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Low sidelobe pattern using Woo filter concepts
Int. J. Electron. Commun. (AEÜ) 59 (2005) 499 – 501 www.elsevier.de/aeue
LETTER
Low sidelobe pattern using Woo filter concepts Uttara M. Kumaria,∗ , K. Rajarajeswarib , Murali K. Krishnab a Department of Electronics and Communication Engineering, R.V. College of Engineering, Bangalore 560 058, India b Department of Electronics and Communication Engineering, Andhra University, Visakhapatnam 530 003, India
Received 19 May 2004; received in revised form 1 October 2004
Abstract This paper presents a new pulse compression technique that can reduce sidelobes significantly. The code used in this is p4, which belongs to the class of polyphase codes. This retains the merit of strong resistance to the Doppler shift effect. This processing technique, which is proposed in this paper, is useful to reduce peak sidelobe level (PSL) and integrated sidelobe level (ISL) but increases the mainlobe width. The results are compared with the Woo filter. 䉷 2004 Elsevier GmbH. All rights reserved. Keywords: Woo filter; Peak sidelobe level; Integrated sidelobe level; Pulse compression; Range resolution
1. Introduction Pulse compression allows radar to utilize a long pulse to achieve large radiated energy, but simultaneously to obtain the range resolution of a short pulse [1]. Range resolution is the ability of the receiver to detect the nearby targets. Low autocorrelation sidelobes are required to prevent the masking of weak target that occur in the range sidelobes of strong target. Sidelobe reduction techniques are developed to reduce the sidelobes. Lewis proposed sliding window twosample subtractor to reduce the sidelobes for the polyphase codes [2]. Woo and Griffiths developed a sidelobe canceller to reduce peak sidelobe level and integrated sidelobe level [3,4]. This paper presents a new technique to reduce the sidelobes significantly.
2. Merit measures For a general complex-valued sequence an code of length N, the aperiodic autocorrelation function (ACF), ∗ Corresponding author. Tel.: +91 80 23208292; fax: +91 80 28600750.
E-mail address: [email protected] (U.M. Kumari). 1434-8411/$ - see front matter 䉷 2004 Elsevier GmbH. All rights reserved. doi:10.1016/j.aeue.2004.11.045
for k = 0, is given by r[K] =
N−k−1 n=0
an an+k ,
(1)
where k = 0, 1, 2, . . . , N − 1. One of the most commonly used merit figures of aperiodic ACF is peak sidelobe level (PSL), which is the ratio of maximum ACF sidelobe power to ACF mainlobe power. Another important measure of aperiodic ACF is integrated sidelobe level (ISL), which is the ratio of the total power in ACF sidelobes to the power in ACF mainlobe. Well-known polyphase codes with better Doppler tolerance and low range sidelobes such as the step frequency derived Frank and p1 codes, the Butler matrix derived p2 code and linear frequency derived p3 and p4 codes were intensively analyzed by Lewis and Kretschmer in [5–7]. The significant advantage of p1 and p2 code over the Frank code and the p4 code over p3 code is that they are tolerant of received band limitations.
500
U.M. Kumari et al. / Int. J. Electron. Commun. (AEÜ) 59 (2005) 499 – 501
Fig. 1. A schematic diagram of sidelobe reduction using modified Woo filter form-I.
Fig. 2. A schematic diagram of sidelobe reduction using modified Woo filter form-II.
