sdr ofdm waveform design for a ugv/uav communication scenario

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SDR OFDM Waveform Design for a UGV/UAV Communication Scenario Christian Blümm & Christoph Heller & Robert Weigel Received: 30 September 2011 /Revised: 28 October 2011 /Accepted: 7 November 2011 /Published online: 30 November 2011 # Springer Science+Business Media, LLC 2011 Abstract In the course of a national research project, EADS Innovation Works has developed a waveform on a self-designed hybrid Software Defined Radio (SDR) plat- form, consisting of an FPGA (Xilinx Virtex 5) and a GPP (Intel Atom). The waveform realizes a video link between an Unmanned Ground Vehicle (UGV) and its Ground Control Station (GCS). This link is established indirectly with an Unmanned Aerial Vehicle (UAV) acting as a relay, in order to enable non-line-of-sight (NLOS) communication and to increase the communication range. Additionally, a direct video link from the UAV to the GCS is set up simultaneously as well as multiple low data rate control channels between the individual link partners. To cope with diverse operation areas, accompanied by a heterogeneous set of requirements, the waveform must offer outstanding adaptability and flexibility to maximize data rates in each scenario, while maintaining link robustness. The developed waveform is based on OFDM with freely customizable modulation parameters. Time Division Multiple Access (TDMA) is implemented on medium access layer to switch between the different users in the scenario (UGV, UAV, GCS). In this article, we give an overview of the waveform, its implementation on a hybrid platform and its validation for the intended field of application. Keywords SDR . OFDM . Waveform . Data links . UAV . UGV 1 Introduction Autonomous mobile robot systems with UGVs or UAVs are increasingly used for military purposes. For civil use, there is also a wide range of conceivable applications, where small unmanned platforms are more flexible, higher available and cheaper than traditional manned systems. Equipped with adequate sensors, autonomous systems could for instance detect hazardous substances at an early stage and monitor their spread. Other examples are Search and Rescue (SAR) activities, security services, image and video recording, environmental mapping and -monitoring, the surveillance of major sport events, etc. In combination with satellite navigation technology any captured informa- tion could be georeferenced exactly. Typically, the UGV/UAV data is sent without much preprocessing to the GCS. To therefore guarantee a reliable communication link, the special needs of UAVs/UGVs and their environmental conditions must be taken into account [13]. This article focuses on an appropriate self-designed waveform, which is based on an OFDM physical layer in combination with a TDMA medium access (MAC) protocol. This work is part of the project SiNafaR (Safe navigation for autonomous robotic platforms), which was launched by the Bavarian ministry of state for economy, infrastructure, traffic and technology to develop methods and processes for the use of cooperating air and ground robots in civil applications. 2 The Considered Communication Scenario The waveform which is presented here can cope with a broad variety of communication scenarios. Nevertheless, a C. Blümm (*) : C. Heller EADS Innovation Works, Munich, Germany e-mail: [email protected] R. Weigel University of Erlangen-Nuremberg, Erlangen, Germany J Sign Process Syst (2012) 69:1121 DOI 10.1007/s11265-011-0640-8

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Page 1: SDR OFDM Waveform Design for a UGV/UAV Communication Scenario

SDR OFDM Waveform Design for a UGV/UAVCommunication Scenario

Christian Blümm & Christoph Heller & Robert Weigel

Received: 30 September 2011 /Revised: 28 October 2011 /Accepted: 7 November 2011 /Published online: 30 November 2011# Springer Science+Business Media, LLC 2011

Abstract In the course of a national research project,EADS Innovation Works has developed a waveform on aself-designed hybrid Software Defined Radio (SDR) plat-form, consisting of an FPGA (Xilinx Virtex 5) and a GPP(Intel Atom). The waveform realizes a video link betweenan Unmanned Ground Vehicle (UGV) and its GroundControl Station (GCS). This link is established indirectlywith an Unmanned Aerial Vehicle (UAV) acting as a relay,in order to enable non-line-of-sight (NLOS) communicationand to increase the communication range. Additionally, adirect video link from the UAV to the GCS is set upsimultaneously as well as multiple low data rate controlchannels between the individual link partners. To cope withdiverse operation areas, accompanied by a heterogeneousset of requirements, the waveform must offer outstandingadaptability and flexibility to maximize data rates in eachscenario, while maintaining link robustness. The developedwaveform is based on OFDM with freely customizablemodulation parameters. Time Division Multiple Access(TDMA) is implemented on medium access layer to switchbetween the different users in the scenario (UGV, UAV,GCS). In this article, we give an overview of the waveform,its implementation on a hybrid platform and its validationfor the intended field of application.

Keywords SDR . OFDM .Waveform . Data links . UAV.

