Scattered Pilots and Virtual Carriers Based Frequency Offset Tracking for OFDM Systems: Algorithms, Identifiability, and Performance Analysis

In this paper, we propose a novel carrier frequency offset (CFO) tracking algorithm for orthogonal frequency division multiplexing (OFDM) systems by exploiting scattered pilot carriers and virtual carriers embedded in the existing OFDM standards. Assuming that the channel remains constant during two consecutive OFDM blocks and perfect timing, a CFO tracking algorithm is proposed using the limited number of pilot carriers in each OFDM block. Identifiability of this pilot based algorithm is fully discussed under the noise free environment, and a constellation rotation strategy is proposed to eliminate the c-ambiguity for arbitrary constellations. A weighted algorithm is then proposed by considering both scattered pilots and virtual carriers. We find that, the pilots increase the performance accuracy of the algorithm, while the virtual carriers reduce the chance of CFO outlier. Therefore, the proposed tracking algorithm is able to achieve full range CFO estimation, can be used before channel estimation, and could provide improved performance compared to existing algorithms. The asymptotic mean square error (MSE) of the proposed algorithm is derived and simulation results agree with the theoretical analysis

Algorithm-Architecture Co-Design for Digital Front-Ends in Mobile Receivers

The methodology behind this work has been to use the concept of algorithm-hardware co-design to achieve efficient solutions related to the digital front-end in mobile receivers. It has been shown that, by looking at algorithms and hardware architectures together, more efficient solutions can be found; i.e., efficient with respect to some design measure. In this thesis the main focus have been placed on two such parameters; first reduced complexity algorithms to lower energy consumptions at limited performance degradation, secondly to handle the increasing number of wireless standards that preferably should run on the same hardware platform. To be able to perform this task it is crucial to understand both sides of the table, i.e., both algorithms and concepts for wireless communication as well as the implications arising on the hardware architecture. It is easier to handle the high complexity by separating those disciplines in a way of layered abstraction. However, this representation is imperfect, since many interconnected "details" belonging to different layers are lost in the attempt of handling the complexity. This results in poor implementations and the design of mobile terminals is no exception. Wireless communication standards are often designed based on mathematical algorithms with theoretical boundaries, with few considerations to actual implementation constraints such as, energy consumption, silicon area, etc. This thesis does not try to remove the layer abstraction model, given its undeniable advantages, but rather uses those cross-layer "details" that went missing during the abstraction. This is done in three manners: In the first part, the cross-layer optimization is carried out from the algorithm perspective. Important circuit design parameters, such as quantization are taken into consideration when designing the algorithm for OFDM symbol timing, CFO, and SNR estimation with a single bit, namely, the Sign-Bit. Proof-of-concept circuits were fabricated and showed high potential for low-end receivers. In the second part, the cross-layer optimization is accomplished from the opposite side, i.e., the hardware-architectural side. A SDR architecture is known for its flexibility and scalability over many applications. In this work a filtering application is mapped into software instructions in the SDR architecture in order to make filtering-specific modules redundant, and thus, save silicon area. In the third and last part, the optimization is done from an intermediate point within the algorithm-architecture spectrum. Here, a heterogeneous architecture with a combination of highly efficient and highly flexible modules is used to accomplish initial synchronization in at least two concurrent OFDM standards. A demonstrator was build capable of performing synchronization in any two standards, including LTE, WiFi, and DVB-H

GCL Based Synchronization and Time Domain Frequency Offset Correction in OFDM System

Orthogonal Frequency Division Multiplexing (OFDM) is a modulation technique that has become the technology of choice in most wireless communication networks of today. Despite the advantages the OFDM system offers, it has some disadvantages like sensitivity to synchronization and high power-to-average-power ratio (PAPR). Any time offset leads to inter-symbol interference (ISI) whereas any frequency offset results in inter-carrier interference (ICI) and, as a result, the system performance degrades. The studies of preamble based time synchronization show that, the standard PN sequence based preamble in IEEE 802.16a is less robust to frequency offset when used in Park’s method of time synchronization - a method that gives more accurate result as compared to other methods. Time domain channel estimation cannot be carried out in the presence of integer frequency offset. This thesis has three specific objectives. Firstly, to design and evaluate a new preamble by making use of a generalized chirp-like (GCL) sequence that has low PAPR and good time and also frequency correlation properties. Secondly, to design a new receiver scheme that estimates and corrects the integer-frequency offset in the time domain and evaluate its performance. And lastly, having corrected the frequency offset in time domain, to estimate the wireless channel in time domain and evaluate its performance. The results show that, the proposed GCL based preamble has better and more robust performance than the standard PN sequence (IEEE 802.16 standard) based preamble in the time and integer frequency synchronization and also in the time domain channel estimation. In the new receiver scheme, the presence of symmetrical correlation shows that received signal is frequency corrected. The results show that the new receiver scheme is able to detect the symmetrical correlation quite accurately. The receiver also works well even in low SNR environment

