oversampling the signal after DAC

Dear all

I have Nsample OFDM which i received from DAC such as

-0.26405059224367516  0.06408994957370756  0.12561630116446681  -0.25892339627777855  -0.24354180838008874  0.11023471326677702  0.1051075173008804  0.1051075173008804
....

I would like to oversampling these samples by matlab code for suitable with matrix sie in optisystem (the signal will be transmit to fiber in optisystem for simulation... Because the sampling rate DAC is only 4 GSPS, but the sampling rate in optisystem is 16 GSPS. After transmit over fiber, i receive the signal after photodectector, i would like to downsample these samples, how can i downsample them to receive the orginal sample? Thanks for you support

Answer 1

[Star Strider]

If you have the Signal Processing Toolbox, use the resample function to both upsample and downsample your signals.

[Vu]

Dear Sir/Madam But i don't have that one. How can you help me? I try to do as bellow The signal after DAC is s. i will translate to frequence domain then upsample and translate back time domain. Then transmit over fiber ( through optisystem) then receive by PIN. After that i will down sample that signal then continue process s1= fft(s) s2=upsample(s1) s3= ifft(s2) after receive the signal from PIN s4=downsample(s3) But i'm not sure this procdure correct or not. I i don't know wheather i can use the upsample and downsample in this case or not. Please hlep me soon. Thanks alot

[Star Strider]

If you do not have access to the resample function, you can get similar results with interp1. It is part of MATLAB.

I never tried to upsample or downsample in the frequency domain, so I am not certain that is possible, considering that it is complex. I encourage you to experiment.

[Vu]

Dear Sir/Madam You mean that i will interp function to oversampling in time domain at transmiter side then decimate at reciver side to get original signal?

I've tried use s1= interp(s,4) at transmiter. then use s2=decimate(s1,4) the result is not the original signal i don't know how to get the original signal. Please help me soon Thank you so much

[Vu]

Dear my friend You mean that we can oversampling the signal in time domain by the interp function I've tried to use s1=intep(s,os)%which os is oversampling factor then downsample s2=s1(:,1:os:end); to collect original signal But i'm not sure this way. Can we oversampling signal in time domain?

[Star Strider]

Without knowing more, it looks as though you are doing it correctly. You have to supply a vector of values (times) at which you want to interpolate the values. The linspace function might be the easiest way to create that vector.

The decimation appears to be correct. You will have to experiment with your data and your application to be certain it does what you want.

[Vu]

Dear my friend The these function only increase te sample rate, it does not affect to the signal. I mean that if i have 84000 sample with value as:

-0.187814484507834  -0.0685671927568283  -0.426309068009845  -0.175889755332733  0.169927390745183  0.0685671927568283  -0.175889755332733  0.0149059114688757
...

84000 samples will be transmit during 2.1e-5 seconds. when i increase sample rate up 4 times, Number sample i received 336000 sample as:

-0.187814484508603  -0.111428853815155  -0.0564680268947086  -0.0395866887715861  -0.0685671927579779  -0.151223309909352  -0.256127402209661  -0.356664616707606  -0.426309068010649  -0.430862272035892  -0.383134613447975  -0.292431330314335  -0.175889755333293  -0.0635568124113296  0.0389248703199971  0.119418863833141  0.169927390745207  0.192797336632401  0.180525290507363  0.136380022754065  0.0685671927568499  -0.0202136805381460  -0.101103622398209  -0.156716123279345  -0.175889755332638  -0.137173771285759  -0.0779607419403142  -0.0206524107667478  0.0149059114684460  -0.0108125329382146  -0.0533473910451080  -0.0898207374667763
....

all of them will also be transmit during 2.1e-5 second? thanks you so much

[Star Strider]

Communication engineering is not an area of my expertise. (I am more comfortable with signal processing.) I assume that if you interpolate to increase your signal size by 4 and your sample rate by the same amount, it should transmit in the same time.

[Vu]

First of all, i would like to send you many thanks. As your answer, the signal will be oversampling by interp function in time domain once the signal come out DAC? We do not need translate the signal into frequence domain by FFT? Thanks for your reply

[Star Strider]

My pleasure!

I see no reason to take the fourier transform of the signal. What would you do with it in your application?

[Vu]

I try to simulate OFDM signal and baseband signal over fiber by Optisystem. Because sample rate of DAC is limit 4 GSPS. (I think this rate is suitable in practically). I would like to simulate sample rate in optisystem 16 GSPS. So i try find out the way to oversampling the OFDM signal after DAC.