Implementation of a Vehicular NetworkingArchitecture Supporting Dynamic Spectrum Access

Tayyar Rzayev, Yuan Shi, Anastasios Vafeiadis, Srikanth Pagadarai, Alexander M. Wyglinski

Wireless Innovation Laboratory,Department of Electrical and Computer Engineering,

Worcester Polytechnic Institute,Worcester, MA, 01609-2280, USAURL: http://www.wireless.wpi.edu

Email: {tayyar, johnshi, anasvaf, srikanthp, alexw}@wpi.edu.

Abstract—In this paper, we present a prototype of a decen-tralized vehicular communications networking architecture thatis capable of performing dynamic spectrum access (DSA) whileoperating under mobile conditions. Referred to as vehiculardynamic spectrum access (VDSA) networking, our prototype im-plementation employs software-defined radio (SDR) technologyat each of the vehicular wireless nodes that have been pro-grammed to simultaneously perform multi-hop routing and DSA.To evaluate the proposed VDSA implementation, a prototypenetwork consisting of four mobile nodes was deployed in an actualhighway operating environment, where the wireless transmissionlinks between any two vehicular nodes were established usingunoccupied portions of the ISM band between 2.4 GHz and2.5 GHz. The performance of the proposed VDSA networkingarchitecture was assessed experimentally in terms of the comple-tion of end-to-end multi-hop relaying across the four vehicularnodes, the overall data throughput, and the network latency. Theexperimental results show that the proposed network architectureperformed a combination of DSA and multi-hop routing with theefficiency of up to 72.5% per hop

I. INTRODUCTION

S ignificant research efforts are currently being pursued inorder to facilitate ubiquitous, reliable, and efficient wire-

less data connectivity between individual vehicles on the road,as well as between vehicles and roadside wireless networkinginfrastructure [1]. The objective of these research efforts isto further enhance the overall driving experience while on theroad [2–5]. In particular, one application for these advances inwireless connectivity is vehicular safety, where the informationreceived by a driver via wireless data transmissions can beused to enhance his/her perception of the prevailing drivingconditions and be used to improve his/her overall situationalawareness [6].Given the transformative potential of vehicular communica-

tion networks to revolutionize the way we drive, it is expectedthat commercial vehicle manufacturers will introduce thistechnology into successive generations of their products [7–9]. However, as the number of vehicles equipped to performwireless networking increases, coupled with an emerging as-sortment of vehicular wireless applications and standards, theissue of spectrum scarcity will eventually need to be addressedin order to accommodate this rapid growth. To resolve this

issue of spectrum scarcity, the concept of dynamic spectrumaccess (DSA) has been proposed as a viable solution forfacilitating spectrally efficient vehicular communications andincreasing vehicular networking capacity [8, 10, 11].Although there have been various research efforts into

assessing the feasibility of performing DSA within vehic-ular networks, referred to as vehicular dynamic spectrumaccess (VDSA), most of these studies have either exam-ined candidate frequency bands for supporting VDSA net-works [10], defined networking protocols for enabling VDSAcommunications [7–9], or assessed the theoretical capacityof VDSA networks [11]. One research project constructed aVDSA prototype system and evaluated the implementation ina laboratory environment [12]. However, to the best of theauthors’ knowledge, there have not been any actual multi-vehicle, distributed VDSA experiments conducted in actualtraffic conditions.In this paper, we present the outcomes of an experimental

study that assessed the performance of a decentralized VDSAnetworking architecture capable of performing multi-hop rout-ing while operating in actual highway traffic conditions. Thenetwork architecture under study was implemented using acollection of Universal Software Radio Peripheral – Version 2(USRP2) software-defined radio (SDR) platforms [13] runningcustom-built digital communication, digital signal processing,and networking algorithms via the GNU Radio open-sourcesoftware package [14].The rest of this paper is organized as follows: An overview

of the VDSA concept is presented in Section II, while theproposed network architecture is described in Section III. Thecomplete experimental implementation along with results andperformance analysis are presented in Section IV. Finally,several concluding remarks are drawn in Section V.

II. VDSA OVERVIEWA VDSA-enabled network employs wireless transmission

links between any two vehicular communication devices es-tablished across unoccupied portions of licensed spectrum.Consequently, link formation is performed once a vehicularwireless device has identified all the unoccupied frequency

2011 IEEE Vehicular Networking Conference (VNC)

978-1-4673-0047-6/11/$26.00 ©2011 IEEE 33

frequency

PU PUSU SU

Car A

Car B

Car CLABLBC

LBC LAB

UnoccupiedSpectrum

VehicularCommunications

VehicularCommunications

Fig. 1. An illustration of the vehicular dynamic spectrum access (VDSA)concept. Notice how the vehicular secondary (unlicensed) users (SUs) tem-porarily access unoccupied spectrum from primary (licensed) users (PUs) inorder to communicate between individual vehicles on the road.

