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Techniques Analysis of the Interference Suppression Algorithm in Broadband Aeronautical Multi-carrier Communication System
Available online at www.sciencedirect.com
Physics Procedia 33 (2012) 361 – 367
2012 International Conference on Medical Physics and Biomedical Engineering
Techniques Analysis of the Interference Suppression Algorithm in Broadband Aeronautical Multi-carrier Communication System Li Dong-xia, Ye Qian-wen College of Electronic and Information Engineering Civil Aviation University of China Tianjin , China [email protected]
Abstract Out-of-band radiation suppression algorithm must be used efficiently for broadband aeronautical communication system in order not to interfere the operation of the existing systems in aviation L-Band. Based on the simple introduction of the broadband aeronautical multi-carrier communication (B-AMC ) system model, several sidelobe suppression techniques in orthogonal frequency multiplexing (OFDM) system are presented and analyzed so as to find a suitable algorithm for B-AMC system in this paper. Simulation results show that raise-cosine function windowing can suppress the out-of-band radiation of B-AMC system effectively. © by Elsevier B.V. Selection and/or peer review under responsibility of ICMPBE International ©2012 2011Published Published by Elsevier Ltd. Selection and/or peer-review under responsibility of [name Committee. organizer] Keywords: Broadband Aeronautical Multi-carrier Communication(B-AMC), Out-of-band radiation suppression, Windowing
1.
Introduction
The numbers of aircraft, airport and management areas are increased rapidly with the fast development of global civil aviation and Air Traffic Management (ATM) system, which heavily relies upon air-ground communications, has met great challenge. The frequency band currently used for air – ground communications (117.975 – 137.000 MHz) is becoming more and more congested. Eurocontrol expects that the capacity limits for the existing voice based communications system will be reached around 2015-2020[1]. The development of a high-speed broadband aeronautical moving communication system is necessary. According to Eurocontrol and FAA roadmaps, new future air-ground radio system should preferably be deployed in the L-band, called L-band Digital Aeronautical Communication System (L-DACS). B-AMC Broadband Aeronautical Multi-carrier Communication is one of the techniques proposed for the future L-DACS) [2].
1875-3892 © 2012 Published by Elsevier B.V. Selection and/or peer review under responsibility of ICMPBE International Committee. doi:10.1016/j.phpro.2012.05.075
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Li Dong-xia and Ye Qian-wen / Physics Procedia 33 (2012) 361 – 367
B-AMC system is intended to be an inlay system that allows utilization of frequency gaps between two adjacent channels assigned to other systems in L-band such as Distance Measure Equipment (DME), military Tactical Air Navigation (TACAN) system, and Joint Tactical Information Distribution System (JTIDS) so on. Spectrum is not assigned specially for B-AMC system, so utilization rate of aeronautical frequency resources can be improved. How to mitigate the interference from B-MAC towards legacy L-band systems has became a key technique question which must be resolved as soon as possible. The methods that can be used to reduce the out-of-band radiation of B-AMC are investigated in this paper. 2.
System Model
B-AMC is a broadband multi-carrier duplex system based on Orthogonal Frequency Division Multiplexing(OFDM) in which OFDMA OFDM+FDMA is used for the forward link and reverse link. Air-ground (A/G) as well as direct air-air (A/A) communication modes are supported by B-AMC simultaneously. As an inlay broadband system, B-AMC is coexisted with other legacy systems in L-band and is intended to be operated between two adjacent DME channels as illustrated in Figure 1.
Figure 1 Spectrum of B-AMC signal
The un-continuous frequency spectrum of B-AMC must maintain a well characteristic. A great out-of-band radiation (the edge of the spectrum is dropped slowly) of B-AMC can prolong into the DME channels and disturb normal operation of DME severely. So, the modulation sidelobes have to be minimized sufficiently at the B-AMC transmitter under the condition of not degrading the system performance itself. The block diagram of B-AMC transmitter is shown in Figure2. It is same as the OFDM system except that a sidelobe suppression module is added after the series-parallel transfer. The input signal is modulated in PSK or QAM. The receiver performs the opposite process.