UGV

1 Introduction

Autonomous mobile robot systems with UGVs or UAVsare increasingly used for military purposes. For civil use,there is also a wide range of conceivable applications,where small unmanned platforms are more flexible, higheravailable and cheaper than traditional manned systems.Equipped with adequate sensors, autonomous systemscould for instance detect hazardous substances at an earlystage and monitor their spread. Other examples are Searchand Rescue (SAR) activities, security services, image andvideo recording, environmental mapping and -monitoring,the surveillance of major sport events, etc. In combinationwith satellite navigation technology any captured informa-tion could be georeferenced exactly.

Typically, the UGV/UAV data is sent without muchpreprocessing to the GCS. To therefore guarantee a reliablecommunication link, the special needs of UAVs/UGVs andtheir environmental conditions must be taken into account[1–3].

This article focuses on an appropriate self-designedwaveform, which is based on an OFDM physical layer incombination with a TDMA medium access (MAC) protocol.

This work is part of the project SiNafaR (Safe navigationfor autonomous robotic platforms), which was launched bythe Bavarian ministry of state for economy, infrastructure,traffic and technology to develop methods and processesfor the use of cooperating air and ground robots in civilapplications.

2 The Considered Communication Scenario

The waveform which is presented here can cope with abroad variety of communication scenarios. Nevertheless, a

C. Blümm (*) :C. HellerEADS Innovation Works,Munich, Germanye-mail: [email protected]

R. WeigelUniversity of Erlangen-Nuremberg,Erlangen, Germany

J Sign Process Syst (2012) 69:11–21DOI 10.1007/s11265-011-0640-8

Page 2: SDR OFDM Waveform Design for a UGV/UAV Communication Scenario

specific set-up was defined for a proof of concept. It will beintroduced in the following.

UGV communication in rough terrain is often hinderedby a missing line-of-sight (LOS) data link between UGVand the GCS. A promising way to overcome this problem isthe use of a UAV as relay, optimized for acting asautonomously as possible. A second advantage of thisapproach beside the NLOS compatibility is the increasedavailable communication distance between UGV and itsGCS.

Figure 1 depicts the considered communication set-up.The UGV is equipped with scenario-specific sensors and acamera. The UAV is acting as a communication relay forthe UGV, and in addition, it carries another camera tomonitor the UGV. For this set-up unidirectional video linksare needed in parallel to bidirectional control links from theUGV to the GCS (indirect link) and from the UAV to theGCS (direct link). There will be no direct link between theUGV and the GCS.

The main technical needs within this scenario are givenas follows. For each video link, a minimum data rate of3 Mbit/s is required, when low-quality MPEG 2 compres-sion is chosen [4]. However, better video quality is desired,to be traded off against its need for higher bandwidth.

The control links are low-bandwidth data streams andthus negligible for throughput considerations. Anotherdifference between the control link and the video link isthe height of tolerable bit error rate: Video stream errorslead to a decreased video quality, whereas the worst case ofa defective control link would be a complete systemfailure.1 The aspect of different reliability demands of thetwo link types must be kept in mind throughout thecomplete design process, starting with an adequate systeminterface: Both link types provide the Internet Protocol (IP),but differ in the transport layer. For the video link, theminimal protocol User Datagram Protocol (UDP) suffices,whereas the critical control link covers the TransmissionControl Protocol (TCP) to guarantee reliable messagedelivery. The difference is that UDP is based on continuousunidirectional transmission, no matter if the messagereaches its destination or not. In contrast, TCP offers theconcepts of message acknowledgment, retransmission andtimeout [5].

The set-up in Fig. 1 is intended for heterogeneousoperational environments. To comply with their inevitablyvarying demands, the waveform needs to behave differentlyfor maximum performance. Hence, it must be customizableto allow different configuration parameters tailored for

varying environments and varying channel characteristics.The video quality, for example, can be improved dramat-ically under good channel conditions, as soon as thewaveform parameters fit the channel quality.

Scenario- and OFDM- specific major channel challengesinclude high relative speed between the communicationpartners and frequency-selective fading channels, e.g. dueto multipath in urban environments [6].

Beside the waveform, also the SDR hardware, onwhich the waveform is implemented, needs to accountfor the specific scenario requirements. Beside other, themost obvious requirement is a small and lightweightform factor, suitable for mobile robots and flyingplatforms.

In summary, the three most important needs to becovered are the outstanding waveform parameter flexibilityon runtime, the need for two logical links at once (video/control) and the need for compact SDR hardware.

