2_HO Dac Tu and Shigeru SHIMAMOTO, A STUDY OF MULTI-CARRIER TRANSMISSON FOR AIR TRAFFIC...

7/30/2019 2_HO Dac Tu and Shigeru SHIMAMOTO, A STUDY OF MULTI-CARRIER TRANSMISSON FOR AIR TRAFFIC CON.[1].pdf

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Track 1 _ELECTRONICS & TELECOMMUNICATIONS Section A

International Symposium on Electrical & Electronics Engineering 2005 Oct. 11, 12 2005 - HCM City, Vietnam

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HO Dac Tu and Shigeru SHIMAMOTO

Graduate School of Global Information and Telecommunication Studies, Waseda University1-3-10 Nishi-Waseda, Shinjuku-ku, Tokyo, 169-0051 Japan

E-mail: [email protected] ; [email protected]

ABSTRACT

In present times, air traffic control (ATC) systems, which are using a limited VHF band (118-137MHz), have

faced several serious problems especially in system capacity and spectrum usage efficiency. In air navigation,

there is a so-called C band (5030-5150MHz), which is being assigned worldwide for Microwave Landing

System (MLS). In reality, one part of this band is not being used. We evaluated and realized its applicability for

ATC communications, then assume to use in our next evaluations. Based on a typical propagation model in ATC,

we compute Root Mean Square (RMS) delay spread and Doppler shift spread in C band. With many advantages

of multi-carrier transmission technique as theoretical evaluations, we propose two system-models for ATC

communications based on the combination of OFDM and conventional multiple access schemes. They are

OFDM-FDMA (OFDMA) and OFDM-CDMA (MC-CDMA). From these two system-models, we carry outcomputer simulations for the two systems in terms of channel performance. Our results show that, OFDMA is

the most promising candidate for ATC communications in the future. This model deserves the striking selection

for the advanced air-ground digital link that should be considered in ATC communications.

Key words: Air Traffic Control, OFDMA, MC-CDMA, Rice factor.

1. INTRODUCTIONIn air traffic control communication, radio system

between ground and aircraft stations have employed

Very High Frequency (VHF) band [1] which waslimited from 118-137MHz. This system played the

most important function because its mainresponsibility is to provide aircraft route, route

guidance, weather bulletin informationetc to ensure

the safety of each flight. In present time, VHFAmplitude Modulation channel has been used for

voice communications among air traffic controllers

and pilots. The congestion of air traffic has contributed

to system inefficiencies such as aircraft delay,

diminishing airline profits and most seriously a

compromise in the safety of passengers and flight crew.

Although VHF digital link mode 2-VDL2 [2] has

being used a little but it did not show a clear capability

to solve those problems for ATC.In reality other VDL systems such as VDL mode 3

[3] and 4 [4] are also being studied for this

communication but there was no significant difference

between these systems in terms of capability. That was

the reason why ATC communication faced to channel

capacity limitation and an inefficient channel

performance situation. Nowadays, aircraft density has

increased rapidly especially at big airports such as in

European countries, America and Japanetc.

Therefore, necessary information to be transmitted

through radio channel is increasing very fast. Currently,

a part of C band is assigned for Microwave Landing

System (MLS) [1] in air navigation. This band is beingused partly and not popular in all countries because of

a minor commercial success. The unused band

therefore is now assumed to be used for ATC

communications [5]. In the state of arts, OFDM

technology has been considered the most efficient

channel performance for terrestrial mobile

communication under the combination with other

conventional multiple access methods.

We introduced system models for OFDM-FDMA

and OFDM-CDMA used in air traffic control then

compared their channel performance through

computer simulations.

2. WAVE PROPAGATION IN C BAND2.1.Frequency Distribution

According to [1], a so-called C frequency band

which is being used in Microwave Landing System

(MLS) is assigned from 5030MHz to 5150MHz. As

explained above, we assumed to use and now


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International Symposium on Electrical & Electronics Engineering 2005 Oct. 11, 12 2005 - HCM City, Vietnam6

distribute this band for ATC purpose. Figure 1 showed

the two ways to use C band for wide band

communication applications.

