UWB IRF correlator
- 1. Focus on M-sequence sensor electronics IR-UWB: Correlator -Jayendra Mishra
- 2. UWB Principle Of Operation Ultra-wideband sensors stimulate their test objects with pulses. UWB pulses allows following the propagation and the temporal development of energy distribution within the test scenario Multiple chirp signals of sub nanoseconds spread over a large bandwidth are generated to get an impulse response.
- 3. Measurement Complexity Large peak power signals represents some inconvenience of the impulse measurement technique. Idea is to go away from excitations by strong single or infrequent ‘shocks’ and to pass instead to a stimulation of the DUT by many subtle ‘pinpricks’. Binary pseudo-noise codes represent pulse signals which are more careful with the test objects.
- 4. Binary Pseudo-Noise Codes Pulse signals which have spread their energy homogenously over the whole signal length. Hence, their peak voltage remains quite small by keeping sufficient stimulation power. PN-codes are periodic signals which constitute of a large number of individual pulses ostensible randomly distributed within the period. The theory of PN-code generation is closely connected with Galois-fields and primitive polynomials. Summarized by the term binary PN-code they may have three (-1, 0, 1) and more signal levels. Amalgamated properties of two signal classes Periodic-deterministic signals Random signals Pseudo-noise codes examples: Barker code, M-sequence, Golay-Sequence, Gold-code, Kasami-code
- 5. M-Sequence Maximum length binary sequence or shortly termed M-sequence is Simple to generate and provide best properties to measure the Impulse response function IRF. For a given signal power, lowest amplitude and shortest autocorrelation function. No. of flip-flops in shift register determine the length of M-sequence Spares the measurement objects and the requirements onto the receiver dynamic are more relaxed.
- 6. M-sequence Generation Digital shift registers with appropriate feedbacks. Shift the starting point of the M- sequence to control the time lag of an UWB correlator. In figure the register is of the 9th order. Fibonacci structure deals with cascaded XOR-gates which increase the dead time before a new change of the flip-flop states is allowed.
- 7. Technical Challenges IRF measurement not directly possible with a time extended signal such as the M-sequence. The M-sequence signal has to be subjected to an impulse compression to gain the wanted impulse response. The price to pay for reduced voltage in a UWB signal is the chaotic structure of the receive signal, therefore, a correlator is required for interpretation. Intermediate frequency cannot provide a low freq. as the Nyquist bandwidth is huge for 3 GHz of bandwidth. To perform the correlation of a very wideband signal we need. Cross-correlation to access the wanted IRF Digital impulse compression Frequency domain conversion
- 8. Impulse Correlator Objective: to determine the impulse response function of a test object. A correlator is used to detect the presence of signals with a known waveform in a noisy background. The output is nearly zero (below set threshold) if only noise is present. Impulse compression is performed by the correlator
- 9. Impulse Correlation In UWB, Impulse compression using wideband correlation Convolving with a time inverted stimulus x(-t). Technical implementation Cross-correlation The Sliding correlator PN- correlation
- 10. The Sliding Correlator (a.1) Two shift registers of identical behavior make a sliding correlator. First one provides actual band limited stimulus signal at fc, and second serves as reference to perform correlation fs = fc-Δf. (a.2) Both shift registers have to run in parallel with constant time lag during this time. (b.1) Integrator of the product detector is crucial for the noise suppression and it largely determines the overall sensitivity of the sensor device. (b.2) Waveform sweeping over the initial states of each of the shift registers to get sampled version of correlation function. Rather sliding, the correlator jumps in steps of Δtc = 𝑓𝑐−1 . This gives sampled version of correlation function. Sampling interval equals the M-sequence chip duration.
- 11. Block Diagram Of UWB Correlator Digital ultra-wideband correlation joins sliding correlator and stroboscopic sampling. Sub-sampling: data gathering is distributed over several periods by capturing the signal with the lower sampling rate fs. Since No. of chips: Ns = N = 2 𝑛 − 1 ,n is the order. sub sampling is simply controlled by a binary divider. The placement of the anti-aliasing filter depends on the measurement environment.
- 12. Sub-Sampling Control By Binary Divider. Goal is in to organize a precise and simple sub- sampling control. Interleaved sampling approach for single stage binary divider; so, two stage divider would have four periods of measurements. In figure, 3rd order M-sequence with 7 chips or data samples. For every beat of the clock generator the voltage is captured at every second beat. So, first the odd chip numbers are captured and then the even ones. Wanted results: IRF or FRF of DUT.
- 13. Data Reduction By Averaging the most efficient way permitting both a high sampling rate and a continuous processing of the incoming data stream. Shorter the binary divider, higher the efficiency. Synchronous averaging- used before applying other cascaded linear algorithms for data reduction. the total number of data samples reduces by the factor p, Synchronous averaging performs noise suppression by the factor √p.
- 14. Digital Impulse Compression If only round trip time is of interest we can apply correlation between receive signal y(t) and the ideal m-sequence m(t), for faster implementation. Where IRF shape is less important. The known M sequence data samples arranged in the circulant matrix Mcirc are rearranged in Hadamard matrix for faster computing in Fast Hadamard transform (FHT). Figure shows the Hadamard butterfly. ENOB-number of receiver that includes the quantization noise and the thermal noise. FHT-butterfly is used which only includes sum or difference operations which can be executed at high processing speed. the correlation leads to a noise suppression by the factor √Ns = √(2 𝑛 -1). The equation shown in figure is equivalent to the signal with Max. SNR obtained afte FFT or FHT operation.
- 15. Design Tetrahedron Of Digital Ultra-Wideband Correlator Binary divider length- relaxes the speed requirements onto the receiver. Large dividing factor reduces data throughput but also largely reduces sensitivity of receiver. Recording time Tr- number p of averaging controls recording time length. Target speed and acceleration, and the rate of mechanical object oscillations are the dominant parameters restricting the recording time in radar measurements. Pre-processing- An M-sequence sensor is typically equipped with FPGA for data processing as • Data reduction by averaging or Background removal • Digital impulse compression or correlation • Data conversion into frequency domain and error correction, detection; round trip time estimation.
- 16. KNOCK YOUR SELF OUT!