Cancellation of Zigbee interference in OFDM based WLAN for multipath channel


Published on

Zigbee is one of the major sources of interference
in 2.4GHz band for WLANs. It is seen whenever any Zigbee
system is operating near to the WLAN system and
transmitting signal at same frequency, time as of WLAN’s, the
later ones performance detoriate severely. So in this paper an
algorithm is proposed to estimate Zigbee interference
component present in all OFDM based WLANs sub-carriers
and cancel out the Zigbee interference from the received
signal of WLANs receiver for multipath fading channels in
frequency domain. Simulation results shows for high SNR
values full cancellation of Zigbee interference or zero BER is

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Cancellation of Zigbee interference in OFDM based WLAN for multipath channel

  1. 1. ACEEE Int. J. on Network Security, Vol. 02, No. 01, Jan 2011 Cancellation of Zigbee interference in OFDM based WLAN for multipath channel Minakshmi Roy1, H.S. Jamadagni2 1 Indian Institute of Science, CEDT, Bangalore, India Email: 2 Indian Institute of Science, CEDT, Bangalore, IndiaAbstract— Zigbee is one of the major sources of interference II. OVERVIEW OF THE WLAN AND ZIGBEEin 2.4GHz band for WLANs. It is seen whenever any Zigbeesystem is operating near to the WLAN system andtransmitting signal at same frequency, time as of WLAN’s, thelater ones performance detoriate severely. So in this paper analgorithm is proposed to estimate Zigbee interferencecomponent present in all OFDM based WLANs sub-carriersand cancel out the Zigbee interference from the receivedsignal of WLANs receiver for multipath fading channels infrequency domain. Simulation results shows for high SNRvalues full cancellation of Zigbee interference or zero BER ispossible.Index Terms— WLAN, Zigbee, OFDM, OQPSK, narrowband I. INTRODUCTIONRecent development in wireless technology shows that nextgeneration wireless system will provide users with variety Figure 1: Zigbee interference in OFDM based WLANof wireless services assuming coexistence of differentwireless technology at the same time and same place. At IEEE802.11g[7] standard based WLAN uses OFDMpresent IEEE802.11g standard based WLAN which is modulation technique. In OFDM modulation multiple lowintended for wireless network, is one of the most popular data rate carriers are combined by transmitter to form adevice adopted in home, office, institution etc. It can composite high data rate transmission. IEEE802.11goperate within range of 100 meter distance in 2.4GHz ISM supports different data like 6, 9, 18, 36, 48,54Mbps. Use ofband. cyclic prefix makes OFDM signal immune to multipathZigbee is another wireless technology intended for personal effect, inter carrier interference (ICI), intersymbolarea network. It transmit within a range of 10meters in interference (ISI) etc.same 2.4 GHz unlicensed ISM band. As both the system Zigbee[5] system is designed for mainly low cost, lowoperates in same ISM band (i.e 2.405-2.480GHz) whenever power applications. In the 2.4GHz range bit rate of Zigbeethey operate at the same time results in collision. is 250kbs/s. Zigbee uses direct sequence spread spectrumFrom the literature search some techniques, to mitigate (DSSS) to make the signal robust against interference.narrowband interference by estimating interference Zigbee system has 16 different 32-chip sequences. So tocomponent with OFDM null carriers using pseudo inverse make the signal DSSS 4bit information sequence is mappedof narrowband signal’s transfer function [3] or erasing the