In Ultra-Wideband Impulse Radios

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In Ultra-Wideband Impulse Radios Int. J. Communications, Network and System Sciences, 2010, 3, 425-429 doi:10.4236/ijcns.2010.35055 Published Online May 2010 (http://www.SciRP.org/journal/ijcns/) A/D Restrictions (Errors) in Ultra-Wideband Impulse Radios Giorgos Tatsis1, Constantinos Votis1, Vasilis Raptis1, Vasilis Christofilakis1,2, Panos Kostarakis1 1Physics Department, University of Ioannina, Ioannina, Greece 2Siemens Enterprise Communications, Enterprise Products Development, Athens, Greece E-mail: {gtatsis, kvotis, vraptis}@grads.uoi.gr [email protected], [email protected] Received February 4, 2010; revised March 6, 2010; accepted April 20, 2010 Abstract Ultra-Wideband Impulse Radio (UWB-IR) technologies, although are relatively easy in transmission but they present difficulties in reception, in fact the reception of such waveform is a quite complicated matter. The main reason is that in fully digital receiver the received waveform must be sampled at a rate of several GHz. This paper focuses on the impact of the Analog to Digital (A/D) conversion stage that is used to sam- ple the received waveform. More specifically we focus on the impact of the two main parameters that affect the performance of the Software Defined Radio (SDR) system. These parameters are the bit resolution and the time jittering. The influence of these parameters is deeply examined. Keywords: UWB, Impulse Radio, ADC, Jitter Error, Quantization Error 1. Introduction is transmitted. Several modulation schemes are used such as Bi-phase, Pulse Position, On-Off keying etc. [4]. In UWB transmission has recently received great attention this paper we choose Binary Pulse Position Modulation in academia and industry for applications in wireless (BPPM). We consider transmission through indoor mul- communications. A UWB system is defined as any radio tipath environment [5], in the presence of white Gaussian system that has a 10 dB fractional bandwidth larger than noise. The performance of the system is evaluated by the 20% of its center frequency, or has a 10 dB bandwidth bit error probability (BEP) in terms of jitter and quanti- larger than 500 MHz. It is expected that many ap- zation noise. An expression of BEP is derived and nume- proaches used for short-range wireless communications rically results are presented. will be revaluated and a new industrial sector with high data rate will be formed. Fully digital receiver for 2. Analog to Digital Conversion UWB-IR requires the use of A/D conversion and SDR techniques as described below. The RF waveform re- During the A/D conversion additional noise is produced ceived from the antenna is directly digitized from the at the output of the A/D converter due to two main rea- antenna via an A/D conversion stage. Then the digital sons: Quantization and Jitter error. The first is illustrated information derived from the UWB waveform is handled and processed by a DSP. However, this process, intro- in Figure 1 (a) and it is a result of the difference be- duce new signal distortions, due to the new uncertainties tween the analog, continuous input signal and the digi- introduced, that are the jitter error and the quantization tized output of the ADC. The finite ADC resolution error. The latter comes exclusively from the bit resolu- gives the form of the stairs-like signal. If an ADC has a tion of the A/D converter, while time jittering comes bit resolution of N bits, it means that the output signal N merely from the aperture jitter of the ADC, and from is coded at 2 different binary numbers, from 0 to N clock jitter of the sampling circuitry [1,2]. In this paper 2 –1. Let assume that the input signals peak to peak we examine the impact of those two parameters on the amplitude (Vpp ) is the same with the ADC full-scale bit error rate performance of an UWB-IR fully digital voltage range. Then the corresponding quantization step N receiver. In UWB-IR systems a pulse train, consisting of is QV pp /2 . An amplitude value at the input is very short pulses and occupying very large spectrum [3], mapped to the nearest N bit binary number and the Copyright © 2010 SciRes. IJCNS 426 G. TATSIS ET AL. absolute difference between input-output can be from time t1 , corresponding at a multiplicate of sampling zero to Q /2 , thus the quantization error is from period. Due to jitter effect the sample taken by the ADC Q /2 to Q /2. We assume that the input signal can is the one at the time tt1 j , where t j is a random take