An IR-UWB Receiver Design for Low Cost ApplicationsSoon-Woo Lee, Young-Jin Park, Jimyung Kang, and Kwan-Ho Kim

Power telecommunication network research groupKorea Electrotechnology Research Institute, Uiwang city, South Korea

{rheesw, yjpark, jmkang, khkim} gkeri.re.kr

Abstract - In this paper we present an energy collection path ofIR-UWB signal substituting a RAKE structure. Also asbased non-coherent low power and low complexity IR-UWB an alternative to a high complex ADC we use a 1-bit sampler(Impulse Radio Ultra Wide Band) receiver which is robust to change the received pulse of continuous form to discreteagainst multipath fading. The proposed receiver adopts an one. In addition, the required sampling rate of the 1-bitintegrator, a 1-bit sampler and a digital symbol synchronizer for estimating symbols can be much lower than thereplacing with a classical RAKE structure, a high complex ADC saplerand PLL respectively. Since the receiver is composed of such orginal one at the cost of losngh1igh frequency componentsimple modules, we can reduce the total cost drastically. The since the input signal for the 1-bit sampler is an envelope ofperformance of the proposed receiver is simulated based on low frequency component extracted from the received pulseIEEE 802.15.4a channel model. by an integrator. Also a digital pre-processor followed by the

1-bit sampler is designed to improve signal to noise ratioIndex Terms - IR-UWB, 1-bit sampler, pulse decision window (SNR). In addition, for each pulse repetition interval (PRI), a

symbol is determined by several samples within a predefinedI. INTRODUCTION range around an estimated pulse position. The predefined

range is called pulse decision window (PDW) and the positionImpulseradio ultra wideband (IR-UWB) system is basically a of PDW is updated every PRI by the proposed symbolbase-band system which uses narrow pulses having low duty synchronization method. Since the proposed symbol decisioncycle for communication. And these lead the system to have and symbol synchronization methods are based on a certainlow complexity and low power characteristics. Recently many interval, PDW, an effort can be reduced to search and track thestudies have been investigated for applying IRqUWB exact timing of incoming pulses. Besides the immunity of thetechnology to ultra low power network such as Ubiquitous receiver from time-varying channel environment is increased.Sensor Network (USN) and Low Rate Wireless Personal Area This means the design of synchronizer can be simple and aNetwork (LR-WPAN) [1]. classical PLL can be replaced with a simple digital circuit.

In a multi path channel the RAKE receiver is often used to This paper is organized as follows. In Section II weimprove receiving performance [2]. Also a complicated ADC present the proposed receiver structure and its modeling. Thehaving several GHz sampling rates is adopted to detect narrow proposed symbol synchronization method is described inpulse in an IR-UWB receiver [3]-[4]. Furthermore a Section III. Simulation results showing the performance of thesophisticated phase locked loop (PLL) has been designed for receiver in multi path channel environment are given inbeing synchronized with the incoming sub nanosecond IR [5]. Section IV. Finally conclusions are drawn in Section V.To design a low cost receiver, however, these modules shouldbe replaced or modified to simple ones. II. SYSTEM MODEL

An energy detection based non-coherent receiver wasproposed as an alternative structure for low cost and low rate Figure 1 shows the proposed receiver structure. The receiverapplications [6]-[8]. This approach reduces the complexity and can be divided into two parts; one is an analog part consistingpower consumption of an IR-UWB receiver at the cost of of a band pass filter (BPF) with bandwidth W, a squarer, areduced channel spectral efficiency. In [6] and [7], a receiver finite time integrator and the other is a digital part consistingstructure using parallel integrators was introduced and the of a 1-bit sampler, a pre-processor, a symbol detector andcomplexity of the receiver is reduced to a certain level. synchronizer.However adopting parallel structures or a high speed timerkeeps the cost still high. In paper [8], the authors model a non- Received YEt) Y(t)coherent receiver mathematically and they derived the bit Sinal BPF 0/2 (dterror rate (BER) of the receiver. But no practical issues wereintroduced to design a low cost receiver.

The present paper therefore aims at developing a practical Data Symbol SymboI( Pre-1benergy collection based non-coherent IR-UVvB receiver for Sync. Detect processor Samplerlow rate that allows low complexity and low power Fg rpsdrcie tutrconsumption with high robustness against channel variations. Fg rpsdrcie tutrThe proposed receiver has an integrator and a squarer thateffectively collect dispersed energy among different reflection

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We adopt On/Off Keying (OOK) for data modulation and a T I 2TWsingle bit is represented by one pulse. Thus the transmitted V0 = |n(t)dt= 1 a2, (5-a)signal is given by 02

