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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5
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1answer
7k views

What does the one-tap equalizer used with OFDM accomplish?

I've read that OFDM normally uses a one-tap equalizer. It seems that a one-tap equalizer could only scale and delay a signal. Why is this useful? I thought multipath was handled through the cyclic-...
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1answer
623 views

Barker Sequence for QPSK

I'm trying to implement a QPSK burst modulation and demodulation through a channel and I'm now considering the synchronization part. I've read that Barker sequence were excellent option for this as ...
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1answer
175 views

Rician factor effect

The Rician factor is the ratio between the mean power of the LOS path to the mean power of all other (non-LOS) paths. Increasing/decreasing this factor means increasing/decreasing the mean power of ...
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2answers
8k views

Is there a closed form for the average energy of a QAM constellation?

I'm trying to come up with a formula for the average energy of the signals in a M-QAM constellation. If $b$ is the number of bits represented by each point in the constellation, such that $M=2^b$. For ...
2
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1answer
884 views

Bandwidth of a communication system

As a rule of thumb, the bandwidth of a communication system with a complex signal must be equal to the bit-rate or symbol-rate? Besides, after reading this post, it is not clear for me how a band-...
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1answer
648 views

Manchester decoding

Is there any method of decoding a Manchester encoded signal (IEEE 802.3) that is independent of the bit period? Suppose the data I want to send is $\rm 10110$. The Manchester code of this data will ...
2
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1answer
142 views

Laser diode bitrate datasheet

I'm a student studying communications engineering. I'm trying to understand why this particular laser diode is made for 155 Mbps and 622 Mbps. I'm thinking that the bitrate is entirely dependent ...
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2answers
1k views

Bandpass filter BPSK

I have a question concerning BPSK and the bandpass filter that seems to be sometimes placed before the receiver circuit (the receiver is a mixer and lowpass filter). Basically, from what I've noticed,...
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6answers
49k views

Understanding the Matched Filter

I have a question about matched filtering. Does the matched filter maximise the SNR at the moment of decision only? As far as I understand, if you put, say, NRZ through a matched filter, the SNR ...
3
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1answer
194 views

What are the options for turning a bit stream into a byte stream?

I am creating a system (modulator and demodulator) for broadcasting binary data through an analog signal. I'm using BPSK with great success – at least, I'm generating great graphs. Green is input ...
4
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1answer
1k views

OFDM training symbol format

For OFDM synchronisation, we are using two training symbols according to the Schmidl and Cox method of frequency synchronization and timing synchronization. The paper says: The first OFDM training ...
3
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1answer
578 views

Appropriate sampling frequency for bpsk modulated signal

I'm modulating a message with particular baudrate, say 2MHz using BPSK , Carrier frequency(Fc) is 8MHz, What should be appropriate Sampling Frequency(Fs) following Nyquist criteria, Should it be ...
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2answers
5k views

Need for Oversampling in OFDM and symbol time determination

I am trying to implement OFDM on gnuradio. I have two questions. The sampling frequency which is slightly higher than available bandwidth satisfies Nyqvist criterion for complex baseband signals. So ...
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3answers
559 views

OFDM coarse freq estimation

I am trying to read OFDM synchronization paper by Schmidl and Cox. The math is slightly overwhelming. What is the formula that they use in implementation that distinguishes between coarse estimation ...
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1answer
5k views

Determining sampling rate in OFDM demodulation

I have seen that in OFDM, the sampling rate is much lesser than twice the bandwidth of signal. why is it so? How is sampling rate calculated in OFDM? Does it depend on the modulation type used inside ...
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1answer
331 views

Find the time lag for a wave

suppose that we have a wave which has some peaks in some indexes like 2,12,25,... and otherwise zero. I want to extract the time lag or lags between peaks of a signal. could anyone help me ?
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2answers
444 views

Fixed point channel model for OFDM system

I'm testing an OFDM system implemented in fixed point. The data format is Q11. My system work's fine but I need to test it under some channel for evaluation of the design before field testing. The ...
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1answer
454 views

Remove spurious peaks of Hilbert transform

I want to eliminate spurious peaks of Hilbert transform for finding glottal closure in the linear prediction (LP) residual. I have the following 4 steps: Down-sample. Hilbert Transform. Identify ...
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1answer
4k views

Apply AWGN noise to QPSK-OFDM symbol

I'm working on an OFDM-QPSK based system and I'm having some trouble to match the BER of my system with the theoretical curve. My OFDM implementation fills all the subcarriers and have t_symbol s ...
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2answers
8k views

How to normalize the power of a QAM signal?

