Time Synchronization for Wireless Sensor Networks - Adaptive Signal Processing in Wireless Communications - page 370

Research works on time synchronization in the context of WSN roughly began to appear
in 2002, where [5], for the first time, pointed out NTP cannot be directly applied to WSN
and described some important characteristics and design principles of time synchroniza-
tion in WSNs. In the same year and the next, two important time synchronization proto-
cols for WSNs were reported: Timing-sync Protocol for Sensor Networks (TPSN) [17] and
Reference Broadcasting Synchronization (RBS) [18]. These two protocols set the stage for
two fundamental approaches of time synchronization in WSNs. After that, many exten-
sions and generalizations of TPSN and RBS, and many different synchronization schemes
based on other ideas, have been proposed in the literature. Notice that symbol timing
synchronization in the physical layer, which is critical for accurate symbol detection at
the receiver, is a different problem and is out of the scope of this chapter.
The purpose of this chapter is threefold. First, this chapter summarizes the funda-
mental features and theoretical results encountered in time synchronization of WSNs.
Second, it represents the survey of existing time synchronization protocols for WSNs,
focusing mainly on the signal processing aspects, the most recent developments in this
field. Finally, this chapter discusses the need for adaptive time synchronization schemes
for WSNs, analyzes the features of the recently reported adaptive time synchronization
protocols, and proposes several research directions for improving their performance.
The rest of this chapter is organized as follows. In section 13.2, the general clock model
for time synchronization is first introduced and analyzed. Some important features
that have to be considered when designing time synchronization protocols for WSNs
are presented. Additionally, various delay components in timing message delivery are
categorized. Section 13.3 presents three general and fundamentally different time syn-
chronization approaches: sender-receiver, receiver-receiver, and receiver-only synchro-
nization. These basic approaches are analyzed and compared to illustrate the common
and different characteristics in clock synchronization of WSNs. Section 13.4 categorizes
and surveys the existing synchronization protocols and relates them to the results pre-
sented in section 13.3. In section 13.5, results concerning the importance and effective-
ness of adaptive time synchronization schemes are presented, and the most important
adaptive synchronization protocols are introduced as well. Finally, section 13.6 summa-
rizes and concludes this chapter.

13.2 Signal Models for Time Synchronization

13.2.1 Definition of Clock

Every individual sensor in a network has its own clock. The counter in a sensor is
increased in accordance with the zero-crossings or the edges of the periodic output sig-
nal of the local oscillator. When the counter reaches a certain threshold value, an inter-
rupt is created and delivered to the memory. The frequency of the oscillator and the
threshold value determine the resolution of the clock. Ideally, the clock of a sensor node
should be configured such that C ( t ) = t , where t stands for the ideal or reference time.
However, due to the imperfections of the clock oscillator, the clock function of the i t h
node is modeled as