coherent and noncoherent ..

matlab coherent non-coherent

hi,

what's the difference between coherent and noncoherent communications ?

cos wt + theta

as far as i remember , coherent is usin a local osc. in the Rx syncronized with the Tx to detect the signal (using multiplier) while non-coherent involves usin something like matched filter

quadrature non coherent method matlab

Also coherent works a few dB better than noncoherent at low SNR while at high SNR they are the same because other impairments dominate the errors. Examples are nonlinear phase across the signal spectrum frequency range and amplitude nonliearities (IMD) between signal spectrum lines.

non-coherent qpsk receiver

Coherent systems need carrier phase information at the receiver and they use matched filters (or correlation receiver) to detect and decide what data was sent , while noncoherent systems do not need carrier phase information and use methods like square law (push detection or energy detection) to recover the data.

It's evident that coherent systems need synchronization algorithms to synchronize the local osc. with the transmitter osc. and hence they require more complexity and costs .
In terms of performance coherent systems are much better than noncoherent systems.

regards

difference coherent and noncoherent communication

Hi
for example tx send x(t)cos(wt), and in rx signal x(t)cos(wt+teta) is received. If in receiver local oscillator is in phase(coherent) with transmitter, it can demodulate received signal by mixing it with
cos(wt+teta) and extracting x(t), but if it is not in phase with transmitter(noncoherent) for example 90deg out in phase, the demodulator output is ZERO so receiver should use another techniq to demodulate information from received signal for example envelope detector.

difference between coherent and noncoherent

i have a read a few times that if Your data is differentially encoded than You dont need to know the phase. You only need to know the incoming frequency and the PLL is there do that for You.

Now i dont understand how i can avoid phase detection with differentilal encoding.

For example: I have differentially encoded QPSK. My tx signal is

x=a(t)cos(wt)-b(t)sin(wt) a(t) and b(t) are pulse shapes and 'w' is my carrier

on recevcer my I signal is demodulated with cos(wt+theta) and quadrature signal Q is demodulated with sin(wt+theta.)

Consequently , neglecting high freq components, the inphase and quadrature components are
Yi= 1/2a(t)cos(theta)-b(t)sin(theta)
Yq=1/2a(t)cos(theta)+b(t)sin(theta),

now despite the fact that we have differenyial signals, our received signals not only have cos but sine as well and theta is always varying as well. In short i can not understand the significance of differential encoding, especially in my present case.

can any one elaborate?

br

we all

Guess the problem is with your understanding of what is "differential". In a diffrential modulation scheme the modulation done to the carrier depends not only on the bit that you want to transmit, but also the previously sent bit.

For example, you would transmit a(t)cos(wt) if the bit you want to send is the same as the previous bit; if the present bit is different from the last, you would send -a(t)cos(wt). Now, at the reciever, you only need to measure the _change_ in phase of the carrier, not the absolute phase. This you can do by comparing the current phase with the phase that was recieved one bit period back. There is no need to generate carrier frequency with correct phase.

So the signal model for QPSK wont be as you have written. You will have to put in a phase term (just inversion) that depends on the last I bit and the current I bit (also for Q bits)

a(t)(2*mod(I(i) + I(i-1),2) - 1) cos(wt) + a(t)(2*mod(Q(i) + Q(i-1),2) - 1) cos(wt)

For demodulation, there will be a comparison with previous bit period's. OF course as assumption for the "first bit" needs to be made.

-b

hello,

i urgently need matlab m files for coherent and non coherent modulaor and demodulator.
please help me.
waiting for your positive
regards,

Re: non-coherent qpsk receiver

amihomo said:

Coherent systems need carrier phase information at the receiver and they use matched filters (or correlation receiver) to detect and decide what data was sent , while noncoherent systems do not need carrier phase information and use methods like square law (push detection or energy detection) to recover the data.

It's evident that coherent systems need synchronization algorithms to synchronize the local osc. with the transmitter osc. and hence they require more complexity and costs .
In terms of performance coherent systems are much better than noncoherent systems.

regards

Click to expand...

--Dear friends and amihomo
i want to implement an oqpsk non-coherent receiver in verilog.but i don't have a clear idea about how it's structure can be.can you introduce me some refrences that have a detail look at such receivers structure and circuitry??
thanks