12-08-2014, 11:54 AM
seminar report on Interference Evaluation of Different Wireless Systems Operating in 2.4 GHZ ISM BAND
[attachment=66563]
Abstract
There are different wireless technologies that share the same 2.4GHzunlicensed ISM frequency band. Such technologies usually operate in proximity and have to co-exist with each other. For example, Wi-Fi uses the same frequency band that is used by Zigbee and Bluetooth. Thus, an interference problem arises which causes loss of data packets being transmitted. In this paper, we have discussed the mutual interference between different technologies that operate in 2.4GHz frequency band and suggest ways to resolve interference.
INTRODUCTION
Today, most radio technologies like ZigBee, Bluetooth employ the 2.4 GHz Industrial, Scientific and Medical (ISM) bands frequency band, which is also used by Wi-Fi. It is anticipated that some interference will result from all these technologies operating in the same environment. Wi-Fi, Bluetooth, and cordless phones are considered the broadest systems in the 2.4 GHz band, in addition to the other no networking systems such as microwave ovens, which may radiate electromagnetic radiation in the 2.4 GHz band. Signal comparison of wireless systems operating in the 2.4-GHz bands shown in figure 1 [2].
Sharing of the spectrum between these technologies raise serious problems, leading to a degradation of performance, dropping of packets, decreasing of the throughput and malfunctioning of the whole network The interference could threaten the security and the functionality of the network
Wi-Fi
Wi-Fi uses DSSS, with each channel being 22 MHz wide, allowing up to three evenly-distributed channels to be used simultaneously without overlapping each other. The basic data rate for the DS system is 1 Mb/s encoded with differential binary phase shift keying (DBPSK) [6]. Similarly, 2 Mb/s rate is provided using differential quadrature Phase shift keying (DQPSK) at the same chip rate of11 × 106 chips/s. Higher
Bluetooth
Bluetooth is a short range (0–10 m) wireless link technology aimed at replacing non interoperable proprietary cables that connect phones, laptops and other portable devices together. Bluetooth operates in the ISM frequency band starting at 2.402 GHz and ending at 2.483 GHz in the USA and Europe. 79 RF channels of 1 MHz width are defined. The signal is modulated using binary Gaussian Frequency Shift Keying (GFSK). The raw data rate is defined at 1 Mb/s. A Time Division Multiplexing (TDM) technique divides the channel into 625 μs slots. Transmission occurs in packets that occupy an odd number of slots (up to 5). Each packet is transmitted on different hop frequency with a maximum frequency hopping rate of 1600 hops/s. Connected Bluetooth devices are grouped into networks called piconets; each piconet contains one master and up to seven active slaves. The channel-hopping sequence of each piconet is derived from the master's clock. All the slave devices must remain synchronized with this clock. A slave packet always follows a master packet transmissions illustrated in figure 3[1], which depicts the master’s view of the slotted TX/RX channel. There are two types of link connections that can be established between a master and a slave: the Synchronous Connection-Oriented (SCO) and the Asynchronous Connection-Less (ACL) link. The SCO link is a symmetric point-to-point connection between a master and a slave where the master sends an SCO packet in one TX slot at regular time intervals, defined by TSCO time slots.
Wireless USB
Wireless USB uses a radio signal similar to Bluetooth but uses DSSS instead of FHSS. Each wireless USB channel is 1MHz wide, allowing wireless USB to split the 2.4 GHz ISM band into 79 RF channels of 1MHz wide. Wireless USB devices are frequency agile, in other words, they use a "fixed" channel, but dynamically change channels if the link quality of the original channel becomes suboptimal. Wireless USB uses pseudo-noise (PN) codes to encode each information bit. Most Wireless USB systems use two 32-chip PN codes allowing two information bits to be encoded in each 32-chip symbol. This scheme can correct up to three chip errors per symbol and can detect up to 10 chip errors per symbol. Although the use of 32-chip (and sometimes 64-chip) PN codes limits the data rate of Wireless USB to 62.5 kb/s, data integrity is much higher than Bluetooth, especially in noisy environments.
ZigBee
ZigBee also uses a DSSS radio signal in the 868 MHz band (Europe), 915 MHz band (North America), and the 2.4 GHz ISM band (available worldwide). In the 2.4-GHz ISM band sixteen channels are defined; each channel occupies 3 MHz and channels are centered 5 MHz from each other, giving a 2-MHz gap between pairs of channels [1]. ZigBee uses an 11-chip PN code, with 4 information bits encoded into each symbol giving it a maximum data rate of 128 Kbps. The physical and MAC layers are defined by the IEEE 802.15.4 Working Group and share many of the same design characteristics as the IEEE 802.11b standard.
Figure 4.RF spectrum of ZigBee
E. 2.4 GHz cordless phones
Cordless phones appear to come in 900MHz, 2.45 GHz or 5.8GHz. Mostly 2.4GHz cordless phones use spread spectrum techniques –DSSS or FHSS and most of them use a channel width of 5 to 10 MHz
F. Microwave ovens
There are also non-data transmitting devices operating in these bands, specifically in the 2.4 GHz range. The most common of these unintentional interferers is the Microwave Oven (MWO).Microwave ovens operate in the ISM bands and are largely used in residential and commercial environments. Microwave ovens interfere with WLANS operating in 2.4GHz and affect the bit-error rate.