bands via spectrum sensing, as well as identified the frequencyposition of all the beacon signals corresponding to nearbytransmitting vehicles. Moreover, by characterizing the statisti-cal behavior of the wireless spectrum environment, each vehic-ular wireless device can differentiate between an actual signaland background noise. Finally, using network protocols thatsupport multi-hop routing throughout the distributed vehicularnetwork, vehicles can establish communications between eachother without the need for centralized infrastructure.As an example, we refer to Fig. 1 where “Car A” communi-

cates with “Car B” by forming a secondary (unlicensed) user(SU) transmission link LAB across some unoccupied spectrumthat is located between two primary (licensed) user (PU)signals. Similarly, wireless communications between “Car B”and “Car C” is established via the transmission link LBC, whichis also located within a region of unoccupied spectrum. Thus,using the experimental network architecture presented in thispaper, the feasibility and overall performance of a VDSA-based multi-hop relay network is evaluated. The proposedarchitecture to be studied, implemented, and evaluated ispresented in the next section.

III. PROPOSED NETWORK ARCHITECTURE

The implementation of a VDSA communication system canbe subdivided into the transmitter and receiver designs. Thetransmitter design is mainly focused on finding spectral whitespaces that could be used to send messages within the VDSAnetwork. On the other hand, the receiver design is mostly basedon the probabilistic analysis of spectrum in order to determinethe transmitter carrier frequencies.

A. Transmitter DesignOne of the primary concerns when transmitting information

within our proposed VDSA networking architecture is thepotential for interference with existing PU and SU signalslocated within the spectral vicinity. In order to avoid anyspectral collision with these signals, we designed a two-stepapproach: (i) Determine portions of the spectrum that are freeof PU signals, and (ii) Analyze those portions to avoid SUsignals. A flow diagram describing the overall design of thetransmitter is shown in Fig. 2.Finding Primary User - Unoccupied Frequencies: The

proposed VDSA architecture relies on a combination of aspectrum database consisting of known PU frequency locationsand spectrum sensing of SU signals. The spectrum databaseis linked to geographical locations and defines the frequencyranges of static PU signals that would be considered “off-limits” to SU transmission. Without loss in generality, the fre-quency locations of the PU signals were artificially generatedduring the VDSA experiments in order to enable us to observethe behavior of the SU transmissions. Note that database-assisted VDSA for static PU signals have been created, suchas for digital television channels [10].Avoiding Secondary Users: In order to avoid secondary

users, a “listen-before-talk” approach was devised. That is,each radio is in Receive Mode by default and it recordsthe frequency values of the SU transmissions. As soon asthe radio is switched to Transmit Mode, it is aware of thefrequency locations of the SU transmissions and thus it canproceed with selecting the most optimal carrier frequency fortransmission. In this work, “most optimal” carrier frequency isdetermined with an intelligent and environment aware method,referring to a frequency location within the wireless spectrumthat is furthest from existing PU transmissions, as well as notoverlapping with existing SU signals.Message Generation and Protocol: Once information about

both PU and SU signals have been obtained, the messageis then generated. For this network architecture, a messageformat was devised in order to facilitate the successful transferof information from one vehicle to another. The informationincluded in this message format are: (i) the identificationnumber of the radio that initiated the transmission of thismessage, (ii) the type of hazard warning to be issued toother vehicles within the VDSA network, (iii) the currentmessage hop count, (iv) the time stamp when the messagewas generated, and (v) the identification number of the radiothat most recently relayed the warning message. In its currentimplementation, the proposed architecture given this messageformat can theoretically support up to 1000 radios, 100 typesof hazard warnings, 100 hop counts, with the time stamp inthe hh/mm/ss format.

B. Receiver DesignThe receiver design relies heavily on the statistical charac-

terization of the wireless spectrum. Referring to Fig. 2, thepower spectral density (PSD) is derived from the spectrumsensing measurements and all peaks exceeding three standard

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StartAvoidPrimaryusers

ChooseVacant

Frequency

AvoidSecondaryusers

Generatethe messageaccording toprotocol

Transmit themessage

End ofTxLoop

Fig. 2. Flow chart of the proposed transmitter design showing how it searches for and avoids both PUs and SUs, as well as determine the most optimallocation for the next SU carrier frequency, prior to secondary transmission.

Yes

No

Yes

Print themessagefor the user

Scan 20windows of5 Mhz each

Try to decodeat theoutlyingfrequency

Record theMessage andTransmissionFrequency

Find all 3-sigmaoutliers

Continue tothe nextoutlier

Start

Anymessagereceived

?

Anyotheroutlierleft?

