"Designers of Internet of things (IOT) products now tend to use WiFi based wireless connection because it is widely deployed and easy to understand. However, any type of RF function is complex and requires compliance testing. Without the corresponding expertise, the development speed may slow down, especially if the designer chooses to design the RF part from scratch.
One way to speed up the design process is to choose from many available pre certification modules. Therefore, this paper will discuss the advantages of WiFi in wireless applications before introducing how to use modules and related design tools to design products.
Why WiFi?
WiFi is one of many popular short-range RF technologies for wireless communication using 2.4 GHz industrial, scientific and medical (ISM) unlicensed spectrum allocation. This technology is based on IEEE 802.11 specification, and its different variants have different throughput and a variety of digital coding methods.
Compared with low-power Bluetooth (Bluetooth LE) and ZigBee, it is relatively power consuming, expensive and requires considerable processor resources. However, its speed is also amazing. From the lowest version 802.11b with an original data rate of 11 MB / s to the impressive 600 MB / s version n, no other open standard 2.4 GHz technology can match it（ See digi key article "comparison of low power wireless technologies".)
Which WiFi to choose?
One thing WiFi variants have in common is that all WiFi operating specifications are developed by the WiFi alliance. As the management organization of WiFi brand and specification, the alliance determines the data structure, encryption technology, frequency, packet configuration and sub protocol used by WiFi local area network (LAN).
Importantly, WiFi can also use 5 GHz spectrum allocation to further improve throughput and reduce potential interference by avoiding communication in the crowded 2.4 GHz band. The disadvantage is that the transmission range will be reduced and the obstacle penetration is poor.
There are several WiFi protocols: IEEE 802.11b/g works in the 2.4 GHz band, IEEE 802.11a/ac works in the 5 GHz band, and IEEE 802.11n radio can work in the above two bands.
IEEE 802.11b was adopted in 1999 and provides data rates of 5.5 and 11 MB / s. now it is generally only used in traditional systems. However, the modern N-version radio has built-in support for version b, so that the modern system can be used with the traditional system.
IEEE 802.11g was adopted in 2003 and uses modulation techniques different from the original protocol to achieve data rates of up to 54 MB / s. In practical applications, due to the forward error correction algorithm, the available data rate is usually halved. Version G is backward compatible with version B.
IEEE 802.11n was adopted in 2009 and introduced multiple input multiple output (MIMO) antenna technology, which can encode multiple synchronous "spatial streams" and increase the data rate to 216 MB / S (assuming that the channel width is 20 MHz and the transmitter adopts three spatial streams). 802.11n also specifies a wider channel of 40 MHz by connecting two 20 MHz channels, increasing the throughput to 450 MB / s. Devices that support three spatial streams are limited to higher end laptops, tablets, and access points (APS). There are more devices supporting two spatial streams, but they are still limited to laptops, tablets and the latest generation of smartphones.
IEEE 802.11a is the same as version g in most respects except that it operates in the 5 GHz band. The maximum data rate is 54 MB / s. At present, it is generally considered that 802.11a is a traditional protocol.
IEEE 802.11ac was adopted in 2013 to provide eight spatial streams and channel widths up to 160 MHz to further improve throughput. Commercial products have just entered the market and are still very expensive. At least in the initial stage, this technology may only be used for very high-end consumer products.
The 2.4 GHz band is allowed to be allocated to 11 (the United States), 13 (most of the rest of the world) and 14 (Japan) 20 MHz channels. The 83 MHz bandwidth supports only three non overlapping WiFi channels (1, 6 and 11) (Figure 1).
In order to avoid conflicts caused by adjacent WLAN using any of 11 to 14 channels, manufacturers usually design their equipment to communicate in non overlapping channels. For example, WiFi radio waves with excessive interference in channel 1 can be switched to channel 6 or 11 to find an interference free environment.
To facilitate spectrum sharing, WiFi includes a contention mechanism to fairly allocate bandwidth to access points (APS) using the same channel. AP communication time running on congested channels is limited, and the time when data can be received or sent will be affected.
WiFi for Internet of things
It should be noted that WiFi Based on IEEE 802.11 specification only defines the physical layer (PHY) and data link layer of the communication protocol. The data link layer includes media access control (MAC) and logical link control (LLC). However, Internet WiFi connection is everywhere, and its phy and data link layer are usually integrated into a complete TCP / IP protocol stack. The protocol stack ensures Internet interoperability, which is usually (but not always) software provided by WiFi connection solution providers. The rest of this article will discuss WiFi solutions using TCP / IP stack (Figure 2).
WiFi, as a key technology connecting smart phones, portable computers and personal computers to the Internet, has occupied a place. At the same time, it is rapidly diversifying and becoming a basic technology of the Internet of things.
When Internet interoperability and throughput are more important than power consumption, WiFi driven Internet of things devices provide a convincing solution for transmitting information directly from wireless sensors to the Internet. WiFi IOT sensors can directly connect to the Internet without the help of other complex network layers such as IPv6 low-power wireless personal area network (6LoWPAN).
WiFi can be used as a cost-effective "gateway", in which the unit based on multi protocol Bluetooth Le / ZigBee / WiFi system on chip (SOC) gathers data from multiple low-power wireless sensors and forwards this information to the cloud.
It is worth noting that low-power WiFi is emerging. This technology named "halow" is based on IEEE 802.11ah standard. It makes full use of the ultra-low duty cycle used by other low-power wireless technologies to minimize power consumption. Its power consumption is expected to be only about 1% of that of conventional WiFi chips. Hailow works in the 900 MHz ISM band, and its transmission distance is nearly twice that of the current WiFi. However, the technology compromises in terms of throughput, which is said to be roughly equivalent to the maximum raw data rate of 2 Mb / s of Bluetooth le.
