Impact of the transmission channel on GALILEO signal performance

Acta Asrmnnuticn

Pergamon www.elsevier.com/locate/actaastro

PII: SOO94-5765(02)00060-7

Vol. 51, No. l-9, pp. 285-293, 2002 0 2002 Published by Elsevier Science Ltd Printed in Great Britain 0094-5765102 $ - see front matter

IMPACT OF THE TRANSMISSION CHANNEL ON GALILEO SIGNAL PERFORMANCE Michel Monnerat,

Bruno Lobert, Alcatel Space Industries, Toulouse miche!.monnerat~space.a!cate!~fr

spectrum allocation which determines the radio environment (interference, multiple electric access), and more generally the propagation channel nature. In order to contribute to the design of an optimised GALILEO signal, able of suiting at best the users expectations, Alcate! Space Industries has developed a simulation too!, RxSim, which aims at assessing as far as possible the real user performances. This paper proposes first to describe this simulation too!, and then to show the results of such an approach addressing two key issues which are on the one hand the wave form optimisation with respect to multipath environment, and on the other hand the design of an aviation-dedicated signal considering the jamming DME/TACAN problems. The considered waveforms are BPSK, SRC and BOC.

BIOGRAPHY M. Monnerat is a graduated engineer from ENSICA (Toulouse, France). B.Lobert is a graduated engineer from IDN (Lille, France) and IRR (Toulouse, France) and is currently in charge of the Engineering Navigation Department of Alcate! Space Industries. ABSTRACT In the framework of the GALILEO signal definition, this paper endeavours to address two signal optimisation issues through results of simulations. The first point refers to the choice of the signal wave form, trading off a square root raised cosine and a square BPSK signal in several channel models. The second point refers to the study of the impact of DME/TACAN interference in E5 band with diffrent modulation schemes. Impact of BOC modulation is compared with other modulation schemes. 0 2002 Published by Elsevier Science

SIMULATION

TOOLS DESCRIPTION

In order to check out the performance of the Navigation signals, Alcate! Space Industries has developed a complete simulation too! whose main part stands in a receiver simulation too!, called RxSim. The too! is developed under MATLAB Environment and is completely modular, so that the implementation of any simulation can be seen as a gathering of the necessary elements to build the systems to be simulated (Modulations, channel, receiver loops characteristics etc . . .). The complete tool is designed to be able : . to perform the generation of the navigation signal spread with the desired sequence modulated with BPSK, SRC or BOC. with or without a pilot tone Bit encoding with a FEC or not l To mode! the transmission channel, through a transfer function :

Ltd.

INTRODUCTION In the definition process of a Radio Navigation Satellite System, the direct interface between the user and the entire system stands in the supplied signal. It plays a great role in the final navigation performances, which makes it a comer stone of the system. On the other hand, the signal design and its optimisation can not be done independently from the receiver processing techniques. That is to say that the capacities of any signal can not be estimated overshadowing the user component segment and its capability to achieve the best performances with given signals while taking into account associated constraints (e.g. cost, technologic limitations,..). The other key issue of the signal performance estimation is the

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/z(t,r) =

2 A, (r)exp[jo, (r)jqr - e, T, )

3

where

I=,

A,(t) and S,(t) are independent random processed representative of the fading amplitude and phase. l At last to simulate a completely modular receiver architecture. The receiver simulation tools RxSim is composed by l a front end filtering, implemented through a 71h order butterworth filter, whose cut frequency can be adjusted. l Carrier recovery and tracking loop through I’LL or Costas loop l Code delay recovery and tracking loop l Data demodulation, The RxSim tools is finally able to simulate and process a navigation signal with or without a pilot tone component, choosing the modulation type, with the possibility to encode and interleave the message This generic&y of the RxSim tools is illustrated through three key signal definition issues. WAVE FORM MULTIPATH REJECTION CAPABILITIES This section addresses a key trade off in the signal design process : the choice of the modulation. It compares the performance of two modulation types, Square Root Cosine filtered signal (noted SRC) Square BPSK (noted BPSK) with a particular focus on their behaviour towards the multipath rejection, which is a key source of bias in the synchronisation accuracy. Results will be compared with simulations of BOC performance. Wave form description The transmitted signals for both modulation given by expression s(r)=&g(+T,)

