Chapter 1 Introduction
1.4 Organization of the Thesis
In this thesis, Chapter 1 describes the main purpose and goal of this research.
Chapter 2 presents the background information of the ZigBee and its specifications.
Chapter 3 develops the signal propagation model for ZigBee WSN by using the RSSI measurements in both indoor and outdoor environments. Chapter 4 and 5 present the monitoring software for network link quality and the improvement of data dropout by applying the integration of IME and CMD for the ZigBee WSN. Finally, achievements and contributions of this thesis are summarized in Chapter 6.
Chapter 2
Principle and Implementation of ZigBee
ZigBee is a new wireless technology guided by the IEEE 802.15.4 Personal Area Networks standard. It is primarily designed for the wide ranging automation applications and to replace the existing non-standard technologies. Some of its primary features are[10][11][12]:
standards-based wireless technology
interoperability and worldwide usability
low data-rates
ultra low power consumption
very small protocol stack
support for small to excessively large networks
simple design
security
reliability
In this chapter, the ZigBee technology will be discussed in detail so that the common reader can have an overview of this technology. This thesis is carried out in corporation with FineTek, which want to evaluate the suitability of ZigBee in industrial application. The ZigBee’s implementation on FineTek will also be
2.1 ZigBee Alliance
ZigBee Alliance is an association of companies working together to define an open global standard for products with low-power wireless networks. The intended outcome of ZigBee Alliance is to create a specification defining that how to build different network topologies with data security features and interoperable application profiles. The association includes companies from a wide spectrum of categories, from chip manufactures to system integration companies. Until now, the Alliance has become a large and thriving ecosystem of organizations providing everything product manufacturers need to create ZigBee products, include that radio semiconductor chips, design houses, software companies, support tools, and testing.
2.2 ZigBee and IEEE 802.15.4
The name ZigBee is said to come from the domestic honeybee that uses a zig-zag type of dance to communicate important information to other hive members. This communication dance (the "ZigBee Principle") is what engineers are trying to emulate with this protocol a bunch of separate and simple organisms that join together to tackle complex tasks.
The goal IEEE had when they specified the IEEE 802.15.4 standard was to provide a standard for ultra-low complexity, ultra-low cost, ultra-low power consumption and low data rate wireless connectivity among inexpensive devices. The raw data rate will be high enough (maximum of 250 kb/s) for applications like sensors, alarms and toys.
2.2.1 Components of the IEEE 802.15.4
(1) The network coordinator maintains overall network knowledge. There can be only one coordinator in a network; it has the ability to communicate with any device in the network. The coordinator is really the core component of the ZigBee network as proper working of this component is compulsory for the network to achieve the desired communication results. [13] [14].
(2) The Full Function Device (FFD) supports all IEEE 802.15.4 functions and features specified by the standard and it can be function as a network coordinator.
Additional memory and computing power make it ideal for network router functions or it can be used in network-edge devices (where the network touches the real world).
One other responsibility is that it searches the other FFDs and the RFDs to create the communication link so that the transfer of data can be made possible to reach the desired node. [13] [14].
(3) The Reduced Function Device (RFD) carries limited (as specified by the standard) functionality to lower cost and complexity. It is generally found in network-edge devices. The RFD can be used where extremely low power consumption is a necessity.
2.2.2 Network topologies
Figure 2.1 shows three types of topologies that ZigBee supports: star topology, peer-to-peer topology and cluster tree. which add to the properties of the network and how the data will be transferred or how the communication carried out.
Figure 2-1 ZigBee network topologies [4]
(1) Star Topology
In the star topology, the communications are established between devices and a single central controller, called the PAN coordinator. Applications that could get benefits from this topology include home automation, personal computer (PC) peripherals, toys, and games.
After an FFD is activated for the first time, it may establish its own network and become the PAN coordinator. Each star network chooses a PAN identifier, which is not currently used by any other network within the radio sphere of influence. This allows each star network to operate independently.
(2) Peer-to-peer Topology
In peer-to-peer topology, there is also one PAN coordinator. In contrast to star topology, any device can communicate with any other device as long as they are in
healing. Applications such as industrial control and monitoring, wireless sensor networks, asset and inventory tracking would be benefits from such a topology. It also allows multiple hops to route messages from any device to any other device in the network. It can provide reliability by employing multipath routing.
