# How to preserve base band signal's bandwidth

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Glo 2014-12-2

I am using a baseband signal with bandwidth 100 kHz, which I then multiply with a local oscillator of frequency 1.5 MHz. Now, when I see the output after the up-conversion, the base band signal is up-converted to 1.5 MHz frequency, but, the original bandwidth of 100 kHz is not preserved. Rather, the bandwidth is also stretched to 1.5 MHz (the same as the local oscillator's frequency).
How can I up-convert my signal while keeping the original bandwidth of 100 kHz?
I have been trying to look into a lot of things, but couldn't find any solution.
Any help or guidance would be valuable.

### 采纳的回答

Puneet Rana 2014-12-2
For up-conversion you typically need to first increase the sample rate of your input through (multistage) interpolation and then multiply by local oscillator. See the example: http://www.mathworks.com/help/dsp/examples/digital-up-and-down-conversion-for-family-radio-service.html
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Glo 2014-12-9
Thanks, a lot. My problem is solved as I performed interpolation through the RRC filter's upsampling option such that the sample rate of Local Oscillator and input symbols is same. Then I multiplied with the complex exponential.
One side question has arisen from this. After transmission i am using an AWGN channel block. Which time should I specify in the AWGN block option "symbol period"? Should it be the sample time of my binary input, or the sample time after interpolation?
Details of model: I have used 1/100e3 sample time for my digital input (B.W = 100 KHz). For my local oscillator (F_lo = 10 MHz), I have used 1/(4*10e6) sample time. So in the end I have to interpolate/upsample 400 times (4*10e6/100e3).

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