Wimax timing and frequency synchronization based on training sequence

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1 Wimax timing and frequency synchronization based on training sequence Benzarti Majdi, Messaoudi Mohamed, Hasnaoui alem University of Tunis El Manar YCOM Laboratory ENIT Tunis, Tunisia Abstract In all wireless communication systems and especially in ODM and MIMO-ODM system synchronization is crucial. The receivers do not know the beginning and the end of the ODM symbols, so the placement of the T window. Moreover, they have their own sampling frequency asynchronous with that of the transmitter. These two points show two synchronization concepts: time and frequency. An error of time synchronization in the reception may cause loss of the orthogonality of subcarriers. This is why it is important to have a robust synchronization algorithm and know its effect on system performance. everal estimation techniques for time and frequency offset have been proposed in the literature, but varied test conditions make comparison difficult. In this paper, we use chmidl and Cox algorithm as a symbol timing synchronization algorithm for ODM system. We also see the preamble structure using the equations provided in the IEEE 82.6d. imulation results show the effect caused by carrier frequency offset (CO) and timing frequency offset (TO) on the performance of an ODM system in a WiMAX context. Keywords Wimax, ODM, ynchronization, TO, CO, reamble. I. INTRODUCTION An ODM system is very sensitive to a carrier frequency error which destroys the orthogonality between the carriers. This effect will dramatically degrade system performance. The research reported in this paper addresses the problem of the frequency and time-synchronization and estimation of the ODM channel in Wimax systems. ynchronization is divided into two parts, time synchronization and frequency synchronization. Time synchronization is done, first by the coarse synchronization of estimating the beginning of each received frame, and the other by the fine synchronization which detects the beginning of each ODM symbol in the received frame. The principle of the frequency synchronization is to find the phase shift between the frequency in the transmission and the local frequency receiver. In this paper, we achieved a study of the different existing sequences to compare the efficiencies of each of these sequences for synchronization in ODM system. II. YTEM MODEL Like any telecommunications system, Wimax system consists of a transmitter, a channel, and a receiver. A basic system level block diagram of Wimax transmitter using ITT is presented in ig. In the transmitter, at first the serial binary sequence coming from the data source is converted to a sequence of complex valued symbols. WiMAX supports different modulation schemes; In our simulations we use 6QAM Modulation. The mapped data were serial to parallel and grouped into a number of bits. Afterwards, the mapped data enter into the ODM modulation which consist of assembling ODM frame, 256 IT and cyclic prefix insertion. After this the parallel to serial converter () converts the data to serial form and readily transmits over the channel. Mapper 6 QAM ilot, DC, Guard I T Add C ODM modulation ig. The mandatory parts of WiMAX Transmitter The functional blocks which compose the WiMAX receiver as shown in fig2 which are the reverse functional blocks of WiMAX transmitter. At the receiver the cyclic prefix is removed. After the demodulation and equalization tasks, the equalized R Tx $3. 25 IEEE

2 symbols are demapped to bits and afterwards the demapped data enter the channel decoder. Demapper Channel equalizer I T Remove C Rf Rx ODM demodulation ig2. The mandatory parts of WiMAX Receiver A. Time synchronization The first important task in a MIMO-ODM system is time synchronization, which is divided into two phases[2]: a) The frame synchronization or coarse time synchronization: The task of frame synchronization is to estimate the start of a frame by using a preamble. b) The symbol synchronization or fine time synchronization: The symbol synchronization task is used to identify the beginning of the symbol ODM in a bit. The symbol synchronization in a system MIMO-ODM consists in positioning the T window on the train of samples received. B. requency synchronization The frequency synchronization is one of the most important tasks in WiMAX systems. On transmission, the system has its CTX sampling frequency to generate the different samples of the ODM signal which are then transposed on CRX carrier frequency. In reception, the receiver does not know the CRX frequency, so it is asynchronous in both frequency and phase. Its carrier frequency can also be shifted with the CRX frequency. The differences will therefore cause different types of errors so-called timing or rhythm[3]. The CO represents the difference between the carrier frequency of the transmitter and the receiver. We note the major sources of CO: a) The phase shift between the frequency of the transmitter and receiver: Due to the phase difference between the frequency of the transmitter and receiver, the signal after modulation is centered on a frequency δ f instead of being centered about DC(MHz), where δ f= CRX - CTX. The carrier frequency offset (CO) is shown in ig3. b) Doppler Effect: The Doppler Effect is another source of CO. In the case of mobile receivers, the carrier frequency at the receiver ( CRX) can vary due to the Doppler effect. c) The difference with the sampling frequency: The gap between the sampling frequencies between the data source ( ETX ) and the destination( ERX) is another source of CO. C. reamble structure ig3. Carrier frequency offset (CO) ixed pattern referred as preamble is used for time, frequency and channel synchronization in many of the wireless systems. In this part we will see how this preamble is generated using the equations provided in IEEE 82.6d. The long preamble of the downlink in WiMAX consists of two consecutive ODM symbols. ymbol has four times 64 sequences. ymbol2 has 2 times 28 sequences. In the frequency domain, these symbols are represented by 4x64(k) and 2x28(k) according to the equation mentioned in igure-. These two preamble symbols are derived from a sequence ALL(k) of the form as mentioned in the same figure. ALL sequence for all the bandwidth is mentioned in the IEEE ODM physical layer specifications [4]. ± ± j k all(-,) = { k = 4x64(k) = { 2 ALL(k) k mod (4) = k mod (4) 2x28(k) = { 2 ALL (k) k mod(2) = k mod (2) After that, Arranging the preamble symbol and symbol2 as per 256 IT structure and taking IT and Adding C to the time domain preamble symbol and symbol2. The structure of first and second preamble is shown in ig4. ()