There are already a couple of promising SDRplatforms available, which address similar goals. Insome cases, even with OFDM reference designs provid-ed. On the open-source side, two of the most popularplatforms are GNU Radio with its Universal SoftwareRadio Peripheral (USRP) family [7, 8] and the RiceUniversity’s Wireless Open-Access Research Platform(WARP) [9–11]. In the commercial field, the popularSFF SDR Development Platform from Lyrtech [12] orSundance’s SDR ‘baseband’ developers’ platform series(e.g. SDR 3.0 or SDR 4.0) [13] come close to therequirements of our communication scenario. However,among all mentioned platforms and others [14], nooptimal solution was found which could be used withoutmajor adaptations regarding the hardware, the waveformor both. Consequently, the decision was made to design acompletely new SDR OFDM waveform from scratch for anovel hardware platform.

Radio link UAV-GCS

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narrowband, high-available control-data link, bidirectional, about 115 kbit/s (TCP/IP)

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Figure 1 The UGV/UAV communication scenario.

1 Even in case of a system failure, the UAV can be prevented fromcrashing by navigating it with a redundant remote control, as used formodel helicopters

12 J Sign Process Syst (2012) 69:11–21

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3 Hardware Platform

A compact three-staged hardware stack was developedaccording to the photo in Fig. 2 and the correspondingblock diagram in Fig. 3. It is used in exactly the sameconfiguration for the UGV, the UAV and the GCS. Itcombines a nanoETXexpress-SP Board from Kontron witha 1.6 GHz Intel Atom Processor Z530 [15], a Xilinx Virtex5 FPGA, coming as Express Card version from PicoComputing [16] to be inserted in a PCIe-based slot of theKontron board and a self-designed interface board. Assystem interface, this platform features Gigabit Ethernet toconnect to the mission control computers of UGV, UAVandthe GCS. Via this interface, video and control data streamsare exchanged in an IP packet format. The Kontron GPPboard serves as router here.

3.1 The Interface Board

The hardware platform is combinable with any propri-etary or commercial-off-the-shelf (COTS) RF transceiv-er hardware. For this reason, a front-end interface boardwas designed to offer analog baseband I/Q signal ports.The board comprises a dual 16 bit 500 MSPS DAC forthe transmitter path (currently 14 of 16 available bits inuse) and two 14 bit 125 MSPS ADCs for the receiverpath.

A downside of the Virtex 5 FPGA Express Card isits limited number of free I/O ports. This number wouldbe too small for the exchange of full 14 bit I and Qsignals. The workaround to this problem is I/O portexpansion with a second FPGA, a low-cost XilinxSpartan 3. Therefore, data is exchanged between theVirtex 5 and the interface board on both edges of theclock signal. More precisely, I data is exchanged onrising edges and Q data on falling edges. This doubledata rate transfer (DDR) is then resolved to single datarate transfer using the Spartan 3.

Low pass filters are included at the frontend I/O portsagainst aliasing effects.

3.2 The Virtex 5 FPGA

In order to achieve high signal processing performance, allthe OFDM baseband processing is done within the XilinxVirtex 5 FPGA. Beside baseband processing, another majorfunction block of the FPGA is the TDMA MAC control,realized as a finite state machine (FSM).

Multiple first-in-first-out (FIFO) buffers are imple-mented for temporary IP packet storage in order todecouple the data stream, as it passes from the GPP-based application layer to the FPGA-based MAC layer.As already mentioned, this interface is realized withPCIe, and it serves not only for exchanging payloaddata, but also for configuring the waveform: All flexiblewaveform parameters are set on runtime accordingPCIe-fed signals, which are stored in a configurationRAM register in the FPGA. Polling this configurationregister for every new data frame allows frame-wiseparameter updates.

Beside payload data and parameter control, the thirdsignal type which is transmitted via PCIe containsinformation about relevant internal FPGA signals. Thisinformation is required to evaluate the suitability of thecurrent waveform parameter set. Details of these monitor-ing signals are given in Section 7.

3.3 The Atom General Purpose Processor

The FPGA configuration register which was described inthe last section is updated by the GPP, more precisely by itscentral component, the waveform control block. This blockis primarily an easy-to-use handler for changing anywaveform parameter on runtime, implemented in Clanguage. The user can conveniently control it via a self-designed QT-based GUI, as depicted in Fig. 4. A secondtask of this GUI besides setting parameters is to display theaforementioned monitoring signals for real-time perfor-mance checking.Figure 2 Photo of the hardware stack.

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Page 4: SDR OFDM Waveform Design for a UGV/UAV Communication Scenario

For the Ethernet interface, an adequate driver wasimplemented also in C language. It offers a standard Linuxnetwork interface for the IP payload data, with theopportunity to set priorities for the different ports, namelythe video port and the control port. As a consequence, thecomplete data link is fully transparent to the applicationsoftware. It does not even notice that a wireless link insteadof a cable underlies.

Since the GPP runs under Linux OS, any standardapplication software is easily applicable to the SDRplatform. For this project, applications as the camera driveror video compression software are running on the IntelAtom. However, since this is no primary focus of thisarticle, we will not go into details here.

