Master Synchronization in Physical-Layer Communications of Wireless Sensor Networks
© Lin Zheng et al. 2010
Received: 15 April 2010
Accepted: 23 December 2010
Published: 10 January 2011
Synchronization in physical layer of wireless sensor networks is critical in restricting complexity of tag node and power consumption. Considering the master-to-tag communication (i.e., receiving signal from a master or anchor node by tag nodes), we propose a scheme on the basis of the principles of feedback control, to transfer the signal acquisition functionality from the tag receivers to the master nodes in a cluster. Furthermore, the algorithm of timing acquisition and phase adjustment do work in the master transmitter, and the tag nodes just need feedback results of the phase detection. The tag nodes do not require complicated clock or phase adjustment circuit any more or estimation in synchronization either. Thus, this master synchronization method reduces the complexity of tag nodes and power consumption. Due to the large random time delay in the wireless feedback loop, there exists the problem of stability and convergence in the acquisition. We analyze it and present a feasible scheme for the proposed master synchronization. In order to reduce acquisition time and cost in feedback, a two-step master acquisition algorithm is proposed. The acquisition performance under nonideal channel is analyzed, and further verified by simulations.
Recent years, the research on wireless sensor networks (WSNs) has attracted many focuses from academic, military, and industrial community. In many cases, including outdoor applications, a large amount of sensor tags not only sense but also exchange the gathered information. Thus, how to satisfy the requirement of the sensor tags, including small size, low complexity, and low-power consumption to ensure a long-time maintenance-free work, becomes a primary challenge in practice . The development of physical-layer technologies is still required for further reducing power consumption in sensor nodes. In this area, the conventional timing recovery in a receiver costs significant power and complexity. On account of this, this paper introduces a master synchronization differing from the traditional mechanism in wireless communication to simplify transceiver and lower the power consumption in tag nodes. Its feasibility and performance are studied in this paper.
At present, the reduction of power consumption in nodes communication process is mostly considered by the methods in Medium-Access Layer (MAC) designs. Literatures have contributed several energy-efficient MAC protocols [2–4]. In addition, the low-power wireless passive sensor networks (WPSN) are attracting considerable attention due to the low-cost tags . To supply the energy source of transceiver, the cluster node feeds the passive RFID tags with RF power. Even so, the absorbed energy can just offer temporary and close interaction between nodes. It is difficult to meet the requirement of communication distance and data rate in common applications .
Carrier acquisition, bit, and frame synchronization process are the essential conditions in physical layer of wireless data communication . However, the low-power research for synchronization subsystem is often overlooked, ignoring that it comprises over 15% of the physical layer die area in several common wireless standards, such as Bluetooth and 802.11. In WSNs, the acquisition and tracking scheme implemented in receiver is commonly adopted to physical-layer communication as it used to be [8, 9]. It has the advantages of fast acquisition and high synchronization precision, but a shortcoming that the receiver must have a significant chip area, controllable clock (such as voltage-controlled oscillator, VCO), or a high-rate sampler and signal processor, which cost more than 15% system power. In ultrawideband communications, the high-precision controllable delay line is even required in receiver for acquiring narrow pulses which also leads to high circuit complexity and power consumption .
The analog or digital methods are utilized for synchronization acquisition in a conventional receiver. The analog phase-locked loop (PLL) using feedback control is to achieve the carrier and phase synchronization. Timing recovery by adaptive digital signal processing adopts the open-loop frequency or phase offset estimation which still use the principles of feedback iteration on the received cyclostationary signal. Different from traditional acquisition, the proposed master synchronization transfers the timing recovery and clock control to the master node, while the tag receiver only requires to generate and feedback the error control signals. That distributes the synchronization functionality into transmitter and receiver to meet the low-complexity and low-power consumption requirements of a tag node. Since the error control information can be fed back only through a wireless channel, shortening acquisition time and lowering the power consumption of tag nodes are the key issues to the proposed synchronization scheme. Due to the large time delay caused by feedback loop in wireless link, the stability of the proposed distributed feedback-control synchronization may be influenced greatly. The stability analysis and a feasible protocol is given below.