End ofRx Loop

No

Fig. 3. Flow diagram of the proposed VDSA receiver design.

deviations of the mean PSD are identified, i.e., three-sigmaoutliers. The center frequency for each of the three-sigmaoutlier peaks is located, and the intercepted message is thendemodulated and decoded.Spectrum Sensing and Statistical Analysis: The first step in

establishing a communication link between any two vehicleswithin the proposed VDSA networking architecture is toperform energy detection across the wireless spectrum ofinterest. Energy detection is a simple form of discriminatingbetween noise and signals across a frequency band via theuse of an energy detection threshold [9]. Since the frequencylocations of the VDSA transmissions are not known a priori,and that there might be frequency offsets between any tworadios due to radio frequency (RF) front-end imperfections, itis necessary for the radios lock onto the transmission energy of

the other in the frequency domain. To differentiate between asignal and spurious noise that may appear to be a signal in thefrequency domain, the value of the energy detection thresholdwas selected such that only three-sigma outliers [15] of theenergy detector were identified as potential signals.Center Frequency Detection: Once the three-sigma outliers

have been identified with respect to their frequency locations,the surrounding noise is filtered out and the process of finetuning the center frequency commences. This is accomplishedby integrating the possible signal PSD over signal bandwidth,and then finding the frequency value that partitions the overallsignal energy by half assuming a symmetric PSD. This processis repeated several times using different decimation rates untilthe desired center frequency accuracy is achieved (in this work,the error ≤ 61 Hz) using the expression:

error ≤ 100MSamples/s2× 400decimation× 2048Samples . (1)

Once the center frequency of the potential signal has beendetermined, the receiver attempts to demodulate and decodethe signal at that center frequency using the process defined inFig. 3. Decoded messages and their center frequency valuesare stored for later use, while the messages are printed tothe user. One example of later use of this information is inthe “listen-before-talk” procedure discussed earlier, where theprevious frequency locations of the SUs are used in the processfor determining an appropriate location to transmit.

C. Multi-hop Routing for VDSA NetworkingWhen a vehicle detects some kind of driving or road hazard,

it proceeds with broadcasting a hazard message to other nearbyvehicles in order to alert them to the hazard. For example,in Fig. 1, Car A detects a hazard and immediately alertsall nearby vehicles, in which case Car B detects the hazardmessage and automatically relays it to any vehicles in itsvicinity such as Car C. Although Car A can broadcast itshazard message to all nearby vehicles, its transmit power levelsare constraint due to the fact that it is an SU transmitter and itsinterference level must be kept at a minimum. Consequently,only the nearest vehicle (Car B) will receive the message andreact accordingly.In order to enable the message travel a sufficient distance, a

multi-hop routing network is created for this VDSA test-bed.In this network, each radio receiving a message will in turnsend it to the next one, thus enabling the information to berelayed from vehicle to vehicle. If a vehicle is the initial sourceof the message (e.g., Car A), it generates the message using

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Antenna Mount ofTransmitter and Receiver

Car C Car B Car A

Spectrum Analyzer

Fig. 4. A photograph of the actual VDSA networking experiment conductedon 12 March 2011 at 12:50 PM. Picture taken from car D. The figureshows the VDSA network implemented on the highway I-190 north ofWorcester, MA, USA. As indicated on the figure, the leading car is performingspectrum sensing and the rest of the cars form the proposed VDSA networkingarchitecture.

the transmitter design shown in Fig. 2. On the other hand,if the vehicle is not detecting a road hazard, the networkingdevice defaults to the receiver design shown in Fig. 3 andstarts scanning the wireless spectrum for possible hazardmessages on the road. To prevent flooding of the network,two limiting factors are employed, namely: (i) transmissiontime, and (ii) hop count. Longer transmission times limit theamount of radios able to use the spectrum. In the VDSAnetworking experiments describing in the following section,the transmission time of each radio was arbitrarily limitedto 1 minute, which was determined empirically via actualexperimentation. It is also undesirable for a message to berelayed over very large distances, where the hazard may notpossess any sort of significance or priority for vehicles thatare too far to be affected by the hazard. Thus, to prevent this,we limited the hop count of each message, i.e., the numberof times each message hops or in other words travels betweentwo nodes (or two radios) of the network.

