Tutorial on 802.11n PHY layer

By Sathish Viswanathan
May 16, 2007 -- wirelessnetdesignline.com

Wi-Fi's biggest advantage is that it provides mobility and coverage. But early versions of Wi-Fi did not achieve data rates on par with the wireline network. Recent advancements in wireless research and smart antenna technology made it possible for Wi-Fi networks to remove this bottleneck, thereby providing access to the users with extended range and increased throughput.

The IEEE 802.11n WLAN standard was created to implement these upgrades. This three-part article will review its evolution from earlier 802.11 standards and discuss the major aspects of the 802.11n PHY layer.

WLAN standards review

The IEEE 802.11 WLAN standard was first defined in 1997 for indoor communication between computers and the mobile devices within a range of 150 meters. It consists of physical-layer (PHY) and medium-access-channel layer (MAC) specifications (IEEE 802.11 1999).

The 802.11-complaint devices use the 2.4-GHz ISM band for its operation. The PHY layer techniques used in this standard are frequency hopping spread spectrum (FHSS), direct sequence spread spectrum (DSSS) and infrared (IR) communication. The maximum data rate that can be achieved using these techniques is 2 Mbps.

The MAC mechanism used is carrier sense multiple access with collision avoidance (CSMA/CA). This is achieved by physical carrier sensing and virtual carrier sensing techniques. With the motivation to increase the data rate of WLANs in the same frequency, an enhancement to the PHY specification of 802.11 was standardized as IEEE 802.11b.

In this standard, the PHY layer uses DSSS and achieves a maximum data rate of 11 Mbps (IEEE 802.11b 1999). There is no significant change in the MAC as compared to the basic 802.11 standard. In 1999, another PHY specification for enhancing the data rate of the system was also standardized and is called as IEEE 802.11a.

Since the 2.4 GHz band is used by microwave ovens, Bluetooth and other devices, the 802.11a standard uses 5 GHz for its operation. This standard also uses a spectrally efficient transmission scheme called as orthogonal frequency division multiplexing (OFDM).

The maximum data rate obtained is 54 Mbps with a rough bandwidth of 20 MHz (IEEE 802.11a 1999). In 2003, another PHY specification was arrived to collectively provide the PHY features of 802.11b and 802.11a in the 2.4GHz. This is standardized as IEEE 802.11g.

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