(1)

k

Where a, E {-1,I)represents chip

is

the value

of the kth

g(t) is the shaping function For the SRC modulation , g(t) is given by

4r Is,&

cosp+~)~~J+ tbrlT,‘Cl

=7 1- (4r+-)* c

where r is roll off factor. It allows to tune the chip rate function of the desired bandwidth B, r, =(l+r)/B The

T, is the chip duration. SRC

signal

is shaped

with

gsRCat the

transmission level, and is filtered by the same filter at the receiver level, which leads to a Raised cosine filtered modulation. The auto correlation function of the resulting signal is given by sin@/ r,) cos(mt/ r,) g(t)=n rlT l-(2rt/T,)* f For the square BPSK, the shaping function

is

I0 else It is well known that the signal bandwidth is one of the major parameters which determines the multipath error. The present analysis rests on a 4MHz bandwidth as an example of application of El and E2 bands allocated to navigation services during WRC 2000. A SRC signal at 3.069Mc/s with a roll off of 0.2 and a BPSK 2.046Mcls has been retained, conserving a multiple of the base clock rate of 1.023MHz for the chip rate. Figure 1 gives the two corresponding spectra : it can be seen that, although most of the energy is in the main lobe for both cases, the BPSK signal gets higher side lobes. The following paragraph will show the impact of reducing these side lobes with an additional filtering.

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A theoretical analysis’ shows that narrow correlator techniques give better results for SRC when the signal is filtered within the main lobe of the modulation. Conversely, performance is dramatically improved for BPSK when processing also the side lobes of the modulation. This is illustrated by Figure 3 shows the multipath envelop obtained for a SRC and BPSK filtered in a 4MHz bandwidth (6 dB relative attenuation), with various values of discriminator spacing. Note that the reduction of the discriminator spacing has a limited impact.

Freq (MHZ,

Figure 1: SRUBPSK

spectra

The Power Spectral Density modulation is given by :

BOC

I 1 sin(-)?f-

IW)J2 = f,

of

287

?f’

2s, ticos($)

=

J

where f, is the sub carrier frequency and f, is the chip rate. The classical terminology is m=f,/l.O23 MHz and BOC(m,n) where n=fJl.O23 MHz. The next figure shows the spectrum of BOC( 14,2), which is well adapted to emission in El and E2 bands with carrier frequency in Ll GPS (1575.42 MHz). BOC(l4,2)

nonnslisrd spe~Irum

Figure 2: BOC(14,2)

spectrum

Figure 3: Multipath envelop for a 4MHz bandwidth

On the other hand, Figure 4 shows the impact of the receiver front filter width. The multipath envelop is represented considering a spacing of A = O.OSchip for 4 to 20 MHz bandwidth. It comes out that the performances are dramatically improved for the BPSK when the bandwidth is increased. On the other hand the receiver filter bandwidth has no impact on the SRC signal since its bandwidth is intrinsically limited to 4MHz.

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perform a simulation thanks to RxSim.

of the receiver

behaviour

Simulations The channel model adopted already been described’.

in this study

has

Channel modelling

Figure 4: Impact of the band width The same approach is applied to BOC( 10,5) waveform. Results are displayed in Figure 5 and compared with BPSK. Clearly, BOC yields better results than BPSK with equivalent bandwidth. This effect is ‘in the same direction as the reduction of thermal jitter, due to the increase of Gabor bandwidth. MvlQ& .nn@8%,I .eoc(ll.q ‘BPSKO.l.d n-z. n-mW”.lll*. MO5 ‘r I 1

;..-..._-.__L_

-._--_-,:

I

The received signal is viewed as the sum of a direct path affected by a random fading component, which follows a Rice distribution and several indirect paths affected by an random weight whose amplitude follows a Rayleigh distribution and whose phase is uniformly distributed in [0 2~1. Let x(t) be the transmitted signal and s(t) the received signal. s(r)= x(l).a&?i*(‘) Where -

