1 Timing and Carrier Synchronization in Wireless Communication Systems: A Survey and Classification of Research in the Last Five Years Ali A. Nasir, Salman Durrani, Hani Mehrpouyan, Steven D. Blostein, and Rodney A. Kennedy Abstract—Timing and carrier synchronization is a fundamen- communication schemes. Hence, there has been considerable tal requirement for any wireless communication system to work research recently in synchronization techniques for novel properly. Timing synchronization is the process by which a communication strategies. receiver node determines the correct instants of time at which to sample the incoming signal. Carrier synchronization is the Aim: The aim of this paper is to provide a survey and process by which a receiver adapts the frequency and phase of classification of the research in the field of synchronization for its local carrier oscillator with those of the received signal. In this wireless communication systems that spans the last five years paper, we survey the literature over the last five years (2010-2014) (2010-2014). This is not an easy task given the large number and present a comprehensive literature review and classification of papers dealing with synchronization and its associated chal- of the recent research progress in achieving timing and carrier synchronization in single-input-single-output (SISO), multiple- lenges in both current and emerging wireless communication input-multiple-output (MIMO), cooperative relaying, and mul- systems. The critical need for such a survey is highlighted tiuser/multicell interference networks. Considering both single- by the fact that the last comprehensive survey paper on syn- carrier and multi-carrier communication systems, we survey and chronization was published nearly a decade ago [10]. While categorise the timing and carrier synchronization techniques survey papers on synchronization for wireless standardization proposed for the different communication systems focusing on the system model assumptions for synchronization, the synchroniza- have recently appeared [11, 12, 13], these surveys do not tion challenges, and the state-of-the-art synchronization solutions overview the state-of-the-art published research. and their limitations. Finally, we envision some future research In this survey, we overview the relationships among the directions. published research in terms of system model and assump- Index Terms—Timing synchronization, carrier synchroniza- tions, synchronization challenges, proposed methods, and their tion, channel estimation, MIMO, OFDM. limitations. We also highlight future research directions and important open problems in the field of synchronization. The main intended audience of this survey paper is anyone I. INTRODUCTION interested in or already working in synchronization. Our hope Motivation: The Wireless World Research Forum (WWRF) is that this survey paper would enable researchers to quickly prediction of “seven trillion wireless devices serving seven immerse themselves in the current state-of-the-art in this field. billion people by 2020” [1] sums up the tremendous challenge Moreover, by highlighting the important open research issues facing existing wireless cellular networks: intense consumer and challenges, we believe the paper would stimulate further demand for faster data rates. Major theoretical advances, such research in this field. Since this paper is not intended to be a as the use of multiple antennas at the transmitter and receiver tutorial on synchronization, we deliberately avoid presenting (MIMO) [2, 3], orthogonal frequency-division multiple access mathematical details and instead focus on the big picture. arXiv:1507.02032v1 [cs.IT] 8 Jul 2015 (OFDMA) [4], and cooperative relaying [5, 6, 7] have helped Background and Scope: Synchronization is a common meet some of this demand and have been quickly incorporated phenomenon in nature, e.g., the synchronized flashing of into communication standards. These technologies also form fireflies or the synchronous firing of neurons in the human a core part of next generation cellular standards, 5G, which is brain [14, 15]. In wireless communications, timing and carrier under development [8, 9]. synchronization is a fundamental requirement [16]. In general, In order to fulfill the demand for higher data rates, a critical a wireless receiver does not have prior knowledge of the requirement is the development of accurate and realizable physical wireless channel or propagation delay associated with synchronization techniques to enable novel communication the transmitted signal. Moreover, to keep the cost of the paradigms. Such synchronization techniques allow communi- devices low, communication receivers use low cost oscillators cation systems to deliver higher data rates, e.g., through the which inherently have some drift. In this context: use of higher order modulations and utilization of cooperative 1) Timing synchronization is the process by which a receiver node determines the correct instants of time at which to Ali A. Nasir, Salman Durrani and Rodney A. Kennedy are with the Research School of Engineering, Australian National University, Canberra, ACT 2601, sample the incoming signal. Australia (Email: fali.nasir, salman.durrani, [email protected]). 2) Carrier synchronization is the process by which a re- Hani Mehrpouyan is with the Department of Electrical and Computer En- ceiver adapts the frequency and phase of its local carrier gineering, Boise State University, Idaho, USA (Email: [email protected]). Steven D. Blostein is with the Department of Electrical and Computer Engi- oscillator with those of the received signal. neering, Queen’s University, Canada (Email: [email protected]). For instance, requiring two watches to be time synchronized 2 means that they should both display the same time. However, directly affecting the frequency stability of the oscillator [28]. requiring two watches to be carrier synchronized means that The HPA nonlinearities refer to the operation of the HPA they should tick at the same speed, irrespective of what time in its nonlinear region when working at medium and high- they show [17]. Note that channel estimation, which is an power signal levels. The influence of these RF impairments is inherent requirement for synchronization, is not the main focus usually mitigated by suitable compensation algorithms, which of this paper. For a recent survey and tutorial on channel can be implemented by analog and digital signal processing. In estimation alone, please see [18]. this paper, the focus is on timing and carrier synchronization Major advances in timing and carrier synchronization such and RF impairments are outside the scope of this paper. as pilot symbol assisted modulation [19], are used in present For a detailed discussion of RF impairments, the reader is day cellular networks to achieve carrier accuracy of 50 parts referred to [29]. In cases where RF impairments (typically per billion and timing accuracy of 1 µs (±500 ns) [11]. The I/Q imbalance or phase noise) are considered in conjunction requirement in future wireless networks is towards tighter with timing and carrier synchronization, they are identified accuracies, e.g., timing accuracy of 200 ns, to enable location- separately in the classification. based services [12]. Hence, there is a need for new and more Methodology: Synchronization is generally considered as a accurate timing and carrier estimators. In general, in order to subfield of signal processing. According to Google Scholar, 9 quantify the performance of any proposed estimator, a lower out of the top 10 publication avenues in signal processing are bound on the mean-square estimation error can be derived. IEEE journals [30]. Hence, we used the IEEEXplore database The bounds are also helpful in designing efficient training to search for papers on timing and carrier synchronization. sequences. In addition, for multiple parameters needed, say, Synchronization in wireless communication systems is an for the joint estimation of timing and carrier frequency off- active area of research and there are a very large number of sets, these bounds include coupling information between the papers on this topic in IEEEXplore. For example, a general estimation of these parameters. For example, if the bound search with the words “timing synchronization” yields close to suggests very low coupling between the estimation of timing 19; 000 papers (admittedly not all papers would fit the scope and carrier frequency offsets, this implies that these parameters of this survey). can be estimated separately without any significant loss in We selected papers (in December 2014) by searching for the estimation performance. In particular, there usually exist words “frequency offset” OR “frequency offsets” OR “timing strong coupling between channel and carrier frequency offset offset” OR “timing offsets” in IEEEXplore metadata only. In estimation and their joint estimation is helpful to achieve order to focus on the important recent advances, we limited improved estimation accuracy [20, 21]. our search to all journal papers published in the last 5 years Although timing and carrier synchronization is necessary for only, i.e., from 2010-2014. Also, we limited the search to successful communication, it cannot provide a common notion the following conferences: ICC, GLOBECOM, VTC, WCNC, of time across distributed nodes. Clock synchronization is SPAWC, and PIMRC, because it was found that these confer- the process of achieving and maintaining coordination