GB2561226A

GB2561226A - Method and apparatus for managing pulse radar compression settings

Info

Publication number: GB2561226A
Application number: GB1705573.2A
Authority: GB
Inventors: Thoumy François; Achir Mounir
Original assignee: Canon Inc
Current assignee: Canon Inc
Priority date: 2017-04-06
Filing date: 2017-04-06
Publication date: 2018-10-10
Anticipated expiration: 2037-04-06
Also published as: GB2561226B; GB201705573D0

Abstract

The present invention concerns a pulse radar system comprising a transmitter configured to obtain an opposed sequence of first, second, third and fourth codes 8.1,8.2, the opposed sequence of codes being composed of two pairs of complementary codes. codes of the two pairs having different bit lengths, codes of a same pair having a same bit length, the first and second codes belonging to one of the two pairs and the third and fourth codes belonging to the other pair, wherein sidelobes of a signal obtained by cross correlation of a transmitted signal encoded using the second code and a reflected signal resulting from reflexion of the transmitted signal, and sidelobes of the signal obtained by cross correlation of a another transmitted signal encoded using the fourth code and another reflected signal resulting from reflexion of the another transmitted signal have opposed polarities: generate a modulation signal resulting from modulation of pulses encoded with the opposed sequence of codes; and transmit the modulation signal.

Description

(54) Title of the Invention: Method and apparatus for managing pulse radar compression settings Abstract Title: Method and apparatus for managing pulse radar compression settings (57) The present invention concerns a pulse radar system comprising a transmitter configured to obtain an opposed sequence of first, second, third and fourth codes 8.1,8.2, the opposed sequence of codes being composed of two pairs of complementary codes, codes of the two pairs having different bit lengths, codes of a same pair having a same bit length, the first and second codes belonging to one of the two pairs and the third and fourth codes belonging to the other pair, wherein sidelobes of a signal obtained by cross correlation of a transmitted signal encoded using the second code and a reflected signal resulting from reflexion of the transmitted signal, and sidelobes of the signal obtained by cross correlation of a another transmitted signal encoded using the fourth code and another reflected signal resulting from reflexion of the another transmitted signal have opposed polarities: generate a modulation signal resulting from modulation of pulses encoded with the opposed sequence of codes; and transmit the modulation signal.

8.2

8.3

8.4

8.5

8.6

Fig.8

1/14

2/14

Fig. 2c (Prior art)

3/14

-►-►-► τ τ τ

-►-►-►

Τ+Δ Τ+2Δ Τ+3Δ

Fig. 3

4/14

X1 Τχ

X1 Rx

X2Tx

X2 Rx

X3Tx

X3 Rx

X4Tx

X4 Rx

Fig. 4

5/14

SR sequences

LR sequences

SR seq. LR seq.

K

5.1

5.2

Fig. 5

6.1

JL

6.2

JL·

6.3

JL

6.4

6.5

6.6

Fig. 6

6/14 autocorrelation output

3.5

2.5

1.5

0.5

-0.5

7.1

7.2 δ

time code 10 code 15

Fig. 7a

Ύ........^:....................¹.....................^;.....................^f....................’..................

i ......Θ......code 140 i ( j ( .......ΐ........code 065 ((

Q.

4i

X.

o □

S3

0·

-2i

7.3

Yk.

7.4 time

Fig. 7b

7/14

8.1

8.2

8.3

8.4

8.5

8.6

Fig. 8

8/14

N1 bits codes

N2 bits codes

9.1

XI transmitted XI received

X2 transmitted X2 received

X3 transmitted X3 received

X4 transmitted X4 received

Fig. 9

9/14

Q factor in dB

Fig. 10

10/14

Fig. 11a

SNR in dB

Fig. 11b

11/14

Fig. 12

12/14

Fig. 13

13/14

SNR	10	12	14	16	18	20	22	24	26
Nb acq=50	Seq 1	Seq 1	Seq 1	Seq 1	Seq 1	Seq 2	Seq 2	Seq 2	Seq 2
Nb acq=20 0	Seq 1	Seq 1	Seq 2	Seq 2	Seq 2	Seq 2	Seq 2	Seq 2	Seq 2

Fig. 14a

SNR		10	12	14	16	18	20	22	24	26
Nb acq=50	Seq 1	21,39	21,93	22,36	22,69	22,94	23,13	23,26	23,35	23,41
Seq 2	18,95	19,91	20,77	21,65	22,41	23,10	23,73	24,24	24,65
Nb acq=200	Seq 1	22,70	22,95	23,13	23,26	23,35	23,41	23,45	23,47	23,49
Seq 2	21,68	22,43	23,10	23,73	24,22	24,66	24,97	25,21	25,39

Fig. 14b

14/14

15.1

15.2

15.3

15.4

15.5

15.6

15.0

Fig. 15

METHOD AND APPARATUS FOR MANAGING PULSE RADAR COMPRESSION SETTINGS

The present disclosure concerns a method and a device for managing pulse radar compression settings. It concerns more particularly the reduction of side lobes effects in presence of mobile targets.

Pulse radars work by emitting a signal composed of regular pulses at a given frequency. The pulse is typically coded and modulated to allow long pulses while preserving a bandwidth corresponding to a short pulse, knowing that a short pulse is favourable to get a lower range resolution value, and then a better range resolution.

When such a coded pulse is reflected by a target and this echo is received back by the radar, it is processed so as to obtain a compressed signal which exhibits a main lobe whose position on the distance axis corresponds to the distance of the target (hereafter named “range”). But the compressed signal also exhibits secondary lobes of lower amplitude, known as side lobes, from either side of the main lobe on the distance axis. These side lobes may not be a problem in presence of a single target as the main lobe may be easily identified. However, in presence of a second target, especially if the second target has a lower radar cross section (RCS) compared with the first target, this second target may be difficult to detect. The second target will generate its own main lobe with its side lobes. As the second target has a lower RCS, its main lobe has a lower amplitude than the one of the main lobe of the first target, and may be mixed up with the side lobes of the first target when both targets are closed. We recall here that the radar cross section (RCS) is a measure of how detectable a target is with radar. A larger RCS indicates that a target is easier to detect, it generates a stronger echo when hit by the radar pulse. The smallest distance between two targets that may be discriminated by the radar is called the “range resolution” of the radar.

A known technique to litigate the side lobes effect is the use of complementary codes, as for example Golay or Spano codes. Using this technique, successive pulses are encoded with complementary codes. The corresponding received signals are processed with their related matched filters. These complementary codes have the effect of generating compressed signals at the output of the matched filters with the same main lobe and polarityinversed side lobes. By summing these compressed signals, the main lobe is strengthened while side lobes are cancelled, and a processed signal comprising processed echoes is obtained. It becomes easy to detect the main lobes corresponding to several targets, even if these targets are close and with different RCS. At least this is true for static targets.

In order to quantify the performance of a code in term of side lobe reduction, a quality factor named “Q factor” may be computed as the ratio of the amplitude of main peak to the sum of the amplitudes of all side lobes.

Considering a mobile target, successive echoes related to a same target exhibit a phase shift due to the Doppler effect. This phase shift alters the complementarity of the side lobes of the compressed signals. This results in a degradation of the side lobes cancellation. The higher the phase shift, the higher is the degradation of the side lobes cancellation. The phase shift is known to increase with the velocity of the target and with the time interval between two consecutive pulses of the transmitted pulse signal. This interval is known as the PRI (Pulse Repetition Interval). Therefore the lower is the PRI, the better is the side lobes cancellation.

