ZigBee technology is a short-range wireless communication technology with a unified standard. Its name comes from the observation of people dancing the "Z" in the honey bee collecting process. The bees are small in size, require less energy, and can transmit collected pollen. Therefore, ZigBee is used to represent wireless communication technologies with low cost, small size, low energy consumption, and low transmission rate. This paper introduces the structure of ZigBee network, analyzes the ZigBee RF performance, and discusses the ZigBee RF test method.
1 Introduction
ZigBee is one of the new technologies that will have a huge impact on the 21st century. Compared with traditional networks, wireless sensor networks are data-centric self-organizing wireless networks with fast temporary networking and network topology dynamics. It is highly resistant to damage and does not require the construction of network infrastructure. Based on these characteristics, ZigBee is widely used in military, environmental monitoring, smart home, building condition monitoring, complex mechanical monitoring, urban transportation, space exploration, and security testing in airports and large industrial parks. Environmental monitoring is an aspect of wireless sensor network applications. Sensor networks have obvious advantages in the field of environmental monitoring, and can provide a new means for environmental monitoring with more accurate, larger data volume and less impact on the environment.
ZigBee technology is widely used in environmental monitoring, smart home, building automation, industrial control and other fields due to its low cost, low power consumption, large network capacity, short transmission delay and high reliability.
2 ZigBee technology features and network structure
(1) Technical characteristics of ZigBee
â— Low power consumption: Because ZigBee's transmission rate is low, the transmission power is only 1mW, and the sleep mode is adopted, the power consumption is low, so the ZigBee device is very power-saving. It is estimated that ZigBee devices can last up to 2 years using only two 5th batteries, which is beyond the reach of other wireless devices.
â— Low cost: The initial cost of the ZigBee module is around $6, which is estimated to drop to $1.5 to $2.5 soon, and the ZigBee protocol is royalty-free. Low cost is also a key factor for ZigBee.
â— Short delay: The communication delay and the delay from the sleep state are very short. The typical search device delay is 30ms, the sleep activation delay is 15ms, and the active device channel access delay is 15ms. Therefore, ZigBee technology is suitable for applications with time-critical wireless control (such as industrial control applications).
â— Large network capacity: A star-shaped ZigBee network can accommodate up to 254 slave devices and one master device, and the network is flexible.
â— Reliable: A collision avoidance strategy (CSMA-CA) is adopted, and dedicated time slots are reserved for communication services requiring fixed bandwidth, avoiding competition and collision of transmitted data. The MAC layer uses a fully acknowledged data transmission mode, and each transmitted packet must wait for the acknowledgement from the receiver. If there is a problem during the transfer, you can resend it.
â— Security: ZigBee provides a packet integrity check function based on Cyclic Redundancy Check (CRC), which supports authentication and authentication. It uses AES-128 encryption algorithm, and each application can flexibly determine its security attributes.
(2) Equipment and network topology in the ZigBee network
It is divided into a full-featured device (FFD) and a reduced-function device (RFD) according to the function of the device.
â— Full-featured devices have more memory and computing power, with all 802.15.4 features and all features. There is a controller function that provides bidirectional transmission of information.
â— Streamlined function devices only have limited functions to control cost and complexity. They are only used as terminal devices, using small memory and small protocol stacks for easy implementation.
Divided into Zig-Bee coordinator, ZigBee router and ZigBee terminal device according to their role in the network.
â— The ZigBee Coordinator is at the top of the network. It is always working and has a stable and reliable power supply. It contains all the network information and is the most complicated of the three types of devices. It has large storage capacity and strong computing power. Can send network beacons, establish a network, manage network nodes, store network node information, and so on. They can only be undertaken by the FFD.
â— ZigBee routers must have data storage and forwarding capabilities and route discovery capabilities. In addition to completing application tasks, you must also support the connection of its child devices, maintenance of routing tables, and so on. They can only be undertaken by the FFD.
â— ZigBee terminal equipment is the simplest structure and function. It is powered by battery and sleeps most of the time, maximizing energy saving and extending battery life. It can be borne by the FFD, but it is mainly borne by the RFD.
Three topologies are defined in the IEEE 802.15.4/ZigBee protocol, namely, star structure, cluster tree structure, and mesh structure. See Figure 1 for details.
