Embedded Digital SISO Radar using Wireless Open Access Research Platform for Object Detection and RCS Measurement

Published Online November 2015 in MECS

Patented radar technologies [1] are now available that has major advantages eliminating the most of the disadvantages of either pulse or continuous wave approaches. Those are based on the use of a digital pseudo-random binary code, similar to that Spread Spectrum technology in Wireless mobile communications. The technology uses DSSS (Direct Sequence Spread Spectrum) signals to create noise like modulation, making the transmitted signal virtually undetectable. Spread spectrum radio has long been used in military communications because of this advantage. Thus the waveform of choice will be Phase coded Pulse Compression using digital techniques instead of Linear

Frequency modulation LFM based analog techniques [2]. Sometimes pulse compression radars have been called spread spectrum radars. The pulse compression nature of this signal processing provides significant protection against normal Interference. Therefore, DSSS radar [3] will be aimed for the development. Scientists and Technologists involved in the development of radar and remote sensing systems all over the world are now trying to involve themselves in saving of manpower in the form of developing a new application of their ideas in embedded integrated Digital radar [4]. Digital or DSSS Radar has been a vibrant scientific field for the last few years [5]. Over the years, digital radar systems have developed considerably, and contemporary advanced radars use Software Define Radio (SDR) based development to minimize the size of Radar which may be useful in human daily life for home security or medical purposes, etc. In software radio, most signal processing is performed by software and reconfigurable digital hardware, so it offered some flexibility that is impossible in traditional radio implementation. Its architecture allows of software programmability to many parameters such as bandwidth, central frequency, modulation and demodulation scheme, coding and decoding schemes, link layer protocol and so on. However, software radio does not imply completely remove the hardware, it means to replace some fixed application specific hardware with flexible and reconfigurable hardware that may be programmed for desired functionality. Since its flexibility and robustness, software radio already been applied in communication, moreover it will be the future develops direction of radio. At the same time this design thought and method also completely suit in the radar design. In our design we have tried to develop DSSS radar using an embedded WARP SDR kit [6], in which polyphase code is used in the radar transmitter to provide the spreading effect, whereas a correlation based receiver is designed in the same SDR kit for the target detection and for Radar Cross section (RCS) measurement. In radar, a target is characterized by its radar cross section (RCS) function [7]. This work is basically extension of previous DSSS Radar development work done at Sikkim Manipal Institute of Technology (SMIT) [8] [9]. There authors had developed a DSSS Radar using Arbitrary Waveform generator as transmitter, RF up and down conversion section designed by RF components and a Digitizer along with vector signal analyzer as receiver but this set up is heavy in size and a combination of several costly equipments. Our efforts convert this huge setup in a single board environment which is a very low cost solution also.

• II.    Warp Sdr Hardware

The Wireless Open-Access Research Platform (WARP) is a scalable and extensible programmable wireless platform, built from the ground up to prototype advanced wireless networks. WARP combines high-performance programmable hardware with an open-source repository of reference designs. The special feature of this hardware is as follows: [10]

• •    Xilinx Virtex-6 LX240T FPGA

• •   2 programmable RF interfaces, each with:

o 2.4/5GHz transceiver (40MHz RF bandwidth)

o Dual-band PA (20dBm Tx power)

o Shared clocking for MIMO applications

• •    FMC HPC expansion slot

• •   2 gigabit Ethernet interfaces

• • DDR3 SO-DIMM slot

• •    FPGA config via JTAG, SD card or flash

• •    User I/O:

o USB-UART o  12 LEDs o  2 seven-segment displays

• o   4 push buttons

• o   4-bit DIP switch

•   16-bit 2.5v I/O header [11]

Fig.1. WARD Hardware board with 2 Radio Cards

• III.    Radar Code Generation

Baseband transmitted Signal of the Radar System mainly consists of pulsed PN (P4 code) sequence generation. P4 code used in SS Radar operation gives better SNR at the receiver. The Spreading signal is formed by multiplying a pulse width of 5 usec with Polyphase code having chip rate of 200 ns [= 1/Bandwidth]. SDR board is highly programmable using MATLAB. In first stage authors have concentrated their development to generate P4 code through SDR Board. P4 codes are beneficial in modern radar, having MPSK (Mary Phase Shift Keying) property, and are normally derived from the phase history of Frequency-modulated pulse. P4 codes can be expressed as: [12]

φ i = π (i -1) (i - 1-M)/M,           (1)

Whereφi = i th order phase.

M= no of phase states

The values of φi are computed using equation 1 which is generated by Matlab coding. A PC along with Matlab is connected through SDR for hardware realization of the P4 code. The P4 code is generated and communicated over a RF transmitter which is connected with a 2.4 GHz parabolic dish antenna. Another parabolic dish is connected with SDR receiver which is receiving the RF signal and processed through SDR. P4 code is recovered and saved in a file which is plotted in fig 4. Baseband code generation is experimented over communication system set up is shows in fig 2. Generated Baseband code using WARP SDR is shown in fig 3 and recovered baseband IQ P4 in receiver side is shown in fig 4.

Fig.2. System Setup in Communication Mode.

Fig.3. Generated Baseband code through SDR

Fig.4. Received I Baseband zoomed Data from SDR board

• IV.    System Bandwidth Selection

By controlling the transmit bits author have achieved 30 MHz bandwidth as shown in figure 5, around 350 samples are transmitted to get desired bandwidth. The high bandwidth signal is advantageous for the Radar experiment. The author have tried the Radar experiment with 5 MHz bandwidth also which is not impressive, so 30 MHz P4 code is transmitted towards the target and after reflecting back the correlation receiver is generating a corellation peak if the code is matched. This receiver is working based on the matched filtering process which is used for 3G mobile communication also. [13]

Fig.5. Signal Bandwidth set to 30 MHz

• V.    Siso Radar Experiment

In outdoor experiment [14], Transmit and Receive antenna are placed at outdoor environment. Flat plates are used to place in front of the Radar as target which is located at the opposite side of the Antennas, as our bandwidth is 30MHz the range resolution is calculated to 5mt. That means our Radar are able to identify different targets separately if Targets are separated with minimum 5mt distance from radar as well as from each others. If Target distance is less then 5mt then multiple target peaks will not be resolve separately and it will show multiple targets as single one. Indoor Part of our Radar which is basically the Baseband Part, consisting of SDR Board and the PC in which Matlab is running is shown in figure 6. WARP SDR board is having two radio cards inbuilt with it. One Radio is programmed as Transmitter and another one is kept as receiver.

Fig.6. SDR Setup with PC

Fig.7. Target Detection Setup

Fig.8. Target Detection of 1mt/1mt Flat Plate

Two horn antennas are connected with the RF cards using environment using those long RF cables which is mainly the RF Part of our experiment, is shown in figure 7. Using above system author has performed Radar experiment. In figure 8 the target detection is shown, peak is generated by the corellation between the transmitted and received signal, received signal is saved in a file using LAN port which is connected between the board and PC. Received RF signal is processed by SDR board and baseband P4 code in receiver side is saved in a file in the connected PC. The file is saved in mat format using Matlab programming. Received code and transmitter reference is corellated and target peak is generated. In this experiment an another peak is also present in the result window which is known as RF leakage, it is generated due to direct leakage of transmitted signal in between two RF cards present in SDR board as both the cards are embedded very nearby in small SDR hardware . Significantly the amplitude of the received signal is improved using corellation gain which is detected at sample number 351.

• VI.    Multi-Target Detection