Analysis and Simulation of Relay Assisted Pulse Position Modulation Scheme using UWB System

In modern world of communication system, a coded M-ary pulse position modulation (PPM) scheme for transmitted reference ultra-wideband (TR-UWB) systems is proposed. The modulation level M in conventional M-ary PPM TR-UWB scheme, is defined only with a number of possible pulse positions Z. So, Z radio frequency (RF) wideband delay lines are required in order to map data bits into the proper pulse position. The number of required delay lines in the proposed scheme is reduced by mapping data bits in both different orthogonal codes and pulse position on frame level of the signal, producing a modulation level of M = KZ. The analytical model for realistic IEEE standard UWB channel models is developed to evaluate the performances of the proposed coded M-ary PPM scheme, The performances of the proposed scheme are compared with these of the conventional 4-ary PPM scheme for the same modulation level M. The results show that the hardware complexity is lower in terms of number of required RF delay lines with the proposed coded M-ary PPM scheme, while it achieves approximately the same bit error probability (BEP), higher data rate and higher bandwidth efficiency. However, by increasing the number of used orthogonal codes the minimum number of frames per one information symbol rises and consequently the maximum achievable data rate is limited. The trade-off between the number of orthogonal codes and target data rate thus should be made. Keywords—PPM, cooperation, relay, diversity, power allocation, Ultra-wideband, decode-and-forward, DF, performance analysis, cooperative diversity, correlated noise. INTRODUCTION A. Objectives: 1. Create understanding about 4-ary PPM. 2. Demonstrate the need for using PPM. 3. Propose and analysis a novel method for implementing the 4-ary PPM that is more efficient in using the available bandwidth in a multiuser access system B. Introduction and Overview: Ultra Wideband (UWB) technology is the primary candidate for the physical layer of the upcoming standards for wireless personal area networks, since it provides reliable high-speed data transmission at short ranges over severe multipath conditions. It also exhibits robust Multiple Access (MA) performance with little interference to other communication systems sharing the same bandwidth due to its very low Power Spectral Density (PSD). It also offers a promising solution to the RF spectrum drought by allowing new services to coexist with current radio systems with minimal or no interference. The advantage of avoiding the expensive spectrum licensing fees is achieved using this coexistence that providers of all other radio services must pay. The fundamental characteristic of UWB is the extremely large bandwidth, which is required since very narrow pulses of appropriate shape and sub nanosecond duration, are being used by the transmitted signal. One of the most widely studied schemes for UWB communications employs Pulse Position Modulation (PPM) combined with Time Hopping (TH) as its multiple access technique. The UWB pulses are time hopped within a fixed time window (frame) and each transmitted symbol is spread over several pulses in order to facilitate multiple users. In PPM the position of each pulses varied by each instantaneous sampled value of the modulating wave in relation to the position of a recurrent reference pulse, it used exclusively for transferring digital signals and cannot International Journal of Engineering Research and General Science Volume 3, Issue 3, May-June, 2015 ISSN 2091-2730 171 www.ijergs.org be used with analog systems. Also it used for transferring simple data and is not effective at transferring files. Due to the important role of the M-ary PPM modulation technique, we will focus our study on its performance and propose a novel improvement for such a technique with main target of improving the overall system performance under interference dominated system. C. Applications: Pulse position modulation has many purposes, especially in RF (Radio Frequency) communication such as, pulse position modulation is used in remote controlled aircraft, cars and boats. Also it’s often used in optical communication, such as fiber opticswhich has a little or no multipath interference, we can see how PPM is used in optical fibers, i.e. sending a laser pulse in a random location after dividing the frame into number of frames. Fig.1. Pulse Position Modulation in optical fibers PULSE POSITON MODULATION Pulse Position Modulation, sometimes known as pulse phase modulation is used for digital signal transition. It is used in fiber optics and IR (infrared) remote controls where there is a lack of interference; this technique uses pulses of the same breath and height but is displaced in time from some base position according to the amplitude of the signal at the time of sampling. The Pulse Position Modulation (PPM) is a modulation technique designed to achieve the goals like simple transmitter