APPLICATION NOTE Atmel AT02845: Coexistence between ZigBee and Other 2.4GHz Products Description Atmel MCU Wireless This document introduces all kinds of 2.4GHz RF products. It focuses on analysis of ZigBee and other 2.4GHz RF products and concludes on the interferences with ZigBee and ZigBee s coexistence possibility with other 2.4GHz RF products in the same space. The document will help to make ZigBee work well under the complex electro-magnetic environment. Features Coexistence and interference between ZigBee and other 2.4GHz products Introduction of all kinds of 2.4GHz products ZigBee s anti-interference characteristic analysis Coexistence possibility analysis How to make ZigBee coexist with other 2.4GHz products
Table of Contents 1. Overview... 3 2. Introduction of 2.4GHz RF Products... 4 2.1 ZigBee / IEEE 802.15.4... 4 2.2 Wi-Fi / IEEE 802.11a/b/g... 4 2.3 Bluetooth / IEEE 802.15.1... 4 2.4 Wireless USB... 5 2.5 Other 2.4GHz Products... 5 3. Anti-interference Characteristic Analysis of ZigBee... 6 3.1 DSSS (Direct Sequence Spread Spectrum)... 6 3.2 CCA (Clear Channel Assessment)... 6 3.3 Dynamic Channel Selection... 6 3.4 Acknowledged Transmission and Retries... 6 4. Coexistence Analysis... 7 4.1 Coexistence between ZigBee and Wi-Fi... 7 4.2 Coexistence between ZigBee and Bluetooth... 8 4.3 Coexistence between ZigBee and Wireless USB... 8 4.4 Coexistence between ZigBee and Cordless Telephone... 8 4.5 Coexistence between ZigBee and Microwave Oven... 8 5. Conclusion... 9 6. Reference... 11 7. Revision History... 12 2
1. Overview ZigBee products based on IEEE 802.15.4 are acting more and more important roles in IOT and Smart Grid applications, however, with the development of short range radio technology, more and more 2.4GHz ISM band of radio systems are being widely adopted in more fields, so designers have to deal with increasing interferences caused by other radio communication devices. To make a ZigBee network work well, it is necessary to study on the mutual coexistence and interference between ZigBee (based on IEEE 802.15.4) and other 2.4GHz radio systems such as Wi-Fi, Bluetooth, Wireless USB, other interference sources including 2.4GHz cordless phones, and microwave ovens. The 2.4GHz Industrial, Scientific, and Medical (ISM) unlicensed frequency band is shared by many RF technologies, such as IEEE 802.11b/g Wi-Fi, ZigBee, Bluetooth, Wireless USB, cordless phone, etc. Due to its low transmit power feature, ZigBee is potentially vulnerable to the interferences introduced by these RF technologies rather than vice versa. In this document, the coexistences and interferences between ZigBee and other 2.4GHz technologies are mainly focused on. 3
2. Introduction of 2.4GHz RF Products 2.1 ZigBee / IEEE 802.15.4 The IEEE802.15.4 is a part of the IEEE family of standards for physical and link layers, the standard is designed to address applications with requirements including low data throughput, low power, short transmitting range and low cost. The IEEE802.15.4 supports two PHY options based on DSSS (Direct sequence spread spectrum). The 2.4GHz PHY uses Q-QPSK modulation, whereas 780/868/915MHz uses BPSK (binary phase shift keying ) modulation.both of its 2.4GHz and 868/915 MHz can offer good BER (bit error rate) performance at low SNR (signal to Noise Ratio). The IEEE802.15.4 physical layer offers 31 channels, 4 in 780MHz band for China (IEEE 802.15.4c), 1 in 868MHz band for Europe,10 in 915MHz for North America,16 in the 2.4GHz throughout of the world. Atmel AT86RF212B is the latest generation of the Atmel Sub-GHz chip, which supports 780/868/915MHz bands. The nominal radio data rates on these four frequency bands are 20kbps, 40kbps, and 250kbps. ZigBee over IEEE 802.15.4, defines specifications for low-rate WPAN, provides self-organized, multi-hop, and reliable mesh networking with long battery lifetime. Currently, ZigBee is widely used in WSN and IOT applications. Because 2.4GHz band is unlicensed RF band throughout the world, this paper mainly focuses on coexistence among 2.4GHz RF products, rather than Sub-GHz RF products, thus Sub-GHz ZigBee products are not studied on in this document. 