CHAPTER 3 BER EVALUATION OF IEEE COMPLIANT WSN

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1 CHAPTER 3 EVALUATIO OF IEEE COMPLIAT WS 3. OVERVIEW Appliations of Wireless Sensor etworks (WSs) require long system lifetime, and effiient energy usage ([75], [76], [7]). Moreover, appliations an fulfill their requirements if ommuniation through the network is reliable. This is beause the nature of the ommuniation hannel used for sensor data transmission is erroneous due to the presene of thermal noise, interferene from outside the network, and interferene due to large sale and small sale fading omponents arising from multipath propagation. The reliability of the ommuniation link generally depends on the Bit Error Rate (). This greatly affets network performane in terms of throughput, delay, and energy [77], [78]. Although there have been several studies on sensor network performane analysis over varying fading hannel onditions, none of them are diretly appliable to the WS senario as they have not onsidered sensor nodes based on ZigBee tehnology. ZigBee tehnology uses the globally available, liense-free.4 GHz frequeny band. It enables wireless appliations using a standardized set of high-level ommuniation protools sitting atop ost-effetive, low-power digital radios based on the IEEE standard for Wireless Personal Area etworks (WPAs). In this work, a WS based on the IEEE standard ([79]-[8], [6]) whih has been widely aepted as an industrial standard for sensor nodes is onsidered. The main harateristi features of this standard inludes support for various network topologies, diret, and indiret data transmission, ontention and sheduling based medium aess ontrol methods, and flexible addressing shemes. Heneforth, in this thesis, a wireless sensor node based on ZigBee tehnology using IEEE RF transeiver will be referred to as an IEEE ZigBee transeiver.

2 3 This work aims to derive an analytial expression for of WS in the presene of interferers under various hannel onditions ranging from no fade to severe fading senarios, i.e., AWG and Rayleigh fading hannels ([8]-[84]). Data Zigbee P Zigbee Bits to i s Symbol y r Chip to Symbol to î Zigbee to P Zigbee Data Bits Symbol Chip Symbol Regroup Group Map Remap O-QPSK Modulator Fading Channel n AWG oise O-QPSK Demodulator Figure 3.. Blok Diagram of Zigbee IEEE transeiver 3. IEEE ZIGBEE TRASCEIVER The IEEE standard has been adopted by Zigbee for WS tehnology. The salient features of Zigbee inlude low ost, reliable data transfer and ease of implementation. The PHY layer of IEEE standard an be operated in three unliensed frequeny bands namely 858 MHz, 95 MHz and.4 GHz. Aordingly, the standard speifies three different physial media: (i) Diret Sequene Spread Spetrum using Binary Phase Shift Keying (BPSK) operating in the frequeny range of 868 MHz at a data rate of Kbps, (ii) Diret Sequene Spread Spetrum using BPSK operating in the frequeny band of 95 MHz at a data rate of 4 Kbps, (iii) Diret Sequene Spread Spetrum using Offset-Quadrature Phase Shift Keying (O-QPSK) operating in the frequeny band of.4 GHz at a data rate of 5 Kbps. For analysis purposes in this work, sensor nodes that use IEEE Zigbee transeiver operated at higher data rate physial media (.4 GHz/5 Kbps) whih is an internationally used liense free ISM frequeny band is onsidered. The omplete blok diagram of IEEE Zigbee transeiver system onsidered in this study is shown in Figure 3.. It involves spreading and modulation of input bits. 3.. Bits to Zigbee symbol grouping In the first stage, inoming bits are grouped into k bits so as to represent a Zigbee symbol. The IEEE standard speifies the value of k as 4. Therefore, 4 LSBs of

3 4 eah otet of input information stream i shall map into one Zigbee symbol s and 4 MSBs of eah otet shall map into the next Zigbee symbol. Within eah otet, the least signifiant symbol is proessed first and the most signifiant symbol is proessed seond. Table 3. Zigbee Symbol to Chip Mapping Zigbee Symbol Chip Values ( ) 3.. Zigbee Symbol to Chip mapping These four bits are used to selet one of the 6 nearly orthogonal Pseudo-random oise (P) sequenes to be transmitted. Eah Zigbee symbol shall be mapped onto a 3- hip P sequene as shown in Table 3.. The P sequenes are related to eah other through yli shifts and the suessive seleted P sequenes are onatenated and sent to the O-QPSK modulator O-QPSK Modulation The hip sequenes representing eah Zigbee symbol are modulated onto the arrier using O-QPSK with half sine pulse shaping. Even indexed hips are modulated

