Not a wireless engineer? Got a C in your college class on Maxwell's equations? And you want to add wireless functionality to your next project? Don't worry about it— it's easier than you think.
Wireless can be a great choice to give a project cache or distinguish it from the competition. Just follow a few key guidelines, and you can graduate to the level of wireless engineer.
For the sake of reference, "short-range wireless" here doesn't refer to cell phones or WiMAX. But it does include all of the technologies you've heard of—Bluetooth, IrDA, ISM, Wi-Fi, and ZigBee— and a few specialized designs worthy of mention. Ultra-Wideband, another option, is covered in a separate article in this issue (see "UWB Seizes The USB Terrain... And That's Just The Start"). All of these technologies are authorized under the FCC's Part 15 unlicensed rules and regulations. Go to www.fcc.gov for a copy of Part 15 of the U.S. Code of Federal Regulations (CFR) Title 47— Telecommunications.
This document should become your bible for future projects. Specifically, look at section 15.231, which covers the 260- to 470MHz band, and section 15.249, which covers the popular 902- to 928-MHz band. Parts 90 and 95, which are separate from Part 15 in the Code of Federal Regulations, may be more appropriate for your application.
Similar regulations exist in other countries. For instance, the European Telecommunications Standards Institute (ETSI) guidelines are designated EN 300 220, while Japan has the ARIB T67 regulations.
THE DESIGN PROCESS
STEP 1—SPECIFY THE APP AND CHARACTERISTICS: As in any design adventure, step one in your wireless design is to specify the application and define the parameters. What exactly do you want to do? Write it out.
Telemetry, which includes home/building environmental monitoring, automatic meter reading, medical, automotive (temperature, tire pressure), and industrial sensor monitoring, is one of the more popular applications. Remote control for garage doors, toys, remote keyless entry, HVAC, security/alarms, and industrial control is another favorite.
STEP 2—SPECIFY KEY CHARACTERISTICS: It's critical that you specify the project's important characteristics, such as its desired range; environment; power consumption limits; whether it's fixed or portable; the modulating signal (analog or digital?); the need for transceivers instead of just a transmitter or receiver; inputs; outputs; interfaces; and other relevant specifications. Some measure of performance, like bit error rate or reliability, should be included if applicable as well. Among the key project specs to be keenly aware of are:
• Range: Short-range wireless technology covers a wide scope of distances, from a few inches to many miles. You need to pinpoint this or at least get a range of ranges as closely as possible, since the technology you select depends on knowing such information.
• Environment: Is it indoors or outdoors? Will you have good line of sight between transmitter and receiver, or will it be through walls, floors, or trees? Is the environment loaded with noise from electrical systems and devices like motors, or are other wireless devices being operated nearby? As a general rule, the higher the frequency, the shorter the range.
• Link: Is the communications point-to-point (P2P), point-to-multipoint (P2M), or multipoint-to-point (M2P)? A single P2P link is the easiest to work with, of course. But your application may require multiple monitoring points, as in telemetry, so M2P is what you need. Or, if you must control multiple devices from a single location, you need P2M. Also, consider whether the link is simplex (one-way, broadcast) or duplex (two-way). And is that half or full duplex?
• Type of information: Will your source of information be analog or digital? These days, most sources will be digital codes or data, but you can handle analog if it's essential. You will need analog-to-digital conversion and digital-to-analog conversion. Under section 15.231 in the 260- to 470-MHz band, you can only transmit short data bursts. Also, no voice or video is permitted. But you can carry voice, video, or any other intelligence in the 902- to 928-MHz band in either analog or digitized form.
• Data rate: What is the maximum data rate necessary? Most telemetry and control are very low speed, less than 100 kbits/s. But you can accommodate rates up to several hundred megabits per second.
• Network: Will your project be a simple P2P link or part of a network? Networking assumes you will be talking to a host or one or more other nodes. Will you be needing a mesh network?
• Security: Does your wireless link involve critical data that needs to be secure? These requirements will affect your choice of protocol and technology.
