Millimetre Wave Short Range Communications for Advanced Transport Telematics

I5

Telecommunication Systems

Millimetre Wave Short Range Communications for Advanced Transport Telematics (*)

Oreste Andrisano, Marco Chiani, Velio Tralli DEIS, Universith di Bologna, Viale Risorgimento 2, 40136 Bologna - Italy Mario Frullone Fondazione U. Bordoni, 40044 Pontecchio Marconi (Bologna) - Italy Colin R. Moss Marconi Radar and Control Systems, Leicester, England.

Abstract. This paper presents the results of a research programme to investigate short range millimetre wave communications links for road transport telematics applications. The work was carried out by the University of Bologna, the Fondazione U. Bordoni and Marconi Radar and Control Systems Limited. Financial support was provided by PROMETHEUS programme, which is itself a EUREKA research project and the ECC DRIVE programme.

The paper provides a theoretical assessment of the effects of multipath as applied to millimetre wave vehicle-to-vehicle and vehicle-to-roadside communications links. A digital link, based on phase shift keying, is assumed. Performance improvements resulting from the use of both antenna diversity and coding are assessed.

1 .INTRODUCTION

In recent years a number of European organisations have undertaken research programmes to define the design criteria for a telecommunications network orienta- ted towards road traffic applications. Such networks are aimed at improving both traffic flow and safety. Two indirect consequences will be an improvement in the economics of road transport and a reduction in the amounts of pollution produced by road vehicles.

Both the PROMETHEUS (EUREKA) and DRIVE (ECC) research programmes have placed special emphasis on short range communications systems which permit both intervehicular and vehicle-to-roadside communications [ I - 61. The latter operate in conjunction with an appropriate roadside communication infrastructure. This will include long range communication systems which will be available in the com- ing years at European level. The systems, which are often called Advanced Transport Telematics ( A m ) systems, will form part of an Integrated Road Transport Environment (IRTE).

Since the beginning of the IRTE research activity the

(*) Work performed within PROMETHEUS and DRIVE propram- mes. PROMETHEUS is funded in Italy by CNR (Progetto Finallirzn- to Trasporti 2)

authors have emphasized the necessity for efficient use of the spectrum when implementing new communication systems for ATT: in this regard the frequency spectrum usage for fixed or mobile ATT applications is limited to frequencies higher than 5 GHz.

There are a number of existing transport telematics links in operation which use frequencies in the region of 5 GHz. These are single application links which provide vehicle-to-roadside communication for toll collection purposes. Multi-application vehicle-to-vehicle and vehicle-to-roadside links could require great bandwidth and radio channels availability. As a consequence, there is now a wide body of support for links to operate at much higher frequencies and, in particular, at millimetre wave.

Significant examples of applications for multi-application short range intervehicular and vehicle-to-roadside links are: - exchange of information, such as vehicle speed, ac-

celeration and driver intentions, between vehicles travelling on a highway and thereby acting as part of a driver safety and collision avoidance system;

- convoy driving; - route guidance; - vehicle location; - collection of all categories of tolls even when vehi-

cles are travelling at high speed on an highway or in

Vol. 4. NU. 4 Ju ly -August lY93 403

16 Orate Andrisano, Marco Chiani, Velio Tralli, Mario Frullone, Colin R. Moss

heavy urban traffic congestion; - provision of parking information and booking

parking places; - provision of information on situation at airports,

ferry terminals and railway stations, i.e. to act as a link with public transport systems. The higher millimetre wave frequencies permit a si-

gnificant reduction in component size compared with those required for 5 GHz operation. There is, however, a specific advantage in using the 60 GHz frequency band as the basis for a short range link. This is the presence of an oxygen absorption band at this frequency which results in an overall atmospheric attenuation equivalent to 15 d B h . This high level of attenuation would be a great disadvantage to any long range link, but it can be exploited in the case of short range applications because it significantly reduces the range interval at which any given transmission channel can be reused. This is an important factor when many vehicles are present on a highway and results in more efficient use of the available spectrum. It is for this reason that this paper concentrates on 60 GHz link systems. The paper also assesses the effects of rain on system performance.

Any radio communication system is potentially af- fected by multipath propagation effects caused by reflections of the surrounding objects. In this instance the objects will be other vehicles, roadside buildings or ve- getation, crash barriers or even the road surface itself. A 60 GHz system is no exception, especially in an IRTE environment [7, 8, 91; it is necessary, therefore, to in- corporate countermeasures in order to meet system performance requirements. The main countermeasures are message coding and error correction techniques and the use of antenna diversity. The improvements resulting from the use of both these countermeasures are evaluated in the paper.

It is considered that extensive use of 60 GHz communication, achieving large area coverage, will necessi- tate its integration with different, long-range communication systems which will be available in the forthco- ming years at an European level (the new GSM mobile radio system, and successive system developments, which are under investigation within other European research programmes).

