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Cdma

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CDMA Tutorial

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Intuitive Guide to Principles of Communications www.complextoreal.com

Code Division Multiple Access (CDMA)
The Concept of signal spreading and its uses in communications Let’s take a stright forward binary signal of symbol rate 2.

Figure 1 – A binary information signal To modulate this signal, we would multiply this sequence with a sinusoid and its spectrum would look like as In figure 2. The main lobe of its spectrum is 2 Hz wide. The larger the symbol rate the larger the bandwidth of the signal.

Figure 2 – Spectrum of a binary signal of rate 2 bps Now we take an another binary sequence of data rate 8 times larger than of sequence shown in Fig. 1.

Copyright 2002 Charan Langton

www.complextoreal.com

CDMA Tutorial

2

Figure 3 – A new binary sequence which will be used to modulate the information sequence Instead of modulating with a sinusoid, we will modulate the sequence 1 with this new binary sequence which we will call the code sequence for sequence 1. The resulting signal looks like Fig. 4. Since the bit rate is larger now, we can guess that the spectrum of this sequence will have a larger main lobe.

Figure 4 – Each bit of sequence 1 is replaced by the code sequence The spectrum of this signal has now spread over a larger bandwidth. The main lobe bandwidth is 16 Hz instead of 2 Hz it was before spreading. The process of multiplying the information sequence with the code sequence has caused the information sequence to inherit the spectrum of the code sequence (also called the spreading sequence).

Figure 5 – The spectrum of the spread signal is as wide as the code sequence The spectrum has spread from 2 Hz to 16 Hz, by a factor of 8. This number is called the the spreading factor or the processing gain (in dBs) of the system. This process can also

Copyright 2002 Charan Langton

www.complextoreal.com

CDMA Tutorial

3

be called a form of binary modulation. Both the Data signal and the modulating...