992 IEEE TRANSACTIONS ON VEHICULAR TECHNOLOGY, VOL. 46, NO. 4, NOVEMBER 1997

Multihop R-ALOHA for IntervehicleCommunications at Millimeter Waves

Roberto Verdone, Member, IEEE

AbstractWith reference to road transport information (RTI)applications, such as cooperative driving, short-range intervehiclecommunications in a highway environment are investigated inthis paper. The research in this field indicates the suitability ofthe 6064-GHz band.Due to the distributed nature of the intervehicle communication

system, an R-ALOHA protocol is considered; multihop (MH) andsingle-hop (SH) strategies are compared. Network performanceis assessed by considering the joint impact of random access,interference, thermal noise, propagation, and packet captureeffect. Several figures of merit are analyzed and discussed: packetsuccess probability (PSP), system stabilization time (SST), firstsuccess time (FST), and deadline failure probability (DFP). Net-work performance is evaluated either by an analytical approachor by a software tool able to simulate a one-lane highway sce-nario. Both steady-state and transition situations are considered.System performance in terms of PSP (in the presence of two-ray Rice fading, noise, and interference with antenna diversityand selection combining) is analytically evaluated to validate thesimulation tool and to prove the suitability of an MH networkstrategy. The simulation approach allows the evaluation of theimpact of protocol parameters on network performance, withreference to nonsteady-state situations.

Index TermsHighway, intervehicle, millimeter waves, multi-hop.

I. INTRODUCTION

WITH THE purpose of designing a road traffic man-agement system, either in urban or in highway envi-ronments, intervehicle communications will play a relevantrole for road transport information (RTI) applications such ascollision avoidance and cooperative driving, aimed at improv-ing traffic safety and efficiency [1], [2]. Several EuropeanResearch Programs (such as DRIVE and PROMETHEUS)have dedicated efforts to this field in the past years. Sys-tem designers are considering higher and higher frequencyranges having larger bandwidth capacity; more precisely,microwave and millimeter wave systems are investigated inEurope [3][5].In a highway environment, information exchange (about

speed, acceleration, etc.) between vehicles has to be allowedeither in the presence or in the absence of fixed infrastructure:this can be achieved by means of short-range intervehiclecommunication links [6], [7]. In this sense, the use of the

Manuscript received April 7, 1996; revised September 9, 1996. This workwas performed under contract with Progetto Finalizzato Trasporti 2, CNR,Rome, Italy.The author is with CSITE-CNR, University of Bologna, 40136 Bologna,

Italy.Publisher Item Identifier S 0018-9545(97)05116-5.

6064-GHz band is suggested by the presence of a peak inthe oxygen absorption at those frequencies, allowing efficientspatial filtering effects. In Europe, the 63.5-GHz band has beenrecommended [8].In this paper, a short-range communication system at 63.5

GHz for highway environments and cooperative driving appli-cations, is considered. Due to the need for distributed networkmanagement, a suitable multiple-access protocol has to beproposed. Moreover, since propagation impairments can causehigh levels of unavailability of the service, proper transmissiontechniques have to be exploited [9], [10].In the literature, some recent papers have been dedicated

to intervehicle communication systems at millimeter waves[9][14], but usually investigating transmission or networkaspects separately.The aim of this paper is to evaluate network performance

by taking propagation impairments, transmission techniques,random access and network structure into account: the effectsof fading and cochannel interference are also considered.In [11], narrowband and spread-spectrum modulation sys-

tems were compared in the framework of an S-ALOHAprotocol, and the former showed superior performance in thegiven scenario. In [12] and [13], an S-ALOHA network ina real environment was investigated. However, for coopera-tive driving applications, vehicles have to send their statusinformation to the neighborhood mobiles periodically; hence,a protocol allowing a form of channel reservation shouldbe considered. An R-ALOHA network was investigated in[15], but an ideal channel was assumed for performanceevaluation. R-ALOHA and its modifications were proposedfor intervehicle communications also in [6] and [7]This paper starts from the conclusions drawn in [15], and

its first aim is to show the impact of the propagation channelon network performance when R-ALOHA is used; a one-lanescenario, where vehicles are separated by a fixed distance,is considered. Frame synchronization is assumed, which canbe obtained either by means of roadside infrastructures sup-porting the intervehicle system, or by exploiting a distributedalgorithm [16].The number of mobile terminals that should receive the

information referred to a given vehicle [defining the so-calledzone of relevance (ZOR)] is a parameter of relevance, from thepoint of view of both the application and the communicationprotocol [17]; moreover, the information could be transmittedeither by broadcasting to all the vehicles in the ZOR, orby means of multihop (MH) links, where mobile terminalsretransmit received packets. In this paper, MH and single-hop

00189545/97$10.00 1997 IEEE

VERDONE: MULTIHOP R-ALOHA FOR INTERVEHICLE COMMUNICATIONS 993

(SH) R-ALOHA systems are compared, in order to underlinethe suitability of the MH strategy in the given context.In the paper, several figures of merit are analyzed: packet

success probability (PSP), system stabilization time (SST), firstsuccess time (FST), deadline failure probability (DFP). Theperformance is assessed as a joint effect of fading, noise andinterference: suitable channel characterization at 63.5 GHz isgiven by means of a two-ray Rice model taking the effectof the road-reflected wave into account. With the purpose ofevaluating network performance, two means are exploited.1) An analytical procedure [14], [18][20] allowing the

evaluation of PSP in the joint presence of thermalnoise, cochannel interference, Rician fading, and antennadiversity with selection combining.

