Frequency band Max. transmit power with 6 dBi antenna gain 5.150–5.250 GHz
40 mW (2.5 mW/MHz)
5.250–5.350 GHz
200 mW (12.5 mW/MHz)
5.725–5.825 GHz
800 mW (50 mW/MHz)
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734 21 Wireless Local Area Networks Solution f 200 Hz
Minimum number of chips
20
Number of tones contained in spreading bandwidth (
B ss )
2
20
1,048,576
1,048,576
B ss 200
B ss 209,715,200
209.715 MHz
Chip rate
1.2
20
24 kchip/sec
Processing gain (
G p )
209.715
10
6
1.2
10
3
174,762.5
Symbol rate
1.2
10
3
3
400 symbols/sec
Number of chips per symbol
24
10
3
400
60
Example 21.3 The IEEE 802.11a WLAN uses a 64-subchannel implementation of multicarrier
modulation (MCM, i.e., OFDM). Forty-eight subcarriers are used for information
transmission, 4 subcarriers for pilot tones are used for synchronization, and 12 are
reserved. Each subchannel has a symbol rate of 250 kilo symbols per second (ksps).
The occupied bandwidth is 20 MHz. Find the bandwidth of a subchannel. What is
modulation effi ciency? What is a user symbol rate? If 16-QAM modulation is used,
what is the user data rate if the information bits are encoded with a rate of
3
/
4
? If the
guard time between two transmitted symbols is 800 ns, what is the time utilization
effi ciency of the system?
Solution Total number of subcarriers
48
12
4
64
Bandwidth of subchannels
20
10
6
64
312.5 kHz
Modulation effi ciency
250
312.5
0.8 symbols/sec/Hz
User symbol transmission rate
48
250
1.2 Msps
User bit per symbol
4 for 16-QPSK modulation
•
•
•
•
•
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User data rate
3
4
4
12
36 Mbps
Symbol duration
1
250
10
3
4000 ns
Time utilization effi ciency
4000
4800
0.83
