Internationa Journa of Software Engineering and Its Appications Vo. 6, o. 3, Juy, 0 A Frame Synchronization Method with Robustness to the Effects of Initia SFO in DRM Systems Ki-Won Kwon, Seong-Jun Kim,Yong-Suk, Park and Jong-Ho Paik* Mobie Convergence Patform Research Center, Korea Eectronics Technoogy Institute, S. Korea Department of Mutimedia, Seou Women s University, S. Korea kwonkw@keti.re.kr, ksjuny@keti.re.kr, yspark@keti.re.kr, paikjh@swu.ac.kr Abstract In this paper, we propose a synchronization method for Digita Radio Mondiae (DRM) receivers. In order to satisfy the advanced synchronization requirement of DRM receivers, the proposed method eiminates the initia samping frequency synchronization (SFO) process in conventiona synchronization methods. In the proposed method, samping frequency tracking is performed after integer frequency synchronization and frame synchronization. Different correation agorithms are appied to detect the first frame of the Orthogona Frequency Division Mutipexing (OFDM) demoduation symbo with SFO. A frame detection agorithm that is robust to SFO is seected based on the performance anaysis and simuation. Simuation resuts show that the proposed method reduces the time spent for initia SFO even if SFO is present in the DRM signa. In addition, it is verified that inter-ce differentia correation used between reference ces is robust to the effect of initia SFO. Keywords: DRM, Frame Detector, Synchronization. Introduction OFDM has been chosen as the transmission method for severa broadcasting standards, such as digita radio broadcasting (DAB), digita video broadcasting (DVB), and DRM. DRM is an OFDM-based internationa digita radio broadcasting standard that can substitute the anaog AM radio in frequency bands under 0 MHz []. DRM has been receiving the attention of broadcasters throughout the word due to its arge coverage. Currenty, DRM is being serviced in various regions around the gobe such as Europe and China. There is an unavoidabe deay invoved from the point when the user powers up the device to pay sound or data service. Since such deay does not occur in anaog broadcast receivers, it has become a key performance evauation eement of the receiver (especiay, automotive and portabe mobie receivers), and DRM receiver engineers must consider the receiver time deay as a receiver design parameter. The DRM receiver synchronization consists of severa synchronization steps reevant to symbo time, carrier frequency, and samping frequency. Therefore, there is opportunity for processing speed improvement agorithmicay, compared to other pre-processing processes such as decoding and data parsing []. The DRM standard uses reference ces which perform the same function as the OFDM piots for synchronization. Initia synchronization is done by seecting a method conforming to the DRM specification from the existing initia synchronization methods based on OFDM piot symbos or by changing the existing method to meet the DRM specification requirements. As a method for initia synchronization for DRM, it has been proposed which sequentiay performs symbo time offset (STO) synchronization, fractiona frequency offset synchronization, and SFO 7
Internationa Journa of Software Engineering and Its Appications Vo. 6, o. 3, Juy, 0 synchronization, and then performs integer frequency offset (IFO) synchronization and frame detection in the frequency domain [3]. However, this method needs to use mutipe OFDM symbos during SFO estimation for initia SFO synchronization. Therefore, it is not suitabe for high-speed synchronization. Another synchronization method proposed extracts more than two types of synchronization parameters from the same received signa to perform simutaneous estimation. This method can estimate STO and carrier frequency offset (CFO) or CFO and SFO synchronization simutaneousy [4]. If this method is impemented using parae processing architecture, the estimation steps needed for initia synchronization can be reduced. However, the estimation agorithms proposed for this method are based on reguar training symbos or frequency synchronization of piot based burst mode OFDM communication systems, making them unsuitabe for DRM which is a continuous mode broadcasting system based on irreguar piots. In this paper, we propose a synchronization architecture that is carried out after symbo time synchronization in order to sove initia high-speed synchronization issues in DRM receivers. The proposed architecture omits the initia SFO which does not affect much the quaity of synchronization performance in order to reduce the