Figure 21.8 DSSS PHY PPDU frame

726 
21 Wireless Local Area Networks
Each DSSS PHY channel occupies 22 MHz of bandwidth and allows for 
three noninterfering channels spaced 25 MHz apart in the 2.4 GHz frequency 
band (see Figure 21.9). With this channel arrangement, a user can confi gure 
multiple DSSS networks to operate simultaneously in the same area. Table 21.5 
lists the DSSS channels used in different parts of the world. Fourteen frequency 
25 MHz
25 MHz
22 MHz
#11
#6
#1
2.4 GHz
2.412 GHz
2.437 GHz
2.462 GHz
Minimum
channel spacing
between center
frequencies
Figure 21.9 Channel spacing for IEEE 802.11 DSSS networks.
Table 21.5 DSSS channels for different parts of the world.
Channel 
number
Frequency 
GHz
North 
America
Europe
Spain
France
Japan
1
2.412
x
x
2
2.417
x
x
3
2.422
x
x
4
2.427
x
x
5
2.432
x
x
6
2.437
x
x
7
2.442
x
x
8
2.447
x
x
9
2.452
x
x
10
2.457
x
x
x
x
11
2.462
x
x
x
x
12
2.467
x
x
13
2.472
x
x
14
2.483
x
Ch21-P373580.indd 726
5/3/07 10:58:16 PM

channels are defi ned for operation across the 2.4 GHz frequency band. In North 
America 11 frequencies are used ranging from 2.412 to 2.462 GHz. In Europe 
13 frequencies are allowed between 2.412 and 2.472 GHz. In Japan only channels 
at the 2.483 GHz frequency are permitted.
The maximum allowable radiated power for DSSS PHY varies from region 
to region (refer to Table 21.6). The transmit power is directly related to the range 
that a particular implementation can achieve.
Example 21.1
A QPSK/DSSS WLAN is designed to transmit in the 902–928 MHz ISM band. 
The symbol transmission rate is 0.5 Mega symbols/sec. An orthogonal code with 
16 symbols is used. A bit error rate of 10
5
is required. How many users can be 
supported by the WLAN? A sector antenna with a gain of 2.6 is used. Assume 
interference factor 
0.5 to account for the interference from users in other cells 
and power control effi ciency 
0.9. What is the bandwidth effi ciency?
Solution
B
w
928 
902 
26 MHz
 
R
b
R
s
log
2
16 
2 Mbps
G
p
26
2
13
P
b
10
5
1 
2
erfc
E
b
 
N
0
E
b
 
N
0
10 dB
M
G
p
 
(
E
b
/
N
0
)
1 
1 
13
10
1 
1 
0.5
2.6 
0.9 
2
2 
2 
26
0.16 bps/Hz 

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