2) A simulation tool able to model the one-lane highwayenvironment under investigation. It allows the simulationof nonsteady-state situations.

Both the analytical and the simulation approach take intoaccount channel impairments due to fading, antenna diversity,the mo-demodulation techniques employed and the strategy ofreceived power measurement; the packet capture effect, in theframework of the R-ALOHA protocol, is also considered.As a first step in the paper, PSP is evaluated by means

of the analytical procedure. Its determination allows propertesting of the simulation tool results; moreover, starting fromthe analytical approach, a suitable comparison between theperformance of the SH and MH strategy can be carriedout. Then, with the aim of stressing the impact of protocolparameters on network performance in the real environmentconsidered, nonsteady-state situations are analyzed through theuse of the simulation tool.In conclusion, the combined approach enables proper inves-

tigation of system sensitivity, in a real environment, to severalparameters such as the number of vehicles in the network,the number of slots per frame, bit rate, intervehicle distance,the SH or MH strategies, power requirements, etc. The impactof channel propagation, movement of vehicles and antennadiversity on network performance is evaluated.The paper is organized as follows. In Section II, the com-

munications scenario is introduced. In Section III, the maincharacteristics of the intervehicle link at 63.5 GHz are sum-marized. Section IV introduces the analytical method for linkperformance evaluation proposed in [14]. Then, the assump-tions regarding the network model are described (Section V).The performance comparison between an SH and an MHapproach is shown in Section VI. Section VII presents thesimulation tool for analyzing network behavior in nonsteady-state situations. Finally, numerical results are given.

II. THE COMMUNICATIONS SCENARIOA one-lane highway scenario is investigated: a group ofmobiles separated by an intervehicle distance is

considered, as shown in Fig. 1. From a communications pointof view, mobile users are distinguishable by means of theiridentification number (ID).A parameter of relevance in network design is the number

of terminals, which have to receive the information about a

Fig. 1. One-lane highway scenario.

Fig. 2. SH and MH strategies.

given vehicle, in its neighborhood. It defines the ZOR [17]. Inthis paper, it is assumed that the status of a given mobile has tobe known by the closest terminals: of these behindand ahead. Different values of are considered.An R-ALOHA protocol is investigated: the time resource is

subdivided into frames of duration seconds, each of themconsisting of slots. Each vehicle exploits the radio channelto transmit packets containing its status to the closestmobiles in its ZOR, according to (see Fig. 2) the following.1) An SH strategy: the vehicle broadcasts the packets to

the mobiles directly2) An MH strategy: the vehicle transmits the packets to

the two closest terminals ; these retransmit theinformation in the following frame (together with theirown data), playing as repeaters.

For cooperative driving applications, a channel bit rate inthe order of 10 Kb/s is sufficient to allow the necessaryinformation to be exchanged [17]. However, data have to beexchanged cyclically, in order to permit proper control oftraffic flow, with a period of about 100 ms [17]. Thus, theframe duration has to be in the order of 100 ms, whereas slotduration, will depend on In the paper, as a referencecase, the overall bit rate is fixed at Mb/s.The protocol has distributed management, and, as a conse-

quence, channel (slot) reuse is obtained, at distancewhere is an integer (random) value.

III. THE INTERVEHICLE LINK AT MILLIMETER WAVESDue to the choice of a contention protocol and to the

implicit reuse strategy introduced by the network, each packetis received in an interfered environment, where one vehicleplays the role of useful transmitter and are interferers. Ingeneral, only the two closest interfering terminals need to beconsidered.

A. Vehicle EquipmentEach vehicle is equipped with a communication device and

four directional antennas (see Fig. 3): two of them radiatingin the backward direction and two in the forward.Each couple of antennas represents a two-branch diversity

system, where the two elements are vertically separated by adistance The two couples are separated by a distance

In [9], the efficiency of height in comparison withhorizontal diversity was shown. Let us consider the link

994 IEEE TRANSACTIONS ON VEHICULAR TECHNOLOGY, VOL. 46, NO. 4, NOVEMBER 1997

Fig. 3. Vehicle equipment.