service start deay in DRM receivers. Instead, it compares and seects a frame detection method which is not greaty infuenced by the SFO. The rest of the paper is organized as foows. In section II, an overview of DRM is given. In section III, the conventiona DRM synchronization method and the need for advanced synchronization are expained. The proposed DRM synchronization method is expained in section IV, foowed by its performance anaysis through simuations in section V. The paper is concuded in section VI.. System Mode DRM is highy frequency efficient due to its use of OFDM moduation. It can easiy dea with frequency seective fading, but on the downside, the overa reception performance fuctuates depending on the performance of synchronization. The DRM physica ayer is designed to provide spectrum efficiency greater than bits/s/hz in the 9 to 0 khz AM band, in order to provide high-quaity audio services in diverse transmission environment of the AM frequency band. The transmission frame structure consists of Fast Access Channe (FAC) which contains the channe information and service reevant information required by the receiver, Main Service Channe (MSC) which carries audio and data payoad, and Service Description Channe (SDC) which carries the channe coding parameters for MSC and the mutipex structure of audio and data signas. In order to receive DRM, frame synchronization must be performed to detect the start of a frame. Due to the frequency characteristics of DRM services, five types of transmission modes are defined in order to effectivey dea with the diverse environment depending on terrestria transmission. The spectrum occupancy information is transmitted in the FAC. Therefore, if the FAC is decoded without any errors in the 4.5 khz bandwidth from the center frequency during the synchronization process, channe estimation of the designated occupied bandwidth is carried out after obtaining the occupied bandwidth information from the FAC. DRM defines reference ces to perform synchronization and channe estimation. The frequency reference ce is ocated continuousy in the sub-carrier frequencies corresponding to 750, 50, and 3000 Hz of each OFDM symbo, and it is used for frequency synchronization. The time reference ce exists ony in the first OFDM symbo of each transmission frame, and it is used for frequency synchronization and frame synchronization. The gain reference ce is spit in each OFDM symbo, and it is used for channe estimation. A reference 8
Internationa Journa of Software Engineering and Its Appications Vo. 6, o. 3, Juy, 0 ces have a coefficient of, and the gain reference ces ocated at the extremes have a coefficient of [, 5]. 3. Conventiona Synchronization Method The conventiona synchronization method of DRM is summarized in this section. First, the detection of 5 types of DRM transmission mode is carried out. Transmission mode detection is done by obtaining the correation the received signa using the guard interva and usefu symbo duration for each mode. If the transmission mode is determined, synchronization process is carried out using the reference ces ocated within 4.5 khz from the center frequency regardess of mutipe occupancy bandwidths. Coarse symbo timing synchronization is performed which is ater corrected by fractiona CFO estimation. Then, after SFO estimation and correction, the signa is changed to frequency domain through FFT, and IFO estimation and frame detection are performed. Timing offset and frequency offset are continuousy tracked after the initia synchronization process described above [3]. The osciator frequency error of the transmitter and receiver creates SFO which causes OFDM symbo timing to drift. This causes FFT window starting point to change, as we as, intersymbo interference and inter-carrier interference. It obstructs stabe DRM frame synchronization resuting in the overa receiver performance degradation [6, 8]. In addition, initia samping frequency synchronization is performed in order to estimate and correct the initia SFO caused by the use of ow-cost osciators in many conventiona DRM receivers. However, initia synchronization process becomes onger since SFO estimation and correction is necessary for samping frequency synchronization. This in turn means deayed service start time for the users. Therefore, advanced synchronization technique is seen as an important eement of receiver performance. 