If I have, for example a 16-QAM signal, and the constellation points are (3,3),(3,1),(1,3),(1,1),...(-3,-3). Now I have a simulated in coming signal with arbitrary ...
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3answers
5k views

How to find the group delay of IIR filters in Python/SciPy

I sent a square wave signal (red) through a IIR filter (butterworth in this case), to make it more "realistic". Then there's a slight group delay in my output signal (blue). Is there a way to find out ...
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1answer
10k views

How to perform carrier phase recovery in software?

What are the options for recovering the phase of a BPSK signal in software? The only resources I can find online contain circuit diagrams – there seems to be no one willing to explain this to people ...
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1answer
140 views

Does any error correction code still work in such situation?

I'm looking for a kind of error correction code or solution that can correct my codeword in this case: My message holds $k$ bits, and $2k$ bits codeword (rate is $1/2$) is produced by the generator ...
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1answer
5k views

Determine the optimum receiver and the corresponding $P_{eM}$ for an AWGN channel

I have a source that emits $M$ equiprobable messages, which are assigned signals $s_1, \dots,s_M,$ that are equidistant by $a$. That is, if we plot the $s_k$ signals in a horizontal axis they are dots ...
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0answers
698 views

How to encode a byte array to sound file and then convert back?

This question is moved from Stack Overflow. Sorry about this noob question, because I never work with matlab and signal processing before. Here is what I want to do: I have a fixed length of byte ...
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3answers
3k views

Why is an interleaver often placed between an outer block code and an inner convolution code?

It seems that when doing FEC with a concatenated-code, an interleaver is often placed between the outer block code and the inner convolution code. The explanations I've seen say that this is because ...
4
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1answer
2k views

Finding out modulation index and DC offset

I have a question form my teachers, and I cannot understand why I can find out the modulation index form the figure. The question provide a Figure like this: And the information signal is a ...
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4answers
5k views

Why does OFDM use cyclic-prefix while QPSK uses root-raised cosine filters?

My understanding is that BPSK/QPSK/... are often implemented with root-raised cosine (RRC) filters to reduce ISI and bandwidth. OFDM can be understood to be BPSK/QPSK/... implemented on multiple ...
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4answers
14k views

How is symbol synchronization with OFDM done?

I'm trying to understand how symbol synchronization is done in OFDM using pilot tones, cyclic prefixes, or any other technique. I've read the following answers which provide some explanation, but I ...
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2answers
859 views

Meaning of Q and I in all-software QPSK communication systems?

Q and I seem to come up a lot in discussions of digital communications, such as the comprehensive answer provided to this question: FFT window phase syncronising It seems like the Q and I channels ...
2
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1answer
540 views

Intersymbol Interference — Root raised cosine filter redundant if doing impulse response equalization?

I'm implementing DBPSK (differential binary phase-shift keying) over audio (44.1kHz sample-rate) using software. Eventually I'll use multiple audio frequencies at once, so I'm using FFTs and inverse ...
5
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1answer
209 views

How to send / receive a short impulse in an extremely noisy signal with high timing accuracy?

I am pretty new to DSP. I need a computer to transmit data to a receiver through audio signals in a loud outdoor environment. (I am writing the software for both sender and receiver so I can design ...
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2answers
3k views

Why do you get instantaneous phase by calculating correlation with a carrier signal?

I am somewhat new to DSP. I'm trying to implement BPSK by following this Python QAM tutorial. My code seems to work, but I really just ported the tutorial's Python code to C++, and do not understand ...
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3answers
2k views

Common methods of transmitting digital data through an extremely noisy audio signal?

