802.11n PHY layer--A tutorial

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.

Formation of 802.11n group
Even though the maximum PHY layer rate is around 50 Mbps, the net throughput obtained is only 60 percent of it in the indoor applications. To increase the net throughput on par with Ethernet, the task group 'n' was formed in January 2004 and many proposals were reviewed to achieve this goal.

The three main proposals are WWiSE, TGnsync and EWC (WWISE 2005, TGnsync 2005 and EWC 2006). All the three proposals utilize multiple transmit and multiple receive antennas called as multiple-input multiple-output (MIMO) technology.

This allows one to transmit multiple independent data streams simultaneously to increase the spectral efficiency. This is also known as spatial multiplexing. To counter the multipath nature of the channel, OFDM coding is used along with the MIMO technology.

Therefore, the final system is categorized as a MIMO-OFDM system. The three proposals mandate the 20 MHz operation and support for 40 MHz operation. They also mandate the interoperability with the legacy 802.11a/g systems.

The maximum PHY data rate that can be achieved with four transmit antennas in 40 MHz is around 500 Mbps. On the MAC side, all the proposals support frame aggregation, block acknowledge (BACK) and MAC header compression.

In January 2006, the EWC proposal was finalized as the draft for the 802.11n standard. Apart from the above features, this supports advanced techniques for optional modes. They are adaptive beamforming, space time block coding (STBC), and low density parity coding (LDPC) for increased range and reliable communications.

In this article, only the PHY layer part of the 802.11n standard is discussed in a detailed manner and the whole content is based on the first draft of the standard.

The flow of the tutorial is as follows. First, we will discuss the objectives of the 802.11n standard. Under this, the techniques and the challenges in achieving them are briefed. In the next section, the different modes of 802.11n network operation and the preamble structures that are used in each mode are described.

Finally, a typical 802.11n transmitter and a receiver are described.

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