4. Proposed Synchronization Method 4.. Synchronization Structure Reduce the Time Spent for Initia SFO In this paper, we propose a new synchronization method which omits the initia SFO estimation and correction for initia high-speed synchronization. The initia SFO estimation and correction has itte effect on data reception performance. Estimation methods are compared and seected for IFO estimation and frame detection with SFO. The proposed method eiminates this part in the initia stages of synchronization making fast synchronization possibe. In genera, the aowabe SFO range in DRM receivers is reguated by the manufacturers. The DRM receiver deveoper seects an osciator that can satisfy the range constraint and designs the receiver. However, the use of high-precision and highaccuracy osciator is expensive. Therefore, ow-price osciators are used which cause SFO in the received signa. In such a case, samping frequency synchronization is needed. In order to estimate the SFO, a minimum number of symbos need to be observed, and the estimated SFO needs to be corrected. This process consumes synchronization time [7]. The proposed method does not perform initia SFO estimation and correction initiay, acceerating the synchronization speed. Since the initia detection and correction for the samping frequency is omitted, the OFDM symbo after FFT may incude SFO, causing synchronization performance degradation in subsequent steps. Therefore, IFO estimation and frame detection methods, which reduce the effects of SFO, are used to prevent degradation of synchronization performance. After the initia synchronization process, STO, CFO and SFO are repeatedy tracked and corrected. 9
Internationa Journa of Software Engineering and Its Appications Vo. 6, o. 3, Juy, 0 4.. Frame Detection Performance Anaysis If the initia SFO estimation and correction processes are omitted and integer CFO estimation and frame detection are carried out after FFT for initia high-speed synchronization in DRM receivers, its performance is infuenced by the initia SFO, and the degree of infuence depends on the characteristics of the agorithm used. Therefore, it is important to seect an agorithm that is robust to the effects of initia SFO in order to achieve more accurate IFO estimation and frame detection. In this paper, we compare and anayze two reference ce based correation methods suitabe to be appied to OFDM symbos in frequency domain after FFT. Since the effects of SFO are infuenced equay to both IFO and frame detection processes, ony frame detection process is observed. The simpest reference ce based correation method is to use the correation between the reference ce and the received signa. Assuming idea symbo timing synchronization, the -th OFDM reception symbo after FFT infuenced by SFO can be represented as foows [7, 9]. j / / k j s k j s g k R k e e e X ( k) H ( k) ( k) () s where ξ is the SFO, and α(.) is the distortion coefficient of subcarrier due the SFO. j s k / e represents the phase coefficient due to SFO. X (k) is the data symbo assigned to the k-th subcarrier in the -th OFDM symbo transmitted., g, and s represent the FFT size. s = g +. k is expressed in genera as the residua CFO and STO, since STO is 0, it can be approximated as the residua frequency offset ϵ. H (k) and (k) represent channe frequency response and noise, respectivey. We define the method using the correation vaue between the reference ce and the received symbo as method and express it as foows [0]. RC * RC RC () ˆ arg max R P k X P k k where = 0,,,, S n- and S n is the number of OFDM symbos per frame. P RC [k] is the position of the k-th reference ce. R (P RC [k]) is the k-th reference ce of the -th symbo received. X (P RC [k]) is the k-th reference ce transmitted, i.e. a known reference ce. RC refers to the number of a reference ces within 0 to 4.5 khz bandwidth. In order to anayze equation (), equation () is substituted into equation () as foows. RC ˆ s j s / j s PTRC k/ arg max e e H ( P ) RC k k arg max (3) where s j s / e RC j s PTRC k/ and ( ) e H P RC k. k 0
Internationa Journa of Software Engineering and Its Appications Vo. 6, o. 3, Juy, 0 The residua frequency offset vaue (ϵ) and the SFO in are very sma vaues (units of ppm). Therefore, ξ and yieds the foowing equation. is approximated. Appying this to equation (3) ˆ argmax (4) The reference ce is aperiodic and paced irreguary. Therefore, P RC [k] in can be spread in a FFT areas and infuenced greaty by SFO. In order to improve the performance in equation (3), the vaue of needs to be increased. Under the assumption that there is amost no channe frequency response changes between consecutive reference ces (i.e. H (k) H (k+)), the correation method based on the received reference ce and its consecutive reference ce can be used