Both 2.45 and 5 GHz wireless LANs (WLANs) are widely used in consumer applications. However, for industrial and more mission critical applications, there are performance and interference requirements and characteristics that need to be given serious consideration before choosing which band to use.
This article will explain the main differences between 2.45 and 5 GHz WLAN operation, before highlighting the advantages of designing industrial wireless devices using 5 GHz WLANs compared with 2.45 GHz systems. The article will then present some recently released IEEE 802.11x modules and design tools for 5 GHz industrial applications, along with practical design tips.
Comparing 2.45 GHz and 5 GHz
The license-free 2.45 GHz Industrial, Scientific and Medical (ISM) band is particularly popular for consumer and industrial WLANs. Wi-Fi, based on 2.45 GHz IEEE 802.11x technology, for example, is well established for domestic wireless applications because it offers the range, bandwidth and wall-penetrating properties required for today’s mobile consumer electronics applications.
But 2.45 GHz Wi-Fi does have its drawbacks, notably limited provision for avoiding interference from close by Wi-Fi installations and numerous other 2.45 GHz protocols. For example, Bluetooth, ZigBee and proprietary applications such as cordless phones.
This susceptibility to RF interference makes 2.45 GHz WLANs less than ideal for industrial applications where a reliable link is essential. However, WLAN chip suppliers and equipment makers provide technology for operation in the alternative license-free, 5 GHz ISM allocation. The allocation is less crowded and provides for a greater number of non-overlapping channels, considerably easing interference. A further advantage is the potential for increased bandwidth that comes with higher frequency communication.
Demystifying the Wi-Fi alphabet
There are several Wi-Fi protocols: IEEE 802.11b/g operate in the 2.45 GHz band, IEEE 802.11a/ac are designed for 5 GHz band operation, while IEEE 802.11n radios can operate in both bands.
IEEE 802.11b was adopted back in 1999 and offered data rates of 5.5 and 11 Mbps. It is now largely found only in legacy systems. However, support for b is built into contemporary n radios so that modern systems can operate with legacy systems.
IEEE 802.11g was adopted in 2003 and used a different modulation technique to the original protocol to achieve data rates up to 54 Mbps. While backwardly compatible with b, this compatibility comes at a cost to airtime, which can reduce throughput by up to 50 percent compared to a g only environment.
IEEE 802.11n was adopted in 2009 and introduced multiple input, multiple output (MIMO) antenna technology for encoding several, simultaneous “spatial streams”, boosting the data rate to 216 Mbps (assuming 20 MHz channel width and a three spatial stream transmitter). 802.11n also specifies a wider, 40 MHz channel, formed by bonding two 20 MHz channels, which boosts throughput to 450 Mbps. Devices which support three spatial streams are limited to higher end portable computers, tablets and access points (APs). Two spatial stream devices are more plentiful, but still limited to portable computers, tablets and the latest generation of smartphones.
IEEE 802.11a is identical in most respects to g, except it operates in the 5 GHz band. The maximum data rate is the same, 54 Mbps. Today 802.11a is largely considered a legacy protocol.
IEEE 802.11ac was adopted in 2013 and offers eight special streams and channel widths of up to 160 MHz to further boost throughput. Commercial products are only just hitting the market, remain expensive and, at least in the first instance, is likely to be employed for only very high-end consumer products. Today, the most cost-effective solution for developers targeting the 5 GHz band are IEEE 802.11n dual-band chipsets. The rest of this article considers only 802.11n technology.
Channel limitations of 2.45 GHz WLANs
The regulations for IEEE 802.11n in the 2.45 GHz band allow for between 11 (in the U.S.), 13 (in most of the rest of the world) and 14 (in Japan) 20 MHz channels. Because the 2.45 GHz ISM band predates the development of IEEE 802.11 regulations, it comprises a rather restrictive 83 MHz width with space for no more than three non-overlapping Wi-Fi channels (1, 6 and 11) (Figure 1).