IV. COMPLETE VDSA NETWORK IMPLEMENTATIONA. Experimental SetupThe experiment consists of five cars as shown in Fig. 4. The

leading car was equipped with a spectrum analyzer and each ofthe remaining cars was equipped with a USRP2 SDR platformwith its antennas anchored to the exterior of the vehicle. Theexperiment was carried out from Worcester, MA, USA alongInterstate I-190 north to Leominster, MA, USA, followed byheading east along Route 2 until the intersection with InterstateI-495, where the proposed VDSA network proceeded southuntil the intersection between Interstate I-495 and Interstate I-290, where the vehicles finally headed back west on InterstateI-290 to Worcester, MA, USA.During the experiment, Car A of the VDSA network was

continuously broadcasting hazard messages, while the remain-ing three cars constantly performed spectrum sensing over a100 MHz wide frequency band from 2.4 GHz to 2.5 GHz. Thereceiver would lock on to the frequency and start to decode

Frequency (in GHz)

Primary User VDSA Network

Tim

e (in

sw

eeps

)

(a) Schematic illustrating the spectrum measurements collected on the high-way using the spectrum analyser

Car A

Car B

Car C

Car D

Frequency (in GHz)

Tim

e (in

Sw

eeps

)

(b) Experiment conducted on March 12, 2011 at 12:50 pm. The figure showsthe VDSA network implemented on a highway. As indicated the car in thefront is performing spectrum sensing and the rest of the cars are part of thenetwork.

Fig. 5. Illustration of the VDSA network architecture.

the message as soon as transmission was detected using thethree-sigma rule. If the message was successfully decoded andrecorded by the USRP2, it switched to transmit mode andbroadcasted the message to the rest of the cars. In the eventthat no message was decoded, or the decoded message didnot correspond to the desired network protocol, the USRP2ignored the transmission and started another iteration of spec-trum sensing. Each radio was connected to a laptop that wouldrecord and time stamp every received message for performanceanalysis. The multi-hop wireless access protocol described inSection III-C and used in these experiments is shown in Fig. 6.

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TABLE INUMBER OF RELAYING SCENARIOS ACHIEVED DURING ACTUAL VDSA

NETWORKING ARCHITECTURE EXPERIMENT. NOTE THAT CAR AINITIATED A TOTAL OF 80 HAZARD MESSAGES.

Relaying Scenarios Num. Successful TransmissionsCar A 80

Car A → Car B 58Car A → Car B → Car C 33

Car A → Car B → Car C → Car D 13

B. Analysis and Discussion

Without loss in generality, the frequency locations of virtualPU signals were defined prior to the actual experiment, asindicated in Fig. 5. This resulted in two occupied frequencybands from 2.41 GHz to 2.44 GHz and from 2.46 GHz to2.485 GHz. After performing spectrum sensing, the proposedVDSA networking architecture was able to select a channelthat was relatively far away from the virtual PU signals.Fig. 5(b) shows a close-up of the VDSA multi-hop relayoperation during the experiment. Car A initiated the hazardmessage at 2.449 GHz and then the message was received byCar B. After Car B successfully decoded the message, Car Bstarted relaying the message at 2.452 GHz. Car C was ableto pick up the transmission by the end of transmission forCar B. The message from Car B was successfully decoded,thus, car C started broadcasting the emergency message. It wasobserved that after transmission of Car B stopped, Car D wasable to receive the message from Car C and transmit it at thefrequency that was abandoned by Car B. Table I shows thestatistics of the relaying performance for the proposed VDSAnetworking architecture during this experiment. In particular,we observe that a total of 80 messages were initiated by carA, out of which 58 transmissions were successfully relayed toat least car B, in turn 33 were further relayed to car C, and 13transmissions were relayed through all three cars to car D. Weevaluated the throughput, or efficiency of each link as the ratioof successful relays of the next node to those of the previousone. For instance, the efficiency of link AB was evaluated tobe:

E f f iciency=100%× 58TransmissionsbycarB

80TransmissionsbycarA. (2)

V. CONCLUSION

A prototype VDSA networking architecture using a col-lection of SDR platforms capable of performing DSA andmulti-hop relaying was successfully designed, implemented,and evaluated. Based on several software modules for thetransmitter design, receiver design, and the multi-hop re-laying protocols, we were able to successfully demonstratethe VDSA concept experimentally in actual highway trafficconditions across unlicensed spectrum with virtual PU signalsinserted. This work will enable additional research into thedeployment of VDSA networking architectures in licensedwireless spectrum, such as digital television, where spectrum

occupancy databases will be employed to mitigate potentialspectral collisions with PU signals.

ACKNOWLEDGMENT

The authors would like to thank the generous technicaland financial support from the Toyota InfoTechnology CenterU.S.A. for this research. Moreover, the authors would like toexpress their sincerest gratitude to Mr. Si Chen, Ms. Di Pu, Mr.Jingkai Su, Ms. Zhu Fu, Mr. Sean Rocke, and Mr. John PaulSyriopoulos for their assistance during the VDSA experimentsconducted on 12 March 2011, as well as to Mr. Si Chen forhis feedback during the preparation of this manuscript.

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YesNo

No

No

Yes

Is this carin an

accident?

Is the hopcount

exceeded?

Anymessagereceived?

Start

Yes

Returnto Start

Rx Loopscan 100 MHz

Tx Loopfor 1 minute

Fig. 6. Flow diagram of the multi-hop wireless access routing protocol.

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