Fig. 1Frequency distribution of C band case 1

Fig. 2 Frequency distribution of C band case 2

2.2.Analysis of Rice FactorIn the experiment [5], received signal distribution

belongs to Nakagami-Rice distribution because it was

in the multi-path propagation model (direct path and

reflected paths). According to this model, the ratio

between direct path power and reflected paths powerwhich was so-called Rice factor [6]. This factor was a

typical parameter which reflects wave propagation

condition. Figure 3 showed the dependency of Rice

factor on distance d from the aircraft to the ground

station [7].

Fig. 3 The relation between Rice factor and distanced

In general, each flight belongs to four typical phases

including Parking, Taxi, Take-off/Arriving and En-

route [8]. During these phases, Rice factor at

Parking/Taxi phase was very low and it increased

gradually in Take off/Landing and En-route phases.

Table 1 illustrated some numerical values of this factor

calculated from Fig.3

Table 1 Typical Rice factor at different phases of a flight

Phase name Distance range Value range

Parking 0.0-0.21 km 0 dB

Taxi 0.21-2.1 km 0- 7 dB

Take-off/Arriving 2.1-20 km 10 dB

En-route 20-300 km 15 dB

In reality, to introduce the wave propagation of C

band in ATC, we need to do many experiments and

computations which have been done in [6], [8].Therefore we would like to continue with the main

point in this paper was the evaluation of OFDM-

FDMA and OFDM-CDMA in the consideration of

channel performance.

3. OFDM-FDMA FOR ATC3.1System ModelOrthogonal Frequency Division Multiplexing (OFDM)

was not a new technique, but its application in a

special communication like air traffic control is new.

With OFDM, it was expected to satisfy therequirements of the next generation air traffic control

systems. In ATC, time delay and Doppler shift were

the most considered problems to any radio system

especially in ATC, because of its long distance and a

very high speed of aircraft. These problems were

expected to be solved when applying OFDM

technology.

Figure 4 described a model of OFDM-FDMA

system designed for air traffic control purpose. In this

model, each aircraft was assigned by one sub-carrier

or one sub-carrier frequency. Signals from users werefirstly mapped and rectangular pulse making process

then modulated at their own sub-carrier frequencies

respectively. Modulation scheme was chosen as QPSK.

Modulated signal was converted into time domain

through IFFT block then additional processes were

also done before transmitting. Channel in this situation

was Rice fading channel and noise was assumed as

AWGN.


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Fig. 4 OFDM-FDMA model for air traffic control

In reality, depends on the required data bit rate,

differential modulation schemes could be appliedproperly. In this case the bit rate was 128kbit/s

therefore QPSK was suitable for data modulation

scheme. In order to avoid inter carrier interference

(ICI) and inter symbol interference (ISI), adding guard

time and a guarantee spectrum in design were

indispensable [9][10]. By this way, such interferences

were eliminated and channel performance of OFDM

system was improved significantly.

3.2Simulation Parameters and ResultsTable 2 illustrated all parameters used for OFDM-

FDMA simulations. In C band, Doppler spread wascalculated at 400Hz [7] when ground antenna gain was

set at 20dBi. To ensure ICI/ISI was low, sub-carrier

spacing was designed many times larger than Doppler

spread value and symbol duration was much larger

than spread delay maximum value. Therefore, a value

of 20 KHz was designed for sub-carrier spacing and

60s was for total symbol duration. Spread delay has a

limited value of 2s max5s if ground antenna gain

was set at 1020dBi. This value need to be increased if

ground station antenna gain gets smaller.

In addition, to increase the channel performance

by reducing the error rate, Forward Error Correction-

FEC, with convolutional coding and soft detection

Viterbi decoding were applied. The code rate wasselected as R=1/2 and constraint length as K=7.

Table 2 Parameters for OFDM-FDMA simulation

Based on those parameters, the following figure

showed the simulation result:

PPaarraammeetteerr nnaammee VVaalluueeProperties of Channel Propagation

Frequency band C band

Carrier frequency f0=5100 MHz

System bandwidth B = 2.56 MHz

Spread delay maximum max= 25 s

Doppler spread maximum 300-400Hz

Aircraft speed 1000km/h

Ground antenna 1020dBiRice factor K K=5,10,15dBProperties of OFDM System

Modulation QPSK

OFDM symbol duration TS= 50 s

Guard interval time TG =10 s

Total symbol duration TOFDM=60s

Carrier bit rate 128kbit/s

No. of carriers NC= 128

FFT length 128

Sub-carrier spacing fC=20KHz

Modulation QPSK

Doppler shift frequency 5000 Hz

Guard interval length 8 (NC/16)

Encoding/Decoding Convolutional Code

Code rate R=1/2 and

constraint length L=7Viterbi decoding

with soft detection


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Fig. 5 Result of OFDM-FDMA channel performance

According to figure 5, at BER required for voice

communication in ATC was 10-3 [3], an Eb/N0 was

required at least 13dB (at Rice factor K=15dB).

