Digital Signal Processing Reference
In-Depth Information
Ta b l e 6 . 1
PHY parameters considered
Performance Model
Energy Model
PHY Control Dimensions
P
FE
=
P
FE
=
W
=
20 MHz
200 mW
Back-off (dB) {6 to 16}
E
DSP
=
E
DSP
=
B
=
250 kBaud
250
P
Tx
(dBm) {0 to 20}
nJ
/
symbol
E
DSP
=
N
c
=
48
8
.
7nJ
/
bit
Modulation
N
Mod
{
BPSK, QPSK,
16
−
QAM,
64
−
QAM
}
Block
=
288
P
idle
=
200 mW
Code Rate
B
c
{
1
/
2
,
2
/
3
,
3
/
4
}
N
f
=
10 dB
L
frag
=
1024 Bytes
6.4.1 Energy-Performance Anticipation
Four control dimensions for the physical layer have been introduced to steer energy
and performance of these OFDM transceivers. As function of these control dimen-
sions, the energy
E
Tx
(
K
i
)
needed to send and the energy
E
Rx
(
K
i
)
needed to receive
a unit of
L
frag
bytes need to be characterized.
We realistically assume that the energy consumption of the digital baseband
is a linear function of the number of symbols to decode (
E
DSP
and
E
DSP
in en-
ergy/symbol). For the turbo coder, the energy cost depends on the number of bits
to decode (
E
DSP
in energy/bit) at the receiver [77]. The block size used for the
turbo coding is 288 bits. Based on current implementations [78], the frequency syn-
thesizer, Analog-to-Digital Converter (ADC), Digital-to-Analog Converter (DAC),
Low-Noise Amplifier (LNA) and filters are assumed to have a fixed front-end power
consumption
P
FE
as given in Table
6.1
. The time needed to wake-up the system
(stabilization time for the Phase-Locked Loop (PLL) in the frequency synthesizer)
is assumed to be 100 µs, which is optimistic but can be achieved when designing
frequency generators for this purpose. Application layer frames are fragmented at
the link layer into
L
frag
-sized fragments. We obtain the following expressions for
the energy needed to send or receive a fragment of length
L
frag
, as a function of the
current knob settings:
P
PA
+
P
FE
B
bit
E
DSP
E
Tx
=
+
×
8
L
frag
,
(6.11)
N
c
×
N
Mod
×
B
c
P
FE
E
DSP
E
DSP
E
Rx
=
B
bit
+
N
c
×
N
Mod
×
B
c
+
×
8
L
frag
.
(6.12)
Finally, to complete the model, we introduce a term
P
idle
that denotes the power
consumption when the transceiver is idle (Table
6.1
).
Obtaining the actual values for energy consumption for each of the configura-
tions only depends on the fragment size, and the analog and digital baseband power
consumption values as function of the configuration settings. In practice, this in-
formation is obtained very fast by transmitting a
L
frag
packet once (requiring 0.1
to 1.3 ms depending on the configuration) using e.g. 1000 configurations (hence
1
.
3
10
−
3
×
×
1000
=
1
.
3 s for a complete system profile). It is acceptable to spend
