Timing and carrier synchronization in wireless communication systems: a survey and classification of research in the last 5 years

Motivation: The Wireless World Research Forum (WWRF) prediction of seven trillion wireless devices serving seven billion people by 2020 [1] sums up the tremendous challenge facing existing wireless cellular networks: intense consumer demand for faster data rates. Major theoretical advances, such as the use of multiple antennas at the transmitter and receiver (multiple-input multiple-output (MIMO)) [2, 3], orthogonal frequency-division multiple access (OFDMA) [4], and cooperative relaying [5‐7] have helped meet some of this demand and have been quickly incorporated into communication standards. These technologies also form a core part of the next-generation cellular standards, 5G, which is under development [8, 9].

In order to fulfill the demand for higher data rates, a critical requirement is the development of accurate and realizable synchronization techniques to enable novel communication paradigms. Such synchronization techniques allow communication systems to deliver higher data rates, e.g., through the use of higher-order modulations and utilization of cooperative communication schemes. Hence, there has been considerable research recently in synchronization techniques for novel communication strategies.

Aim: The aim of this paper is to provide a survey and classification of the research in the field of synchronization for wireless communication systems that spans the last 5 years (2010–2014). This is not an easy task given the large number of papers dealing with synchronization and its associated challenges in both current and emerging wireless communication systems. The critical need for such a survey is highlighted by the fact that the last comprehensive survey paper on synchronization was published nearly a decade ago [10]. While survey papers on synchronization for wireless standardization have recently appeared [11‐13], these surveys do not overview the state-of-the-art published research. For an overview of the state-of-the-art in synchronization research prior to 2010, see the 2009 special issue on synchronization in wireless communications of the EURASIP Journal on Wireless Communications and Networking [14].

In this survey, we overview the relationships within the published research in terms of system model and assumptions, synchronization challenges, proposed methods, and their limitations. We also highlight future research directions and important open problems in the field of synchronization. The main intended audience of this survey paper are those interested in or already working in synchronization. This survey paper aims to enable researchers to quickly immerse themselves in the current state of the art in this field. Moreover, by highlighting the important open research issues and challenges, we believe the paper would stimulate further research in this field. Since this paper is not intended to be a tutorial on synchronization, we deliberately avoid presenting mathematical details and instead focus on the big picture.

Background and scope: Synchronization is a common phenomenon in nature, e.g., the synchronized flashing of fireflies or the synchronous firing of neurons in the human brain [15, 16]. In wireless communications, timing and carrier synchronization are fundamental requirements [17]. In general, a wireless receiver does not have prior knowledge of the physical wireless channel or propagation delay associated with the transmitted signal. Moreover, to keep the cost of the devices low, communication receivers use low-cost oscillators which inherently have some drift. In this context, timing synchronization is the process by which a receiver node determines the correct instants of time at which to sample the incoming signal and carrier synchronization is the process by which a receiver adapts the frequency and phase of its local carrier oscillator with those of the received signal. Particularly, depending on the specific communication systems, synchronization definition/procedure could be very different. According to 3rd Generation Partnership Project, a terminology referred to as cell search has been widely used to represent an entire synchronization procedure, which may constitute both initial and target cell searches. Timing synchronization may consist of frame/slot/symbol/chip synchronizations, residual timing tracking, first arrival path search (in terms of OFDMA), multi-path search (in terms of CDMA), etc. Similarly, carrier synchronization may imply integer/fractional frequency offset estimation (in terms of OFDMA), coarse/fine frequency offset estimation (in terms of CDMA), residual frequency offset tracking, etc.

Major advances in timing and carrier synchronization such as pilot-symbol-assisted modulation [18] are used in present-day cellular networks to achieve carrier accuracy of 50 parts per billion and timing accuracy of 1 μs (±500 ns) [11]. The requirement in future wireless networks is toward tighter accuracies, e.g., timing accuracy of 200 ns, to enable location-based services [12]. Hence, there is a need for new and more accurate timing and carrier estimators. In general, in order to quantify the performance of any proposed estimator, a lower bound on the mean-square estimation error can be derived. The bounds are also helpful in designing efficient training sequences. In addition, for multiple parameters needed, say, for the joint estimation of timing and carrier frequency offsets, these bounds include coupling information between the estimation of these parameters. For example, if the bound suggests very-low coupling between the estimation of timing and carrier frequency offsets, this implies that these parameters can be estimated separately without any significant loss in the estimation performance. In particular, there usually exists strong coupling between channel and carrier frequency offset estimation and their joint estimation is helpful to achieve improved estimation accuracy [19, 20].

