Wireless Security.pdf - PDF Archive

154 Chapter 6
Table 6.5 : US FCC specified U-NII channels used in the 802.11a OFDM PHY
RF Band
Frequency
Range (GHz)
Channel
number
Centre
frequency
(GHz)
Maximum
transmit
power (mW)
U-NII lower band 5.150–5.250 36 5.180 50
40 5.200
44 5.550
48 5.240
U-NII middle band 5.250–5.350 52 5.260 250
56 5.280
60 5.300
64 5.320
U-NII upper band 5.725–5.825 149 5.745 1000
153 5.765
157 5.785
162 5.805
Each of the 20 MHz wide channels accommodates 52 OFDM subcarriers, with a
separation of 312.5 kHz ( 20 MHz/64) between centre frequencies. Four of the
subcarriers are used as pilot tones, providing a reference to compensate for phase and
frequency shifts, while the remaining 48 are used to carry data.
Four different modulation methods are specified, as shown in Table 6.6 , which result in a
range of PHY layer data rates from 6 Mbps up to 54 Mbps.
The coding rate indicates the error-correction overhead that is added to the input data
stream and is equal to m /( m n ) where n is the number of error correction bits applied to
a data block of length m bits. For example, with a coding rate of 3/4 every 8 transmitted
bits includes 6 bits of user data and 2 error correction bits.
The user data rate resulting from a given combination of modulation method and coding
rate can be determined as follows, taking the 64-QAM, 3/4 coding rate line as an
example. During one symbol period of 4 μ s, which includes a guard interval of 800 ns
between symbols, each carrier is encoded with a phase and amplitude represented by
one point on the 64-QAM constellation. Since there are 64 such points, this encodes
6 code bits. The 48 subcarriers together therefore carry 6 48 288 code bits for each
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