4 5 onto the In-phase (I) arrier and odd indexed hips are modulated onto the Quadraturephase (Q) arrier. Beause eah Zigbee symbol is represented by a 3-hip sequene, the hip rate is 3 times the symbol rate. To form the offset between I-phase and Q-phase hip modulation, the Q-phase hips shall be delayed by hip time T with respet to I- phase hips, where T is the inverse of the hip rate. During eah symbol period, the least signifiant hip is transmitted first and the most signifiant hip, 3, is transmitted last. The inoming hip sequenes to the O-QPSK modulator are modulated onto a arrier with half-sine pulse shaping Communiation hannel The ommuniation hannel onsidered for study is a fading hannel environment. Fading may be due to multipath propagation, referred to as Multipath fading, or due to shadowing from obstales affeting the wave propagation, referred to as shadow fading. Thermal noise, interferene from other users and interferene from iruit swithing transients an ause errors in deteting the pulses. The presene of refletors in the environment surrounding a transmitter and a reeiver reate multiple paths that a transmitted signal an traverse. As a result, the reeiver gets superposition of multiple opies of the transmitted signal, eah traversing a different path. Eah signal opy will experiene differenes in delay, attenuation, and phase shift while travelling from soure node to the sink node. This an result in either onstrutive or destrutive interferene, amplifying or attenuating the signal power at the reeiver. Strong destrutive interferene is referred to as deep fade, and may result in temporary failure of ommuniation due to severe drop in the hannel SR. The modulated signal y is transmitted through the hannel gets affeted by white noise n and other fading parameters depending upon the hannel onditions. In this work, AWG, and Rayleigh fading hannels are onsidered to study network performane over varying hannel harateristis. i.e., no fade to severe fading situations O-QPSK Demodulation The reeiver setion of IEEE Zigbee Transeiver onsists of bloks to perform reverse operations as that of the transmitter. This inludes demodulation, hip to Zigbee symbol remapping, and finally Zigbee symbol to bit regrouping. Even

5 6 indexed hips are retrieved from the I-phase arrier and odd indexed hips from the Q- phase arrier to determine the estimate of transmitted hip sequene Chip to Zigbee Symbol remapping Despreading is done to get bak the Zigbee symbols,, whih would be provided to the next stage. This is done by finding the lose approximation of the reeived hip sequene with that of the transmitted one and hoosing the orresponding Zigbee symbol for further proess Zigbee Symbol to Bits regrouping Finally, Symbol to bits regrouping is performed to get bak the original transmitted information, î. 3.3 DERIVATIO FOR IEEE COMPLIAT WS In this Setion, an analytial expression for of WS under various hannel onditions ranging from no fade to severe fading senarios is derived, i.e., for AWG and Rayleigh fading hannels. The speifiations of IEEE Zigbee transeiver operated with.4 GHz band is as follows: The data modulation sheme used here is Diret Sequene Spread Spetrum-Offset Quadrature Phase Shift Keying (DSSS- OQPSK). By using an offset, the phase hanges in the ombined transmitted signal never exeed 9. O-QPSK provides a very good performane when the hannel has omponents with signifiant non-linearity. Eah O-QPSK signal element at the input of the modulator onsists of two hips, and hene O-QPSK symbol energy an be represented as E E. In the generalized form, O-QPSK symbol energy an be rewritten as E E log M, (3.) where M denotes M-ary signaling. Here, onsider M 4. Moreover, O-QPSK signal energy, E, at the modulator input is represented in terms of bit energy as follows: As k bits are grouped into a Zigbee symbol, the Zigbee symbol energy is given by Es k E b. (3.)

6 Also, eah Zigbee symbol is represented using L 3 hips. Hene, Zigbee symbol energy an also be represented in terms of the spreading gain as E E L, ktb where L, i.e., 3, (here). Therefore, from Equation (3.), the bit energy E b an T be written in terms of hip energy as s 7 E b EL. (3.3) k The O-QPSK modulated signal is now written as follows: E t E t y T T T T t os osf t sin sin f t, t T (3.4) where T is the hip duration, energy, whih is equal to twie the hip energy, duration whih is equal to twie the hip duration f is the arrier frequeny, E is the O-QPSK symbol E, and T is the O-QPSK symbol The first term in Equation (3.4) represents the In-phase omponent of the transmitted signal, whereas the seond term represents the Quadrature phase omponent whih is delayed by hip duration, T, to realize a 9 phase hange. This an be denoted as T. y I t E t os T T, T t T, E t and yq t sin, t T T T. (3.5) The use of half sine pulse shaping eliminates any amplitude variations and turns O-QPSK into a onstant envelope modulation. Half sine pulse shaping [85] is desribed as p t t sin, t T T, otherwise (3.6)