X(f- ,h),@(‘) +A h=I

a, (1) is a Rician process, ar(t), k > 0 is Rayleigh process,

-

&k((t) is an uniformly

-

[0,2x] process. rt a fixed delay.

distributed

over

The fading & multipath components (Rice and Rayleigh process) are characterised by a Doppler bandwidth depending on the relative Doppler between the direct path and the echoes. The f, V,,,,,kr lc, where maximum Doppler is then VMAr is the holder speed, c the light celerity and

f, the carrier frequency. Figure 5: Multipath envelop, BOC(14,2) vs. BPSK It must be noted that this theoretical approach is limited to very static scenarios. Indeed, the receiver generally moves so that multipath is not stationary : the phase differences between the direct and the reflected signals are not constant but change randomly with variations rapidity depending on the relative dynamic between the receiver and the reflecting surfaces. In order to quantify the real capabilities of both wave form, it is necessary to define a more realistic multipath model and then to

Simulation

results SRC vs. BPSK

RxSim is used to check out the behaviour of both signal types. The simulated environment aims at describing the environment that can be found for a high speed holder, which can be a train or an aircraft in final approach. The assumption made is that the signal is affected by a diffuse component with a very low Doppler (1Hz) and one multipath component with a variable delay and a Doppler bandwidth of 420Hz, which approximately corresponds to a 300km/h holder speed. The corresponding attenuation are given in the following table

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The simulated GALILEO signal is centred in the E2 band, spread with a 3.069McIs in the case of SRC signal, and 2.046Mc/s in the case of a BPSK. The date rate is taken equal to 150 bit/s with a convolutional error correcting is code whose octal transfer function (131,171).

The receiver parameters following characteristics,

are tuned

with the

41

:

:

D

I

2

:

:

3 4 Mulllpath delay IS,

: 5

I

Pre-detection Bandwidth

300Hz

Loop Bandwidth

20Hz

The simulations is carried out for several multipath delays. For each one, a 100s simulation is done. Figure 6 shows the mean code phase error obtained for a SRC signal, and BPSK signal, filtered in 12 MHz and 16MHz. The spacing of the correlator is 0.05 chip. Unr* *r. WDl 6 : : : : I

Figure 6: Simulation

7 r

10’

Figure 7: Mean code phase error for various roll off. Impact of the pre-detection

2”6 Order Carrier Loop

I

6

results

The simulations confirms the conclusions of the previous chapter, i.e. the BPSK allows a great improvement of the multipath rejection. The same simulation has been performed with various roll off factor, in Figure 7. The spacing of the correlator has an impact on the multipath rejection but the mean error keeps being much higher than the one found with BPSK. The roll off factor seems to have a very limited impact.

bandwidth

As mentioned in the channel modelling description, the multipath components is multiplied by an attenuation coefficient which follows a gaussian law, characterised by a given bandwidth, here 420 Hz. The pre-detection band width acts like a filter towards this source of noise reducing the power of the multipath component. Here the pre-detection bandwidth is limited to 300Hz, due to the symbol rate. The multipath Doppler bandwidth is 420 Hz, so that the finally viewed power of the multipath component is -6 + IO * log lO(300 / 420) = -7.46&Z. Nevertheless the power of the multipath can still be reduced, reducing the pre-detection bandwidth. This can be done through the use of a pilot tone. Figure 8 shows the results of the simulation of the same channel using a pilot tone, which leads to be able to use a pre detection bandwidth of the code phase lock loop of 10 Hz and 100 Hz for the carrier lock loop. The multipath power is thus decreased by a factor 16.23 dB. The maximum error is lower than 20 cm for SRC and quite the same for BPSK. So that the difference between SRC and BPSK is very reduced and is quasi non sensitive.