Accelerate WiFi Based Design
Designing WiFi IOT solutions from scratch can reduce costs and provide opportunities to fully optimize the performance of wireless products. However, designers need to have considerable professional knowledge of GHz RF hardware, be familiar with TCP / IP protocol, and adhere to the long test and verification process in accordance with the compliance certification specifications of relevant standards.
Some helpful reference designs provided by semiconductor suppliers can be used as a basis for accelerating the development process. However, such schematic diagrams can only be regarded as a starting point; Small changes in the impedance of magnetic components, substrates, tracks and circuits can have a significant impact on performance, and often require multiple design iterations to solve the problem.
A faster way to achieve a satisfactory design is to select a module that has completed assembly, testing, verification and compliance certification. These products can be quickly integrated into WiFi Internet of things solutions to speed up the time to market.
Many chip suppliers provide all variants and related development tools of IEEE 802.11 module for Internet of things applications. The basic module usually integrates WLAN baseband processor and RF transceiver, power amplifier (PA), clock, RF switch, filter, passive components and power management.
Because the WiFi based TCP / IP protocol stack is a complex firmware that is difficult to monitor, microprocessor resources that can support advanced operating systems (OS) such as Linux or Android are needed. Common drivers for operating systems that manage the WiFi stack are available from hardware providers, while other drivers, such as those required by wince and a range of real-time operating systems, are provided through third parties.
Typically, designers need to find suitable microprocessors, passive components for forming matching circuits, and 2.4 and / or 5 GHz antennas. However, some modular solutions contain embedded processors, while others contain complete and effective solutions.
WiFi module for all situations
Silicon Labs' bluegiga brand wf111 is a good example of a low-cost WiFi module designed for Internet of things applications such as point of sale terminals, remote security cameras and medical sensors. The device provides Internet connection via WiFi version b, g or n. The product operates only at 2.4 GHz, with a maximum data rate of 72 MB / s and a link budget of 114 DBM (17 DBM transmitter power output and - 97 DBM receiver sensitivity). Its power supply voltage is 1.7 to 3.6 V, TX peak current is 192 Ma and Rx peak current is 88 ma.
Wf111 contains a built-in antenna (or connector for external antenna), which is specially used with external host microprocessor. The device is controlled by the host microprocessor using a secure digital input / output (SDIO) interface operating in 1-bit or 4-bit mode. The SDIO interface allows the host microprocessor to directly access IEEE 802.11 functions.
Since the chip supplier expects that wf111 will be used in the close range of Bluetooth Le sensor, the product has built-in up to six hardware control lines to manage wireless coexistence. The control line ensures coordinated communication between WiFi and Bluetooth devices to avoid synchronous packet transmission when WiFi is close to Bluetooth Le devices. Such transmissions typically degrade link performance (Figure 3).
The wl1801 of Texas Instruments (TI) further integrates with Bluetooth by integrating IEEE 802.11 a / B / g / N and Bluetooth / Bluetooth Le transceiver into the same device. Due to the built-in interoperability with WiFi and Bluetooth protocols, such modules are an ideal solution for the above Internet of things gateway devices.
The device can operate at 2.4 and 5 GHz WiFi with a maximum data rate of 54 MB / s and a link budget of 115 DBM (18.5 DBM transmitter power output and - 96.5 DBM receiver sensitivity). Its operating voltage range is 2.9 to 4.8 V, TX peak current is 420 Ma, RX peak current is 85 ma. These modules are FCC, IC, ETSI and telec certified.
Wl1801 is equipped with WiFi and Bluetooth stack, but it must be paired with appropriate microprocessor, 32 kHz crystal and antenna to form a complete solution. Ti recommends using its Sitara series microprocessor, such as am3351, which is an arm that can support Linux, Android or real-time operating system, WiFi driver and Bluetooth Le stack ® Cortex ®- A8 kernel device. The microprocessor drives WiFi operation through SDIO interface and Bluetooth through UART (Fig. 4).
Murata's lbee5zz1md module further improves the integration through the built-in processor and pre installed WiFi firmware stack. Although matching the processor with the radio can simplify the process, the disadvantage is that developers are subject to the processor hardware selected by the module manufacturer and may face an unfamiliar development environment.
Murata module provides Internet connection via WiFi version b, g or n. The device operates only at 2.4 GHz, with a maximum data rate of 65 MB / s and a link budget of 100 DBM (2 DBM transmitter power output and - 98 DBM receiver sensitivity). It uses a 3.3 V power supply with TX peak current of 300 Ma and Rx peak current of 45 ma.
The module pairs WiFi MAC / baseband / radio with stm32f412 arm cortex-m4 core microprocessor of STMicroelectronics. The module includes on-board crystal, matching circuit and 2.4 GHz antenna, and 32.786 kHz crystal can be added. Stm32f412 processor includes UART, SPI, I2C and other interfaces (Figure 5).
The module comes with a TCP / IP protocol stack and an electric imp operating system for connecting to the electric imp cloud service. This is very useful for designers who are not yet familiar with third-party cloud service providers and how to upload and access data. The electric imp Development Center website provides development guidance.
U-blox's Nina w132 is an example of how far a modular solution can let designers go. The device integrates WiFi and Bluetooth Le functions, host processor, power management, independent 16 MB flash memory and a 40 MHz crystal.
Internet connection via WiFi 802.11b, g or n. The device operates only at 2.4 GHz, with a maximum raw data rate of 54 MB / s and a link budget of 112 DBM (16 DBM transmitter power output and - 96 DBM receiver sensitivity). It uses a 3.3 V power supply with a TX peak current of 320
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