7 8 The orthonormal basis funtions t and t for O-QPSK are given by and t t os, T T T T Correspondingly, OQPSK signal is written as t T t T, t sin, t T. (3.7) where yand limits by T and y t y t y t, t T (3.8) y orrespond to the integration of the produts, t t T, and t t and y y between the between the limits and T, respetively, as given y y T T T y y t t E, t t dt E. dt (3.9) ote that both integrals are evaluated for a time interval equal to twie the hip duration. Both lower and upper limits of integration of the produt are used to evaluate the oeffiient y, shifted by hip duration T as ompared to that of y, beause offset T is introdued in the Quadrature phase omponent. In the ase of an AWG hannel, the reeived signal is given by r t y t n t, where y t is the transmitted signal and n t is the sample funtion of a white Gaussian noise proess of zero mean and power spetral density. For optimum detetion, we have r r y y where nand n are noise omponents. n, n, T t T, t T, (3.) 3.3. derivation for IEEE Zigbee RF Transeiver under an AWG Channel The possible message points y, y, y 3, and y 4 are as follows: E, E, E, E, E, E, and E, E. Suppose message 4, ( y 4 ), is the transmitted signal, the reeiver will make a orret deision

8 provided that the reeived signal point represented by the observation vetor r lies inside z 4. Therefore, for a orret deision, when 9 y 4 t is transmitted, the elements r and r of the observation vetor, r must be both positive. Probability of orret deision, P, an be defined as the onditional probability of the joint event r and r given that signal y t 4 was transmitted. Sine random variables r and r are independent, P an be written as the produt of onditional probabilities. Furthermore, r and r are Gaussian random variables with onditional mean E and variane. Therefore, P p r y dx pr y 4 exp r E dr exp r E dr. 4 dx (3.) r E r E Substituting z into Equation (3.), we have P E exp z. dz. (3.) u We know that erfu exp z du Equation (3.) an be written as (Setion 8.5, page 88 of [86]). ow, E exp. E P z dz erf (3.3) Therefore, P an be obtained as E E P erf erf. 4 (3.4)

9 The Symbol Error Probability (SEP) of an IEEE Zigbee transeiver is given by 3 E E p e P erf erf. (3.5) o 4 Substituting the term E from Equation (3.3) into Equation (3.5) and simplifying, we obtain the of an IEEE Zigbee transeiver as erf log M sin erf log M sin (3.6) M 8 M where the reeived instantaneous SR,. It is found that the value of erf x E b is negligibly small for x 3. Hene, ignoring the higher order terms in Equation (3.6), we obtain of IEEE Zigbee RF transeiver as erf log M sin. (3.7) M Combining Equations (3.6) and (3.7), the of ZigBee RF transeiver under AWG hannel an be represented as D erf D, erf D 3 8 (3.8) erf D, D 3 where D M sin log M derivation for IEEE Zigbee RF transeiver in the presene of interferene IEEE devies an be reasonably expeted to oexist, i.e., to operate in proximity with other wireless devies. The proposed standards haraterized for oexistene are IEEE 8.b, IEEE 8.5.3, and IEEE In this setion, the of IEEE based sensor network is determined under a wide range of interferene onditions.

10 3 Suppose that there exist devies in an environment and let n P be the reeived power due to transmission of the n th devie. Consider devie as the desired user operating with.4 GHz DSSS speified by IEEE and other (-) devies, i.e., devies,... -, as interferers that operate under the same frequeny band, but are allowed to oexist with IEEE suh as WLA or Bluetooth wireless devies. Sine the interferer power levels are not equal, a more general definition of Central Limit Theorem (CLT) holds good. Aording to CLT, it is assumed that the effet of interferers an be approximated by a Gaussian random variable, when the number of interferers in the environment is very large. Therefore, referring to (E.6) on page 636 of [87], the expression for an IEEE Zigbee RF transeiver in the presene of interferene an be written as sin log 3 8 sin log 3 i b i i b i AWG M M PT L k erf M M PT L k erf (3.9) where P and P i are the reeived power levels of desired and interferene users, respetively. Grouping the onstants in the denominator of both the terms in Equation (3.9), we have 3 i b i PT L k D. Therefore, Equation (3.9) an be simplified as D D erf D D erf AWG 8. (3.) For ease of analysis, Equation (3.) an be further simplified as follows:

