A Real-Time, Fully-Software Aeronautical VHF tx/rx waveform ...

A Real-Time, Fully-Software AeronauticalVHF tx/rx waveform based on the

SCA-compliant OSSIE/USRP2 platformMario Di Dio, Andrea Morelli and Marco Luise

University of Pisa - Dip. Ingegneria dell’Informazione - Via Caruso - 56122 Pisa, ItalyEmail: [email protected]; [email protected]; [email protected]

Abstract—This paper describes the architecture, im-plementation and experimental results of a proof-of-concept, real-time, fully-software SCA-compliant tx/rxwaveform for VHF aeronautical communications basedon ETTUS Research’s Universal Software Radio Pe-ripheral v2.0 (USRP2) platform. According to the Soft-ware Defined Radio (SDR) paradigm, C++ softwaremodules performs all the signal processing functions toimplement the Aeronautical VHF standard. They weredeveloped from scratch by the authors in a SDR de-velopment framework (OSSIE) compliant to the Soft-ware Communications Architecture (SCA) guidelines.The resulting waveform can be considered as SCA-compliant and so enjoys the portability and interop-erability features which are characteristics of this ar-chitecture. The paper describes the main implementa-tion details, validation tests and computational perfor-mances, together with the design choices allowing real-time performance on an off-the-shelf personal com-puter.

Index Terms—VHF Aeronautical Communications,OSSIE, Software Communications Architecture, Soft-ware Defined Radio, USRP, Baseband Processing.

I. INTRODUCTION AND MOTIVATION

AERONAUTICAL radio communications is thesubject of several analogue and digital stan-

dards, like VHF telephones and VHF data link (VDL)[1], in order to transfer vocal and data information be-tween the aircraft and the ground stations during allthe phases of the flight (landing, take-off,..).As described in [1], the frequency spectrum from

118 MHz to 137 MHz is assigned to aeronauticalcommunications. According to such standard, an ana-logue voice channel has a bandwidth of 8.3 KHz,resulting in 2280 available voice channels on the

This work was in part supported by the European Commis-sion in the framework of the FP7 project SANDRA (contractn. 233679)

whole aeronautical frequency spectrum. The mod-ulation used is legacy amplitude-modulation (AM)with modulation index 0.85. VHF aeronautical equip-ment is commonly based on Application Specific In-tegrated Circuits (ASICs) implementing all the digi-tal signal processing functions in hardware (HW). Insuch HW architecture, software (SW) is used only forhouse-keeping functions and user interfaces. A fully-software implementation of a tx/rx chain for VHFaeronautical communications is not generally consid-ered because of the large economies of scale of thosesingle-standard products.

On the other hand, the implementation of a newflexible and reconfigurable radio terminal capableof switching among different and heterogeneous ra-dio communications standards is actually investigatedin some research projects, like SANDRA (Seamlessaeronautical networking through integration of datalinks, radios, and antennas), financed by the EuropeanCommission and strongly supported by manufactur-ers. The key-concept of this approach is the Soft-ware Defined radio (SDR) paradigm in which easily-reconfigurable hardware (General Purpose Processor(GPP), Digital Signal Processor (DSP), Field Pro-grammable Gate Arrays (FPGA)) is used as a com-putational asset for executing all the software-definedsignal processing functions. Also, the SW frameworkhas to provide flexibility and interoperability betweenthe software modules and the hardware platforms.This is the aim of the Software Communications Ar-chitecture (SCA) [2]. SCA is a non-proprietary, openarchitecture framework providing complete interop-erability across different SDRs that implement differ-ent radio communications standards (that are calledwaveforms in the SCA parlance), as well as highportability among heterogeneous hardware platforms.SCA is not a system specification, as it is intended

7th Karlsruhe Workshop on Software Radios

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Fig. 1. Software architecture of a SCA-compliant SDR.