and receiver circuitry, constant bandwidth, noise performance and the power efficiency and constant transmitter power. The amplitude of the pulse in Pulse Position Modulation is kept constant as in the case of the FM and PWM to avoid noise interference. Unlike the Pulse Width Modulation the pulse width is kept constant to achieve constant transmitter power. The modulation is achieved by varying the position of the pulse from the mean position according to the variations in the amplitude of the modulating signal. The Pulse Position Modulation (PPM) can be actually easily generated from a PWM waveform which has been modulated according to the input signal waveform. The Pulse Position Modulation can be demodulated both synchronously and asynchronously. The synchronous demodulation requires synchronization of the receiver with the transmitter and hence it is complex. The quality will be comparatively less when using the asynchronous demodulation technique, but with an advantage of very simple circuit for demodulation. A. Block Diagram of PPM There are different methods for extracting the message signal from a PPM wave synchronously and asynchronously. The asynchronous demodulator uses a low pass filter to filter out the message signal from the modulated wave. The implementation of a PWM modulator is represented in following block diagram. Fig.2. Simple PPM Block diagram International Journal of Engineering Research and General Science Volume 3, Issue 3, May-June, 2015 ISSN 2091-2730 172 www.ijergs.org B. PPM Generation: The PPM required for this project is generated from a PWM wave which is modulated with the message signal. This message signal used here is a pure sine waveform generated using the Wien Bridge Oscillator (WBO). A ramp signal is generated with the help of a RC charging circuit and a comparator IC. Another comparator IC which is having ramp signal as one of its input and the message signal as other can produce a PWM wave at its output. This Pulse Width Modulation wave is then used to generate the PPM wave using a mono-stable multi-vibrator. The given block diagram of the PWM generation circuit is given below: Fig.3. PPM Generation C. Sine Wave Generator The circuit which is based on the Wien Bridge Oscillator (WBO) circuit. The WBO circuit can produce distortion less sinusoidal sweep at its output. This circuit is designed in such a way that both the amplitude and frequency of the oscillator can be adjusted using potentiometers. The sine wave generator is adjusted to produce a waveform of frequency 1 KHz. The Ramp generator used in this circuit is designed with an RC charging circuit and an op-amp. The RC charging circuit is connected to the output of the op-amp and the voltage across the capacitor is connected to one of the input of the op-amp. The variable pin of a potential divider is connected to another input of the op-amp to which divides the voltage from the output of the op-amp. The ramp waveform is applied to one of the input of another comparator circuit and the output of the comparator circuit will be a PWM waveform. The PPM generation is achieved with the help of a mono-stable multi-vibrator designed using a 555 timer IC. D. Ramp Generator: The Ramp generator used in this circuit is designed with an RC charging circuit and an op-amp. The RC charging circuit is connected to the output of the op-amp and the voltage across the capacitor is connected to one of the input of the op-amp. The variable pin of a potential divider is to another input of the op-amp connected to which divides the voltage from the output of the op-amp. E. Comparator: The ramp waveform is applied to one of the input of another comparator circuit and the output of the comparator circuit will be a PWM waveform. Features 1. On-board message signal with variable amplitude 2. Three different frequency message signal 3. On Board carrier signal 4. PPM Modulation using Timer IC 5. PAM Demodulation using Low Pass Filter 6. Amplifier using Op-Amp 7. Internal Power Supply +5V , +12V/ 500 mA 8. Number of test point to study the PPM system 9. User friendly front panel block diagram F. PPM Vs. PAM: PPM is superior to PAM and PDM in the sense that it has higher noise immunity since the only thing the receiver needs to do is to detect the pressure of the pulse at the correct time. The amplitude and duration of the pulses are irrelevant. Pulse Amplitude Modulation (PAM), like PPM is a form of signal modulation where it differs the message information is encoded in the amplitude of a series of signal pulses. Pulse Amplitude Modulation is an analogy pulse modulation scheme in which the International Journal of Engineering Research and General Science Volume 3, Issue 3, May-June, 2015 ISSN 2091-2730 173 www.ijergs.org amplitude of train of carrier pulse is varied according to the sample value of the message signal. Pulse Duration Modulation (PDM), is a pulse modulation technique that transmits analogue signals. PDM is not dependent on the height of the pulse but does depend on its duration.