Abstract:
A method for transmitting a radar signal comprises the step of transmitting a series of pulses, each of the pulses being separated in time by an interpulse period, and each of the pulses in the series being modulated in accordance with a different character of a first code

Description:
FIELD OF THE INVENTION  
         [0001]    The invention relates generally to the field of radar, and more particularly to the application of error detection/correction techniques and/or coding techniques to a radar system.  
         BACKGROUND OF THE INVENTION  
         [0002]    Radar (which is an acronym for RAdio Detection And Ranging) is a technique used to detect objects at a distance through the transmission of electromagnetic energy (usually at RF or microwave frequencies). Radar systems are used for a wide variety of purposes, including meteorological purposes (e.g., the NEXRAD system), e.g., to detect storm systems. Radars are used on board planes and ships to detect and track objects, both on the surface and in the air, for both military and commercial applications. Examples of such systems include AN/SPQ-9(b) naval radar system available from Northrop Grumman Norden Systems, Inc., which is used to detect surface vessels and small, low-flying objects such as missiles, and the Pathfinder commercial shipboard radar system available from Raytheon Corp., which is used to detect other ships and for navigational purposes. Radars are also used for collision avoidance on automobiles. It is also known to use radars for imaging, both for celestial and terrestrial objects. For example, the Joint Surveillance Target Attack Radar System (JSTARS) uses a synthetic aperture radar (SAR) mounted on the underside of a converted 707 airframe to form ground images which can reveal the presence of military vehicles such as tanks.  
           [0003]    Radar systems can be either continuous wave or pulse types. In both types of systems, a portion of the energy transmitted toward an object by a transmitting antenna is reflected toward a receiving antenna to provide information about the object. In a continuous wave radar system, electromagnetic energy (typically at RF or microwave frequencies) is continuously transmitted, while in pulse radar systems, electromagnetic energy is transmitted in short bursts, or pulses, at a frequency referred to as the pulse repetition frequency, or prf. The prf is often chosen based on the maximum expected range at which target detection is desired. This range is referred to as the unambiguous range. In such pulse systems, the pulses have a period, or pulse width, which is typically short as compared to the period between the pulses (which is the inverse of the prf). After a pulse is transmitted, a receiver “listens” for echoes of the transmitted pulse reflected by an object. If an echo is received, the object has been detected.  
           [0004]    In pulse systems the range of an object can be determined by determining the time between the transmitted pulse and receipt of the echo (assuming that the range is less than the unambiguous range). If the transmitting and/or receiving antenna (the same antenna is often used for both functions) has directional properties, information concerning the bearing of the target may also be revealed by the echo.  
           [0005]    Radar may also be used to determine relative velocity of an object with respect to the radar system. For example, in Doppler radar systems, frequency and/or phase shifts in the echo can reveal the relative velocity between object and system. In such pulse radar systems, echoes received from several pulses are often “batched” or combined (e.g., added and averaged). The echoes from a plurality of pulses that are combined in this fashion are often referred to as a batch. In a surveillance radar application, in which a rotating directional antenna with a beam width on the order of 5-10 degrees and a rotation rate of on the order of 1-10 seconds is utilized, batches of 2-16 pulses are typical, whereas batches of 50-250 pulses are more common to high resolution radar systems with non-rotating antennas, such as the type utilized in automobiles for collision avoidance. Batching serves to both increase the signal-to-noise ratio of the echo and to decrease the amount of processing required.  
           [0006]    With radar systems, like any communication system, it is desirable to transmit information at the lowest cost and highest speeds possible. Radar systems, again like other communications systems, are also subject to degradation in performance resulting from interference and noise. This is especially true for radar systems such as collision avoidance radar systems on automobiles, in which each radar transmitter/receiver in the system is subject not only to interference from other transmitters in the system and/or on the vehicle, but also is potentially subject to interference from multiple other vehicles in the vicinity. Improving the performance of radar systems in the face of interference and noise has long been a goal of radar system engineers. While much progress has been made over the 60 or so years since radar has been in existence, improvement is still desirable and necessary.  
           [0007]    What is needed is a system and method for reducing the effects of interference and noise in a radar system.  
         SUMMARY OF THE INVENTION  
         [0008]    The present invention meets the aforementioned needs to a great extent by the present invention, in which known coding/error correction techniques are applied to radar systems to improve performance. In one highly preferred embodiment of the invention, particularly applicable to pulse radar systems, a direct sequence spread spectrum (also referred to as code division multiple access, or CDMA) technique is utilized. In this embodiment, a batch of pulses is treated as a symbol. The symbol is modulated by a pseudo-noise (PN) code, with each chip of the code corresponding to the period associated with a single pulse (i.e., the chip rate corresponds to the prf). The modulation technique is preferably bi-phase shift keying (BPSK); however, polyphase coding (e.g., QPSK) may also be used. The received echoes from each of the pulses in the symbol are combined, or correlated, as a function of the PN code. Viterbi decoding may be employed by the receiver. The PN code may be of any of the types commonly used in other communications systems, e.g., Gold codes, Walsh codes, Turbo codes, etc. In one preferred embodiment, the system also employs pulse coding. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]    A more complete appreciation of the invention and many of the attendant features and advantages thereof will be readily obtained as the same become better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein:  
         [0010]    [0010]FIG. 1 is a schematic diagram illustrating a plurality of pulses and interpulse periods comprising a batch in a conventional process.  