4 Baseband Processing

Coming with the benefits of great spectral efficiency androbustness in multipath environments, OFDM is the commu-nication scheme of choice when UAV systems require highdata rates in heterogeneous outdoor scenarios [17, 18].Further advantages of OFDM are its efficiency in imple-mentation based on FFT/IFFT processing and its efficiencyin channel equalization within the frequency domain [19].

Fortunately, there was no claim for complying with anycommon OFDM wireless standard like DVB, IEEE 802.11aor WiMAX, so there was any freedom to pick up commonideas and disregard others. Just to name one example for anadoption from IEEE 802.11a, the synchronization algorithmand the Automatic Gain Control (AGC) setting was imple-mented according Fort’s and Eberle’s proposal [20, 21].

The broad parameter flexibility must not only apply tothe waveform in general but for each logical link (videolink, control link) independently of the other. So the idea isto have two sets of parameter settings, one referring to arobust channel with low throughput for control data andanother one providing high throughput at the expense ofreliability for video data. Since subsets of subcarriers arefreely assigned to the individual links (video/control), theycan be transmitted simultaneously. This technique iscommonly known as OFDMA (orthogonal frequencymultiple access).

Figure 5 gives a simplified overview of the wholefunctionality of the central part of the waveform, which isthe FPGA part. Control signals are illustrated with thinarrows and data signals with thick arrows. All functionblocks are self-designed, except for the IFFT/FFT, the En-and Decoder and the PCIe Interface, which are standardXilinx IP cores.

Figure 4 GUI for parameter control and monitoring.

14 J Sign Process Syst (2012) 69:11–21

Page 5: SDR OFDM Waveform Design for a UGV/UAV Communication Scenario

As depicted at the top of Fig. 5, the entire frontendconfiguration, namely the AGC and transceiver setting, isrealized with simple FSMs. At the lower left part ofFig. 5, the PCIe interface is shown, which serves forvideo and control data exchange via IP frames onnetwork layer. These IP frames will be serialized tobitstreams within the High-Level Data Link Control(HDLC) blocks on the transmitter side (TX). On thereceiver side (RX), similar HDLC blocks de-serialize thebitstreams again.

A similar structure of what is shown from thoseHDLC blocks on to the right-hand edge of Fig. 5 canbe found as standard OFDM baseband processing chain inevery OFDM book (disregarding the OFDMA aspect)[22]. Within this processing chain, the bitstream ismapped to I/Q constellation points (frequency domain)and finally transformed to I/Q time domain signals usingan IFFT.

Although the PCIe interface runs natively with65.2 MHz, the ideal clock for the baseband domaincould differ from that for an optimal match with aspecific frontend. For this reason, two internal clockdomains are realized with the aid of a digital clockmanager and buffers, one for PCIe and one for thebaseband domain.

Since there is no need to change the frontend on runtime,there is also no runtime-configurable baseband clockrequired. Instead, the clock is defined before synthesizingthe design. With the 100 MHz oscillator, which drives theFPGA, a 100 MHz baseband domain and a 48 MHz werealready realized and tested.

5 Runtime Flexible TDMA

The channel access of the different users (UGV, UAV andGCS) is organized with TDMA. As depicted in Fig. 6, thetop layer of the time schedule is made up of a periodicallyrepeated TDMA superframe. This superframe is then sub-divided into 64 TDMA slots or less, all of the same length.Within each TDMA time slot, only one user is transmittingwhile the rest is listening. This means, a half-duplexcommunication system is provided.

Once the traffic demand of the different users isidentified, the slots can be portioned accordingly: Themore traffic a user requires, the more slots are reserved forhim to transmit.

The users are not equal. One master node must bechosen to set the timing of the superframe by inserting asuperframe start indicator in the first slot of the schedule.

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Since the master node must be directly visible to all otherusers, it must be the UAV in our scenario. When the UGVor the GCS join the network, they must wait for thesuperframe start indicator to come, before they cansynchronize and actively participate.

Besides synchronizing, this first slot is also used tospread information about important TDMA settings like thenumber of slots currently in use and their allocation. This inturn means that the master node is able to change theTDMA slot allocation on runtime.

At the start of each slot, a short idle time is hold in order toremedy propagation delays and clock drifts and to ensure thatall users are listening. (called Action Point Offset TX in Fig. 6)

6 Runtime Flexible OFDM

The key feature of the presented waveform is its parameterflexibility. One flexibility aspect was already introduced inthe last section, namely the TDMA slot allocation.

In the following, all the OFDM waveform parameterswhich are runtime-configurable are listed and explained.All of them have in common the trade-off betweenbandwidth and reliability.