• Protocol: Do you need to comply with a specific protocol or standard, like Bluetooth, Wi-Fi, or ZigBee? Or can you devise your own protocol to fit the application?
• Interfaces: What data interfaces do you need (RS-232, SPI, USB, etc.)? In simple applications, you may only require a bus line on the microcontroller.
• Power consumption: If your device is to be remote or portable, battery life is a critical factor. You will need to choose chips or modules and protocols with sleep modes and low duty cycle operation to eke out maximum life.
STEP 3—PICK A TECHNOLOGY: Based on your list of the above specifications, use the table to choose a technology. You may find that only one fits, or that two or more can be used. And while the table will help you zero in on a specific choice, there are a few additional factors to consider in your selection.
Bluetooth is by far the most widely used wireless standard. It beats out Wi-Fi by a factor of 10 or more. Bluetooth's success is primarily tied to cell-phone and cell-phone headset usage, but it's also found in various other applications (e.g., computer peripherals). On top of that, it's the number-one audio wireless technology for remote headphones and speakers and other audio uses.
Bluetooth offers an extended range of speed options. The basic data rate is 1 Mbit/s, but an enhanced data-rate version (EDR) at 3 Mbits/s is available. An UltraWideband version with a data rate to 480 Mbits/s will arrive after 2008 for video and other very high data-rate needs.
Finally, Bluetooth is capable of basic networking. It can talk to up to seven other Bluetooth nodes in what's called a piconet. In turn, these nodes can talk to one another in more extended scatternets.
But while Bluetooth has great potential in many applications, it doesn't suit everything. Due to the complexity of its protocol and the related stack, it's overkill for some simpler applications. Yet for some predefined applications like audio, its formalized profiles and certifications are second to none.
IR wireless, like Bluetooth, has a broader scope than you may think. Virtually every remote control in the world uses IR. It works well and is very low in cost. The major problem is a more limited range. It also needs an unobstructed line-of-sight path. Ideal range is about 1 m within a ±15° cone, but longer range is possible. The IrDA standard provides data rates to 16 Mbits/s, and the modules are very inexpensive.
The ISM band is one of the easiest standards to use. ISM best suits ultra-simple control or monitor applications. Its data rate rarely exceeds 100 kbits/s, and it's typically much less than that. There's a wide choice of frequencies, but most applications employ the 315-, 433.92-, and 902- to 938-MHz (915 MHz is popular) and 2.4-GHz bands.
There's no formal protocol. So if you plan to use these simple, low-cost devices, you'll need to devise your own. Analog Devices' line of ADF70xx ISM chips includes a software package that helps you apply the company's chips and put together a simple protocol to solve your problem.
Cypress Semiconductor's WirelessUSB line uses direct-sequence spread-spectrum (DSSS) in the 2.4-GHz band to provide low-speed wireless to human interface devices (HID) like keyboards and mice. Its basic data rate is only 62.5 kbits/s, but a 1-Mbit/s version is available as well. It's also good for multipoint-to-point applications. A simple protocol is available, leaving you free to focus on other aspects of the design.
Now about a decade old, Wi-Fi has seen continual improvement over the years. It's mainly found in wireless local-area networks (LANs). Occasionally, you'll see it in short-range monitor and control applications.
Wi-Fi is a complex standard, but it provides extended range to 100 m and even more with a power amplifier and directional antenna. Data rates range from 11 Mbits/s to well over 100 Mbits/s with the new 802.11n standard, which is overkill for most short-range applications. Power consumption is proportionally higher.
Ultra-Wideband (UWB) is another technology for very high speeds. It can accommodate data rates from 53 to 480 Mbits/s, but over a distance of less than 10 m. Its standard has focused on implementing a wireless version of the ubiquitous USB interface standard for computer peripherals and other devices. Other potential applications include video.
