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20.3 Survey of Unlicensed Wireless Networks using barcode writer for none control to generate, create none image in none applications. About Micro QR Code The purpose none none of this section is to give the reader a flavor of the various wireless networks operating in the unlicensed frequency bands. This section is not intended as a detailed description of each of the various wireless networks, just a brief summary. This section is organized by wireless network technology: WLAN, WPAN, WMAN, WRAN, etc.

20.3.1 Wireless Local Area Network (WLAN) The most popular wireless local area network (WLAN) standard is IEEE 802.

11, of which there have been a number of amendments. Some of these amendments have been to introduce new physical (PHY) layers, while other amendments have been to introduce new capabilities in the medium access control (MAC) layer. The original 802.

11 standard issued approximately 10 years ago in 1997 [5]. The original standard included three different PHY layers. When the 802.

11 standard was originally issued operation in the ISM bands required use of spread spectrum technology. There were two reasons for this FCC rule. First by spreading the signal the power spectral density of the unlicensed transmission would be spread out over a wider frequency range resulting in less interference to narrow band systems.

Second, the unlicensed system is less susceptible to narrowband interference due to the processing gain benefit at the receiver. The two RF PHY layers in the original 802.11 standard were the frequency hopping spread spectrum (FHSS) PHY and the direct sequence spread spectrum (DSSS) PHY.

Both of the RF PHY operate in the 2.4 GHz ISM. Coexistence of Unlicensed Wireless Networks 475 frequency ba none none nd. There was also an infrared (IR) optical PHY which was never implemented in the industry. The FHSS PHY transmits at 1 or 2 Mb/s, which hops over 79 1-MHz channels.

This PHY is no longer popular; however, the Bluetooth SIG leveraged parts of this PHY in the Bluetooth specification. The DSSS PHY transmits at either 1 or 2 Mb/s using BPSK or QPSK respectively. The signal is spread with an 11-chip Barker sequence.

In 1999 the IEEE issued two high-rate PHY layer amendments: 802.11a and 802.11b.

The 802.11b PHY [6], and extension of the original DSSS PHY, became quickly popular partial due to the establishment of the Wi-Fi alliance (originally called the wireless Ethernet Compatibility Alliance). The 802.

11b PHY added 5.5 Mb/s and 11 Mb/s data rates to the original 1 and 2 Mb/s data rates. The bandwidth of the signal remained unchanged.

By increasing the data rates while maintaining the signal bandwidth, the spread spectrum processing gain is reduced, resulting in signals that are more susceptible to interference. This is a common theme, that with limited bandwidth and a demand for higher data rates, the signals require higher SNR and hence are often much more susceptible to interference. On the other hand, higher data rates tend to lead to shorter packet durations, which with bursty interference shorter packets tend to be much less susceptible to interference.

Also in 1999 [7], the IEEE issued 802.11a which is an orthogonal frequency division multiplexing (OFDM) PHY operating in the 5 GHz frequency band. Due to the use of the 5 GHz frequency band this PHY initially did not become as popular due to the cost of RF circuits in that band at the time.

However, 802.11a has now become quite popular. Also, the bandwidth available at 5 GHz is significantly greater than that at 2.

4 GHz which makes this frequency band attractive. The 802.11a PHY supports data rates from 6 Mb/s up to 54 Mb/s.

These data rates use modulations ranging from BPSK up to 64-QAM and code rates ranging from rate to rate . Over the last several years the IEEE has been developing a higher-rate PHY layer using multiple input multiple output (MIMO) antenna technology and wider bandwidth signals (i.e.

, 40 MHz versus 20 MHz channels). This new PHY is currently only available as a draft since the standard has not been ratified by the IEEE. When ratified the amendment will be 802.

11n. This new amendment to 802.11 not only includes a new PHY but also many enhancements to the MAC layer to improve the overall throughput available to the user.

For many more details about the draft 802.11n see 8 of this book. IEEE 802.

11n will be able to operate in either the 2.4 GHz or the 5 GHz frequency band. In terms of coexistence with other wireless devices in the band we can make several generalizations, prior to performing any simulations or detailed analysis.

First, as an interferer, when the 20 MHz channels are used, 802.11n causes similar interference to the previous 802.11 PHYs.

However, as a victim of interference this new PHY is more susceptible to interference, since the data rates have been increased within a given bandwidth. This increase in spectral efficiency comes at a price of an increase in the required SNR which results in higher sensitivity to interference from other wireless systems. Finally, the IEEE is also currently developing a PHY for operation in the 3650 MHz frequency band.

This band, from 3650 to 3700 MHz does not allow unlicensed wireless networks. However, it does allow non-exclusive licensed operation. Since the licenses are non-exclusive it is quite possible for multiple wireless networks, potentially designed according to different standards or specifications, to operate in the same region.

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