The outline of this paper is as follows. In Section 2 the master synchronization models and three algorithms for wireless sensor networks are introduced. In Section 3, the stability of these distributed feedback control methods are analyzed, and a stable acquisition is proposed in Section 4. The performance of the single-step pulse master synchronization is also deduced in Section 4. In Section 5, a two-step master synchronization is given to reduce the acquisition time, and its performance is analyzed either. Numerical results are presented in Section 6, and conclusions are drawn in Section 7.
2. Master Synchronization Modeland Algorithm
The master synchronization presented in this paper aims at the synchronization problem when a master sends commands or messages to tag nodes. This synchronization method is based on feedback control principles. Since the phase-locked loop is the most classical feedback control method, we analyze the master synchronization by using the similar model of phase-locked loop at first [12, 13].
Considering first-order loop model above, where there is no loop filter, the total delay in forward and reverse link is equivalent to . Consistent with the conventional principles of PLL, the phase of signal received in tag node has a difference from its local phase at the locked status. To achieve acquisition by this loop, the signal of local oscillator in receiver is requested with a same period as the received signal. The stability and performance of synchronization is also impacted by loop gain , and feedback delay .
When the correlation output exceeds the threshold, that is to say, the phase difference between received pulse and local phase in tag node is less than a pulse width, it can be determined that the acquisition is achieved. The phase detector is a gated on-off control function. The advantage of pulse synchronization feedback loop is obvious that the tag receiver transmits no feedback when correlation output is lower than threshold, that is , and the master adjusts its phase by itself until it receives a feedback from tag receiver. That means tag receiver will not consume energy until the condition of synchronization acquisition is satisfied. Assisted by the multiple access code, the above scheme can not only achieve synchronization acquisition, but also activate a mass of tag receivers. It is intelligible that such a master synchronization has the advantages of low-power consumption and low complexity for the tag nodes in WSNs.
3. Stability Analysis
The master synchronization separates feedback control into two parts in transmitter and receiver. However, the stability of this feedback loop is affected by adding the long delay caused by the forward and reverse wireless link. Since the phase detector is often a nonlinear feedback unit with respect to the phase, it is hard to obtain a closed-form solution of determining stability or convergence under most situations.
Obviously, the analysis of sawtooth-waveform-based system is similar with that of SPLL based. Because of linear output of the sampling phase detector , its stability analysis result is closer to above theoretical value.
4. Applying Master Synchronization in WSNs
In order to reduce the power consumption of tag nodes, unnecessary feedback must be avoided in the master synchronization. In our proposed scheme, the acknowledgements are just transmitted at the time of satisfying the timing condition in tag receiver. That means a little energy may be cost in feedback. Therefore, it may be very beneficial to low-power working of the tag receiver. However, there are some problems to be solved in the following aspects. First, the stability of the mast synchronization is influenced by the long-delay feedback. Second, the precise synchronization required a small loop gain which leads to large acquisition time. Third, the detection probability and the false-alarm probability caused by the wireless channel increase the average acquisition time and the transmitted energy in tag nodes obviously.
4.1. Stable Acquisition
The intuitive reason of instability in master synchronization is that the feedback control error departs from the right phase in master node due to the large delay. A simple method to ensure the stability of pulse master synchronization is by modulating the phase information into corresponding phase signal transmitted to tag node. After the tag node acquires this phase signal, that is, correlation exceeds the preset threshold, the carried phase information is fed back by the acknowledgement. Accepting the acknowledgement, the master node adjusts the VCO or NCO according to the phase information. Consequently, the phase of received signal would just agree with the phase of local oscillator in tag receiver.
4.2. Performance Analysis
The master synchronization is similar to the process of single dwell serial acquisition. In the traditional serial acquisition, the receiver needs to adjust the phase of local correlation template until it reaches consensus with that of the received signal. Different from serial acquisition, the adjustment of signal phase is transferred to the master transmitter in WSNs and the feedback has to be sent in wireless link. If the master synchronization requires a precision in the interval of , the searching stepsize requires to be less than . Noting that the synchronization accuracy of ultra-wideband impulse is affected by many factors, such as the searching stepsize, impulse waveform, multipath environment, modulation scheme, and decision threshold [17, 18], it is out of the range of this paper and should be further studied for the proposed scheme.
