Page 435 - From GMS to LTE

Wireless Local Area Network (WLAN) 421

Not-HT short Not-HT long
training field training field Non-HT PLCP info MAC Frame

L-STF L-LTF L-SIG Data
8 s 8 s 4 s

Non-HT
PLCP header Non-HT
PLCP Info HT PLCP info HT training fields

L-STF L-LTF L-SIG HT-SIG HT-S+LTF Data
8 s 8 s 4 s 8 s x*4 s

HT-mixed PLCP header

HT-GT-STF HT-LTF1 HT-SIG HT-S+LTF Data
8 s 8 s 8 s x*4 s

HT Greenfield header

Figure 6.18 PLCP header variants.
a backward‐compatible PLCP header is used. This header can also be decoded by
802.11g devices and includes a number of additional bytes. Also, fewer OFDM subchan-
nels are used. If 802.11b devices are additionally present in the network, a CTS packet
has to be sent preceding the data frame using HR/DSSS modulation. In addition, an
802.11n‐compatible device has to be aware of the 802.11n functionalities supported by
the receiver. This is required to control the OFDM modulation (e.g. using a short GI)
and to allow the choice of a 20 MHz or a 40 MHz channel. Furthermore, the number of
MIMO channels used and the coding rate depend on the capabilities of the receiver.
Even this already quite extensive list does not consider additional optional 802.11n
functionalities, which are described in more detail below.
For battery‐driven devices, it is important that the WLAN chip uses as little energy as
possible while no data is being transferred. In general, the PS mode described earlier is
used for this purpose. Some applications that periodically transmit data, such as VoIP,
might prevent the use of this PS scheme, however, as the WLAN chip has to monitor
the channel for incoming data and therefore energy is wasted. For such situations, the
optional 802.11n PSMP enhancement has been specified. With PSMP, a client can
negotiate a transmission and reception pattern with the AP. If granted, the AP estab-
lishes a PSMP window and informs the client as to times at which data can be sent and
received. The client then only activates its transceiver during the agreed window to
receive data packets. Once the downlink window expires, an uplink opportunity win-
dow can be implicitly used without prior reservation of the channel. During all other
times, the client’s receiver can be fully deactivated to save power.