IEEE 802.11ad Microwave Wi-Fi / WiGig Tutorial

notes and details of the IEEE 802.11ad Microwave Wi-Fi standard to provide data throughput rates of up to 6Gbps at frequencies around 60 GHz.

The IEEE 802.11ad standard is aimed at providing data throughput speeds of up to 7 Gbps. To achieve these speeds the technology uses the 60 GHz ISM band to achieve the levels of bandwidth needed and ensure reduced interference levels.

Using frequencies in the millimetre range IEEE 802.11ad microwave Wi-Fi has a range that is measured of a few metres. The aim is that it will be used for very short range (across a room) high volume data transfers such as HD video transfers. When longer ranges are needed standards such as 802.11ac can be used.

As part of the marketing, the scheme will be known by the name WiGig after the Wireless Gigabit Alliance that endorses the system.

Wireless Gigabit Alliance

In order to provide the industry support and an easy marketing name, the IEEE and Wireless Gigabit Alliance have worked together on developing the IEEE 802.11ad WiGig standard.

To this end, the WiGig MAC/PHY specification aligns exactly with the 802.11ad standard. This provides industry standardisation, industry recognition, input from industry to ensure that the standard is realisable and also meets the industry needs, and it also provides an easy marketing name.

The Wireless Gigabit Alliance was formed to provide a single multi-gigabit wireless communications standard among consumer electronics, handheld devices and PCs, and drives industry convergence using unlicensed ISM (industrial, scientific and medical) 60 GHz spectrum.

802.11ad salient features

The table below gives a summary of the salient features of 802.11ad.

In addition to the tabulated details, the system uses a MAC layer standard that is shared with current 802.11 standards to enable session switching between 802.11 Wi-Fi networks operating in the 2.4 GHz, and 5 GHz bands with those using the 60 GHz WiGig bands. In this way, seamless transition can occur between the systems.

However the 802.11ad MAC layer has been updated to address aspects of channel access, synchronization, association, and authentication required for the 60 GHz operation.

Physical layer

The WLAN system uses frequencies in the 60GHz unlicensed spectrum. Dependent upon geography these are located between 57 GHz and 66GHz. 

The ITU-R then recommends the use of four channels, each 2.16 GHz wide with centre frequencies of 58,32, 60.48, 62.64, and 64.80 GHz. It can therefore be seen that only channel 2 with its centre frequency of 60.48 GHz is globally available. This is recommended to be the default channel.

The signal spectrum and spectral mask needs to ensure that the signal is maintained within a certain bandwidth. The spectral mask shows the mask for the spectrum.

802.11ad Spectral Mask

One of the main forms of modulation used is OFDM. This is a key element of the overall modulation and RF signal format, providing the capability for high data rates while supplying good resilience against multiple paths.

The 802.11ad PHY supports three main signals with different modulation.

• Control PHY, CPHY:   Providing control, this signal has high levels of error correction and detection. Accordingly it has a relatively low throughput. As it does not carry the main payload, this is not an issue. It exclusively carriers control channel messages.
  
  The CPHY uses differential encoding, code spreading and BPSK modulation.
• Single Carrier PHY:   The SCPHY employs single carrier modulation techniques: BPSK, QPSK or 16-QAM on a suppressed carrier located on the channel centre frequency. This single has a fixed symbol rate of 1.76 Gsym/sec. A variety of error coding and error coding modes are available according to the requirements.
• Orthogonal Frequency Division Multiplex PHY, OFDMPHY:   As with any OFDM scheme, the OFDMPHY uses multicarrier modulation to provide high modulation densities and higher data throughput levels than the single carrier modes.
  
  The modulation format SQPSK is Spread QPSK and involves using paired OFDM carriers onto which the data is modulated. The two carriers are maximally separated to improve the robustness of the signal in the presence of frequency selective fading.
• Low Power Single Carrier PHY, LPSCPHY:   This 802.11ad signal uses a single carrier as the name implies, and this is to minimise the power consumption. It is intended for small battery devices that may not be able to support the processing required for the OFDM format.

802.11ad beam management

One of the features of WiGig microwave Wi-Fi is the aspect of antenna beam management. The very high frequencies used means that the antennas are very small and this makes the development, manufacture and use of the phased arrays required for this a feasible proposition.

The beam-forming is accomplished using a bi-directional training sequence that is appended to each transmission. This enables the system to shape the transmit and / or the receive beams to achieve the optimum link properties. This enables the system to overcome any movement of the transmitter, receiver, or objects between them that might alter the path characteristics.

By Ian Poole