From now, we start with the system model and theevaluation of channel performance with OFDM-

CDMA system.

4. OFDM-CDMA FOR ATC4.1System ModelThe principle of OFDM-CDMA combination is to

distribute the information of one user on several sub-

carriers by spreading. To obtain several sub-carriersNcin a fixed bandwidth B, OFDM is used [11], resulting

in a sub-carrier bandwidth of B/Nc. By using

orthogonal Walsh-Hadamard, spreading sequenceswith lengthL, a maximum ofKmax=L users can shareLsubcarriers simultaneously without interfering each

other in the noise-free case [12] [13]. The complex-

valued data symbols D(k) of each user are multiplied

with the corresponding spreading sequence C(k),

resulting in the source vectors S(k). Up to Kmax=L of

these source vectors are added to generate the spread

vectorS. This vector is modulated on the Nc=L sub-

carriers of an OFDM system, resulting in the time

domain vectorxby an IFFT transformation.

Fig. 6 MC-CDMA model in case of one user

In this situation, we selected the number of sub-

carriers and spreading sequence length as the same

value. This means, after spreading, each signal will be

processed in one sub-carrier. Actually in the multi-user

environment, at one time there will be many

transmitted signal and one sub-carrier will process

many signals come from differential users. These

signals were even on the same sub-carrier, but they

were orthogonal each other. That was the reason why

the interference could be eliminated.

With the idea applied in case of one user. We

would like to introduce the completed model which

was built for the whole systems under the multi-user

environment in air traffic control.

In ATC communications, still line of sight (LOS)

condition for propagation was applied. This feature

was specified by K factor at typical values:5, 10 and

15dB, which correspond to typical phases in any flight.

This system model was designed with the intention to

compare with OFDM-FDMA system. That was the

reason why we also chosen 128 users, the same user

bit rate, same modulation scheme and under the same

Rice fading channel.


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Fig. 7 OFDM-CDMA model for air traffic control

Signal from users were encoded by convolutional

code with code rate R=1/2; constraint length L=7.

Encoded signal was modulated by QPSK method and

then spread by Walsh Hadamard sequence. Spread

signals from all users were added to create a serial

signal. This signal was divided for many sub-carriers.

Through IFFT block, signal was processed by adding

the guard time to ensure the overlap between OFDM

symbols. In the receiver side, opposite processing was

carried out and get the final signal to the receivers. The

following table showed a complete set of parameters

used for computer simulation done for OFDM-CDMA

system under the same condition with the OFDM-

FDMA system. The purpose is to do computer

simulation for the two systems in the same conditionof channel propagation and OFDM system properties.

Table 3 Parameters for OFDM-CDMA simulation

4.2Simulation Parameters and ResultsSimilar to section 3.2 that stands for the result of

OFDM-FDMA channel performance simulation, this

section also showed the simulation result for OFDM-

CDMA system based on the parameters which were

set in Table 3. The result in this case will be done in

the same way in section 3.2 (means to calculate BitError Rate-BER). In addition, all the parameters were

set to make the both channel performance results

comparable.

The next figure showed the channel performance

of OFDM-CDMA system with 128users and bit rate

per user was 128kbit/s.

Fig. 8 Result of OFDM-CDMA channel performance

According to figure 8, at BER required for voice

communication in ATC was 10-3 [3], an Eb/N0 was

required at least 15dB (at Rice factor K=15dB).From now, we start with the system model and the

evaluation of channel performance with OFDM-

CDMA system.

