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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Parameter selection for log distance path loss model

I am currently referring to the following book MIMO-OFDM Wireless Communications with Matlab in which the log distance path loss model is given by the equation: $P_{LD}(d)[dB] = PL_F(d_0) + 10nlog(d/...
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2k views

Carrier frequency offset and carrier leakage

Recently I study frequency Offset estimation due to mismatch in carrier frequency from the oscillator at the receiver. I understand how this cause problem can cause a phase shift. But I also saw that ...
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1answer
58 views

How to write time waveform of transmitted sequence with certain chip rate?

I have been studying a book which describes how to write the waveform of sequence that has certain chip rate $T_c$. The sequence with chip rate $T_c$ is for example $$X=[-1 -1 +1 +1 +1 +1 +1 +1 +1 -...
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6k views

Simulating BPSK Costas Loop in Matlab

I am trying to implement BPSK demodulator in FPGA. First, I am modeling the system in MATLAB. Now, I am trying to design Costas Loop for carrier synchronization. This is the block diagram I'm trying ...
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2answers
938 views

SFBC Alamouti Scheme LTE

A sentence I copied from book. That a 2$\times$1 Alamouti SFBC is referred to as a rate 1 code, two symbols are sent over two adjacent subcarriers. This question is regarding this SFBC encoder (...
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1answer
249 views

A sequence OFDM modulated with normalization factor

The following sequence is used to modulate 52 subcarriers. Twelve out of 52 are non zero. I was wondering $S_{[–26, 26]} = \sqrt{13/6} \{0, 0, 1+j, 0, 0, 0, –1–j, 0, 0, 0, 1+j, 0, 0, 0, –1–j, 0, 0, ...
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1answer
307 views

Questions regarding OFDM, FFT, IFFT

The questions below are for OFDM, IFFT, FFT operations. Example 1 In OFDM modulation, a stream of parallel symbols are input to an IFFT operation of size $M$ for example and the output is time ...
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1answer
2k views

Pilot symbols in IEEE 802.11a

In IEEE 802.11a standards, there are many fields. Of which is the Data field which has 52 available sub-carriers used for transmission of coded bits. If the subcarrier locations are numbered from 32,-...
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1answer
68 views

Output signal in digital communication

In digital communications systems we say that the received signal can be expressed as $$y(t)= x(t)+ n(t)\tag1$$ if x(t) is the transmitted signal and $n(t)$ is the noise. I am wondering whether a ...
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3answers
401 views

General questions regarding OFDM modulation

In Orthogonal frequency division multiplexing (OFDM), data symbols are modulated using an N pt-IFFT operation to produce ONE output OFDM Assume we have total of $N$ QAM symbols which are input to ...
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1answer
312 views

Nyquist rate and oversampling

Assuming we have time domain signal $$x(t)\,\,\,\,\,\,\, 0\leq t< T$$ and we decide to upsample the signal by factor $L\geq1$. Then the samples of this oversampled time domain signal are ...
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1answer
3k views

Differential BPSK vs BPSK what are the advantages and disadvantages?

Recently I have come across different variation of the sane modulation techniques. I am quite confused about the advantages of different constellations. In particular, what are the advantages of using ...
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2answers
2k views

MMSE Equalizer in communication system

This question is regarding applying minimum mean square operation at the receiver side of communication system. I have read in paper that Given an output signal from communication system $$R(k)= H(...
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2k views

spectral mask and power spectral density

I have been reading a paper that specifies the spectral mask for certain communication devices attempting transmit over a channel Can someone explain 1) importance if this spectral mask, is it a ...
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1answer
7k views

BPSK vs $\pi/2$ BPSK, what the benefits?

I have come across modulation techniques that have $\pi/2$ phase shifts from the regular constellations. For example $\pi/2 $ BPSK vs constellation the regular BPSK. Other examples include $\pi/2 $ ...
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51 views

Why can CPRI links not be wireless?