Figure 1: Wi-Fi channel allocations in the 2.45 GHz ISM band allow for only three non-overlapping 20 MHz channels (one, six and eleven.) Most commercial implementations are designed to operate using these three channels but can be switched to other channels. (Image source: Cisco.)
To avoid the clashes that could otherwise result from adjacent WLANs using any of the 11 to 14 channels, manufacturers typically design their equipment to communicate in just the non-overlapping ones. For example, a Wi-Fi radio experiencing excessive interference in channel 1 can be switched to channels 6 or 11, in an effort to find an interference-free environment.
In the early days of wireless communication, such operation usually solved the interference problem, but with the increasing number of Wi-Fi installations and other wireless devices which now crowd the band, the lack of overlapping channels is beginning to limit Wi-Fi’s reliability and scalability. Worse still, the lack of overlapping channels makes it particularly difficult to take advantage of the greater data rate of IEEE 802.11n’s 40 MHz bonded channels.
In industrial and commercial environments, the problem is exacerbated because Wi-Fi’s wall-penetrating properties (a strength in domestic dwellings) can become a drawback in shared commercial and industrial premises. With just three non-overlapping channels, interference from other users quickly becomes a problem. This problem is made more acute because unlike a wired LAN, Wi-Fi is a half-duplex system, allowing information to be either sent or received, but not simultaneously.
Wi-Fi does include contention mechanisms that share bandwidth fairly between access points (APs) using the same channel. An AP operating on a congested channel experiences limited airtime, affecting when it can receive or send data. In turn, as more connected devices access a given AP, the airtime each device receives decreases. At busy public hotspots, for example, restricted airtime manifests itself as long idle periods interspersed with data arriving in short bursts.
The advantages of 5 GHz Wi-Fi
For critical commercial and industrial applications, 5 GHz Wi-Fi overcomes the 2.45 GHz version’s interference and airtime challenges. Until recently, this wasn’t an easy option to implement because few 5 GHz products were commercialised, but the ratification of IEEE 802.11n has led to several commercial solutions hitting the market.
The 5 GHz ISM band offers advantages because it was allocated for license-free usage specifically with IEEE 802.11a in mind. As a result, the band extends from 5.725 to 5.875 GHz (150 MHz). In most of the world, the band has been extended to include three other sub-bands, referred to as Unlicensed National Information Infrastructure (UNII) bands. The result is 725 MHz of spectrum for 5 GHz Wi-Fi operation.
The exact allocations vary slightly across the world, but in the U.S., based on Federal Communications Commission (FCC) regulations, the UNII bands are:
- 5.150 to 5.250 GHz (UNII-1 - 4 Channels 36 - 48);
- 5.250 to 5.350 GHz (UNII-2 - 4 Channels 52 - 64);
- 5.450 to 5.710 GHz (UNII-2e - 12 Channels 100 - 144);
- [ISM allocation 5.725 to 5.875 GHz (UNII-3 - 5 Channels 140 - 165).]
The wider spectrum allocation allows for assignment of many more non-overlapping 20 and 40 MHz channels, with a minimum of 5 MHz separation (Figure 2). The result is a significantly reduced chance of clashes with other 5 GHz Wi-Fi users. Even if clashes do occur, there are plenty of options to move away from sources of interference. Figure 2 illustrates the 20, 40, 80 and 160 MHz channels available for 5 GHz Wi-Fi in the U.S., along with the future allocations (green and red).
Figure 2: Wi-Fi channel allocations in the 5 GHz ISM plus UNII bands allow for many more non-overlapping 20, 40, 80 and 160 MHz channels than the 2.45 GHz ISM band. Designers can use this flexibility to significantly decrease interference and airtime disruptions. (Image source: Cisco)
In addition to significantly decreased interference and airtime disruptions, 5 GHz Wi-Fi’s higher frequency operation does offer the possibility of greater data rate compared with 2.45 GHz Wi-Fi for given operating conditions. However, because the IEEE802.11n protocol was designed to cater to both frequency bands, there’s no immediate gain in data rate with 20 MHz channels. Nevertheless, there is greater scope in the 5 GHz allocation to employ n’s 40 MHz bonded channels to double the data rate. Further increases up to 1,560 Mbps will be realised when commercial ac devices become more widely available due to its 160 MHz channel width.
