Page 244 - From GMS to LTE

230 From GSM to LTE-Advanced Pro and 5G

as possible. 3GPP has hence decided to use a different transmission scheme, referred
to as Single Carrier Frequency Division Multiple Access (SC‐FDMA). SC‐FDMA is
similar to OFDMA but contains additional processing steps, as shown in Figure 4.6.
In the first step, shown in the top left of the figure, the input signal is delivered.
Instead of dividing the data stream and putting the resulting substreams directly on
the individual subcarriers, the time‐based signal is converted to a frequency‐based
signal with an FFT function. This distributes the information of each bit onto all
subcarriers that will be used for the transmission and thus reduces the power differ-
ences between the subcarriers. The number of subcarriers used depends on the
signal conditions, the transmission power of the device and the number of simulta-
neous users in the uplink. Subchannels used for uplink transmissions are encoded
with 0. This frequency vector is then fed to the IFFT as in OFDMA, which converts
the information back into a time‐based signal. For such a signal, it can be mathe-
matically shown that the PAPR is much lower than that obtained without the addi-
tional FFT.
At the receiving end shown in the lower part of Figure 4.6, the signal is first demodu-
lated and then sent to the FFT function as in OFDMA. To get the original signal back,
the resulting frequency signal is given to an IFFT function that reverses the initial
processing step of the transmitter side.
Apart from the additional processing step on the transmitting and receiving sides, the
same physical parameters are used as for the downlink direction.

SC-FDMA f t Modulation and
FFT 0 IFFT A amplification
0
1101110001 c 01
d 10 t

0
A
ab cd e f
f t
IFFT FFT Amplification and
A demodulation
Detect c 01
d 10
t
1101110001
A

ab c d e f

Figure 4.6 Principles of SC‐FDMA for uplink transmission. Source: Wiley 2009. Reproduced with
permission from John Wiley and Sons Ltd.