A comparison of wi-fi and wimax with case studies - Florida State ...
A comparison of wi-fi and wimax with case studies - Florida State ...
A comparison of wi-fi and wimax with case studies - Florida State ...
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3.3.1.4.4. HR-DSSS <strong>and</strong> CCK<br />
Figure 3-4 DSSS PLCP<br />
The original modulation method (Barker code) used in DSSS, for 802.11, only achieved the<br />
speeds <strong>of</strong> 1 <strong>and</strong> 2 Mbps. For commercial use, this is not an acceptable amount. In the data stream,<br />
each barker word carries one or two bits depending on the data rate. To improve the speed, it is<br />
necessary to increase the capacity <strong>of</strong> each symbol. That means more complicated phase angle<br />
changes. For example, the DQPSK receiver could at least detect four cycle phase differences.<br />
More cycle phase differences <strong>wi</strong>ll cause the smaller phase changes. It ends up <strong>wi</strong>th the need <strong>of</strong> a<br />
highly accurate <strong>and</strong> costly receiver.<br />
Therefore, the next st<strong>and</strong>ard 802.11b had a different modulation method known as<br />
Complementary Code Keying (CCK). This method splits the data stream into code symbols<br />
composed <strong>wi</strong>th 8 bits. By a complex mathematic procedure, the CCK can encode four or eight<br />
bits per code word <strong>wi</strong>th 8-bit code symbols. Also, this procedure can help receivers to identify<br />
different codes easily, even in interference or a multipath fading environment. By using the CCK<br />
process, the speed can reach 5.5 Mbps or 11 Mbps.<br />
The process <strong>of</strong> CCK is quite similar to the DS chipping process. Figure 3-5[17]. The<br />
difference between them is that CCK does not use the static repeating code word, as for example,<br />
Barker Code. The code word is converted from the data. These code words are used to transmit<br />
data <strong>and</strong> spread the signal. Phase angle also plays an important role here, but this time it <strong>wi</strong>ll not<br />
cost as much as DQPSK.<br />
18