Using a pulse compression technics creates a blind zone for the radar: depending of the length/duration of the transmitted pulse and the range of a target, a target close to the radar may reflect the transmitted pulse before the end of the transmitted pulse which will prevent the correct detection of the target. The blind range for a radar system is the distance occupied by the transmit pulse and the setup time for the receiver.

In real conditions, the complementarity of the codes is disrupted by the noise and the Q factor achieved is dependent not only upon the code sequence but also upon the noise level. The code sequence which gives the higher Q factor varies with the SNR (Signal to Noise Ratio).

In order to decrease the noise in the signal before to process it for the target detection, a filtering function is usually applied: each code sequence is transmitted many times and the received signals are filtered, for instance averaged, to generate a “low noise received signal” which is further processed for the target detection.

In the following of this text, the reception of a signal encoded with a code sequence and reflected on a target is named “acquisition”.

Then, a change of the number of acquisitions modifies the level of noise at the input of the target detection process and has an impact on the relationship between the Q factor and the code sequence. However, increasing the number of acquisition filtered elevates the latency of the detection. Then, the number of acquisitions should be limited as much as possible to reduce the latency of the detection.

The invention proposes to improve the sidelobes cancellation and then to improve the detection of close targets with significantly different RCS when the targets are moving.

A first aspect concerns the generation of a new code sequence that provides an improved quality factor, by combining sequences assigned to short and long range detections. In some embodiments, applying a correction factor in the radar signal processing enables to improve again the reduction of the sidelobes, therefore increasing the Q factor.

A second aspect of the invention is to select and apply a coded pulse sequence, depending on a measured SNR value, and a look-up table associating for each SNR value, the coded pulse sequence providing the best Q factor.

According to a first aspect of the invention there is provided a pulse radar system comprising a transmitter configured to:

- obtain an opposed sequence of first, second, third and fourth codes, the opposed sequence of codes being composed of two pairs of complementary codes, codes of the two pairs having different bit lengths, codes of a same pair having a same bit length, the first and second codes belonging to one of the two pairs and the third and fourth codes belonging to the other pair, wherein sidelobes of a signal obtained by cross correlation of a transmitted signal encoded using the second code and a reflected signal resulting from reflexion of the transmitted signal, and sidelobes of the signal obtained by cross correlation of a another transmitted signal encoded using the fourth code and another reflected signal resulting from reflexion of the another transmitted signal have opposed polarities;

- generate a modulation signal resulting from modulation of pulses encoded with the opposed sequence of codes;

- transmit the modulation signal.

According to an embodiment, the system further comprises a receiver configured to:

- receive a reflected signal resulting from transmission of the modulation signal;

- applying a cross correlation function to the transmitted signal and the corresponding received signal, both related to pulses encoded with the same code, for each code of the sequence;

- adding the results of the cross-correlation functions for the first and second codes;

- adding the results of the cross-correlation functions for the third and the fourth codes;

- obtain a signal to be used for targets detection by the pulse radar system by adding the results of the two additions.

According to an embodiment, the transmitter or the receiver is further configured to:

- apply a correction factor to obtain even amplitude of sidelobes for signals associated with the two codes having different bit lengths.

According to an embodiment, the receiver is further configured to:

- detect long range target using the obtained signal; and

- detect short range target using a second signal obtained by the addition of the results of the cross-correlation functions for the shortest codes.

According to another aspect of the invention there is provided a pulse radar system comprising:

a transmitter configured to:

- obtain a plurality of sequences of codes based on complementary codes;

- generate a modulation signal resulting from modulation of pulses encoded with one sequence of the plurality;

- transmit the modulation signal; and a receiver configured to:

- determine a signal to noise ratio of the received signal;

- select the sequence among the plurality giving a best quality factor for the determined signal to noise ratio;

- configure the transmitter to use the selected sequence.

According to an embodiment:

- the selection of the sequence among the plurality giving the best quality factor for the determined signal to noise ratio is made based on curves giving for each sequence the quality factor based on the level of signal to noise ratio.

According to an embodiment:

- the selection of the sequence among the plurality giving the best quality factor for the determined signal to noise ratio is made based on a look-up table giving for each sequence the quality factor based on the level of signal to noise ratio.

According to an embodiment:

- receiving a reflected signal resulting from transmission of the modulation signal comprises realising a number of acquisitions of the reflected signal, the number of acquisitions being initialized to an initial number of acquisitions; and the receiver is further configured to:

- determine if the quality factor provided by the selected sequence is higher than a predetermined quality threshold; and if not:

- increase the number of acquisitions until:

o the quality factor provided by the selected sequence is equal or higher than the predetermined quality threshold; or o a maximum number of acquisitions has been reached.

According to an embodiment:

- at least some of the sequences of the plurality of sequences are opposed sequence of codes, an opposed sequence of codes being composed of two pairs of complementary codes, codes of the two pairs having different bit lengths, codes of a same pair having a same bit length, the first and second codes belonging to one of the two pairs and the third and fourth codes belonging to the other pair, wherein sidelobes of a signal obtained by cross correlation of a transmitted signal encoded using the second code and a reflected signal resulting from reflexion of the transmitted signal, and sidelobes of the signal obtained by cross correlation of a another transmitted signal encoded using the fourth code and another reflected signal resulting from reflexion of the another transmitted signal have opposed polarities.

According to another aspect of the invention there is provided a method to generate an opposed sequence of codes to be used in a pulse radar system comprising:

- obtaining a first pair of complementary codes;

- obtaining a second pair of complementary codes, the bit length of both codes being different;

- generating the opposed sequence of codes by ordering both pairs of code, sidelobes of a signal obtained by cross correlation of a transmitted signal encoded using the second code and a reflected signal resulting from reflexion of the transmitted signal, and sidelobes of the signal obtained by cross correlation of a another transmitted signal encoded using the fourth code and another reflected signal resulting from reflexion of the another transmitted signal have opposed polarities.

According to another aspect of the invention there is provided a transmission method by a transmitter in a pulse radar system comprising:

- obtaining an opposed sequence of codes, the opposed sequence of codes being composed of two pairs of complementary codes, codes of the two pairs having different bit lengths, codes of a same pair having a same bit length, the first and second codes belonging to one of the two pairs and the third and fourth codes belonging to the other pair, wherein sidelobes of a signal obtained by cross correlation of a transmitted signal encoded using the second code and a reflected signal resulting from reflexion of the transmitted signal, and sidelobes of the signal obtained by cross correlation of a another transmitted signal encoded using the fourth code and another reflected signal resulting from reflexion of the another transmitted signal have opposed polarities;

- generating a modulation signal resulting from modulation of pulses encoded with the opposed sequence of codes;

- transmitting the modulation signal.

According to another aspect of the invention there is provided a transmission method in a pulse radar system comprising:

by a transmitter:

- obtaining a plurality of sequences of codes based on complementary codes;

- generating a modulation signal resulting from modulation of pulses encoded with one sequence of the plurality;

- transmitting the modulation signal; and by a receiver:

- receiving a reflected signal resulting from the transmission of the modulation signal;

- determining a signal to noise ratio of the received signal;

- selecting the sequence among the plurality giving the best quality factor for the determined signal to noise ratio;

- configuring the transmitter to use the selected sequence.

According to an embodiment, one of the sequences obtained being an opposed sequence generated according to the method of the invention.

According to an embodiment, the pulse radar system according to the invention is further configured to generate an opposed sequence of codes according to the method of the invention.