ZigBee is designed for short-range monitoring and control. It features the lowest power consumption of any of these technologies. In its simplest form based on the IEEE 802.15.4 standard, it's good for P2P, M2P, or P2M applications. By adding the ZigBee Alliance stack, mesh networking becomes possible, thereby extending the range and reliability of any node. Lots of chips and modules are available. Just be aware that if you use the ZigBee stack, you'll face not only the certification standards, but royalty payments as well.
If you're operating in a noisy environment, select a modulation method that helps mitigate the noise. Any FSK-related (frequency shift keying) modulation is good. DSSS is even better with its BPSKbased (binary phase-shift keying) modulation, but it's more complex and more expensive. Amplitude shift keying/on-off keying (ASK/OOK) is the simplest method by far, but it requires shorter ranges and lower noise environments for best performance.
Your application also may affect the frequency of operation. According to the Friis equation for free space power, the lower the frequency (greater the wavelength), the greater the overall range for a given power. This means using the lower unlicensed bands in the 260- to 470-MHz range as opposed to the higher 900-MHz and 2.4-GHz bands if maximum range is critical. On the other hand, the lower the frequency, the greater the antenna size. That may turn into a real problem for some portable or mobile applications. This is a key tradeoff to consider.
Finally, one critical issue that may affect your choice is security. Bluetooth, IrDA, and ISM band don't provide it, but it's available with UWB, Wi-Fi, and ZigBee.
STEP 4—MAKE OR BUY?: In this step, you must conclude what's the best course of action—do you make it or buy it? If your project requires an embedded approach with minimum cost, you'll probably select your chips and design your own solution. If the application permits, you can finish your design faster by incorporating an existing module.
The modules have everything you need for the wireless connection, including the antenna in some cases. They cost a bit more, and modules usually will be larger and more expensive. However, they're an excellent option for some applications with low volume and flexible size/cost requirements. The less you know about wireless, the better off you'll be with the module approach.
STEP 5—THE ANTENNA: Don't forget the antenna. It's a critical mechanical part of all wireless devices. In fact, factor the antenna in from the beginning, because an application's success depends on it.
STEP 6—BECOME FCC FRIENDLY: Plan for Federal Communications Commission testing. If you've designed the wireless device, you'll need to secure FCC approval to use and sell it. The FCC requires verification that your product meets the emission limits established for your category of device. Furthermore, all intentional radiators must have full FCC certification.
While you can test the device yourself, most companies subcontract that responsibility to one of the many organizations established for this purpose. A Yahoo or Google search under "FCC testing" will bring up dozens. Be sure to budget this service into your plans.
ESTIMATING SIGNAL STRENGTH AND PATH LOSS
You can use some basic formulas to calculate your initial estimates of range, power, and other link characteristics. The basic equation is:
Pr = (PtGtGrλ2) / (16π2d2)
where Pr is the power received; Pt is the transmit power; Gt is the transmit antenna power gain; Gr is the receive antenna power gain; d is the distance between transmitter and receiver in meters; and is the wavelength in meters where = 300/fMHz. There are two key factors to recognize:
• The received power is a function of the square of the wavelength. Thus, the lower the frequency, the greater the received power. Higher frequencies are usually beneficial because the antennas are much smaller. But the range is less for a given power.
• Received power is a function of the square of the distance between transmitter and receiver. Your design goal is to balance range against power and frequency.
This equation assumes a clear line-of-sight (LOS) path between transmit and receive antennas, so it doesn't account for the penetration of walls, trees, or other obstacles. Also, the equation is valid only if the separation between the transmit and receive antennas is sufficient to be in the far field.
All electromagnetic waves have a near field and a far field. The near field is mostly the magnetic field, so the transmit and receive antennas act more like transformer primary and secondary windings. The far field is the real combined electromagnetic fields or radio wave. It's approximately a distance greater than D2/λ, where D is the largest dimension of the antenna (typically one half-wavelength at the operating frequency, or 468/fMHz). For the best estimates, assume that the far field is greater than 10 wavelengths from the antenna to be safe.
The antenna gains in the formula are referenced to an isotropic (spherical) source. This is a gain of 1. Most practical antennas, such as a half-wave dipole or a quarter-wave ground plane, are directional. As a result, they represent a power gain of 1.64 or 2.15 dB.