The kind of communication to be investigated, to be generally seen as a short-range communication, poses several problems both from a technological and theoretical point of view. This paper analyses the design criteria of a 60 GHz communication system, especially considering the propagation due to road reflection and more generally multipath propagation effects, and assesses the performance of 60 GHz communications links in terms of error probability and outage probability, starting from the propagation models recently presented in the literature [7, 8, 91. System performance is also evaluated by handling the distance between vehicles and the antenna heights as random variables. Nu- merical examples are reported and performance behaviour is discussed. Moreover, an overview of the main subsystems is given with reference to the prototype which has been built by Marconi Radar and Control Systems in cooperation with Standard Elektric Lorentz [6, 10, 111. The numerical analysis assumes a 60 GHz transmission frequency. The results of this analysis are completed with the results of a trials programme which used a prototype 60 GHz vehicle-to-vehicle link operating in a wide range of road traffic environments.

2. SYSTEM DESCRIPTION

Use of short range 60 GHz communication can lead to interesting ATT functions: e.g. driving support application andor traffic control applications can be obtained through vehicle-to-vehicle or vehicle-to-roadside communication systems. As far as vehicle-to-vehicle communication is concerned. one potential application relates to cooperative driving: in this case the vehicle communication network has the structure of a dynamic local radio network.

Fig. I shows a typical operating environment for a multi-application millimetre wave link. Any given vehicle is required to communicate with up to 30 other vehicles in the forward direction and a similar number of vehicles in the rearward direction. These numbers have been derived as a result of initial modelling of the highway scenario [ 111 and represents the numbers of vehicles which can either directly or indirectly influence the given vehicle.

GIVEN VEHICLE OIRECTLY IHFLUENCING

OTHER VEHICLE: WllHlN REIRYIARO curse o r mpLuemcE'

Fig. 1 - Network structure for il typical highway.

I NFLUCnCe'

blillimetre Wave Short Range Communications for Advanced Transport Telematics 17

Modelling [2, 111 has shown a requirement for a link range of up to 300 m, a beamwidth of 15" (to 3 dB points) and a transmitter power output of 20 mW. These requirements were taken as a basis for the study described in this paper. The literature shows that other studies have indicated maximum range requirements of up to 500 m.

Anyway, prior to any implementation of an ATT system it will be necessary to confirm the transmission power requirements and the link budget through suitable theoretical evaluation. It will also be necessary to develop appropriate multiple access protocols. The latter will allow the system to operate with many vehicles in close proximity to each other such as will normally be the case in heavy traffic conditions. These aspects are outside the scope of the work presented in this paper.

A prototype 60 GHz transmission system was developed under the ECC DRIVE initiative to meet the requirements of vehicle-to-vehicle and vehicle to roadside communication [6, 101. The on-board system inclu- des three separate units: - TransmitterAIeceiver Millimetre Wave Head - Signal Processor - User interface and display.

It should be stated that a production version of the Millimetre Wave Head could be small enough to be mounted even behind the vehicle front and rear number plates. Two such units will be required, one at the front and one at the rear of the vehicle. They may also be mounted at the top of the front and rear windows if this proves more appropriate. On account of this configuration, it will permit mass production at low unit cost.

The prototype link used for the trials programme conforms to the following configuration. A one way link is sufficient for trials purposes. A two way link would be required for any operational ATT system. The link transmitter comprises a frequency locked loop sta- bilked Gunn oscillator, a differential phase shift modu- lator and an antenna. The receiver contains two stages of IF mixing, an automatic frequency control to lock on to the received signal and finally, the demodulating and decoding units. The trials system also incorporated a PC based monitoring system to monitor link performance [ l l ] .

The configuration of a roadside unit is essentially the same as that of an on-board systems, except that the user interface and vehicle inputs are replaced by inter- faces with the IRTE communication network.

3. TRANSMISSION ANALYSIS

The research work has proceeded on both theoretical and experimental bases. From a theoretical point of view an approximate evaluation of vehicle-to-vehicle system performance was given in [ 2 ] .

In this paper we intend to report a complete link performance estimation based on a theoretical, annlyti-

cal approach to the problem of defining system availability in the real situation as shown in Fig. 2. The analysis will be presented as a general one, both as regards the coding and modulation functions, and the channel model, including the propagation medium.

d

I \ I

Fig. 2 - Multipath propagation effeas due to road and environment reflection.

3.1. Models for the propagation medium

To define and evaluate the performance limits when the system operates in an actual environment, the first step is characterization of the channel model. In this regard, diffractions effects are not included in the model, because it has been shown [7] that the Fresnel first elli- psoid may be taken to be free, at least in most common situations.

Fig. 2 shows the way multipath operates. Radiation is transmitted both by the direct path and via a single reflection at the road surface. The coherent nature of the microwave radiation will result either in a summing or a subtraction of the signals produced by the two paths. This effect has been modelled by a two-path deterministic representation. The remainder of the received power is due to multipath effects produced by surrounding reflections. This has been modelled on a statistical basis, thus the global received envelope has to be described by means of a random process, according to Rayleigh and Rice statistics [7, 81. Long term fading effects, such as shadowing, described in [9], are not taken into account here.