to be implementation-independent, but rather a set ofdesign constraints. If a developer designs an SDR ac-cording to such design rules (Fig. 1), the resultingSW should be portable onto another SCA implemen-tation regardless of the operating system or hardwarethat implementation is based on. This feature encour-ages the implementation of SCA-compliant wave-forms implementing heterogeneous and interoperableradio communications standards (VHF maritime andaeronautical, GSM, TETRA, and even broadcastingstandards like Digital Video Broadcasting (DVB)).This paper describes the implementation of a real-

time, fully-software SCA-compliant waveform foranalogue VHF aeronautical communications, basedon the well-known Ettus Research’s Universal Soft-ware Radio Peripheral v2.0 (USRP2) hardware [3] .Specifically, the USRP2 provides the digital to ana-logue conversion (DAC) and baseband-to-RF con-version in the transmitter and the RF-to-basebandconversion including analogue to digital conversion(ADC) in the receiver. According to the fully-software paradigm, all baseband transmitter/receiverfunctions are implemented through efficient C++modules that were developed from scratch at the Uni-versity of Pisa, Digital Signal Processing for Commu-nication Laboratory (DSPCoLa).

II. THE OSSIE/USRP2 SDR PLATFORM

Any SDR system can be segmented into its ownSW and HW sections that are customized on the ba-sis of project requirements. In this work, both thetransmitter and the receiver are developed on an open-source SW/HW platform that we will shortly de-scribe.

Fig. 2. OSSIE SDR development environment.

A. SW Section

The SW section of the project was implemented ona C++-based SDR framework, called OSSIE [4]. Itwas developed at Wireless@Virginia Tech and rep-resents a SW environment for implementing SCA-compliant waveforms (Fig. 2). The software packageincludes:

• an SDR core framework based on SCA;• the Waveform Workshop, a set of tools for rapiddevelopment of SDR components and wave-forms applications;

• an evolving library of pre-built components andwaveform applications.

The framework is conceived to provide a valid de-velopment environment and remove the complexityrelated to the implementation of the SCA hierarchi-cal software architecture. Within OSSIE, unlike otherSDR frameworks like GNURadio, C++ is responsi-ble for the single functional block implementations,while the interconnection among such blocks is pro-vided, as stated by the SCA standard [2], by means ofCORBA interfaces and ports communication. Com-munication from/to the USRP2 is obtained by includ-ing the libusrp2 external C++ library in a suited soft-ware device, that we call USRP2, which abstracts intoour SW the HW platform.A certain amount of ad-hoc modifications to theOSSIE primitives that manage communicationsfrom/to the USRP2 were also introduced to exploitthe full functionalities of the described HW platform.

B. HW section

The peripheral used to transmit/acquire the sig-nal in this project is the well-known USRP2 system,which was developed by Matt Ettus [3] and was uni-versally adopted by the GNURadio community. The


156

Fig. 3. Functional block scheme of the implemented transmitter chain.

USRP2 is a board with open design and drivers. Itconsists of:

• 4 ADC at 100 MSamples/s with a resolution of14 bit;

• 4 DAC at 400 MSamples/s with a resolution of16 bit;

• Digital downconverters/upconverters with pro-grammable decimation/interpolation rates;

• 1 Gigabit Ethernet interface;• 2 Gbps high-speed serial interface for expansion;• extension sockets to connect a wide variety ofRF daughterboards;

Daughterboards are responsible for RFup/downconversion. In order to cover as manyfrequency bands as possible, several daughter-boards were developed: receivers, transmitters ortransceivers do exist which collectively cover theRF spectrum from DC to about 5 GHz. The RFfront-end used in the transmitter is the transceiverdaughterboard WBX which operates from 50 MHz to2.2 GHz while on the receiving side the RF front-endis provided by the receiver daughterboard TVRXcovering the frequency spectrum from 50 MHz to860 MHz. In both cases complex-valued I-Q samplesare sent over a Gigabit Ethernet interface (interleavedreal/imaginary parts, both represented as a signedshort int on 2 bytes). The maximum samplingfrequency sustained by the Gigabit Ethernet interfaceis 25 Msamples/ s corresponding to 800 Mb/s overthe interface, with which a full spectral window of 25MHz can be processed with no loss.

