# Tag Info

Accepted

### Average Energy of modified QAM Constellation

The procedure is always the same. You need to compute the expectation $E\{|A_k|^2\}$, where $A_k$ are the complex symbols of the constellation: $$E\{|A_k|^2\}=\sum_kP_k|A_k|^2\tag{1}$$ $P_k$ is the ...
• 90k
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### Clock recovery using Mueller and Muller adds noise affecting EVM or SNR (Two cases - GNU Radio & Python code)

Unlike the Gardner Loop, the M&M synchronizer should be performed after the RRC filter in the receiver for best performance. With cases of high RRC alpha, the M&M won't work as expected ...
• 51.2k
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### Carrier Frequency Offset, Rotating Constellation and Correlator

In the frequency domain a signal with a carrier offset looks like the following- This is usually modeled as the desired baseband signal convolved (in the frequency domain) with a complex tone with ...
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### Carrier Frequency Offset, Rotating Constellation and Correlator

The correlator is just a filtering operation. So in the case of passband pulse-amplitude modulation (PAM), such as QPSK or QAM, the (noiseless) received analytic signal before demodulation and ...
• 90k
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### OFDM symbol acquisition block in GNU Radio gives rotating constellation even if there is no carrier offset. How is this possible?

Got me at that one! The "OFDM symbol acquisition" block is in fact not from gr-digital (where your other OFDM blocks come frome), but from gr-dtv, where it is used ...
• 30.6k
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### Why is Error Vector Magnitude (EVM) measured as an RMS value?

You've got it – zero mean noise still has power. That power happens to be its variance. RMS of such signals thus happens to be the noise amplitude's standard deviation – and give a sensible number to ...
• 30.6k
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### What techniques are available for correcting constellation rotation due to phase ambiguity?

In my experience from the space industry, both near-earth and deep-space communication system resolve phase ambiguity by means either unique word detection or different flavors of differential ...
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### How does carrier frequency offset effect the constellation diagram in OFDM?

so let's act as if we had only one active subcarrier "at a time", just to ignore interference between the different subcarriers. The frequency offset in time domain means a time offset in frequency ...
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### Carrier Frequency Offset, Rotating Constellation and Correlator

The original post mentions : "I have been further reading up on carrier recovery methods". Just for clarification - even though this thread is a few years old - did you mean symbol recovery ...
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### P2 symbol constellation in DVB-T2

Ok, question solved. I found out that indeed L1 pre/post constellations are not rotated. The constellation above comprises: L1 pre (BPSK) L1 post (64-QAM) Data PLP from P2 symbol (rotated 16-QAM) ...
• 41

### What causes this type of distortion in a constellation I/Q plot?

Despite the great answers by Dan and Olli - I am convinced this is indeed just plain ISI. In my case it's introduced by the droop of the analog filters after the DAC cutting into the passband of my ...
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### What causes this type of distortion in a constellation I/Q plot?

If there is capacitance between signal and ground, a resistor-capacitor (RC) filter may be formed. It can also be an intentionally added filter, like in original poster's answer. An RC low-pass filter ...
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### What causes this type of distortion in a constellation I/Q plot?

I don’t think this is necessarily additionally introduced ISI (beyond the ISI of the pulse shaping filter itself, which is zero when there is no timing offset), but may be the result of timing offset ...
• 51.2k

### Decision region when bit/symbol mapped to multiple points on the constellation

I believe that this is actually a perfectly valid question intended to check your approach to poorly-defined problems where you need to come up with a framework that allows making some reasonable ...
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### Why is 16 APSK arrangement 4 + 12?

The arrangement on constant circles for APSK leads to lower peak to average ratio in comparison to QAM which is a primary motivation for the arrangement. To then optimize the symbol error rate given ...
• 51.2k
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### Complex Filter (Modelling a Channel ) convolved with complex baseband modulation gives error *Plots inside -Updated*

(Poster and I have iterated on this in a chat here: https://chat.stackexchange.com/rooms/131792/isi ) The OP mentions that the filter should not impact the signal since the signal is within the ...
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### How exactly do I plot a constellation diagram?

Typically a constellation diagram is not just the raw RF values because that would be extremely disorganized. You would end up with a bunch of points in a gaussian distribution about the origin (...
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### How to do QAM modulation and plot the constellation diagram for a complex signal (I+jQ)?

Here is a simpler example of a 16QAM waveform, which will hopefully clear up the confusion. The following is a map of 4 bit binary words (as 0 to 15) to a complex symbol on the 16QAM constellation. ...
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### Frequency Error/CFO effect on Constellation for OFDM

Assuming no timing/sampling error, the time domain samples of the m-th OFDM symbol (index from 0) are y[n] = x\left[m(N+N_p)+N_p+n\right] \exp\left(j2\pi \frac{\epsilon}{N} \big(m(N+N_p)+N_p+n\big) \...
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### How to do QAM modulation and plot the constellation diagram for a complex signal (I+jQ)?

1.- ONE real signal generates I + 1j*Q this is ; ONE In-phase channel and ONE Quadrature channel that in turn generate ONE constellation, the typical square QAM ...
1 vote
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### A vanilla BPSK set up, but constellation disgram is a line

The constellation diagram will only look like what you're expecting if you view it after matched filtering at 1 sample per symbol - i.e. the constellation plot should be displaying your ideal sampling ...
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1 vote
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### Why does fine CFO cause rotation of the constellation?

CFO is modeled like this $x(t) = s(t) e^{j2\pi f_{\text{CFO}}t}$. This is a phase rotation only and does not effect the magnitude. Convince yourself of it by breaking $e^{j2\pi f_{\text{CFO}}t}$ up ...
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### Frequency Error/CFO effect on Constellation for OFDM

I detail the relationship of frequency offset to ICI below, but if this isn't at all clear, please proceed to the bottom with related links, references and background information that should further ...
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1 vote
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You would need to recover some timing information from the transmitted signal. A common way is to transmit some sort of preamble before the rest of the data in your frame that is transmitted. The ...
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### QPSK constellation from baseband signal

The following depicts a QPSK Carrier Recovery loop that works on the baseband I and Q samples following proper decisions from a Timing Recovery Loop. For more information on this implementation see ...
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### Phase versus time graph of QPSK

You're right that the phase plot doesn't match the signal plot. Note that it also shows more symbols than the signal plot. Those two plots do not refer to the same bit sequence, and the phase plot is ...
• 90k
1 vote
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### Understanding unexpected i/q calculation on a downsampled signal

Because the decimate function does not try to keep power constant. Remember that decimation is not inherently an LTI operation; so, probably, the authors of that function simply did not care too much ...
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1 vote
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### Aliasing in digital communication system

If the signal breaks the Nyquist criterion we'll only get a part of the signal. This needs to be clarified: the frequencies above Nyquist will fold back into the Nyquist band, so it's not just that ...
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### Zeros inserted in QAM Constellation

As others have mentioned, it sounds like you are talking about the null carriers in an OFDM system: These bins are used to mitigate leakage from adjacent RF bands 2. Perhaps your tutor meant ISI in ...
1 vote
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### How to scale measured QPSK points to determine magnitude error?

Even though have AGC etc in your system, it is not ensured that the received constellation points will lie exactly at the points that you would expect from the TX constellation. There can be ...
• 6,218

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