Design and Implementation of Ultra-Wideband Impulse Radio Transmitter Giorgos Tatsisa, Constantinos Votisa, Vasilis Raptisa, Vasilis Christofilakisa,b, Spyridon K. Chronopoulosa, Panos Kostarakisa a
Physics Department, University of Ioannina, Panepistimioupolis, Ioannina, 45110, Greece Siemens Enterprise Communications, Enterprise Products Development, Athens, 14564, Greece Emails:
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Abstract. This paper describes the design and implementation of an UWB Impulse Radio transmitter. This UWB transmitter produces very short pulses with possibility of time-shifting to be used in cases of Pulse Position Modulation (PPM). The transmitter, which is based on step recovery diode (SRD), can operate at 50Mbps with high repetition rate. Experimental results show that, ultra short pulses with duration of about 1nsec and spectrum exceeding 2GHz at -10dB, are produced and transmitted. Keywords: UWB, Impulse Radio, Software Radio, SRD, Transmitter BPPM, QPPM. PACS: 84.40.Ua
INTRODUCTION Ultra-Wideband Impulse Radio is one of the most promising technologies of the recent years. UWB technology has many benefits owing to its ultra-wideband nature, which include high data rate, availability of low-cost transceivers, low transmit power and low interference. Impulse Radio is based on transmitting ultra short (in the order of nanoseconds) and low power pulses. Impulse radio is advantageous in that it eliminates the need for down-conversion and allows low-complexity transceivers. It also enables various types of modulations to be employed, including on–off keying (OOK), pulse-amplitude-modulation (PAM), pulse-position-modulation (PPM), phase-shift-keying (PSK). This paper describes the design and implementation of a transmitter that enables the transmission of ultra short pulses, in the order of nsec, supporting Pulse Position Modulations (i.e. BPPM, QPPM) and On-Off Keying as well. Experimental measurements shown a good repetition rate at 50Mpps, and the pulse bandwidth at -10dB exceeds 2GHz.
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TRANSMITTER DESIGN The main purpose of this transmitter is to generate a particular sequence of UWB pulses that can be used in Impulse Radio communications. Fig. 1. shows the sequence of a Binary Pulse Position Modulation scheme (BPPM). The two symbols (0, 1) are distinguished by a time delay Δ (BPPM index). We can assume that the non-delayed pulse represent bit ‘0’ and delayed represent bit ‘1’.
FIGURE 1. BPPM pulse sequence of bits [0 0 1 0 1 0] a) Pulse shaper output, b) Modulator output
The transmitter consists of two distinctive parts. The first produces square pulses that are modulated as PPM and it is implemented almost completely with digital elements. The heart of this pulse generator-modulator is a FPGA (Field Programming Gate Array). The second part is a pulse shaper that takes the modulated square pulses and gives to them the form of an UWB impulse that has the shape of a gauss monocycle (1st derivative Gauss). Fig. 2., shows a simple block diagram of the transmitter.
FIGURE 2. UWB-IR Transmitter diagram
Pulse Modulator In order to create the pulse sequence of Fig. 1b., the digital part of the transmitter first creates the square-like pulse sequence shown in the Fig. 2. The block diagram of the pulse generator-modulator is shown in Fig. 3. This logic circuit is implemented in a FPGA. The data bits are pre-stored in a memory (ROM), which has an address bus as input and a data output of width 1bit. The address bus has a width that depends on the number of data (for example 10bits for 1024 data bits). The output of the ROM (data) is the select input of a multiplexer which selects as output, either the non-delayed (for data ‘0’) or the delayed (for data ‘1’) clock input. The time delay D represents the BPPM index Δ. This delay is produced by connected an external transmission line into two I/O of the FPGA. The length of this transmission line defines the delay time. Internal buffers in the FPGA can also produce a time delay but
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this is not an accurate way, because the exact propagation time of the implemented buffers is unknown, although after the programming of the FPGA this propagation time is constant (with slight variance by temperature). The inverter INV1, connected between clock and counter acts as a half clock period delay, to ensure that the rising edge of the clock pulse inserts to multiplexer before the counter change its output, since it is positive edge triggered. The clock source may be an external crystal oscillator, which is embedded, or a clock generator via coaxial cable connection. As it can be seen from figure 3, the output pulses of the modulator are actually a delayed version of the clock source pulses. There are no divisions of the source clock. This gives, theoretically, the opportunity to achieve the maximum clock rate of the FPGA. The inverter INV2, at the OUT of the modulator is needed for next stage of transmitter, the pulse shaper.