3. Woo filter and modified Woo filter
Table 1. PSL and ISL comparison for various pulse compression schemes
4. Results and conclusion The peak sidelobe level and integrated sidelobe level for Woo filter and for modified Woo filter of form-I and form-
PSL ISL PSL ISL
Code length (N)
p4 code (dB)
Woo filter (dB)
Modified Woo filter form-I (dB)
Modified Woo filter form-II (dB)
1000 1000 2000 2000
−36.36 −19.97 −39.37 −21.47
−57.83 −30.1 −63.89 −33.1
−88.57 −71.31 −97.65 −79.1
−88.67 −72.60 −97.71 −80.0
0 -20 -40 Filter Response in DB
Woo filter is a modified linear combination of matched filter for linear FM derived phase codes [3,4]. The two correlation filters 1 and 2 are combined together to produce a single discrete filter called Woo filter. 1 is the autocorrelation function of the original code and 2 is generated by correlating the original code with the conjugate signal of itself but shifted by one bit. The signal consists of 1000 sub pulses each having uniform length while their corresponding Woo filter has 999 elements. Uniform sidelobe patterns have been achieved throughout the entire range bins except at the beginning and ends of the sidelobe sequences. The modified version proposed in this paper is formulated by adopting post compression processing. This way Woo filter is restructured which is shown in Figs. 1 and 2. In form-I one bit shifted version of the code is combined with two bit shifted version and later the resulting signal is subjected to correlation. This structure is shown in Fig. 1 as form-I. In form I, one bit shifted p4 signal is transmitted and the same is considered as the reference signal. Two bit shifted p4 signal is considered as the received signal expecting one bit delay in transmitting and receiving process. The reference signal is added to the received signal and then correlated. In the form-II the signal is directly combined with one bit shifted version of itself and the resulting signal is subjected to correlation process. This structure is shown in Fig. 2 as form-II. In this form II, p4 signal is transmitted and the same is considered as the reference signal at the receiver. One bit shifted p4 signal is considered as the received signal expecting one bit delay in transmitting and receiving process. The reference signal is added to the received signal and then correlated. Here matched filter is used as the autocorrelator. In both the forms, PSL and ISL have shown significant improvement in reduction over Woo filter. We observed the same result for both form I and form II except that the sidelobes are more uniform in form I than in form II. PSL and ISL are reduced significantly and Doppler tolerance is improved but increases the mainlobe width. Mismatch loss and SNR loss are not effected by this new scheme.
-60 -80 -100 -120 -140 -160 -180 0
200
400
600
800 1000 1200 1400 1600 1800 2000 Sample Number
Fig. 3. Pulse compression output generated by modified Woo filter of form-I for p4 code of length 1000.
II for p4 code of length N = 1000 are shown in Table 1. The modified Woo filter of form-I and form-II responses are shown in Figs. 3 and 4. From the results it is observed that, PSL and ISL are reduced by significant amount in form-I and form-II. For length N =1000, around 32 dB of total PSL gain and 42 dB of total ISL gain is achieved by using modified Woo filter compared with Woo filter and 52 dB improvement of PSL and ISL over ordinary p4 code. Peak sidelobe level and integrated sidelobe level are equal for both the forms. Form-I exhibits the uniform sidelobe level whereas in form-II this property was not achieved. The proposed model
U.M. Kumari et al. / Int. J. Electron. Commun. (AEÜ) 59 (2005) 499 – 501
References
0 -20 -40 Filter Response in DB
501
-60 -80 -100 -120 -140 -160 -180 0
200
400
600
800 1000 1200 1400 1600 1800 2000 Sample Number
Fig. 4. Pulse compression output generated by modified Woo filter of form-II for p4 code of length 1000.
enhances the performance of pulse compression radar but at the sacrifice in range resolution.
[1] Skolnik M. Introduction to radar system. New York: McGrawHill; 1981. [2] Lewis B. Range sidelobe reduction technique for fm derived polyphase codes. IEEE Trans Aerospace Electron Syst 1995;29:834–40. [3] Lee W, Griffith H. A new pulse compression technique generating optimal uniform range side lobe and reducing integrated sidelobe level. IEEE International Radar Conference, 2001. p. 441–6. [4] Lee W, Griffith H. Pulse compression filters generating optimal uniform range sidelobe level. Electron Lett 1999;35:873–5. [5] Frank R. Polyphase codes with good nonperiodic correlation properties. IEEE Trans Inform Theory 1963;IT-9:43–5. [6] Lewis B, Kretschmer FF. A new class of polyphase pulse compression codes and techniques. IEEE Trans Aerospace Electron Syst 1981;AES-17:364–72. [7] Lewis B, Kretschmer F. Linear frequency modulation derived polyphase pulse compression codes. IEEE Trans Aerospace Electron Syst 1982;AES-18:637–41.