PPaarraammeetteerr nnaammee VVaalluuee

Properties of channel propagation

Frequency band C band

Carrier frequency f0=5100 MHz

System bandwidth B = 2.56 MHz

Spread delay maximum max= 25 s

Doppler spread maximum 300-400Hz

Aircraft speed 1000km/h

Ground antenna 1020dBi

Rice factor K K=5,10,15dB

Properties of channel propagation

Spreading code Walsh Hadamard

Sequence length 128 codes

Modulation QPSK

OFDM symbol duration TS= 50 s

Guard interval time TG =10 s

Total symbol duration TOFDM=60s

Carrier bit rate 128kbit/s

No. of carriers NC= 128FFT length 128

Sub-carrier spacing fC=20KHz

Modulation QPSK

Doppler shift frequency 5000 Hz

Guard interval length 8 (NC/16)

Encoding/Decoding Convolutional Code

Code rate R=1/2 and

constraint length L=7

Viterbi decoding

with soft detection


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5 CONCLUSIONFrom the both figures 5 and 8, they showed that

the curve lines in figure 5 were always below the curve

lines in figure 8. This means that, the probability of

Error from channel performance simulations in

OFDM-FDMA system was lower than that in OFDM-

CDMA system.

In reality, this difference could be understood by

knowing that in OFDM-FDMA system, each user

signal was processed separately by different

frequencies. Meanwhile, in OFDM-CDMA system,

all-user signals were processed in a sub-carrier

bandwidth. The processing in the second system

showed a possibility to cause interference between

these signals. But in the first system, such interference

will be avoided more perfectly.

From this conclusion, combine with one of our

result [14], this study showed the best channel

performance in air traffic control communication, it

was the combination of OFDM and FDMA.

In addition, the special requirements and operation

methods of ATC system made OFDM-FDMA system

advantages compared to other systems.

REFERENCES

[1] Aeronautical Information Publication Japan, CivilAviation Bureau - JCAB, 2004.

[2] P. Delhaise, D. Desperier and B. Roturier, VDL2Model 2 Physical layer validation report. EATMPReference Number COM5-11-0501, EUROCONTROL,April 2001.

[3] Manual on VHF Digital Link VDL Mode 3 (ICAO),Doc 9805 AN/763, 2002.

[4] E. Haas, P. Hoher, H. Lang and U.-C. Fiebig, FutureVHF Architecture Implementation Study, WP5000:Physical layer; Final Report. EUROCONTROL, June1999.

[5] Guan X., Ho D. T., Y. Tsuda, S. Shimamoto, J.Kitaori, Y. Nakatani, S. Kato, A Study on 5GHz DigitalData Communication for Air Traffic Control, TechnicalReport of IEICE, SANE 2004-35, July 2004.

[6] Rappaport, T. S., 1996, Wireless Communications,Principles and Practice, Prentice Hall, New Jersey, ISBN0-13-461088-1.

[7] Ho D. T., Y. Tsuda, S. Shimamoto, J. Kitaori and S.Kato, The Next Generation Air to GroundCommunication System Using for Air Traffic Control,2005 IEEE/ACES International Conference on WirelessCommunications and Applied ComputationalElectromagnetic, Catalog No. 05EX1049, ISBN: 0-7803-9068-7, Hawaii, U.S.A., Apr. 2005.

[8] E. Haas, Aeronautical channel modeling, IEEE

Transactions on Vehicular Technology, Vol.51, No. 2, pp.254-264, March 2002.

[9] K. Fazel, S. Kaiser, 2003, Multi-Cairrer and SpreadSpectrum Systems, Wiley, England, ISBN 0-470-84899-5

[10]L. Hanzo, M. Munster, B.J. Choi and T. Keller,OFDM and MC-CDMA for broadband multi-usercommunications, WLANs and Broadcasting, IEEE Press,2003.

[11]S.B. Weinstein and P.M. Ebert, Data Transmissionby Frequency-Division Multiplexing Using the DiscreteFourier Transform, IEEE Transactions onCommunications, vol. COM-19, pp. 628-634, October1971.

[12]J.G. Proakis, Digital Communications, New York:McGraw-Hill, 3rd edition, 1995.

[13]K. Fazel and L. Papke, On the performance ofconvolutionally-coded CDMA/OFDM for mobilecommunication system, in Proceedings IEEEInternational Symposium on Personal, Indoor and MobileRadio Communications (PIMRC93), Yokohama, Japan,pp. 468-472, September 1993.

[14]HO D. T., Y. TSUDA, S. SHIMAMOTO and J.KITAORI, C band and OFDM for air traffic controlcommunications system, 32nd AIC InternationalConference, Halong, Vietnam, May 2005.


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2_HO Dac Tu and Shigeru SHIMAMOTO, A STUDY OF MULTI-CARRIER TRANSMISSON FOR AIR TRAFFIC...

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