3 Where Zl[n] = IDT { Zl[k] } Taking the N-point T of the received samples, let δ denote the normalized TO, ε denote the normalized CO and Y l[k] denote the lth received symbol at the kth subcarrier. ig4. irst and second symbols preamble structure The resulting cross correlation function between first and second symbols preamble is shown in ig5. Cross-correlation irst symbol preamble spectrum Lag a) Effect of TO In order to see the effects of the TO, consider the received signal in the frequency domain by taking the T of the time domain received samples Xl = [n + δ] N n=, given as[4]: Yl[k] = N N- n= xl [n + δ]e j2πkn = N N- n= { N- p= X l(p) e j2π(n+δ)n }e j2πkn = N N- p= xl [p]e j2πpδn N- p= e j2πp(p k)n =xl[k] e j2πkδn (3) ig6 and ig7 show the received symbols in the signal constellation for the case when the estimated starting point of ODM symbol coincides with the exact timing, and the case when the estimated starting point of the ODM symbol is after the exact point (TO=). 2.8 econd symbol preamble spectrum Constellation 6-QAM before TO effect.6.8 Cross-correlation Lag ig5 Cross correlation between first and second symbols preamble III. EECT O TO AND CO The received baseband signal under the presence of CO ε and TO δ can be expressed as: In-hase ig6. The 6QAM constellation before TO effect yl[n]= IDT {yl[k]}= IDT{Hl[k] Xl[k] + Zl[k] } (2) = N H N k= l[k]x l [k] e j2π(k+δ)(n+ε)n + Zl[n]

4 .5 Constellation 6-QAM after TO effect, TO = Constellation 6-QAM with effect of CO = In-hase ig7. The 6QAM constellation after TO effect (TO=) In-hase ig8. Constellation of received symbols with CO ε=.263 b) Effect of CO Let f c and f c denote the carrier frequencies in the transmitter and receiver, respectively. Let f offset denote their difference (i.e., f offset = f c f c ). Let us define the normalized CO, ε, as a ratio of the CO to subcarrier spacing Δf, shown as:.5 Constellation 6-QAM with effect of CO =.7 ε = f offset Δf rom Equation (2), the time-domain received signal can be written as[4]: N yl[n] = H N k= l[k]x l [k] e j2π(k+ε)nn + Zl[n] (4) ig8 and ig9 show two consecutively received ODM symbols with different CO values where the effects of the channel, TO, and noise are ignored. It is clear from this figure that amplitude and phase distortion becomes severe as CO increases In-hase ig9. Constellation of received symbols with ε=.7