2.2 Wi-Fi / IEEE 802.11a/b/g Wireless fidelity (Wi-Fi) includes IEEE 802.11a/b/g standards for wireless local area networks (WLAN), which are commonly used today to provide wireless connectivity in the home, office, and some commercial establishments. Wi-Fi is a popular technology that allows an electronic device to exchange data wirelessly over a computer network, including high-speed Internet connections. The Wi-Fi Alliance defines Wi-Fi as any "wireless local area network (WLAN) products that are based on the Institute of Electrical and Electronics Engineers' (IEEE) 802.11 standards". The IEEE 802.11a amendment to the original standard was ratified in 1999. The IEEE 802.11a standard uses the same core protocol as the original standard, operates in 5GHz band, and uses a 52-subcarrier orthogonal frequency-division multiplexing (OFDM) with a maximum raw data rate of 54Mbit/s, which yields realistic net achievable throughput in the mid-20 Mbit/s. The IEEE 802.11b and 802.11g are amendments to the IEEE 802.11 specification that extends throughput from 54Mbit/s to 600Mbit/s using the same 2.4GHz band as 802.11b. The IEEE 802.11b and 802.11g operate in total of 14 channels available in the 2.4GHz band, each with a bandwidth of 22MHz and a channel separation of 5MHz. WLAN output powers are typically around 20dBm and operate within a 100m range. This specification under the marketing name of Wi-Fi has been implemented all over the world. 2.3 Bluetooth / IEEE 802.15.1 Bluetooth, also known as the IEEE 802.15.1 standard, is a RF technology standard for exchanging data over short distances (using short-wavelength radio transmissions in the ISM band from 2400 2480MHz) from fixed and mobile devices, creating personal area networks (PANs) with high levels of security. Bluetooth is created by telecom vendor Ericsson in 1994. 4
Bluetooth operates in the range of 2400 2480MHz, which is in the globally unlicensed Industrial, Scientific and Medical (ISM) 2.4GHz short-range radio frequency band. It uses a radio technology called frequency-hopping spread spectrum. The transmitted data is divided into packets and each packet is transmitted on one of the 79 designated Bluetooth channels in a pseudo-random pattern. Each channel has a bandwidth of 1MHz. The first channel starts at 2402MHz and continues up to 2480MHz in 1MHz steps. It usually performs 1600 hops per second, with Adaptive Frequency-Hopping (AFH) enabled. 2.4 Wireless USB Wireless USB is a short-range, high-bandwidth wireless radio communication protocol created by the Wireless USB Promoter Group, which is primarily designed as a wireless option for computer input devices such as mouse and keyboards. Now it is also targeting the wireless sensor networks. Wireless USB devices operate for months on alkaline batteries and require regular recharging. Wireless USB uses the DSSS frequency modulation instead of FHSS. Each Wireless USB channel is 1MHz wide, allowing Wireless USB to split the 2.4GHz ISM band into 79 1MHz channels, similar to the Bluetooth. Unlike Bluetooth, Wireless USB devices support frequency agility, that is, they use a fixed channel, but dynamically change channels if the link quality of the channel becomes bad. Wireless USB uses pseudonoise (PN) codes to encode each information bit. Most Wireless USB systems use two 32 chip PN codes, allowing two information bits encoding in each 32 chip symbols. This scheme can correct up to three chip errors per symbol and can detect up to 10 chip errors per symbol. 2.5 Other 2.4GHz Products Many cordless telephones, car alarms and microwave ovens use the 2.4GHz frequency. 5
3. Anti-interference Characteristic Analysis of ZigBee The anti-interference performance analysis studied in the document mainly refers to Co-Channel Interference, namely, the interference from other RF technologies, which seriously impacts radio features of ZigBee products based on IEEE 802.15.4 standard. IEEE 802.15.4 adopts many mechanisms to enable it to coexist with other 2.4GHz radio products. 3.1 DSSS (Direct Sequence Spread Spectrum) DSSS, the spreading technique employed by IEEE 802.15.4, makes use of a pseudo-random code sequence, often called a chipping sequence, which is transmitted at a maximum rate called the chip rate. The chipping sequence is used to directly modulate the basic carrier signal- hence the name direct sequence and to encode the data being transmitted. DSSS technique extends band of frequencies, which can reduce the occupancy of an interference source and get better coexistence with other interference sources. 