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