LETTER
Low sidelobe pattern using Woo filter concepts Uttara M. Kumaria,∗ , K. Rajarajeswarib , Murali K. Krishnab a Department of Electronics and Communication Engineering, R.V. College of Engineering, Bangalore 560 058, India b Department of Electronics and Communication Engineering, Andhra University, Visakhapatnam 530 003, India
Received 19 May 2004; received in revised form 1 October 2004
Abstract This paper presents a new pulse compression technique that can reduce sidelobes significantly. The code used in this is p4, which belongs to the class of polyphase codes. This retains the merit of strong resistance to the Doppler shift effect. This processing technique, which is proposed in this paper, is useful to reduce peak sidelobe level (PSL) and integrated sidelobe level (ISL) but increases the mainlobe width. The results are compared with the Woo filter. 䉷 2004 Elsevier GmbH. All rights reserved. Keywords: Woo filter; Peak sidelobe level; Integrated sidelobe level; Pulse compression; Range resolution
1. Introduction Pulse compression allows radar to utilize a long pulse to achieve large radiated energy, but simultaneously to obtain the range resolution of a short pulse [1]. Range resolution is the ability of the receiver to detect the nearby targets. Low autocorrelation sidelobes are required to prevent the masking of weak target that occur in the range sidelobes of strong target. Sidelobe reduction techniques are developed to reduce the sidelobes. Lewis proposed sliding window twosample subtractor to reduce the sidelobes for the polyphase codes [2]. Woo and Griffiths developed a sidelobe canceller to reduce peak sidelobe level and integrated sidelobe level [3,4]. This paper presents a new technique to reduce the sidelobes significantly.
2. Merit measures For a general complex-valued sequence an code of length N, the aperiodic autocorrelation function (ACF), ∗ Corresponding author. Tel.: +91 80 23208292; fax: +91 80 28600750.
E-mail address: [email protected] (U.M. Kumari). 1434-8411/$ - see front matter 䉷 2004 Elsevier GmbH. All rights reserved. doi:10.1016/j.aeue.2004.11.045
for k = 0, is given by r[K] =
N−k−1 n=0
an an+k ,
(1)
where k = 0, 1, 2, . . . , N − 1. One of the most commonly used merit figures of aperiodic ACF is peak sidelobe level (PSL), which is the ratio of maximum ACF sidelobe power to ACF mainlobe power. Another important measure of aperiodic ACF is integrated sidelobe level (ISL), which is the ratio of the total power in ACF sidelobes to the power in ACF mainlobe. Well-known polyphase codes with better Doppler tolerance and low range sidelobes such as the step frequency derived Frank and p1 codes, the Butler matrix derived p2 code and linear frequency derived p3 and p4 codes were intensively analyzed by Lewis and Kretschmer in [5–7]. The significant advantage of p1 and p2 code over the Frank code and the p4 code over p3 code is that they are tolerant of received band limitations.
500
U.M. Kumari et al. / Int. J. Electron. Commun. (AEÜ) 59 (2005) 499 – 501
Fig. 1. A schematic diagram of sidelobe reduction using modified Woo filter form-I.
Fig. 2. A schematic diagram of sidelobe reduction using modified Woo filter form-II.