According to another aspect of the invention there is provided a computer program product for a programmable apparatus, the computer program product comprising a sequence of instructions for implementing a method according to the invention, when loaded into and executed by the programmable apparatus.

According to another aspect of the invention there is provided a computer-readable storage medium storing instructions of a computer program for implementing a method according to the invention.

At least parts of the methods according to the invention may be computer implemented. Accordingly, the present invention may take the form of an entirely hardware embodiment, an entirely software embodiment (including firmware, resident software, micro-code, etc.) or an embodiment combining software and hardware aspects that may all generally be referred to herein as a circuit, module or system. Furthermore, the present invention may take the form of a computer program product embodied in any tangible medium of expression having computer usable program code embodied in the medium.

Since the present invention can be implemented in software, the present invention can be embodied as computer readable code for provision to a programmable apparatus on any suitable carrier medium. A tangible, nontransitory carrier medium may comprise a storage medium such as a floppy disk, a CD-ROM, a hard disk drive, a magnetic tape device or a solid state memory device and the like. A transient carrier medium may include a signal such as an electrical signal, an electronic signal, an optical signal, an acoustic signal, a magnetic signal or an electromagnetic signal, e.g. a microwave or RF signal.

Embodiments of the invention will now be described, by way of example only, and with reference to the following drawings in which:

Figure 1 illustrates the output of the matched filter in case of use of a Barker code and in case of presence of two close targets having different RCS;

Figures 2a, 2b and 2c illustrate the principle of the complementary codes;

Figure 3 illustrates the transmission and reception of a sequence of four codes;

Figure 4 illustrates the signal processing applied at the receiver side of the radar;

Figure 5 illustrates an example of an arrangement of two sequences of codes that may be used to combine short range detection and long range detection;

Figure 6 illustrates an example of arrangement of two sequences of codes according to an embodiment of the invention;

Figure 7a and 7b illustrate two examples of the outputs of the autocorrelation functions applied on each code used in the sequences;

Figure 8 illustrates the flowchart describing a method to determine a new sequence of codes according to an embodiment of the invention;

Figure 9 illustrates the signal processing applied at the receiver side of the radar in an embodiment of the invention;

Figure 10 illustrates an example of the curves giving the quality factor as a function of the phase shift achieved with various code sequences;

Figure 11a and lib illustrate two examples of curves giving the quality factor as a function of the SNR for a given code sequence and for a given phase shift;

Figure 12 illustrates a flowchart describing the selection of a sequence with regard to the SNR according to an embodiment of the invention;

Figure 13 illustrates a flowchart describing another embodiment of the invention;

Figure 14a and 14b illustrate examples of look up table which may be used;

Figure 15 is a schematic block diagram of a computing device for implementation of one or more embodiments of the invention.

When applying pulse compression techniques using coded pulses, the received signal is processed by a matched filter which applies a cross correlation between the received signal and the transmitted one in order to output a compressed signal. When a single target is in the detection field, the matched filter output a compressed signal with a main lobe, which position of the highest value on the distance axis corresponds to the range of the target, and other lobes, having lower energy, surrounding the main lobe. By analogy with antenna radiation pattern, the lobes around the main lobe are named “side lobes”.

It should be noted that a radar system comprises a transmitter and a receiver. In some embodiments, the transmitter and the receiver are provided within a single device, while in some other embodiments, the transmitter and the receiver may be provided as two separate devices.

Figure 1 illustrates the output of the matched filter in case of use of a Barker code and in case of presence of two close targets having different RCS. Barker code is a typical known code used in pulse radar for compression of pulses.

Figure 1 illustrates the issue encountered to detect a second target having a low RCS and close to a first target having a high RCS. In the matched filter adapted to the Barker code, the main lobe 1.1 related to the signal of the second target (having the lowest RCS) is combined with the side lobes of the signal of the first target (having the highest RCS). Then, depending of the value and the position of the main lobe corresponding to the second target with regard to the side lobes of the signal of the first target, the main lobe may not be visible. Therefore the detection of the second target which has the lowest RCS may fail. In the illustrated case, the main lobe related to the second target is visible but its value is very close to some surrounding side lobe values and its detection may be difficult. It is to be noted that Figure 1 is a simulation. In real conditions, noise may be present, thus increasing the difficulty of detection.

When using the complementary codes technique to cancel side lobes, several pulses are successively sent, each pulse being encoded with a specific code.

At the reception side, the matched filters are applied on the received signals, and the outputs of the matched filters are summed to obtain a processed signal. Thanks to the property of the complementary codes, in ideal case, the side lobes related to the pulses cancel each other out: the amplitude of the main lobe is doubled while the amplitudes of the side lobes are nulled.

Figures 2a, 2b and 2c illustrate the principle of the complementary codes. It shows the outputs of the matched filters in case of use of a Golay complementary sequence and the presence of two close static targets having different RCS. It illustrates how the use of complementary sequences like Golay pairs can solve the issue illustrated by Figure 1 when the targets are statics.

Here, two Golay codes of length 4 are used as an example. The Golay code A is [1 1 1 0] and the Golay code B is [1 1 0 1],

Figure 2a illustrates a first compressed signal on output of a first matched filter, resulting from transmission of a pulse coded with code A. The signal corresponds to the detection of two targets, where the main target corresponding to main lobe 2.1 has a RCS of 1, while the second target corresponding to main lobe 2.2 has a RCS of 0.05. The main target is at a distance of 20 meters from the radar, while the second target is at a distance of 22 meters. The pulse has a frequency of 4 GHz and the bandwidth is 0.5 GHz.

Main lobe 2.1 has an amplitude of 32, main lobe 2.2 of 3 and the side lobe 2.3 has an amplitude of 7. The horizontal axis represents the range in meters, and the vertical axis represents the amplitude of the compressed signal. Alternatively, the horizontal axis may represent time (Figures 7a and 7b).

Figure 2b illustrates a second compressed signal on output of a second matched filter, resulting from transmission of a pulse coded with code B. The elements having the same reference as in Figure 2a are similar to the corresponding elements of this figure. The main lobes corresponding to both targets are still present. The side lobe 2.4 has an amplitude of 7. For each target, at a given range (e.g., 20.5), side lobes (e.g., 2.3 and 2.4) have the same amplitude (e.g., 7) but complementary polarities (e.g., one is positive and the other one is negative).

Figure 2c illustrates the processed signal obtained after the summation of the first and second compressed signals output by the matched filters, namely the summation of Figure 2a and Figure 2b. Thanks to the property of the complementary codes, in ideal case, the part of the signals due to the side lobes generated by the filters in response to reception of the signal reflected on the targets cancel each other out during the summation. Main lobe 2.1 due to the first target is doubled (e.g., 32*2=64). The main peak 2.2 due to the second target having the lowest RCS becomes visible although its low value due to the low RCS of that target.

Considering a moving target, the change of position of the target degrades the side lobes reduction or cancellation. When the target is moving, the phase of the reflected signals varies between a pulse and the following one. Thus, the signals output by the matched filters are no more complementary for the side lobes and the cancellation of the side lobes is not completed.

Knowing the velocity of the target v, the time interval between the complementary pulses PRI and the carrier frequency f, the value a of the phase shift between two consecutive echoes related to a same target can be computed as:

2.n.2.v.f.PRI a =c c being the light speed.

In order to simplify the demonstration, we consider now the Golay code pairs of length 4 bits used in Figures 2a, 2b and 2c.

When applying pulse compression techniques using coded pulses, the received signal is processed by a matched filter which applies a cross correlation between the received signal and the transmitted one. Here, two cross correlations are applied, and the outputs of the correlators are summed.