any random value within a quantization step, with variable, assuming normally distributed with zero mean equal probability. Therefore the distribution of quantiza- and standard deviation j . The corresponding voltage tion error is uniform and its probability density function error is then, VVttjj()()11 Vt. By rewriting this f ()x , is shown in Figure 1(b). Obviously, it has a mean expression we have, of zero and it is easy to prove that the standard deviation Vt()()11 tj Vt of quantization error is q Q /12, as follows, VVttj ()()11jj Vt t (1) t j Q Q 32 122 QQ 222 22 2 q xfxdx() Q xdx xdx For small t we can approximate the expression in QQQ 0 24 12 j 2 the brackets with the first derivative of Vt() [6], and The second error that concerns our study is called jitter obtaining, error and it is a result of the non infinite timing precision dV() t ' VttVtjjj ()1 (2) of the sampling procedure and the ADC imperfections. dt The fact is that there is an uncertainty at the sampling tt 1 time which causes an uncertainty at the input voltage of the signal. This effect is shown in Figure 2. Let the input 3. Signal Model Description signal at an A/D converter be Vt(). We focus at the The transmitted pulses have the form of a Gauss mono- cycle, i.e. the first derivative of a standard Gauss pulse. Figure 3 shows a schematic representation of the BPPM modulated transmitted signal. The bit period is Tf (fra- me period) and the time offset represents the modu- lation index. Time is divided into frames, the period of e d tu li mp A V(t) ∆tj ∆Vj Time (a) f (x) t1 t1 + ∆tj t Figure 2. Jitter error effect. 1/Q Tf Tf -Q/2 Q/2 x (b) Figure 1. (a) Digitization of an analog continuous signal 0 1 0 (dotted line) to discrete and quantized signal (normal line); (b) Uniform distribution f ()x of quantization error x . Figure 3. BPPM signaling. Copyright © 2010 SciRes. IJCNS G. TATSIS ET AL. 427 the frames is Tf . We determine the symbol by its posi- All the vectors has length, NTff fs, where Tf is tion within each frame. A logic “0” is a pulse at the be- the frame period and fs is the sampling frequency. The ginning of the frame, while a logic ‘1’ is delayed by a templates for the two symbols ( xx01, ) are the transmit- small amount of time . The modulated pulses wave- ted symbols for ‘0’ and ‘1’ respectively after the channel, form s()t is expressed by Equation 3 below, n is Gaussian process, representing total additive noise, N1 with a mean value of zero and a double side power spec- tral density, N /2. s()tEwtjTb bfj ( ) (3) 0 j0 The channel impulse response, corresponding to the IEEE 802.15.3a model [5] for indoor multipath environ- where, wt() is the pulse shape (first derivative of a L1 ments, is given by ht() ( t ), where L is Gaussian pulse) normalized to have total energy l 0 ll the total resolvable channel paths, ll, are the gain wt()2 dt 1, E is the energy per bit, T is the b f coefficient and time delay, respectively, for the corre- sponding l path. Thus the transmitter signal after the frame period, bj is the j-th bit, is the BPPM modu- lation index, N is the total number of transmitted channel is expressed as follows, pulses. is related to the pulse width with the rela- L 1 w (4) xt() ( s h )() t astll ( ) tionship Tp 2 w , where Tp represents the width of l 0 the pulse. The modulation index is chosen to satisfy the orthogonality of the transmitted symbols, i.e., where (*) denotes convolution. The received discrete signal r is given in Equation 5 wtwt() ( ) dt 0. We choose greater than the below, pulse duration, i.e., T . p r=x n nj nq (5) where, n NN(0,22 ), / 2 is the total additive 4. Theoretical Analysis of Error Probability nn 0 noise at the receiver, n is the noise vector due to jitter j error and by using Equation 2. we have: n N(0, 2 ), In order to derive an expression for the probability of j jit T error, we consider the transmit and receive system model 22'' jit j xx and nq is the noise term due to shown in Figure 4. The transmitted signal, s()t , de- QQ Q scribed above, propagates through a multipath channel quantization noise i.e. nqq U (,), . with impulse response ht(). Then it is converted from 22 12 analog to digital using an A/D converter. As mentioned The derivative x' is calculated from Equation 3 and above the input signal is sampled at the ADC frequency Equation 4, as follows, and quantized with corresponding ADC resolution. For ddL 1 xt() ast ( ) the detection of the symbols, a matched filtering tech- ll dt dt l 0 (6) nique is used.
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