into any of the 32 chip sequence which results in 2Mchip/ssub carriers[1] can be found. But all this techniques having chip rate. Then chips are modulated by Offset-QPSK.there own limitation. In this paper an algorithm isproposed to cancel Zigbee interference in OFDM based III. INTERFERENCE MODEL AND CANCELLATIONWLANs. In the proposed cancellation algorithminterference component in all the sub carriers are estimated In this section first a mathematical model of Zigbeeand cancelled. It is assumed that WLAN is monitoring the interference is given, and then an algorithm is proposed tochannel before transmission using spectrogram [2]. From estimate and cancel Zigbee interference from OFDM basedthis it can detect any Zigbee signal interference. WLAN receiver. The paper is organized as follows: In section 2 The received signal in the OFDM receiver for the kthOverview of OFDM system and Zigbee interference subcarrier is given by[9] [5][6] :modeling is given. In section 3 Cancellation algorithm,estimation of frequency, power of Zigbee and channel tap Rk = H kU k + I k + N k (1)value is given In section 4 simulation results are givenSection 5 conclude the paper summarizing of the Where U k is the complex MQAM signal and H k denotessimulation results. the kth channel component corresponds to equivalent channel transform matrix and I k is the interference 35© 2011 ACEEEDOI: 01.IJNS.02.01.194
  2. 2. ACEEE Int. J. on Network Security, Vol. 02, No. 01, Jan 2011component corresssponds to kth sub-carrier of OFDM or S Zn,l = BZn,l • Yn 2 (6)system and N k is the AWGN. Zigb where Y n2 = [ D 0 D 1 ]T , n is any positive integerThe idea is to model interference I k as function oftransmitted unknown binary data to model the interference ⎛ πt ⎞ (7)as given below: P (t ) = cos ⎜ ⎟ c o s 2π f h t ⎝ 2TZ ⎠ p −1 ⎛ πt ⎞ Ik = ∑a n=0 n,k Dn + Ck (2) Q (t ) = sin ⎜ ⎟ sin 2 π f h t ⎝ 2TZ ⎠ (8) The value of p depends on the number of unknown Where subscript l denotes the lth Zigbee channel andtransmitted binary data of Zigbee system transmitted within subscript n is given to denote nth time instant. Matrix BZone symbol duration of the OFDM system. The values of n,l is an N×2 matrix consisting of 2 columns of Pl (t), Ql (t) an , k , Ck can be found from the fixed parameters of both multiplied by the square root of the power P. In equationsystem like modulation type, pulse shape used in the (6) the superscript “2” denotes that matrix, Yn 2 isZigbee system , tap values of the receiver low pass filter,cyclic prefix remove matrix, FFT matrix in OFDM generated from 2 unknown binary data. Similarly for preceiver. (where p is even) unknown binary data can be written as:A. Interference Model of Zigbee signal in OFDM receiver S Zn,l = BZpn,l • Yn p p (9) This subsection show how to model the Zigbee pinterference for each of the OFDM subcarrier as a function Where S Zn,l , BZpn,l Yn p is of dimension N1×1, N1×p,of unknown binary data transmitted within a symbol ,duration of OFDM signal. p×1 respectively. The superscript “p” denotes the number of binary Zigbee data transmitted during one the OFDM T The OQPSK modulated signal can be written as[5]: symbol period and Ynp = ⎡D0 D D2.... Dp−1 ⎤ where D0,D1…Dp- 1 ⎣ ⎦ 1are binary data transmitted by Zigbee system. So vector of sz (t ) = PD0 p (t ) cos 2π f ht Zigbee samples p S Zn,l depend on the particular (3) + PD1 p (t − TZ ) sin 2π f h t combination binary data vector x j = { D 0 D1 D 2 ....D p −1 } . Where D0 , D1 ∈ {+1, −1} denotes binary data for in Therefore, to denote the dependency on a particular combination of data