Constellation diagram for QPSK modulation

I need to plot (in MATLAB R2011a) constellation diagram of a QPSK modulated signal after AWGN was added to the signal. My current MATLAB code looks like this:b=round(rand(1, Nbits)); fill=10; T=1/1000;fs=120000;fc=4000;t=0:2/fs:length(b)*T-1/fs;for k=1:floor(Nbits/2) %Dividing pair and odd bits even(k)=b(2*k);endfor k=1:round(Nbits/2) odd(k)=b(2*k-1);endE=A*sqrt(T/2);for k=1:length(even) %NRZ code if (even(k) == 1) evenNRZ=[evenNRZ ones(1, fill)*E]; else evenNRZ=[evenNRZ -ones(1, fill)*E]; endendfor k=1:length(o...Read more

qpsk - Gardner timing recovery and constellation diagram

I would like to ask for some guidance regarding 2 issues mainly QPSK and Gardner algorithm for timing recovery.I am having as input a set of IQ samples (10 samples per symbol) to which I need to provide timing recovery.Having red Gardner’s landmark paper on that concept (A BPSK/QPSK Timing-Error Detector for Sampled Receivers) I understand that in a simple BPSK case where transition exists (in a pulse always) the two first terms (x[nT]-x[(n-1)T]) of the formula e={x[nT]-x[(n-1)T] }x[nT+T/2] cancel out and the error approaches zero. In cases whe...Read more

High Transmission Power : BPSK or QPSK?

In BPSK,the constellation consists of 2 points (equiprobable) spaced at distance of $\sqrt{E_b}$ each from the origin.So the average power from the constellation can be obtained by:For BPSK, $P = \frac{E_b+E_b}{2} = E_b.$For QPSK, 4 points which are equiprobable are spaced at distance of $\sqrt{2E_b}$ from origin each, that is they are placed on vertices of a square of side $2\sqrt{E_b}$. So, for QPSK, $P = \frac{2\cdot4\cdot E_b}{4}=2E_b.$So, does this mean that the power required for QPSK is more than BPSK when the probability of error is sam...Read more

constellation diagram - How to scale measured QPSK points to determine magnitude error?

I am trying to determine the magnitude error between demodulated I,Q values and reference values on a constellation plot for a QPSK signal. The reference points are at (1,0),(0,1),(0,-1), and (-1,0). Of course as the amplitude of the input signal increases, the measured points move further away from the references. My (limited) understanding is that measured points would be near the expected to make the resulting constellation useful - where the proximity is affected by the quality of the signal, but not the amplitude.Is there a standard scalin...Read more

qpsk - BER performance of a space-time code over Rayleigh channel model

For a given space-time code, and in general terms, how would one proceed in the evaluation of the BER performance of this code ?In order to be more clear, say i have 16 code matrices(who's elements belong to QPSK modulation) of size 2x2 that could be sent over a Rayleigh flat fading channel by a MIMO system with 2 transmit antennas and 2 receive antennas.How would one proceed in the evaluation of the BER of such system ? Thank you!...Read more

Phase rotation for QPSK

I have two SDRs and I want to do a simple test: I want to send data values using QPSK modulation with coordinates: (1,1), (-1, 1), (1, -1), (-1, -1), but on Rx site I got all possible values on the circle created by rotation of the square. Why phases of received signal are changed?...Read more

How to interpret transitions in a QPSK constellation diagram?

I have one doubt regarding how to analyze a constellation diagram. It is from a QPSK transmitter. The constellation diagram looks typically as books (as a square as you can see in Figure 1), but for shorts periods it changes as you can see in Figure 2. We don't know if it is a problem with the transmitter, a problem with the data that is being modulated, or if it is a bad configuration on the Spectrum Analyzer. What do you think about that? How can we interpret the constellation diagram and the transitions? Thanks in advance.UPDATE: 27/02/2018 ...Read more

qpsk - Understanding the over sampling factor effects before a Matched Filter?

I am trying to design a matched filter at the receiver side to a half sine pulse shaping function sent by the transmitter of an OQPSK transceiver. I am not sure I understand the effect of varying the over sampling factor of the ADC right before the matched filter (the minimum is twice the symbol rate). I think it may have something to do with the quality of the timing synchronization. I'm using an early-late gate synchronization algorithm, which needs at least 3 samples per symbol. I've also read in literature that the over sampling is typicall...Read more