         [0011]    [0011]FIG. 2 is a time-domain representation of a single pulse showing bi-phase modulation according to an seven element Barker code according to a conventional process.  
         [0012]    [0012]FIG. 3 is a schematic diagram illustrating the pulses in the batch of FIG. 1 after modulation by a pseudo-noise code according to an embodiment of the present invention.  
         [0013]    [0013]FIG. 4 is a hardware block diagram of a radar system employing the modulation scheme of FIG. 2 according to an embodiment of the present invention.  
     
    
     DETAILED DESCRIPTION  
       [0014]    The present invention will be discussed with reference to preferred embodiments of the invention. Specific details, such as types of modulation schemes and lengths of codes, are set forth in order to provide a thorough understanding of the present invention. The preferred embodiments discussed herein should not be understood to limit the invention. Furthermore, for ease of understanding, certain method steps are delineated as separate steps; however, these steps should not be construed as necessarily distinct nor order dependent in their performance.  
         [0015]    [0015]FIG. 1 illustrates a series of pulses  110  processed as a batch  100  with a period TB. The pulses  110  of FIG. 1 are symbolic representations of actual pulses. Each pulse  110  represents a period of time in which a radar system transmits electromagnetic energy. In the period of time between pulses, referred to as the inter-pulse period, the radar system listens for echoes of the pulses reflected by objects. Each pulse  110  has a period T P , which is sometimes referred to as the pulse width. An expanded time-base view of a pulse  110  is illustrated in cut out section  120 . As can be seen from section  120 , each pulse has been modulated.  
         [0016]    [0016]FIG. 2( a ) is a diagram of the symbolic representation of the pulse  110  of FIG. 1, while FIG. 2( b ) is a representation of a corresponding waveform. Referring now to FIG. 2( a ), it can be seen that pulse coding (also referred to as pulse modulation) has been applied to the pulse in a manner well known in the art. The pulse width T P  has been divided into a number of sub-pulses referred to as chips, each with an equal sub-pulse period, or sub-pulse width, T C . Each of the sub pulse periods T C  is associated with a + or − symbol. As can be seen with reference to FIG. 2( b ), no phase modulation of the signal  220  occurs if the corresponding sub-pulse is associated with a + symbol, whereas a phase shift of 180 degrees occurs for those sub-pulse periods T C  associated with a − symbol. The pattern of +s and −s is referred to as a code (which in this case is a 7 bit Barker code), and the process of modulating the pulse in accordance the code is referred to as pulse coding or pulse modulation. The modulation of a single pulse with a code shall be referred to herein as intrapulse coding. Although a 7 bit Barker code is illustrated in FIG. 2, it is possible to use Barker codes or other codes (e.g., of different lengths. Bi-phase coding/modulation (also referred to as BPSK, or binary phase shift keying) is illustrated in FIG. 2. However, polyphase (more than two phase) coding, e.g., Frank coding or Quadrature Phase Shift Keying (QPSK), may also be employed. As discussed more fully in M. Skolnik, Radar Handbook, 2 nd  ed. 1990 at pp. 10.15-10.26, the entirety of which is hereby incorporated by reference herein, phase coding is a form of pulse compression, which allows for additional power transmission and/or range resolution as compared to an uncompressed pulse.  
         [0017]    Referring now back to FIG. 1, it can be seen that the exact same pulse  110  is transmitted a large number of times during the batch period T B . As discussed above, the number of pulses in a batch can vary, but is typically on the order of 200 pulses per batch in a collision avoidance system for use with automobiles. The pulses in the batch  100  of FIG. 1 can be thought of as a unit of information, i.e., a symbol. Using spread spectrum techniques, it is possible to improve the performance of the communication (radar) system by coding this information unit/symbol. In particular, the symbol/batch can be coded using a direct sequence spread spectrum technique, which can be performed by modulating the symbol using any codes known for this purpose, including pseudo-noise (PN) codes such as those that are commonly used in CDMA applications.  
         [0018]    Treating a series of pulses (such as the series of pulses in a batch) as a symbol and modulating each of the pulses in the batch in accordance with a code shall be referred to herein as interpulse modulation (as distinguished from intrapulse modulation, which refers to coding a single pulse). These codes are preferably orthogonal; that is, they have high autocorrelation (correlation with itself) with low cross-correlation (correlation with other codes). This means that when decorrelation is performed using any code other than the same code used to modulate the symbol, the output is approximately zero. It is possible to use codes such as Gold codes, Walsh codes and Turbo codes. A short code scheme, in which the code length equals the symbol length, may be used when the number of pulses in a batch is sufficiently large to provide for a sufficient number of unique codes. Long code schemes, in which the code length exceeds the symbol length, may also be used, especially where the number of pulses in a batch is small. Any type of modulation may be used, but in highly preferred embodiments of radar systems, especially those employing BPSK intrapulse modulation, BPSK modulation is preferred as this takes advantage of the bi-phase modulator that is already present the system.  