11 AWG erf G erf G 3, (3.) 8 DD where G. Following Equation (3.8), the expression in an environment D D involving interferene an be obtained as AWG G erf 8 G G, erf, G 3. (3.) erf G derivation for IEEE Zigbee RF transeiver under a Rayleigh fading hannel The probability of error an be alulated easily for a multipath hannel that has slow, flat fading with respet to a symbol period. Assume that the number of independent paths between the transmitter and reeiver is L. In polar format, the jl magnitude and phase of the fading hannel gain are denoted by l and l : h l l e, where l, L referred to as l th path, l. The fading hannel amplitude and the fading hannel phase are l and l respetively. Essentially, the reeived instantaneous SR of the h l E b, an be assumed to be fixed over the duration of the deision interval of one symbol. Therefore, the resultant reeived instantaneous SR at the input L E b l of the reeiver due to L multipaths are given by l, where l. The statistial distribution onsidered for analysis in this work inludes Rayleigh fading distribution. The average probability of error for a Rayleigh fading hannel an be omputed by integrating over a fading distribution. So, the an be obtained by averaging the onditional error probability with respet to the instantaneous SR, as follows: L l Ray AWG f Ray d, (3.3)

12 where Ray f is the Probability Density Funtion (PDF) of, when the fading is Rayleigh [85]. It an be noted that depends on the squared value of. Sine is Rayleigh distributed, it is found that has a Chi-square distribution with two degrees of freedom. In partiular, the PDF of an be expressed as E b 33 f Ray exp,, (3.4) where is the mean value of the reeived SR (ratio of the signal energy per bit to Eb noise spetral density) defined as E E, where E is the average Rayleigh fading power denoted as. Substituting Equations (3.) and (3.4) into Equation (3.3), an expression for of IEEE Zigbee transeiver under Rayleigh fading hannel is obtained as G erf G Ray erf exp d. (3.5) 8 Using integration by parts to solve the integral in Equation (3.5), the expression of for IEEE Zigbee transeiver under a Rayleigh fading hannel is obtained as Ray og 4 tan G G, (3.6) G 8 og G where G k 3L M M log M log M sin T sin b i P i. 3.4 UMERICAL RESULTS AD DISCUSSIOS The analysis of WS based on IEEE Zigbee transeiver is performed using MATLAB. umerial results are obtained for various fading hannel

13 onditions by varying system speifiations suh as the number of input bits used to represent the Zigbee symbol, k, number of modulation levels used in O-QPSK modem of Zigbee transeiver, M, mean of the Rayleigh fading parameter, and spreading fator L. The number of potential interferers present in the environment that operate with the same frequeny band as IEEE is hosen as 5. Figure 3. shows the variation of as a funtion of SR for different values of bits per Zigbee symbol over an AWG hannel. It is found that there is a signifiant degradation in the performane for higher values of k, suh as 6, and k= k=4 k=6 k= o. of interferers = 5 M=4 L = SR (db) Figure 3. of IEEE Zigbee transeiver for various values of bits per symbol in the presene of interferers in an AWG hannel environment. This is beause as the number of quasi orthogonal Pseudo oise (P) sequene to be transmitted is equal to k, inreasing the value of k will lower the orthogonality between the P sequenes. This will lead the reeiver to make erroneous deisions on the reeived hip sequene resulting in inreased. It is also observed that there is signifiant improvement in performane as the SR value inreases from db to 5 db. However, under low SR ondition (oisy environment), i.e., from -5 db to db, the is high for all values of k.