The cross correlation of code A is given by:

xcorr(A) = [+4,+1,0,-1]

Considering a null phase shift as code A is taken as the reference.

The cross correlation of code B is given by:

xcorr(B) = [+4, — 1,0, +1]. e^ia

Considering the phase shift a between the two echoes.

When summing the two codes, the cross correlation becomes: xcorr(code) = [4. (l + e^La), 1 — e^{i a}, 0,-1+ e^{i a}]

It is to be noted that the side lobes are no more null and that they depend on the value of the phase shift a. It is noted that for a static target, the phase shift a is null and the sum gives:

xcorr(code) = [8,0,0,0]

with a perfect cancellation of the side lobes.

In order to quantify the performance of the code in term of side lobe reduction, a quality factor named “Q factor” may be computed as the ratio of the amplitude of main peak to the sum of the amplitudes of all side lobes. Other methods which quantify the difference between the main lobe amplitude versus at least one of the side lobe amplitudes may also be considered to evaluate the Q factor. In logarithmic form, the quality factor is defined by:

Q = 10.log₁₀ abs(main lobe)

Zj abs(side_lobej) j being the index of the side lobes.

In case of the Golay code pairs of length 4 bits with a moving target, the quality factor becomes:

Q = 10.log₁₀

—e^ia|² + |-1 + e^ia|²

4.(1 + e^ia)

The reduction of sidelobes may be improved by using special codes sequences. When the targets move, the phase of the pulses reflected by the targets increases with the time. Therefore, the sums 1 - e^ja and -1 + e^ja are no longer null which lead to sidelobes generation.

However, selecting particular codes sequences allow reducing the sidelobes. These sequences may be defined by approximating an exponential function at first order. This is illustrated by Figure 3 and 4.

Figure 3 illustrates the transmission and the reception of a sequence of four codes X1, X2, X3 and X4 with a PRI of T. These four codes are assumed to have the same length here. If the target is moving, the delay between the reception of two successive pulses increases. For example, X2 is received at a time T + Δ after the beginning of the reception of X1, X3 is received at a time

T + 2Δ after the reception of X2, X4 is received at a time T + 3Δ after the reception of X3. Δ being the extra time due to the increase of the distance between the radar and the target, that increase of distance being proportional to the speed of the target v (considered as constant at the time scale of the radar pulse) and the time interval between two PRI: Δ =2*v*T.

Figure 4 illustrates the signal processing applied at the receiver side of the radar. Each received code is correlated with the corresponding transmitted one. The cross correlation of X1 is summed with the cross correlation of X2. The cross correlation of X3 is summed with the cross correlation of X4. These two sums are summed to give the output S.

S = ^X_k. = + X₂. e^ja + X3. el^2a + X4. eJ-^3a k=l

At the outputs of the cross correlators, the sidelobes are no longer complementary, the resulting sum has non-null sidelobes.

Assuming that a is low, then: e^jka -1+j.k.a

S = XI+ X2+X3+X4 + j. a. (X2 + 2.X3 + 3.X4)

If X1, X2 are complementary codes and X3, X4 are complementary codes:

XI + X2 = Cl.<5(.); X3 + X4 = C2.<5(.) = (Cl + C2). <5(. j+j.a. (X2 + 2. C2. <5(.) + X4) = C4. δ(.) +

C5. j. a. (X2 + X4)

Where :

C1 to C5 are positive integers and <5(.) is the Dirac function.

If X2 and X4 are complementary codes, <5(.) is a Dirac function leading to a cancelation of the sidelobes. Then selecting X2 as complementary code of

X1, X4 as complementary code of X3 and X4 as complementary code of X2 enables the contention of sidelobes.

It is to be noted that the above demonstration approximates e^{J ka} at the first degree, then in real world the contention of the sidelobes is not complete but their levels decrease strongly. Also, when a increases, the approximation is less correct and the sidelobes increase as well.

Therefore using Spano or Pezeshki code sequences (rather than Golay code), which are based on the above calculus, allows the reduction of sidelobes for a given target speed, meaning obtaining a higher Q factor with regard to those achieved with classical complementary codes like Golay codes. In other words, using Spano or Pezeshki code sequences instead of classical complementary codes, the same level of the sidelobes (given Q factor) are achieved for a higher speed of the target. Spano sequences have been described in “Complementary Sequences with High Sidelobe Suppression Factors for ST/MST Radar Applications”, E. Spano and 0. Ghebrebrhan, IEEE Trans. On Geoscience and Remote Sensing, 1996. Pezeshki sequences have been described in “Doppler Resilient Waveforms with Perfect Autocorrelation”, Ali Pezeshki et Al, IEEE Transaction on Information Theory 2007.

Using a pulse compression technics creates a blind zone for the radar. Depending of the length/duration of the transmitted pulse and the range of a target, a target close to the radar may reflect the transmitted pulse before the end its transmission which will prevent the correct detection of the target. The blind range for a radar system is the minimum distance which the target must have (relatively to the radar) to be detected. It may correspond to the distance related to the duration of a pulse and optionally to the setup time necessary for the radar to switch from transmission to reception.

Then, considering a null setup time for the receiver and a coded pulse, the blind zone can be computed as follows BZ=0,5*c*Tsp*code_length with c the speed of light, Tsp the sub-pulse duration and codejength the length of the code in number of sub pulses.

Considering a bandwidth of 1GHz, the sub pulse duration is Tsp=1ns; then, for 8bits code the blind zone goes up to 1,2m and for 32bits code the blind zone goes up to 4,8m.

As a result of, for applications requiring detection of targets both at short and long distances, it is required to use two different codes: one short code which enables to get very short blind zone but which has limited range detection capacity and one longer code which enables higher range detection but which has a higher blind zone.

Figure 5 illustrates an example of an arrangement of two sequences of codes that may be used to combine short range detection and long range detection.

The transmitter sends periodically sequences adapted to short range (SR) detection and sequences of codes adapted to long range (LR) detection.

The sequences 5.1 related to short range detection are composed of a pair of complementary codes which are alternatively used to code the pulses to transmit, the length of the complementary codes being short, for instance 4 or 8 bits, to reduce the blind zone of the radar. The time interval between successive pulses is named “pulse repetition interval” PRI.

The sequences 5.2 related to long range detection are also composed of a pair of complementary codes, the length of the complementary codes being long enough, for instance 32 bits, to be able to detect targets at the specified maximum range.

The number of sequences, meaning the number of repetition of the complementary pairs, to apply in each mode of detection of targets may be different. In an example, 4608 repetitions are used for short range sequences and 18432 for long range sequences. The process of detection in each mode is done by considering solely the sequences related to the considered mode, here short or long mode. It is to be noted that the detections of targets in each range area are completely independent and each detection process doesn’t take advantage of the other process.

Figure 6 illustrates an example of arrangement of two sequences of codes according to an embodiment ofthe invention.

Here the sequences adapted to short range detection and the sequences of codes adapted to long range detection are interlaced before transmission.

The transmitted signal can be considered as a repetition of a new sequence which is composed of pulses encoded with a pair of complementary codes related to the short range detection (codes A, B) followed by pulses encoded with a pair of complementary codes related to the long range detection (codes C,D). Transmission of each pulse is separated by a pulse repetition interval (PRI).

In the radar receiver part, a processor extracts the pairs 6.4, 6.5 and 6.6 of complementary codes related to the short range detection to detect the targets in the short range. The detection of the targets in the long range is done by considering all the sequences 6.1, 6.2 and 6.3 of complementary codes.