         [0019]    An exemplary modulation scheme employing both intrapulse and interpulse modulation for a batch/symbol  300  according to one embodiment of the invention is illustrated in FIG. 3. The batch  300  comprises a plurality of pulses  310 ,  311 . Some of the pulses  310  are intrapulse modulated in the same fashion as the pulse  110  of FIG. 1; that is, the pulses have been divided into sub-pulses (or chips), with each of the chips either left unchanged or phase shifted by 180 degrees in accordance with a 7 bit Barker code. Other of the pulses  311  are phase shifted versions of the pulses  310 , with a phase shift of 180 degrees. Thus, the pulses in the batch  300  have been interpulse modulated. The pattern by which the pulses  310 ,  311  are shifted or left unshifted is dictated by a code associated with the system. The received echoes are de-correlated (de-spread) using the identical code.  
         [0020]    In practice, each radar transmitter is assigned a different PN code, much in the same way that each cell phone is assigned a different PN code in a typical CDMA system. These radar transmitters may be located on a single vehicle or may be located on a plurality of vehicles. Thus, the invention may be used to reduce interference between radar transmitters on the same automobile as well as interference from other vehicles when used in an automobile collision avoidance radar system. If a short code scheme is used, the number of different possible codes depends upon the number of “chips” in the symbol, which in this case is the number of pulses in the batch. In a system that batches  200  pulses, a large number of orthogonal codes are available for assignment to individual transmitters.  
         [0021]    A block diagram of an embodiment of a system  400  according to one embodiment of the invention is illustrated in FIG. 4, which is similar to FIG. 1 of the aforementioned co-pending application. In FIG. 4, a microwave source  10  is stabilized by FM modulation module  11 . The output of the microwave source is amplified and buffered (to prevent frequency pulling) by buffer amplifier  12 . The output of buffer amplifier  12  is input to a pulse former  414 , which is controlled by switch driver  15  to form a pulse with the desired pulse width. The pulse from the pulse former  414  is input to bi-phase modulator  16 , which performs the intrapulse and interpulse bi-phase modulation. The bi-phase modulator  16  is controlled by the output of an XOR gate  401 . One input to the XOR gate  401  is connected to pulse modulator driver  17 , which is responsible for supplying a waveform that corresponds to the Barker code for intrapulse modulation (pulse compression) purposes. Thus, the waveform output by pulse modulator driver  17  corresponds to the waveform of FIG. 2( a ). The other input to the XOR gate  401  is connected to batch modulator driver  402 , which uses the PN code stored in the PN code memory  403  to output a waveform that varies in accordance with the PN code over the batch period to perform the interpulse modulation function. Thus, the XOR gate  401  combines the waveforms produced by the pulse modulator driver  17  and the batch modulator driver  402  in order to control the bi-phase modulator  16  to produce the interpulse and intrapulse-modulated output symbolically represented in FIG. 3. The output of the bi-phase modulator is then input to transmit/receive switch  18 , which is controlled by switch driver  28  such that the output of the bi-phase modulator  16  is connected to amplifiers  25  and  26  for transmission through antenna  30  during the transmit cycle, and input to the local oscillator inputs of I and Q mixers  44  and  45  of the receiver during the receive cycle.  
         [0022]    In a highly preferred embodiment, the functions performed by the elements  15 ,  17 ,  28 ,  401 ,  402  and  403  enclosed by dashed line  409  are implemented in a single integrated circuit such as a programmable gate array or an ASIC. However, the elements in dashed line  409  may also be implemented using discrete components.  
         [0023]    Signals received through antenna  31  are amplified by amplifiers  40 ,  41  and then divided and input to mixers  44 ,  45 , which mixes the input signal with a replica of the transmitted pulse and a replica of the transmitted pulse that is phase-shifted by 90 degrees (to form the quadrature portion of the received signal), respectively. The downconverted output of the mixers  44 ,  45  are then connected to integrators  47 ,  48 , which perform the decorrelation function corresponding to the intrapulse modulation. The output of the integrators  47 ,  48  are then Doppler-filtered by filters  49 ,  50 . The output of the Doppler filters  49 ,  50  are connected to switches  52 ,  54 , which are controlled by sampler  51  to perform an integrate-and-dump process as is well known in the art. The output of the switches  52 ,  54  are connected to second stage integrators  53 ,  55 , respectively.  
         [0024]    The microwave frequency source  10  is employed in and received signals are down-converted directly to baseband in FIG. 4. As will be readily understood by those of skill in the art, it is also possible to employ include an intermediate frequency (IF) oscillator in the transmitter and downconvert the received signals to IF before they are downcoverted to baseband. In such a scheme, the bi-phase modulation may be performed at IF instead of at the transmitted frequency as shown in FIG. 4. Furthermore, the decorrelation scheme illustrated in FIG. 4 is but one method for decorrelating the received signal. Other forms of decorrelation, including Viterbi decoding, may also be used.  
         [0025]    An interpulse modulation scheme has been illustrated and preferred embodiments of a radar system employing an interpulse modulation scheme has been set forth. Obviously, numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein.