All the bits transmitted by the OFDM PMD in
the data portion are scram-
bled using a frame-synchronous 127-bit sequence generator. Scrambling is used to
randomize the service, PSDU, pad bit, and data patterns, which may contain long
strings of binary 1s or 0s. The tail bits are not scrambled.
All information
contained in the service, PSDU, tail, and pad are encoded
using convolutional encoding
R
1
/
2
,
2
/
3
, or
3
/
4
corresponding to the desired
data rate. Puncture codes are used for the higher data rates.
PLCP Header
Coded-OFDM: Rate
indicated
by signal symbol
Reserved
1-bit
Length
12-bits
Parity
1-bit
Tail
6-bits
Service
16-bits
PDSU
Tail
6-bits
Pad
bits
Rate
4-bits
PLCP Preamble
12-Symbols
Signal
(1) OFDM Symbol
Data: Variable
Number of OFDM Symbols
PPDU
Coded OFDM
BPSK Rate = 1
2
Figure 21.12 OFDM PLCP preamble, header, and PSDU.
Table 21.7 PSDU data rate.
Rate (Mbps)
Modulation
Coding rate
Signal bits
6 BPSK
R
1
/
2
1101
9
BPSK
R
3
/
4
1111
12
QPSK
R
1
/
2
0101
18
QPSK
R
3
/
4
0111
24 16-QAM
R
1
/
2
1001
36 (optional)
16-QAM
R
3
/
4
1011
48 (optional)
64-QAM
R
2
/
3
0001
54 (optional)
64-QAM
R
3
/
4
0011
21.5 IEEE 802.11 WLAN
731
Ch21-P373580.indd 731
5/3/07 10:58:20 PM
732
21 Wireless Local Area Networks
In OFDM modulation, the basic principal of operation is to divide a high-speed
binary signal to be transmitted into a number of lower data rate subcarriers. There are
48 data subcarriers and 4 carrier pilot subcarriers for a total of 52 nonzero subcar-
riers defi ned in IEEE 802.11a. Each lower data rate bit stream is used to modulate a
separate subcarrier from one of the channels in the 5 GHz band. Intersymbol interfer-
ence is generally not a concern
for a lower speed carrier; however, the subchannels
may be subjected to frequency selective fading. Therefore, bit interleaving and convo-
lutional encoding is used to improve the BER performance. The scheme uses integer
multiples of the fi rst subcarrier, which are orthogonal to each other.
Prior to transmis-
sion, the PPDU is encoded using a convolutional coded rate
R
1
/
2
, and the bits are
reordered and bit interleaved for the desired data rate. Each bit is then mapped
into a complex number according to the modulation type and subdivided into
48 data subcarriers and 4 pilot subcarriers. The subcarriers
are combined using
an inverse fast Fourier transform and transmitted. At the receiver, the carrier is
converted back to a multicarrier lower data rate form using an fast frequency
transform (FFT). The lower data subcarriers are combined to form the high rate
PPDU. Figure 21.13 shows IEEE 802.11a OFDM PMD.
The 5 GHz frequency band is segmented into three 100 MHz
bands for opera-
tion in the United States. The lower band ranges from 5.15–5.25 GHz, the middle
band ranges from 5.25–5.35 GHz, and the upper band ranges from 5.725–5.825 GHz.
The lower and middle bands accommodate 8 channels in a total bandwidth of
200 MHz and the upper band accommodates 4 channels in a 100 MHz bandwidth.
The frequency channel center frequencies are spaced 20 MHz apart. The outermost
channels of the lower and middle bands are centered 30 MHz from the outer edges.
In the upper band the outermost channel centers are 20 MHz from the outer edges.
Transmit
OFDM PMD
PPDU
Bit Interleaving
and
Mapping
Inverse
FFT
Symbol
Shaping
QAM
Modulator
Convolution
Encoder
Receive OFDM PMD
PPDU
AFC and Clock
Recovery
PSK/QAM
Modulator
Convolution
code Decoder
FFT
Bit
Deinterleaving
and
Demapping
