"The recent development is largely focused on the Internet of Things (IoT) connected to the data using sensors. The final product can wear electronic equipment from smartphones, health and fitness class (Figure 1) and family Automation, to smart meters and industrial controls. These products have ultra low power, low cost and small volume design constraints.
This topic will discuss and compare several main options for low-power wireless technology. It will discuss the basic principles of each technology and its key operating properties, such as frequency bands, network topology support, throughput, range, and coexistence. In addition, this topic also includes examples of solutions.
Low power
Engineers now have many options in low-power wireless technology, including low-power Bluetooth, Ant, ZigBee, RF4CE, NFC, Nike + and Wi-Fi, etc. Based radio technology, and infrared technologies advocated by Infrared Data Association (IRDA) .
However, this extensive choice increases the difficulty of the selection process. Each technique is weighed between power consumption, bandwidth and range. Some technologies are based on open standards, while other technologies remain exclusive. What makes the situation more complicated, in order to meet the needs of the Internet of Things, the new wireless interface and protocols continue to emerge. One of them is low-power Bluetooth.
Low Power Bluetooth
Low power Bluetooth begins with a Nokia Research Center, a project called WiBree. In 2007, the technology was used in the Bluetooth Technology Alliance (SIG). In 2010, SIG introduced this technology when introducing Bluetooth 4.0 (V4.0), as an ultra-low power of Bluetooth technology.
This technology extends the Bluetooth ecosystem to a smaller battery capacity such as wearable electronic equipment. It uses microan average current in target applications, complementary "traditional" Bluetooth commonly used in smartphones, audio headsets, and wireless desktop computers.
The technology operates in the 2.4 GHz industry, research and medical (ISM) bands, suitable for transmitting data from compact wireless sensors or other peripherals that can use fully asynchronous communication. These devices rarely send a small amount of data (i.e., a few bytes). Its duty cycle ranges from once a minute to once a minute, or longer.
Starting from Bluetooth V4.0, the Bluetooth kernel specification defines two chip types: one is a low-power Bluetooth chip; another Bluetooth chip uses a modified stack, plus the basic rate of integration previous versions (B) / enhancement Type Data Rate (EDR) Physical Layer (PHY) and low power (LE) PHY ("Br / EDR + LE") makes it interoperable with all of the standards and chip variants. Low-power Bluetooth chips can be interoperable with other low-power Bluetooth chips and Bluetooth chip that conforms to Bluetooth V4.0 or higher.
In many consumer applications, low-power Bluetooth chips can work with Bluetooth chips, but due to enhanced standards in 4.1, 4.2 and 5, low-power Bluetooth chips are increasingly used as a separate device.
Recently launched Bluetooth 5 Specifications will increase the original data rate of low-power blue to 2 Mb / s, and provide 4 times higher than previous versions. Please note that the maximum throughput and the maximum range cannot be implemented at the same time, which is a traditional trade-off. The Bluetooth SIG recently used Bluetooth Mesh 1.0 for supporting the technology in the mesh network topology. Part 3 of this series will be described in more detail.
For a comprehensive overview of low-power Bluetooth, see "Low-power Bluetooth SOC and tools of Bluetooth 4.1, 4.2, and 5). The LAN is capable of responding to the Internet of Things Challenges (Part 1)."
What is Ant?
Ant is an ultra-low power wireless protocol that works in the 2.4 GHz ISM band, comparable to low power Bluetooth. It is designed to be designed for a low-power Bluetooth, which is designed for a button battery, and has a battery life of several months. The protocol is released by Dynastream Innovations (a Canadian company, it has incorpted Garmin) in 2004. Dynastream Innovations does not produce chips, but designers can get their firmware from the 2.4 GHz transceiver of Nordic Semiconductor and Texas Instruments (Ti), which produces Nordic Semiconductor and Texas Instruments (TI). However, the company also offers a range of RF modules running Ant protocol and fully testing and certified, these modules have hardly need to design integration work, and have been certified.