Low Complexity Time Synchronization Algorithm for OFDM Systems with Repetitive Preambles

In this paper, a new time synchronization algorithm for OFDM systems with repetitive preamble is proposed. This algorithm makes use of coarse and fine time estimation; the fine time estimation is performed using a cross-correlation similar to previous proposals in the literature, whereas the coarse time estimation is made using a new metric and an iterative search of the last sample of the repetitive preamble. A complete analysis of the new metric is included, as well as a wide performance comparison, for multipath channel and carrier frequency offset, with the main time synchronization algorithms found in the literature. Finally, the complexity of the VLSI implementation of this proposal is discussed. © 2011 Springer Science+Business Media, LLC.This work was supported by the Spanish Ministerio de Educacion y Ciencia under grants TEC2006-14204-C02-01 and TEC2008-06787.Canet Subiela, MJ.; Almenar Terre, V.; Flores Asenjo, SJ.; Valls Coquillat, J. (2012). Low Complexity Time Synchronization Algorithm for OFDM Systems with Repetitive Preambles. Journal of Signal Processing Systems. 68(3):287-301. doi:10.1007/s11265-011-0618-6S287301683IEEE 802.11a standard (1999). Wireless LAN medium access control (MAC) and physical layer (PHY) specifications: high-speed physical layer in the 5 GHz band.IEEE 802.11 g standard (2003). Wireless LAN specifications: Further higher data rate extension in the 2.4 GHz band.IEEE 802.16-2004 (2004). Standard for local and metropolitan area networks, part 16: Air interface for fixed broadband wireless access systems.Lee, D., & Cheun, K. (2002). Coarse symbol synchronization algorithms for OFDM systems in multipath channels. IEEE Communications Letters, 6(10), 446–448.Park, B., Cheon, H., Ko, E., Kang, C., & Hong, D. (2004). A blind OFDM synchronization algorithm based on cyclic correlation. IEEE Signal Processing Letters, 11(2), 83–85.Beek, J. J., Sandell, M., & Börjesson, P. O. (1997). ML estimation of time and frequency offset in OFDM system. IEEE Transactions on Signal Processing, 45(7), 1800–1805.Ma, S., Pan, X., Yang, G., & Ng, T. (2009). Blind symbol synchronization based on cyclic prefix for OFDM systems. IEEE Transactions on Vehicular Technology, 58(4), 1746–1751.Schmidl, T., & Cox, D. (1997). Robust frequency and timing synchronization for OFDM. IEEE Transactions on Communications, 45(12), 1613–1621.Coulson, A. J. (2001). Maximum likelihood synchronization for OFDM using a pilot symbol: Algorithms. IEEE Journal on Selected Areas in Communications, 19(12), 2495–2503.Tufvesson, F., Edfors, O., & Faulker, M. (1999). Time and frequency synchronization for OFDM using PN-sequence preambles. Proceedings of the Vehicular Technology Conference (VTC), 4, 2203–2207.Shi, K., & Serpedin, E. (2004). Coarse frame and carrier synchronization of OFDM systems: a new metric and comparison. IEEE Transactions on Wireless Communications, 3(4), 1271–1284.Minn, H., Zeng, M., & Bhargava, V. K. (2000). On timing offset estimation for OFDM Systems. IEEE Communications Letters, 4, 242–244.Minn, H., Bhargava, V. K., & Letaief, K. B. (2003). A robust timing and frequency synchronization for OFDM systems. IEEE Transactions on Wireless Communications, 2(4), 822–839.Minn, H., Bhargava, V. K., & Letaief, K. B. (2006). A combined timing and frequency synchronization and channel estimation for OFDM. IEEE Transactions on Communications, 54(3), 416–422.Park, B., Cheon, H., Ko, E., Kang, C., & Hong, D. (2003). A novel timing estimation method for OFDM systems. IEEE Communications Letters, 7(5), 239–241.Chang, S., & Kelley, B. (2003). Time synchronization for OFDM-based WLAN systems. Electronics Letters, 39(13), 1024–1026.Wu, Y., Yip, K., Ng, T., & Serpedin, E. (2005). Maximum-likelihood symbol synchronization for IEEE 802.11a WLANs in unknown frequency-selective fading channels. IEEE Transactions on Wireless Communications, 4(6), 2751–2763.Larsson, E. G., Liu, G., Li, J., & Giannakis, G. B. (2001). Joint symbol timing and channel estimation for OFDM based WLANs. IEEE Communications Letters, 5(8), 325–327.Troya, A., Maharatna, K., Krstic, M., Grass, E., Jagdhold, U., & Kraemer, R. (2007). Efficient inner receiver design for OFDM-based WLAN systems: algorithm and architecture. IEEE Transactions on Wireless Communications, 6(4), 1374–1385.Yang, J., & Cheun, K. (2006). Improved symbol timing synchronization in IEEE 802.11a/g wireless LAN systems in multipath channels. International Conference on Consumer Electronics. doi: 10.1109/ICCE.2006.1598425 .Manusani, S. K., Hshetrimayum, R. S., & Bhattacharjee, R. (2006). Robust time and frequency synchronization in OFDM based 802.11a WLAN systems. Annual India Conference. doi: 10.1109/INDCON.2006.302775 .Zhou, L., & Saito, M. (2004). A new symbol timing synchronization for OFDM based WLANs under multipath fading channels. 15th IEEE International Symposium on Personal, Indoor and Mobile Radio Communications. doi: 10.1109/PIMRC.2004.1373890 .Kim, T., & Park, S.-C. (2007). A new symbol timing and frequency synchronization design for OFDM-based WLAN systems. 9th Conference on Advanced Communication Technology. doi: 10.1109/ICACT.2007.358691 .Baek, J. H., Kim, S. D., & Sunwoo, M. H. (2008). SPOCS: Application specific signal processor for OFDM communication systems. Journal of Signal Processing Systems, 53(3), 383–397.Van Kempen, G., & van Vliet, L. (2000). Mean and variance of ratio estimators used in fluorescence ratio imaging. Cytometry, 39(4), 300–305.J. Melbo, J., & Schramm, P. (1998). Channel models for HIPERLAN/2 in different indoor scenarios. 3ERI085B, HIPERLAN/2 ETSI/BRAN contribution.Abramowitz, M., & Stegun, I. A. (1972). Handbook of mathematical functions. Dover.López-Martínez, F. J., del Castillo-Sánchez, E., Entrambasaguas, J. T., & Martos-Naya, E. (2010). Iterative-gradient based complex divider FPGA core with dynamic configurability of accuracy and throughput. Journal of Signal Processing Systems. doi: 10.1007/s11265-010-0464-y .Angarita, F., Canet, M. J., Sansaloni, T., Perez-Pascual, A., & Valls, J. (2008). Efficient mapping of CORDIC Algorithm for OFDM-based WLAN. Journal of Signal Processing Systems, 52(2), 181–191