3.2 CCA (Clear Channel Assessment) The clear channel assessment (CCA) is performed according to at least one of the following three methods: Energy above threshold, CCA reports a busy medium upon detecting any energy above the Energy Detect Threshold. Carrier sense only, CCA reports 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. Carrier sense with energy above threshold, CCA reports 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. 3.3 Dynamic Channel Selection ZigBee allows dynamic channel selection, a scan function steps through a list of supported channels in search of beacon, receiver energy detection, link quality indication. A feature called frequency agility is specified in the ZigBee standard to improve the robustness of ZigBee networks, according to this function, if interference is detected and reported in the current channel, a ZigBee network may move to a clear channel based on some mechanisms. ZigBee's operation in 2.4GHz band is aided by the choice of 16 available channels. The frequency agility function makes using these extra channels easier. When a network is first formed the node seeks a channel with the least noise or traffic. If overtime extra traffic appears or noise becomes present, the host application scans for a better channel and moves the whole network to the new channel allowing the network to adapt overtime to changing RF environments. 3.4 Acknowledged Transmission and Retries To ensure the reliability of transmission, an acknowledge frame delivery protocol is defined in IEEE 802.15.4-2003 standard. If a receiving device fails to receive or handle the received data frame for any reason, the message will not be acknowledged. If the sender does not receive an acknowledgement, it assumes that the transmission was not done successfully and retries the data frame transmission. This strategy is particularly useful to deal with frequency hopping interferences, such as from Bluetooth, which may interfere with a first transmission attempt but will usually hop to a different part of the spectrum for the retry. 6
4. Coexistence Analysis 4.1 Coexistence between ZigBee and Wi-Fi The IEEE 802.11 Wireless LAN (WLAN) standard adopts DSSS and operates in a total of 14 channels available in the 2.4GHz band, numbered 1 to 14, each channel with a bandwidth of 22MHz and a separation of 5MHz. The IEEE 802.15.4 is also based on DSSS. A total of 16 channels are available in the 2.4GHz band, numbered 11 to 26, each with a bandwidth of 2MHz and a channel separation of 5MHz. Both ZigBee and Wi-Fi are based on DSSS. The signal is spread over a larger bandwidth, so narrow-band interferers block a smaller overall duty of the signal, allowing the receiver to receive the signal correctly. Because both are in 2.4GHz band, so co-channel interference should be taken into account. Wi-Fi networks using the same or overlapping channels with other RF products will more or less influence the communication of other RF productions. Figure 4-1. Wi-Fi and ZigBee Overlapping Channels in 2.4GHz ISM Band The overlapping channels between 802.11 and 802.15.4 are shown in Figure 4-1, which shows channel 15, 20, 25, and 26 are not overlapped with Wi-Fi channels, these channels are of negligible interference. For each Wi-Fi channel, there are four overlapping ZigBee channels, two channels of them are at the edges and another two are close to the center frequency of the Wi-Fi channel. In fact, the interference level on the two channels close to the center is higher than those on the edges. PER (Packet error rate) has close relationship with distance (between interference source and receiver) and differences of center frequencies (between interference source and receiver). They can even coexist within very short range (2 meters) even when difference of center frequencies is considerable, however, when their center frequencies are very close, they can coexist only out of long distance (even dozens of meters). It shows that, if the interference source is more far from receiver, they can coexist better. Channel occupancy detection and dynamic channel selection are very important to keep good coexistence. 7