I have been studying about CRAN and the CPRI links (fronhaul links) and have observed that CPRI links are always either fiber cable or some electrical interface. Why is this needed? Is it related to ...
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162 views

Modulation of data bits into complex symbols in MATLAB

In communication theory, a modulator takes the input sequence and and maps it to some complex numbers. In MATLAB the following commands can do that. ...
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1answer
1k views

How to compute the free distance in TCM (Trellis-Coded Modulation)?

I would like to be able to compute the so-called "free distance" for a TCM with a rate 2/3 convolutional code (two inputs, three outputs) inside that I plan to use for 8PSK. My previous assumption was ...
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1answer
477 views

64 QAM versus lower order modulation

This question is regarding digital modulation. If 64 QAM always provides rates greater than BPSK or 16 QAM, then why do systems always use higher order modulation of QAM? Thanks
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2answers
382 views

What is the output of IFFT operations, continuous or discrete time?

Assume we have a sequence of symbols $$S= \{0, 0, 1+j, -1-j, 0 ,0 , 1+j,-1+j\}$$ and that this sequence is a frequency domain sequence and will be input to an IFFT operation. Is the output a ...
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1answer
208 views

Doppler Model, how does it affect the fading impulse response?

I have been reading a paper that mentions Doppler model that may be included in the fading characteristics of impulse response. One example of bell shaped Doppler Spectrum is the following $$S(f) = \...
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1answer
24 views

Carrier sense multiple access, MAC layer

A medium access control MAC mechanism in wireless local area network and Ethernet is called Carrier Sense Multiple Access. There are two different types of this CSMA 1- CSMA/Collision Detection 2- ...
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1answer
125 views

How to implement path loss in wireless communications?

I would like to implement path loss propagation model in a cellular network scenario. I know that the path loss can be modeled as follows: $$\mathrm{PL}(d)\propto\left(\dfrac{d_{ij}}{d_0}\right)^\...
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1answer
746 views

Transmission capacity in “duobinary coding”

In "duo-binary signalling" how exactly is the transmission capacity increased? Because , when we encode the binary bits, the bit duration remains the same, but the amplitude will change. Then how can ...
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2answers
853 views

OFDM modulator in LTE

My understanding of what an OFDM modulator does, is that it takes a parallel sequence of QAM symbols of size M, pads it with zeroes corresponding to the zero subcarriers, then passes them through an ...
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1answer
2k views

LTE questions (subcarriers and cyclic prefix)

Assume we have transmission bandwidth of 10 MHz, in this case the total number of subcarriers (FFT size) present is 1024. According to LTE standards, 600 out of 1024 subcarriers are data subcarriers. ...
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1answer
257 views

Questions Downlink OFDMA in LTE

I am looking for a clear and precise explanation of how an OFDMA modulator in downlink LTE operates. I have many books that explain it but each has its own way and do not answer my questions ...
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1answer
2k views

what are the advanatages of using a scrambler

What is the purpose of using a scrambler in communication systems? The link wiki_for_scrambling says that it is used to randomize data. But the data is already random (like speech,audio,video,etc..), ...
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1answer
797 views

Question regarding 16 QAM digital modulation

Below is an example of a digital modulation system using 16-QAM. I have copied this example from MATLAB communications toolbox, ...
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32 views

Basic “filtering” in digital communications [duplicate]

I am completely stuck on solving some problems for my digital communications class. Junior level undergrad class, so fairly basic. I have been searching around all day for related articles etc but can'...
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1answer
105 views

Visualising a Z-transformed Transfer Function?

For designing any analog filter and various other outputs of filter we use laplace transform,I can visualise a laplace transform like for ex. s[X(s)] can be ...
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2answers
457 views

Question related to Principle of Communication

Why the frequency of carrier wave is always kept at least $10$ times higher than the highest frequency of base band signal? Can you explain me this with a practical example?
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145 views

Basic “filtering” in digital communications

I am completely stuck on solving some problems for my digital communications class. Junior level undergrad class, so fairly basic. I have been searching around all day for related articles etc but can'...
0
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0answers
45 views

Find the essential bandwidth of rect function? i want the ans by using Energy spectral density and then essential bandwidth… [duplicate]

I am trying to find EBW of rect function but i am unable to solve the integral after finding the fourier transform and then apply parseval's theorem?
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1answer
351 views

What is the theoretical upperbound of information that could be transmitted by a device per second?