In order to express the received power P, due to both direct and road-reflected rays, let us now consider the scenario depicted in Fig. 2. The transmitting and receiving antenna heights, h, and h,, are in practice much smaller than transmission range, d, whence the following approximation is appropriate:

4lr h, h, P, z pro [ 1 + r? + 2 r cos cp], cp = - - (1) A d

where Pro is the reflection-free level, r is the reflection coefficient, which assumes values near to - I , and h is the wavelength corresponding to the 60 GHz carrier frequency (5 mm). Hereafter we will consider r G - 1 that gives

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18 Oreste Andrisano, Marco Chiani, Velio Tralli, Mario Frullone, Colin R. Moss

The dependence of P, on distance d can be eviden- ced by introducing a reference received power Pref as the free-space received power at distance d,, which depends on systems parameters as transmitted power, P,. antenna gains, G, and G,, and wavelength. So we have:

2

Pref = 4 [ - 4 nAdref] G,Cr

and

(3)

where (Y is the attenuation (dB/km) due to rain and oxygen absorption. In the following we will refer to d,, = 100 m.

The system analysis may be developed further by considering a more accurate model of the propagation medium which incorporates rnultipath fading (MPF) effects due to reflections in the surrounding environment. These are assumed to be independent of both frequency and time.

The combination of the direct and road-reflected paths with the MPF gives rise to a received power, P, which can be statistically described as a R i c h random variable (r.v). This approach, already presented in [8], leads to the following probability density function (p.d.f.):

p p ( P ) = R ( P , c, P ) =

where is the mean Fceived power level, c is the Rice factor and R (P, c, P ) is used to indicate the Rician p.d.f. with parameters c and p.

is in turn given by: - P = P,+ PM (6)

where P, is the mean power due to the multipath component. The Rice factor, c, is given by

(7)

It should be noted that, by virtue of ( 2 ) . P, is a function of the distance d and actenria. heights, so the same dependence holds for c and P.

Following [8] is useful to introduce:

406

Pro

PM c, = -

which represents the Rice factor in absence of the road- reflected ray. In our application c,, takes values between 3 dB and 10 dB.

As an example Fig. 3 gives the behaviour of P, , Pro, as a function of d, having fixed some typical values

for system parameters [ 101: antenna heights h, = hr = = 1.5 m, emitted power P, = 13 dBm, antenna gains G, = 18 dB (transmission) and G, = 9 dB (reception). giving P,, = - 68 dBm, c, = 5 dB, attenuation due to rain = 17 dB/km and due to oxygen absorption = 15 dB/km. Rain attenuation has been estimated assuming a rainfall rate of 50 m d h , which on average occurs for a time not greater than two minutes per year [7 ] . In the figure the Pro value is represented by the dashed line.

-90 - p,,

-100 - L 2 -110

p -120

I

u

0

H -130

2 - 1 LO

-150

I1

ll

I 4 \ \ . .

\

I .

100 150 200 230 300

d Im I

Fig. 3 - Received power as a function of distance d . Antenna heights h, = h, = h,, =1.5 m. emitted power P, = I 3 dBrn. antenna gains C, = 18 dB (transmission). C, = 9 dB (reception). c,, = 5 dB. atienua- tion due to n i n and oxygen absorption u = 32 dB/krn.

A simple comparison of (7) and (8) leads to:

c = 4 c, sin'( :) ( 9 )

that shows the dependence of c on distanced and antenna. heights h, , h, . So, c takes values in the range (0. 4 c, ) The behaviour of quantity c is similar to that shown in Fig. 3 with reference to P,. The presence of various minima is also found by fixing distance d and varying antenna heights.

Now we define the conventional signal-to-noise ratio at the receiver as

(10) w=-

where P is given by (5) and the noise power is evaluated over a bandwidth equal to the bit rate Br starting from the two-sided power spectral density, N,, . An analogous dc- tinition holds with reference to a retlection-free condition ( r = 0), represented by P,. giving W,,,. and when P

P

2 No Br

E r r

blillimetre Wave Short Range Communications for Advanced Transport Telematics 19

is substituted by the reference P,, , giving W,, . Due to the proportionality between P and conventional signal to noise ratio, the same p.d.f. can be extended to r.v. W. Considering a Rice p.d.f. for W, ply (W) requires defini- tion of Rice factor c as in (7), and mean value of W

where the last expression is obtainable from (4), (7) and (8). It is evident, as a consequence of (7), that c tends to

zero as P, tends to zero; in this case the Rice p.d.f. as in ( 5 ) degenerates to a Rayleigh p.d.f. with = P,. This condition, which is represented by the minima in Fig. 3, holds for some values of h, h,ld. So, it is possible to have, at the same distance d, different values of c due to small antenna heights variations.

Practical situations often demand that system performance is modelled to represent a dynamic situation, i.e. with d , h,, h, varying as the vehicles move. In practice a static model, referred to fixed values of these parameters, may also be used to represent the dynamic situation in cases where vehicles are moving very slowly. Under these circumstances, worst case propagation conditions might occur for a comparatively long period hence a static model is an important mechanism for as- sessing worst case performance with slow moving vehicles. By indicating the Rician p.d.f. with parameters c and w as R (x , c. w) the p.d.f. of W in “static” condition is:

The corresponding cumulative density function (c.d.f.) is

P,(s)(x) = Prob { W < x } =

where Q(.,.) is the Marcum function.