There are some potential downsides to 5 GHz Wi-Fi. Cost and complexity are increased because the spectrum allocation that extends beyond the ISM band is shared with weather and military radar systems, which must take priority. To ensure coexistence with these critical systems, 5 GHz Wi-Fi must comply with two features that were adopted as part of the IEEE 802.11h amendment to the IEEE 802.11 standard: Transmit Power Control (TPC) and Dynamic Frequency Selection (DFS).
TPC is of little consequence to Wi-Fi system developers because it only applies in the rare instance that a system features very high transmit power and antenna gain. In contrast, DFS compliance is required for all APs and connected devices operating on any of the shared bands. APs (and any associated controllers) manage the coexistence on behalf of all connected devices.
Prior to transmitting on a DFS channel, an AP “listens” for critical systems and flags the channel as unavailable if a signal is detected. During operation, the AP continues to monitor shared channels and switches to an alternative if a previously quiet channel is activated.
Another drawback of 5 GHz Wi-Fi is its shorter range and poor penetration of walls and ceilings due to its higher operating frequency. In like-for-like applications, a 2.45 GHz radio will provide a given signal strength at up to 50 percent greater range than a 5 GHz radio. The solution for commercial and industrial installations is to increase the number of APs such that connected devices are always in range of a strong signal.
Although it can be expensive, increasing the number of APs in a 5 GHz Wi-Fi LAN is much simpler than doing so with 2.45 GHz systems because of the availability of many more non-overlapping channels. APs can be positioned within range of a neighboring AP’s signal, but use a different channel allocation to avoid channel sharing and subsequent airtime reduction for a connected device.
An increased number of 5 GHz Wi-Fi APs can be a further advantage in industrial installations that employ mobile transceivers (for example, on forklifts and pallets). These transceivers frequently roam to locate the nearest access point. Roaming time is reduced if more APs are installed, reducing the time the mobile transmitter is ‘off air’ (Table 1).
Spectrum allocation
ISM (83 MHz)
ISM + UNII (725 MHz (FCC))
Channels
3 non-overlapping
23 non-overlapping (FCC)
IEEE 802.11 standard
b, g, n
a, n, ac
Range
Longer
Shorter
Data rate
Lower
Higher (but not currently limited by protocol)
Interference
Higher
Lower
Table 1: Comparison of 2.45 and 5 GHz Wi-Fi. The key advantage of 5 GHz Wi-Fi is the number of non-overlapping channels in a less-populated band, leading to much lower interference and greater airtime. (Data source: Digi-Key Electronics)
Designing 5 GHz Wi-Fi systems
Designing a Wi-Fi solution from the ground up brings the benefits of lower BOM, decreased product size, and an opportunity to fully optimize the wireless product’s performance. However, doing so demands a high degree of RF expertise at gigahertz frequencies, considerable testing, and a long-winded process of verification to the standard’s specification for compliance certification. There is a good selection of application notes and white papers from silicon vendors to guide designers through this process.
For the less experienced, modules are a good alternative. Modules are assembled, tested, verified and compliance certified wireless products that can be quickly incorporated into a Wi-Fi solution, easing design complexity and accelerating time-to-market.
IEEE 802.11n modules and associated development tools are readily available from several silicon vendors. One popular example comes from Texas Instruments’ (TI) WiLink family. The WL1807MOD is a certified module offering 2.45 and 5 GHz Wi-Fi with two antennas and an industrial temperature grade rating. The device is FCC, ETSI/CE and TELEC certified for AP (with DFS support) and connected device applications.
The module integrates RF, power amplifiers (PAs), clock, RF switches, filters, passives and power management. It features WLAN baseband processor and RF transceiver support for IEEE 802.11n (plus a, b and g).
