Page 428 - From GMS to LTE

414 From GSM to LTE-Advanced Pro and 5G

Middle frequency of a Figure 6.15 Simplified representation
subchannel of OFDM subchannels.
Amplitude

Frequency

No interference from neighboring
channels at the middle frequency
of a subchannel

influence the amplitude of neighboring subchannels. OFDM does not transmit data by
changing the phase of the carrier but by changing the amplitudes of the subchannels.
Depending on the reception quality, a varying number of amplitude levels are used to
encode a varying number of bits.
To demodulate the signal, the receiver performs a Fast Fourier Transformation (FFT)
analysis for each transmission step. This method calculates the signal energy (ampli-
tude) over the frequency band. The simplified result of an FFT analysis is shown in
Figure 6.15. The x‐axis represents the frequency band instead of the time as in most
other graphs. The amplitude of each subchannel is shown on the y‐axis.
Table 6.3 gives an overview of the datarates offered by the 802.11g standard. In practice,
an algorithm dynamically selects the best settings depending on reception conditions.
Under ideal transmission conditions, 64‐Quadrature Amplitude Modulation (64‐
QAM) can be used in the subchannels. Together with a 3/4 convolutional coder (three
data bits are coded in four output bits) and a symbol speed of 250,000 symbols/s, a
maximum speed of 54 Mbit/s is reached (216 bits per step × 250,000 symbols/s = 54
Mbit/s). It is to be noted that a similar convolutional coder for increasing redundancy is
also used for GSM and UMTS (see Section 1.7.6).
802.11g client devices and APs are backward compatible with slower 802.11b devices.
This means that 802.11g APs also support 802.11b client devices, which can only com-
municate with a speed of up to 11 Mbit/s. 802.11g client devices can also communicate
with older 802.11b APs. However, the maximum datarate is then, of course, limited to
11 Mbit/s. As slower 802.11b devices are not able to decode OFDM modulated frames,
802.11g devices in mixed configurations have to transmit a CTS packet to themselves
for the reservation of the air interface prior to transmitting a frame. This ensures that
802.11b and g devices can be used simultaneously in a BSS. Furthermore, the PLCP
header of a frame is sent at 1 Mbit/s for all devices to be able to receive the header cor-
rectly. While these procedures ensure interoperability, performance is reduced by about
20% because of the extra overhead of the CTS frames, which can only be sent at a maxi-
mum speed of 11 Mbit/s. Owing to these disadvantages, a ‘G‐only’ mode can be acti-
vated in some APs to avoid this extra overhead.
Under ideal conditions, a maximum transfer rate of about 2500 kB/s can be observed
in an 802.11g BSS. If two wireless devices communicate with each other, the maximum
speed drops to about 1200 kB/s, as all frames are first sent to the AP, which then for-
wards the frames to the wireless destination device. As mentioned earlier, the 802.11e
standard aims to overcome this problem by standardizing direct client‐to‐client