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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3.3.1.5. IEEE 802.11a <strong>and</strong> OFDM<br />
3.3.1.5.1. Background<br />
Because there are lots <strong>of</strong> non-802.11 signals that also existed in the 2.4 GHz b<strong>and</strong>, this b<strong>and</strong><br />
is <strong>of</strong>ten very crowded. For achieving higher speed <strong>and</strong> capacity, 802.11 Task group A (TGa)<br />
came out <strong>wi</strong>th a new st<strong>and</strong>ard. It used a 5 GHz un-licensed b<strong>and</strong> <strong>and</strong> a different transmission<br />
method (OFDM). Although this st<strong>and</strong>ard was published in 1999, the practical hardware was not<br />
available until the end <strong>of</strong> 2001. At <strong>fi</strong>rst, 802.11a used the UNII b<strong>and</strong> <strong>and</strong> was speci<strong>fi</strong>ed for the<br />
U.S. only. Later, two similar st<strong>and</strong>ards were published. 802.11h is for Europe <strong>and</strong> 802.11j is for<br />
Japan. [08][30][31][43][44]<br />
OFDM (Orthogonal Frequency Division Multiplexing) was developed in the late 1960s. It is<br />
not a new theory however, because <strong>of</strong> the <strong>wi</strong>reless technology it revived again. OFDM has<br />
several features. First, it uses multiple subcarriers to do a single transmission. Second, for<br />
simplifying equalization at receiver, it transforms scattered channels into parallel narrowb<strong>and</strong><br />
subchannels. Third it is based on simple mathematics. OFDM is related <strong>wi</strong>th FDM (Frequency<br />
Division Multiplexing), that both divide b<strong>and</strong><strong>wi</strong>dth into divisions which is called carriers or<br />
subcarriers <strong>and</strong> use them for transmitting information. [08][17][29][33][36]<br />
3.3.1.5.2. Principles<br />
When transmitting data, OFDM chooses channels which overlap in frequency domain, but<br />
<strong>wi</strong>ll not disturb <strong>wi</strong>th each other. Figure 3-7[17] These overlapping subcarriers are de<strong>fi</strong>ned by<br />
mathematical calculation, therefore, they can travel individually, <strong>and</strong> this relationship is called<br />
orthogonal. With this feature, OFDM can increase capacity in the <strong>fi</strong>xed b<strong>and</strong><strong>wi</strong>dth. Thus the<br />
overall performance is improved. [08][17][29]<br />
Figure 3-7 FDM <strong>and</strong> OFDM<br />
Although OFDM has better performance, there is still a cost. Inter-symbol interference (ISI)<br />
is a common problem when transmitting data. It mainly occurs when the delay that happens in<br />
different paths is too large <strong>and</strong> causes a later copy to shift onto a previously arrived copy. The<br />
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