Fig. 4. Two-ray Rice model.

between a transmitting and a receiving mobile: antenna heightswith respect to flat terrain are assumed to fluctuate dueto ground unevenness around mean values and

for the transmitting vehicle, andfor the receiver. The instantaneous values of

heights will be denoted by and respectively,or more generally by and Fluctuations are modeled bya sinusoidal law with frequency 1 Hz and peak deviationThe transmitted power is denoted by transmitter and

receiver antenna gains by and , respectively. Thereceiver is characterized by means of its noise figure

B. Propagation ModelAt 63.5 GHz, the specific attenuation due to oxygen absorp-

tion is roughly equal to dB/Km. Moreover, rainfallcan introduce large values of the specific attenuation due torain, Let us denote by the overallspecific attenuation.In the given scenario, a flat fading channel can be assumed

[4], [22]; constant power at each antenna is received during aslot time. On the other hand, it is assumed that in consecutiveslots power samples are independent [12]. In the following,indicates the useful received power. Let us also denote bythe th interfering received power.In [9], the channel model for intervehicle links in the

6064-GHz band in highway scenarios, was described; a two-ray Rice model, based on the coherent cumulation of the directand road-reflected (with reflection coefficient paths, andthe incoherent addition of a multipath power component, waspresented. For the sake of completeness, the relations holdingin the two-ray Rice model are shown [9].With reference to a generic link (either useful or interfering,

see Fig. 4), let us denote by the distance between thetransmitting and receiving mobiles (referred to, e.g., forwardantennas); the distance between antennas is

(1)

where antenna number 1 is assumed to be the transmitting one,whereas can denote either or In the given scenario,

, where is an integer. A reference received powercan be introduced as the free-space received power at

distance , which depends on link budget parameters andwavelength [9].As far as the direct path is concerned, let us denote by

and the useful received power and link range, respectively;similarly, and are the th interferer direct path receivedpower and link range, respectively Thedistances between antennas are denoted by and Wehave [21]:

(2)

(3)

(4)

The total received average power for the generic link isgiven by the sum

(5)

where is the power due to the direct and road-reflectedwaves (when present), the average multipath power. De-pending on the presence of line-of-sight (LOS) conditions, wemay have two different situations, described in the following.1) LOS link: in the presence of direct and road-reflected

paths, the Rice factor describing the statistic of thereceived power is given by

(6)

where is the road-reflection coefficient, thereflection-free (direct) path power (corresponding to

VERDONE: MULTIHOP R-ALOHA FOR INTERVEHICLE COMMUNICATIONS 995

and for the useful and th interfering signals,respectively), and

(7)

is the phase shift between the direct and the road-reflected waves. Under these conditions, the ratio

(8)

and are the two degrees of freedom of the model.2) NonLOS (NLOS) link: when the direct and road-

reflected paths are obstructed, the received signalstatistic is Rayleigh distributed; hence

(9)

whereas the average multipath (overall) power canbe determined by choosing a proper value for the pa-rameter

(10)

where represents the received power in case of freespace propagation (corresponding to or Underthese conditions, is the only degree of freedom of themodel.

In both cases, the probability density function of the short-term useful power can be generally written as

(11)

where is the average multipath received power [generallydenoted by in (5)] from the useful user given by

(12)

where the Rice factor, which, from (6) and (8), is equal to(13)

and represents the modified zero-order Bessel function ofthe first kind.Similar expressions hold for the th interfering signal by

replacing and with andIt is worth noting that the useful and interfering signals

can generally come from a LOS or a NLOS link, dependingon the receiver and transmitter relative positions in the lane;since in this paper we assume a fixed (when is chosen)monodimensional scenario, the following considerations canbe made.1) When the signal comes from one of the two closest

vehicles, we have a LOS link, characterized by a givenRice factor and (two-ray Rice fading) [9].

2) When the signal comes from vehicles at a larger dis-tance, we have a NLOS link because of the obstructiondue to the closest vehicles, so and canbe chosen (Rayleigh fading).

TABLE ISYSTEM AND LINK BUDGET PARAMETERS

EXPLOITED IN THE PAPER

Some analytical evaluations based on the experimentalresults of [4] suggest the values dB andfor LOS conditions and dB for NLOS.As far as the system and link budget parameters (trans-

mitted power, antenna gains, etc.) are concerned, the valuesreported in Table I, based on existing RF devices, are exploitedthroughout the paper.

IV. LINK PERFORMANCELink performance can be defined in terms of PSP and outage

probability.