6.1 OFDM Frame Layout

As shown in Fig. 6, each TDMA slot contains an OFDMframe. It makes no sense to flexibilize every aspect of theframe structure. Specifically, the preamble structurewhich is used for calibrations at the start of each OFDMframe must be static. It is loosely adopted from the IEEE802.11 standardization [20] and comprises the followingfour parts: An AGC Preamble for signal detection andAGC setting, a Slot Sync for coarse frequency offsetestimation and timing synchronization, an SF Sync, whichis only included in case the current TDMA slot is the first

one in the superframe schedule2 and a Pilot Symbol forchannel and fine frequency offset estimation [23, 24].

Apart from this static preamble structure, the presentedwaveform offers a variety of configuration options for theOFDM frame layout:

6.1.1 OFDM Symbols per Frame

Since the preamble is not usable for payload transmission,the OFDM frame should be generally chosen as long aspossible in order to maximize the data-to-preamble ratio.

However, because of the essential tasks of the preamble,like AGC setting or coarse frequency offset estimation, thepreamble must appear repetitively within the transmittedstream. With other words, a rise of the frame length directlyimproves the data throughput, but is limited due to the needfor preamble-based calibration.

In order to account for specific channel characteristics,the preamble repetition rate is flexible, as the number ofOFDM symbols within one frame can be adapted.

One example for the need to adapt the preamblerepetition rate is a varying sample clock offset betweenthe sender and the receiver platform. This could e.g. resultfrom temperature changes between the UGV and the UAV,which would directly affect the oscillators.

The induced frequency offset could be compensated bypilots only up to this point when the offset equals one fullclock cycle at the end of the frame [25]. Here, the termpilots refers not only to those within the pilot symbol of thepreamble, but also to the scattered pilots, which areexplained in Section 6.2.3.

In the above-stated case, the frame length would need tobe reduced in order to perform coarse frequency offsetestimation and timing synchronization earlier, before theoffset compensation capability of the pilots is reached.

6.1.2 Cyclic Prefix length

Without cyclic prefix, transmitting OFDM symbols over afading multipath channel leads to inter-carrier-interferences(ICI) and inter-symbol-interferences (ISI). The reason is thespectral discontinuity between OFDM symbols, which dis-perses into the payload part of the symbol and thus causes aloss of subcarrier orthogonality. However, the OFDM symbolcan be cyclically extended by copying the last portion of thesymbol and appending it as cyclic prefix to the front of thesymbol. This way, the discontinuity still remains, but is nolonger present within the payload part of the symbol.

For the receiver, the cyclic prefix is useless and will bediscarded. Consequently, it should be set as short as possible,but at least as long as the channel impulse response.

2 The SF Sync is not compliant with the IEEE 802.11 standardization

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ymb

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c

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ync

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Win

do

win

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licP

refi

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licP

refi

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FFT length: 64 FFT length: variable FFT length: variable

SF SyncAGC Preamb

Win

do

win

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win

g

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win

g

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win

g

OFDM

Symbols

Figure 6 TDMA / OFDM time schedule.

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6.1.3 Windowing Length

Thanks to the cyclic prefix, the spectral discontinuity betweenOFDM symbols is no longer an obstacle for correct datatransmission. Nevertheless, this spectral blurring is stillexistent and poses a problem for using adjacent frequencychannels. A solution is windowing, which denotes simplycross-fading between consecutive OFDM symbols. To attain asharp spectral edge of the OFDM signal, the fading must be assmooth as possible, which means long windowing.

Just like the cyclic prefix, also the windowing portion ofthe OFDM signal is discarded without being used by thereceiver and thus it should be short, too.

6.2 OFDM Subcarrier Usage

According to the TX part in Fig. 5, the OFDM SymbolAssembler allocates the mapped data to the subcarriers,which are then fed to the IFFT block. On the RX side, thisprocedure is reversed with the aid of the OFDM SymbolDe-Assembler. Especially this part of the baseband pro-cessing is implemented to provide highest flexibility:

6.2.1 Number of Subcarriers / FFT Length

An essential challenge of the presented scenario is the highspeed the UAV is able to reach. The top speed depends onseveral aspects and can reach up to 120 km/h. With theUGV moving in the opposite direction, the relative velocitywould be even higher. Specifically, in OFDM communica-tion, high speed leads to much shorter coherence time andlarger Doppler frequency spread, especially for the case thata high RF carrier frequency is used. As a direct result, theorthogonality among subcarriers gets lost and ICI degradesthe performance [26]. The solution to this problem is tobroaden the frequency spacing between the subcarriers forincreased tolerance against orthogonality loss. This can beachieved by decreasing the FFT length, because thisparameter is indirect proportional related to the frequencyspacing. So in order to strengthen against Doppler shift, anadequate FFT length out of the following can be chosen: 8,16, 32, 64, 128, 265, 512 or 1,024.

