AWGN Noise with OFDM Signal

Hi, I'm simulating an OFDM system with Multipath and AWGN channels. One
thing that i'm observing is that whenever I simulate the system on baseband
the AWGN command adds almost the correct noise but whenever I modulate the
baseband signal to a carrier and then add the noise, it adds very little
noise compared to previous case. I have implemented the system as follows.
===============================================
function CHN_OUT = channel_multi_td(PARAMS, mod_output)

% mod_output is OFDM transmitter output

mod_out_abs = abs(mod_output);
mod_out_ang = angle(mod_output);

chn_out_mean = mean(mod_out_abs);

% power amplifier model

mod_out_amp_abs =
mod_out_abs./sqrt(1+(mod_out_abs/(chn_out_mean*10^(PARAMS.sat_level/10))).^2);
mod_out_amp_ang = mod_out_ang;

if PARAMS.bypass_pa == 0
    mod_output_pa = mod_out_amp_abs.*exp(j*mod_out_amp_ang);
else
    mod_output_pa = mod_output;
end

% multipath channel

if PARAMS.bypass_multipath == 0
    CHN_OUT_MULTI = filter(PARAMS.channel, mod_output_pa);
else
    CHN_OUT_MULTI = mod_output_pa;
end

% awgn channel

if PARAMS.bypass_awgn == 0
    CHN_OUT_AWGN = awgn(CHN_OUT_MULTI, PARAMS.snr, 'measured');
else
    CHN_OUT_AWGN = CHN_OUT_MULTI;
end

% carrier frequency offset

freq_offset =
2*pi*PARAMS.freq_off*PARAMS.subcarrier_spacing/PARAMS.symbol_rate;
CHN_OUT = CHN_OUT_AWGN.*exp(j*freq_offset*(0:length(CHN_OUT_AWGN)-1));
=========================================================

does anyone has any clue why it adds little noise in the case when it is
modulated over a carrier.. ?

best regards,

ubaid

On Jul 25, 3:06&#4294967295;am, "Ubaid Abdullah" <ubaid_abdul...@yahoo.com> wrote:
> Hi, I'm simulating an OFDM system with Multipath and AWGN channels. One
> thing that i'm observing is that whenever I simulate the system on baseband
> the AWGN command adds almost the correct noise but whenever I modulate the
> baseband signal to a carrier and then add the noise, it adds very little
> noise compared to previous case.
[snip]

The MATLAB awgn function works on a sample-by-sample basis.
So you may have to normalize according to the oversampling factor,
and also according to the number of information bits per sample or
symbol.

Is this for a homework assignment?