Using both the short range sequences and the long range sequences for the long range detection, under additional constraints regarding complementarity between the short and long range sequences, allows improving the sidelobes reduction. This reduction is further improved by applying a correction factor to the short range received sequences in order to even the amplitude level of the short range sidelobes, meaning the sidelobes of the signal obtained by the cross correlation of the short range codes, and the amplitude level of the long range sidelobes meaning the sidelobes of the signal obtained by the cross correlation of the long range codes. Determining thoroughly a new code sequence by combining the sequences assigned to short and long ranges leads to an improvement of the quality factor for a given phase shift which allows improving the detection of a second target with a lower RCS close to a main target with a higher RCS, the main target being moving.

It should be noted that the correction factor may be actually applied directly at the outputs of the cross correlation functions related to the short range received sequences or even at their inputs. Similarly, an inverse correction factor may be applied at different location of the process of the long range received sequences instead of the short range received sequences. The actual location of application of the correction factor does not matter as long as it allows obtaining even amplitude of sidelobes for the short range and the long range signals before their addition.

Figure 7a and 7b illustrate two examples of the outputs of the autocorrelation functions applied on each code used in the sequences.

Figure 7a illustrates the outputs of the autocorrelation function applied to each short complementary codes used in a short range sequence. Here codes are 4 bits complementary codes ‘1000’ noted ‘10’ and ‘1 101’ noted ‘15’ using octal notation.

Looking at the autocorrelation signal of code ‘10’ (circle markers), it can be noticed that the polarities of the sidelobes are alternatively positive and negative, the first polarity, namely the first sidelobe 7.1 with the level ‘1 ’, being positive.

Looking at the autocorrelation signal of code ‘15’ ('+’ markers), it can be noticed that the polarities of the sidelobes are alternatively positive and negative, the first polarity, namely the first sidelobe 7.2 with the level ‘-T, being negative.

One can check here than these codes are complementary codes as when the outputs of the autocorrelation functions are summed, the obtained signal has no more sidelobes.

On the same way, Figure 7b illustrates the outputs of the autocorrelation function applied to each long complementary codes used in long range sequences. Here codes are 8 bits complementary codes ‘01100000’ noted ‘140’ and ‘00110101’ noted ‘065’.

The same observation can be done except that here, the levels of the first sidelobes 7.3 and 7.4 are +3 and -3.

Figure 8 illustrates the flowchart describing a method to determine a new sequence of codes according to an embodiment of the invention.

In step 8.1, a pair of complementary codes A, B suitable for short range detection is obtained. As a short blind zone is expected, these codes shall have a short length. It may be for instance 4 bits codes, like codes 10 and 15 of

Figure 7a.

In step 8.2, a pair of complementary codes C, D suitable for long range detection is obtained. In order to get a high compression gain, which is needed to detect targets set at a high distance, these codes shall have a high length. It may be for instance 8 bits codes, like codes 140 and 065 of Figure 7b.

In step 8.3, the autocorrelation functions of each code are computed.

Optionally, curves like the ones shown in Figures 7a, 7b are plotted.

In step 8.4, for each autocorrelation function output, the polarity of the first sidelobe is determined. The polarity is positive if the amplitude of the sidelobe is characterized by positive values. The polarity is negative if the amplitude of the sidelobe is characterized by negative values.

In step 8.5, for each autocorrelation function output, the magnitude of the first sidelobe is determined. Note that step is optional; it will be used for receiver optimization, namely the application of a correction factor CF, used in some embodiments of the invention.

In step 8.6, a new sequence X1 Χ2 Χ3 Χ4 is determined by re-ordering the previous complementary codes, each part of the sequence being one code among the complementary codes. The new sequence is composed of the four codes A B C D arranged in order to get opposed polarities of the first sidelobes for X1 and X2, X3 and X4, X2 and X4 with Xn being the nth code transmitted. Sequences of four codes, composed of two pairs of complementary codes, the two pairs having different bit lengths, having opposed polarities of the first sidelobes for the first and second code, the third and the fourth code and the second and the fourth code will be called “opposed sequences” in the following. This definition may be extended to sequences of arbitrary number of codes. The sequence is composed of two groups of codes, each group comprises pairs of complementary codes having the same bit length, codes of each group having different bit length, sidelobes of codes of one group have opposed polarities compared to the codes of the other group.

For instance for the codes A=10, B=15, C=140, D=065 the arrangements may be:

15 065 140 which give polarities of the first sidelobes: + -- +

Or

10 140 065 which give polarities of the first sidelobes: - + + Or

065 140 10 15 which give polarities of the first sidelobes: - + + Or

140 065 15 10 which give polarities of the first sidelobes: +-- +

Figure 9 illustrates the signal processing applied at the receiver side of the radar in an embodiment of the invention. It illustrates more particularly the combination of the outputs of the cross correlation functions to get the signal to be used for detecting targets by the pulse radar for an opposed sequence.

It can be noticed that at the receiver side, the mathematical operation used for the pulse compression is the cross correlation between each signal transmitted using a code and the received signal corresponding to this code, followed by the sums of all cross correlation signals.

During the process of generation of the sequence, the cross correlation function is an autocorrelation function applied on the transmitted signal which is equivalent to apply a cross correlation function between the transmitted signal and an ideal received signal, meaning without any distortion.

The signal processing is based on the use of four cross correlation functions (9.1, 9.2, 9.3, 9.4), 3 adders (9.5, 9.6, 9.8), and one optional multiplier 9.7.

Each cross correlation function receives at its inputs one of the transmitted signal, corresponding to one code, and the received echo of that signal, corresponding to the same code, which may have been reflected on at least one target. As the pulses of the transmitted signal are encoded using sequences of codes which are transmitted and received in a synchronous way, it is obvious to determine the code used in pulses of the received signal.

For instance the cross correlator 9.1 computes the cross correlation between pulses of the transmitted signal encoded with the code 10 and pulses of the received echo encoded with the code 10.

In the same way, the cross correlator 9.2, respectively 9.3, respectively 9.4 compute the cross correlation between pulses of the transmitted signal encoded with the code 15, respectively 140, respectively 065 and pulses of the received echo encoded with the code 15, respectively 140, respectively 065.

It has to be noted that the cross correlators 9.1 and 9.2 process pulses encoded with codes having same length of Nl bits while the cross correlators 9.3 and 9.4 process pulses encoded with codes having same length of N2 bits.

The outputs of the cross correlators 9.1 and 9.2 are summed by the adder 9.5, benefiting from the complementary property of the codes XI and X2. The outputs of the cross correlators 9.3 and 9.4 are summed by the adder 9.6, benefiting from the complementary property of these codes X3 and X4.

In Figures 7a and 7b described above, it has been noticed that the magnitude of the first sidelobe varies with the length of the code. Then, before to sum the signals outputs by the adders 9.5 and 9.6, it is advantageous to apply a correction factor CF to the signal related to the shorter codes. This is done by the multiplier 9.7. It should be noted that the correction factor may be applied at diverse locations without changing its technical effect. An inverse signal may also be applied to the signal related to long codes.

Summing with the adder 9.8 the magnified signal issued from the multiplier 9.7 with the signal issued from the adder 9.6 enables to benefit from the property of the newly generated opposed sequence, namely that X2 and X4 have opposed polarities of their sidelobes. Thanks to this property, the first sidelobes obtained at the outputs of the adders 9.5 and 9.6 have opposed polarities. Therefore, as these first sidelobes are not null due to the move of the targets, the addition by the adder 9.8 contributes to reduce the sidelobes of the signal S. The use of the correction factor CF, by giving to these sidelobes the same level of amplitude, helps to improve this reduction.