6.2.2 Allocating Video, Control or None of Both

All subcarriers can be fully flexible occupied either withvideo data, control data or they are just left open. This hasbasically three advantages:

First of all, the throughput of the video link can bedynamically adjusted at the expense of the control linkthroughput and vice versa, which allows furtherprioritization.

Second, each RF transceiver offers a specific band-width to be usable for transmission. To maximizethroughput, the waveform must fill all subcarriers lyingwithin this bandwidth up with data and set the rest tozero as guard bands. With other words, any giventransceiver bandwidth is supported with this option. Ofcourse, this parameter is strongly related to the FFTlength. Changing one of both parameters requireschanging the other one as well.Third, beside the guard band, another reason for settingsubcarriers to zero is to mask out strongly fadedfrequencies.

Figure 7 gives a simple demonstration of the change ofsubcarrier on runtime. The figure refers to RF transmission ofthe OFDM waveform at 1.5 GHz center frequency. The toppart of the figure shows the currently measured signalspectrum and the bottom part gives its history with differentcolors for different signal power. The older the measuredspectrum, the more it is shifted downwards. This spectrogramclearly shows that the bandwidth increases over time. This isbecause the 512 FFT OFDM signal was changed from using20 subcarriers, then 40 and finally 60 for payload data.

6.2.3 Scattered Pilots

In the previous section, three options for subcarrierallocation were presented. In order to account for thescenario-specific problem of fast changing channels, yetanother option is provided, which is the use of subcarriersas scattered pilots.

As already mentioned, the first OFDM symbol of eachOFDM frame is filled with pilots, which are known data toidentify the transfer function of the system. The correctionvalues calculated with these pilots for each subcarrier are

Figure 7 Over-the-air measured spectrogram: occupied OFDMsubcarriers are changed from 20 to 40 to 60.

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applied to the following subcarriers of the following OFDMsymbols of this frame, assuming that the channel character-istics do not change meanwhile.

Since this assumption applies less and less with fastchanging channels, the pilots must occur more often withinthe data stream in order to update the correction valuesmore frequently. To meet this goal, two options areconceivable, which is either reducing the number of OFDMsymbols per frame, or, the more elegant solution, placingadditional pilots among the payload data.

The second solution, the concept of scattered pilots isimplemented as follows. Before checking for a specificsubcarrier in a specific OFDM symbol, whether it should beallocated with video or control data or none of both, there is aprioritized request, if the specific subcarrier shall be occupiedby a pilot. This request refers to a systematic placement of thepilots within the frame, using up to three completelyindependent patterns. These patterns are defined in terms oftheir spacing over the subcarriers and their spacing over theOFDM symbols. In addition, the start position of each of thepatterns is defined by the according OFDM symbol numberand the according subcarrier number. Overlapping the patternsis possible. The pattern placement is completely runtime-flexible, simply by spreading the information about eachpattern’s subcarrier- and symbol-spacing, as well as its startposition to all communication partners (UGV, UAVand GCS).

Figure 8 gives two simple examples of how the pilotscan be distributed over the frame. Each row refers to anOFDM symbol and each line to a specific subcarrier (withDC in the middle and the guard bands at the edges). For thestructure in Fig. 8a, all three flexible pilot patterns areneeded, while for Fig. 8b, one pattern is enough.

The question remains, how and when to change thescattered pilot structure. This challenge of optimizing pilot-aided channel estimation/equalization for OFDM systems isextensively studied in literature [27–30]. In order to reducethis complex topic to a rule of thumb, one could split up theproblem of fast changing channels into two extreme shapes:Fast changes over frequency, e.g. due to a long delay spreadand fast changes over time, e.g. due to a high Dopplerspread [31, 32]. For fast changes over frequency, the patternin Fig. 8a could be an adequate solution, while for fastchanges over time the pattern of Fig. 8b would be moreadequate.

The classical method to process scattered pilots is thesample-and-hold approach, meaning that each time, ascattered pilot subcarrier is received, the initially (or last)determined correction factor is updated for this subcarrier,while all other correction factors stay the same. However,especially in case of fast changing channels over timeaccording Fig. 8b, the second provided method is bettersuited: For this method, the pattern must be chosen in a wayto have more than one scattered pilot within one OFDM

symbol. Each time, an OFDM symbol with scattered pilotsubcarriers is received, all correction values of the completesymbol are updated in terms of interpolating linearly overall subcarriers of this OFDM symbol.

With an additional control signal provided, the sample-and-hold method can be changed on runtime to the linearinterpolation method and vice versa.