When considering a “dynamic” situation d, h, and h, must be described as r.v.’s; in this case the c.d.f. of W can be expressed as:

P,(D)(x)= j: 1 j f w ( x ; h,, h,, d ) _ _ _ _ --

where P , (x ; h, , h, , d ) = f, (n(x) is the “static” c.d.f. of Was a function of the parameters d , h, and / I , , which are assumed to be independent with p.d.f. given by p,, (.), p , (.) and p/,, (.) respectively. It is convenient to write h, = h, + Ah, and h, = h,, + Ah, where ti,, denotes

‘ I

the nominal height and Ah,, Ah, the random variations. As Ah,, Ah, <<h,, , the following approximation holds

47t I?, ;1 li

cp 3 - -(ho +Ah, + Ah,)

hence we can consider two r.v.’s, Ah = Ah,+ Ah, and distance d. This position is able to simplify integration (14).

This forms a suitable basis for a performance comparison which will be developed in the following sections.

3.2. System performance

An algorithm for the evaluation of system availability will be presented and applied with the aim of defining the performance limits of vehicle-to-vehicle communication operating under flat fading conditions. This assumption is justified because time and frequency selectivity produce negligible effects at the typical bit-rate (500 - 1000 kbit/s) of actual practical interest [6, 71. The influence of the previously defined p.d.f.’s will be discussed. Among the possible countermeasures to be adopted against multipath propagation we will consider in this paper “low cost” diversity techniques, avoiding more complex methods presented in [ 121: we will also emphasize the role of coding in achieving low cost so- lutions.

System performance will be evaluated in terms of error probability, which is in turn a function of the random signal-to-noise ratio. In this regard the analysis will be simplified by assuming that the communication link is free from intersymbol interference. Binary Phase Shift Keying will be assumed in conjunction with coherent reception (CPSK), differential encoding with coherent reception (DECPSK) and differential encoding with differentially coherent reception (DPSK) in both cases using a matched filter receiver. Exact camer recovery will be considered in the coherent case.

The average bit error probability is given by:

based on p.d.f. pw (w), either in “static” or “dynamic” conditions, and on the well known function Pb (W) cha- racterizing the modulation choice.

In the “static” situation, with DPSK, handling of (16) gives:

- I 1 1 p -- - W exp 1-c Tj I+-=-

I+- c + l

b - 2

Generally, integration (16) can be solved by numrri-

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20 Oreste Andrisano, Marco Chiani, Velio Tralli, Mario Frullone, Colin R. Moss

cal methods. It should be noted that w depends through (10) on W, (considering reflection-free propagation), c and c, (dependent on the way the various reflected rays interfere).

A more accurate system analysis may be developed by defining outage probability by means of the cumulative distribution function of W, as follows:

Pout = Prob [ P, > P, } = Prob [ W < Wo } (18)

which requires the consideration of a threshold, maximum value. P,,, and consequently a minimum signal to noise ratio, W,.

The above error analysis may now be extended to an entire packet containing a significant number of data bits. Let the packet error probability be P, defined as the probability of a packet being at the cutput of the de- coder different from the initial transmitted packet. It is also necessary to make the following assumptions when extending the analysis: - the transmission channel can be modelled as quasi-

stationary, thus W assumes the same random value during the whole packet time; in [9] the hypothesis has been experimentally checked in most cases

- each packet contains m information bits; - no other transmission impairments are taken into ac-

count (due to non perfect bit and packet synchronization, carrier recovery, etc.);

- P, will be evaluated by means of the bit error probability and. where possible, known formulas for the memoryless Binary Symmetric Channel (BSC) will be applied. In this regard when differential encoding is adopted, the bit error probability before and after differential decoding, Pb and P, respectively, are re- lated according to

P, = 2Pb (1 - P, ) (19)

In the following we will distinguish two cases:

the packet, so we have - no Forward Error Correction (FEC) is provided in

Pc ( W j = 1 - [ l - P, (Wj]'" (20)

- Forward Error Correction is provided by means of a Reed-Solomon code, RS(n.k), with s-bit symbols (this corresponds to m = k . s information bits), able to correct up to t symbol errors, so we have

where P, is the symbol error probability defined as

Moreover, when differential decoding is adopted, the 408

error probability for a group of s contiguous bits will be approximated with

with good approximation when s is greater than 4.5.

packet error probability as These definitions allow us to define the average

and the outage probability referred to packet error probability as

P , , = P r o b ( P , > P , , ) = P r o b { W < W , , } = P , (W,,)

(25)

where P, (.) is the c.d.f of W defined in (13) and (14) according to static or dynamic situation respectively. Wop is obtainable from P, by means of (17) and (18). The previous analysis allows us to evaluate system performance in term of packet error probability and to estimate the improvement which can be obtained by using FEC.