The correction factor CF can be determined when computing the autocorrelation function of each code during the determination of the sequence. It may be done by measuring the magnitude of the first sidelobe for each of the two code sequences. An optimal CF value is given by the ratio between the magnitude of the first sidelobe of the longer code and the magnitude of the first sidelobe of the shorter code. In the example, the magnitude for the 4 bits code is equal to 1 and the magnitude for the 8 bits code is equal to 3 then CF is set to 3.

It should be noted that the addition of the multiplier 10.7 improves the contention of the sidelobes but is not mandatory. Even without the correction factor the proposed sequence gives better results, namely a higher Q factor, that the single code pairs used in prior art.

Figure 10 illustrates an example of the curves giving the quality factor Q as a function of the phase shift a, Q = /(a), achieved with various code sequences.

On that figure, the variations of the Q factor values with regard to the phase shift (which is proportional to the speed of the target) have been plotted for various sequences of codes by simulation, without noise addition (high SNR).

The lower curve (the one with '+' markers) gives the variation of the Q factor when a first sequence of long complementary codes are used (e.g., 140/065).

The curve with 'x' markers gives the variation of the Q factor when a second sequence of short complementary codes are used (e.g., 10/15).

The curve with square markers gives the variation of the Q factor when a third sequence is considered. This third sequence is obtained by combining the short complementary codes (e.g., 10/15) and the long complementary codes (e.g., 140/165) without applying the method according to an embodiment of the invention. This means that the generated sequence is not opposed. Therefore, the correction factor is not applied either. Here it leads to create the sequence {10, 15, 140, 065}. The Q factor obtained is the same that the one achieved with only short codes.

The curve with star markers gives the variation of the Q factor when a fourth sequence is considered. This fourth sequence is obtained by combining the short complementary codes and the long complementary codes, it is an opposed sequence. The combination is done by applying the method according to an embodiment of the invention as illustrated by Figure 8. There is no application of correction factor. This leads here to generate the sequence <10, 15, 065, 140}. An improvement of the Q factor of more than 1 dB with regard to another sequence is visible.

The upper curve with diamond markers gives the variation of the Q factor when the fourth sequence is considered and the correction factor is applied. It illustrates the gain which is obtained with that preferred embodiment of the invention: the value of the Q factor is at least 3dB higher than the Q factor achieved with another complementary code.

In other words, using the fourth sequence, an opposed sequence and using the correction factor, instead of a single pair of complementary codes, the same value of Q factor can be achieved for a speed of the target which is 65% higher.

For instance, the minimum value of the Q factor of 20dB can be achieved for sequence 10/15/140/065 with a phase shift up to 0.057 while, for sequence 10/15/065/140, it can be achieved with a phase shift up to 0.094. As the phase shift is proportional to the speed of the target, with the code 10/15/065/140 an higher speed of target can fulfill the Q factor constraint than with the code 10/15/140/065.

In the previous embodiments, a new code sequence, the opposed sequence, has been designed and used, combining code sequences dedicated to short and long range detection. By using opposed code sequences, the quality factor may be improved. However, the design of the new code sequence does not take into account the level of SNR in the received signal.

In real conditions, the complementarity of the codes may be disrupted by the noise and the Q factor achieved is dependent not only upon the code sequence but also upon the noise level. As illustrated in Figures 11a and 11b, the code sequence which gives the higher Q factor is not always the same: it varies with the Signal to Noise Ratio (SNR).

In order to decrease the noise in the signal before processing it for target detection, a filtering function is usually applied: each code sequence is transmitted many times. For example the value of 18432 repetitions of the Long range sequence has been used. The received signals are filtered, for instance averaged, to generate a “low noise received signal” which is further processed for the target detection.

Then, a change of the number of code sequences transmitted and filtered modifies the level of noise at the input of the target detection process and has an impact on the relationship between the Q factor and the code sequence. However, increasing the number of acquisitions elevates the latency of the detection. Then, the number of acquisitions should be limited as much as possible to reduce the latency of the detection.

Therefore, in other embodiments, the selection of a coded pulse sequence may depend on a measured SNR value. For example, a look-up table associating for each SNR value, the coded pulse sequence providing the best Q factor might be used. In some embodiments, the number of acquisitions which determine both the SNR and the latency may be adapted in order to get the lowest latency for a given expected quality factor.

This aspect of the invention, namely the selection of a coded pulse sequence depending on a measured SNR value, will now be described.

Figure 11a and lib illustrate two examples of curves giving the quality factor as a function of the SNR for a given code sequence and for a given phase shift a. Figure 11a is achieved for a number of acquisitions equal to 50 while Figure lib is achieved for a number of acquisitions is equal to 200.

Both figures have been drawn considering both sequences {10 15 065 140} and {10 15 140 065}, a target velocity of lOm/s, a carrier frequency of 60GHz, a PRI of lps and a SNR varying from 0 to 60dB.

Figure 11a shows that, for a number of acquisitions equal to 50, the sequence 10/15/065/140 gives a higher Q factor than the sequence 10/15/140/065 when the SNR is greater than 20dB.

Then in order to get the higher Q factor possible, the sequence 10/15/065/140 shall be used when the SNR is greater than 20dB and the sequence 10/15/140/065 shall be used when the SNR is lower.

Figure lib shows that for a number of acquisitions equal to 200, the sequence 10/15/065/140 gives a higher Q factor than the sequence 10/15/140/065 when the SNR is greater than 14dB. Here, in order to get the higher Q factor possible, the sequence 10/15/065/140 shall be used when the SNR is greater than 14dB and the sequence 10/15/140/065 shall be used when the SNR is lower.

The SNR value which corresponds to the crossing point of the curves is called the SNR threshold (SNR_thr).

Alternatively, a look-up table may be used instead of these. Some examples of such look-up tables are illustrated on Figure 14a and 14b. In this case, the SNR threshold can be defined as the value of the SNR which is the boundary between the zone where sequence 1 gives the higher Q factor and the zone where sequence 2 gives the higher Q factor.

Figure 12 illustrates a flowchart describing the selection of a sequence with regard to the SNR according to an embodiment of the invention.

First step 12.1 is the initialization step where the parameters of the radar are set. Among them, at least two sequences may be selected, based on the radar specifications, the considered range or the considered blind zone. The SNR threshold which corresponds to the point where the two curves giving the quality factor as a function of the SNR related to these sequences cross each other is determined and obtained.

In a step 12.2, a first signal encoded with a first sequence among the two selected ones is transmitted, any of the two sequences may be considered.

In a step 12.3, the signal reflected by a/at least one target is received.

In a step 12.4, the Signal to Noise Ratio (SNR) of the received signal is measured according to known technics.

In step 12.5 the measured SNR is compared with the value of the SNR threshold which has been defined with regard to the two code sequences considered.

If the measured SNR is greater or equal than the SNR threshold value, in a step 12.7 the code sequence which gives the highest Q factor for high SNR is selected.

If the measured SNR is lower than the SNR threshold value, in a step 12.6 the code sequence which gives the highest Q factor for low SNR is selected.

Alternatively to the use of curves, look up tables may be applied, and then steps 12.5, 12.6 and 12.7 are replaced by another step 12.5 where the best sequence is obtained by reading the look up table.

