Page 434 - From GMS to LTE

420 From GSM to LTE-Advanced Pro and 5G

Space Time Block Coding (STBC) diversity support (described in more detail below);
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Power Save Multipoll (PSMP) support, an enhanced PS mechanism;
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optional MIMO beamforming support (see below);
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support of optional dynamic antenna selection methods (see below).
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The ‘HT Information’ parameter (element ID 61) is the second new parameter
contained in beacon frames. This parameter is used by the AP to inform clients as to
which HT functionalities are currently used in the network and which must not be used,
for example, to preserve backward compatibility. The parameter indicates the following:
Whether 40 MHz transmissions may be used or if transmissions must be limited to
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the primary 20 MHz channel.
The operation mode of the network: Greenfield, HT‐mixed, non‐member protection
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modes (to protect transmissions of clients that communicate with other APs that use
the same channel).
If there are devices in the network that do not support Greenfield mode.
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Activation of overlapping BSS protection. If the AP detects beacon frames of other APs
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on the same channel that do not support HT extensions or operate in mixed mode, it
can instruct clients with this bit to also activate mixed‐mode support. Neighboring APs
that detect this bit but do not detect non‐HT‐capable clients do not have to set the bit.
This ensures that HT‐capable networks can coexist with non‐HT‐capable networks on
the same channel and limits the use of such measures to areas where it is necessary.
If a secondary beacon is sent, the AP informs client devices whether the beacon frame
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was sent on the primary 20 MHz channel of a 40 MHz channel or on the secondary
20 MHz channel.

The HT capability and HT information parameters are also included in association,
reassociation and probe‐response frames. Client devices are additionally informed of
all necessary parameters and the current configuration during initial communication
and when reselecting to a different AP in the same network.
In addition to the HT parameters, 802.11n compatible APs also broadcast 802.11e
QoS parameters in beacon frames, as discussed in more detail in Section 6.8.
During the association procedure, each client device in return informs the AP of its
HT capabilities. APs can adapt transmissions to individual client devices by using only
the supported transmission options. An AP can therefore communicate over a 40 MHz
channel with two MIMO streams and a short GI with one device while a frame for a
device with fewer capabilities is sent over a 20 MHz channel with an 800 nanoseconds
GI and without using MIMO.
Owing to the required backward compatibility for 802.11b, g and a devices and the
many optional extensions of 802.11n, a device can choose from many data transfer
options before transmitting a frame. If a frame is sent to an 802.11b device, HR/DSSS
modulation has to be used and an appropriate coding rate is selected depending on the
current signal conditions. For 802.11g devices, OFDM modulation is used with fewer
subchannels (non‐HT format) compared to transmissions to 802.11n devices. Also, an
802.11g PLCP header has to be used. OFDM is also used for a transmission between
two 802.11n devices. Compared to 802.11g, however, a shorter PLCP header is used,
which contains HT‐specific information (HT Greenfield mode). If 802.11n and 802.11g
devices are present in the network (HT‐mixed mode), as shown in Figure 6.18,