In the case that there are error decisions of acquisition, the master synchronization has a problem of mistaking non-synchronous status as synchronous status, which is defined by false alarm probability. On the contrary, the synchronous status may be detected as non-synchronous status either. Its probability is , where is the detection probability. By repeatedly sending -periodic synchronization signaling for confirmation, the false alarm is eliminated with a penalty of delay. If the synchronization is not confirmed, the serial search continues. Missing synchronization, the feedback system has to achieve acquisition in the next period of the serial search, which results in a long acquisition time.
where is the signal bandwidth, is the signal-to-noise ratio (SNR), and is the inverse function of function. According to the equation, the false alarm probability decreases when the detection probability decreases. Obviously, a minimum feedback times can be obtained with the small detection and false alarm probabilities. Unfortunately, the small detection probability would also greatly extend the acquisition time.
5. Two-Step Acquisition
The pulse master synchronization has the advantages that single feedback and low complexity in tag receiver are required under ideal condition. At the same time, a main problem of master acquisition is that a highly precise synchronization with narrow pulse signal costs long time to be achieved. Take the ultra-wideband pulse for example, the longest acquisition time is . In this view, we propose a two-step acquisition scheme to limit acquisition time and feedback times in master synchronization, while guaranteeing the precision.
The two-step acquisition consists of two master acquisition processes. The main difference between the two steps is the widths of synchronization pulses transmitted by master node. The two processes are as follows.
Although the two-step acquisition scheme may increase the feedback times and power consumption, the cost is acceptable and the reduction of acquisition time is remarkable. Suppose that the detection probability and false alarm probability remain unchanged in the two steps of master synchronization, the acquisition time can be further derived by the fact of the same processes in the two search steps. The two-step acquisition time is given by (12).
By assigning as in (14), Figure 7 also illustrates the two-step acquisition curves of average acquisition time and average feedback times. The average acquisition time of the proposed two-step scheme is much less than that of the single-step master acquisition with a same . At the same time, the difference of feedback times between the single-step and two-step master acquisitions is small at the region of short acquisition time. With a big , the feedback times of two-step search is even less than that of the single-step acquisition.
In this section, the feasibility of the proposed master synchronization and the correctness of the analysis are demonstrated by simulations. We implement point-to-point simulations because the master synchronization is a physical-layer method in wireless sensor networks. The research of the proposed master synchronization mainly focuses on three contents including the stability, the acquisition time, and the feedback time (power consumption). The stability margins of master synchronization is obtained and analyzed. To break the stability constraint of the pulse master synchronization, the phase information is proposed to be carried by acknowledgement. The simulations of the acquisition time and the feedback time to verify the analysis are mentioned in the following part.
The simulation condition includes the period of pulses denoted by , the pulse width , and the feedback delay . Suppose that the false alarm probability in tag receivers and the feedback detection probability are and , respectively. In the two-step scheme, the optimum width of transmitted pulses in first step is according to (14). That determines the phase search range of the second step.
Even though the proposed synchronization may be unsuitable for the conventional transceivers due to its slow acquisition and feedback requirement, it significantly reduce the complexity and power consumption in physical-layer communication of tag nodes. Aiming at the case that low burst data rate and existing master node in cluster of a WSN, the pulse master acquisition presents feasibility and a longer communication distance than the RFID-based techniques. From the analysis and simulation results, the two-step pulse scheme has a better performance for the acquisition time and power consumption in tag node. It is an interesting method in physical-layer synchronization of wireless sensor networks. And what is more, the synchronization tracking is another problem to be solved even in low burst-rate communications. Fast estimation algorithms may be implemented to further reduce the complexity and power consumption with this distributed synchronization architecture.
This work was supported by National Natural Science Foundation (project 60962001, project 61071088) of China, and Guangxi Nature Science Foundation (project 0731026) in China.
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