T 2TW

s(t)= d p(t-iTp) , (1) L = f(rB(t) + nB (t))2 dt= s (a, +ai)2 (5-b)i=-0o

w poTp is If we assume that the noise nB(t) bandwidth is W and twowhere p(t) is the transmitted pulse with duration of d, sided nosipcrldnitssN2 ,i 5-)ad(-)ithe pulse repetition interval (PRI) and d E , is the

sded nose spectral density is N o2,ai in (5-a) and (5-b) is a

information data. Gaussian random variable with zero mean and varianceThe received signal after the Rx antenna is given by 2N02W. And a, in (5-b) is the sample value of rB(i/2W), where

rB(t) is band limited version of r(t). Therefore V0 has a chi-y(t) = r(t) + n(t) , (2) square distribution and V1 has a non-central chi-square

where distribution with non-centrality parameter A given byr(t) = w(t) * h(t).

St A Y a,~~~~~~~12T rB_f(t)dt . 6w(t) is the t deviation of s(t) representing antenna effect, h(t) 2WN a,=A (6)is an impulse response of UWB channel, n(t) is zero mean

o2 o

AWGN(Additive White Gaussian Noise) and <*> is aconvolution operator. Also we can observe that A means signal to noise ratio. For

The output signal Y(t) after being integrated by on-the-fly large 2TW (>250), by central limit theorem, V0 and V1 can beintegrator depicted in Fig. 1 can be written as follows, approximated Gaussian random variable of N(2TW, 4TW) and

N(2TW+X,4(TW+ 2)) respectively. If the threshold value of theI

t 1-bit sampler shown in Fig. 1 is set to VT, probability of falseY(t) fy2(r)dr, (3) alarmpfand probability of detection Pd is as follows [9].

t-T1 VT-2TW

where T is an integration interval, yB(t) represents the band P 2rf(V21TW (7-a)limited version of y(t) by BPF shown in Fig. 1. Here, we I V 2TW-assume that the integration time T is smaller than the PRI for Pd = -er T -T (7-b)preventing inter symbol interference (ISI). Simultaneously T is 2 2 W +large enough to gather the impulse energy dispersed in themultipath fading [8]. where binary symmetric channel (BSC) is assumed and

The incoming signal Y(t) is sampled by 1-bit sampler and 2the sampled signals YD(n) are enhanced by a digital pre- erfc(z)= fexp(-xbdx.processor as shown in Fig. 1. Here, we assume that Nc points zare sampled in single pulse duration dp. The pre-processor is asort of rectangular filter fc(n) of size Nc as defined by Equ. For symbol decision it is important to know whether a pulse(4). And convolution between YD(n) and fc(n) is performed exists or not in every PRI since the OOK modulation is used.since the symbols are decided based on the pre-processor To check a pulse, total NPDW consecutive samples beingoutput of YD(n)*fc(n). around an expected pulse position are used as shown in Fig. 2

for every PRI. Here, we define time interval including thef

1 0< n < Nc NPDW samples as a Pulse Decision Window (PDW) and thefc (n) ={o, otherwise (4) position of PDW is updated in every PRI by the proposed

symbol synchronization method.

For analyzing the receiver performance, in (3), we consider PDWonly particular interval of (0,j) and assume that the bandwidth - - - - - - - - - - - - - - - - - - - - -

ofyB(t) is W. It is known that a sample function of duration T,3of a processor which has a bandwidth W is described Capproximately by a set of sample values 2TW in number. As VI

mentioned before we use OOK modulation and we canconsider (3) into two cases; yB(t) is noise alone (5-a) and YB(t) _____________________o___l___l _iS signal plus noise (5-b) [9] n n+l n+N PD(! 1-

Fig. 2 Example of Pulse Decision Window (PDW)

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Then a symbol is decided by following rule: If there is no by the symbol decision process described in chapter 11 aboutsample of NC in magnitude it is regarded that the PDW doesn't the NI PDWs. If all the symbols are decided as bit 'I's, weinclude a pulse (defined as bit '0'); otherwise, the PDW regard that the assumed synchronization point To is correctincludes a pulse (defined as bit '1'). Thus in the symbol and finish the initial symbol acquisition process and define thedecision process the probability of false alarm Pf and the point as TSYNC. Otherwise, the acquisition process goes back toprobability of detection Pd are given by first step and restarts from the next sample.