Figure 2 Block diagram of the B-AMC transmitter
Assume the number of subcarrier of B-AMC system is N. In frequency domain, signal on each subcarrier can be expressed as
363
Li Dong-xia and Ye Qian-wen / Physics Procedia 33 (2012) 361 – 367
sn x
dn
sin
x xn
, n 1,2, , N
x xn
(1)
In formula(1) x means normalization frequency, defined as
x
f
f0 T0
(2) where f0 denotes center frequency and T0 the OFDM symbol duration not including guard bandwidth, xn is the normalization frequency of the nth subcarrier. The transmitted signal spectrum of B-AMC system is expressed as N
s x
sn x
(3) Sidelobe suppress module is an important part of B-AMC transmitter, in which B-AMC signal are processed according to a certain suitable algorithm and the out-of- band radiation can be reduced highly. It can assure the B-AMC system is coexistent with DME, TACAN system in L-band. n 1
3.
Sidelobe Suppression Algorithm
Now, there are many sidelobe suppression methods related to broadband OFDM system, such as windowing of the transmission signal, cancellation carriers and subcarrier weighting [3].Although B-AMC system is built based on OFDM technique, its demands for the sidelobe suppression are higher than general OFDM system because B-AMC is an inlay system. Only highly effective sidelobe suppression algorithms can be used in B-AMC. 3.1 Windowing In windowing method, each OFDM symbol is multiplied with a windowing function in time domain before transmission, which can make the edges of symbol are slowly dropped to zero. The widely used two windowing functions are rectangular window and rasied-cosine pulse window. Rectangular window function is defined as 1 T T , t (4) pr t T 2 2 0, otherwise Rasied-cosine pulse window is defined as 1 2 prc t
1 cos 2
1, T 1 2
t ,0 t T
T
(5)
t T
1 cos 2
t T T
,T
t
1
T
0, otherwise
1 where T denotes the OFDM symbol duration not including guard bandwidth and , 0 the roll-off factor. Windowing is simple and realized easily, but the signal is enlarged in time domain that leads to the transmission speed decrease. Normally, this method is applied with other methods together. 3.2 Cancellation Carriers Insertion In this method, few so-called Cancellation Carriers (CC) are inserted on the left and right hand side of the used OFDM spectrum. The CCs are not used for data transmission, but carry complex weighting
364
Li Dong-xia and Ye Qian-wen / Physics Procedia 33 (2012) 361 – 367
factors which are determined such that the sidelobes of the CCs suppression the sidelobes of the original transmit signal.[4] Cancellation Carriers insertion does not lead to ISI due to the signal spreading. But a certain amount of Tx power has to be spent for the CCs, and thus, is not available for data transmission. This is resulting slight degradation of power used for transmitting useful signal and also the bit error rate (BER) performance. Subcarrier Weighting Subcarrier weighting (SW) demands all subcarriers are used for data transmission, but weighted by real-valued factors which are used in order to minimize the sidelobes [5]. The weighting factors are chosen according to following condition 2
min Sg , constrain g
where
S
s1 , s2 ,
, sN
g
2
g min
N gn
g max
is the signal sample set in the optimization range and g is the
weighting factor vector, which is meeting g min
1
1 , g max 1
1
1 1
g max g min
Subcarrier weighting can not waste frequency resource and prolong signal in time domain. Because different subcarriers receive unequal amounts of transmission power, it can induce a slight loss in BER performance. The loss is further increased with enlarging the value . The three methods above can be used individually. Also two of them can be combined. The results of sidelobe suppression using these three methods have a large difference. 4.