3. Woo filter and modified Woo filter
Table 1. PSL and ISL comparison for various pulse compression schemes
4. Results and conclusion The peak sidelobe level and integrated sidelobe level for Woo filter and for modified Woo filter of form-I and form-
PSL ISL PSL ISL
Code length (N)
p4 code (dB)
Woo filter (dB)
Modified Woo filter form-I (dB)
Modified Woo filter form-II (dB)
1000 1000 2000 2000
−36.36 −19.97 −39.37 −21.47
−57.83 −30.1 −63.89 −33.1
−88.57 −71.31 −97.65 −79.1
−88.67 −72.60 −97.71 −80.0
0 -20 -40 Filter Response in DB
Woo filter is a modified linear combination of matched filter for linear FM derived phase codes [3,4]. The two correlation filters 1 and 2 are combined together to produce a single discrete filter called Woo filter. 1 is the autocorrelation function of the original code and 2 is generated by correlating the original code with the conjugate signal of itself but shifted by one bit. The signal consists of 1000 sub pulses each having uniform length while their corresponding Woo filter has 999 elements. Uniform sidelobe patterns have been achieved throughout the entire range bins except at the beginning and ends of the sidelobe sequences. The modified version proposed in this paper is formulated by adopting post compression processing. This way Woo filter is restructured which is shown in Figs. 1 and 2. In form-I one bit shifted version of the code is combined with two bit shifted version and later the resulting signal is subjected to correlation. This structure is shown in Fig. 1 as form-I. In form I, one bit shifted p4 signal is transmitted and the same is considered as the reference signal. Two bit shifted p4 signal is considered as the received signal expecting one bit delay in transmitting and receiving process. The reference signal is added to the received signal and then correlated. In the form-II the signal is directly combined with one bit shifted version of itself and the resulting signal is subjected to correlation process. This structure is shown in Fig. 2 as form-II. In this form II, p4 signal is transmitted and the same is considered as the reference signal at the receiver. One bit shifted p4 signal is considered as the received signal expecting one bit delay in transmitting and receiving process. The reference signal is added to the received signal and then correlated. Here matched filter is used as the autocorrelator. In both the forms, PSL and ISL have shown significant improvement in reduction over Woo filter. We observed the same result for both form I and form II except that the sidelobes are more uniform in form I than in form II. PSL and ISL are reduced significantly and Doppler tolerance is improved but increases the mainlobe width. Mismatch loss and SNR loss are not effected by this new scheme.
-60 -80 -100 -120 -140 -160 -180 0
200
400
600
800 1000 1200 1400 1600 1800 2000 Sample Number
Fig. 3. Pulse compression output generated by modified Woo filter of form-I for p4 code of length 1000.
II for p4 code of length N = 1000 are shown in Table 1. The modified Woo filter of form-I and form-II responses are shown in Figs. 3 and 4. From the results it is observed that, PSL and ISL are reduced by significant amount in form-I and form-II. For length N =1000, around 32 dB of total PSL gain and 42 dB of total ISL gain is achieved by using modified Woo filter compared with Woo filter and 52 dB improvement of PSL and ISL over ordinary p4 code. Peak sidelobe level and integrated sidelobe level are equal for both the forms. Form-I exhibits the uniform sidelobe level whereas in form-II this property was not achieved. The proposed model
U.M. Kumari et al. / Int. J. Electron. Commun. (AEÜ) 59 (2005) 499 – 501
References
0 -20 -40 Filter Response in DB
501
-60 -80 -100 -120 -140 -160 -180 0
200
400
600
800 1000 1200 1400 1600 1800 2000 Sample Number
Fig. 4. Pulse compression output generated by modified Woo filter of form-II for p4 code of length 1000.
enhances the performance of pulse compression radar but at the sacrifice in range resolution.
[1] Skolnik M. Introduction to radar system. New York: McGrawHill; 1981. [2] Lewis B. Range sidelobe reduction technique for fm derived polyphase codes. IEEE Trans Aerospace Electron Syst 1995;29:834–40. [3] Lee W, Griffith H. A new pulse compression technique generating optimal uniform range side lobe and reducing integrated sidelobe level. IEEE International Radar Conference, 2001. p. 441–6. [4] Lee W, Griffith H. Pulse compression filters generating optimal uniform range sidelobe level. Electron Lett 1999;35:873–5. [5] Frank R. Polyphase codes with good nonperiodic correlation properties. IEEE Trans Inform Theory 1963;IT-9:43–5. [6] Lewis B, Kretschmer FF. A new class of polyphase pulse compression codes and techniques. IEEE Trans Aerospace Electron Syst 1981;AES-17:364–72. [7] Lewis B, Kretschmer F. Linear frequency modulation derived polyphase pulse compression codes. IEEE Trans Aerospace Electron Syst 1982;AES-18:637–41.