The interference with Wi-Fi, caused by ZigBee, is smaller than the interference with ZigBee, caused by Wi-Fi, it is because ZigBee s bandwidth (2MHz) is much smaller than Wi-Fi s bandwidth (22MHz), so ZigBee is a kind of narrowband interference source to Wi-Fi. 802.11b/g/n adopts spread spectrum technology, which can greatly restrain interference signal, in addition, for most ZigBee products the power is conditioned to 0dbm (1mW), which is not enough to pose a threat to Wi-Fi products, its power is far less than that of IEEE 802.11b/g, 20dbm (100mW), they can coexist very well if necessary measures are adopted. 4.2 Coexistence between ZigBee and Bluetooth Bluetooth adopts FHSS technology, which supports 79 channels with each 1MHz bandwidth. Its working frequency quickly hops 1600 times per second. Even if there are several kinds of 2.4GHz RF systems, the hopping system only interferes with other RF systems for a little time period, other RF systems can operate without influence in most of the time. ZigBee is a DSSS system, not a kind of frequency hopping system, so there is only one time channel overlap in 79 times, if a Bluetooth device transmits in a frequency that overlaps with the ZigBee channel, then the ZigBee device randomly backs off while the Bluetooth quickly hops to another frequency, so Bluetooth does not disturb ZigBee products in most instances, they can coexist very well. 4.3 Coexistence between ZigBee and Wireless USB Wireless USB is based on DSSS frequency modulation instead of FHSS, with the bandwidth of 1MHz for every channel, like Bluetooth, Wireless USB also has 79 channels. The difference between them is that Wireless USB adopts DSSS, while Bluetooth adopts FHSS. Wireless USB supports frequency agility, even if Wireless USB adopts fixed channel, however, if LQI becomes bad, it can dynamically change its channel. In most instances, CSMA and small transmission duty cycle help to guarantee good transmission with low data loss rate. In fact ZigBee can also support frequency agility. Wireless USB checks noise level of the channel per 50ms, if its frequency overlaps that of ZigBee, Wireless USB will change to a new clear channel to avoid the interference between them, so Wireless USB and ZigBee can well coexist. 4.4 Coexistence between ZigBee and Cordless Telephone 2.4GHz cordless telephones do not adopt a technology compliant with a special standard. Some of them adopt DSSS, in fact, most of them adopt FHSS. There is always a key for channel selecting on a Cordless telephone adopting DSSS and a fixed channel, which makes user manually change channel, however, there is no such key on a cordless telephone based on FHSS, it is because such telephones always keep changing their frequencies. Bandwidth of most 2.4GHz cordless telephones is 5~10MHz. Cordless telephones transmit radio waves with considerable energy, so cordless telephones are interference sources to many RF systems. The worst cases, a cordless telephone based on FHSS can black out the communication of a ZigBee PAN, which is because its bandwidth (5~10MHz) is wider than bandwidth of Bluetooth (1MHz), which results in more interference than Bluetooth, in addition, cordless telephones always transmit considerable power so that ZigBee device keeps retransmitting data package. To avoid such interference from cordless telephones, ZigBee should be kept away from cordless telephones based on FHSS and ZigBee s channel should not be overlapped with the channels of cordless telephones based on DSSS. 4.5 Coexistence between ZigBee and Microwave Oven Microwave oven is also a common interference source in 2.4GHz frequency band, meanwhile, it is the most unpredictable and dispersed RF source. Output energy intensity and frequency distribution of each microwave oven are not the same. Some designs of microwave ovens can cut off electromagnetic radiation better than others. According the test in [6], 0.5% ~ 2% of Data frames is seriously destroyed when transmitting within the scope of one meter. The interference is negligible when transmitting outside the scope of one meter. 8
5. Conclusion Through above analysis, such conclusion can be drawn, namely, ZigBee has very good anti-interference performance and can coexist with other 2.4GHz RF systems when necessary measures are taken. ZigBee Network is robust and reliable, which has been proved in many complex RF environments. There are many large consumer trade shows of electronic items in Las Vegas, where there are thousands of Wi-Fi, Bluetooth and other ZigBee devices all operating in very close range and very large numbers of devices sharing the 2.4GHz band, and it is very rare that any of the Atmel ZigBee demos are affected by the environment. The following methods should be adopted to reduce the effects of interference on a ZigBee network. Non-overlapping Channel Selection Bluetooth adopts