Page 445 - From GMS to LTE

Wireless Local Area Network (WLAN) 431

For high‐quality signal conditions the optional OFDM PHY can be used, to which
MCS 13 to 24 have been assigned. MCS 13 uses SQPSK and a coding rate of ½, which
results in a raw physical layer transmission speed of 693 Mbit/s. MCS 24 uses 64‐QAM
and a coding rate of 13/16 for a physical layer transmission speed of 6.757 Gbit/s in a
2 GHz channel. A 512 point FFT (Fast Fourier Transformation) is used to decode 355
subcarriers spaced 5.15625 MHz apart. This results in a total bandwidth use of
1830.47 MHz. The 355 subcarriers are used for 16 pilot channels with a known refer-
ence signal, 3 empty DC channels and 336 data carriers. Further PHY details can be
found in [15].
On the MAC layer, 802.11ad is organized in a different way compared to the PHYs
described before. Access to the channel is split into Beacon Intervals (BI). At the begin-
ning of each BI a Beacon Header Interval (BHI) with the following zones is used for
channel management purposes:
Beacon Transmission Interval (BTI): The access point uses beamforming to send
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beacon frames in different directions. This is required for beacon frames to reach
devices which the access point is not yet aware of.
Association Beamforming Training (A‐BFT): This section of the BHI is used to
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calibrate transmit antenna configurations towards destination devices.
Announcement Transmission Interval (ATI): Here, frames are exchanged between a
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client device and an access point for beamforming management.
The Data Transmission Interval (DTI) follows the beacon header and is used, like in
other PHYs, for transferring user data frames and MAC layer control frames such as
frame acknowledgements. Access to the channel can either be contention‐based with a
distributed coordination function, or, optionally, contention‐free, as will be described
in more detail in the section on Quality of Service below.
At the end of each beacon interval a Sector Level Sweep (SLS) phase is appended for
beamforming training operations. During this time a client device asks another device
(e.g. the access point) to send several frames with a specific antenna pattern. The client
device then evaluates which antenna configuration it should use for receiving user data
frames. In addition an optional beam refinement protocol phase can be used to further
refine the beamforming settings.
In practice 802.11ad access points and client devices usually also implement other
PHYs for the 2.4 and 5 GHz band. As the range of 60 GHz is much more limited and
directional compared to the lower‐frequency PHYs, a fast session‐transfer procedure to
and from the 60 GHz spectrum is part of 802.1ad for reacting to losing and regaining
60 GHz connectivity. In transparent mode a device can seamlessly switch to and from
the 60 GHz band and continue an ongoing data transmission. This is transparent to
applications, which will only notice a change in transmission speed. When fast session
transfer is configured, a timer is used on both sides of a radio link. If the link suddenly
fails the timer countdown starts and each side performs a session transfer if reestablish-
ment of the radio link did not succeed before the timer reached zero. The fast session‐
transfer functionality can also be used when a device uses a lower‐frequency band and
the radio link in the 60 GHz band is reestablished. Further details on this and other
MAC layer functionalities can be found in [16].
As in other PHYs it is interesting to note that quite a bit of overhead has to be deducted
from the physical layer datarate usually quoted in product advertisements to get the