IFFT fixed point and AWGN channel

Hi,

I am building an fixed point OFDM for 802.16,using 2^10 quantisation(ie
the floating point values are multiplied by 1024). The system was working
fine, so I decided to test with AWGN channel. The problem is that the code
for AWGN that I have is for Floating point.

I normalised the output of the IFFT by- 

IFFt symbol = Symbol / sqrt(real*real+imag*imag), and converted this to
float. Then I added noise. Since I am using 2^10, I multiplied this final
noisy signal with 1024, and reconverted back to fixed point.

I am getting very poor BER(10e-2 even at 10db). Now if I remove the IFFT
and FFT, I get good BER.

I tested the IFFT-FFT by itself, taking a MATLAB output as reference. I
get very good performance, very close to MATLAB results(floating point,
converted to fixed point at end of calculation).

The only problem I can think of is that way I am adding noise to IFFT is
wrong. Does anyone know how we add AWGN noise to fixed point IFFT- am I
missing any steps?

Thanks
Shan

shan_100 said the following on 17/03/2006 17:11:
> I am building an fixed point OFDM for 802.16,using 2^10 quantisation(ie
> the floating point values are multiplied by 1024). The system was working
> fine, so I decided to test with AWGN channel. The problem is that the code
> for AWGN that I have is for Floating point.
> 
> I normalised the output of the IFFT by- 
> 
> IFFt symbol = Symbol / sqrt(real*real+imag*imag), and converted this to
> float.

If you're doing the division in fixed point, won't you get horrendous 
rounding errors?


> I am getting very poor BER(10e-2 even at 10db). Now if I remove the IFFT
> and FFT, I get good BER.

10dB what?  SNR or Eb/No?  What constellation are you using?



-- 
Oli

>If you're doing the division in fixed point, won't you get horrendous 
>rounding errors?


I convert it to float for adding AWGN noise.

>> I am getting very poor BER(10e-2 even at 10db). Now if I remove the
IFFT
>> and FFT, I get good BER.
>
>10dB what?  SNR or Eb/No?  What constellation are you using?

Eb/N0 with QPSK

shan