It will be apparent that FEC alone is inadequate under static conditions where the Rice factor, c, approaches 0. A typical example of this condition is where two vehicles drive at a relative distance such that the direct and road-reflected transmission paths produce mutually cancelling signals.

Thus, diversity techniques may be used to overcome the occurrence of power minima. Diversity techniques have been investigated, also in conjunction with coding. The example of 2-branches selection height diversity has been considered and compared with horizontally spaced antenna diversity. Height diversity technique has considerable merit for vehicle applications because of the high sensitivity of power minima position with respect to antenna height variation.

When considering height diversity, we assume for the first branch a receiving antenna height h , = h, and for the second branch h, = h, + ah. Let the corresponding Rice factors be c, and c, and the signal-to-noise ra- tios w,, w,. Due to selection strategy, we can write:

Now, if we assume Sh sufficiently greater than h such that the two paths are statistically independent with reference to multipath propagation:

P,(s) (x) = Prob ( W < x } = Prob [ W, < x and W2 <I) =

= P, , 'S) (x) . P, ,(S) ( x ) (27)

The last formula can be also used in the case of switched diversity, when the switching threshold is greater than x. On the other hand, when two horizontal-

E T T

Millimetre Wave Short Range Communic ,ations for Advanced Transport Telematics 21

ly spaced antennas'are adopted:

As far as other diversity techniques are concerned, it is worth observing that frequency diversity would behave as height diversity, and, at least for the static situation, time diversity as space diversity, but with more bandwidth resource.

So, system performance will be investigated, for the purpose of testing system sensitivity to the propagation characteristics, which will be mainly referred to an highway environment.

4. NUMERICAL AND TRIALS RESULTS

The algorithms developed above are used in this section to provide a quantitative assessment of system performance. The results of the assessment enable the sensitivity of a link system to a variety of propagation conditions, such as would be experienced in a highway environment, to be established.

The roles of coding and diversity as potential countermeasures to adverse propagation effects are discussed and finally some numerical examples are given assuming a 60 GHz carrier frequency.

[ 121 shows that the analytical approach developed in this paper maybe extended to cover the case of channel selectivity despite the fact the approach considers only nonselective propagation effects.

4.1. Niirnerical results

The results presented in [ 2 , 7, 81 have been used as a basis for a representative numerical assessment of system performance.

System design criteria will be discussed, both in a static condition (handling d, h,, h, as parameters) and in a dynamic situation where d and Ah are assumed to be r.v.'s. In this regard the distance between vehicles can be assumed a uniform r.v. over a specified range (100 - 300 m. or 100 - 500 m.) or a gaussian r.v. (with mean value do and standard deviation ud) and antenna heights can be assumed gaussian r.v.'s (e.g. with mean value h, = 1.5 m and standard deviation uh = 0.2,O. 1,0.05 m.).

In all the following figures, system parameters are fixed to some typical values for this application: c, = 5 dB, a = 32 dBkm, h, = 1.5 m., m = 180, P,, = - 68 dBm and Wmf = 39 dB. It is now possible to determine system sensitivity to the propagation characteristics.

First, Fig.4 shows average bit error probability as a function of distance d , for varying the c value and the modulation format, in the absence of coding. As is clear, whatever the c value, the DPSK system performs worse than the CPSK. Moreover, we can note the very large sensitivity in performance to the c value chosen. Actually, the curves with c = 0 and c = 4c, represent the boundaries of the performance obtainable.

VOI. 4. NO. 4 July - August I993

LO' ,

10-10 1 100 200 300 400 500

d [m 1

Fig. 4 - Average bit-error probability as a function of distance d. Sys- tem parameters as in the previous figure.

A more accurate performance description can be achieved by means of outage probability. As an example, Fig. 5 shows Pa,, as a function of bit error probability P, assuming CPSK modulation (with c = 0, c = c, and c = 4c,) and the value of W, at distance d = 300 m. It is important to note the significant decrease in performance with decreasing c values.

LO-'

10''

LO"

i lo-+ L

- - - - - - - - - - - - c=O (Raylergh) - c=c,=5 d B

- -. .. . . . . . . . . . . c=4c ,=l l dB . . \ .

\

Fig. 5 - Outage probability vs. threshold bit-error probability for a CPSK system (distanced = 300 m).

Fig. 6 displays average error probability as a function of distance d with c set to its maximum and minimum values of 4c, and 0 respectively. The following operating conditions are also assumed: CPSK modulation format with no coding; CPSK with various forward error correction (FEC) schemes, each with the same number of information bits per packet. Any attempt at a full system optimization as a function of FEC schemes lies outside the scope of this paper. The three FEC schemes selected in Fig. 6 are merely representative samples of typical block coding schemes. The case with c = 0 emphasises the behaviour in the worst propagation conditions. I t should be noted that the improvements due to coding are quite limited, even with code-rate 0.5 (fa-

409

22 Orate Andrisano, Rlarco Chiani, Velio Tralli, Mario Frullone, Colin R. Moss

ding is assumed to be "stationary" over the entire dura- tion of a codeword), and not so sensitive to coding format and redundance. In fact no difference is noted between RS (24.12) and BCH (182,90) cases. A different conclusion could be obtained through a model assuming a fading variation on a bit-by-bit base. Howe- ver, our assumptions are in good agreement with the experimental results presented in [7].