5 IV. TIMING AND REQUENCY YNCHRONIZATION OR WIMAX YTEM chmidl and Cox [5] proposed a preamble with a repetitive structure [A A]. A is a time sequence that can be generated with an IT of size of N2 of a N sequence (sequence, - pseudo-random) where N is the length of the preamble. The N sequence is chosen so that the AR (eak to Average ower Ratio) of the preamble is low, which reduces the impact of signal clipping at the digital analog converter DAC (Digital to Analog Converter). In reception timing metric is calculated by taking the autocorrelation of the received signal. The method of chmidl-cox is simple, but the variance of the time estimator is important that degrades the performance of synchronization. The first block of the receiver is the synchronization block. This block is presented in time domain. This block is applied in order to detect beginning of ODM received frames and to align the modulators and the demodulator s local oscillator frequencies. A block diagram of this scheme is shown in ig. Mapper 6 QAM ilot, DC, Guard I T Add C ynchronization preamble R Tx chmidl & Cox proposed two special training sequences for timing synchronization and carrier synchronization [6]. Training sequence is used to achieve timing synchronization and the fractional frequency offset estimation. There is a different relationship between training sequence and training sequence2. It can be used to complete the integer frequency offset estimation. It can be used to complete the integer frequency offset estimation. Dem ODM demodulation Remove C ync Rf Rx Timing synchronization of this algorithm is achieved by finding the ideal sampling point. The ideal sampling point was in the position of maximum of the timing judgment function M(d). The timing judgment function M(d) is expressed as expression(5)[7]. M(d) = p(d)2 q(d) 2 (5) Where, p(d) and q(d) can be expressed by expression (6) and (7), respectively. (d)= L m= r d+m r d+m+l (6) R(d)= L m= r d+m+l 2 (7) we denote r the complex sample types of the received signal, L is the number of samples in a half symbol and d is the time index of the first sample in a window of 2*L = N ample Types. The block that we have specifically studied in our transmission system is the synchronization. This block is to insert the synchronization preamble in the time domain at the beginning of each ODM frame sent. The characteristics of the synchronization preamble have been described in section II.C. ig. Wimax transmission system with synchronization V. IMULATION REULT The simulation parameters are based on fixed WiMAX 82.6d standard. The physical level parameters used in the simulation are given in table. Table. imulation arameters considered for simulation imulation arameters tandard IEEE 82.6 (ixed WIMAX) Channel Bandwidth 5 Mhz ource Coding Convolutif (2) Cyclic refix Length 8 (32) Constellation 6-QAM Useful symbol period Tb 44,44 (μs) Gard Time Tg=TbG(μs), (μs) ub-carrier spacing (KHz).2 IT Length 256 Data ub-carrier Used 2 Number of pilot ub-carrier 8 Upper guard 28 Lower guard 27 Channel Model RAYLEIGH irst, we show the 6QAM constellation before and after timing and frequency synchronization. Then we simulate the

6 ME performance with synchronization. inally, BER results of WiMAX receiver tested in a multipath-channel with AWGN. The results of 6QAM constellation after timing and frequency synchronization are shown in ig. and ig2. Constellation 6-QAM with TO synchronization BER curve for 6-QAM with TO = - -2 chmidl & Cox sync AWGN Theoritical ig3. BER performance for 6QAM modulation with TO= In-hase ig. Constellation 6-QAM with TO synchronization Constellation 6-QAM with CO synchronization BER curve for 6-QAM with CO = chmidl & Cox sync AWGN Theoritical ig4. BER performance for 6QAM modulation with CO= In-hase ig.2 Constellation 6-QAM with CO synchronization ig3. show the BER performance for 6QAM modulation with TO= and ig4. how the BER performance for 6QAM modulation with CO=.263. VI. CONCLUION The effects of carrier frequency offset (CO) and timing frequency offset (TO) on the received samples are analyzed and explored to develop a Timingfrequency synchronization and channel estimation block. imulation results show that the use of synchronization can improve the Wimax system performance. REERENCE [] Khalid Taher Mohammed Al-Hussaini, Creating and performing wimax physical layer,28. [2] Jen-Ming Wu, Chun-Hung Chou, Baseband ampling Clock requency ynchronization for WiMAX ystems. [3] Zhen shang, Xuehong mao, Jinkang zhu, A novel frequency synchronization method for wireless ofdm systems [4] IEEE tandard for Local and metropolitan area networks art 6: Air Interface for ixed Broadband Wireless Access ystems,24.

7 [5] Yong oo Cho, Jaekwon Kim, Won Young Yang, Chung G. Kang, MIMO-ODM WIRELE COMMUNICATION WITH MATLAB. [6] Timothy M. chmidl and Donald C. Cox, Robust requency and Timing ynchronization for ODM, IEEE transactions on ommunications, vol. 45, no. 2, december 997. [7] Emmanuel Bouquet, Design and implementation of a radio modem CODM in the 2.45GHz band for strong applications mobility,27.