The major wireless systems operating in 2.4GHz band can be summed up as shown in Table 1[2].
COLLISION AVOIDANCE
between Wi-Fi and Bluetooth
When a Bluetooth and a Wi-Fi are operating in the same area, the single 22 MHz-wide Wi-Fi channel occupies the same frequency space as 22 of the 79 Bluetooth channels which are 1 MHz wide. When a Bluetooth transmission occurs on a frequency that lies within the frequency space occupied by a simultaneous Wi-Fi transmission, some level of interference can occur, depending on the strength of each signal. Wi-Fi’s collision-avoidance algorithm listens for a quiet channel before transmitting. This allows multiple Wi-Fi clients to efficiently communicate with a single Wi-Fi access point. If the Wi-Fi channel is noisy the Wi-Fi device does a random back off before listening to the channel again. If the channel is still noisy the process is repeated until the channel becomes quiet; once the channel is quiet the Wi-Fi device will begin its transmission. If the channel never becomes quiet the Wi-Fi device may search for other available access points on another channel. Wi-Fi networks using the same or overlapping channels will co-exist due to the collision avoidance algorithm, but the throughput of each network will be reduced. If multiple networks are used in the same area it is best to use non-overlapping channels such as channels 1, 6, and 11. This allows each network to maximize its throughput since it will not have to share the bandwidth with another network. Interference from Bluetooth is minimal due to the hopping nature of the Bluetooth transmission. If a Bluetooth device transmits on a frequency that overlaps the Wi-Fi channel while a Wi-Fi device is doing a "listen before transmit", the Wi-Fi device will do a random back off during which time the Bluetooth device will hop to a non-overlapping channel allowing the Wi-Fi device to begin its transmission.
Between Zigbee and Wi-Fi
The coexistence of Wi-Fi and ZigBee exerts a serious challenge for operating the two technologies in the same environment. Sharing of the spectrum between these technologies raise serious problems, which lead to a
CONCLUSION
Coexistence, and ultimately simultaneous operation, between various wireless technologies is a desirable goal. These technologies are expected to grow rapidly over the next few years, offering new levels of portability and convenience, and many critical usage models require collocation and simultaneous operation of these standards in the same device. Systems-level approaches that address coexistence offer the potential to dramatically reduce, if not eliminate, interference between these two systems. Such robust wireless-system design technology will become increasingly important in the unlicensed bands as Bluetooth, Wi-Fi, and other unlicensed wireless technologies proliferate.
[attachment=66563]
Abstract
There are different wireless technologies that share the same 2.4GHzunlicensed ISM frequency band. Such technologies usually operate in proximity and have to co-exist with each other. For example, Wi-Fi uses the same frequency band that is used by Zigbee and Bluetooth. Thus, an interference problem arises which causes loss of data packets being transmitted. In this paper, we have discussed the mutual interference between different technologies that operate in 2.4GHz frequency band and suggest ways to resolve interference.
INTRODUCTION
Today, most radio technologies like ZigBee, Bluetooth employ the 2.4 GHz Industrial, Scientific and Medical (ISM) bands frequency band, which is also used by Wi-Fi. It is anticipated that some interference will result from all these technologies operating in the same environment. Wi-Fi, Bluetooth, and cordless phones are considered the broadest systems in the 2.4 GHz band, in addition to the other no networking systems such as microwave ovens, which may radiate electromagnetic radiation in the 2.4 GHz band. Signal comparison of wireless systems operating in the 2.4-GHz bands shown in figure 1 [2].
Sharing of the spectrum between these technologies raise serious problems, leading to a degradation of performance, dropping of packets, decreasing of the throughput and malfunctioning of the whole network The interference could threaten the security and the functionality of the network
Wi-Fi
Wi-Fi uses DSSS, with each channel being 22 MHz wide, allowing up to three evenly-distributed channels to be used simultaneously without overlapping each other. The basic data rate for the DS system is 1 Mb/s encoded with differential binary phase shift keying (DBPSK) [6]. Similarly, 2 Mb/s rate is provided using differential quadrature Phase shift keying (DQPSK) at the same chip rate of11 × 106 chips/s. Higher
Bluetooth
Bluetooth is a short range (0–10 m) wireless link technology aimed at replacing non interoperable proprietary cables that connect phones, laptops and other portable devices together. Bluetooth operates in the ISM frequency band starting at 2.402 GHz and ending at 2.483 GHz in the USA and Europe. 79 RF channels of 1 MHz width are defined. The signal is modulated using binary Gaussian Frequency Shift Keying (GFSK). The raw data rate is defined at 1 Mb/s. A Time Division Multiplexing (TDM) technique divides the channel into 625 μs slots. Transmission occurs in packets that occupy an odd number of slots (up to 5). Each packet is transmitted on different hop frequency with a maximum frequency hopping rate of 1600 hops/s. Connected Bluetooth devices are grouped into networks called piconets; each piconet contains one master and up to seven active slaves. The channel-hopping sequence of each piconet is derived from the master's clock. All the slave devices must remain synchronized with this clock. A slave packet always follows a master packet transmissions illustrated in figure 3[1], which depicts the master’s view of the slotted TX/RX channel. There are two types of link connections that can be established between a master and a slave: the Synchronous Connection-Oriented (SCO) and the Asynchronous Connection-Less (ACL) link. The SCO link is a symmetric point-to-point connection between a master and a slave where the master sends an SCO packet in one TX slot at regular time intervals, defined by TSCO time slots.