The WiLink module is promoted as a “connectivity solution” and as such does not include a supervisory microprocessor or 2.45 and 5 GHz antennas with associated matching circuits. To ease the design of the peripheral circuitry, TI offers a development board, the WL1837MODCOM8l, which incorporates a WiLink module, the 2.45 and 5 GHz antennas’ footprints (chip antennas must be added) and the controlled impedance traces required for a fully functioning RF circuit.
The WiLink module can be paired with TI’s AM1808EZWT4 Sitara processor running a high-level OS, such as Linux or Android. However, the development board makes it relatively easy for developers to work with other processors. Drivers for some OSes are available from TI, while additional drivers, such as those needed for WinCE and a range of real-time OSes, are supported through third parties.
The processor also requires a Wi-Fi and Secure Digital I/O (SDIO) driver, a Wi-Fi Protected Access (WPA) Supplicant (to implement security protocols) and a peripheral 32 kHz crystal (Figure 3).
Figure 3: TI’s WiLink module is a fully compliant 2.45/5 GHz Wi-Fi solution with industrial temperature rating. To implement a full system the module requires a microprocessor, OS, Wi-Fi driver, antenna and matching circuits. TI offers reference designs to help with this. The version shown here also incorporates Bluetooth functionality. (Image source: Texas Instruments)
For developers who would rather avoid the hassle of working with a discrete microprocessor, and designing antenna footprints and matching circuits, u-blox offers its ODIN-W2 series module. The device combines 2.45/5 GHz Wi-Fi (IEEE 802.11a/b/g/n), Bluetooth and Bluetooth low energy RF transceiver, host ARM® M4F microprocessor, 24 MHz crystal, power supply, filters and an internal antenna in a 15 x 22 mm package (Figure 4).
Figure 4: u-blox’s ODIN-W2 series module integrates a 2.45 and 5 GHz Wi-Fi (plus Bluetooth) RF transceiver with an ARM M4F host processor. The module also features IEEE 802.11d enabling configuration to the ISM and UNII bands available in the country in which it operates. (Image source: u-blox)
The module also comes with complete with Wi-Fi driver, IP stack, and an application for wireless data transfer, (plus an embedded Bluetooth stack) all configurable using AT instructions from the company’s commands manual.
The ODIN-W2 is supported by an evaluation kit (EK), the EVK-W262U. The EK enables configurations over a single USB interface that provides both power and high-speed data transfer. There are two ways to configure the module. The first uses the company’s s-center software tool which can be downloaded from the u-blox website. A graphical user interface (GUI) allows the developer to configure the module using u-blox’s AT commands. More complex applications can be configured using the EK to directly access the ARM M4F microprocessor. The EK is compatible with ARM’s mbed Integrated Development Environment (IDE) so a developer can write and compile application code using the EK before porting to the microprocessor for debugging, test and verification. The module also supports a serial interface which allows a developer to connect their own choice of microprocessor (and application) if they prefer not to use the ARM IDE.
One particularly useful feature of the u-blox module for 5 GHz Wi-Fi applications is built-in support for IEEE 802.11d, which allows ODIN-W2 devices to self-configure and operate according to the regulations of the country in which they operate. This gives the device access to as many channels as possible. The module initially performs a scan to ascertain the local regulatory domain, and then sets the channels according to an FCC, ETSI or “World” schedule (Figure 5).
Figure 5: The ODIN-W2 module supports an algorithm (represented by this state transition diagram) to automatically scan which 5 GHz Wi-Fi channels are available in the country of operation. (Image source: u-blox)
Conclusion
The 2.45 GHz band is becoming increasingly congested and is relatively narrow. Both these factors conspire to undermine reliability of Wi-Fi operation in commercial and industrial applications. In contrast, 5 GHz Wi-Fi uses a quieter and much wider band, with many more allocated channels. Fewer users and an increased number of channels ease interference and airtime restrictions. IEEE 802.11n specification for 5 GHz operation also allows the use of 40 MHz bonded channels as well as forthcoming 80 and 160 MHz channels to considerably boost data rate.
Using widely available 5 GHz modules and associated development tools designers can take advantage of the higher-frequency option.
Reference
- “Enterprise Mobility 8.1 Design Guide”, Cisco, November 2016.