Suppose I would like to stream data from a device in a single second. What would be the theoretical upperbound of the amount of information (in bits, bytes, pixels...) that could be sent during this ...
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140 views

Sub-carriers spacing in DVB-T2

Looking at DVB-T2 technical specifications I see that sub-carriers spacing has, in the worst case, a value of few hundred hertz, (something like 279 Hz). 32K sub-carriers are used to reach such a low ...
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1answer
293 views

Gaussian, Rayleigh, and Exponential RVs

I am doing some simulations on the performance of OFDM systems in multi-path channels and I am confused about something. It is explained below: Assume that we have a multi-path channel with a number ...
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3answers
436 views

Can OFDM modulate analog signal?

I suppose that OFDM can't modulate analog signal. The basic mathematical description for OFDM is $$ v(t)=\sum_{k=0}^{N-1}X_{k}\exp(j2\pi k t /T),\quad 0<t<T$$ In this equation, $X_k$ is a ...
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1answer
3k views

Rectangular 8 QAM bit error rate calculation

I am simulating an 8-QAM communication system over an AWGN channel for various signal to noise ratios, in order to plot the average bit error. However I have not been able to derive an equation for ...
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3answers
3k views

cyclic prefix in ofdm

In ofdm systems after applying IDFT we take the last N/x samples and place them before these N samples. This is called cyclic prefix(CP).So why to use a cyclic prefix? Why is the CP known to combat ...
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1answer
1k views

I/Q Sampling and Demodulation

A bit stream is mapped to symbols during baseband modulation, each symbol (say for QPSK) represents two bits. Each symbol has a corresponding I & Q value, which in QPSK, are used as the amplitude ...
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1answer
1k views

Relationship between “received” signal power and “transmitted” energy per bit?

In digital communications textbooks (e.g., Digital Communications by Bernard Sklar), the relationship between the "received" signal power $P_r$ and the "transmitted" energy per bit $E_b$ is given as ...
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2answers
858 views

carrier frequency difference

Papers use wireless device-dependent radio-metrics as fingerprints. A radio-metric is a component of radio signal, amplitude, frequency and bandwidth. Each device creates a unique set of radio-metrics ...
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1answer
255 views

Transmit bitstream from USB over radio

I am extremely new to this and I'm learning by building stuff myself so please let me know if this is too vague and Ill elaborate if I can. Im sure this is something very simple, but cant figure out ...
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2answers
5k views

Sampling rate in Wifi 802.11 -> 20 MHz enough with a BW of 20 MHz?

In a Wifi 802.11n context, I'm looking to find the sampling rate to perform some processing on the signal and I've something I cannot understand : in the book Next Generation Wireless LAN's: 802.11n ...
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1answer
617 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
3k views

OFDM transmitter bandwidth

Theory Suppose an OFDM transmitter with four subcarriers is implemented using the structure shown below. If the serial data input rate is 40 bps, each parallel stream will run at 10 bps. ...
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2answers
1k views

Simulating noise in OFDM

I'd like to simulate a certain $E_s/N_0$ in a baseband OFDM MATLAB modem to generate SER curves. I randomize 50 QAM symbols with $N$ subcarriers each in frequency domain, add null frequencies (guard ...
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1answer
198 views

sampling a signal below nyquist rate

This question is for people working in multi rate signal processing. Using compressive sensing techniques, we can still recover a signal by sampling below nyquist rate(by decimating the signal). How ...
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1answer
61 views

Legal issues when using a paper [closed]

I'm just curious about the copyright issues when publishing a paper on the Internet. Can some simply take the idea I explained in a manuscript and implement it for his own work? (e.g. using some ...