Wireless USB
Wireless USB uses a radio signal similar to Bluetooth but uses DSSS instead of FHSS. Each wireless USB channel is 1MHz wide, allowing wireless USB to split the 2.4 GHz ISM band into 79 RF channels of 1MHz wide. Wireless USB devices are frequency agile, in other words, they use a "fixed" channel, but dynamically change channels if the link quality of the original channel becomes suboptimal. Wireless USB uses pseudo-noise (PN) codes to encode each information bit. Most Wireless USB systems use two 32-chip PN codes allowing two information bits to be encoded in each 32-chip symbol. This scheme can correct up to three chip errors per symbol and can detect up to 10 chip errors per symbol. Although the use of 32-chip (and sometimes 64-chip) PN codes limits the data rate of Wireless USB to 62.5 kb/s, data integrity is much higher than Bluetooth, especially in noisy environments.
ZigBee
ZigBee also uses a DSSS radio signal in the 868 MHz band (Europe), 915 MHz band (North America), and the 2.4 GHz ISM band (available worldwide). In the 2.4-GHz ISM band sixteen channels are defined; each channel occupies 3 MHz and channels are centered 5 MHz from each other, giving a 2-MHz gap between pairs of channels [1]. ZigBee uses an 11-chip PN code, with 4 information bits encoded into each symbol giving it a maximum data rate of 128 Kbps. The physical and MAC layers are defined by the IEEE 802.15.4 Working Group and share many of the same design characteristics as the IEEE 802.11b standard.
Figure 4.RF spectrum of ZigBee
E. 2.4 GHz cordless phones
Cordless phones appear to come in 900MHz, 2.45 GHz or 5.8GHz. Mostly 2.4GHz cordless phones use spread spectrum techniques –DSSS or FHSS and most of them use a channel width of 5 to 10 MHz
F. Microwave ovens
There are also non-data transmitting devices operating in these bands, specifically in the 2.4 GHz range. The most common of these unintentional interferers is the Microwave Oven (MWO).Microwave ovens operate in the ISM bands and are largely used in residential and commercial environments. Microwave ovens interfere with WLANS operating in 2.4GHz and affect the bit-error rate.
The major wireless systems operating in 2.4GHz band can be summed up as shown in Table 1[2].
COLLISION AVOIDANCE
between Wi-Fi and Bluetooth
When a Bluetooth and a Wi-Fi are operating in the same area, the single 22 MHz-wide Wi-Fi channel occupies the same frequency space as 22 of the 79 Bluetooth channels which are 1 MHz wide. When a Bluetooth transmission occurs on a frequency that lies within the frequency space occupied by a simultaneous Wi-Fi transmission, some level of interference can occur, depending on the strength of each signal. Wi-Fi’s collision-avoidance algorithm listens for a quiet channel before transmitting. This allows multiple Wi-Fi clients to efficiently communicate with a single Wi-Fi access point. If the Wi-Fi channel is noisy the Wi-Fi device does a random back off before listening to the channel again. If the channel is still noisy the process is repeated until the channel becomes quiet; once the channel is quiet the Wi-Fi device will begin its transmission. If the channel never becomes quiet the Wi-Fi device may search for other available access points on another channel. Wi-Fi networks using the same or overlapping channels will co-exist due to the collision avoidance algorithm, but the throughput of each network will be reduced. If multiple networks are used in the same area it is best to use non-overlapping channels such as channels 1, 6, and 11. This allows each network to maximize its throughput since it will not have to share the bandwidth with another network. Interference from Bluetooth is minimal due to the hopping nature of the Bluetooth transmission. If a Bluetooth device transmits on a frequency that overlaps the Wi-Fi channel while a Wi-Fi device is doing a "listen before transmit", the Wi-Fi device will do a random back off during which time the Bluetooth device will hop to a non-overlapping channel allowing the Wi-Fi device to begin its transmission.
Between Zigbee and Wi-Fi
The coexistence of Wi-Fi and ZigBee exerts a serious challenge for operating the two technologies in the same environment. Sharing of the spectrum between these technologies raise serious problems, which lead to a
CONCLUSION
Coexistence, and ultimately simultaneous operation, between various wireless technologies is a desirable goal. These technologies are expected to grow rapidly over the next few years, offering new levels of portability and convenience, and many critical usage models require collocation and simultaneous operation of these standards in the same device. Systems-level approaches that address coexistence offer the potential to dramatically reduce, if not eliminate, interference between these two systems. Such robust wireless-system design technology will become increasingly important in the unlicensed bands as Bluetooth, Wi-Fi, and other unlicensed wireless technologies proliferate.