# Questions tagged [digital-communications]

Digital Communication is the physical transfer of data (a digital bit stream) over a point-to-point or point-to-multipoint communication channel.

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### data rate and PN code

I have a question about the spreading sequence for CDMA. Normally, we use a PN code but imagine that we use a cosine wave combined with a PN code where the PN code has for length $T * fs$. With $T$ is ...
192 views

### Why does power spectral density of OFDM signal changes with number of zero subcarriers

I am trying to plot the power spectral density (PSD) of an OFDM signal with $N_{dsc}$ subcarriers and $N_{zeros} = N_{fft}-N_{dsc}$ zero subcarriers. I am using the matlab function pwelch() to plot ...
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### Time delay estimation between two complex signals(IQ)

I need to time align two complex signals. The two signals obtained were at different RF frequency before digitisation. So,I have two complex signals(IQ) and I need to do delay estimation and apply ...
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### guard interval calculation for multi-cell massive MIMO

I am interested in a multi-call massive-MIMO system modeled as in the works of T. Marzetta, for example: "Inter-Cell Interference in Noncooperative TDD Large Scale Antenna Systems". In these works, ...
57 views

### Time frequency analysis for a spreading signal

I am a beginner in digital communications: I am studying the spread spectrum communication and I have a question on the spreading signals. For example I have 2 spreading signals and I do a time ...
27 views

### phase shift for a spreading signal

For a chirp spread spectrum with $2$ users, I can use an up-chirp and down-chirp for the spreading signals. This is my questions: 1) These $2$ signals have a phase shift. If my $2$ users are at ...
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### explanation on the mimo technique

In MIMO systems we have: $y=Hx+n$ with $H$ is a matrix $N_r \times N_t$. But the previous equation is only true for non-frequency selective channels. What is the formula for frequency-selective ...
304 views

### Why are the mark and space tones of these FSK signals reversed?

I'm working on a project involving FSK-modulated signals. I'm trying to reverse-engineer these signals to determine the preamble pattern used, because the protocol is proprietary. Shown below is a ...
108 views

### Frequency synchronization in OFDM

Could you please help me understanding how the null sub-carriers can be used to estimate the carrier frequency offset and thus performing frequency Synchronization at the receiver?
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### Normalizing OFDM after IFFT

I had a question about normalizing OFDM transmit power after IFFT across each subcarrier. I have simulated OFDM with BPSK and according to different website I have used ...
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### Confusion in GMSK Demodulation

I'm trying to implement this GMSK demodulator. Is the input to deviator in degrees? do I have to change it to radians? Does the output of the deviator has to be NRZ or Have to convert it to NRZ? I'm ...
150 views

### Viterbi Decoder without traceback

How can I use Viterbi algorithm for a real time scenario where I cannot afford to have delay in the tracing back the best path to the initial state?
I am considering a straight forward simulation setup in matlab. I am transmitting a random sequence $\mathbf{x}$ of $\{+1,-1\}$ over a two-tap ISI channel $\mathbf{h}=[h_0 \hspace{2mm} h_1]$. The ...