Figure 1 Several basic network models
In a star topology, the network is controlled by a coordinator. The coordinator is responsible for initializing and maintaining the network and all other devices in the network that communicate directly with the coordinator as end devices. In a mesh or cluster tree network, the coordinator is responsible for starting the network and setting some key parameters, but the network can be extended by the router. In a cluster tree network, routers use hierarchical routing policies to transmit data and control information. Tree networks typically use beacon-based communication modes. The Mesh network allows full peer-to-peer communication where the router does not send regular IEEE 802.15.4 beacons.
3 ZigBee RF test main content and analysis
ZigBee's test specification mainly tests the performance of the whole machine according to the IEEE 802.15.4 standard. The overall performance test includes transmitter and receiver performance tests. The main requirements of the transmitter radio are divided into two parts: ensuring the quality of the transmitted signal in the useful channel and preventing the frequency component from unwanted emissions. The receiver should have the ability to combat a wide range of interfering signals, thereby ensuring reliable demodulation of the wanted signal and avoiding excessive sensitivity to transmission. The ZigBee launch test and receive test are detailed below.
(1) Transmitter test
Transmitter Test Principle: To analyze the transmitter's transmitted signal, first use an ideal receiver to convert the transmitted signal into a digital baseband signal. Then perform signal processing in the numbers to get the corresponding indicators. In actual testing, a high-performance spectrum analyzer or signal analyzer is generally used as an ideal receiver. After an ideal receiver and AD conversion to obtain IO data, a series of pulse shaping filtering, interpolation, and decimation on the data will obtain the initial measurement signal, and then the amplitude, phase, and frequency of the measurement signal, and time. The correction is to obtain the corrected measurement signal, and then the measurement signal is descrambled, despread, demodulated, remodulated, respread, and then scrambled, and the reference signal is obtained through the shaping filter, and finally the corrected measurement signal and reference are obtained. The comparison calculation of the signals yields various indicators of the measurements within the channel.
For the transmitter, the following test items are mainly included in the transmitter test indicators:
â—Transmitter signal output power
The transmitter transmit power should not interfere with other devices and systems, so the output power of ZigBee must meet the requirements of the indicator. The maximum transmit power in different regions is different in the corresponding frequency band, and the maximum transmit power in China is 10mW (EIRP).
â—Power lifting
â— Spectrum emission template
For wireless data communication ZigBee, the transmitter's spectrum emission template and spurious emission characteristics directly affect its interference with existing radio services. The spectrum power of the transmitter should be lower than the specified limit. The limit of the spectrum power template in China is shown in Table 1. For both the relative and absolute limits, the average power spectrum mean is measured using a resolution bandwidth of 100 kHz. For the relative limit reference power is the highest average power spectral density at the center frequency fc ± 600 kHz.
â— Launching spurs
Transmitter stray radiation is caused by undesired transmitter phenomena (eg, harmonic radiation, parasitic radiation, intermodulation products), but does not include out-of-band radiation. The spurious emission limits are limited to the frequency range from the edge of the channel, which is greater than 2.5 times the useful bandwidth. To improve measurement accuracy, sensitivity, and efficiency, the resolution bandwidth can be less than the measurement bandwidth.
â—Center frequency tolerance
The emission center frequency tolerance should be within ±40 ppm of the maximum frequency.
â— Constellation error
â— Error Vector Magnitude (EVM)
EVM refers to the error vector (including the magnitude and phase loss), which characterizes the vector difference between the ideal error-free reference signal (physical layer specification reference signal) and the actual transmitted signal at a given time (see Figure 2). From the EVM parameters, we can understand the amplitude error and phase error of an output signal.
Figure 2 error vector calculation
Figure 2 Error Vector Calculation EVM is an indicator to measure the overall modulation quality of an RF system. It is defined as the error between the measured signal and the ideal signal on the signal constellation. It is used to indicate the modulation accuracy of the transmitter, modem, PA, mixer, Transceivers, etc. will have an impact on it. The accuracy of the transmitter modulation is determined by the measurement of the EVM. The transmitter's EVM should be less than 35% when measuring 1000 chips. The actual transmitted signal received shall be measured on the baseband IQ chip after the receiving system is restored.
(2) Receiver test
Receiver test principle: The purpose of the receiver test is to evaluate whether the overall performance of the wireless data communication receiving part meets the design and acceptance requirements. Generally, the test port is located at the RF input port, and the standard signal source is used to generate the RF test signal, fed into the receiver, and then the output code stream is tested to evaluate the receiver performance.
