> REPLACE THIS LINE WITH YOUR PAPER IDENTIFICATION NUMBER (DOUBLE-CLICK HERE TO EDIT) < 1 Performance of Chirped-FSK and Chirped-PSK in the Presence of Partial-band Interference Ramen Dutta, Andre B. J. Kokkeler, Ronan v. d. Zee, Mark J. Bentum University of Twente, Enschede, The Netherlands email: [email protected], [email protected], [email protected], [email protected], 0 10 Abstract—To improve interference robustness of wireless communication, spread spectrum techniques are often used. We use the chirp spreading technique along with FSK and PSK -5 binary modulation schemes to obtain interference robust radio 10 communication. The performance of chirped-FSK and chirped- PSK modulation through a white gaussian noise channel is FSK-C BER FSK-NC simulated assuming a synchronized clock between transmitter -10 PSK 10 and the receiver. We analyzed and simulated the error FSK-C with Interfernce probability (BER) of the overall system in the presence of partial FSK-NC with Interfernce band of interference in the channel. The simulated BER is close PSK with Interfernce to the estimated BER and they prove the superior performance of -15 10 chirp-based modulation in the presence of interference. 0 5 10 15 Index Terms— Interference-robust, Chirped spread spectrum, SNR FSK, PSK. Figure 1 : Interference effect on BER of PSK and FSK (Coherent and non-coherent) I. INTRODUCTION the binary modulation techniques, FSK and PSK schemes are here are a large number of applications targeting for most interference robust [4]. We simulated the BER Trobust and energy-efficient Wireless Sensor Network performance of binary FSK (coherent and non-coherent) and (WSN) communication. There are large number of PSK receiver for an AWGN channel including a partial band literatures improving the communication energy efficiency of of interference. The BER curve of these modulation schemes the WSN to increase the battery lifetime such as [1]. However, gets deviated (Interference power 4dB less than signal power, interference robustness is not addressed enough although 10MHz wide and evenly spaced around the carrier frequency) communication in most WSNs has to happen in the largely as shown in Figure 1. crowded ISM frequency bands [2]. A global ISM band in the Though interference robustness is a common challenge in all frequency range from 2.4GHz to 2.5GHz is widely chosen for wireless communication systems, it is even a bigger problem WSNs as a good tradeoff between antenna size and power in wireless sensor network application, where available energy consumption. This band of frequency is highly occupied by is very limited. Interference robust schemes which add applications such as WLAN, zigbee, Bluetooth, cordless significant amount of power, can be used in other high- phone, wireless USB, microwave oven etc. The sensor performance communication systems but not in sensor network, therefore has to co-exists with one or more of these networks because of limited energy resources. Therefore we short range radio standards. A serious effect of the need a simplified robustness scheme which can tradeoff other interference can occur if proper care is not taken into account performance metrics such as bandwidth efficiency or data rate, [3]. Hence interference robustness from those signals of but does not increase power consumption considerably. various standard is necessary for reliable communications between sensor nodes. II. SPREAD SPECTRUM USING CHIRPED CLOCK Among the ISM band standards, Wi-Fi (Wireless LAN) Spread spectrum techniques are useful to mitigate narrow transmits signals with a largest bandwidth of 22MHz. Where a band interference [5, 6]. The most popular spread spectrum cordless phone signal has a bandwidth of 10MHz, all other techniques, direct sequence spread spectrum (DSSS) and signals are narrow band. So, a narrowband rejection scheme frequency hopping spread spectrum (FHSS) can achieve high will still be effective against interference for most of the performance but at the cost of large power consumption [7]. standard applications. Another technique, which is called the chirped spread Basic binary modulation techniques are generally used in spectrum, is proposed for RADAR [8] and communication [9] wireless sensor radio transceivers because of a simple and using surface acoustic wave (SAW) devices for spreading and comparatively low power transceiver architecture. However, dispreading the signal in the transmitter and receiver those techniques are prone to in-band interferences. Among respectively. The external SAW device is not very suitable for wireless sensor nodes which intend to be as small as possible > REPLACE THIS LINE WITH YOUR PAPER IDENTIFICATION NUMBER (DOUBLE-CLICK HERE TO EDIT) < 2 where 1 is the initial phase of the signal. 