WiGig: IEEE 802.11ad 60GHz Microwave Wi-Fi

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Wi-Fi tends to be used for short distances, typically across a room, and increasingly very high volumes of data need to be transferred very quickly. IEEE 802.11ad, WiGig is able to provide a solution, giving very high throughput rates and using frequencies in the microwave section of the spectrum at frequencies around 60GHz.

As a result it is sometimes called 60GHz Wi-Fi or Microwave Wi-Fi.

The microwave section of the radio spectrum has a number of major advantages. It is possible to use a very wide bandwidth to enable high throughput data, and also good re-use of the spectrum is possible. As signals do not travel over such great distances and are absorbed by buildings, etc, high levels of re-use can be obtained without users on the same channel experiencing interference.

As part of the marketing, the scheme will be known by the name WiGig after the Wireless Gigabit Alliance that endorses the system, but it is also sometimes referred to as 60GHz WiFi.

Wireless Gigabit Alliance

To promote and advance the use of microwave Wi-Fi an alliance called the Wireless Gigabit Alliance was set up. Working with the IEEE, both organisations are committed to the advancement of the standard. It has been standardised as IEEE802.11ad, but its marketing name is WiGig.

Accordingly, 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.

It is anticipated that as microwave technology becomes cheaper and the requirement for spectrum increases, WiGig technology will become widespread in its use.

WiGig 802.11ad salient features

The table below gives a summary of the salient features of 802.11ad - 60GHz Wi-Fi.

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.

WiGig physical layer

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

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.

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 WiGig 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.

WiGig 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.

  Written by Ian Poole .
  Experienced electronics engineer and author.

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