Pulsed Radar Bi-Phase Coding for Ghost Pulse Cancellation
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Solution Overview
Problem
Pulsed radar systems face issues with noise and unwanted reflections from ambiguity ranges, leading to distorted signal reconstruction and incorrect range profiling, particularly in swept-threshold receivers.
Innovation Solution
Implementing a bi-phase coding scheme with pseudo-random sequences that ensure equal numbers of positive and negative polarities for each threshold value, using concatenated substrings to cancel unwanted reflections while reinforcing desired signals, and applying signal chopping to remove noise.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional pulsed radar transmits repeated pulses at PRF, then continuous monitoring capability is achieved, but unwanted reflections from ambiguity ranges and noise distort the received signal
Solution Approach 1:
The patent applies periodic action by transmitting pulses with a specific repetition frequency (PRF) and using a pseudo-random binary sequence (PRBS) that repeats after a defined period. The PRBS modulates the pulse polarities in a periodic pattern (e.g., + - + - for length 4), creating predictable signal behavior that enables correlation-based detection. This periodic modulation allows the receiver to distinguish desired echoes from unwanted reflections through autocorrelation processing, as the desired signal aligns with the known PRBS pattern while noise and ghost pulses do not.
Solution Approach 2:
The patent changes the polarity parameter of transmitted pulses according to a pseudo-random binary sequence. Instead of transmitting uniform positive pulses, the system alternates pulse polarities (+1 or -1) based on the PRBS values. This parameter change creates a coded signal where the desired echo, when correlated with the transmitted code, produces a strong peak, while unwanted reflections from ambiguity ranges produce suppressed responses due to code mismatch.
2Measurement precision
If swept-threshold receiver is used to reconstruct signal, then range profiling capability is improved, but distortion occurs due to unequal positive and negative pulse counts at each threshold
Solution Approach 1:
The patent ensures that the PRBS is designed with balanced properties, where the number of +1 and -1 values are equal (or differ by at most one for odd lengths). This parameter balance in the transmitted code ensures that during swept-threshold reception, each threshold level encounters an equal number of positive and negative polarity pulses. Consequently, the thermometer-coded output from the comparator remains unbiased, preventing signal distortion in the reconstructed range profile.
Solution Approach 2:
The system uses autocorrelation processing as a form of feedback verification. The received signal is correlated with the transmitted PRBS sequence, and the resulting autocorrelation function provides feedback on signal quality. When the PRBS is properly balanced, the autocorrelation peak at zero lag is maximized while sidelobes are suppressed, confirming that the threshold sweeping has produced an accurate range profile without distortion.
3Object-affected harmful factors
If pseudo-random sequences are used to cancel unwanted reflections, then ghost pulse cancellation is achieved, but signal reconstruction becomes complex
Solution Approach 1:
The patent uses a periodic PRBS sequence that repeats after a defined length (e.g., 15 or 31 bits). This periodicity allows the receiver to process signals in repeating cycles, where each cycle independently cancels ghost pulses from specific ambiguity ranges. The regular structure of the PRBS enables efficient implementation of correlation-based cancellation algorithms, reducing computational complexity compared to aperiodic sequences.
Solution Approach 2:
The system changes pulse polarities according to the PRBS values (+1 or -1), creating a coded signal structure. At the receiver, the same PRBS sequence is used to modulate a local replica signal for correlation processing. This parameter-based coding approach simplifies ghost pulse cancellation because the known code sequence allows direct subtraction or correlation-based rejection of unwanted reflections, avoiding the need for complex adaptive filtering or iterative processing.
Data Source
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AI summary
According to a first aspect, the invention provides a pulsed radar comprising a transmitter; wherein the pulsed radar is arranged to generate a string of binary values; wherein the transmitter comprises a pulse generator arranged to generate a pulse signal comprising a series of transmit pulses with polarities determined in accordance with the string of binary values; wherein a first substring comprises a first series of values; wherein a second substring comprises a second series of values; wherein the second substring is different from the first substring; and wherein each value in the second series of values is either the same as or different from the corresponding value in the first series of values according to a repeating pattern; and wherein the string of binary values comprises at least the first substring and the second substring concatenated together and each optionally being reversed before concatenation. By transmitting two related series of pulses, the transmitter can achieve better noise cancellation and/or better rejection of reflections from unwanted ambiguity ranges. The repeating pattern can be selected so as to form a relationship between the polarities of reflections from one ambiguity range as they appear in a different ambiguity range. This relationship can be used to ensure cancellation of unwanted pulse reflections, while ensuring that the reflections of interest are summed, i.e. reinforced.