Simulation Results
To achieve a better sidelobe suppression and to be able to fulfill the requirements on sidelobe suppression in the B-AMC systems, sidelobe suppression algorithms are applied within the whole continuous frequency spectrum, although some of which are not used by B-AMC system. For simplicity, some simulation parameters are selected. It is assumed that the B-AMC system uses channels with 500 kHz bandwidth. This bandwidth is used with 48 subcarriers, each modulated with a binary phase-shift keying (BPSK) symbol and a 64-point FFT is used. That can be expressed as N 64 , N d 48 . Gaussian fading channel is assumed in the B-AMC systems. Two methods for sidelobe suppression, CCs and windowing are simulated in this paper. In Figure 3 and 4, the normalized signal spectrum density (PSD) of the OFDM signal with conventional rectangular windowing averaged over all possible symbol vectors is compared to the PSD of an OFDM signal with rasied-cosine pulse windowing for different roll-off factors . The spectrum edges of rasied-cosine signal are fallen more quickly than the rectangular windowing. When rasied-cosine windowing with = 0.1 is applied the sidelobe is suppressed by about 20 dB in Figure 3. For a larger roll-off factors, 0.2 , the out-of-radiation is further reduced, like Figure 4 ,and the effect of sidelobe suppression is more obvious. So rasied-cosine windowing is more suitable for B-MAC system. Windowing has better sidelobe suppression performance and lower complexity, also its realization is easier than other sidelobe suppression methods. So it has applied widely in broadband system.
Li Dong-xia and Ye Qian-wen / Physics Procedia 33 (2012) 361 – 367
Figure 3 Comparison of signal spectrum with rectangular and raise cosine window( =0.1)
Figure 4 Comparison of signal spectrum with rectangular and raise cosine window( =0.2)
365
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Li Dong-xia and Ye Qian-wen / Physics Procedia 33 (2012) 361 – 367
Figure 5 Comparison of signal spectrum with/without cancellation carriers
The sidelobe suppression achievable with the insertion of CCs for B-MAC system is shown in Figure 5. At both sides of the spectrum two CCs are inserted. The weighting factors of CCs are real value because the modulation type is real mapping. The comparison of the spectra with and without CCs reveals that by the insertion of CCs the sidelobe are reduced by 5 dB in the regarded frequency range. So broad band carrier signal can make a low disturb to DME channel. But a certain mount of power is spent for the CCs and less power is available for data transmission, the signal-to-noise ratio (SNR) is decreased. When a larger amount of transmission power is available for the CCs a better suppression is achieved, but at the same time system performance further degrades. CCs and windowing methods can be used for sidelobe suppression in B-MAC system separately or combined, and better suppression effect can be achieved for the latter. 5.
Conclusion
How to avoid the mutual interference between B-AMC and exiting L-band system DME/TACAN has became one of the focus techniques in development of future broad band air-ground communication. Several exiting sidelobe suppression methods are analyzed. The suppression effect using CCs and windowing methods for B-MAC system are simulated in this paper. It has been shown rasied-cosine windowing can suppress the sidelobe more valid than other methods and more suitable to use in B-AMC system. Acknowledge This work was sponsored by the National Nature Science Foundation of Tianjin (09JCYBJC00600) and CAUC Special Foundation of Central University (ZXH2010B004).
Li Dong-xia and Ye Qian-wen / Physics Procedia 33 (2012) 361 – 367
References [1] Haindl B., Sajatovic M., Rihacek C., Prinz J., Schnell M., Haas E., and Cosovic I., "B-VHF –A Multi-Carrier Based Broadband VHF Communications Concept for Air Traffic Management," in Proc. Of 2005 IEEE Aerospace Conference, Big Sky, MT, USA,March 2005. [2] Rokitansky C.H., Ehammer M., Gräupl T., Schnell M., Brandes S., Gligorevic S., Rihacek C., and Sajatovic M., “B-AMC a system for future broadband aeronautical multi-carrier communications in the L-BAND”, Digital Avionics Systems Conference, 2007. DASC '07. IEEE/AIAA 26th. [3] Brandes S., Cosovic I.,and Schnell M., “Techniques for ensuring co-existence between B-VHF and legacy VHF systems”,Aerospace Conference, 2006 IEEE. [4] Brandes S., Cosovic I., and Schnell M., “Sidelobe suppression in OFDM systems by insertion of cancellation carriers,” in Proc. of IEEE 62nd Semiannual Vehicular Technology Conference(VTC Fall’05), Dallas, TX, USA, September 2005. [5] Cosovic I., Brandes S., and Schnell M., “A technique for sidelobe suppression in OFDM systems,” in Proc. of IEEE Global Telecommunications Conference(Globecom’05), St.Louis, MO, USA, November 2005.
367
Physics Procedia 33 (2012) 361 – 367
2012 International Conference on Medical Physics and Biomedical Engineering
Techniques Analysis of the Interference Suppression Algorithm in Broadband Aeronautical Multi-carrier Communication System Li Dong-xia, Ye Qian-wen College of Electronic and Information Engineering Civil Aviation University of China Tianjin , China [email protected]
Abstract Out-of-band radiation suppression algorithm must be used efficiently for broadband aeronautical communication system in order not to interfere the operation of the existing systems in aviation L-Band. Based on the simple introduction of the broadband aeronautical multi-carrier communication (B-AMC ) system model, several sidelobe suppression techniques in orthogonal frequency multiplexing (OFDM) system are presented and analyzed so as to find a suitable algorithm for B-AMC system in this paper. Simulation results show that raise-cosine function windowing can suppress the out-of-band radiation of B-AMC system effectively. © by Elsevier B.V. Selection and/or peer review under responsibility of ICMPBE International ©2012 2011Published Published by Elsevier Ltd. Selection and/or peer-review under responsibility of [name Committee. organizer] Keywords: Broadband Aeronautical Multi-carrier Communication(B-AMC), Out-of-band radiation suppression, Windowing
1.
Introduction
The numbers of aircraft, airport and management areas are increased rapidly with the fast development of global civil aviation and Air Traffic Management (ATM) system, which heavily relies upon air-ground communications, has met great challenge. The frequency band currently used for air – ground communications (117.975 – 137.000 MHz) is becoming more and more congested. Eurocontrol expects that the capacity limits for the existing voice based communications system will be reached around 2015-2020[1]. The development of a high-speed broadband aeronautical moving communication system is necessary. According to Eurocontrol and FAA roadmaps, new future air-ground radio system should preferably be deployed in the L-band, called L-band Digital Aeronautical Communication System (L-DACS). B-AMC Broadband Aeronautical Multi-carrier Communication is one of the techniques proposed for the future L-DACS) [2].
1875-3892 © 2012 Published by Elsevier B.V. Selection and/or peer review under responsibility of ICMPBE International Committee. doi:10.1016/j.phpro.2012.05.075
362
Li Dong-xia and Ye Qian-wen / Physics Procedia 33 (2012) 361 – 367
B-AMC system is intended to be an inlay system that allows utilization of frequency gaps between two adjacent channels assigned to other systems in L-band such as Distance Measure Equipment (DME), military Tactical Air Navigation (TACAN) system, and Joint Tactical Information Distribution System (JTIDS) so on. Spectrum is not assigned specially for B-AMC system, so utilization rate of aeronautical frequency resources can be improved. How to mitigate the interference from B-MAC towards legacy L-band systems has became a key technique question which must be resolved as soon as possible. The methods that can be used to reduce the out-of-band radiation of B-AMC are investigated in this paper. 2.
System Model
B-AMC is a broadband multi-carrier duplex system based on Orthogonal Frequency Division Multiplexing(OFDM) in which OFDMA OFDM+FDMA is used for the forward link and reverse link. Air-ground (A/G) as well as direct air-air (A/A) communication modes are supported by B-AMC simultaneously. As an inlay broadband system, B-AMC is coexisted with other legacy systems in L-band and is intended to be operated between two adjacent DME channels as illustrated in Figure 1.
Figure 1 Spectrum of B-AMC signal
The un-continuous frequency spectrum of B-AMC must maintain a well characteristic. A great out-of-band radiation (the edge of the spectrum is dropped slowly) of B-AMC can prolong into the DME channels and disturb normal operation of DME severely. So, the modulation sidelobes have to be minimized sufficiently at the B-AMC transmitter under the condition of not degrading the system performance itself. The block diagram of B-AMC transmitter is shown in Figure2. It is same as the OFDM system except that a sidelobe suppression module is added after the series-parallel transfer. The input signal is modulated in PSK or QAM. The receiver performs the opposite process.
Figure 2 Block diagram of the B-AMC transmitter
Assume the number of subcarrier of B-AMC system is N. In frequency domain, signal on each subcarrier can be expressed as
363
Li Dong-xia and Ye Qian-wen / Physics Procedia 33 (2012) 361 – 367
sn x
dn
sin
x xn
, n 1,2, , N
x xn
(1)
In formula(1) x means normalization frequency, defined as
x
f
f0 T0
(2) where f0 denotes center frequency and T0 the OFDM symbol duration not including guard bandwidth, xn is the normalization frequency of the nth subcarrier. The transmitted signal spectrum of B-AMC system is expressed as N
s x
sn x
(3) Sidelobe suppress module is an important part of B-AMC transmitter, in which B-AMC signal are processed according to a certain suitable algorithm and the out-of- band radiation can be reduced highly. It can assure the B-AMC system is coexistent with DME, TACAN system in L-band. n 1
3.
Sidelobe Suppression Algorithm
Now, there are many sidelobe suppression methods related to broadband OFDM system, such as windowing of the transmission signal, cancellation carriers and subcarrier weighting [3].Although B-AMC system is built based on OFDM technique, its demands for the sidelobe suppression are higher than general OFDM system because B-AMC is an inlay system. Only highly effective sidelobe suppression algorithms can be used in B-AMC. 3.1 Windowing In windowing method, each OFDM symbol is multiplied with a windowing function in time domain before transmission, which can make the edges of symbol are slowly dropped to zero. The widely used two windowing functions are rectangular window and rasied-cosine pulse window. Rectangular window function is defined as 1 T T , t (4) pr t T 2 2 0, otherwise Rasied-cosine pulse window is defined as 1 2 prc t
1 cos 2
1, T 1 2
t ,0 t T
T
(5)
t T
1 cos 2
t T T
,T
t
1
T
0, otherwise
1 where T denotes the OFDM symbol duration not including guard bandwidth and , 0 the roll-off factor. Windowing is simple and realized easily, but the signal is enlarged in time domain that leads to the transmission speed decrease. Normally, this method is applied with other methods together. 3.2 Cancellation Carriers Insertion In this method, few so-called Cancellation Carriers (CC) are inserted on the left and right hand side of the used OFDM spectrum. The CCs are not used for data transmission, but carry complex weighting
364
Li Dong-xia and Ye Qian-wen / Physics Procedia 33 (2012) 361 – 367
factors which are determined such that the sidelobes of the CCs suppression the sidelobes of the original transmit signal.[4] Cancellation Carriers insertion does not lead to ISI due to the signal spreading. But a certain amount of Tx power has to be spent for the CCs, and thus, is not available for data transmission. This is resulting slight degradation of power used for transmitting useful signal and also the bit error rate (BER) performance. Subcarrier Weighting Subcarrier weighting (SW) demands all subcarriers are used for data transmission, but weighted by real-valued factors which are used in order to minimize the sidelobes [5]. The weighting factors are chosen according to following condition 2
min Sg , constrain g
where
S
s1 , s2 ,
, sN
g
2
g min
N gn
g max
is the signal sample set in the optimization range and g is the
weighting factor vector, which is meeting g min
1
1 , g max 1
1
1 1
g max g min
Subcarrier weighting can not waste frequency resource and prolong signal in time domain. Because different subcarriers receive unequal amounts of transmission power, it can induce a slight loss in BER performance. The loss is further increased with enlarging the value . The three methods above can be used individually. Also two of them can be combined. The results of sidelobe suppression using these three methods have a large difference. 4.
