The average transmitted power of a conventional radar will increase with increasing the transmitted pulse duration 7-8. However, this technique decreases the range resolution ability of the radar by decreasing the bandwidth of the received signal. In order to give increased pulse width without compromising range resolution, a technique we use that provides for the transmission of a long pulse that has a bandwidth corresponding to the short pulse duration in the receiver 8. Then the received echo will process using a compression filter to give a short pulse response to the main lobe of width 1/B that does not depend on the pulse width of the transmitted pulse 8.
Pulse compression technique enables radar to detect targets with sufficiently fine range resolution while remaining within the peak power limitations of the transmitter 8. For a pulse Doppler radar to track a target, the target amplitude following Doppler filtering must exceed that of the clutter residual. Thus, the pulses which may occur within an interval must choose a right value while designing a Costas based radar, to keep the Doppler return radar echoes by keeping the blind speed to an integer value. Blind speed = The above relation defines that the Blind speed will depend on transmitted pulse width and pulse repetition interval. By coding the pulse accordingly using Costas method, the range ambiguity and thus the Doppler approaches can be solved considerably. Costas codes are frequented-coded waveforms and it is a burst of the contiguous encoded pulse waveform, with a different frequency of a finite set of equally spaced frequency.These signals are a variety of frequency modulated radar signals which is frequency coded through a Costas sequence. In Costas coding, a long duration transmitting pulse of width T is divided into N number of sub-pulses, and that is a finite set of equally spaced frequency signal 9.
These are a class of time-frequency-coded waveforms 9. Costas signals are of the class that they possess good correlation characteristics and are better suited for fading channel with additive noise. The coding of Costas sequences are carrying out here and thus the phase information is updated during each cycle by the phase accumulator, also the Doppler frequency approaches which we merged herein coding with OFDM to generate pre-modulation Gaussian Filter based Costas coding.The phase information from the phase accumulator drives the DAC. Here we use pre-modulation Gaussian filtering on the transmitter side to obtain satisfactory prior efficiency and constant envelope properties.
By the application of above-mentioned technique offers coherent detection capability better BER performance and efficiency. This filter produces an efficient output but from the fact that the impulse response of this system is a sharp peak with relatively negligible overshoots. However, an OFDM signal can be modulated either in phase or frequency for radar applications, which increases power and amplifier efficiency 10. Thus, the amplifier operates in the region near to saturation and fading in multipath also increases. This will result in poor range resolution for radar with OFDM. The primary drawback of OFDM is that there will be high amplitude variations of the modulated waveform and this produces high peak-to-average power ratio (PAPR) 11. The reduction in peak-to-average power ratio (PAPR) problem associated with radar with modulated waveform mentioned here can be used to enhance the performance. It is widely used to analyze the variation of the output waveform.
When the signal is passed through a pre-modulation filter with 0.5 modulation index then it is called as GMSK modulation 9.This helps in suppressing high- frequency components. In a radar system, it comprises of a transmitter and a receiver, having a fair SNR by using a GMSK filter with a name as a pre-modulation filter 9. The reason is that this filter produces better accuracy and increased efficiency while filtering the signal components. In this paper, the same filter design will also use in the receiver section and is equally proportioned in the form of matched filter. Pulse shaping is an important task performed on the transmitter side and in order to reduce the effective bandwidth during transmission suitable Gaussian low pass filter is used 5. The choice of bandwidth B and bit-rate T is a compromise between spectrum efficiency and BER performance 1.
Hence, smaller BT leads to compact spectrum and more ISI. Hence, these parameters suitably are chosen to reduce ISI while doing this paper. The above mentioned Gaussian filter introduces overlapping of the transmitted signal, and this causes the degradation and it is small if the 3dB bandwidth bit duration product (BT) is greater than 0.5 value 9. This ISI occurred by the channel can be controlled by filtering the shaped pulses5. Earlier initially, the input data is in the form of NRZ, data and it is given to a pulse shaping filter (Gaussian filter) before feeding them to the LFM modulator.
Digital phase modulation system involves sending information through channel either by varying the physical quantities and it is smoothed by using pulse shaping filter. The requirement of pulse shaping filter is effective to nullify the spectral leakage, reducing ISI and channel width5.These results in a very compact signal spectrum and better utilization of available bandwidth. At the receiving end, the matched filter eliminates the reflected echo to overlap with a subsequent symbol period 11. II.