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DME/TACAN

-02

..___

i . . . . . . . . .

j

j

i

j

j

I

0

0,

02

03

04 05 delay (-2 Wei)

06

07

08

Figure 8: Multipath error with pilot tone component if we Considering a urban application, supposed a speed of 50 km, the Doppler bandwidth of the multipath is approximately 70 Hz. The reduction of the multipath component power is 8.5 dB, so that it is still very useful to use a pilot tone. Nevertheless it is not the case for a pedestrian whose Doppler Bandwidth would be approximately 4 Hz. On the other hand, this shows the importance of diminishing the data rate, in order to allow to increase as much as possible the pre detection bandwidth. Conclusion To conclude it can be stated out that When using only the main lobe, SRC is better than BPSK towards multipath rejection When the side lobes can be used, narrow allows to techniques correlators dramatically improve the performances of the BPSK, and then the BPSK modulation gives much better performances than the SRC. The use of pilot tone allows to dramatically decrease the impact of the multipath in presence of dynamic so that the difference between SRC and BPSK becomes thin. There is a great advantage decreasing the data rate towards the multipath rejection, in presence of dynamic.

ISSUES

An other key issue requiring simulations refers to the jamming resistance of the signal. The band spanning over 115 I- 12 15 MHz is an ARNS band. It is thus a good candidate to receive a GALILEO signal destined to the civil aviation. Nevertheless, it is already occupied by Distance Measuring Equipment (DME) and TACtical Aeronautical Navigation (TACAN) systems. These systems generates pulsed interferences. It is then very important to study the compatibility between GALILEO and these systems. A previous paper’ proposes a theoretical approach to this problem. The assumption made on the receiver architecture (use a blanking device etc.. .) can be found in * as well as the description of the DME/TACAN signals. It uses the theoretical approach to perform analysis on the degradation sustained by a receiver onboard an aircraft flying over Europe. It comes out from this study that : . maximum degradations due to the DME/TACAN beacons occur at high flight level . a wide band signal (10.23Mc/s) is much more affected by the degradation than a narrow band (1.023Mc/s) signal (8.2 dB for the wide band signal vs. 1.3 for the narrow band). However the wide band signal is more attractive for several reasons and particularly for its robustness towards local jammers which the aircraft could meet during the landing phase. Simulations carried out with RxSim2,9 have allowed a more precise assessment of the degradation in worst case, i.e. in the area of Europe where jamming is the most severe. Results yield 6 dB for the wide band case and 2 dB for the narrow band case. The objective of the present work is now to assess the impact of the waveform over the degradation due to jamming environment in E5. SRC waveform The assumption for the SRC modulated signal in narrow band are the same as defined above, i.e. 3.069 Mcps chip rate and 0.22 roll-off. For the wide band signal, chip rate is 15.345 Mcps. Bandwidth is then identical to BPSK assumptions and leads to fair comparison. Carrier frequency is 1202.025 MHz.

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Figure 9 shows the evolution of degradation as a function of flight level, for both wide band and narrow band cases. Figure 10 displays the degradation map over Europe for the narrow band case, whereas Figure 11 displays the wide band case. Maximal degradation is 7.2 dB for the wide band case and 1.8 dB for the narrow band case.

Figure 11: SIN degradation by DME/TACAN on wide signal at FL=400 BOC waveform

Figure 9 : Equivalent degradation of Signal over Noise Density Ratio

BOC( 10,5) has been selected for a first comparison because it has the same equivalent bandwidth as the wide band signals (both SRC and BPSK). Figure 12 displays the degradation map for the worst case flight level (corresponding to 40000 ft altitude for an aircraft). Carrier frequency is 1186.68 MHz. Maximum degradation turns out to be 7.7 dB.

Figure 10 : S/N degradation by DME/TACAN on narrow band signal at FL=400 Figure 12 : BOC(lO,S),

Flight level =400

Another example is given with BOC(8,4) which has approximately the same first-lobe bandwidth as SRC and BPSK previously considered waveforms. Carrier frequency is here 1202.025 MHz. Degradation map is displayed in Figure 13. Maximal degradation is then 6.9 dB.

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Table‘ 1 : receiver loop assumptions

Figure 13: BOC (8,4) Flight level = 400, Interest of FDAF The previous results have been obtained considering a receiver supplied with a blanking device. But other techniques to fight against jammers can be envisaged. A study of Frequency Domain Adaptive Filtering (FDAF) has been carried out. Architecture of receiver implementation of this time-frequency technique is shown in Figure 14.

