Page 430 - From GMS to LTE

416 From GSM to LTE-Advanced Pro and 5G

implemented in practice. The following section describes the new functions of the High
Throughput (HT) PHY and the MAC layer extensions that are defined as mandatory in
the standard. In addition, options that are used in practice today are described.
An easy way to increase throughput is to increase the channel bandwidth. In addition
to the 20 MHz channels, it is now also possible to use a 40 MHz channel. In practice, this
was already implemented by many vendors in a proprietary fashion in 802.11g products.
As a consequence, devices from different manufacturers were often not interoperable at
higher speeds. In practice, all 802.11n‐capable PCs and WLAN APs support 40 MHz
channels in a standardized manner today. Some 802.11n‐compatible smartphones, how-
ever, are still limited to 20 MHz channels.
In addition to using a wider channel bandwidth, the number of OFDM subchannels
on a 20 MHz carrier has been increased from 52 to 56. The subchannel bandwidth of
312.5 kHz, however, remains the same. The additional subchannels occupy bandwidth
that was not used up to this point, at the right and left sides of the carrier. The number
of pilot subchannels remains unaltered at four. If two channels are combined to be a
40 MHz channel, 114 subchannels are used for data transmission and six subchannels
are used as pilot channels, that is, for channel estimation. Table 6.4 compares the PHY
carrier parameters of 802.11g to the 20 MHz and 40 MHz bandwidth options of 802.11n.
The initial 802.11 specification required the transmission of an ACK frame to con-
firm the correct reception of each data frame. This is important to ensure that frames
are correctly received over the comparatively unreliable air interface and to be able to
retransmit faulty data as quickly as possible. The disadvantage of this approach is that
the air interface is not used efficiently. To reduce this overhead, the 802.11n standard
has specified a method to aggregate frames on the MAC layer, allowing frames to be
transmitted together. This is much more efficient when transmitting large amounts of
data as only a single ACK is required for the aggregated frames. The maximum aggre-
gated frame size is 65,535 bytes. The disadvantage of this method is, however, that in
case of a transmission error, the aggregated frame has to be completely retransmitted.
Figure 6.16 compares the default data transmission to a transmission in which the
frames are aggregated.
Another air interface parameter that was optimized is the OFDM guard interval (GI).
This is required between two OFDM symbols to reduce the interference between
consecutive symbols. In practice, a GI of 400 nanoseconds is sufficient in most transmission
environments when compared to the 800 nanoseconds used previously. This significantly

Table 6.4 Comparison of PHY carrier parameters of 802.11g vs. 11n.

20 MHz non‐HT (as 802.11g) 20 MHz HT 40 MHz HT

Number of carriers 48 52 108
(2 × 54)
Number of pilot carriers 4 4 6
Total number of carriers 52 56 114
(2 × 57)
Unused carriers at the center 1 1 3