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Figure 3-9 OFDM Modem
3.3.1.5.3. The IEEE 802.11a’s OFDM and its PLCP and PMD
OFDM is a good transmission technology, but TGa did not use the full version. They
modified their own one.
When building an OFDM network, there are some important parameters which are
bandwidth, delay and bit rate. The bandwidth is fixed for 802.11a. When considering delay, it is
necessary to have guard time. Usually the ratio of guard time and delay is two to four times as
big. And the symbol time is five times of the guard time. Thus TGa chose 800 ns as guard time
and 4 µs as symbol time. The bandwidth of an operation band is 20 MHz and the theoretical
highest speed is 54 Mbps. Higher capacity could have higher throughput. However it will
decrease the number of operation channels. Therefore, how to achieve a reasonable balance is an
important design issue. [17][29] - [31][39]
The modulation technology for OFDM is shown in Table 3-4[17]. To reach a higher speed,
TGa has not only changed the transmission method, but also used another modulation technology
QAM (Quadrature Amplitude Modulation). Figure 3-10[55] For preventing the BER (Bit Error
Rate) increase which caused by certain channel’s signal fading, the error correction code is
added to the all sub-channels. The error correction code is called “Convolutional Code”, and this
kind of OFDM sometimes is named COFDM (Coded OFDM). Here is the related research:
[36][39][40] – [42]
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