(3) Cluster-tree Topology
The cluster-tree network is a special case of a peer-to-peer network in which most devices are FFDs and an RFD may connect to a cluster-tree network as a leave node at the end of a branch. Any of the FFD can act as a coordinator and provide synchronization services to other devices and coordinators. Only one of these coordinators however is the PAN coordinator. The PAN coordinator forms the first cluster by establishing itself as the cluster head (CLH) with a cluster identifier (CID) of zero, choosing an unused PAN identifier, and broadcasting beacon frames to neighboring devices. A candidate device receiving a beacon frame may request to join the network at the CLH. If the PAN coordinator permits the device to join, it will add this new device as a child device in its neighbor list. The newly joined device will add the CLH as its parent in its neighbor list and begin transmitting periodic beacons such that other candidate devices may then join the network at that device. Once application or network requirements are met, the PAN coordinator may instruct a device to become the CLH of a new cluster adjacent to the first one. The advantage of this clustered structure is the increased coverage area at the cost of increased message latency.
protocol architecture, and as a framework for developing protocol standards. The entire point of the model is to separate networking into several distinct functions that operate at different levels. Each layer is responsible for performing a specific task or set of tasks, and dealing with the layers above and below it. An illustration of the general OSI-model and where ZigBee is in the model can be seen in Fig. 2-2.
Figure 2-2 The OSI model
2.2.4 Physical layer
The IEEE 802.15.4 specification operates on three different frequency bands, in order to conform to regulations in Europe, Japan, Canada and the United States [15].
Table 2-1 describes the frequency bands and data rates. Totally 27 channels are available across the different frequency bands, as described in Table 2-2.
protocol architecture, and as a framework for developing protocol standards. The entire point of the model is to separate networking into several distinct functions that operate at different levels. Each layer is responsible for performing a specific task or set of tasks, and dealing with the layers above and below it. An illustration of the general OSI-model and where ZigBee is in the model can be seen in Fig. 2-2.
Figure 2-2 The OSI model
2.2.4 Physical layer
The IEEE 802.15.4 specification operates on three different frequency bands, in order to conform to regulations in Europe, Japan, Canada and the United States [15].
Table 2-1 describes the frequency bands and data rates. Totally 27 channels are available across the different frequency bands, as described in Table 2-2.
protocol architecture, and as a framework for developing protocol standards. The entire point of the model is to separate networking into several distinct functions that operate at different levels. Each layer is responsible for performing a specific task or set of tasks, and dealing with the layers above and below it. An illustration of the general OSI-model and where ZigBee is in the model can be seen in Fig. 2-2.
Figure 2-2 The OSI model
2.2.4 Physical layer
The IEEE 802.15.4 specification operates on three different frequency bands, in order to conform to regulations in Europe, Japan, Canada and the United States [15].
Table 2-1 describes the frequency bands and data rates. Totally 27 channels are available across the different frequency bands, as described in Table 2-2.
Table 2-1 Frequency bands and data rates [4]
Table 2-2 Channels and center frequency [4]
Center
In the physical layer, the conversion of the binary data to a modulated signal in the 2450 MHz frequency band could describe as the functional block diagram in Fig.2-3. The numbers show how the binary data "0000b" that is converted to a baseband chip sequence with pulse shaping:
PHY
868 868-868.6 300 BPSK 20 20 Binary
915 902-928 600 BPSK 40 40 Binary
2450 2400-2483.5 2000 O-QPSK 250 62.5 16-ary
Orthogonal
Figure 2-3 Modulation and Spreading (2) Bit to symbol
The first step is to encode all the data in the PHY Protocol Data Unit (PPDU) from binary data to symbols [16]. Each byte is divided into two symbols and the least significant symbol is transmitted first. For multi-byte fields, the least significant byte is transmitted first, except for security related fields where the most significant byte is transmitted first.
(3) Symbol to chip
Each data symbol is mapped into a Pseudo-random (PN) 32-chip sequence. The chip sequence is then transmitted at 2 MChip/s with the least significant chip ( ) transmitted first for each symbol. Table 2-3 shows the data symbol with corresponding chip values.
Table 2-3 Symbol to chip mapping Figure 2-3 Modulation and Spreading (2) Bit to symbol
The first step is to encode all the data in the PHY Protocol Data Unit (PPDU) from binary data to symbols [16]. Each byte is divided into two symbols and the least significant symbol is transmitted first. For multi-byte fields, the least significant byte is transmitted first, except for security related fields where the most significant byte is transmitted first.