SYSTEM DESCRIPTION In this section, we give the details of Costas coding technique and how the modulation is undergone on the transmitter side. Here constant amplitude based OFDM is used to provide a constant amplitude with a relatively low PAPR value. This method explained here which involved a signal transformation from the frequency domain to time domain prior to amplification and an inverse process will be at the receiver during demodulation. Costas coding offers better correlation capabilities and is better suited for spectrum analysis. The signal transformation which is done in the time domain converts these variations of the signal into a constant mean power called as Constant Amplitude.
This method offers a very low peak to average power ratio13 reduction for the conventional OFDM. This requires two steps of algorithm design. Step1: Time domain signals from OFDM source block having several subcarriers which are superpositioned using pre-coding.Step2: The pre-coding weights are optimized to minimize the amplitude variations. Here the phase accumulator will produce a sequence of phase signal. This phase signal is transformed into equivalent sine waveform by using a look-up-table. The pre-modulation filter helps to convert in suppressing the high-frequency components to a smooth signal.
The filter output is either frequency and phase modulated to an LFM signal and then convert this signal to an analog waveform. In Fig (1) the phase accumulator along with D/A converter will generate a sine wave. The phase accumulator that adds each pulse to generate a sine wave signal having a frequency Fout. Fout is given by Fout= …
…..
….
…
……
……(1)Where M is the resolution of phase accumulator, is the clock frequency and N is the number of pulses that count by the accumulator. where M is the resolution of the tuning word (24 to 48 bits, depending on DDS design), and N is the number of pulses of, matching the smallest incremental phase change of the phase accumulator’s output word.
Therefore, the tuning word defines the output frequency as a fraction of the reference clock frequency. The encoder here used to code the binary data sequence that can be used into its equivalent Costas code of length N. This helps the receiver to overcome the effect of noise and interference encountered in the transmission.
Especially Costas coding technique is used to enable unambiguous range and Doppler measurement and at the same time minimising Crosstalk between frequencies for a particular range In order to reduce the ISI problems occur during transmission suitable cyclic prefix codes are inserted between the blocks. The resulting part of the peak signal is clipped off in order to avoid the higher PAPR values to the smallest value. This maintains the peak to the average ratio to a low value. However, an OFDM signal can be modulated either in phase or frequency for radar applications, which increases power and amplifier efficiency 10. Thus, the amplifier operates in the region near to saturation and fading in multipath also increases. This will result in poor range resolution for radar with OFDM.
The primary drawback of OFDM is that there will be high amplitude variations of the modulated waveform and this produces high peak-to-average power ratio (PAPR) 11. The reduction in peak-to-average power ratio (PAPR) problem associated with radar with modulated waveform mentioned here can be used to enhance the performance. It is widely used to analyze the variation of the output waveform.
When the signal is passed through a pre-modulation filter with 0.5 modulation index then it is called as GMSK modulation 9. This helps in suppressing high-frequency components. In a radar system, it comprises of a transmitter and a receiver, having a fair SNR by using a GMSK filter with a name as a pre-modulation filter 9. The reason is that this filter produces better accuracy and increased efficiency while filtering the signal components. In this paper, the same filter design will also use in the receiver section and is equally proportioned in the form of matched filter. Pulse shaping is an important task performed on the transmitter side and in order to reduce the effective bandwidth during transmission suitable Gaussian low pass filter is used 5.
The choice of bandwidth B and bit-rate T is a compromise between spectrum efficiency and BER performance 12. Hence, smaller BT leads to compact spectrum and more ISI. Hence, these parameters suitably are chosen to reduce ISI while doing this paper.
The pre-modulation Gaussian filtering introduces ISI in the transmitted signal, but the degradation is small if the 3dB bandwidth bit duration product (BT) is greater than 0.5 value 9. This ISI occurred by the channel can be controlled by filtering the shaped pulses5.
Earlier initially, the input data is in the form of NRZ data, and it is given to a pulse shaping filter (Gaussian filter) before feeding them to the LFM modulator. Digital phase modulation system involves sending information through channel either by varying the physical quantities and it is smoothed by using pulse shaping filter. The requirement of pulse shaping filter is effective against eliminating spectral leakage, reducing the channel width and also to eliminate the ISI 5. This result in a very compact signal spectrum and better utilization of available bandwidth. At the receiving end, the matched filter eliminates the reflected echo to overlap with a subsequent symbol period 11.