A. Capture ModelA threshold capture model is assumed in this paper: having

fixed the desired link quality in terms of bit-error probability,since received power in a packet time is assumed to be

constant, the PSP can be described as

(14)where

conventional signal-to-noise ratio;minimum value of required to obtainthe desired link quality in the absence ofinterference;total interference short-term received power;signal-to-interference protection ratio relatedto

The signal-to-noise threshold and the protection ratiodepend on the transmission system and the multiple-accessmethod considered, the countermeasures employed, and thequality desired. The conventional signal-to-noise ratio at thereceiver can be defined as

(15)

Noise power is evaluated over a bandwidth equal to the bitrate , starting from the two-sided power spectral densityof thermal noise , which depends on the receiver noisefigure Let us also define the threshold useful power

In [20], it was shown that for a minimum shift keying(MSK) system with limiter-discriminator (LD) detection, the

996 IEEE TRANSACTIONS ON VEHICULAR TECHNOLOGY, VOL. 46, NO. 4, NOVEMBER 1997

Fig. 5. Outage probability as a function of interferer position: (a) without and (b) with height diversity.

values dB and dB correspond to a bit-error probability (when two interferers are present)Here, we assume that this quality level is sufficient (in thepresence of a suitable coding scheme) to capture, with highprobability, the packet [23]. So, in the numerical results,as a reference case, dB and dB arefixed.In [14] and [19], an analytical procedure to derive outage

probability defined as

(16)has been proposed. Since from (14) and (16) we have

(17)the same analytical procedure of [14] and [19] can be exploitedto derive PSP. In Appendix A, the approach proposed in[14] and [19] is summarized; it takes thermal noise, Ricefading, and interference into account in the presence of antennadiversity with selection combining: the antenna having thehighest level of measured total short-term received poweris connected to the receiver [24]. The analytical method isapproximated: the validity of the approximation was checkedin [14], [18], and [19] by comparison with Monte Carlosimulation results and other approaches.

B. Numerical EvaluationsIn order to stress the importance of the combined effects

of the propagation channel and the multiple-access protocol,the transmission of a packet from the closest vehicle, instatic conditions (that is, neglecting antenna fluctuations), isconsidered.By fixing the intervehicle distance m, in Fig. 5

outage probability is shown as a function of the position in

the lane of the closest interferer with respect to the receiverThe relative position of the useful transmitter is equal toone (that is, a LOS useful link is considered). The performanceis evaluated either in the presence or in the absence ofantenna diversity. The propagation channel model describedin Section III is used: the values of system parameters arethose described in Table I. Fig. 5 shows that is almostunitary when (since the useful and the interferinglinks have the same range), that is, the PSP is about zero;however, because of the obstruction of the direct and road-reflected waves for interfering signals coming from distantvehicles, is very small (that is, PSP is almost unitary)whenNow, in order to emphasize the role played by the direct

path in the link performance, results are shown in the absenceof interference as an ideal case to which the network tends.Fig. 6 shows as a function of , with diversity; the choiceof the link budget parameters allows large value of PSP upto several hundred of meters. In Fig. 7, is displayed as afunction of the position in the lane of the transmitter withrespect to the receiver with diversity. Several values of

are exploited.

V. THE NETWORK MODELA brief description of the R-ALOHA protocol considered

is given in this section.At first, a vehicle contends for the radio channel in a

frame by transmitting in a randomly chosen slot a packetin both directions (backward and forward) and waiting foracknowledgments (acks) from the closest terminalsof these behind and ahead). This is the contention phase.Upon reception of all the acknowledgments during the frame,the vehicle will use the same slot in the following frames; this

VERDONE: MULTIHOP R-ALOHA FOR INTERVEHICLE COMMUNICATIONS 997

Fig. 6. Outage probability as a function of intervehicle distance for a LOS link in the absence of interference.

Fig. 7. Outage probability as a function of useful transmitter position in the absence of interference.

will be the contention-free phase. If an ack is missing, a slotwill be randomly chosen in the next frame and the contentionphase will continue.The choice of the slot to be used for transmission in a frame

during the contention phase is made on the basis of a tableindicating which slots are busy and which are free. This tableis achieved by means of total received power measurements,and consequent comparison with a given threshold carriedout by each vehicle.

In the contention-free phase, a packet can be lost due tofading; if this happens, the same slot will be exploited in thefollowing frame until a chosen time-out expires.The following simplifying assumptions are made throughout

the paper.1) Packets: they have the same duration of one slot.2) Framing: perfect frame synchronization is obtained by

means of a global time reference.

998 IEEE TRANSACTIONS ON VEHICULAR TECHNOLOGY, VOL. 46, NO. 4, NOVEMBER 1997

3) Acknowledgments: if a terminal receives a packet cor-rectly, its ack has a probability of success equal to one;acks can be transmitted by means of a time division du-plexing (TDD) technique and a dedicated frame section.It is not the intention of this paper to define how acksshould be sent to transmitting vehicles.

4) Power measurements: they are carried out during non-transmitting slots; hence, the slot just exploited can bereused in the next frame. The power measured is the sumof the short-term received powers at the two antennas.

5) Mobile identity: each vehicle has knowledge of theID of the closest terminals ahead and thebehind; this can be obtained by means, e.g., of a pe-riodical procedure in which, at regular intervals, eachmobile transmits in a round-robin fashion a short packetinforming the closest terminals of its location [25].Thus, vehicles know how many acks are expected tobe received.

6) Destination addressing: each transmitted packet containsthe ID of vehicles which have to receive it; a mobilediscards any packet which is not intended for it orit retransmits the information in the following frame(together with its own data) if the MH strategy is usedand the packet is to be received by other vehicles in itsneighborhood [25].

VI. SINGLE-HOP AND MULTIHOP STRATEGIESThe aim of this section is the comparison between the

performance of an SH and an MH strategyin a R-ALOHA framework and a monodimensional scenario.Let us assume a steady-state situation, where each vehicle

in the lane is in the contention-free phase and the distributionof the time resource is optimal: vehicle 1 transmits during slot1, vehicle 2 during slot 2, , vehicle during slot ,vehicle during slot 1, and so on. That is, the sameslot is reused at distance Let us also consider, asan example, a value of large enough to have noise-limitedlinks (Fig. 5 shows that is sufficient for this purpose,i.e., has to be chosen).The choice of mainly affects the ability of the network

to react to situations of danger (e.g., a warning from a vehicleahead due to an accident) and the PSP.Let us define, as a quality measure for the comparison, the

steady-state DFP (SS-DFP) defined as- (18)

where is the packet delivery delay and is a giveninteger number. The SS-DFP obviously depends on PSP,which, as shown in Fig. 7, is highly affected by the relativepositions of active terminals in the lane; for this reason,in the following PSP(m) will denote the PSP at distance

At first, let us evaluate PSP(m) for SH and MH strategy.1) SH: for the sake of simplicity, let us consider only

the transmission to the farthest vehicle in the ZORwe have

(19)

where the dependence of on the relative positions oftransmitter and receiver has been emphasized.

2) MH: let us assume that a packet to be repeated is retrans-mitted only in the subsequent frame; by concentratingon the farthest vehicle in the ZOR, we can calculate the

as a function of the PSP(1) of each hop(20)

Now, the SS-DFP, whose dependence on and will beexplained in the following, can be evaluated as

-

(21)

SH: it can be easily found that

(22)hence

-

(23) MH: if is smaller that we have SS-DFP since at least frames areneeded for hops; on the other hand, when

(24)

hence

-

(25)

The strategy having the smallest value of SS-DFPis the most appropriate since it determines the largest valuesof probability of having a fast transmission of informationthrough the lane. As an example, Figs. 8 and 9 show SS-DFP as a function of for intervehicle distance equal to 200m, in the interference-free case considered in Fig. 7, by fixingdifferent values of From the figures, it can be noted that forvalues of slightly larger or equal to an MH strategyoffers better performance than SH, in terms of SS-DFP.Let us note that the delay introduced by the MH scheme can

be larger, in mean; however, if frame length and bit rate aresuitably chosen, the delay remains within acceptable bounds.For this reason, in the following an MH strategy will be

considered. On the other hand, it is worth noting that theimpact of the strategy on network performance has to beconsidered also in nonsteady-state conditions. This will bedone in the next sections.

VERDONE: MULTIHOP R-ALOHA FOR INTERVEHICLE COMMUNICATIONS 999

Fig. 8. Steady-state deadline failure probability as a function of N for the SH and MH strategy; div

= 200 m and Nz

= 4 and 6.

Fig. 9. Steady-state deadline failure probability as a function of N for the SH and MH strategy; div

= 200 m and Nz

= 8 and 10.

VII. THE SIMULATION TOOLA software simulation tool has been developed to analyze an

R-ALOHA network in a one-lane highway scenario; the aim ofthe computer program is the evaluation of the impact of proto-col parameters (such as, e.g., the number of slots per frame) onsystem performance. The multiple-access protocol, the channelpropagation and packet capture model implemented are thosedescribed in previous sections.

Fig. 10. Test scenario for validation purposes.

For the purpose of testing and validating the computerprogram, with particular emphasis on the channel propagation

1000 IEEE TRANSACTIONS ON VEHICULAR TECHNOLOGY, VOL. 46, NO. 4, NOVEMBER 1997

Fig. 11. PSP as a function of useful distance when interference is at di

= 200 m; comparison between the analytical approach (line) and thesimulation results (circles).

Fig. 12. Probability distribution of SST with reference to number of frames; the impact of Nr

;N

s

= 16; N

v

= 9; and div

= 200 m.

and packet capture modules, the following situation has beeninvestigated, by means of either the analytical approach pre-sented in Section IV, or simulation: PSP is found as a functionof useful distance for a LOS link between a transmitting,T, and a receiving, R, mobile, when an interfering vehicle, I,is transmitting from a fixed distance m on an NLOSlink (see Fig. 10). Fig. 11 shows the comparison obtained

by means of the two approaches for some values of usefuldistance. The results validate the computer program.The simulation tool enables the evaluation of the probability

distribution of some figures of merit defined in the following.Although it can be exploited to investigate either steady-stateor nonsteady-state conditions, in this paper the latter case isanalyzed since the intervehicle communication network must

VERDONE: MULTIHOP R-ALOHA FOR INTERVEHICLE COMMUNICATIONS 1001

be able to react quickly to unpredictable situations. Thus, aworst case situation is considered, as in [15]: let us assume thatall vehicles in the lane simultaneously compete for a channelstarting from a memoryless state, that is, no information onfree and collision slots is available.The probability distributions of SST and FST are evaluated

as a function of the number of slots per frame the numberof vehicles in the lane intervehicle distance and thenumber of mobiles which have to receive a packet1) SST is defined as the number of frames that elapse until

all the vehicles in the lane have successfully lockedonto a slot (by transmitting a packet and receiving theexpected number of acknowledgments: let us call thisthe success event) [15]; it depends on

2) FST is defined as the number of frames that elapseuntil a given mobile succeeds in transmitting its packet(success event) [15]. It is not a function of as longas is sufficiently large.

Their probability distributions are estimated by counting thenumber of times the observed success event happens forthe first time at the th frame from the start of simulation:the probability is given by the ratio between

and the total number of iterations,can be determined in a similar way.From the above-mentioned figures of merit, the nonsteady-

state DFP (NSS-DFP) can be defined as- (26)

where is the maximum delay required in the successfultransmission of one packet.It is worth noting that SST, FST, and NSS-DFP are defined

with reference to the number of frames, as in [15]. However,when different network configurations are investigated byvarying the number of slots per frame, nonhomogeneousparameters are obtained. So, in the next section when isconsidered as a variable parameter, SST and FST are describedwith reference to the number of slots instead of number offrames

VIII. NUMERICAL RESULTSThe simulation campaign was based on iterations

for each set of input parameters. In this section, the resultsobtained are presented.Some of the system parameters have been fixed: they

are summarized in Table I. It is worth recalling that thepropagation model is chosen in accordance with Section III.Height diversity with selection combining, fluctuations ofantennas due to ground unevenness, and two-ray Rice fading(in LOS links) are considered.Power thresholds have been fixed, and their values are re-

ported in Table II; the value of has not been optimized, sofurther investigation could lead to slightly better performanceresults (see [26]).

A. The Impact ofFig. 12 shows the probability distribution of SST (in

terms of the number of frames) when varying for

TABLE IIVALUES OF POWER THRESHOLDSEXPLOITED IN THE SIMULATION

TABLE IIILIST OF ACRONYMS EXPLOITED

THROUGHOUT THE PAPER

and m. The impact ofon the reaction time of the network is evident. It is clearthat by choosing an MH network , a much fasterallotment of channels is obtained.Hence, in the following will be fixed.

B. The Impact of Channel PropagationFig. 13 shows the probability distribution of SST when

fixed and the value ofthe intervehicle distance varies from 20 to 400 m. InFig. 14, the same performance results are shown in the ab-sence of antenna diversity. In both cases, the curves inthe figures are almost indistinguishable; this underlines thefact that up to several hundred of meters, network behavioris determined by protocol optimization rather than by linkperformance.

C. The Impact ofIn Fig. 15, the probability distribution of FST in terms of

the number of frames is displayed; andm are fixed. The minimum value of the number neededto approximate an infinitely long lane of vehicles has beenchosen as a function of the number of slots per frame.varies from 6 up to 12. From the figure, it seems that byincreasing , smaller and smaller values of the mean FSTcan be obtained. However, as already noted, when varying

, the number of frames is not a homogeneous measureof time. So, Fig. 16 shows the same results with reference tonumber of slots. Finally, in Fig. 17, the NSS-DFP is depictedas a function of delay expressed in time slots. This lastfigure points out that when is larger than ten, no increasein the performance is achieved.

IX. CONCLUSIONSMH and SH R-ALOHA networks for intervehicle communi-

cations in a highway environment at 63.5 GHz, with referenceto cooperative driving applications, have been studied. Severalconclusions can be carried out. Table III reports the list ofacronyms exploited throughout the paper.

1002 IEEE TRANSACTIONS ON VEHICULAR TECHNOLOGY, VOL. 46, NO. 4, NOVEMBER 1997

Fig. 13. Probability distribution of SST with reference to number of frames; the impact of div

;N

s

= 16; N

v

= 9; and Nr

= 2:

Fig. 14. Probability distribution of SST with reference to number of frames; the impact of div

when antenna diversity is neglected; Ns

= 16;N

v

= 9;

and Nr

= 2:

1) R-ALOHA protocols can be exploited for the applicationconsidered since they provide good reaction times todangerous situations and offer an efficient means for thepurpose of allowing periodical data exchange betweenvehicles.

2) MH strategy performs better than SH in the givenenvironment because of the monodimensional structureof the scenario.

3) A link budget has been found, based on existing RFdevices, giving good link performance up to severalhundred of meters.

4) The impact of channel characteristics, given that onlyLOS links between adjacent vehicles in the lane areexploited, is negligible.

5) The number of slots per frame can be chosen accordingto the performance analysis carried out in Fig. 17, where

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Fig. 15. Probability distribution of FST with reference to number of frames; the impact of Ns

;N

r

= 2 and div

= 200 m.

Fig. 16. Probability distribution of FST with reference to number of slots; the impact of Ns

;N

r

= 2 and div

= 200 m.

it has been shown that a number of slots per framelarger than ten does not improve system performance.It is worth noting that different values of couldlead to different numerical conclusions; moreover, theimpact of on system performance also depends onthe intervehicle distance. Nevertheless, from the analysisshown, the presence of an optimum value of the numberof slots per frame is underlined.

On the other hand, some of the conclusions could depend onthe given one-lane scenario; the effect of the same parameterssuch as, e.g., on system performance could be different ina multilane scenario. This will be investigated in future work.

APPENDIX ATHE ANALYTICAL PROCEDURE TO DERIVE PSP

This methodology is based on considering the probabilitydensity function of total interfering short-term received

1004 IEEE TRANSACTIONS ON VEHICULAR TECHNOLOGY, VOL. 46, NO. 4, NOVEMBER 1997

Fig. 17. NSS-DFP with reference to number of slots; the impact of Ns

;N

r

= 2 and div

= 200 m.

power to be Gaussian, with mean

(27)

and variance

(28)

From this assumption, an expression for is determined.In the absence of antenna diversity, we have [19]

(29)

Let us now consider antenna diversity and denote by thetotal received short-term power

(30)Moreover, in order to distinguish signals at each antenna,

and denote the useful total interference andtotal short-term power at the th antenna placed at height

respectively Signals are assumed to beindependently faded at each antenna.By defining at the th antenna, an

outage condition occurs if the received useful power isless than the threshold or the ratio between useful andtotal interference received power is less than Letus denote by a Boolean variable, which indicateswhether an outage condition occurs at the th antenna.

Due to the combining method chosen, at the combineroutput outage can occur in three different cases.i) .ii) , but (antenna 1

is selected).iii) , but (antenna 2

is selected).The probability of outage can be evaluated as the sum of

the probabilities of the three events i), ii), and iii)(31)

The two latter components of the sum would be zero ifinterference did not cause errors in choosing the antenna withthe largest useful power. The three terms of (31) are nowreported [14]i) is given by [24]:

(32)where is the outage probability at the th antenna and isdetermined by means of (29) by properly choosing the meanpower values related to the th antenna.ii) Prob(ii) can be derived by means of a simplified ap-

proach: with the aim of deriving Prob(ii), we neglect thepresence of noise; moreover, as far as interference is consid-ered, a Rayleigh probability density function is assumed [14].So, we have

(33)

VERDONE: MULTIHOP R-ALOHA FOR INTERVEHICLE COMMUNICATIONS 1005

where and denote the average value of total inter-ference at antennas 1 and 2, respectively.iii) Prob(iii) can be evaluated in a similar way as Prob(ii),

leading to

(34)It is worthwhile recalling that the approximated method has

been checked in different conditions, and its validity has beenproven in [14], [18], and [19]; when interference is due to alow number of terminals and NLOS links (as in the scenarioconsidered in this paper), the accuracy of the model is shownto be very high.

REFERENCES

[1] Special issue on intelligent vehicle highway systems, IEEE Trans.Veh. Technol., vol. 40, no. 1, 1991.

[2] Final demonstration of PROMETHEUS project, Paris, France, Oct.1994.

[3] A. Plattner, 60 GHz mobile communications, propagation and tech-nology requirements, in 23rd European Microw. Conf. (EuMC93),Workshop on Mobile Communications, Madrid, Spain, Sept. 1993, pp.118120.

[4] W. Schafer, Channel modeling of short range radio links at 60 GHz formobile intervehicle communication, in Proc. IEEE Veh. Technol. Conf.VTC91, St. Louis, MO, May 1991, pp. 314319.

[5] J. Kaltwasser and J. Kassubek, A new cooperative optimized channelaccess for inter-vehicle communication, in Veh. Nav. Inform. Syst. Conf.VNIS94, Yokohama, Japan, Aug. 31Sept. 2 1994, pp. 145148.

[6] T. Hatakeyama and S. Takaba, A network architecture of the inter-vehicle packet communication system, in Veh. Nav. Inform. Syst. Conf.VNIS94, Yokohama, Japan, Aug. 31Sept. 2 1994, pp. 159164.

[7] Y. Inoue and M. Nakagawa, MAC protocol for inter-vehicle com-munication network using spread spectrum technique, in Veh. Nav.Inform. Syst. Conf. VNIS94, Yokohama, Japan, Aug. 31Sept. 2 1994,pp. 149152.

[8] CEPT T/R 22-04, Harmonization of frequency bands for road transportinformation systems (RTI).

