which are responsible of transporting the received signal from its central carrier frequency to the intermediate or baseband frequency.

The main difference between the local replicas of receivers A and B is the removal of the frequency components of the subcarriers. This means that whereas for the type A receivers the local replica is simply the shaped PRN code, for the type B receivers the local replica also contains the frequency carrier of each subcarrier.

The mathematical expression of the local replica, ( ), of all the subcarriers of the type A receiver is:

(

where ( ) is the shaped PRN code of the signal. The mathematical expression of the local replica of each individual subcarrier n, * ( ), of the type B receiver is:

The total local replica, * ( ), of type B receiver is,:

The most important part of previous expression is that the code delay affects equally the shaped PRN code and the subcarrier frequency component.

If this condition is not fulfilled, the receiver does not obtain the correlation function of Figure 4. Another important characteristic is that the local replica of the type B receiver is generated independently from the generation (and estimation) of the received FMT signal central carrier frequency. This means that values of the frequency components of the generated local replica are the values of the equivalent baseband FMT signal.

The integration time of the receiver's correlator block is another parameter to be defined for the proposed FMT signals. In fact, due to the SRRC shaping of the PRN code chips, each SRRC shaped PRN code sequence transported by each different subcarrier has a larger duration than the PRN code period. This means that successive integrations are overlapped if the correlator block integrates the entire length of a received SRRC shaped PRN code sequence. However, this overlapping increases the complexity of the receiver. Therefore, this paper proposes to limit the integration time to the exact duration of the PRN code period since the gain obtained by integrating the entire length of the received SRRC shaped PRN code sequence is negligible as is preliminary presented.

The limitation of the integration time to the PRN code duration raises the question about the truncation of the generated local replica. Two options are considered:

1. The local replica is truncated at the PRN code period: the tails of the SRRC shaped chips which are located