Figure 14: FDAF block diagram The algorithm simply consists of calculating the DFT (Discrete Fourier Transform) of the received signal using the FFT algorithm. By comparing the signal spectrum in the frequency domain with a certain prescribed threshold, a filter pattern G(f) is determined, defined to be equal to zero if the spectral modulus is greater than the threshold and one otherwise. This leads to eliminate the frequency components of the interference together with those of the signal in the interference bandwidth. Simulations have been carried out with receiver loop assumptions shown in Table 1.

The code loop implements a +0.5 chip corre :lator. Signal waveform is the wide band BPSK . The DFT is based on 64 points. The PRF (Pulse Repetition Frequency) has been set to 2700 Hz for DME and 3600 Hz for TACAN. It comes out a significant improvement of the degradations, which pass from around 7 dB to 4.8 dB. A more important improvement was expected (3 dB degradation has been already announcedg). This discrepancy may be explained by a more focused zoom on the worst case area over Europe, which might have disclosed spots where degradation is worse than previously seen. CONCLUSION This paper addresses two key issues in the GALILEO signal definition process. the optimisation of the wave form design regarding the effect of the propagation channel distortion with a particular attention paid on the multipath conditions. the impact of the DMEiTACAN interference in the E5 bands. The multipath rejection capabilities have been analysed for two kinds of modulations, SRC and BPSK. Two aspects can be highlighted : l In static conditions, when only using the main lobe of the modulation, the SRC gives slightly better results than the square BPSK. However the use of the side lobes improves dramatically the performances of the BPSK, giving then much better results than the SRC. l In dynamic conditions, the study of a signal containing no pilot tone leads to the same conclusions as the ones found in static conditions. Nevertheless, when the receiver is animated compared to the echoing surface, a pilot tone allows to filter the multipath component making the difference between SRC and BPSK much thinner.

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The impact of the DME/TACAN in the ES band is studied through simulation. It comes out that a solution implementing a signal based on a double component structure, wide band plus narrow band, appears to be basically more robust compared with a single wide band. Nevertheless precise simulations of the environment and the receiver behaviour shows that the impact of DME/TACAN on a wide band signal is less important than expected by the theoretical approach2. Impact of DME/TACAN in E5 has been studied for different waveforms : BPSK, SRC, BOC. It comes out that waveform impact is negligible with respect to resistance to jamming : around 2 dB degradation for narrow band and around 7 dB for wide band Moreover, at the cost of an increase of the receiver complexity, implementing a timefrequency filtering, FDAF, it comes out that the resistance of wide band signal towards DME/TACAN can be significantly improved, while maintaining the possibility to implement a pilot tone which presents many interests such as the multipath rejection as mentioned previously.

ACKNOWLEDGEMENTS The results of this paper have been drawn up in the framework of the GALA study, for the European Commission, the GalileoSat study for ESA, the SIGNAL study for CNES, and ALCATEL internal studies.

REFERENCES ’ Civil GPS/WAAS signal design and interference environment at 1176.45 MHz : results of RTCA SC 159 WG 1 activities Hegarty & van Dierendonck ION GPS’99 2 INNOVATIVEGNSS2 NAVIGATIONSIGNAL GNSS 2000 M. Monnerat, B. Lobert, C. Bourga, S.

Journo 3 Simulation of DME/TACAN interference into RNSS receiver in 1150- 12 15 MHz band M. Poncelet SE28(99)-100, 07.10.99, London 4 Ground based DME/TACAN (over Europe) interference on possible aircraft on board RNSS receiver in DME band. ANFR SE 28(99)89 ’ Analysis of M Code Signal Interference with C/A Code Receivers John W. Betz, GNSS P 2000, Washington. 6 Signal Choice for Galileo Compared Performances of two Candidate Signals L. Lestarquit, CNES, P-A Brison, CNES/DGA ION NTM 2000 ’ Signal Design and Transmission Performance Study for GNSS2 1998,ESA. s GALA Modelling tool document P. Erhard GALA study ’ Performance Analysis of a GALILEO Receiver Regarding the Signal Structure, Multipath and Interference conditions P. Erhard, M. Monnerat, B. Lobert GNSS’200 1, Seville

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