6.3 Modulation Scheme

Lowering the modulation scheme and therewith the resolutionof the I versus Q constellation diagram is the traditionalcountermeasure against a variety of channel effects, includingnoisy channels. This is done at the expense of the bits-per-subcarrier ratio. The waveform presented here supportsBPSK, QPSK, 16QAM and 64QAM. This modulationspectrum is the state-of-the-art selection of modern OFDMbased wireless standards like WiMAX, IEEE 802.11a or3GPP Long Term Evolution (without BPSK).

6.4 Code Rate of Convolutional Encoder/Viterbi Decoder

A convolutional encoder is typically used by the sender toenrich the payload with redundancy. This is done in a waythat allows the receiver to identify and to correct transmis-

DC

f

t

a)

Unused CarrierData CarrierFixed Preamble PilotsDC

f

Flexible Pilot Pattern 1Flexible Pilot Pattern 2Flexible Pilot Pattern 3

b)

t

Figure 8 Examples of scattered pilots patterns: a optimized forspectrally dynamic channels, b optimized for temporally dynamicchannels.

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sion errors with the aid of a Viterbi decoder, completelywithout further inquiry [33].

For the presented waveform, a Viterbi decoder IP corefrom Xilinx was applied [34]. Its code rate, meaning theratio of payload to redundancy, is flexible from 1/2 to 1/7.With puncturing, the code rate can be increased up to 3/4 or7/8 [35].

7 System Performance Monitor

In the previous section, all run-time flexible OFDM param-eters of the waveform were introduced. The question remainshow to choose them in an adequate way for optimalperformance in terms of the current nonideal channelcharacteristics. For evaluating the temporary system perfor-mance, two general monitoring tools are provided: A bit errorrate tester (BERT) and different options to analyze the I versusQ constellation diagram.

7.1 Bit Error Rate Monitor

During normal operation, the control and video data isexchanged via PCIe as described in Section 4. However,when the BERT is applied, the system is alternatively fedfrom a source for pseudo random bit sequences (PRBS). Atthe receiver part, a second PRBS generator is synchronizedwith the incoming data stream in order to evaluate the biterror rate. Since this number is directly transmitted to theGPP and indicated with the QT-based GUI, real-timemeasurements are available.

Of course, error rate testing is also possible with IPpinging, but this would only provide information about theIP packet loss rate and not about the bit error rate. Thepacket loss rate is less significant, because a packet will bediscarded, no matter if there was only one false bit in thepacket or multiple ones.

7.2 I/Q Constellation Monitor

Bit error rate testing is appropriate to evaluate the overalltransmission robustness across the complete communicationsystem. However, in case the bit error rate increases, its reasonsmust be found withmore detailed analysis on particular effects,which result from specific nonideal transmission conditions.For this purpose, the receiver’s I/Q signal stream is tappedbefore and after the channel equalization block accordingFig. 5. These signals are once more directly transmitted to theGPP and visualized with the QT-based GUI as constellationdiagram (I versus Q signals). Thus, any effect whichinfluences the amplitude or phase of the complex modulatedOFDM signal is visualized. A comprehensive overview ofpotential effects of nonideal transmission is given in [36].

The developed GUI not only visualizes the I/Q constella-tion, but also provides with the error vector magnitude (EVM)a corresponding performance metric. The EVM describes thedeviation of the measured transmitted signal against a perfect,theoretical signal [37]. The GUI supports plotting the EVMversus subcarrier or versus symbol. This can be crucial inallowing the operator to quickly locate the source of errors,such as strong faded frequencies (EVM versus subcarrier) orfast changing channels (EVM versus symbol).

8 Designing and Testing the Waveform

The process of developing the waveform can be dividedinto three major steps.

At first, the software/hardware design: The PCIe andEthernet drivers, as well as the waveform control wereimplemented in C language. The FPGA design was done witha hybrid approach of pure VHDL and a model driven designenvironment based on MathWorks Matlab/Simulink includingthe System Generator for DSP from Xilinx. This approachallows at a very early stage elaborated simulations, e.g. with theSimulink multipath Rayleigh or Rician fading channel models.

Second, testing on the SDR hardware was performedwithin a predefined environment. More precisely, the fadingchannel simulator R&S AMU200A was deployed to stressthe waveform with a set of specific static and dynamicfading scenarios, as well as Doppler and AWGN noise real-time simulations.

The final step for the waveform to pass is the test with theUGV, the UAV and a GCS in the open field. There are twotypes of UAVs in use. On the one hand this is the self-designedUAV in Fig. 1 with the maximum speed of 70 km/h and onthe other hand this is a Swiss UAV, NEO S-300 series, whichflies with up to 120 km/h [38]. The applied UGV is the roverMERLIN, which was designed for robust operations in harshoutdoor environments. Its maximum speed is 12 km/h [39].