Although Ant is a proprietary radio frequency protocol, interoperability can be supported by Ant + management network. Ant + helps to achieve interoperability between Ant + Alliance member devices, but also help to complete the collection, automatic transmission and tracking of sensor data. Interoperability ensures through the device profile; An ANT + device that implements a particular device profile can interoperate with any other Ant + device that implements the same device profile. New products must be interoperable through Ant + authentication tests. This certification is managed by Ant + Alliance.
Ant and Ant + were initially directed at motion and fitness segment, but recently the product has been used in the field of family and industrial automation. The agreement is still growing. Recently announced the launch of Ant Blaze is an Internet of Thunder applications for enterprise-oriented networking. (See Section 3.)
How about ZigBee?
ZigBee is a low-power wireless specification that uses PHY and Media Access Control (MAC) based on IEEE 802.15.4. In addition, it runs the protocol controlled by the Zigbee Alliance. This technology is designed for network networks in the field of industrial and home automation (which makes it ahead of some competitive techniques).
ZigBee's operating band is 2.4 GHz ISM band and 784 MHz (in China), 868 MHz (in Europe) and 915 MHz (in the United States and Australia). The data rate varies between 20 kb / s (868 MHz band) to 250 kb / s (2.4 GHz band). ZigBee uses a 2 MHz channel having a 6 MHz of 5 MHz, so spectrum efficiency is reduced due to unused assignments.
ZigBee PRO is released in 2007, providing more features required for steady deployment, including higher security. ZigBee Alliance has just announced the launch of the Zigbee Pro 2017, which is a network network that can simultaneously operate in the frequency band of 2.4 GHz and 800 - 900 MHz ISM. (For more information, see Part 3 of this series of articles.)
Does RF4CE meet all conditions?
Consumer Electronic RF (RF4CE) is based on ZigBee, but protocols customized for RF remote control requirements. In 2009, the following four consumer electronics were standardized by RF4CE: Sony, Philips, Panasonic and Samsung. This technology has been supported by multiple chip vendors, including Microchip, Silicon Labs, and Texas Instruments. The intended use of RF4CE is used as a device remote control system, such as for a television set top box. This technique uses radiofrequency to overcome the interoperability of infrared (IR) remote control, aiming line, and functional limited defects.
Recently, RF4CE faces the fierce competition from low-power bluebearing and ZigBee for remote control applications.
What is Wi-Fi?
Wi-Fi based on IEEE 802.11 is a very efficient wireless technology; however, the technology needle optimizes only the use of high-speed throughput transmission, and does not optimize for low power consumption. Therefore, Wi-Fi is not suitable for low power consumption (button batteries). In recent years, this technology has achieved some improvements in reducing power consumption, including the IEEE standard 802.11V (specifying the configuration of the client device in connection to the wireless network) and other revisions.
IEEE 802.11AH (Wi-Fi "Halow") is released in 2017, working on the 90 MHz ISM band, which can achieve lower power consumption and wider compared to Wi-Fi versions of the 2.4 GHz and 5 GHz bands. Range. (See Section 3.)
NIKE + can be used as a choice?
Nike + is a special wireless technology developed for sportswear manufacturers Nike for fitness market. It is mainly used to connect the NIKE "pedometer" integrated 2.4 GHz radio chip and the Apple mobile device responsible for analyzing and providing the collected data. Since the new generation of smart phones use the same technology, Nike + hardware is still favored by a group of fitness enthusiasts, but has begun to decline. Nike has given the product of its wireless aerobic band, and it will focus on the smartphone software application.
The NIKE + system based on special wireless technology is still used in wireless mouse and keyboards. If there is no interoperability requirement, similar techniques (such as Nordic Semiconductor's NRF24LE1) can indeed provide performance that can be comparable to low-power blue tooth without satisfying standard compliance.
Do Irda resolved short-distance communication issues?
The Infrared Data Association (IRDA) consists of approximately 50 companies and has issued multiple infrared communication protocols in the name of IRDA. IRDA is not radio frequency based technology, but uses a modulated pulse of infrared light to transmit information. The main advantage of this technology is that the built-in security (because it is not a radio frequency), very low bit error rate (BER) (which can improve efficiency), no need to regulate compliance certification and low cost. This technology also has a high-speed version that provides a transmission rate of 1 Gb / s.
