Page 243 - From GMS to LTE

Long Term Evolution (LTE) and LTE-Advanced Pro 229

f t
Modulation and
OFDM(A) a 11 IFFT A amplification
b 01
1101110001 c 11
t
d 00
e 01
A

abcd e f

f t
a detect FFT Amplification and
b detect A demodulation
1101110001 c detect

d detect
t
e detect
A

ab cd e f

Figure 4.5 Principles of OFDMA for downlink transmission. Source: Wiley 2009. Reproduced with
permission from John Wiley and Sons Ltd.

In UMTS, users are scheduled in the code domain and, with HSPA, additionally in the
time domain. In LTE, users are scheduled in the frequency domain, that is, at a certain
point in time several users can receive data on a different set of subcarriers. In addition,
users are scheduled in the time domain.

4.3.2 SC‐FDMA for Uplink Transmission
For uplink data transmissions, the use of OFDMA is not ideal because of its high Peak
to Average Power Ratio (PAPR) when the signals from multiple subcarriers are combined.
In practice, the amplifier in a radio transmitter circuit has to support the peak power
output required to transmit the data and this value defines the power consumption of
the PA device regardless of the current transmission power level required.
With OFDM, the maximum power is seldom used and the average output power
required for the signal to reach the base station is much lower. Hence, it can be said that
the PAPR is very high. The overall throughput of the device, however, does not corre-
spond to the peak power but instead corresponds to the average power, as this reflects
the average throughput. Therefore, a low PAPR would be beneficial to balance power
requirements of the transmitter with the achievable datarates.
For a base station, a high PAPR can be tolerated as power is abundant. For a
mobile device that is battery driven, however, the transmitter should be as efficient