14 35 Similar kind of analysis for different values of k, is made under Rayleigh fading hannel, as shown in Figure 3.3. The error performane of WS fading hannels follows Figure 3.. It is observed that under severe fading hannels, the impat of varying the number of bits per Zigbee symbol has lower effet on performane. k= k=4 k=6 k=8 - o.of interferers = 5 M= 4 L = SR (db) Figure 3.3 for IEEE Zigbee transeiver over a Rayleigh fading hannel for different values of bits per symbol in the presene of interferers. From the above disussions, it is lear that inreasing the number of bits per Zigbee symbol of the order of inreases the. But, for IEEE Zigbee standard speified value, i.e., k = 4, value loser to.5 an be ahieved at higher SR values in the range of db to 5 db under Rayleigh fading hannel whereas of. an be obtained for AWG environment. The olloated interferene greatly inreases the values under severe fading onditions. Therefore, inreasing the number of bits per Zigbee symbol inreases. However, improved performane an be ahieved by operating WS with less number of bits per Zigbee symbol, i.e., k to redue the error rate in an interferene environment. The performane of WS is also studied by varying the number of modulation levels used in a Zigbee transeiver, under AWG, and Rayleigh fading hannels as shown in Figures 3.4 and 3.5 respetively. It is observed that as the number of modulation levels is inreased, the of WS is inreased. This is due to the fat

15 36 that when the number of modulation levels is inreased, the transmitted signal elements are losely loated in their signal spae onstellations, and is very diffiult for the deision devie to disriminate the estimate of the transmitted signal element from the signal set, and the probability of making erroneous deision is high. M=4 M=8 M=6 M=3 - - o.of interferers = 5 k = 4 L = SR (db) Figure 3.4 for IEEE Zigbee transeiver over an AWG hannel for various M-ary modulation shemes in the presene of interferers. Hene, it is lear from Figure 3.4 that for M = 3, the urve does not show muh variations when SR is varied. However, for a Rayleigh fading hannel, the performane is still worse as shown in Figure 3.5, i.e., value (for M = 4) obtained for SR= 5 db, is approximately.5. For other values of M, say M = 8, 6, and 3 the analysis shows very poor performane ompared to M = 4. Sine M = 4 is the IEEE Zigbee standard speified value, inreasing the number of modulation levels to more than 4 has no role in improving network performane in fading hannels. Hene, from numerial results, we an study that the number of modulation levels an remain the same as mentioned in the IEEE Zigbee standard to ahieve better performane. Figures 3.6 and 3.7 depit the performane by varying the length of P spreading sequene for AWG, and Rayleigh fading hannel respetively. The effet of

16 37 interferers an be studied by varying the length of spreading sequene used to transmit the sensor information. The inreased number of o-loated interferers in the environment ontributes more interferene to the desired signal transmission. M=4 M=8 M=6 M=3 - o.of interferers = 5 L = 3 k = SR (db) Figure 3.5 for IEEE Zigbee transeiver over a Rayleigh fading hannel for various M-ary modulation shemes in the presene of interferers. This effet of interferene on the desired signal transmission an be greatly redued by inreasing the spreading fator as despreading at the reeiver with larger P sequene makes the interferer signals to appears as wideband noise with lower Power Spetral Density (PSD) when ompared to the desired signal. values of Figure 3.8 shows the performane of a Rayleigh fading hannel for various, where E. It is also lear that for severe fading hannels like Rayleigh, inrease in the value of from to 5 shows signifiant improvement in values.

17 L =6 L =3 L =64 L = SR (db) Figure 3.6 for IEEE Zigbee transeiver over an AWG hannel for different spreading sequene lengths in the presene of interferers. - L =6 L =3 L =64 L =8 o.of interferers = 5 M = 4 k = 4 - o.of interferers = 5 M= 4 k = SR (db) Figure 3.7 for IEEE Zigbee transeiver over a Rayleigh fading hannel for different spreading sequene lengths in the presene of interferers.

18 39 - = =5 = =5 - o. of interferers = 5 M=4 k=4 L = SR (db) Figure 3.8 of IEEE Zigbee transeiver over a Rayleigh fading hannel for different values of in the presene of interferers. 3.5 SUMMARY In this hapter, the exat expression for of WS onsisting of sensor nodes based on IEEE Zigbee transeiver is derived for AWG and Rayleigh in the presene of potential interferers. umerial results for are obtained by varying IEEE standard speified physial layer parameters and hannel fading parameters. It is found that Zigbee transeiver with O-QPSK modem having M = 4, and representing a Zigbee symbol with lower number of bits shows improved error performane when it is operated in the SR range of db to 5 db. Also, it an be shown that inreasing the length of the spreading sequene will ombat interferene that predominates in the wireless environment, and provides lower. Following this study, the simulation model of IEEE Zigbee transeiver is arried out using MATLAB and is disussed in the next hapter. Further, the performane of various Forward Error Corretion (FEC) odes inorporated in IEEE Zigbee transeiver that improves performane and energy effiieny is also found. These simulations are done for both Fading and Erasure hannel environment.