- unprotected

- - - 3RS(24,12) RS(45,30)

2BCH( 182.90)

I&. 10 ' -

10- -

lo-' LOO 200 300 400 500

d [rn 1

Fig. 6 - Effect of coding on the average packet-error probability for a CPSK system in a static condition. For each group of curves (from top to bottom): unprotected, RS (45, 30) s = 6.3 RS (24. 12) s = 5 interleaved codewords. 2 BCH ( I 82.90) interleaved codewords.

Now, in order to show how system performance is influenced by the variations of c and Pro with distance d (having fixed other system parameters), we report in Fig7 average packet error probability as a function of d , for CPSK systems without countermeasures. In the same figure, the effects of height random variation due to vehicle motion are also shown: Ah is modelled as gaussian r.v. with standard deviation a,, as in the figure. These variations drastically smooth the ripple in performance tending to the behaviour as in Fig.6, where a constant c value was assumed.

LO'

10-

Id 10-

h gauss. u,=lO crn h gauss. u,=ZO crn

lo- '

" ' I I00 200 300 400 500

d [rn 1

Fig. 7 - Effect of random height variation on the averqe bit-error probability for a CPSK system. Antenna heights: fixed, gaussian r.v. with standard deviation u,, = 10 cm and with v,, = 10 cm.

As i n the previous figure, Fig. 8 shows average packet error probability as a function of d with the purpose of comparing diversity and coding techniques. Performance is evaluated with reference to an unprotected system and a system which adopts coding, space or height diversity. As far as the coding scheme adopted is concerned, it should be precised that the two RS-codewords are interleaved symbol by symbol and the packet error probability has been lower-bounded with the codeword error probability. Large improvements can be observed when applying height diversity with respect to coding and even with respect to space diversity. In fact. height diversity can exploit the correlation in the two ray components of the received power. So, we can improve system performance by using a second antenna with proper selected height. As an example, in Fig. 8 we enhanced the performance by controlling c , and c2: at d = 180 m (where c , presents a minimum) we produced a maximum for Rice factor c2 by choosing 6h = 15 cm. It should be precised that, with the specified system parameters, the countermeasures adopted are not able to produce a significant performance improvement when distance d approaches 500 m.

10'

10-

I L-

lo-'

lo-'

lo-'@ I00 200 300 400 500

d [m 1

Fig. 8 - Average packet error probability as a function of distanced in a static condition: comparison between code (2 RS (30. 15) s = 5 in - terleaved codewords). space and height diversity (antenna spacing 611 = 15 cm).

The analysis of performance sensitivity with respect to distance is useful to assess the transmission range, when a quality or an availability requirement is fixed. Taking the behaviour as i n Fig. 8 into account, we could define a reliability range, as the distance range where the requirement is always fulfilled, or a distance coverage, as the fraction of a given range where a requirement is fulfilled. Hence, we can define distance coverage, D,,, as the ratio between the sum of the distance intervals where, for example, c is less than a given value, P,, , and the overall distance range of interest. I t follows that:

-110

Millimetre Wave Short Range Communications for Advanced Transport Telematics JI I

p,,

10-2

10-3

10-4

As an example, if D,, = 0.70 over the range (100 m- -500 m) for P,, = we can say that an average packet error probability equal to or less is granted over 70% of the distance range considered. The following table shows some of these results obtained from Fig. 8 .

Fig. 9 more clearly explains system sensitivity to distance, through a worst case evaluation of the packet error probability. I t displays the envelope of the curves shown in Fig. 8.

D," Unprot. Code Space div. Height div. Height div.

+ code

0.64 0.70 0.77 0.86 0.89

0.49 0.55 0.7 1 0.83 . 0.86

0.38 0.45 0.63 0.76 0.84

protected system, 275 m when coding is adopted and 410 m when height diversity is adopted. It should be observed that the reliability range would be wider if we did not consider rain attenuation. Moreover, height diversity could lead to better performance if we moved the minimum in the figure toward d = 300 m by increa- sing ah. This can be checked in Fig. 11 where performance of height diversity is shown as in the previous figure, but with different antenna separation.

_ I ' ,.-.--, ,.-.'-. I /' -. .. . ' /'

\,. / /' \ . -2 7' -.

c. I' '.

I+;+, r , \

\ , unprotected ' - - ' ,

I h . div. Sh=15 c m

h . div. Sh:25 crn

- - , . . . . . . . . . _ _ _ h . div. 6h318.75 c m I ' . .:'

I

I I

Table 1

, , ,,- ,'-- unprotected I I ,' ,' .. . . . ... .. ZRS( 30.15)

- - - space div. - - - - - - height div.

,' ,I

'. ./' h . div. + ZRS(30.15)

I LOO zoo 300 400 500

d Lrn 1

Fig. 9 - Worst case average packet error probability as a function of distance do in a static condition: comparison between code (7 RS (30. 15) s = 5 interleaved codewords), space diversity, height diversity (antenna spacing 6h = 15 cm). code in conjunction with height diver- slty.

