Digital Signal Processing Reference
In-Depth Information
Ta b l e 2 . 2
802.11a
transmission rates
Data Rate
(Mbps)
PHY mode
Modulation
N
Mod
Code Rate
B
c
1
BPSK
1
/
2
6
2
BPSK
3
/
4
9
3
QPSK
1
/
2
12
4
QPSK
3
/
4
18
5
16-QAM
1
/
2
24
6
16-QAM
3
/
4
36
7
64-QAM
2
/
3
48
8
64-QAM
3
/
4
54
information is transmitted via multiple orthogonal subcarriers. For 802.11a, 48 sub-
carriers and 4 pilot subcarriers are used, covering a total of 20 MHz in bandwidth
including zero carriers. This modulation scheme is very effective for broadband,
hence high-speed, transmission since it efficiently handles the frequency selectiv-
ity of the channel. Moreover, it can be implemented elegantly using Fast Fourier
Transform (FFT) and Inverse FFT (IFFT) digital blocks.
Eight transmission rates have been standardized (Table
2.2
), making transmission
rate a possible control dimension. A transmission rate is set by configuring both the
symbol modulation (
N
Mod
) and the Forward Error Correction (FEC) code rate (
B
c
).
The current value chosen is communicated in the
RATE
field in the Physical Layer
Convergence Protocol (PLCP) header of the PHY Protocol Data Unit (PPDU).
Finally, the output power
P
Tx
is limited to 30 dBm or 1 W. The output power can
be considered as a PHY control dimension, next to the already introduced digital
PHY control dimensions modulation and code rate. As the output power is a result
of the Power Amplifier (PA) settings, it is referred to as an analog or front-end PHY
control dimension in the remainder of this topic. In Chap. 6, we model the impact of
those control dimensions on the effective energy and performance, assuming a real-
istic implementation of the analog and digital modem. However, the output power
also has implications on the channel access of a CR, as is discussed in Chap. 7.
2.2.1.2 The IEEE 802.11n Physical Layer
The IEEE 802.11n standard supports data rates up to 600 Mbps. This is mainly
achieved using Multiple-Input-Multiple-Output (MIMO) techniques, where up to
4 spatial streams are used in parallel. MIMO techniques that are supported by the
standards are space division multiplexing (SDM), transmitter beam-forming and
space-time block coding (STBC). The use of MIMO systems mainly increases the
throughput and coverage as compared to single antenna systems. Clearly, the possi-
bility to vary the MIMO scheme as function of throughput requirements and channel
condition can be considered to be a new flexibility offered by the 802.11n PHY.
Next, the 802.11n offers the possibility to use up to 2 802.11a channels in par-
allel, which is referred to as channel bonding. As a result, the bandwidth can vary