2.5 2) Robustness by spectrum spreading The interference rejection is achieved due to spectral 2.48 spreading of the signal over the spreading bandwidth (BC). The ratio of these two bandwidths gives a measure of the 2.46 interference robustness that can be achieved. If the 2.44 interference bandwidth is Bi ,the ratio equals: Bi Frequency (GHz) r = (6) 2.42 W B C 2.4 Another performance parameter of spread spectrum communication is the time-bandwidth product, which is also 0 2 4 6 8 10 12 Time (us) called processing gain [7] which is defined as: Figure 2 : Up-down chirp signal = G BC T (7) where T is the chirp time. The spectrum of a chirp pulse can -10 be expressed accurately by Fresnel sine and cosine integrals -20 [8]. A simplified and reasonable approximation for a chirp spectrum can be found in [11] for a time-bandwidth (T-B) -30 product greater than 10. The time-bandwidth product -40 determines the spectral response of a chirp waveform. For example for a time-bandwidth product of 100, 98% of the -50 Magnitude signal power remains within the chirp bandwidth. We used T- B product of 200 and corresponding spectral response of an -60 up-down chirp as shown in Figure 3. -70 III. MODULATION -80 2.3 2.4 2.5 2.6 A. Chirp and BFSK Frequency (GHz) Figure 3 : Spectral spreading of chirp signals We use a chirped clock as a carrier frequency and modulate in size. Moreover SAW devices can give a loss of 20 to 30dB the message in FSK style over the chirped frequency. The [10]. Instead of using SAW devices we generate a chirp clock ‘1’and ‘0’ bit of the message can be represented by two chirps by feeding a ramp voltage to a voltage controlled oscillator with the same frequency-time slope and separated by ∆f as (VCO). This way, conventional radio mixers (multipliers) can shown in Fig. 4(a). be used to down-convert the chirped-carrier-modulated data. ()= ⎛ π + 1 μ 2 ⎞ s1 t A cos⎜2 f1t t ⎟ (8) 1) Chirped theory ⎝ 2 ⎠ ⎛ 1 ⎞ (9) A clock is chirped when the frequency of the signal is s ()t = Acos⎜2πf t + μt 2 ⎟ changing with time. For a linear chirp, the rate of change is 2 2 ⎝ 2 ⎠ constant. The angular frequency can be represented as: The modulated signal will be switched between those two ω = ω ± μ ⋅ O t (1) chirps depending on the message. An ideal modulator block where, ‘+’ and ‘-‘ are used for up-chirp and down-chirp diagram is shown in Fig. 4(b). Here the chirped clock is respectively. Non-linear chirps are those where µ is a function indicated by the input of the voltage controlled oscillator as it of time. For a continuous up-down chirp, shown in the Fig. 2, can be practically generated. the starting frequency (ωo) will be different for a up-chirp and 1) Chirped-FSK demodulation a down-chirp. ω =ω + μ()− An FSK receiver can be coherent or non-coherent. In the (t) O t nT For even n=0,2,4.. (2) coherent receiver we assumed that the exact chirp functions and, (8) and (9) are available at the receiver after proper frequency ω =ω + − μ()− For odd n=1,3,5.. and phase synchronization. This detection is based on (t) O B t nT (3) correlation and integration over symbol time (Tb) as shown in For only one up-chirp signal, the instantaneous phase can be Figure 5. written from (1) as: In this case the optimum coherent FSK receiver used for 1 chirped-FSK communication is realized. The probability of φ = ω(t)dt = ω t + μt 2 + c (4) ∫ O 2 1 error (or bit error rate) of detection done by this coherent FSK receiver in the presence of white Gaussian noise can be Therefore an up-chirp signal can be represented as: expressed as [12]: ()= ⎛ω + 1 μ 2 + φ ⎞ sUC t Acos⎜ 0t t 1 ⎟ (5) ⎝ 2 ⎠ > REPLACE THIS LINE WITH YOUR PAPER IDENTIFICATION NUMBER (DOUBLE-CLICK HERE TO EDIT) < 3 9 x 10 Fc 2.46 1 Fc 0 2.45 TX FREQ 2.44 2.43 Frequency (f) 2.42 2.41 2.4 Figure 5 : chirped BFSK coherent receiver 0 0.5 1 1.5 2 -5 TIME x 10 ⎛ E ⎞ ⎛ E ⎞ = ⎜ b ⎟ + ()− ⎜ b ⎟ Message PCFSK−CH rW Q 1 rW Q (13) Q ⎜ N + I ⎟ ⎜ N ⎟ D ⎝ O O ⎠ ⎝ O ⎠ Tb Register If the interference bandwidth is Bi, then we can write, S E P T P / R B b = S b = S b = SIR i Chirp (14) I O IO PI / Bi Rb Signal Cos(ω1t) where signal and interference power are PS and PI respectively S and their ratio (SIR) is the signal to interference ratio. Using this, (13) can also be written as: Chirp Signal ⎛ ⎞ Cos(ω2t) ⎜ ⎟ 1 P = r Q⎜ ⎟+ ()1−r Q()SNR Figure 4 : chirped BFSK (a) signal and (b) ideal modulator CFSK−CH W W (15) ⎜ −1 −1 R ⎟ ⎜ SNR + SIR b ⎟ ⎝ Bi ⎠ ⎛ E ⎞ = ⎜ b ⎟ = () PCFSK Q Q SNR (10) 2) Non-coherent BFSK modulation ⎜ N ⎟ ⎝ O ⎠ Coherent FSK demodulation has the disadvantage that it needs where Eb is the energy per bit and No is the power spectral time consuming phase synchronization along with frequency density of white gaussian noise and Q(.) is Malcolm’s Q synchronization.