III. TRANSMITTER IMPLEMENTATIONWe describe in this section the main constituent

functions of the transmitter. The functional blocks arerepresented in Fig. 3, and all of them, except for theFFT block which is based on a standard routine [5],were implemented from scratch in order to have fullcontrol of the available system resources and to reachreal-time performance.

A. ControllerThe first block is the Controller. Its main func-

tion is to dynamically allocate the computational re-sources with respect to the number of active channelswe intend to transmit. This block represents the inter-face with the user which sets the number of the activechannels, the center frequencies of the channels to beused for transmissions, and the location of the audiosources.

B. Data Acquisition and AM ModulationThe first part of the chain is described as fol-

lows:• the Data Acquisition block acquires from thecontroller the location of the audio sources andreads from local mass storage or from the audioboard the digital audio file sampled at a samplingfrequency of 25 KHz;

• the Short Int-to-Float Conversion is necessaryto provide more accuracy to the subsequent pro-cessing blocks;

• the Fast Fourier Transform (FFT) block per-forms a 1024-point FFT at a sampling rate of 25KSamples/s;


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Fig. 4. Functional block scheme of the implemented receiver chain.

• the AM modulator operates in the frequency do-main and performs AM modulation of the inputdata with a modulation index of 0.85;

• the Low-Pass Filter limits the bandwidth of thesignal to 3 kHz in order to be compliant withthe strict constraints described on the standard[1]. For each channel, the low-frequency 123frequency samples out of the total of 1024 areretained.

C. Channel multiplexing and RF-front endThe second part of the chain is shortly described as

follows:• the Frequency Disposition and Interpolationblock collects the frequency samples of the dif-ferent channels and arrange them on their spec-tral positions according to the specifications de-livered by the controller block;

• the IFFT performs a 1024000-point IFFT. Theoutput stream has a sampling rate of 25 MS/scorresponding to a 25 MHz bandwidth;

IV. RECEIVER IMPLEMENTATIONIn this section we describe the implementation of

the receiver section that pairs with the transmitter de-scribed in III. The receiver has been split in two sub-chains: the channel sensing algorithm and the chan-nel demodulation. In the first phase of reception it isnecessary to detect the active channels in the rasterof the 2280 VHF available channels; afterwards, theuser selects the desired channel(s), the USRP sets itsreception parameters according to this choice, and fi-nally demodulation starts.

A. The USRP2 ControllerThe USRP2 Controller block is responsible for

the dynamic change of the reception parameters of

the USRP2 board. The correspondence between thechannel name and its central frequency is stored in atable, called channel map. According to the informa-tion received from the channel selection, it sets on theUSRP2 the proper center frequency, the HW decima-tion factor, the gain of the RF front-end, and redirectsthe output stream of the USRP2 towards the demodu-lation chain.

B. Channel Sensing Algorithm

The aim of this sub-chain is to reveal the activechannels scanning all the channel positions on theVHF aeronautical spectrum. In order to match theaccuracy of the algorithm with the available compu-tational resources, the spectrum (19 MHz) has beendivided into 76 sub-bands of 250 kHz which are iter-atively analysed. The energy-based algorithm is de-scribed as follows:

• the FFT module receives 5000 complex samplesfrom the USRP2 buffer at the rate of 25 KSam-ples/s and performs a 5000-points FFT;

• the Channel Energy Estimation block isolatesthe contributions of the different channels (166complex samples per channel) and calculates theenergy associated to each channel as an averageof 10 consecutive FFT observations. Then theenergy threshold is calculated as the arithmeticalaverage of the 2280 estimated channel energies.A channel is considered as active if its estimatedchannel energy is larger than the threshold;

• the Channel Selection module presents to theend-user the list of the active channels. Af-ter the user choice, this block communicatesthe channel number to demodulate to theUSRP2 Cotroller.


158

Fig. 5. Spectrum of the parallel transmission of several audiostreams on a 25 MHz spectrum window.