FHSS, it can well coexist with ZigBee products. Wireless USB adopts DSSS, however, its bandwidth is only 1MHz, at the same time, Wireless USB supports frequency agility, so Wireless USB almost does not interfere ZigBee network, and vice versa. Wi-Fi adopts DSSS technique, and it supports 14 channels in 2.4GHz, each with a bandwidth of 22MHz and a channel separation of 5MHz. Overlapping channels can cause interference with a ZigBee network, so a channel centre-frequency offset of 7MHz is recommended to achieve acceptable coexistence with Wi-Fi, These non-overlapping Wi-Fi channels (1, 7, and 13 for Europe, and 1, 6, and 11 for North America) should be chosen to avoid Wi-Fi interference where non-overlapping Wi-Fi channels have been allocated. Physical Separation and Safe Distance Bluetooth adopts fast FHSS technique, according test result in [6], ZigBee and Bluetooth coexist very well when outside of 0.75m scope, no physical separation should be considered. Keeping ZigBee devices at least eight meters away from a Wi-Fi Access Point (AP) can ensure acceptable cochannel IEEE 802.15.4 performance. As mentioned, when a microwave oven is kept one meter away from ZigBee devices, its interference with ZigBee devices transmission can be negligible, so physical separation should be over one meter scope when deploying a ZigBee network. Keeping ZigBee devices one meter away from Microwave ovens can get ideal transmission, which is verified in [6]. For cordless phones, it is suggested that ZigBee devices are kept ten meters from DSSS cordless phones and five meters from FHSS cordless phones. Enabling Network Layer Frequency Agility In ZigBee PRO stack, a ZigBee device can perform dynamic channel selection in response to channel impairment. BitCloud, Atmel ZigBee PRO Stack Suite, supports this feature. If a ZigBee device is developed based on BitCloud and its FREQUENCY_AGILITY feature is enabled, it can detect (by doing energy detection scan) whether a channel, on which the network is operating, is overloaded. If such situation happens the device sends a report to the network manager. To handle this report, the network manager must also have the FREQUENCY_AGILITY feature enabled. The network manager then checks the energy level on all channels available, selects a different channel and sends a broadcast command to the network, ordering to switch to the new channel. For further information on Frequency agility, contact Atmel support. BitCloud - ZigBee PRO: http://www.atmel.com/tools/bitcloud-zigbeepro.aspx Network planning and optimization In order to achieve the best transmission performance and avoid interference from other RF systems, a field survey should be performed to evaluate the RF environments before deploying a ZigBee network, sniffing and logging results over a period of time. Occupied channels and occupied duty should be analyzed, which will help to choose a non-overlapping channel, at the same time, to a ZigBee device, PER test should be performed on a specific installation site. Atmel offers a tool to do these operations, which is Wireless Composer as an extension for Atmel Studio 6. Wireless Composer (Figure 5-1) is located under Tools/Extension Manager within Studio 6. To learn more or download Atmel Studio 6 free of charge, visit www.atmel.com/atmelstudio. 9
Figure 5-1. Wireless Composer of Atmel Studio 6 10
6. Reference [1]. www.zigbee.org [2]. ZigBee Alliance, ZigBee specification v1.0, http://www.zigbee.org/),2004 [3]. IEEE Std.802.15.4, IEEE Standard for Wireless Medium Access Control(MAC) and Physical Layer (PHY) Specifications for Low-Rate Wireless Personal Area Networks (LR-WPANs), 2003 [4]. IEEE Std.802.11, IEEE Standard for Wireless LAN Medium Access Control(MAC) and Physical Layer (PHY) Specification, 1997 [5]. IEEE Std.802.15.1, IEEE Standard for Wireless Medium Access Control(MAC) and Physical Layer (PHY) Specifications for Wireless Personal Area Networks (WPANs), 2002 [6]. Compatibility of IEEE 802.15.4 (ZigBee) with IEEE 802.11 (WLAN), Bluetooth, and Microwave Ovens in 2.4GHz ISM-Band, Test Report v.03.steinbeis -Transfer Centre, Embedded Design and Networking, University of Cooperative Education, Loerra, September 2004 [7]. ZigBee Wi-Fi Coexistence White paper and Test Report, Schneider Electric Innovation Department 11
7. Revision History Doc. Rev. Date Comments 42190A 09/2013 Initial document release 12
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