input vector the equation(9) is writtenphase , quadrature phase component of OQPSK signal and as: p(t ) is the shaping pulse which is given by[5]: S Zn,l, j = BZn,l • Yn,pj p p (10) ⎛ πt ⎞ p (t ) = cos ⎜ ⎟ 0 ≤ t ≤ 2TZ (4) p ⎝ 2TZ ⎠ The received Zigbee data vector S Z n,l in OFDM =0 elsewhere receiver is passed through a low pass filter of impulse And f h is the centre frequency among any of the 16 Zigbee and Ts OFDM response hlpf(t). Let Ts be the samplingchannels of the Zigbee and P is the signal power. Let thereare N samples for the TZ symbol duration, then the time of the OFDM and BT signal respectively. Then theequation(3) can be written in matrix form as given: filtered Zigbee samples in OFDM system will be[3]: N1 −1 ( ⎡P n2T ⎢ l ) Z Q n2TZ l ( ) ⎤ ⎥ i ( n) = ∑ hlpf ( nTsOFDM − mTsZigbee )SZn,l,j ( m) p (11) (5) ⎢P ( n2TZ +TS ) ⎢l ( Q n2TZ +TS l ) ⎥ ⎥ m=0 ⎢ ⎥ ⎡D ⎤ SZn,l = P ⎢. 0 ⎥ ⎢D ⎥ After passing through the cyclic prefix removal block ⎢. ⎥⎣ 1 ⎦ the above equation can be written in matrix form as: ( ⎢l Z ) ( ⎢P n2T + N −1 T Q n2T + N −1 T ( )S l Z ( )S ) ⎥ ⎥ i NFFT = Rcp • hlpf • SZn,l,j = h1 • Bn,l •Yn,j p ⎢ ⎥ p p ⎣ ⎦ (12) Rcp is cyclic prefix removal matrix of dimension NFFT ×(Ncp +NFFT ), Ncp is the length of cyclic prefix. hlpf is generated from tap value of receiver low pass filter. Τ i N FFT = ⎡ i0 i1 .. iΝ FFΤ ⎤ , h1 = Rcp • hlpf , ⎣ -1 ⎦ 36© 2011 ACEEEDOI: 01.IJNS.02.01.194
  3. 3. ACEEE Int. J. on Network Security, Vol. 02, No. 01, Jan 2011 After FFT operation in the OFDM receiver the vectorconsist of interference component for the OFDM sub 2MHz Widecarriers is given by : Zigbee Spectrum p p I j = F • h1 • BZ n,l • Yn, j (13) Sub carriersWhere F is 64×64 FFT matrix. And fstart ffinish 1 2…. k …………. NFFTI = [I 0, j , I 1, j , I 2, j ....I k , j ....I 63, j ]T .Let Ik denotes the interference corresponding to the kth 22 MHz wide WLAN Spectrumsubcarrier and Fk denotes the kth row of the FFT matrix Figure 2: Zigbee inside WLAN frequency bandF, then Ik is expressed as: C. Channel estimation and Interference power estimation: p p I k, j = Fk • h1 • BZ n,l • Yn, j (14) Training signal are used to estimate both channel taps and the instantaneous power of the interference signal. Let According IEEE802.11g standard [4] one OFDM symbol uTrain is the set of training signal after IFFT in theperiod TOFDM=4µs and one Zigbee[5] symbol period transmitter of an OFDM system.TZ=.5µs. As Zigbee is using O-QPSK data per cos or sinpulse will remain unchanged for 2TZ=1µs. So it can befound for 4us of OFDM signal there will be 16 unknown uTrain = F H • U Train = ⎡u0 u1 . .. . u N FFT −1 ⎤ ⎣ ⎦ (18)data for Zigbee (combining both sin and cos pulse’s data),basically which implies 9 combination of unknowns. As If u1Train is the received matrix at the receiver before FFTOFDM will remove the cyclic prefix length which is one operation then it is given by:fourth of the symbol duration, that is 0.8 µs, so for the rest3.2 µs there will 8 unknown Zigbee data. Now Yn, j is a p u1Train = Rcp • H 0 • Tcp • uTrain (19)8×1 matrix consist of 8 unknown binary data and matrix % = H •u (20) Train pBZ n,l is of N1×8.Let % where H is NFFT× NFFT circulant matrix which is given p below:B Z n,l = [C1 C2 C3 C4 C5 C6 C7 C8] ( 15) ⎡h0 0 . . . . . . . . 0 hL-1 . . h2 h1 ⎤So the eq(14) can be rewritten as: ⎢h h 0 . . . . . . . . 0 h . h3 h2 ⎥ ⎢ 1 0 L-1 ⎥ 7 ⎢. . . . . . . . . ⎥ (21) ⎢ ⎥ I k,l, j = ∑a D n=0 n n (16) ⎢ hL-2 hL-3 . h1 h0 0 . . . . . . 0 hL-1 H = ⎢hL-1 . . . . h1 h0 0 . . . . . 