The key to using this formula is to estimate the path loss in dB. This is the attenuation of the path due to the distance between the transmit and receive antennas. You can estimate path loss with the expression:
dB loss = 37 dB + 20log(fMHz) + 20log(d)
Here, d is the distance or range in miles, where 1 mile is about 1610 meters. Knowing path loss and transmit power for a given set of antenna gains, you can determine the necessary receiver sensitivity:
Pr = Pt – PL
Assuming a path loss of 90 dB and a transmit power of 10 dBm (10 mW), the required receiver sensitivity is:
Pr = 10 – 90 = –80 dBm
Receiver sensitivity is the key to longer range and greater link reliability for a given transmit power and antenna gains. Look to get the most receiver sensitivity as is possible. Some of the newer designs can achieve receiver sensitivities up to –120 to –130 dBm.
Dozens of great chips and modules exist for almost any application. Newer products include ADI's recently introduced ISM band chips and some ZigBee products from Microchip Technology.
Analog Devices' ADF70xx series transceivers operate over the 50-MHz to 1-GHz range. Most versions use FSK or Gaussian FSK (GFSK) with data rates in the 20to 384-kbit/s range. Power output is adjustable from –20 or –16 dBm to +10 or +13 dBm. Receiver sensitivity reaches –125 dBm.
Several versions also accommodate ASK and OOK modulation. Others permit 2FSK, 3FSK, or 4FSK operation so more bits per symbol can achieve higher data rates in narrower channels. Most modules include Gaussian filtering of the data, which helps narrow the transmitted bandwidth and helps to ensure meeting adjacent-channel-power (ACP) specifications.
Analog Devices' SRD (short-range device) Design Studio software assists in wireless link design and simulation using the ADF70xx chips (Fig. 1). With this package, you can quickly and efficiently develop real-time simulations, test various configurations, and troubleshoot potential problems.
The software lets you choose one of three simulation modes: frequency domain, transient analysis, and spectrum analyzer. Also, you can experiment with frequency band, power consumption, sync detection, link analysis, modulation alternatives, data rates, loop filters, and other parameters to optimize your design. This free software can be downloaded at www.analog.com/srddesign
Microchip Technology's MRF24J40 IEEE 802.15.4 2.4-GHz DSSS radio transceiver targets ZigBee applications (Fig. 2). It can be used alone or with the ZigBee Alliance stack. Also, the company's proprietary MiWi protocol is like ZigBee mesh lite. It's a simpler protocol that's applicable in mesh applications using less than 1000 nodes and no more than four hops between nodes. If your requirements exceed those limits, go with ZigBee.
But many applications aren't that large. If the basic 802.15.4 characteristics don't quite cut it, MiWi may be a good alternative. With a single crystal and one of Microchip's popular PIC microcontrollers, you can have a certifiable mesh radio network in short order. In addition, the company's ZENA wireless network analyzer tool is available to simplify the process of configuring the ZigBee and MiWi protocol stacks. It helps reduce code size and optimize your design.
And, check out AMI Semiconductor's ISM band chips. The AMIS-53050 targets industrial applications of less than 1 GHz with FSK/OOK. The AMIS-52150 fits industrial applications using ASK/OOK in the 402- to 405-MHz band. The AMIS52100 operates in the 401- to 406-MHz medical band and suits implantable applications.
ISM BAND CHIP VENDORS
ZIGBEE CHIP VENDORS
BLUETOOTH CHIP VENDORS
• Cambridge Silicon Radio
• Texas Instruments
• Avago Technologies (IrDA)
• Cypress Semiconductor
• Abacom Technologies
• California Eastern Technologies
• Cirronet Inc.
• Digi International
• Lemos International Co. Inc.
• Linx Technologies Inc.
• MaxStream Inc.
• Radiometrix Inc.
WI-FI CHIP VENDORS
• AirGo (Qualcomm)
• Atheros Communications
• Intel Corp.