Digital Signal Processing Reference
In-Depth Information
Fig. 6.4
Adapting the PA gain compression characteristic allows to translate a transmit power or
linearity reduction into an effective energy consumption gain
energy consumption gain. A back-off,
b
, can be used to steer the linearity of the
amplification system versus the energy efficiency of the PA [21]. The back-off is de-
fined as the ratio of the effective PA output power to the saturation output power cor-
responding to the 1 dB gain compression point (Fig.
6.4
(b)). The saturation power
and hence signal distortion for class A amplifiers (used with OFDM) are controlled
by modifying the bias current of the amplifier, which directly influences its energy
consumption (Fig.
6.4
(a)). Since the linearity of the PA determines the distortion
added to the signal, we can write the performance loss due to the PA as function of
b
:
D
i
(b)
. Consequently, we save energy from the increased PA efficiency, provided
we ensure that the received signal to noise and distortion ratio (SINAD) is above
the required sensitivity and do not need to retransmit the packet. For the system to
be practical only discrete settings of the control dimensions are considered (listed in
Table
6.1
).
We further consider the eight PHY rates supported by 802.11a based on four
modulation and three code rates (Table 2.2). Given the above introduced
N
Mod
and
B
c
, the bitrate (
B
bit
) achieved for each modulation-coding pair with
N
c
OFDM car-
riers and symbol rate
B
is given by:
B
bit
=
N
c
×
N
Mod
×
B
c
×
B.
(6.1)