to achieve this. Method is the intercarrier differentia correation method which obtains the correation between the received reference ce and its consecutive reference ce. Method can be expressed as foows. RC * RC j diff ( k) (5) k ˆ argmax R P k R P k e j ( k) e diff where is the phase difference of the consecutive reference ces. The distance between the reference ce and the consecutive reference ce is sma in method. Therefore, the phase change is aso reativey sma. Further anaysis of the received reference ce is as foows. The frame detection resut as shown beow. RC ˆ s j s / js PRC kprc k / arg max e e H ( ) ( ) P k H P k arg max k (6) where s j s / e RC j and s ( PRC kprc k )/ e H ( ) ( ) P k H P k. k The residua frequency offset vaue (ϵ) and the SFO in are very sma vaues (units of ppm). Therefore, ξ and is approximated. Appying this to equation (6) yieds the foowing equation. ˆ arg max (7) P RC [k]-p RC [k+] is different from P RC [k] of frame detection equation (3) for method. P RC [k] of method is infuenced by the SFO depending on the ocation of the reference ce in
Internationa Journa of Software Engineering and Its Appications Vo. 6, o. 3, Juy, 0 the tota FFT domain. P RC [k]-p RC [k+] of method is infuenced by the SFO depending on the difference between the reference ce and the adjacent reference ce. Therefore, method is affected ess by the SFO. The phase shift depends on the difference P RC [k]-p RC [k+]. it is ess infuenced by SFO compared to method. Another difference is H ( P ) RC k of and * H ( P ) ( ) k H P k of. Unike ( ) * H P RC k of, H P k H P k * of E H ( P k) H ( P k ) 4.5484 ( ) ( ) of has an average vaue. It can be seen that it aways has vaues greater than method. Therefore, its performance is better than method. 5. Performance and Resut In this section, we investigate the duration of the initia SFO estimation from the DRM reception signa through simuations. The performance of frame detection depending on the initia SFO for methods and are compared. Then, the method most suitabe for the proposed high-speed synchronization architecture is seected. First, the number of symbos required for SFO estimation and correction was measured. The feedback architecture proposed in [8] was used for samping frequency estimation and correction. A oop gain of /4 was appied for stabe tracking during feedback. Figure shows the SFO estimation error depending on the number of OFDM symbos when the initia SFO is 00 ppm, 50 ppm, and 0 ppm in DRM mode B. As the SFO increases, the number of symbos required for estimation and correction increases greaty. Tabe shows the number of symbos that need to be observed to correct the initia SFO vaues of 00 ppm, 50 ppm, and 0 ppm to error ranges of 50 ppm, 0 ppm, and 5 ppm. As seen in Figure, if the initia SFO is 00 ppm, the number of symbos required for correction to 50 ppm through the estimation and correction process is 8. If correcting to 5 ppm, 79 symbos need to be observed. In DRM mode B, the effective symbo duration is 6.66ms. The proposed method is 6.66ms mutipied by the number of observed symbos faster than the conventiona method for initia synchronization. The correction offset (ppm) goa depends on the performance index required for each receiver. The performance of frame detection and IFO estimation is anayzed when SFO is present. Figure. The Symbo umber for the Initia SFO Synchronization
Internationa Journa of Software Engineering and Its Appications Vo. 6, o. 3, Juy, 0 Figure. Frame Detection Performance According to SFO (channe ) Channe is AWG. In Figure, if method is used under AWG environment, performance degradation occurs due to the effect of SFO when the SFO increases from 0 to 00 ppm. Performance degradation is evident when SR increases. However, in method, frame detection performance degradation is margina even when the SFO increases. Therefore, method is effected ess by the SFO. Method shows performance degradation as SFO increases. Method shows margina degradation, simiar to the resut from AWG. 6. Concusions In this paper, we proposed a method that can perform to robust the effect of initia synchronization required in DRM systems. Conventiona synchronization methods require ong initia synchronization due to initia SFO estimation and correction. To this end, the initia SFO estimation and correction is omitted before FFT, reducing the synchronization duration. In addition, in order to detect frame from the demoduated symbo with SFO, correation agorithms are appied to the reference ce. The correation agorithm robust to SFO was seected based on performance anaysis. Simuation