The disadvantage of infrared technologies is that the range is limited (especially high-speed version), with "sight" requirements and lack of two-way communications in standard implementation. Irda is not particularly energy saving (for each bit) compared to radio technology. For the basic remote application of cost-off design parameters, IRDA can maintain market share, but designers typically specify low power Bluetooth and RF4CE in the case of improving control functions (such as control functions required for smart TV).
NFC's applicability
Near Field Communication (NFC) works in the 13.56 MHz ISM band. At this low frequency, the transmitting and receiving annular antennas are mainly used as the primary and secondary windings of the transformer, respectively. Data transfer is done by a magnetic field instead of accompanying electric field because the latter does not dominate within a short distance. NFC transmits data at rate up to 424 kb / s. As the name suggests, it is suitable for a very short distance communication with a maximum operating range of 10 cm. Because of this limitations, it cannot compete with low-power bluebear, ZigBee, Wi-Fi, and similar technology. NXP USA and other manufacturers provide chips such as CLRC66303 NFC transceivers.
Its key advantage is that "passive" NFC devices (such as payment cards) do not require a power supply, only in the near range of the supply NFC device will become active devices. NFC has been widely used in non-touchless payment techniques, and is used as a method of pairing other wireless technologies such as low-power Bluetooth devices, without having the "intermediary" attack security risk. NFC may obtain a larger market share as a niche application technology that adds other wireless technologies described herein.
Network topology
Low power wireless technology supports up to five main network topologies:
Broadcast: Sends messages from the transmitter to any receiver within the coverage. The channel is unidirectional and does not confirm that the message has been received.
End-peer: The two transceivers are connected to the two-way channel, and the message can be confirmed by the channel and two-way transmission data.
Star: The central transceiver communicates with multiple peripheral transceivers via a two-way channel. The peripheral transceiver cannot communicate directly with each other.
Scan: The central scanning device holds the reception mode, waiting to receive signals emitted from any transmitter from the coverage. Communication is unidirectional.
Network: You can transfer messages from a point from the network to any other points across two-way channels connecting multiple nodes (normally using other features such as hubs and repeaters).
The A, B, C, D, and E in Figure 2 show these network topologies, and Table 1 summarizes the topology of each of the above wireless technical support.
Low power wireless technology performance
scope
It is generally believed that the range of wireless technology is proportional to the result of the transmitter of the transmitter ("link budget") and the transmitter ("link budget"). The improvement of power transmission and sensitivity can effectively improve the signal-to-noise ratio (SNR), so the coverage can be expanded. The SNR can measure the ability of the receiver to properly extract and decode the signal from the environmental noise. In the threshold SNR, BER will exceed the radio specification, causing communication to fail. For example, according to the design of the low-power Bluetooth receiver, the maximum BER tolerance is only about 0.1%.
The maximum power output of the excess 2.4 GHz ISM band is limited by regulatory agencies. Relevant regulations are usually very complicated, but basically specified: the frequency of frequency hopping frequency is less than 75 but at least up to 15 frequency hopping systems, and the peak transmit power measured at its antenna input must be limited to +21 DBM peak; If the full-to-the antenna gain is greater than 6 dBi, the output must also be reduced. In this way, the maximum equivalent all-to-directional radiation power (EIRP) is +27 dBm.
In addition to this specification, low-power wireless technology also includes specification restrictions on transmission power to maximize battery life. The time to limit the radio at high power transmit or receive state can save a large amount of electrical energy, but the RF chip manufacturer can also save energy by +4 DBM by powering the low-power blue tooth maximum transmit power (this is usually, sometimes defining +8 DBM is much lower than the +21 DBM limit specified by the relevant specification).
However, transmit power and sensitivity are not the only factor in the range of wireless devices. Work environment (such as whether there is a ceilingAnd walls), frequency, design layout, mechanical construction, and coding schemes affect the range. The scope is usually for the "ideal" environment, but equipment often uses the environment that is seriously affected. For example, the 2.4 GHz signal encounters a serious attenuation of the human body, so wearable electronic device wearing on the wrist may be difficult to transmit the signal on a smartphone mounted in the backpack, even if they may only be around one meter.