0. . . . . . .0 % ⎥ ⎥ ⎢ ⎥ ⎢0 hL-1 . . h1 h0 0. . . . . . . .0 ⎥ ⎢. . . ⎥ Then a0 = Fk • h1 • C1 ; a1 = Fk • h1 • C2 ; ⎢ ⎥ ⎢. . . ⎥ a2 = Fk • h1 • C3; a3 = Fk • h1 • C4 ; a4 = Fk • h1 • C5 ; ⎢ ⎥ ⎣ 0 . . . . 0 hL-1 . . . . . h1 h0 ⎦ a5 = Fk • h1 • C6 ; a6 = Fk • h1 • C7 ; a7 = Fk • h1 • C8 Now equation (20) is rewritten as:B. Interference frequency estimation u1Train = uTrain • h % (22) Let Δf be the sub carrier spacing of the OFDM sub ⎡u0 u N FFT −1 ....u N FFT − ( L −1) ⎤carriers and kth sub carrier have maximum energy. If the ⎢ ⎥ ⎢ u1 u 0 ...........u N FFT − ( L − 2 ) ⎥OFDM frequency band ranges from fstart to ffinish thenZigbee’s frequency is given by Where u % Train = ⎢. . . ⎥ (23) ⎢ ⎥ f h = round ( f start + ( Δf × k ) ) (17) ⎢. ⎢ . . ⎥ ⎥ ⎢u u u ⎣ N FFT −1 N FFT − 2 N FFT − L ⎥ ⎦Where, ffinish- fstart = BMHz , B is the estimation band ofWLAN. and h = (hL −1 .......h1 h0 ) T and uTrain is a matrix of % dimension NFFT×L consisting training signals. 37© 2011 ACEEEDOI: 01.IJNS.02.01.194
  4. 4. ACEEE Int. J. on Network Security, Vol. 02, No. 01, Jan 2011Replacing h1 • BZn,l = h2 in equation(12), the Zigbee p filter ,FFT matrix of the OFDM receiver and Vk can beinterference in OFDM receiver before IFFT can be written expressed as:as: 8 Vk = Fk • h1 • BZn,l (34) = h2 • Yn, j p N FFT i (24) And 8 x j = Yn, j (35)Now in presence of AWGN noise and Zigbee interferencethe received signal before FFT operation is written in Where Fk is subset of FFT matrix corresponds to thematrix form as: chosen set of p=8 null carriers and is of cdimension 1×NFFT,. h1 is generated from low pass filter response of u1Train = u1Train + i N FFT + n1 ˆ (25) 8 OFDM receiver, BZ n,l is generated from transmittedWhere n1 is AWGN noise frequency, modulation ,pulse shaping characteristics of ˆ % ( u1Train = ( uTrain • h ) + h2 • Yn, j + n1 p ) (26) Zigbee system. Assuming hopping frequency, power of the interference signal has been measured perfectly using OFDM training signals], estimation of interference in ⎡h ⎤ OFDM data signals is done as follows: = [ uTrain h2 ] ⎢ p ⎥ + n1 % (27) 1. For each data input vector x j , where ⎢Yn, j ⎥ ⎣ ⎦ = Ay + n1 (28) j ∈ {0,1,...., 255} , generate Vk and find the interference for each of the set of p null sub-carrier using the equationwhere A = [ uTrain h2 % ] is of dimension NFFT×(L+2p). below: ( ) T Vk x j = Fk • h1 • BZn,l • x j = I k, j . 8 (36) T pand y = h Yn, j is matrix of dimension (L+2p)×1consist of unknown channel vector and unknown Zigbee 2. Take the Difference Ek, j = Yk − I k , j and obtain thesignal vector. From eq(29) unknown parameters of matrix ycan be estimated near to the actual value if noise is magnitude of Ek, j . Average Ek, j over all the chosen setnegligible by the equation given below: of null sub-carriers using the equation below: y = A u1Train − A n1 ˆ -1 -1 (29) 1 p −1 Ej = ∑E p k =0 k, j (37)Let be the vector of estimated vector Zigbee signal fromthen estimated Zigbee samples corresponding to the psymbol will be: 3. Choose xj that will minimize the average error E j )p ) 4. For the above chosen x j obtain the interference S Zn,l, j = BZn,l • Yn,pj p (30) component for all the sub-carriers of one OFDM signal.B. Interference Cancellation Algorithm 5. Subtract the above obtained estimated ZigbeeThe received signal for the null sub carriers of the OFDM interference for each sub-carrier from the received signal.receiver is given by[5]: Repeat the above procedure from step 1 to 5 for the next OFDM signal to cancel out Zigbee interference. Rk = I k + N k (32) IV. SIMULATION RESULTSIt can be shown I k can be expressed as function of In all simulation results SIR is ratio of total