FIGURE 3. Block diagram of modulator (BPPM)
Pulse Shaper The pulse shaper is based on a step recovery diode (SRD), known for the characteristic very rapid transition from its forward to reversed bias. This part is the most critical in order to obtain the pulse shape of an efficient UWB transmission. A lot of study about UWB pulse generators has been done and several designs are proposed, based on a SRD diode [1-5]. The design of this circuit in this paper is shown in figure 4. It consists of a buffer, two amplifiers, capacitors, resistances, two transmission lines, a SRD diode and a Shottky diode. The buffer is used to match the output of the digital circuit (LVTTL) to 50Ω amplifier. It has high input impedance and output impedance of 50Ω. The amplifier has an inverted input, thus an inversion of the pulses is done before the buffer by adding at the output of the digital part an inverter (NOT gate INV2 in figure 3). A positive pulse is driven on the SRD and at the output of the diode a step-like pulse is produced. This pulse splits into two parts, one propagates to forward transmission line to Shottky diode and the other to the transmission line T1 wich is reflected. The reflected pulse is inversed and delayed. The combination with the initial pulse produces a Gauss pulse. Shottky diode acts as a half wave rectifier, eliminating the negative part of the pulses. The second transmission line T2, as for T1, also reflects the signal and the combination with the initial produces the final form of the monopulse. Resistance R1 is 50Ω used for impedance matching. Inductance L is used for ringing level reduction. It is found experimentally that reduces a small noise after the pulse. At the end an amplifier, with inverted input, drives the output pulses to an antenna through a 50Ohm coaxial cable.
FIGURE 4. UWB Pulse shaper schematic diagram
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With modifications to the modulator M-ary PPM schemes are feasible. For example for Quartenary PPM (QPPM), at design in figure 3 the data bits can be sort in memory by two and the multiplexer changes into 4/1. Also three different time delays are required. OOK is also applicable, if we cancel the delay D, in figure 3, by setting the corresponding multiplexer input to zero. This transmitter is fabricated on a FR4 glass epoxy substrate with thickness 1.5mm and dielectric constant of 4.6. In next section the experimental results are given.
EXPERIMENTAL MEASUREMENTS The transmitter was tested and measured with the help of oscilloscope and spectrum analyzer. Fig. 5. shows the output waveform of a single transmitted pulse measured with oscilloscope. The pulse is fairly symmetric, its width is less than 1nsec and it has an amplitude of 2Vpp. In Fig. 6. we can observe a typical BPPM transmitted sequence. The modulation index is 1nsec and it can be shown that the second and third pulse are shifted by this delay. This sequence corresponds to bits [0 1 1 0 0]. The bit period is 20nsec which means that the bitrate is 50Mbps. The transmitter has an on board crystal oscillator of 50MHz, but one may connect an external oscillator to reach greater frequencies. Similar measurements with higher clock frequency showed that this transmitter works properly with a rate of 90 Mbps, but with reduced amplitude. Fig. 7. shows the spectrum of an unmodulated pulse train of 50Mpps. That explains the spectral vertical lines, every 50MHz. The outline of this spectrum is actually the spectrum of a single pulse shown in Fig. 5. It is clear that the bandwidth is over 2GHz at -10dB concluding that it is an efficient UWB transmitter.
FIGURE 5. Measured monocycle with oscilloscope
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FIGURE 6. Measured BPPM pulsetrain [0 1 1 0 0] with oscilloscope
FIGURE 7. Spectrum of monocycle pulse train (50 Mpps)
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CONCLUSION The design and implementation of an UWB-Impulse Radio transmitter analyzed in this paper. The transmitter was tested and measured with oscilloscope and spectrum analyzer. The results shown that the transmitter works properly as a PPM modulator with pulse rate of 50Mpps, while it is possible to reach the rate of 100Mpps. The measured banwidth of the transmitted pulses exceeds 2GHz. Applicable modulation schemes are PPM (BPPM, QPPM) and On-Off Keying.
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Wu Jian-bin, Tian Mao, “A New Short Pulse Generator for Ground Penetration Radar”, IEEE 2007. Alexandre Serres, Yvan Duroc, Tan-Phu Vuong, Jose Ewerton P. de Farias, Glausco Fontgalland, “A New Simple UWB Monocycle Pulse Generator”, IEEE 2006. Jeongwoo Han, Cam Nguyen, “A New Ultra-Wideband, Ultra-Short Monocycle Pulse Generator With Reduced Ringing”, IEEE 2002. Tzyh-Ghuang Ma, Chin-Jay Wu, Po-Kai Cheng, Chin-Feng Chou, “Ultrawideband Monocycle Pulse Generator with Dual Resistive Loaded Shunt Stubs”, Microwave and Optical Technology Letters / Vol. 49, No 2, February 2007. Jeong Soo Lee, Cam Nguyen, Tom Scullion, “New Uniplanar Subnanosecond Monocycle Pulse Generator and Transformer for Time-Domain Microwave Applications”, IEEE Transactions on Microwave Theory and Techniques, Vol. 49, No 6 Tune 2001.
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