This list shows a typical range that ultra-low power technology can be implemented in unobstructed environments without other radio frequency or light source interference:
NFC: 10 cm
High Speed Irda: 10 cm
NIKE +: 10 m
Ant (+): 30 m
5 GHz Wi-Fi: 50 m
ZigBee / RF4CE: 100 m
Low Power Bluetooth: 100 m
2.4 GHz Wi-Fi: 150 m
Low-power Bluetooth using Bluetooth 5 extended range function: 200 to 400 m (depending on the forward error correction coding scheme)
Throughput
Transfer by low power wireless technology includes two parts: Bit (such as packet ID and length, channel, and check, collectible) and delivery information (called "payload"). The ratio of the sum of the payload and overhead adds payload determines the efficiency of the protocol (Figure 3).
"Original" data rate (overhead plus payload) can measure the number of bits per second, which is usually referred to in marketing materials. The payload data rate is always lower than the number. (Part 2 of this series of articles will be described in detail and its efficiency of each protocol and its follow-up effect on battery life.)
Low-power wireless technology typically requires a small amount of sensor information to periodically transfer between sensor nodes and central devices while minimizing power consumption, so bandwidth is usually moderate.
The following list compares the raw data and payload throughput described herein. (Note that these data is the theoretical maximum, and the actual throughput depends on configuration and working conditions):
NIKE +: 2 MB / S, 272 BITS / S (intentionally limit the throughput to one packet per second)
ANT +: 20 kb / s (in burst mode - see below), 10 kb / s
NFC: 424 Kb / s, 106 kb / s
ZigBee - 250 kb / s (under 2.4 GHz), 200 kb / s
RF4CE (same as ZigBee)
Low Power Bluetooth - 1 MB / S, 305 KB / S
High-speed Irda - raw data 1 GB / S, payload 500 kb / s
Low-power Bluetooth with Bluetooth 5 high throughput: 2 Mb / s, 1.4 Mb / s
Wi-Fi: 11 MB / S (the lowest power consumption 802.11b mode), 6 Mb / s
Delay
The latency of the wireless system can be defined as the time between the signal transmits to the received. Although the delay is usually only a few milliseconds, it is an important consideration factor for wireless applications. For example, the low delay is not that important for the application that automatically polls data (possibly per second), but for users who expect users like remote to detect latency between pressing button to subsequent operations Application, low delays may become very important.
The following list compares the delay of the techniques described herein. (Similar note, these values depends on configuration and working conditions.)
Ant: negligible
Wi-fi: 1.5 milliseconds (MS)
Low Power Bluetooth: 2.5 ms
ZigBee: 20 ms
Irda: 25 ms
NFC: Usually polls once per second (but can be specified by the product manufacturer)
NIKE +: 1 second
Note that the low delay of Ant and Wi-Fi needs to receive the device to continue to listen, which will quickly consume battery power. For low power sensor applications, the battery consumption can be improved by extending the ANT messaging period, but the cost is delayed.
Stability and coexistence
Reliable packet transmission has a direct impact on battery life and user experience. In general, if the data package is poor due to the transmission environment, accidental interference from nearby radios or intentional frequency interference cannot be delivered, the transmitter will always try until the packet is successfully delivered. This behavior needs to sacrifice battery life as the cost. In addition, if the wireless system is limited to a single transport channel, its reliability will inevitably decline in a crowded environment.
A radio works in the presence of other radios, is called coexistence. This situation is particularly interesting when the radio is not working in the same device. For example, low-power blue and Wi-Fi in a smartphone. One standard method that achieves coexistence between Bluetooth and Wi-Fi is to use an external signal transmission scheme, which includes a wired connection between each integrated circuit (IC) to freely transmit or receive at each IC. Coordination is carried out. In this article, passive coexisting is an interference avoidance system, and active coexistence is a signal transmission between the chip.
There is a verified method to help achieve passive coexisting, that is, channel frequency hopping. Low-power Bluetooth uses a frequency hop spreadout (FHSS), and is hopped in pseudo-random mode between 37 data channels, thereby avoiding interference. Low-power Bluetooth so-called adaptive frequency hopping (AFH) enables each node to map frequent congestion channels and then avoid these channels in future transactions. The latest version of this specification (Bluetooth 5) introduces an improved channel serialization algorithm (CSA # 2) for increasing the next hopping channel sequenced pseudo-random, thereby improving anti-interference ability.