I am developing software for transmitting data from a computer to a mobile device using audio. Specifically, from the computer's speaker to the device's microphone. (I am developing both the sender ...
4
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3answers
813 views

Optimal DC-free line encoding

I have a block of n bits that I'd like to encode into m bits that have an equal number of zeros and ones. I looked at protocols like 8b/10b but it wastes 25% of the data to achieve balance. As an ...
2
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1answer
2k views

increase of encoding rate increases bandwidth?

I came across this paragraph in an article about signal processing. Addition of redundant bits for improving the error probability leads to bandwidth expansion of the transmitted signal by an amount ...
8
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1answer
462 views

Which digital modulation scheme would be suitable for transmitting digital data through the output of a PC soundcard

I need to output a data stream at $40\textrm{ kbit/s}$ or faster out of a computer system whose only accessible output peripheral is the audio interface. This interface has reasonable specs, $96\...
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1answer
2k views

formula to calculate ber in wireless 802.11 according to distance from tx and rx

I'm looking for a formula to calculate the bit error rate of a wifi communication 802.11a/b/g according to the distance between transmitter and receiver.
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2answers
1k views

Is it possible to transmit and/or receive multiple types of signals at the same time?

Using an inexpensive SDR (such as the USRP or Per Vices Phi), I want to process two or more communication protocols at the same time (e.g. WiFi and GSM). Is it possible to Tx (or Rx) both at the same ...
7
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1answer
231 views

Why would I want to define a modulation index for each tone (DSB-FC)?

So the exercise is basically a signal $f(t)$ that is going to modulate the carrier $A\cos(\omega_ct)$ using a modulation index of $m=1$. I have to find $A$ and the power of the modulated signal: $$ f(...
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1answer
651 views

How to calculate dB of a transmitted power

why if I lose 80% power in the output of a system, Does it mean that I had lost 10×log10(0.2) = -7dB ?
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1answer
6k views

Information Theory - units of channel capacity

In a first course in Information Theory, when the operational interpretation of channel capacity is introduced, it is said to be the highest data rate (in bits/channel-use) of reliable communication. ...
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0answers
3k views

Why does my simulation of the binary symmetric channel disagree with theory?

Could someone please answer me why the simulated performance (using matlab) of a Hamming code over a binary symmetric channel does not fit exacly with the theoretical? Here is my code: ...
2
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1answer
2k views

Why the error probability is given by this expression?

I have seen this equation: $$ \Pr({\epsilon}) = Q({\sqrt{2E_b/N_0}}) \,. $$ where these definitions hold: $$N_0/2 - the \ spectral \ density \ of \ white \ noise$$ $$E_b = PT_b - the \ energy \ ...
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1answer
601 views

How to apply the CA-CFAR methods in practice?

Applying constant false alarm rate is a critical step for estimation of performance of all detectors. Has anybody experience on practical implementation of this estimator for energy detector? Can ...
4
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1answer
356 views

Why is privacy scrambling a standalone block in a communications transmitter?

In some telecommunication systems, scrambling of transmitted data is applied to provide some level of security. For example, the communication system might look like: Data -> Scrambling -> ...
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2answers
7k views

Extracting Binary Magnetic-Strip Card Data from raw WAV

I am faced with a tricky challenge: To extract binary data from an iPhone magnetic strip card reader. This is what the magnetisation on the card looks like: Source Here is the .WAV the iPhone ...
8
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2answers
250 views

What input to use for an AR model of a vowel sound?

I've recorded a 2-sec pronunciation of a vowel sound. The first 0.12 or so seconds of the signal are shown below. Now, I've constructed an auto-regressive (AR) 8th-order model to compress this signal....
10
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1answer
672 views

What are thoughts on best modulation to use for underwater acoustic communications at low frequencies?

I wanted to ping the DSP hivemind for general thoughts on what would be the best modulation type to use for low frequency underwater communications. I have chosen this project since I can learn a lot ...
16
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1answer
5k views

How to estimate and compensate for doppler shift in wireless signals?

I was wondering what good method(s) exist for the estimation (and subsequent compensation) of doppler shift for transmitted signals, be they acoustic or RF, in the context of comms. The question: ...