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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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 ...
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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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Spreading symbols with sequence before transmission

I have been learning IEEE 802.11ad and the following processing algorithm I was wondering what is the need for spreading with Golay sequence. I am guessing it has to with something like spread ...
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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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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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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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Use of Equalizers in communication systems for Intersymbol interference

In communication systems with high data rates, Intersymbol Interference (ISI) is a major problem (ISI). I have read in books that one way to combat this problem is to use time domain equalizers (ZF, ...
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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)= ...
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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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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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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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Matlab : Proper estimation of weights and How to calculate MSE for QPSK signal for Constant Modulus Algorithm

I had asked conceptual Question about Constant Modulus Algorithm. I am implementing the simple steps of the algorithm, as I do not have the cma() built in ...
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Wireless channel bandwidth used by device with smaller bandwidth

Recently I have been investigating wireless local area network. I copied a sentence which I dont understand A 80 MHz device associates with an 160 MHz Basic service set. What does this mean, ...
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Modulation GMSK with I/Q

I have a doubt about something. GMSK Modulated signal is equal to I+Q (with I = cos and Q = sin) or I+i.*Q ? Thank you. Best regards.
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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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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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Scrambler generator polynomial with initial state

A length-127 frame scramble uses the following generator polynomial $$G(D)= D^7+D^4+1$$ If the all ones initial state is used, how can one obtain the 127 bit sequence? I am not interested in the ...
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21 views

Transmitter Block Diagram of Data Field

Below is a transmitter block diagram for data field in WLAN Can anyone explain to me why do we need the following blocks I mention next: 1- Group bits into symbols and what does it actually ...
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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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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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Analog to digital conversion : Help in understanding how to represent quantized orbit

Problem : How do I use a map - The Bernoulli Shift Map to generate binary output using the following concepts that can find its application in coding ? Concept : The Dyadic map also called as the ...
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24 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: ...
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1answer
50 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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Resource Mapping LTE

I am having a hard time understand what resource allocation means in LTE. Below is one example of resource block over one subframe. Can anyone please explain to me, what does this mapping mean? ...
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SFBC Alamouti Scheme LTE

That a 2$\times$1 Alamouti SFBC is referred to as a rate 1 code, two symbols are sent over two adjacent subcarriers. Reference: Fundamentals of LTE Book. This question is regarding this SFBC encoder ...
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Pulse Shaping in Digital communication

I am new here. I hope I can get help. I want to ask a question regarding digital communication. Does anyone know what are pulse shaping used for? Are they used for ISI mitigation or for something ...
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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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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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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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Transmission Bandwidth and Sampling frequency

In LTE standards, for a transmission bandwith of 2.5 MHz, the sampling frequency is 3.84 MHz. I was wondering with this choice, are we violating Nyquist theorem? I found the details in the following ...
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General Questions MIMO Communication theory

The following are questions regarding MIMO communication theory. I have come across a MATLAB implementation of MIMO channel. My questions are not regarding the implementation, but rather regarding the ...
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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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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
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Decoders, Detectors and Demodulators (Digital Communications)

In digital communication systems, we use forward error correction codes such as LDPC, turbo, convolutional codes (to decrease the bit error), then we have to use an appropriate decoder at the receiver ...
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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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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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What is difference between OFDM and GFDM?

I am trying to understand the difference(s) between OFDM (Orthogonal Frequency Division Multiplexing) and GFDM (Generalized Frequency Division Multiplexing) which are used as a multicarrier modulation ...
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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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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 ...
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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 ...
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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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The channel capacity with additive Gaussian noise with changing variance?

As we all know the capacity for a AWGN channel with constant noise variance is just given by $\frac{1}{2}\log(1+SNR)$. What if the variance of the noise changes with the type (assuming the variance ...
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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 ...
3
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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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Linear Block Codes Digital Communication Problem

How to design a (8,4) linear block code with highest possible error correction and error detection capability? I have studied linear block codes topic but if i do the conventional way ,In case of ...
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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
52 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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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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148 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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Interpretation Of 4th-Order Cumulants For Complex Signals

For complex signal $y(n)$, we have the following three 4th-order cumulants: $$ \begin{align} C_{40} & = \operatorname{cum} \left( y(n), y(n), y(n), y(n) \right) \\ C_{41} & = ...