OFDM powertransmitted binary input signal of the Zigbee system as: of all sub-carriers with interferer (BT) power. SNR is the ratio of total OFDM power with additive white Gaussian I k = Vk • x j (33) noise. In figure 6 technique1 refers to the narrowband cancellation method[3] where estimation of transmitted data of narrowband interference and reconstruction of itsWhere x is matrix of unknown transmitted binary inputs waveform is done by measuring interference informationcorresponds one OFDM symbol period and on certain unmodulated null sub-carriers and using erasures[1] refers to the techniques of replacing nulls in the {x∈−1−1−1−1−1−1−1−1,......,11111111 ={ x0 ,x1,........x255} } sub-carriers where the narrowband interference signal has hopped in OFDM frequency range.and Vk is matrix generated depending upon modulation ,pulse shaping of the Zigbee system and equivalent low pass 38© 2011 ACEEEDOI: 01.IJNS.02.01.194
  5. 5. ACEEE Int. J. on Network Security, Vol. 02, No. 01, Jan 2011 Figure6. Comparison of uncoded BER of 64QAMWLAN with different techniques in AWGN for scenario in figure2. Technique1 is the technique Figure 3: Comparison of coded BER for 64QAM OFDM based WLAN used in ref[3]. with and without Zigbee in AWGN V. SUMMARY This paper proposes an algorithm for extracting out Zigbee interference component present in all OFDM subcarriers and cancelling those estimated Zigbee signal’s part from received signal in OFDM receiver. Simulation result shows algorithm. gives significant improvement in performance particularly when SNR value is high, almost zero BER can be obtained in presence of Zigbee interference for OFDM based WLANs. As initial estimation of amplitude or frequency cannot be obtained accurately for low SNR values performance in terms of BER will not improve much using proposed algorithm for low SNR. So applying this algorithm will nullify the Zigbee interference present in all OFDM based WLAN subcarriers and give one a step solution towards coexistence of WLAN and Zigbee systemFigure 4. Comparison of Coded BER for 64QAM OFDM based WLANsin presence of Zigbee interference in flat Rayleigh fading channel using REFERENCESproposed cancellation algorithm [1] Doufexi, A. Arumugam, S. Armour and A. Nix, An Investigation of the Impact of Bluetooth Interference on the Performance of 802.11g Wireless Local Area Networks, pages:680-684,Vol1.,April,2003 [2]Miller, Rob Xu, Wenyuan Kamat, Pandurang Trappe, Wade , Service discovery and Device Indentification in cognitive Radio Networks, Workshop on Networking Technologies for Software Defined Radios (SDR)Networks, San Diego , CA,pges40-47, June 2007. [3] Dan Zhang; Pingyi Fan; Zhigang Cao, A novel narrowband interference canceller for OFDM systems Wireless Communications and Networking Conference, 2004. WCNC. 2004 IEEEVolume 3, Issue , 21-25 March 2004 Page(s): 1426 - 1430 Vol.3 [4] IEEE802.11g standard. [5] IEEE Std 802.15.4-2006 [6]ZhengdaoWang and Geogios B. Giannakis, Wireless Multicarrier Communication: Where Fourier Meets Shannon , Transaction on IEEE Signal Processing,May 2000Figure 5. Comparison of Coded BER for 16QAM OFDM, BPSK-OFDM [7]Peter Klenner, The OFDM multicarrier system,based WLANs in presence of Zigbee interference in flat Rayleigh fading Commmunication Technology Laboratory,-SS 2006channel using proposed cancellation algorithm [8]Henik Schulze, Christian Luders, Theory and Applications of OFDM and CDMA, Wideband Wireless Communications, John- Wiley and Sons, Ltd, 2005. [9]Ramjee Prasad, OFDM for Wireless Communication 39© 2011 ACEEEDOI: 01.IJNS.02.01.194