C. Signal Demodulation and Audio PlaybackAfter channel selection, the USRP2 output stream

is redirected to the demodulation chain, whose com-ponents are as follows:

• the Decimation and Butterworth Low-Pass Fil-ter component performs a decimation operationwith a factor 10 reducing the sampling rate from250 KSamples/s to 25 KSamples/s and so rep-resenting a 25 kHz frequency bandwdith. Thissampling frequency was chosen in order to sat-isfy the requirements of the audio card on thehost Personal Computer (PC). To select the de-sired channel, signal samples are filtered by atenth-order Butterworth low-pass filter with acut-off frequency of 3 kHz (the same cut-off fre-quency used on the transmitter);

• the AM Demodulator performs classical AM en-velope demodulation in the time domain by ex-tracting the module of the complex-valued re-ceived samples;

• the Butterworth Low-Pass Post-Filter was intro-duced to reject the out-of-band spurious compo-nents generated by AM demodulation and to im-prove the audio quality of the demodulated sig-nal;

• the Audio Board Playback component receivesthe data stream from input port, performs theDAC operation and plays the audio signal on theloudspeakers.

V. VALIDATION TESTS AND EXPERIMENTALRESULTS

The implemented waveform was developed, testedand validated at DSPCoLa, University of Pisa, Italyboth for the transmitter and receiver side.At the transmitter side, as stated in section III, all

the signal processing is performed by an off-the-shelf

PC equipped with an Intel Core i7 2670QM proces-sor (2.2 GHz), Ubuntu 11.10 Operating System (OS)and OSSIE 0.8.2. The transmitter was validated usingsome commercial radio devices (ICOM,..) capableto receive aeronautical communications. The mainfeature of the transmitter is the possibility to con-currently transmit in real-time several audio streams,coming from dumped audio files or from real-time ac-quisition devices (audio cards), placing them on arbi-trary RF channels located anywhere in the 25 MHzspectrum window. Our measurements showed thatour transmitter is fully compliant to the strict con-straints on adjacent channel interference described inthe standard [1]. For example, Fig.5 shows the spec-trum of a parallel transmission of 5 audio streams.In this case, a low-cost radio scanner was capableto receive all of the active channels with no interfer-ence and a very good audio quality. The main de-sign choice to implement all signal processing func-tions in the frequency domain makes the computa-tional load practically independent of the number ofactive streams: the computational heaviest functionis the 1024000-point IFFT whose complexity is inde-pendent of the number of active channels. To demon-strate this, we stressed the chain by transmitting morethan 20 parallel channels without obtaining any sig-nificant change on the computational load of the sys-tem, which turns out to be only 20% of the availableCPU resources.

At the receiver side, as described in section IV, thesignal captured by the antenna is sent to the USRP2front-end and then, after baseband conversion andADC conversion, it is routed via the Gigabit Ether-net interface to the host PC. In our lab, we used anoff-the-shelf PC with an Intel Pentium IV, Ubuntu10.04 Operating System (OS) and OSSIE 0.8.1. Asdescribed in [1], we tested the receiver using a mod-ulated pilot tone at 2 kHz which was transmitted atdifferent power levels by an Agilent Signal Genera-tor E4438C. Tests showed that the occupied channelwas correctly revealed as active, in any of the allowedspectral positions, and it was demodulated with verygood audio quality even in low signal-to-noise ratio(SNR) conditions. Fig. 6 shows the spectrum of thesignal at the output of the low-pass filter ready to beplayed on the loudspeakers. The computational loadof the receiver is about 25% of the available compu-tational power.

In addition to the instrumental tests above, wealso performed some subjective, ”real-world” listen-ing sessions, in particular by demodulating the Vol-


159

Fig. 6. Spectrum of the processed signal at the input of audioboard

ume Meteorological (VOLMET) channel transmittedby the Pisa Airport at 128.4 MHz, as well as otheranalogue voice channels, with very good perceivedSNR.