Once the code sequence selected, in a step 12.8, the radar is configured. Namely, the transmitter is set to transmit the selected sequence and the receiver processing is set to process that same sequence.

If the selected sequence corresponds to a sequence that may have been generated by the process illustrated by Figure 8, namely that, X2 being complementary of X1, X4 being complementary of X3, X4 is further complementary to X2, the received sequence may be processed according to the process illustrated by Figure 9.

In next step the radar is in operational mode and the target detection process runs until a request of code sequence re-initialization is done by the user.

When that request is done, the radar goes back in step 12.2.

In another embodiment, the radar may go automatically in code sequence re-initialization, step 12.2, after a certain time or when a change in the propagation of the waves is known (change of position, weather, surrounding objects, ...).

This second aspect of the invention works independently of the choice of the two sequences that are chosen. At least one of the sequences used in these embodiments may be an opposed sequence or not. In our example, one of the sequence is an opposed sequence while the other one is not. When dealing with a four code sequence that is not an opposed sequence, the computation of the signal is still done as described in relation with Figure 9, of course without the correction factor. When at least one of the chosen sequence is an opposed one, the two aspects of the invention may be combined. Both sequences may be processed according to Figure 9. The possible opposed sequences may advantageously use the correction factor as described.

Figure 13 illustrates a flowchart describing another embodiment ofthe invention where the number of acquisitions is adapted in order to get an expected Q factor with a minimum latency, thus a minimum number of acquisitions.

First in a step 13.1 the initialization of the parameters of the radar is made. Among them, the two code sequences to be used by the radar are determined, a default number of acquisitions Nb_acq, a maximum number of acquisitions Nbacqmax and the quality threshold which corresponds to the expected minimum value of Q.

In a step 13.2, a first sequence is transmitted. This sequence may be any of the two sequences considered.

In a step 13.3, a number Nb_acq of acquisitions of the signal reflected by the illuminated scene is done and the acquisitions are filtered to increase the SNR of the filtered signal.

In a step 13.4, the Signal to Noise Ratio (SNR) of the filtered signal is measured.

In a step 13.5, using predetermined data, like simulation curves or lookup tables, and the measured SNR, it is checked if the code sequence 1 enables to get a Q factor higher or equal to the quality threshold. If the result of the test 13.5 is positive, the code sequence 1 is selected in step 13.6.

If the result of the test 13.5 is negative, in step 13.7, using predetermined data, like simulation curves or look-up tables, and the measured SNR, it is checked if the code sequence 2 enables to get a Q factor higher or equal to the quality threshold. If the result of the test 13.7 is positive, the code sequence 2 is selected in step 13.11.

If the result of the test 13.7 is negative, in a step 13.8, the number of acquisitions Nb_acq is compared with the maximum number of acquisitions Nbacqmax. If the current value of Nb_acq is lower than Nbacqmax, in a step 13.13, Nb_acq is increased in order to increase the SNR of the next process filtered signal and the process restarts at the step 13.2.

If the results of the comparison done in step 13.8 is negative, which means that increase of Nb_acq is not possible, during next step 13.12, using predetermined data like simulation curves or look-up tables the sequence which gives the higher Q factor for the measured SNR is selected.

After any of steps 13.6, 13.11 and 13.12 the process goes in step 13.9 where the radar is configured. During this configuration, the transmitter is set to transmit the selected sequence and the receiver processing is set to process that sequence using the number of acquisitions Nb_acq.

If the selected sequence corresponds to a sequence that may have been generated by the process illustrated by Figure 8, namely that, X2 being complementary of Χ1, Χ4 being complementary of Χ3, Χ4 is further opposed to X2, the received sequence may be processed according to the process illustrated by Figure 9.

In next step, step 13.10, the radar is in operational mode and the target detection process runs until a request of code sequence re-initialization is done by the user.

When that request is done, the radar goes back in step 13.2.

In another embodiment, the radar may go automatically in code sequence re-initialization, step 13.2, after a certain time or when a change in the propagation of the waves is known (change of position, weather, surrounding objects, ...).

Figure 14a and 14b illustrate examples of look up table which may be used instead of the curves giving the quality factor in the flowcharts illustrated

Figures 12 and 13.

Figure 14a illustrates an example of a look up table which may be used to define the sequence to be used, meaning the one which gives the highest Q factor, with regard to the measured SNR and the number of acquisitions. This table gives the sequence to be selected for a given SNR for two different numbers of acquisitions. This look up table may be used instead of the curves to define the SNR threshold in the flowchart shown on Figure 12.

Figure 14b illustrates an example of a look up table which may be used to give the Q factor with regard to the sequence, the measured SNR and the number of acquisitions. This table gives the Q factor for a given SNR for the two sequences and for two different numbers of acquisitions. This look up table may be used instead of the curves in the flowchart shown on Figure 13.

Figure 15 is a schematic block diagram of a computing device 15.0 for implementation of one or more embodiments of the invention. The computing device 15.0 may be a device such as a micro-computer, a workstation or a light portable device. The computing device 15.0 comprises a communication bus connected to:

- a central processing unit 15.1, such as a microprocessor, denoted CPU;

- a random access memory 15.2, denoted RAM, for storing the executable code of the method of embodiments of the invention as well as the registers adapted to record variables and parameters necessary for implementing the method according to embodiments of the invention, the memory capacity thereof can be expanded by an optional RAM connected to an expansion port for example;

- a read only memory 15.3, denoted ROM, for storing computer programs for implementing embodiments of the invention;

- a network interface 15.4 is typically connected to a communication network over which digital data to be processed are transmitted or received. The network interface 15.4 can be a single network interface, or composed of a set of different network interfaces (for instance wired and wireless interfaces, or different kinds of wired or wireless interfaces). Data packets are written to the network interface for transmission or are read from the network interface for reception under the control of the software application running in the CPU 15.1;

- a user interface 15.5 may be used for receiving inputs from a user or to display information to a user;

- a hard disk 15.6 denoted HD may be provided as a mass storage device;

- an I/O module 15.7 may be used for receiving/sending data from/to external devices such as a video source or display.

The executable code may be stored either in read only memory 15.3, on the hard disk 15.6 or on a removable digital medium such as for example a disk. According to a variant, the executable code of the programs can be received by means of a communication network, via the network interface 15.4, in order to be stored in one of the storage means of the communication device 15.0, such as the hard disk 15.6, before being executed.

The central processing unit 15.1 is adapted to control and direct the execution of the instructions or portions of software code of the program or programs according to embodiments of the invention, which instructions are stored in one of the aforementioned storage means. After powering on, the CPU 15.1 is capable of executing instructions from main RAM memory 15.2 relating to a software application after those instructions have been loaded from the program ROM 15.3 or the hard-disc (HD) 15.6 for example. Such a software application, when executed by the CPU 15.1, causes the steps of the flowcharts shown in Figures 8, 12 and 13 to be performed.

Any step of the algorithm shown in Figures 8, 12 and 13 may be implemented in software by execution of a set of instructions or program by a programmable computing machine, such as a PC (“Personal Computer”), a DSP (“Digital Signal Processor”) or a microcontroller; or else implemented in hardware by a machine or a dedicated component, such as an FPGA (“FieldProgrammable Gate Array”) or an ASIC (“Application-Specific Integrated Circuit”).

Although the present invention has been described hereinabove with reference to specific embodiments, the present invention is not limited to the specific embodiments, and modifications will be apparent to a skilled person in the art which lie within the scope of the present invention.