(3) Symbol to chip
Each data symbol is mapped into a Pseudo-random (PN) 32-chip sequence. The chip sequence is then transmitted at 2 MChip/s with the least significant chip ( ) transmitted first for each symbol. Table 2-3 shows the data symbol with corresponding chip values.
Table 2-3 Symbol to chip mapping Figure 2-3 Modulation and Spreading (2) Bit to symbol
The first step is to encode all the data in the PHY Protocol Data Unit (PPDU) from binary data to symbols [16]. Each byte is divided into two symbols and the least significant symbol is transmitted first. For multi-byte fields, the least significant byte is transmitted first, except for security related fields where the most significant byte is transmitted first.
(3) Symbol to chip
Each data symbol is mapped into a Pseudo-random (PN) 32-chip sequence. The chip sequence is then transmitted at 2 MChip/s with the least significant chip ( ) transmitted first for each symbol. Table 2-3 shows the data symbol with corresponding chip values.
Table 2-3 Symbol to chip mapping
(4) QPSK Modulation
The modulation format is Offset -Quadrature Phase Shift Keying (O-QPSK) with half-sine pulse shaping, equivalent to Minimum Shift Keying (MSK). QPSK is an efficient way to use the often limited bandwidth. Each signal element represents two bits, the equation below shows how the O-QPSK can be expressed. By using Oset, phase changes in the combined signal never exceeds 90º. In the case, using QPSK the maximum phase change is 180 º. O-QPSK provides a greater performance than QPSK when the transmission channel has components with significant nonlinearity.
( ) = sin 2 , 0 ≤ ≤ 2
0,
(5) Error-vector magnitude
The modulation accuracy of an IEEE 802.15.4 transmitter is determined by an Error Vector Magnitude (EVM) measurement, see Fig. 2-4. EVM is the scalar distance between the two phasor end points representing the ideal and the actual measured chip positions. Expressed in another way, it is the residual noise version of the signal and distortion remaining after an ideal version of the signal has been stripped away.
(2.1)
Figure 2-4 Error vector
, = , + ,
The EVM for IEEE 802.15.4 is defined as shown in Eq. 2.3.
≡ ∑ ∗ 100%
where S is the magnitude of the vector to the ideal constellation point, (δIj, δQj) is the error vector. The transmitter shall have EVM values of less than 35% when measured with 1000 chips.
(6) Transmit power
The transmitter should be capable of transmitting at least -3 dBm. Also, the device should transmit as low power as possible to reduce interference to other devices and systems. The definition of dBm is shown in Equation 2.4.
= 10 1
The EVM for IEEE 802.15.4 is defined as shown in Eq. 2.3.
≡ ∑ ∗ 100%
where S is the magnitude of the vector to the ideal constellation point, (δIj, δQj) is the error vector. The transmitter shall have EVM values of less than 35% when measured with 1000 chips.
(6) Transmit power
The transmitter should be capable of transmitting at least -3 dBm. Also, the device should transmit as low power as possible to reduce interference to other devices and systems. The definition of dBm is shown in Equation 2.4.
= 10 1
The EVM for IEEE 802.15.4 is defined as shown in Eq. 2.3.
≡ ∑ ∗ 100%
where S is the magnitude of the vector to the ideal constellation point, (δIj, δQj) is the error vector. The transmitter shall have EVM values of less than 35% when measured with 1000 chips.
(6) Transmit power
The transmitter should be capable of transmitting at least -3 dBm. Also, the device should transmit as low power as possible to reduce interference to other devices and systems. The definition of dBm is shown in Equation 2.4.
= 10 1
(2.2)
(2.3)
(2.4)
0 = 1 (2.5)
+ 30 = 0 (2.6)
0 = −30 (2.7)
(7) Receiver sensitivity
The receiver sensitivity is defined by two terms. One is Packet Error Rate (PER) which is the average fraction of transmitted packets that are not detected correctly.
The other term is the threshold input signal power that yields a specified PER. In IEEE 802.15.4 a compliant device shall have a sensitivity of -85 dBm or better.
(8) Receiver Engineering Detection (ED)
The receiver Energy Detection (ED) is intended to be used by the network layer as part of a channel selection algorithm. It is an estimate of the received signal power within the bandwidth of an IEEE 802.15.4 channel. No attempt is made to identify or decode signals on the channel. The ED time shall be equal to 8 symbol periods [17].
(9) Link quality Indication (LQI)
The LQI is a characterization of the strength and/or quality of a received packet.