Pf = (NPDW - NC + 1)pNc (8-a) B. Symbol Tracking Process

Pd . (8-b) Though the initial symbol acquisition is succeeded the symboltracking process should be maintained to cope with time-

In (8-a) and (8-b), we can observe the signal-to-noise ratio varying channel condition and/or timing jitter generated by(SNR) of the received signal is exponentially increased by the some hardware components during data transmission. Symbolpre-processor. tracking process begins from TSYNC found in the initial symbol

acquisition process. The center position of the k+Ith PDW,PDWcen,k+l, for k+lth symbol is determined by both the

III. SYMBOL SYNCHRONIZATION tracking offset and the position of the previous PDW asfollows.

As mentioned before, in every PRI, a PDW is set for a symboldecision. Therefore the position of a PDW should be PDWCen,k+l = (PDWcen,k + TpR1) + AkHIIR, (9)synchronized with a position of the incoming pulse. Thesymbol synchronization includes both initial symbol PDWcen,1 = (TSYNC + NITPRI) + Tacquisition process and symbol tracking process. Unliketraditional symbol synchronization algorithm trying to find an where A k,IIR is a filtered version of the kth offset A k whichexact timing synchronized with the received UWB pulse, the represents timing jitter around the ke PDW as illustrated inproposed algorithm is based on searching a PDW, namely a Fig. 4. If there is no pulse (or flag) in the kth PDWwe set A kcertain time interval including received pulse. to zero. A k,IIR and A k are given by

A. Initial SymbolAcquisition Process AkHIR =(1-,g)Ak +/Ak-1IIR, 0 <,8 < 1 (10)

During initial symbol acquisition process, it is assumed that asingle UWB pulse is continuously emitted (bit '1') from the andTx part in every PRI. The initial symbol synchronizationprocess is performed on the samples of the pre-processor flg,k - PDWcen,k, if nflag,k existsoutput Yp(n) ranging from 0 to Nc in magnitude. Ak - , otherwise. (11)

PDwIn (10), X is a feedback coefficient and in (11), nflagk is time

¢3 >5XS , w . index of a flag within the kt PDW. As in (9), we use A k,IIR$ 'U . instead of A k for preventing a sudden change of position of

PDWby noise.

PDWO PDWkI/§z D gog T @4oGog4goo& 17 _ @ S 3 .~~~~~~~~~~........................... ....................

3

PII/OT Tf "> Z2Fig. 3 Example ofthe pre-processor output assuming Nc=3 to explain the o C

initial symbol acquisition process 1 L

As depicted in Fig. 3 we define a bunch of continuous non-zero samples as an island and the first sample being Nc in 0magnitude within an island as a flag. An island being 1ffpk f WIe PDWcenk,lcomposed of pure noise samples may not have aflag. I I

As the first step in the acquisition process, the first island l khaving aflag is searched and then theflag (PIVOT in Fig. 3) is\

assumd assyncroniztion oint,T0. Nxt, ttal N PDW Fig. 4 Example ofthe pre-processor output assuming Nc=3 to explain theare set and the center position of each PDW iS nlTPM±T0,n,H E symbol tracking process{ 1,2,... ,N1} respectively. And then NI symbols are determined

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IV. SIMULATION Figure 6 shows bit error rate (BER) for the proposed receiverversus SNR. If the position of each PDW is perfectly

For evaluating the performance of the proposed IR-UWB synchronized with that of the received UWB pulse, the BER

receiver we perform simulation using UWB channel model 4 Pe is given by Equ. (12) from Equ. (8-a) and Equ. (8-b).(Office NLOS) in IEEE 802.15.4a [10]. In the receiver 1- P (1P2structure as shown in Fig. 1 the BPF is assumed as an ideal p =(f (12)BPF having a passband between 3.1Ghz and 5.1GHz. PRI is 2set to Ims, namely data rate is 1Mbps, and integration time isset to 50nsec by considering both the UWB channel As shown in Fig. 6 we can observe that the simulation resultenvironment and the PRI [8]. And sampling rate of the 1-bit assuming perfect synchronization (circular mark) is similar tosampler is set to 10OMsps (sample per second). The size ofthe the theoretical graph of Pe (diamond mark) and the proposedfilter Nc in the pre-processor is set to 3 and size of PDW, synchronization method performs well since the graph (squareNPDW, is set to 0.2msec which is 20% of the PRI. In the initial mark) is almost identical to perfectly synchronizing casesymbol acquisition process the PDW iteration count NI for (circular mark).verifying an assumed synchronization point is set to 4. As a V. CONCLUSIONreceiver input UWB pulse model described in [11] is adopted.

Figure 5 shows required mean acquisition time for initial In this paper we proposed a low complexity IR-UWB receiversymbol acquisition versus signal-to-noise ratio (SNR) in dB structure in which such major components as a classical Rake,scale. Here, the unit of mean acquisition time is PRI. Since NI an ADC and a PLL are replaced with an integrator, a 1-bitis set as 4, we can observe that mean acquisition time sampler and a PDW based simple digital circuit respectively.converges into 5xPRI as SNR is increasing. The receiver performance is estimated by system modeling

30 and computer simulation and we conclude that the proposedreceiver can be applied to row rate and low cost applications

25 like Ubiquitous Sensor Network (USN) or Low Rate WirelessPersonal Area Network (LR-WPAN).

20

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proposed sync.(square)

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