resuts show that, if the differentia correation method is used between consecutive seected reference ces, it is robust against the effects of initia SFO. The proposed synchronization method is robust against SFO effects and can be used as a key eement technoogy when impementing DRM receivers Acknowedgements This work was supported by the IT R&D program of MKE/KEIT. [003996, Smart Patform Deveopment for Integrating Wordwide Radio Technoogy to Smart Devices]. References [] ETSI ES 0 980, V 3.., Digita Radio Mondiae (DRM); System Specification, (009). [] S. J Kim, K. W. Park, S. H. Park, K. W. Kwon and J. H. Paik, An effective frame detection and FFT window point tracking agorithm for digita radio mondiae receivers, Proc. ICCE 009, (009) January, pp. -. [3] V. Fischer and A. Kurpiers, Frequency synchronization strategy for a PC-based DRM receiver, Proc. APCCS 004, (004) December, pp. 989-99. 3
Internationa Journa of Software Engineering and Its Appications Vo. 6, o. 3, Juy, 0 [4] M. M. Freda, J. F. Weng and T. Le-goc, Joint channe estimation and synchronization for OFDM systems, Proc. IEEE VTC, vo. 3, (004) September, pp. 673-677. [5] F. Hofmann, C. Hansen and W. Schafer, Digita Radio Mondiae(DRM) digita sound broadcasting in the AM bands, IEEE Trans. Broad., vo. 49, no. 3, (003) September, pp. 39-38. [6] F. Cassen and H. Meyr, Frequency synchronization agorithms for OFDM systems suitabe for communication over frequency seective fading channes, Proc. of IEEE VTC, (994) June, pp. 655-659. [7] M. Speth, S. A. Fechte, G. Fock and H. Meyr, Optimum receiver design for wireess broad-band systems using OFDM-Part I, IEEE Trans. Commun., vo. 47, no., (999) ovember, pp. 668-677. [8] T. Poet and M. Peeters, Synchronization with DMT moduation, IEEE Commun. Mag., vo. 37, no. 4, (999) September, pp. 80 86. [9] M. Speth, S. A. Fechte, G. Fock and H. Meyr, Optimum receiver design for OFDM-based broadband transmission Part II: A case study, IEEE Trans. Commun., vo. 49, no. 4, (00) Apri, pp. 57 578. [0] H. ogami and T. agashima, A frequency and timing period acquisition technique for OFDM systems, Proc. IEEE PIMRC, (995) September, pp. 00-05. Authors Ki-Won, Kwon received B.S. and M.S. degrees in computer engineering from Kwangwoon University, Seou, Korea, in 997 and 999, He aso received the Ph.D. degree in the Schoo of Eectrica & Eectronics Engineering from Chung-Ang University, Seou, Korea, in 0. In 999, he joined in Korea Eectronics Technoogy Institute (KETI), Korea, where he is presenty a manageria researcher at the Mobie Convergence Patform Research Center. His research interests are in the area of wireess/mobie communications and digita broadcasting systems Seong-Jun, Kim received the B.S. degree in Eectrica Engineering from Sungkyunkwan University, Suwon, Korea in 00, and the M.S. degree in Information and Communication from Sungkyunkwan University in 004. He was a Research Engineer of Communication system Group, Samsung Thaes by 004. Since 006, he has been with Mobie Convergence Patform Research Center in Korea Eectronics Technoogy Institute (KETI), Seou, Korea. Since March 0, He is aso currenty working toward to Ph.D. degree in Eectrica Engineering at Sungkyunkwan University. His research interests are in the area of wireess/wired and mobie communication systems and broadcasting system. Yong-Suk, Park received B.S. and M.S. degrees in eectrica and computer engineering from Carnegie Meon University in 997 and 998, respectivey. He is currenty a senior researcher at the Mobie Convergence Patform Research Center, Korea Eectronics Technoogy Institute (KETI), Seou, Korea. Before joining KETI in 003, he was with I&C Technoogy and Samsung S, where he worked in projects reevant to wireess networks and system integration. His current research interests are in the areas of smart device patforms and wireess/mobie networks. 4
Internationa Journa of Software Engineering and Its Appications Vo. 6, o. 3, Juy, 0 Jong-Ho, Paik received the B.S., M.S., and Ph.D. degrees in the schoo of Eectrica and Eectronic Engineering from Chung-Ang University, Seou, Korea, in 994, 997, and 007, respectivey. He was a Director with Advanced Mobie Research Center at Korea Eectronics Technoogy Institute (KETI) by 0. He is currenty an assistant professor in the department of mutimedia, Seou Women s University, Seou, since 0. His research interests are in the areas of web-based communication, software testing, wireess/wired communications system design, video communications system design and system architecture. 5
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