Simulation Results
To achieve a better sidelobe suppression and to be able to fulfill the requirements on sidelobe suppression in the B-AMC systems, sidelobe suppression algorithms are applied within the whole continuous frequency spectrum, although some of which are not used by B-AMC system. For simplicity, some simulation parameters are selected. It is assumed that the B-AMC system uses channels with 500 kHz bandwidth. This bandwidth is used with 48 subcarriers, each modulated with a binary phase-shift keying (BPSK) symbol and a 64-point FFT is used. That can be expressed as N 64 , N d 48 . Gaussian fading channel is assumed in the B-AMC systems. Two methods for sidelobe suppression, CCs and windowing are simulated in this paper. In Figure 3 and 4, the normalized signal spectrum density (PSD) of the OFDM signal with conventional rectangular windowing averaged over all possible symbol vectors is compared to the PSD of an OFDM signal with rasied-cosine pulse windowing for different roll-off factors . The spectrum edges of rasied-cosine signal are fallen more quickly than the rectangular windowing. When rasied-cosine windowing with = 0.1 is applied the sidelobe is suppressed by about 20 dB in Figure 3. For a larger roll-off factors, 0.2 , the out-of-radiation is further reduced, like Figure 4 ,and the effect of sidelobe suppression is more obvious. So rasied-cosine windowing is more suitable for B-MAC system. Windowing has better sidelobe suppression performance and lower complexity, also its realization is easier than other sidelobe suppression methods. So it has applied widely in broadband system.
Li Dong-xia and Ye Qian-wen / Physics Procedia 33 (2012) 361 – 367
Figure 3 Comparison of signal spectrum with rectangular and raise cosine window( =0.1)
Figure 4 Comparison of signal spectrum with rectangular and raise cosine window( =0.2)
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Li Dong-xia and Ye Qian-wen / Physics Procedia 33 (2012) 361 – 367
Figure 5 Comparison of signal spectrum with/without cancellation carriers
The sidelobe suppression achievable with the insertion of CCs for B-MAC system is shown in Figure 5. At both sides of the spectrum two CCs are inserted. The weighting factors of CCs are real value because the modulation type is real mapping. The comparison of the spectra with and without CCs reveals that by the insertion of CCs the sidelobe are reduced by 5 dB in the regarded frequency range. So broad band carrier signal can make a low disturb to DME channel. But a certain mount of power is spent for the CCs and less power is available for data transmission, the signal-to-noise ratio (SNR) is decreased. When a larger amount of transmission power is available for the CCs a better suppression is achieved, but at the same time system performance further degrades. CCs and windowing methods can be used for sidelobe suppression in B-MAC system separately or combined, and better suppression effect can be achieved for the latter. 5.
Conclusion
How to avoid the mutual interference between B-AMC and exiting L-band system DME/TACAN has became one of the focus techniques in development of future broad band air-ground communication. Several exiting sidelobe suppression methods are analyzed. The suppression effect using CCs and windowing methods for B-MAC system are simulated in this paper. It has been shown rasied-cosine windowing can suppress the sidelobe more valid than other methods and more suitable to use in B-AMC system. Acknowledge This work was sponsored by the National Nature Science Foundation of Tianjin (09JCYBJC00600) and CAUC Special Foundation of Central University (ZXH2010B004).
Li Dong-xia and Ye Qian-wen / Physics Procedia 33 (2012) 361 – 367
References [1] Haindl B., Sajatovic M., Rihacek C., Prinz J., Schnell M., Haas E., and Cosovic I., "B-VHF –A Multi-Carrier Based Broadband VHF Communications Concept for Air Traffic Management," in Proc. Of 2005 IEEE Aerospace Conference, Big Sky, MT, USA,March 2005. [2] Rokitansky C.H., Ehammer M., Gräupl T., Schnell M., Brandes S., Gligorevic S., Rihacek C., and Sajatovic M., “B-AMC a system for future broadband aeronautical multi-carrier communications in the L-BAND”, Digital Avionics Systems Conference, 2007. DASC '07. IEEE/AIAA 26th. [3] Brandes S., Cosovic I.,and Schnell M., “Techniques for ensuring co-existence between B-VHF and legacy VHF systems”,Aerospace Conference, 2006 IEEE. [4] Brandes S., Cosovic I., and Schnell M., “Sidelobe suppression in OFDM systems by insertion of cancellation carriers,” in Proc. of IEEE 62nd Semiannual Vehicular Technology Conference(VTC Fall’05), Dallas, TX, USA, September 2005. [5] Cosovic I., Brandes S., and Schnell M., “A technique for sidelobe suppression in OFDM systems,” in Proc. of IEEE Global Telecommunications Conference(Globecom’05), St.Louis, MO, USA, November 2005.
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