Cypress Semiconductor As more companies produce products that use the 2.4 GHz band, designers must handle more signals from other sources. The regulations governing unlicensed bands indicate that your device must consider interference issues.
How do designers get the most performance from a 2.4 GHz solution under these demanding conditions? Products tend to work well in a controlled laboratory environment, but at the site they experience significant performance degradation due to other 2.4 GHz solutions. Currently, there are different standards such as Wi-Fi, Bluetooth and ZigBee in the 2.4 GHz band. Most products are implemented by the method provided by the standard setter. However, through the control protocol, the designer can pass other signals through certain measures. The source interference problem is minimized.
In this article, we will explore the various interference control techniques in 2.4 GHz wireless systems and how to use low-level tools to achieve frequency stability in a 2.4 GHz design.
Frequency Hopping Spread Spectrum (FHSS) and Direct Sequence Spread Spectrum (DSSS) are two methods of RF modulation in the unlicensed 2.4 GHz ISM band. Bluetooth uses FHSS, while WirelessUSB, 802.11b/g/a (also known as Wi-Fi) and 802.15.4 (called ZigBee when combined with the upper network layer) use DSSS. All of these technologies work in the global ISM band (ie 2.400"2.483 GHz) (see Figure 1).
Figure 1: Signal comparison of wireless systems operating in the 2.4 GHz band.
The main driver for adoption of Wi-Fi is data throughput. Wi-Fi is typically used for connections between computers and local area networks (LANs) (and indirectly via the LAN). Most Wi-Fi devices today are laptops that can be recharged every day or access points that are powered by utility power, so they are not sensitive to power issues.
Wi-Fi uses DSSS technology with a bandwidth of 22 MHz per channel, allowing three equally distributed channels to be used simultaneously without overlapping each other. The channels used by each Wi-Fi access point need to be manually configured; Wi-Fi customers search for available access points in all channels.
802.11 uses an 11-bit pseudo-random noise (PN) code called a Barker code to encode each of the original data rates of 1 and 2 Mbps. To achieve higher data rates, 802.11b encodes six information bits into an 8-chip symbol by complement keying (CCK).
There are 64 symbols that can be used in the CCK algorithm, requiring each 802.11b radio to include 64 separate correlators (ie, the device used to convert symbols into information bits), which increases the complexity of the radio. Cost, but can increase the data rate to 11 Mbps.
Bluetooth technology focuses on the interoperability of adaptive networking between cellular phones, headsets and PDAs. Most Bluetooth devices require regular charging.
Bluetooth uses FHSS and divides the 2.4 GHz ISM band into 79 1 MHz channels. The Bluetooth device hops 1,600 times per second between these 79 channels in a pseudo-random code. The connected Bluetooth devices are grouped into a network called a piconet; each piconet includes one master and up to seven active slaves. The channel hopping sequence of each piconet is derived from the master's clock, and all slaves must remain synchronized with this clock.
Forward Error Correction (FEC) can be performed on all packet headers by sending each bit in the packet header three times. The Hamming code can also be used for forward error correction of a certain type of data packet payload. The Hamming code adds 50% overhead to each packet, but corrects all one bit errors and detects two errors in each 15-bit codeword (each 15-bit codeword contains 10 information bits).
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