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Discussion Groups | Comp.DSP | BPSK and QPSK have identical BER curve ?

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BPSK and QPSK have identical BER curve ? - Davy - 2006-03-05 06:55:00

Hi all,
I have build a simulation system to test my Error Correcting Code.
And now I add modulation/demodulation module.

When I apply BPSK and QPSK to system, I found their BER versus SNR
curve almost identical, is it right in theory?

My code rate is 0.9, code length >10000.

BTW, I found BPSK and QPSK have identical uncoded BER versus SNR curve,
is above similar?

Any suggestions will be appreciated!

Best regards,
Davy

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Re: BPSK and QPSK have identical BER curve ? - Oli Filth - 2006-03-05 07:02:00

Davy said the following on 05/03/2006 11:55:
> Hi all,
> I have build a simulation system to test my Error Correcting Code.
> And now I add modulation/demodulation module.
> 
> When I apply BPSK and QPSK to system, I found their BER versus SNR
> curve almost identical, is it right in theory?
> 
> My code rate is 0.9, code length >10000.
> 
> BTW, I found BPSK and QPSK have identical uncoded BER versus SNR curve,
> is above similar?
> 

QPSK can be regarded as a pair of orthogonal BPSK systems, i.e. the real 
component is one BPSK system, the imaginary component is the second BPSK 
system.  Because they are orthogonal, they don't interfere (to a good 
approximation), hence the BER curves are largely equivalent.

-- 
Oli

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Re: BPSK and QPSK have identical BER curve ? - Davy - 2006-03-05 07:29:00

Hi Oli Filth,

Oh, I see. Thanks!

Best regards,
Davy

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Re: BPSK and QPSK have identical BER curve ? - Mike Yarwood - 2006-03-05 09:11:00

"Oli Filth" <c...@olifilth.co.uk> wrote in message 
news:LrAOf.27232$b...@newsfe2-win.ntli.net...
> Davy said the following on 05/03/2006 11:55:
>> Hi all,
>> I have build a simulation system to test my Error Correcting Code.
>> And now I add modulation/demodulation module.
>>
>> When I apply BPSK and QPSK to system, I found their BER versus SNR
>> curve almost identical, is it right in theory?
>>
>> My code rate is 0.9, code length >10000.
>>
>> BTW, I found BPSK and QPSK have identical uncoded BER versus SNR
curve,
>> is above similar?
>>
>
> QPSK can be regarded as a pair of orthogonal BPSK systems, i.e. the real 
> component is one BPSK system, the imaginary component is the second BPSK 
> system.  Because they are orthogonal, they don't interfere (to a good 
> approximation), hence the BER curves are largely equivalent.
>
If you plot Eb/No vs probability of bit error.

But you expect to see same PeBit for QPSK when the SNR is 3.01 dB higher 
than for BPSK because, at the same symbol rate, they occupy the same 
bandwidth and the signal energy needs to be twice as big for QPSK to have 
the same energy per bit as BPSK.

Best of luck - Mike

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Re: BPSK and QPSK have identical BER curve ? - sairamesh@gmail.com - 2006-03-07 13:17:00

Hi,
          The probability of bit error  of both the systems are equal
with channel coding and without channel coding.
If you derive analytical expression for BER with channel coding, both
have the same expression.
Best Regards,
-SaiRamesh.

_____________________________
 Free pdf download: Complex Digital Signal Processing in Telecommunications.


Re: BPSK and QPSK have identical BER curve ? - 2006-03-08 11:30:00

>....but you expect to see same PeBit for QPSK when the SNR is 3.01 dB
higher
>than for BPSK because, at the same symbol rate, they occupy the same
>bandwidth and the signal energy needs to be twice as big for QPSK to have
>the same energy per bit as BPSK.


While the above is a perfectly true statement, it glosses
over the fact that the BPSK system conveys one bit per
symbol interval, whereas the QPSK system conveys two
bits per symbol interval.  If the *symbol* rate is the same
(i.e. the bandwidth is the same) and the signal power is
the same for both systems, then the QPSK system has
smaller Eb/No ratio and therefore poorer error-rate
performance (as noted above), but double the data rate
(in bits per second).  If the *bit* rate is the same and the
signal power is the same, then both systems have the
same BER but the BPSK system uses twice the bandwidth.

In comparing apples and oranges, it is best to set the apples
to equal one another and compare the oranges, or set the
oranges equal to one another and compare the apples....

_____________________________
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