Many further modifications and variations will suggest themselves to those versed in the art upon making reference to the foregoing illustrative embodiments, which are given by way of example only and which are not intended to limit the scope of the invention, that being determined solely by the appended claims. In particular the different features from different embodiments may be interchanged, where appropriate.

In the claims, the word “comprising” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude a plurality. The mere fact that different features are recited in mutually different dependent claims does not indicate that a combination of these features cannot be advantageously used.

Claims

1. A pulse radar system comprising a transmitter configured to:

- transmit the modulation signal.

2. The pulse radar system of claim 1, wherein the system further comprises a receiver configured to:

3. The pulse radar system according to claim 2, wherein the transmitter or the receiver is further configured to:

4. The pulse radar system according to anyone of claim 2 to 3, wherein the receiver is further configured to:

- detect long range target using the obtained signal; and

5. A pulse radar system comprising: a transmitter configured to:

- obtain a plurality of sequences of codes based on complementary codes;

- transmit the modulation signal; and a receiver configured to:

- determine a signal to noise ratio of the received signal;

- configure the transmitter to use the selected sequence.

6. The pulse radar system of claim 5, wherein:

7. The pulse radar system of claim 5, wherein:

8. The pulse radar system of anyone claim 5 to 7, wherein:

- increase the number of acquisitions until:

the quality factor provided by the selected sequence is equal or higher than the predetermined quality threshold; or o a maximum number of acquisitions has been reached.

9. The pulse radar system of anyone claim 5 to 8, wherein:

10. A method to generate an opposed sequence of codes to be used in a pulse radar system comprising:

- obtaining a first pair of complementary codes;

11. A transmission method by a transmitter in a pulse radar system comprising:

- transmitting the modulation signal.

12. A transmission method in a pulse radar system comprising: by a transmitter:

- obtaining a plurality of sequences of codes based on complementary codes;

- transmitting the modulation signal; and by a receiver:

- determining a signal to noise ratio of the received signal;

- configuring the transmitter to use the selected sequence.

13. A transmission method according to Claim 12, one of the sequences obtained being an opposed sequence generated according to the method of claim 10.

14. A computer-readable storage medium storing instructions of a computer program for implementing a method according to any one of claims 10 to 11.

Intellectual

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Office

Application No: GB1705573.2

14. The pulse radar system according to any one of claim 1 to 4, further configured to generate an opposed sequence of codes according to the method of claim 10.

15. A computer program product for a programmable apparatus, the computer program product comprising a sequence of instructions for implementing a method according to any one of claims 10 to 12, when loaded into and executed by the programmable apparatus.

16.A computer-readable storage medium storing instructions of a 5 computer program for implementing a method according to any one of claims 10 to 12.

Amendments to the claims have been filed as follows:

1207 18

1. A pulse radar system comprising a transmitter configured to:

- obtain an opposed sequence of first, second, third and fourth codes,

5 the opposed sequence of codes being composed of two pairs of complementary codes, codes of the two pairs having different bit lengths, codes of a same pair having a same bit length, the first and second codes belonging to one of the two pairs and the third and fourth codes belonging to the other pair,

10 wherein sidelobes of a signal obtained by cross correlation of a transmitted signal encoded using the second code and a reflected signal resulting from reflexion of the transmitted signal, and sidelobes of the signal obtained by cross correlation of a another transmitted signal encoded using the fourth code and another

15 reflected signal resulting from reflexion of the another transmitted signal have opposed polarities;

- transmit the modulation signal.

25 - applying a cross correlation function to the transmitted signal and the corresponding received signal, both related to pulses encoded with the same code, for each code of the sequence;

30 --- adding the results of the cross-correlation functions for the third and the fourth codes:

1207 18

3. The pulse radar system according to claim 2, wherein the transmitter or

5 the receiver is further configured to:

4. The pulse radar system according to anyone of claim 2 to 3, wherein

10 the receiver is further configured to:

- detect long range target using the obtained signal; and

- detect short range target using a second signal obtained by the addition of the results of the cross-correlation functions for the shortest codes,

5. The pulse radar system according to anyone of claim 2 to 3, wherein: the transmitter is configured to:

- obtain a plurality of sequences of codes based on complementary codes;

20 - generate a modulation signal resulting from modulation of pulses encoded with one sequence of the plurality;

- transmit the modulation signal; and a receiver configured to:

- receive a reflected signal resulting from transmission of the

25 modulation signal;

-- determine a signal to noise ratio of the received signal;

- configure the transmitter to use the selected sequence.

6. The pulse radar system of claim 5, wherein:

1207 18

7. The pulse radar system of claim 5, wherein:

- the selection of the sequence among the plurality giving the best quality factor for the determined signal to noise ratio is made based on a look-up table giving for each sequence the quality factor based

10 on the level of signal to noise ratio.

8. The pulse radar system of anyone claim 5 to 7, wherein:

- receiving a reflected signal resulting from transmission of the modulation signal comprises realising a number of acquisitions of

15 the reflected signal, the number of acquisitions being initialized to an initial number of acquisitions; and the receiver is further configured to:

-- determine if the quality factor provided by the selected sequence is higher than a predetermined quality threshold; and if not:

20 --- increase the number of acquisitions until:

25 9. The pulse radar system of anyone claim 5 to 8, wherein:

-- at least some of the sequences of the plurality of sequences are opposed sequence of codes, an opposed sequence of codes being composed of two pairs of complementary codes, codes of the two pairs having different bit lengths, codes of a same pair having a

30 same bit length, the first and second codes belonging to one of the two pairs and the third and fourth codes belonging to the other pair, wherein sidelobes of a signal obtained by cross correlation of a

1207 18 transmitted signal encoded using the second code and a reflected signal resulting from reflexion of the transmitted signal, and sidelobes of the signal obtained by cross correlation of a another transmitted signal encoded using the fourth code and another

5 reflected signal resulting from reflexion of the another transmitted signal have opposed polarities.

10. A method to generate an opposed sequence of codes fo be used in a pulse radar system comprising:

10 - obtaining a first pair of complementary codes;

- generating the opposed sequence of codes by ordering both pairs of code, sidelobes of a signal obtained by cross correlation of a

15 transmitted signal encoded using the second code and a reflected signal resulting from reflexion of the transmitted signal, and sidelobes of the signal obtained by cross correlation of a another transmitted signal encoded using the fourth code and another reflected signal resulting from reflexion of the another transmitted

20 signal have opposed polarities.

11. A transmission method by a transmitter in a pulse radar system comprising:

-- obtaining an opposed sequence of codes, the opposed sequence

25 of codes being composed of two pairs of complementary codes, codes of the two pairs having different bit lengths, codes of a same pair having a same bit length, the first and second codes belonging to one of the two pairs and the third and fourth codes belonging to the other pair, wherein sidelobes of a signal obtained by cross

30 correlation of a transmitted signal encoded using the second code and a reflected signal resulting from reflexion of the transmitted signal, and sidelobes of the signal obtained by cross correlation of

1207 18 a another transmitted signal encoded using the fourth code and another reflected signal resulting from reflexion of the another transmitted signal have opposed polarities;

- generating a modulation signal resulting from modulation of pulses

5 encoded with the opposed sequence of codes;

- transmitting the modulation signal.

12. The pulse radar system according to any one of claim 1 to 4, further configured to generate an opposed sequence of codes according to the

10 method of claim 10.

13. A computer program product for a programmable apparatus, the computer program product comprising a sequence of instructions for implementing a method according to any one of claims 10 to 11, when

15 loaded into and executed by the programmable apparatus.