From this figure we could evaluate the reliability range for systems without protection, with code, with space diversity, with height diversity and with height diversity combined with code.

It has been checked that similar behaviour as in the previous figures can be noted by considering the outage probability referred to a given threshold value instead of average packet error probability.

In Fig. 10, the same comparison as in Fig. 8 is shown in terms of outage probability. Here, heights are assumed r.v's (standard deviation a,, = 10 cm) to more clearly understand how performance varies with distance d. If we fix, e.g., P,,(O.OOI) = 0.01 as an availability requirement, we note that, with the previously adopted system parameters, it is granted up to 200 m for the iin-

' 1 LOO 200 300 400 500

d [rn 1

Fig. 10 - Outage probability, for threshold packet-error-probability P,, = 10-I. as a function of' distance between vehicles for a DPSK system. Countermeasures as in the previous figure.

100

10-2

lo-'

t

L 0-6

10-1

I00 200 300 400 500

d [m 1

Fig. I 1 - Effect o f height diversity on the ounpe probability. for rhre- shold pncket-~rror-probability P,"= 10.'. for a DPSK system.

VOI. 4. NO. 4 July - AugtIht l Y Y 3 41 I

24 Oreste Andrisano, Marc0 Chiani, Velio Tralli, Mario Frullone. Colin R. Moss

:I _ _ - 8. ZRS(30.15) 3 8 . h. div. _ - _ - - _

Assuming slightly different antennas' position, e.g. h, = 1.3 m. does not change system behaviour: in this case only achange in the maxima-minima pattern can be observed with respect to the case h, = 1.5 m. However, significant changes can be observed when we adopt antenna heights around 0.5 m, which produces a wide minima at distances near 220 m as shown in Fig. 12. I t should be under- lined that, with these antenna height values, thediffraction effects can not be neglected, at least ford>200 m.

1 OD

10-

a'

10-. - ha= 1.5 m - - - h,=0.5 rn

I00 zoo 300 400 500

d [m 1

Fig. 11 - Effect of antenna position. ho. on the outage probability for threshold packet-error-probability Pco = lo-'. for a DPSK system. An- tenna spacing 6h = 15 cm and 611 = I 2 3 cm for / I , = 1.5 m and ho = 0.5 m respectively.

Finally, the next two figures show the outage probability function referred to packet error probability for both unprotected and protected DPSK systems, in two different typical dynamic situations. In Fig. 13, the scenario where a vehicle tracks another vehicle with nearly the same speed has been modelled, while, in Fig. 14, the situation where a vehicle talks with a set of vehicles uniformly distributed in distance has been considered. It should be noted how it is difficult to obtain a sufficient availability level for distances approaching 500 m,

Fig. 13 - Outage probability as a function of threshold packet-rmr- probability. P,, for a DPSK system in two dynamic scenario: a) u, = I5 cm. distance as a gaussian r.v. with mean value d,, = 450 m and u) = 30 m; b) u - 10 crn. distance as a gaussian r.v. with mean value (1,; = 300 m and :,,-= 20 m:

while a large improvement is easily reached when distance is 300 m or less.

LO0 , I

- B. unprol.

- - - - - - _ _ _ - _ _ - - _ _ - _ _

- B, h: dlv. . . . . . . . . . . . A. unprot.

A , h. div. - - - - - - A , ZRS(30.15)

i 10'' 1 0 - 6 lo-' lo-' I on

p.0

Fig. 14 - Outage probability as a function of threshold packet-error- probability. Pea, for a DPSK system in two dynamic scenario: a) distance as a uniform r.v. in the range [ 100 m - 300 m]; b) distance as a uniform r.v. in the range [ 100 m - 500 m];

A compact description of performance sensitivity to distance in the dynamic scenario where a vehicle tracks another vehicle with nearly the same speed (as in Fig. 13) is shown in Fig. 15: here average error probability is plotted and feasible improvements due to counter- measure techniques can be evaluated with respect to the unprotected case, when considering distance as a gaussian r.v. with mean value do. Here the standard deviation for antenna height and distance are assumed proportional to distance d,, , to give '+,, = 10 cm and ad = 20 m when d,, = 300 m.

/' /'

I'

2 R S ( W L 5 ) ; .......... space d i v . /' - - -

/ unprotected , -

\ height div. /, /'

/ unprotected , -

2 R S ( W L 5 ) ; .......... space d i v . /' - - -

\ height div. /, /'

LOO zoo 300 $00 500

do [m I

Fig. 15 - Average packet error probability as a function of mean distance dc, for a DPSK system in a dynamic condition. Distance as a gaussian r.v. with mean value d,,; u,, and ad proportional to d,, and assuming the values u,, = 20 m and a,, = 10 cm when df, = 300 m.

It is worth remarking that, for each possible scenario under consideration, the outage probability function gives a complete first-order statistical description of r.v. P,.