9 Conclusion and Outlook

In this article, we gave an overview over an OFDM-basedwaveform, designed for communication links amongautonomous robotic platforms.

Though the waveform is applicable to a broad spectrumof scenarios, a specific communication set-up was definedfor validation. Its participants are a UGV, a UAV and aGCS, all accessing the channel by means of runtime-flexible TDMA. In this scenario, the communication focusis on a unidirectional video link from the UGV to the GCS,which is established indirectly, utilizing the UAV as relay.In addition, further direct and indirect links for video andcontrol data among the communication partners are needed.

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The waveform perfectly accounts for the challenges of thisset-up, as it offers two separated logical links for video andcontrol data concurrently in OFDMA manner. Furthermore, itprovides outstanding flexibility according its waveformparameters. The user is able to freely optimize for throughputor for reliability and even more specific problems like fastchanging channels or Doppler spread can be systematicallyand efficiently fought with the right parameter setting.Moreover, not just the waveform in total is adaptable, butspecifically each logical link independent of the other. Alladaption is done dynamically on runtime with a customizedGUI. Besides parameter setting, this GUI also serves assystem performance monitor, as it provides a bit error ratemonitor and an I/Q constellation monitor including EVMscope. This comfortably helps to decide, which parametersetting fits the current channel characteristics best.

The SDR hardware onwhich the waveform is implementedis also ideal for autonomous robotic platforms, since it featuresa very small form factor. Moreover, with TCP/IP and UDP/IPover Ethernet, it offers a universal system interface.

As soon as the waveform is fully tested, the next biggoal is to improve the system with adaptivity features. Upto now, the broad set of waveform parameters is freelyadjusted by the user, who must identify the current channelcharacteristics with the help of the system performancemonitor. In future releases, the waveform itself is supposedto measure, to analyze and to adapt to changing channelcharacteristics automatically.

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Christian Blümm received his Dipl.-Ing. degree in mechatronicsengineering from the Institute for Electronics Engineering at theUniversity of Erlangen-Nuremberg, Germany in 2009. After researchexperience in the field of smart sensors at Siemens Corporate Technologyand Areva N.P., he is currently working towards his PhD degree in thecommunications research department at EADS Innovation Works in thefield of waveform design for software-defined radio.

Christoph Heller has a degree in communications engineering fromthe RWTH Aachen University, Germany and received his PhD fromStuttgart University in 2010. He joined the European AeronauticDefence and Space Company (EADS) in 2005 and is employed in thecommunications research department of EADS Innovation Works.Christoph Heller is heading the development of next-generationwireless data links for aeronautic applications, focusing on digital

baseband processing. He is involved in several EADS internal,national and international research projects dealing with software-defined radio and cognitive radio.

Robert Weigel was born in Ebermannstadt, Germany, in 1956. Hereceived the Dr.-Ing. and the Dr.-Ing.habil. degrees, both in electricalengineering and computer science, from the Munich University ofTechnology in Germany, in 1989 and 1992, respectively. From 1982to 1988, he was a Research Engineer, from 1988 to 1994 a SeniorResearch Engineer, and from 1994 to 1996 a Professor for RF Circuitsand Systems at the Munich University of Technology. In winter 1994/95 he was a Guest Professor for SAW Technology at ViennaUniversity of Technology in Austria. From 1996 to 2002, he hasbeen Director of the Institute for Communications and InformationEngineering at the University of Linz, Austria. In August 1999, he co-founded DICE – Danube Integrated Circuit Engineering, Linz,meanwhile an Infineon Technologies Design Center which is devotedto the design of mobile radio circuits and systems. In 2000, he hasbeen appointed a Professor for RF Engineering at the TongjiUniversity in Shanghai, China. Also in 2000, he co-founded the LinzCenter of Competence in Mechatronics, meanwhile also a company.Since 2002 he is Head of the Institute for Electronics Engineering atthe University of Erlangen-Nuremberg. In 2010, he co-founded thecompany eesy-id.

He has been engaged in research and development on microwavetheory and techniques, integrated optics, high-temperature supercon-ductivity, SAW technology, digital and microwave communicationsystems, and automotive EMC. In these fields, he has published morethan 750 papers and given about 300 international presentations. Hisreview work includes international projects and journals. In 2002, hereceived the German ITG Award, and in 2007 the IEEE MicrowaveApplications Award.

Robert Weigel is a Fellow of IEEE, a member of the Institute forComponents and Systems of The Electromagnetics Academy, and amember of the German ITG and the Austrian ÖVE. He serves onvarious editorial boards and has been editor of the Proceedings of theEuropean Microwave Association (EuMA). He has been member ofnumerous conference steering committees. Currently he serves onseveral company and organization advisory boards. He is an electedscientific advisor of the German Research Foundation DFG.

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