in ofdma transmitter for lte downlink

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mai fouad

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i want to know in the block diagram of ofdma transmitter what is mean by subcarrier mapping ?? how to make that ofdma downlink transmitter.png

is it related to resource grid of the lte resource grid.png
 
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It means that the complex valued modulation symbols obtained as a result of the modulation scheme block (QPSK< 16-QAM, or 64-QAM) are mapped to resource elements in the 2D time-frequency resource grid. Suppose that the UE is assigned 1 resource block for transmission. That is 12 resource elements that can be used during one OFDMA symbol duration. Hope this helps.
 

The x-axis in the second diagram refers to time (in the unit of OFDM symbols), and the y-axis is the subcarrier index. If a user is assigned a set of squares in the second diagram, it means that that user can transmit on those subcarriers in that symbol. As an example, let user 1 be assigned to subcarriers 15-25 in symbol 1, and then silent after that. This means that in the first OFDM symbol, the subcarriers 15-25 would contain data from user 1, and afterwards would be reallocated for some other purpose for successive symbols.
 

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