# Questions tagged [ofdm]

OFDM stands for Orthogonal, Frequency-Division Multiplexing.

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### Does the data rate of ofdm depends on number of subcarriers

I am trying to investigate the performance of OFDM system practically, I am using sampling rate $1 GSps$ giving bandwidth $B = 0.5 GHz$. What I find is when setting the number of subcarriers $N = 64$ ...
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### Why does one of my OFDM subcarriers have a much higher gain than the rest in GNUradio?

I'm using a fairly standard setup after the ofdm flowgraph examples provided by gnuradio. When I look at the absolute value of the received complex symbols in the first sync word (and every other ...
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### how to design the $\text{sinc}(\cdot)$ filter for OFDM oversampled system

We usually zero-padding into the moulated data of OFDM before performing IFFT operation to make the design of anti aliasing filters easier ( the cut off can be less sharp) hence save cost. That ...
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### High error when adding small value of SFO into the OFDM signal

I am trying to emulate the SFO effect for OFDM system, I did it using linear interpolation as shown HERE . That can be explained as if I have the time domain signal $s_n$ representing the output of ...
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### Relation between QAM size and Number of subcarrier in OFDM

I am trying to understand the OFDM system. If i am using 16 QAM with 16 subcarriers that means one constellation point mapped to one subcarrier. What will happen to mapping of constellation point to ...
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### one tap equalizer... what if there is none?

if I have a one tap equalizer system for OFDM, if I don't have an equalizer, what is received? is it just a load of noise?
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### Frequency-domain vs Time-domain OFDM oversampling

I sometimes read some references where the oversampling of OFDM signal is performed in frequency domain, for example, $(N-1)L \times 1$ zeros are inserted in the middle of the modulated signal $s$ ...
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### Does Zero padding cause noise in the high frequency region?

I have an OFDM system. I would like to introduce a fractional delay of (0.2 samples). Therefore, I increase the sampling rate of the OFDM signal by a factor of 10 (via frequency domain zero padding), ...
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### Cross Ambiguity Function Peak Width

I am trying to plot the Cross Ambiguity Function of two OFDM signal (1024 subcarriers) reflected from two targets, which are separated by 30m. The problem I am facing that the CAF peak width at the ...
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### the meaning of management of bandwidth and energy resources

I have read in this paper, Single-Carrier Frequency Domain Equalization, second page, first paragraph, that Single Carriers (SC) cannot offer the same flexibility as OFDM in the management of ...
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### Comparison between two OFDM schemes

I am wondering about two scenarios that I came across for OFDM systems. Say we have a station transmitting to an access point and say we have the following two scenarios: 1- N1 subcarriers and ...
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### Adaptation in time versus frequency domain for OFDM systems 5G NR

I hope someone can help me with this. In 5G NR OFDM system, Bandwidth part switching is helpful for power savings. You reduce the transmission bandwidth and then you save on transmit power. That is ...
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### Does non-orthogonal multicarrier modulation use pulse shaping?

It is well established that OFDM does not use pulse shaping as it destroys the orthogonality of the subcarriers, however, suppose we instead use a form of multi-carrier modulation with non orthogonal ...
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1 vote
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### How to convert discrete-time QAM pulses to a continuous-time signal using the IFFT (for OFDM)?

So I've run into some trouble in trying to plot the PSD of an OFDM signal. Specifically, I cant see how we go from the discrete domain to the continuous in time domain. I have a time vector, which is ...
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### Writing an OFDM modem - subcarriers are interferring

I am trying to write an OFDM modem. I've already written a QAM modem, which I'm using as a basis for this project. The way I'm doing this currently is by spinning up several QAM modulators, each with ...
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### number of subcarrier and transmit power

How does the total transmit power affects the number of subcarriers? For example, if we have 624 subcarrier, can we use the same maximum power (for example the budget is 64 watts) as we will when we ...
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### OFDM Symbol Duration

The OFDM symbol duration is equal to FFT size (number of subcarriers) and guard interval according to some textbooks. However, in some research papers they mention that they implemented (using matlab) ...
56 views

### Can an OFDM signal be demodulated at a sample offset over the cyclic prefix?

OFDM systems typically have a preamble or other synchronization symbols to aid the receiver with fine timing synchronization. There are algorithms, such as Schmidl & Cox (linked below), that ...
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### Can an OFDM waveform be spread (DSSS)?

Is it valid to spread an OFDM waveform using direct sequence spread spectrum (DSSS)? Both operations are linear so it seems like this should work.
1 vote
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### Intercarrier Interference (ICI) with OFDM signals

What is the relationship between Intercarrier Interference (ICI) in OFDM signals and Doppler spread? Is there a mathematical property to determine what OFDM parameters, such as subcarrier spacing, ...
1 vote
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### How can I perform semi-blind non-orthogonal successive interference cancellation (SIC) / source-separation with SISO in 4G/5G Downlink?

Semi-blind non-orthogonal successive interference cancellation in the single antenna case with 4G/5G signals (applies generally though). This is very similar to NOMA in 5G, except that I have no ...
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### Pilot-based channel estimation in OFDM systems with fractional delays

Most studies on OFDM-based communication systems consider that multipath channels consists of a limited set of taps in the discrete-time domain. However, this only holds if the tap delay is a multiple ...
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### Number of Cyclic Prefixes in OFDM

For an L tap channel, we have to add L-1 cyclic prefix(CP) in the starting of N symbols in OFDM. My doubt is that what happens if we change the number of cyclic prefixes, say I take the CP>>L-1 ...
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1 vote
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### Ofdm and subcarrier modulation

I was learning about OFDM systems and the different subcarriers. My understanding is thay each subcarrier carries one symbol. One question come to mind, can these different subcarriers be modulated ...
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### Adding AWGN into a positive real signal