Blind Estimation of OFDM System Parameters for Automatic Signal Identification

Orthogonal frequency division multiplexing (OFDM) has gained worldwide popular­ ity in broadband wireless communications recently due to its high spectral efficiency and robust performance in multipath fading channels. A growing trend of smart receivers which can support and adapt to multiple OFDM based standards auto­ matically brings the necessity of identifying different standards by estimating OFDM system parameters without a priori information. Consequently, blind estimation and identification of OFDM system parameters has received considerable research atten­ tions. Many techniques have been developed for blind estimation of various OFDM parameters, whereas estimation of the sampling frequency is often ignored. Further­ more, the estimated sampling frequency of an OFDM signal has to be very accurate for data recovery due to the high sensitivity of OFDM signals to sampling clock offset. To address the aforementioned problems, we propose a two-step cyclostation- arity based algorithm with low computational complexity to precisely estimate the sampling frequency of a received oversampled OFDM signal. With this estimated sampling frequency and oversampling ratio, other OFDM system parameters, i.e., the number of subcarriers, symbol duration and cyclic prefix (CP) length can be es­ timated based on the cyclic property from CP sequentially. In addition, modulation scheme used in the OFDM can be classified based on the higher-order statistics (HOS) of the frequency domain OFDM signal. All the proposed algorithms are verified by a lab testing system including a vec­ tor signal generator, a spectrum analyzer and a high speed digitizer. The evaluation results confirm the high precision and efficacy of the proposed algorithm in realistic scenarios

