Page 443 - From GMS to LTE

Wireless Local Area Network (WLAN) 429

band available for unlicensed use is located in the 60 GHz range. At 60 GHz, a signal’s
wavelength is only 5 millimeters and hence individual antennas can be very small, which
allows the use of antenna arrays at the transmitter and receiver side to improve range and
signal quality. A major difference compared to WLAN in 2.4 and 5 GHz is the very high
signal absorption rate of 91 db after only 10 meters. As a consequence, data transmis-
sions in this frequency range are unable to penetrate walls and reflection on surfaces is
poor. This limits the indoor use of WLAN at 60 GHz to a few meters and to line‐of‐sight
environments. To improve range and signal quality, beamforming is required to focus
the signal energy at the transmitter towards the receiver. In essence, data transfers in this
frequency band have a quasi‐optical propagation behavior and are limited to in‐room
coverage when used with devices such as notebooks and smartphones. On the other
hand, this limitation is also beneficial as there can be a high number of 60 GHz devices in
very close proximity not interfering with each other. Another application of 802.11ad is
for outdoor wireless point‐to‐point connectivity where larger antennas with a very nar-
row signal beam of 1 degree can be installed and aligned. This way, a distance of several
hundred meters can be bridged with datarates exceeding several hundred Mbit/s.
Above the physical layer, several application protocols have been specified for
applications such as wireless displays and wireless PCI bus extensions. This chapter,
however, focuses on the WLAN physical layer extension for the 60 GHz band as
defined in chapter 21 of the IEEE 802.11 specification [14]. Here, the PHY of 802.11ad
is referred to as ‘Directional Multi‐Gigabit (DMG) PHY’. The name already implies
that the signal has to be directional and no longer omnidirectional as in the 2.4 and
5 GHz bands.
Depending on the regulatory domain, up to four channels with a bandwidth of
2.16 GHz each are available, as shown in Table 6.8. It is important to note at this point
that this channel bandwidth is over 100 times the size of a 20 MHz channel in the 2.4 or
5 GHz band. Even if eight channels are aggregated in the 5 GHz band, the resulting
160 MHz channel is still 12 times narrower. Because of the high signal attenuation, data
transmission is limited to a single stream. As can be seen in Table 6.8, channel 2 has
been assigned for license‐free operation in all regions and it is thus the default channel.
On the physical layer three modulation schemes have been defined. A single‐carrier
transmission option has been specified for raw datarates between 385 Mbit/s and 4.620
Gbit/s. For devices with limited power capabilities this PHY has been extended with a
low‐power option. For even higher speeds up to 6.757 Gbit/s an OFDM‐based transmission
scheme is specified.

Table 6.8 60 GHz channel availability in different regions.

Channel 1 Channel 2 Channel 3 Channel 4
(58.32 GHz) (60.48 GHz) (62.64 GHz) (64.80 GHz)
USA, Canada and Korea
European Union
China
Japan
Australia