[9] O. Andrisano, M. Chiani, V. Tralli, and R. Verdone, Impact of cochan-nel interference on vehicle to vehicle communications at millimiterwaves, in Proc. IEEE Int. Conf. Commun. Syst. (ICCS92), Singapore,Nov. 1992, pp. 924928.

[10] O. Andrisano, M. Chiani, M. Frullone, C. Moss, and V. Tralli, Mil-limeter wave short range communications for advanced transport telem-atics, in ETT European Trans. Telecommun. Conf., JulyAug. 1993,pp. 403414.

[11] O. Andrisano, D. Dardari, and R. Verdone, Code division and timedivision multiple access networks for vehicle-to-vehicle communicationsat 60 GHz, in Proc. IEEE Veh. Technol. Conf. VTC94, Stockholm,Sweden, June 710, 1994, pp. 18591863.

[12] W. Kremer, D. Hubner, S. Hoff, T. P. Benz, and W. Shafer, Computer-aided design and evaluation of mobile radio local area networks in

RTI/IVHS environments, IEEE J. Select. Areas Commun., vol. 11, no.3, pp. 406421, 1993.

[13] R. Hager, R. Mathar, and J. Mattfeldt, Intelligent cruise control andreliable communication of mobile stations, IEEE Trans. Veh. Technol.,vol. 44, no. 3, pp, 443448, 1995.

[14] R. Verdone, Outage probability analysis for short range communica-tion systems at 60 GHz in ATT urban environments, this issue, pp.10271039.

[15] T. Liu, J. A. Sylvester, and A. Polydoros, Performance evaluation ofR-ALOHA in distributed packet radio networks with hard real-timecommunications, in Proc. IEEE Veh. Technol. Conf. VTC95, Chicago,IL, July 2528, 1995, pp. 554558.

[16] W. Zhu and B. Walke, A high precision TDMA frame/slot synchroniza-tion protocol based on a radio clock signalnew results, in PROCOMConf., Stuttgart, Germany, 1991.

[17] W. Kremer, Vehicle density and communication load estimation inmobile radio local area networks (MR-LANs), in Proc. IEEE Veh.Technol. Conf. VTC92, Denver, CO, May 1992, pp. 698704.

[18] M. Chiani, V. Tralli, and R. Verdone, Outage and spectrum efficiencyanalysis in microcellular systems, in Proc. IEEE Veh. Technol. Conf.VTC93, Secaucus, NJ, May 1993, pp. 598601.

[19] G. Mazzini and R. Verdone, Semi-analytical evaluation of outage prob-ability for pico and microcellular mobile radio systems with correlatedshadowing, Electron. Lett., vol. 29, no. 13, pp. 12141215, June 1993.

[20] R. Verdone, Sullefficienza spettrale nelle reti microcellulari e picocel-lulari, Ph.D. dissertation, Univ. Bologna, Bologna, Italy, Mar. 1995.

[21] O. Andrisano, M. Chiani, M. Frullone, V. Tralli, and C. Moss, Propaga-tion effects and countermeasures analysis in vehiclevehicle communi-cation at millimeter waves, in Proc. IEEE Veh. Technol. Conf. VTC92,Denver, CO, May 1992, pp. 312316.

[22] R. Verdone, Time and frequency selectivity effects in vehicle-to-vehiclecommunications at 60 GHz, in Proc. IEEE Veh. Technol. Conf. VTC94,Stockholm, Sweden, June 710, 1994, pp. 17801784.

[23] D. Dardari, A general approach to the evaluation and characterizationof packet radio networks performance, to be published.

[24] W. C. Jakes, Microwave Mobile Communications. New York: Wiley,1974.

[25] V. Wong and C. Leung, Effect of Rayleigh fading in a multihop mobilepacket radio network with capture, IEEE Trans. Veh. Technol., vol. 44,no. 3, pp. 630637, 1995.

[26] R. Verdone, Performance evaluation of R-ALOHA for inter-vehiclecommunications at millimeter waves, in Proc. IEEE Personal, In-door, and Mobile Radio Communications Conf. 1996 (PIMRC96), pp.658662.

Roberto Verdone (M95) was born in Bologna, Italy, on August 6, 1965.He received the Dr. Ing. degree in electronic engineering (with honors)and the Ph.D. degree in electronic engineering and computer science fromthe University of Bologna, Bologna, in March 1991 and October 1995,respectively.Since 1994, he has been a Lecturer in Telecommunications at the University

of Bologna. Since 1996, he has been a Researcher at CSITE-CNR (ResearchCenter for Informatics and Telecommunication Systems of the NationalResearch Council). His research activity is concerned with digital modulation,cellular and mobile systems, multiple access, and spectrum efficiency. Part ofhis work is dedicated to intervehicle and vehicle-to-infrastructure communica-tions at millimeter waves. He was involved in the European research programPROMETHESUS and the national research program TELCO.