Page 242 - From GMS to LTE

228 From GSM to LTE-Advanced Pro and 5G

Table 4.3 Defined bandwidths for LTE.

Bandwidth (MHz) Number of subcarriers FFT size

1.25 76 128
2.5 150 256
5 300 512
10 600 1024
15 900 1536
20 1200 2048

To save bandwidth, the subcarriers are spaced in such a way that the side lobes of each
subcarrier wave are exactly zero at the center of the neighboring subcarrier. This property
is referred to as ‘orthogonality’. To decode data transmitted in this way, a mathematical
function referred to as Inverse Fast Fourier Transformation (IFFT) is used. In essence,
the input to an IFFT is a frequency domain signal that is converted into a time domain
signal. As each subcarrier uses a different frequency, the receiver uses an FFT that
shows which signal was sent in each of the subcarriers at a specific instant in time.
Figure 4.5 shows how the concept works in practice. At the top left, the digital data
stream is delivered to the transmitter. The data stream is then put into parallel streams,
each of which is then mapped to subcarriers in the frequency domain. An IFFT function
is then used to convert the result into a time domain signal, which can then be modu-
lated and sent over the air to the receiver. The receiving end is shown in the lower part
of the figure. After demodulation of the signal, it is fed into the FFT function, which
converts the time domain signal back into a frequency domain representation in which
the individual subcarrier frequencies can then be detected. Finally, the slow data streams
from each subcarrier are assembled again into the single fast data stream, which is then
forwarded to higher layers of the protocol stack.
LTE uses the following physical parameters for the subcarriers:
subcarrier spacing: 15 kHz;
●
length of each transmission step (OFDM symbol duration): 66.667 microseconds;
●
standard cyclic prefix: 4.7 microseconds. The cyclic prefix is transmitted before
●
each OFDM symbol to prevent intersymbol interference due to different lengths in
several transmission paths. For difficult environments with highly diverse transmission
paths, a longer cyclic prefix of 16.67 microseconds has been specified as well. The
downside of using a longer cyclic prefix is a reduced user data speed since the symbol
duration remains the same and, hence, fewer symbols can be sent per time interval.

It is interesting to compare the very narrow subcarrier spacing of 15 kHz to the 200
kHz channels used in GSM to see just how narrow the individual subcarriers are.
Further, the subcarrier spacing remains the same regardless of the overall channel band-
width. For a wider channel, the number of subcarriers is increased while the individual
subcarrier bandwidth remains the same. This is an important concept as this enables
and preserves the channel bandwidth flexibility even beyond the maximum of 20 MHz
specified for LTE in 3GPP Release 8.