Being unlicensed around the world and providing the potential for high bandwidth gigabit links, the 60GHz band has always attracted attention from operators. However, the amount of bandwidth available varies in different regions, the technical challenges of operating at such high frequencies are significant, and connections are generally limited by line-of-sight as millimetre wave wireless signals are easily blocked by buildings and trees.
That said, the IEEE has developed the IEEE 802.11ad standard for operating at 60GHz. This standard, which specifies operation up to 7Gbit/s, also includes a mode of operation for the media access controller to support co-operative operation with existing WiFi 802.11a,b,g and ac MACs, as well as a new physical layer (PHY) that has several different modes. These include a low power mode for transfers at around 1Gbit/s for mobile phones as well as a full multi-channel mode for the full 7Gbit/s link. This is aimed at replacing wires in the home such as the HDMI cable from a set to box to an Ultra HD TV screen with a typical range of up to 10m within a room. It is also possible to use the 802.11ad standard for longer range high bandwidth line-of-sight applications such as the backhaul from small mobile phone radio base stations – together with suitable modifications to the radio and array antenna sub-systems.
Using the 60GHz band also brings some key benefits to the antenna design. Due to the small wavelength (5mm) these links can be achieved with significantly smaller antennas, and this also makes a phased array, beam steering antenna design practical for the backhaul applications. When combined with closed loop electronic control such a beamforming antenna potentially eliminates the alignment challenges of setting up such links as the antenna can automatically converge on the optimum direction connection.
However, the steps to implementing this technology have been significant. In 2013 the WiGig Alliance merged with the WiFi Alliance in order to create a tri-band specification where the MAC is common to all the WiFi standards and in order to create the necessary industry-wide interoperable standard necessary for a high volume chip market for 60GHz implementations.
The specification recommends the use of four channels, each 2.16GHz wide, and there are three modem protocols specified for use in the 802.11ad PHY – Control PHY, Single Carrer PHY and an OFDM PHY. The Control PHY and certain elements of the SC PHY are mandatory whilst other modes are optional. Data channel of 1.76 GHz bandwidth is sampled using an IQ sample rate of 2.64GHz.
The Control PHY (CPHY) has to be extremely reliable and so uses differential encoding with spread spectrum coding and BPSK modulation, coupled with high levels of error correction to provide the control signals at 27.5Mbit/s for the link.
The Single Carrier PHY (SCPHY) mode uses a single carrier modulation such as BPSK, QPSK or 16-QAM for a simple link with a fixed data rate of 1.76 Gsymbols/s that translates to a raw bit rate from 385Mbit/s to 4620Mbit/s.
For higher data rates, Orthogonal Frequency Division Multiplex (OFDM)