VI. CONCLUSIONS AND PERSPECTIVES

This paper described the architecture, implemen-tation, and experimental results of a real-time,fully-software, SCA-compliant tx/rx waveform forVHF aeronautical communications based on theUSRP2/OSSIE platform. Validation tests showedthe developed waveform to be fully-compliant to thespecifications of the standard [1] both at the transmit-ter and at the receiver side, with very good computa-tional performance and subjective listening quality.The implemented waveform has to be considered

as a proof-of-concept, but we believe that the realiza-tion of a SCA-compliant waveform with good compu-tational efficiency can be considered as the first steptowards the realization of a low-cost, high-efficiency,flexible and reconfigurable SDR node in which het-erogeneous aeronautical communications standardscan easily coexist.

REFERENCES

[1] Ground-based VHF hand-held, mobile and fixed radio trans-mitters, receivers and transceivers for the VHF aeronauticalmobile service using amplitude modulation, ETSI Std. EN300 676 V1.5.2, 2011.

[2] SCA 2.2.2 [Online]. Available: http://sca.jpeojtrs.mil/[3] Ettus Research LLC website, [Online]. Available:

http://www.ettus.com.[4] OSSIE, SCA-Based Open Source Software Defined Radio

[Online]. Available: http://ossie.wireless.vt.edu/[5] M. Frigo and S. G. Johnson, “FFTW: An adaptive software

architecture for the FFT”, in Proc. IEEE Int. Conf. on Acous-tics, Speech, and Signal Processing (ICASSP), Seattle, WA,May 1998, pp. 1381–1384.

Mario Di Dio received his cum-laudeB.E. degree in telecommunications en-gineering from the University of Pisa,Pisa, Italy, in July 2007 with a the-sis on wireless cooperative communica-tion. In September 2009 he receivedhis cum-laude M.E. degree from thesame university with a thesis developingthe synchronization and channel estima-

tion/equalization functions for a SDR fully-software DVB-T re-ceiver. Since October 2008 he is actively working with the “DSPfor Communications Lab” (DSPCOLA) of the Dept. of Infor-mation Engineering, University of Pisa, under the supervision ofProf. Marco Luise.Since January 2010 he is attending his PhD studies in radio

transmission systems at DSPCOLA. He is involved in some eu-ropean projects (SANDRA and EULER) focused on implementa-tion of sotware defined radio applications for public security andnext-generation avionics communications. His interests are in theareas of digital communications, signal processing and estima-tion theory. His current research topics focus on efficient signalprocessing algorithms for sofware defined radio, implementationof SCA-compliant waveforms and estimation theory applied tocognitive radio.

Andrea Morelli received the B.E. de-gree in telecommunications engineeringfrom the University of Pisa, Pisa, Italy, inSeptember 2011 with a thesis on TCP al-gorithm variants. Since October 2011 heis working on his master thesis on the im-plementation of a software-defined trans-mitter for VHF communications.

Marco Luise is a Full Professor ofTelecommunications at the University ofPisa, Italy. After receiving his M.E. andPh.D. degrees in electronic engineeringfrom the University of Pisa, he was aResearch Fellow of the European SpaceAgency (ESA) at ESTEC Noordwijk, TheNetherlands, and a Researcher of the Ital-ian National Research Council (CNR), at

the CSMDR Pisa. Prof. Luise co-chaired four editions of theTyrrhenian International Workshop on Digital Communications,was the General Chairman of the URSI Symposium ISSSE’98,and the General Chairman of EUSIPCO 2006 in Florence.Prof. Luise is a Fellow Member of the IEEE and served as

an Editor for Synchronization of the IEEE TRANSACTIONS ONCOMMUNICATIONS, and as an Editor for Communications The-ory of the European Transactions on Telecommunications. Heis the co-Editor-in-Chief of the recently founded InternationalJournal of Navigation and Observation, and acts as the GeneralSecretary of the Italian Association GTTI, Gruppo Telecomuni-cazioni Teoria dell’Informazione. He has authored more than 150publications on international journals and contributions to majorinternational conferences, and holds a few international patents.His main research interests lie in the area of wireless communi-cations, with particular emphasis on CDMA/multicarrier signalsand satellite communications and positioning.


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