E I T

Millimetre Wave Short Range Communications for Advanced Transport Telematics 25

4.2. Trials results

This section describes the results obtained with the prototype system described in [ 111. The pilot link was installed in two cars, the car ahead carried the transmission equipment, the car behind the receiving and evaluation equipment. The bit-rate of the system is about 500 kbit/s in a burst mode. Due to difficult carrier recovery in a mobile environment the signal is a differentially PSK modulated signal: demodulation is obtained by means of a non-coherent process. Antenna heights are approximately about h, = h, = 0.50 m and there is no height diversity. The code is a RS (35, 25) used for both error detection and correction allowing up to 4 symbols to be corrected in this instance and the emitted power is 13 dBm. Both the transmitting and receiving antennas have beamwidths of 15 degrees (to 3 dB points).

The system was designed to give a signal-to-noise ratio at the receiver of 30 dB at 300 m under worst case conditions of atmospheric attenuation and possible mis- alignment between the transmitting and receiving antennas.

Full trials results are presented in [6] and [ 111. They confirm that: - a transmission range of up to 500 m could be achieved

despite the fact that the system was only expected to operate out to 300 m;

- the bit error rate before error correction and given that a given data packet was received, i.e. with correct synchronization, was of the order

- the corresponding packet error rate, again before error correction, was of the order

- the bit error rate after error correction and given that a given data packet was received was less than

- depending on the traffic and highway conditions, outage probability varied between 0 and 20%. This is lower than at first sight might be expected, but it con- firms that, in cases where the direct path is obscured by other vehicles, the system is able to exploit the multipath signals reflected from the road surface, other vehicle and from roadside “furniture”. It was not possible to undertake, as part of the preli-

minary experimental program, specific assessment of outage probability, reliability range and distance coverage D,, .

At the present time a complete comparison between the theoretical and experimental results is not yet possible on account of the necessary refinements both in the theoretical approach and in the experimental tests. This comparison could be probably carried out after more accurate field trials, which will be more extensively developed during the DRIVE 11 project. In this context, the 60 GHz communication technique should be tested in an highway environment.

As yet higher communication protocols layers have not yet been investigated although potentially the system can be adapted to use different communication protocols.

Vol. 4. NO. 4 July - August 1993

5. CONCLUSIONS

Digital short range communication systems will play a fundamental role in the future IRTE, to support vehicle-to- vehicle and vehicle to roadside communications.

In this context 60 GHz communication has been considered due to its large RF bandwidth and immunity to cochannel interference.

System performance has been theoretically evaluated by considering road reflection effects and multipath due to the environment.

More precisely, a methodology has been presented in order to assess system performance in terms of average packet error probability and outage probability. The analytical approach proposed allowed the estimation of the maximum transmission distance available, according to a fixed quality requirement, having chosen system parameters such as transmitter power, antenna gain, etc. The methodology also included the analysis of performance improvements due to coding and antenna diversity. The need of height diversity has been emphasised in order to achieve several hundred metres transmission distance in conjunction with acceptable quality levels.

It is suggested that further theoretical and experimental research and development of 60 GHz communication systems is necessary, in conjunction with further tests in actual ATT environments.

Proper design of Millimetre wave Systems could lead to a comprehensive network of vehicle-to-vehicle and vehicle to roadside digital communication systems. Due to their flexibility, they could play a fundamental role in supporting cooperative driving functions, which are one of the most important applications in the future IRTE.

Further research development are now emphasized: both theoretical and experimental investigations should be developed with reference to system sensitivity to possible impairments, such as multipath propagation effects due to actual environment conditions, e.g. shadowing by lorries, humps and curves of the road, which could require some intelligent system functions.

Future research work in PROMETHEUS and DRIVE could be an ideal context for this applied research work, which should be developed in connection with the implementation phase of ATT systems.

In this perspective, urban traffic applications more than highway applications seem very attractive and should be analysed in the future.

Acknowledgements

The Authors should like to thank the anonymous Re- viewers for the helpful suggestions and comments.

Manuscript received on December 29. 1992.

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J. Altendorf. 0. Andrisano, P. van Berkel, G. Falciaecca, P. Form. M. Frullone. W. Hengeveld. G. Immovilli: Assessment methodology and criteria for RTI tronsmission systems. FIRST Drive Conference (E.C.C.). Brussels. February 1991.

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Wolfgang Schafer: Channel modrllirtg of shorf-range radio links at 60 GHr for mobile intervehicle commurtication. Pro- ceedings of the IEEE Vehicular Technology Conference, St. Louis. May 1991,

H. Bischl. W. Schafer. E. Lutz :The PROMETHEUS 60 GH: channel -fading statistics and simulated pocket error rates. V PROMETHEUS Workshop, Munich, October 1991.

M. Bezler. G. Ploger. G. A. Hurd, R. F. Jones, C. R. Moss: Short range microwave communication for RTl applicariott. DACAR Interim Report (Drive Program. E.E.C.). June 1989.

H. Godde. 1. Ilg. T. Won, M. Jelffs. R. Jones, G. Hurd. C. Moss: Final report on Drive DACAR microwave subproject. (Drive Programme E.E.C.). June 1991,

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