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