ANT supports multiple RF operating frequencies (1 MHz). Once selected, all communications will be performed on one frequency and only channel frequency hopping only occurs when the selected frequency is significantly attenuated.
To mitigate congestion, Ant use time domain multiplex (TDMA) adaptive synchronization scheme, divide a time slot of approximately 7 ms per 1 MHz band. These time slots are repeated according to the ANT messaging period (e.g., each 250 ms or 4 Hz), and the pairing device on the channel communicates during these slots. In practical applications, a 1 MHz band can accommodate dozens or even hundreds of nodes without conflicts. In the case of data integrity, Ant can use "burst" message transfer technology; this is a multi-message transmission technology that uses all available bandwidth and runs to all data transfer.
Some ANT RF channels (such as 2.450 and 2.457 GHz) are allocated and managed by Ant + Alliance for maintaining network integrity and interoperability. The Alliance recommends avoiding using these channels during normal operation.
ZigBee (and RF4CE) uses a direct sequence spread spectrum (DSSS) method compared to the low-power Bluetooth FHSS technology and the TDMA scheme of Ant. During DSSS, the signal is mixed with a pseudo-random code on the transmitter and the signal is extracted on the receiver. This technique effectively improves the signal-to-noise ratio by transmitting a transmitted signal on a broadband (Fig. 4). ZigBee Pro implements another technique called frequency, and the network node scans the clear spectrum and informs the coordinator to use the channel throughout the network. However, this function is deployed in actual operation.
Wi-Fi uses 11 20 MHz channels in the United States, 13 channels in most parts of the world, using 14 channels in Japan. Therefore, there is only three non-overlapping Wi-Fi channels (1, 6, and 11) to provide sufficient space within the 83 MHz width of the 2.45 GHz spectrum. These channels are also used as the default channel. They do not include automatic channel frequency hopping, but if there is an interference problem in the work, the user can switch to another channel manually.
In selected channels, Wi-Fi interference avoidance mechanism is complicated, but basically combines DSSS to combine orthogonal frequency division multiplexing (OFDM). OFDM is a transmission form using many close-range carriers with low rate modulation. Since the signal is orthogonal transmission, the possibility of near-distance mutual interference is greatly reduced.
5 GHz Wi-Fi works in the range of 725 MHz width allocation to allocate more non-overlapping channels. Therefore, compared with 2.4 GHz Wi-Fi, there is significant reduction in chances of interference issues.
Wi-Fi also uses active coexisting techniques, as well as mechanisms that reduce data rates when detecting interference from other radios.
This is the ubiquitous Wi-Fi. Other 2.4 GHz technologies include techniques to avoid conflicts with default Wi-Fi channels (1, 6, and 11). For example, three advertising channels of low-power Bluetooth are located in the gap between the default Wi-Fi channel (Figure 5).
NIKE + uses a proprietary frequency spindle scheme to switch the passage when interference becomes destructive. Due to the minimum data transmission rate and duty ratio of this technology, it is necessary to do so.
IRDA does not implement any form of coexisting techniques. However, as an optical technology, it may only be affected by the bright background lamps containing important infrared components. Because of the short operational distance and aiming line requirement, it is less likely to interfere with each other.
NFC implements some form of coexisting technology, and the reader selects a specific card from the wallet containing multiple NFC cards. Due to the short transmission distance, the interference level between other NFC devices and / or other radios is rare. However, it is worth noting that the 13.56 MHz band in the FM (FM) band has harmonics and is particularly strong in the harmonics of 81.3 and 94.9 MHz. These harmonics may result in clicker noise in the FM receiver in the same way. Anti-conflict technique can be implemented (eg, "offset" or cleaning) can reduce the FM interference effect.
Summarize
There are currently many commonly used low-power wireless technology. Although each technique is designed for battery power and relatively moderate data transmission, they have different ranges, throughput, robustness and coexistence. These different properties are suitable for different applications - even with a large extent overlap. "
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