Design and implementation of a downlink MC-CDMA receiver

Cette thèse présente une étude d'un système complet de transmission en liaison descendante utilisant la technologie multi-porteuse avec l'accès multiple par division de code (Multi-Carrier Code Division Multiple Access, MC-CDMA). L'étude inclut la synchronisation et l'estimation du canal pour un système MC-CDMA en liaison descendante ainsi que l'implémentation sur puce FPGA d'un récepteur MC-CDMA en liaison descendante en bande de base. Le MC-CDMA est une combinaison de la technique de multiplexage par fréquence orthogonale (Orthogonal Frequency Division Multiplexing, OFDM) et de l'accès multiple par répartition de code (CDMA), et ce dans le but d'intégrer les deux technologies. Le système MC-CDMA est conçu pour fonctionner à l'intérieur de la contrainte d'une bande de fréquence de 5 MHz pour les modèles de canaux intérieur/extérieur pédestre et véhiculaire tel que décrit par le "Third Genaration Partnership Project" (3GPP). La composante OFDM du système MC-CDMA a été simulée en utilisant le logiciel MATLAB dans le but d'obtenir des paramètres de base. Des codes orthogonaux à facteur d'étalement variable (OVSF) de longueur 8 ont été choisis comme codes d'étalement pour notre système MC-CDMA. Ceci permet de supporter des taux de transmission maximum jusquà 20.6 Mbps et 22.875 Mbps (données non codées, pleine charge de 8 utilisateurs) pour les canaux intérieur/extérieur pédestre et véhiculaire, respectivement. Une étude analytique des expressions de taux d'erreur binaire pour le MC-CDMA dans un canal multivoies de Rayleigh a été réalisée dans le but d'évaluer rapidement et de façon précise les performances. Des techniques d'estimation de canal basées sur les décisions antérieures ont été étudiées afin d'améliorer encore plus les performances de taux d'erreur binaire du système MC-CDMA en liaison descendante. L'estimateur de canal basé sur les décisions antérieures et utilisant le critère de l'erreur quadratique minimale linéaire avec une matrice' de corrélation du canal de taille 64 x 64 a été choisi comme étant un bon compromis entre la performance et la complexité pour une implementation sur puce FPGA. Une nouvelle séquence d'apprentissage a été conçue pour le récepteur dans la configuration intérieur/extérieur pédestre dans le but d'estimer de façon grossière le temps de synchronisation et le décalage fréquentiel fractionnaire de la porteuse dans le domaine du temps. Les estimations fines du temps de synchronisation et du décalage fréquentiel de la porteuse ont été effectués dans le domaine des fréquences à l'aide de sous-porteuses pilotes. Un récepteur en liaison descendante MC-CDMA complet pour le canal intérieur /extérieur pédestre avec les synchronisations en temps et en fréquence en boucle fermée a été simulé avant de procéder à l'implémentation matérielle. Le récepteur en liaison descendante en bande de base pour le canal intérieur/extérieur pédestre a été implémenté sur un système de développement fabriqué par la compagnie Nallatech et utilisant le circuit XtremeDSP de Xilinx. Un transmetteur compatible avec le système de réception a également été réalisé. Des tests fonctionnels du récepteur ont été effectués dans un environnement sans fil statique de laboratoire. Un environnement de test plus dynamique, incluant la mobilité du transmetteur, du récepteur ou des éléments dispersifs, aurait été souhaitable, mais n'a pu être réalisé étant donné les difficultés logistiques inhérentes. Les taux d'erreur binaire mesurés avec différents nombres d'usagers actifs et différentes modulations sont proches des simulations sur ordinateurs pour un canal avec bruit blanc gaussien additif

A frame synchronization and frequency offset estimation algorithm for OFDM system and its analysis

Orthogonal frequency division multiplexing (OFDM) is a parallel transmission scheme for transmitting data at very high rates over time dispersive radio channels. In an OFDM system, frame synchronization and frequency offset estimation are extremely important for maintaining orthogonality among the subcarriers. In this paper, for a preamble having two identical halves in time, a timing metric is proposed for OFDM frame synchronization. The timing metric is analyzed and its mean values at the preamble boundary and in its neighborhood are evaluated, for AWGN and for frequency selective channels with specified mean power profile of the channel taps, and the variance expression is derived for AWGN case. Since the derivation of the variance expression for frequency selective channel case is tedious, we used simulations to estimate the same. Based on the theoretical value of the mean and estimate of the variance, we suggest a threshold for detection of the preamble boundary and evaluating the probability of false and correct detections. We also suggest a method for a threshold selection and the preamble boundary detection in practical applications. A simple and computationally efficient method for estimating fractional and integer frequency offset, using the same preamble, is also described. Simulations are used to corroborate the results of the analysis. The proposed method of frame synchronization and frequency offset estimation is applied to the downlink synchronization in OFDM mode of wireless metropolitan area network (WMAN) standard IEEE 802.16-2004, and its performance is studied through simulations

Recent Advances in Wireless Communications and Networks

This book focuses on the current hottest issues from the lowest layers to the upper layers of wireless communication networks and provides "real-time" research progress on these issues. The authors have made every effort to systematically organize the information on these topics to make it easily accessible to readers of any level. This book also maintains the balance between current research results and their theoretical support. In this book, a variety of novel techniques in wireless communications and networks are investigated. The authors attempt to present these topics in detail. Insightful and reader-friendly descriptions are presented to nourish readers of any level, from practicing and knowledgeable communication engineers to beginning or professional researchers. All interested readers can easily find noteworthy materials in much greater detail than in previous publications and in the references cited in these chapters