OFDM PMD Operation

OFDM PMD Operation
The operation of the PMD translates the binary representation of the PPDUs into a radio signal
suitable for transmission. The 802.11 OFDM PMD performs these operations by dividing a
high-speed serial information signal into multiple lower-speed sub-signals that the system
transmits simultaneously at different frequencies in parallel.
Operating frequencies for the 802.11s OFDM layer fall into the following three 100MHz unlicensed
national information structure (U-NII) bands: 5.15–5.25GHz, 5.25–5.35GHz, and
5.725–5.825GHz. As shown in Table 5.5, there are 12 20MHz channels, and each band has different
output power limits. In the U.S., the Code of Federal Regulations, Title 47, Section
15.407, regulates these frequencies.
TABLE 5.5 802.11 OFDM Defines Three Frequency Bands Having Different Power
Limitations
Center Maximum Output Power
Channel Frequency (with up to 6 dBi
Frequency Band Numbers (MHz) antenna gain)
U-NII lower band 36 5180 40mW (2.5mW/MHz)
40 5200
(5.15–5.25 MHz) 44 5220
48 5240
U-NII middle band 52 5260 200mW (12.5mW/MHz)
56 5280
(5.25–5.35 MHz) 60 5300
64 5320
U-NII upper band 149 5745 800mW (50mW/MHz)
153 5765
(5.725–5.825 MHz) 157 5785
161 5805
The 802.11 OFDM Physical layer uses a combination of BPSK, QPSK, and QAM, depending
on the chosen data rate (see Table 5.6). Data rates of 6Mbps, 12Mbps, and 24Mbps are mandatory
for all 802.11-compliant products.
TABLE 5.6 802.11 OFDM Modulation Techniques
Code Data
Data Coded Bits per Bits per
Rate Coding Bits per OFDM OFDM
(Mbps) Modulation Rate Subcarrier Symbol Symbol
6 BPSK 1/2 1 48 24
9 BPSK 3/4 1 48 36
12 QPSK 1/2 2 96 48
18 QPSK 3/4 2 96 72
24 16-QAM 1/2 4 192 96
36 16-QAM 3/4 4 192 144
48 64-QAM 2/3 6 288 192
54 64-QAM 3/4 6 288 216
OFDM splits an information signal across 52 separate subcarriers. Four of them are pilot subcarriers
that the system uses as a reference to disregard frequency or phase shifts of the signal
during transmission. A pseudo-binary sequence is sent through the pilot subchannels to prevent
the generation of spectral lines. The remaining 48 subcarriers provide separate wireless pathways
for sending the information in a parallel fashion.
OFDM divides groups (symbols) of 1, 2, 4, or 6 bits, depending on data rate chosen, and converts
them into complex numbers representing applicable constellation points, as shown in
Figures 5.15, 5.16, 5.17, and 5.18. An inverse FFT (fast Fourier transform) combines the subcarriers
before transmission.
The 802.11 OFDM standard requires receivers to have a minimum sensitivity ranging from
82dBm to 65dBm, depending on the chosen data rate. Because of the relatively low power limits
in the lower frequency bands, implementers should carefully consider range requirements of
the application before choosing a particular band.