The following test items are mainly included in the receiver test indicators:
â— Symbol error tolerance
In China, the physical layer symbol rate of O-QPSK/MPSK modulation in the 780MHz band should be 62.5ksymbol/s, and the symbol accuracy is ± 40ppm.
â— Receiver sensitivity
Receiver sensitivity is the minimum input power of the receiver when a certain receive packet error rate is met. The device has a sensitivity of -85 dBm or better when the conditions of Table 2 are met.
â—Maximum input level
The maximum input level of the receiver is the receiver's desired signal at the receiver input, meeting the maximum input power required by Table 2. The maximum input level of the receiver should be greater than or equal to -20dBm.
â— Receiver blocking
The blocking characteristic refers to the case where the interference signal exists outside the desired channel, and the interference signal reduces the quality of the received useful signal by no more than the receiver PER limit, that is, PEB "1%. The adjacent channel is the channel closest to either side of the desired channel, and the alternate channel is the next channel of the adjacent channel.
Adjacent channel anti-jamming should be measured as follows: The required channel should be configured according to the defined 780 MHz O-QPSK/MPSK PHY signal, and the signal power should be set to be 3 dB greater than the measured maximum allowable receiver sensitivity.
â—Energy detection
Channel energy detection provides a channel selection basis for the network layer. The power intensity of the received signal in the target channel is mainly measured. Since the detection itself does not perform the decoding operation, the detection result is the sum of the effective signal power and the noise signal power.
â— Link Quality Indicator (LQI)
The link quality indication provides the network layer or the application layer with the strength and quality information of the wireless signal when receiving the data frame. Unlike the channel energy detection, it decodes the signal to generate a signal to noise ratio indicator. This SNR indicator is submitted to the upper layer processing together with the physical layer data unit. The LQI measurement describes the quality of the received packet. The measurement may use receiver ED, signal to noise ratio estimation, or a combination of both.
â— Clear Channel Estimation (CCA)
The physical layer shall provide at least one of the following methods to reflect the ability of the idle channel to estimate CCA. Judging the signal energy of the channel, if the signal energy is lower than a certain threshold, the channel is considered to be idle; determining the characteristics of the wireless channel, this feature mainly includes two aspects, namely, a spread spectrum signal and a carrier frequency; combining the first two modes, simultaneously detecting Signal strength and signal characteristics give a channel idle decision.
â—CCA measurement parameters
- The ED threshold corresponds to a received signal power that is at least greater than the receiver sensitivity + 10 dB.
- The detection time of the CCA should be 8 symbol periods.
4 ZigBee RF Reference Test Solution
The RF test system of ZigBee terminal is composed of wireless tester, spectrum analyzer and signal generator. On this basis, GPIB or LAN port is used for system control, and test software is developed on the main control computer to realize fully automatic test.
â— Wireless tester: ZigBee signal supporting ZigBee signaling mode and supporting physical layer characteristics specified by IEEE802.15.4.
â— Vector signal source: ZigBee signal supporting the physical layer characteristics specified by IEEE 802.15.4.
â— Spectrum analyzer: Supports ZigBee terminal spectrum test and spurious test, and the bandwidth should reach 30MHz~12.75GHz.
The transmitter test schematic is shown in Figure 3. The ZigBee transmit signal enters the vector signal analyzer and measures each transmitter's specifications.
Figure 3 transmitter test schematic
The normal receiver test schematic is shown in Figure 4. The vector signal generator generates a standard ZigBee signal to the ZigBee component receiver, and ZigBee generates an ACKFrame acknowledge accept. The vector signal analyzer analyzes whether the ACK meets the criteria.
Figure 4 receiver principle test chart
The receiver blocking schematic diagram is shown in Figure 5. The vector signal generator 1 generates the main signal, and the vector signal generator 2 generates the adjacent channel interference signal to be sent to the device under test to test the receiver blocking performance.
Figure 5 Receiver blocking principle test chart
5 Conclusion
ZigBee expects the domestic IoT market to reach 750 billion yuan by 2015, with a compound annual growth rate of over 30%. The construction of smart cities has become an important foothold for operators to promote the Internet of Things. In addition, the Ministry of Industry and Information Technology and the Ministry of Finance have set up special funds to support the development of the Internet of Things. It is reported that the special fund support budget invested in 2013 has increased from 2012 and will exceed 500 million yuan. Industry insiders expect that the Internet of Things will be widely popular in the next 10 years, and its industry scale will far exceed the Internet. It can be seen that ZigBee technology will develop rapidly and its future application range will be quite extensive.
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