The measurement may be implemented using receiver ED, a Signal to Noise Ratio (SNR) estimation, or a combination of these methods. The use of the LQI result by the network or application layer is not part of the IEEE 802.15.4 standard [17].
the desired signal energy to the total in-band noise energy (the signal-to-noise ratio , or SNR) is another way to judge the signal quality. As a general rule, higher SNR translates to lower chance of error in the packet. Therefore, a signal with high SNR is considered a high-quality signal[17].
(11) Clear Chanel Assessment (CCA)
The CCA is used to decide whether the channel is busy or idle and one of the following methods must be supported [17].
CCA Mode 1: Energy above threshold. CCA shall report a busy medium upon detecting any energy above the ED threshold.
CCA Mode 2: Carrier sense only. CCA shall report a busy medium only upon the detection of a signal with the modulation and spreading characteristics of IEEE 802.15.4. This signal may be above or below the ED threshold.
CCA Mode 3: Carrier sense with energy above threshold. CCA shall report a busy medium only upon the detection of a signal with the modulation and spreading characteristics of IEEE 802.15.4 with energy above the ED threshold.
A busy channel shall be indicated by the Physical Layer Management Entity Con-firm (PLME-CCA.confirm) primitive with a status of BUSY. A clear channel shall be indicated by the PLME-CCA. confirm primitive with a status of IDLE.
2.2.5 Medium Access Control Layer
Media Access Control (MAC) has different responsibilities [16]:
The main function of MAC is to carry out the association and disassociation of the network involved. A large number of devices are managed or
It generates the network beacons according to the device, if it is a coordinator
It also performs the function of synchronizing the beacons.
It uses the CSMA-CA channel access mechanism
It uses Guaranteed Time Slot (GTS) mechanism.
It allows different mechanisms to conserve energy like collision avoidance using CSMACA and allowing the device to go into sleep mode.
2.3 Hardware
The Smart development kit from FineTek company were used in this thesis, as show in Fig. 2-5.
Figure 2-5 FineTek ZigBee equipments
The board complete with CC2430 chip complied to system-on-chip (SOC) ZigBee®/IEEE 802.15.4 standard, an industry-standard enhanced 8051 MCU with 128 KB flash memory and 8 KB RAM and with AN040 whip antenna. The board supports the Z-Stack™ protocol stack .
by level transmitters such as electromechanical level measurement, magnetostrictive level transmitter, capacitance level transmitter, magnectic level transmitter, pressure level transmitter. Then data is transmitted via ZigBee communication protocol.
Figure 2-6 FineTek's Level Measurement System
The FineTek system is really is a good monitoring application to observe all levels in the tanks and silos. However, the developed system does not support information of the WSN such as communication reliability and topology of the network. In addition, the application does not deal with the data dropout in the ZigBee WSN.
Chapter 3
Propagation Analysis for ZigBee
In this chapter, the radio frequency behavior of ZigBee devices operating within a real environment will be studied. Three different RSSI measurement models have been carried out to characterize the main propagation features. On the basis of the experimental results, different factors that affect the measurements, such as external factors (e.g. multipath, fading) or internal ones (e.g. hardware device, integrated antennas) have been analyzed. The first approach, made in the outdoor environment, was performed to study both the internal and external factor effects, the second one was performing in the corridor of the building to investigate the signal characteristics in the indoor environment. And the last one is to study how obstacles such as the walls affect signal intensity in indoor environments. Furthermore, to deeply analyze the ZigBee behavior, we have compared the measurements with a suitable propagation model to prove it’s effectiveness. Thus, the ZigBee signal propagation in a complex environment can be predicted by applying the obtained model.
3.1 The Propagation model
The effectiveness of a propagation model depends on how the theoretical approximation can fit with the real measurements. The Log-Distance Path Loss Model
As the reference power at 1m, the equation becomes:
P(d)dB= P(1m)dB- 10.n.log10(d) (3-2)
Where, P(d) is the power in dB to estimate at some particular distance d, P(d0) is the known power at distance d0, n is the path loss exponent, which indicates the decreasing rate of signal strength in an environment, d0 is a reference distance which is close to the transmitter, and d is the distance between the transmitter and the receiver. In general, the exponent n is environment-dependent and in a free space its value is close to 2 [20]. In the indoor case, it will be larger than this value.
However, the Log-Distance Path Loss Model does not show the effect of obstacles such as the wall effect. Therefore, the Wall Attenuation Model [21] is
However, the Log-Distance Path Loss Model does not show the effect of obstacles such as the wall effect. Therefore, the Wall Attenuation Model [21] is