Although timing and carrier synchronization are necessary for successful communication, they cannot provide a common notion of time across distributed nodes. Clock synchronization is the process of achieving and maintaining coordination among independent local clocks to provide a common notion of time across the network. Some wireless networks, such as worldwide interoperability for microwave access (WiMAX), are synchronized to the global positioning system (GPS) [12]. Others, e.g., Bluetooth, wireless fidelity (Wi-Fi), and Zigbee rely on a beacon strategy, where all nodes in the network follow the same time reference given by a master node broadcasting a reference signal [12]. For recent surveys on clock synchronization, please see [21‐25].

In the literature, timing and carrier synchronization techniques are sometimes considered in conjunction with radio frequency (RF) front-end impairments. RF impairments arise as a result of the intrinsic imperfections in many different hardware components that comprise the RF transceiver front ends, e.g., amplifiers, converters, mixers, filters, and oscillators. The three main types of RF impairments are I/Q imbalance, oscillator phase noise, and high-power amplifier (HPA) nonlinearities [26]. I/Q imbalance refers to the amplitude and phase mismatch between the in-phase (I) and quadrature (Q) signal branches, i.e., the mismatch between the real and imaginary parts of the complex signal. Oscillator phase noise refers to the noise in an oscillator, mainly due to the active devices in the oscillator circuitry, which introduces phase-modulated noise, directly affecting the frequency stability of the oscillator [27]. The HPA nonlinearities refer to the operation of the HPA in its nonlinear region when working at medium- and high-power signal levels. The influence of these RF impairments is usually mitigated by suitable compensation algorithms, which can be implemented by analog and digital signal processing. For a detailed discussion of RF impairments, the reader is referred to [28]. In cases where RF impairments (typically I/Q imbalance or phase noise) are considered in conjunction with timing and carrier synchronization, they are identified separately in the classification.

Methodology: Synchronization is generally considered as a subfield of signal processing. According to Google Scholar, nine out of the top ten publication avenues in signal processing are IEEE journals [29]. Hence, we used the IEEEXplore database to search for papers on timing and carrier synchronization. We selected papers (in December 2014) by searching for words “frequency offset” OR “frequency offsets” OR “timing offset” OR “timing offsets” in IEEEXplore metadata only. In order to focus on the important recent advances, we limited our search to all journal papers published in the last 5 years only, i.e., from 2010 to 2014. Also, we limited the search to the following conferences: ICC, GLOBECOM, VTC, WCNC, SPAWC, and PIMRC, because it was found that these conferences contained sufficient numbers of papers to address the synchronization topics.

Using these principles, papers that dealt with timing and carrier synchronization were carefully selected for inclusion in this survey paper. A classification of these papers, with respect to the adopted communication system, is presented in Table 1. Some papers were found to study the effect of timing and carrier synchronization on the performance of various communication systems, but they did not directly estimate or compensate for these synchronization impairments. These papers are summarized in Table 2 for the sake of completeness. However, these papers are not discussed in the survey sections below.

Abbreviations and acronyms: The list of abbreviations and acronyms used in this paper are detailed in Table 3. In the paper, in Tables 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, and 21, “CSI Req.” column indicates (using yes/no value) whether or not channel state information (CSI) is required for synchronization procedure; “CE” column indicates (using yes/no value) whether or not algorithm considers channel estimation (CE); “Est/Comp” column indicates whether algorithm only considers estimation (Est) of parameters or also uses the estimated parameters for compensating (Comp) their effect on system bit error rate (BER) performance; “N/A” stands for not applicable; and “Bound” column indicates whether the paper derives or provides lower bound on the estimation performance.