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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Noise Codes). The most powerful consumptions <strong>of</strong> the DSSS-PHY are to generate chipped data<br />
streams <strong>and</strong> recover data streams from chipped streams. [24] – [28][16]<br />
“Spreading ratio” is an important parameter used to decide how many chips are needed for<br />
one bit. The improper spreading ratio <strong>wi</strong>ll cause the waste <strong>of</strong> b<strong>and</strong><strong>wi</strong>dth. Although to increase<br />
the spreading ratio does help the ability <strong>of</strong> recovering data, it <strong>wi</strong>ll also require higher chipping<br />
rate <strong>and</strong> larger frequency b<strong>and</strong>. And there are prices to pay for increasing the chipping rate. First<br />
is the hardware, the costly high frequency RF components <strong>and</strong> second is the cost <strong>of</strong> enlarging<br />
frequency b<strong>and</strong><strong>wi</strong>dth.<br />
The modulation method used for DSSS are DBPSK (Differential Binary Phase shift Keying)<br />
for 1 Mbps <strong>and</strong> DQPSK (Differential Quadrature Phase shift Keying) for 2 Mbps. The encoding<br />
method is Barker code. [24] – [26][16]<br />
3.3.1.4.2. Against interference<br />
To avoid interference <strong>and</strong> guarantee the fluency <strong>of</strong> the data streams, the two channels,<br />
according to 802.11 rules, must be at least 25 MHz away. The overlapped channels <strong>wi</strong>ll cause<br />
more interference <strong>and</strong> damage the transmitted frames. By using DSSS transmission the<br />
interference problem have been improved to a certain extent. During transmission, because chips<br />
have been added to the data streams, they can get protection from chips. It is exactly like the<br />
function <strong>of</strong> armor. Even if the chips are damaged, the data is still safe. However, if the<br />
interference is really strong, it can still hurt the data very badly, <strong>and</strong> then nothing can be<br />
recovered. [27][28]<br />
3.3.1.4.3. PLCP <strong>and</strong> PMD <strong>of</strong> DSSS<br />
The DSSS-PLCP adds six parts in the DSSS-PPDU. The <strong>fi</strong>rst two parts are PLCP preamble<br />
<strong>and</strong> the other four are headers. The structure is showed in Figure 3-4[21]. The data rate <strong>of</strong> DSSS-<br />
PLCP is 1 Mbps <strong>and</strong> 2Mbps. This preamble is also called a long preamble. The function <strong>of</strong> the<br />
preamble is to help a receiver to synchronize to the incoming frame. The header contains the<br />
information <strong>of</strong> the frame.<br />
The DSSS-PMD is used to operate transmission <strong>and</strong> reception between stations. It is also in<br />
charge <strong>of</strong> modulating <strong>and</strong> de-modulating the PPDUs. The relationship between PLCP <strong>and</strong> PMD<br />
is like a brain <strong>and</strong> a body. PMD is response for action <strong>and</strong> PLCP gives an order. [04] –<br />
[06][16][21]<br />
17