I am trying to add a Additive White Gaussian Noise (AWGN) to a signal whose mean is 0.5. I am working with OFDM for Li-Fi communications whose signal must be real and positive. Normally, The AWGN is ...
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1 vote
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### IFFT and DAC relationship in OFDM Transmitter

Say, for LTE BW 20 MHz, I have total 2048 SubCarrier at IFFT Input leading to 2048 Output (Complex) at IFFT Output, and in 1 OFDM Symbol (useful symbol length) of time 66.67 usec, the sampling rate ...
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### How is SNR Calculated in OFDM Reciever? [closed]

Can you please guide me on the information to calculate SNR for OFDM Reciever? Is OFDM Pilots used for it? And do we need to follow channel estimation like steps of interpolation to determine it? I'm ...
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### OFDM Query : Why In Phase and Quadrature Phase RF Conversion is needed? [duplicate]

OFDM Basic Query- x[n] produced at output of IFFT is in discrete time, so we can use DAC and then RF Upconversion (multiply it with LO Frequency say f) and send it, why are we doing RF up conversion ...
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### MIMO Noise Power Calculation Using SNR

i'm in the process of simulating a MIMO-OFDM transceiver, in which we have $N_T$ transmit and $N_R$ receive antennas ($N_R \geq N_T$), as shown in the image below for the 2x2 case What i'm having ...
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### What's wrong with a filter for subband OFDM?

I am confusing about why it is necessary to pack the QAM symbols onto the frequency bins, such that the allocated range of frequency bins is located in the middle of the subband (as in lines 26-29). ...
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### One subcarrier lost after using scipy resample in an OFDM system with multipath channel

I simulate with Python a simple OFDM communication chain with a multipath propagation channel and no noise. Here are the details: Generate 10k independant batches of 128 random 4-QAM symbols. The 128 ...
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### Why we should pack data symbols in the band center before convolution with a lowpass filter?

I am investigating the spectrum shaping filter for OFDM modulation, and following an example on https://www.mathworks.com/help/comm/ug/f-ofdm-vs-ofdm-modulation.html In the part F-OFDM Transmit ...
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### Filter for ISI Channel

I know that OFDM has many advantages in removing ISI. But I think there is one more way of removing ISI(equalizing or inverting technique). Syppose there is a channel $H(\omega)$ and we have our ...
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### Parsing downlink cellular towers signals

I want to parse signals coming from 4G/5G towers to extract only the ID of the tower and the transmission time (on simplex communication: only downlink). Are these packets (or frames) sent on a ...
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### First element of the complex input array for the IFFT with OFDM

I am simulating OFDM by randomly selecting members of a constellation. That is, randomly selecting the I & Q from a 64QAM constellation. This gives me 64 possible phase & amplitude values. I ...
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### Square Root MMSE

Could anyone please explain how to perform the derivation of the square-root MMSE equation using an extended channel matrix H (as it is said in the "A LOW COMPLEXITY SQUARE ROOT MMSE MIMO DECODER&...
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I have a doubt regarding Pilot signals such as CSI-RS in NR. Using a large number of CSI-RS affects Datarate significantly? or it is negligible? Because I wonder it is meaningful that optimize the ...
1 vote
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### 802.11a OFDM Long Preamble Cyclic Prefix

im currently looking at an implementation of the preamble for OFDM in the 802.11a standard, as shown in the pic below What I dont understand is the cyclic prefix (labelled GI2). Based on this picture ...
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### What is the difference between a guard band and a cyclic prefix in OFDM?

im trying to understand the difference between a guard band, and the cyclic prefix, or at least the relationship between the two. Prior to now, I was under the interpretation that the cyclic prefix ...
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### I am trying to demodulate an OFDM signal

I am trying to demodulate an OFDM signal which was captured using a spectrum analyzer. The transmitted signal contained a 16 QAM modulation format, the received signal and the transmitted signal are ...
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### OFDM transmit and receive steps + RF

Would someone help confirm whether the steps I have here for OFDM communication chain make sense? At the tx bits - symbols - IFFT+CP - precoding - DAC - filter - PA - up conversion At the rx side ...
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### How can I demodulate OFDM signal having low SNR?

So I am trying to demodulate an OFDM signal with SNR = 10 dB, the parameters of modulation are as follows, oversampling factor = 8, cyclic prefix length = 144, number of Sub carriers used = 3600, ...
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### PAPR effect on LNA, ADC and PA

It is well known the OFDM systems suffer from PAPR given that the OFDM symbol is a combination of all of the subcarriers. Subcarrier voltages can add in-phase at some points within the symbol, ...
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### Differences between cyclic prefix, postfix and zeropadding [duplicate]

I was looking into details of OFDM and how the CP is copied last few sample of OFDM and placed in the beginning to cause circular convolution and mitigate is. My understanding is this cyclic prefix ...
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### Frequency Error/CFO effect on Constellation for OFDM

Very often in literature we see that effect of carrier frequency offset is seen as rotating of a constellation points. And explanation is generally that after down converting and sampling the time ...
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### Frequency offset estimation accuracy

To improve frequency offset estimation, we could: Use the second order interpolation / equation in the frequency domain based on the coarse estimation fc, e.g. BPSK frequency offset estimation; Zero ...
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### How to calculate the data rate of OFDM system

I have OFDM system with following parameters, Bandwidth $B = 5$ KHz, Number of sub-carriers $N = 64$, guard interval $N_g = 8$; Sampling frequency $F_s = 10000$ and Modulation order $M$ is QPSK. Is it ...
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### How to calculate received signal power for a Multi-User MIMO OFDM system?

I have a Multi-User MIMO system where there are 2 users served by a base station. The base station consists of 64 transmit elements and the users are equipped with linear antenna array consisting of 3 ...
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