Tagged Questions

OFDM stands for **o**rthogonal, **f**requency-**d**ivision **m**ultiplexing.

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25 views

OFDM violates sampling theorem

The "textbook" OFDM block diagram is as the below figure. But I wonder if it satisfies the sampling theorem. Here, we have N points $X[k]$ before IFFT, and ...
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1answer
46 views

Learning fundamentals of OFDM (Theory & Simulation) [closed]

Will you please suggest me some basic fundamental reference to learn OFDM techniques. I am looking for underneath theory with some basic mathematical explanations of CP, Subcarrier, FFT Size, Pilot ...
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0answers
12 views

OFDM intra-block multipath interference

We know that OFDM eliminates the inter-block intereference. But there are still multipath interference in a single OFDM block(symbol). How does LTE handle this intra-block interference induced by ...
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1answer
42 views

Q and I part in QAM

I have one confusion in QAM about in-phase I and quadrature Q part, i.e. do they transmit identical data or not? For e.g. if i have 11001001 as a byte to be transferred, so will 4-QAM separate it ...
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1answer
38 views

why LTE doesn't use DC subcarrier in the downlink?

I know LTE doesn't use the DC subcarrier for OFDM/OFDMA modulation for the downlink, but no author gives a detailed explanation to this stuff. Can you tell me the reason behind it? Thank you!
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1answer
80 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 understandĀ : in the book Next Generation Wireless LAN's: 802.11n ...
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1answer
130 views

How to calculate bandwidth require for OFDM

I read LTE PHY profile and do not understand the relationship between number of FFT and bandwidth. For example LTE downlink channel bandwidth 1.25MHz and 5MHz have 128 FFTs and 512 FFTs accordingly. ...
2
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1answer
99 views

Time-frequency synchronization algorithm for OFDM continuous stream

I am studying on the implementation of a continuous stream OFDM baseband modem. Googling and also searching here on stackexchange for synchronization techniques I found the descriptions of several ...
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1answer
60 views

How may samples per symbol in ieee 802.11 g

I am doing a baseband simulation for IEEE 802.11 g. The standard uses 64 IFFT length ($3.2 \mu s$) and 20 Msamples per second. This result in one sample per symbol. I am wondering what the structure ...
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1answer
43 views

Additional prefix and suffix required when windowing an OFDM symbol?

To reduce out of band energy, it is common to apply a Raised Cosine filter to the front and back of an OFDM symbol. It is also common to overlap the windowed portions of adjacent symbols to eliminate ...
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2answers
98 views

Hermitian symmetry in OFDM systems

I am trying to understand the usage of Hermitian symmetry in OFDM systems and have a couple of questions regarding this. What is the reason of using the Hermitian symmetry in OFDM? How can we ...
2
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1answer
127 views

Alamouti code after or before IFFT block in OFDM

We are trying to implement Alamouti STBC on underlying ofdm signal. We were thinking as to where to write alamouti coder block. I thought we could write it after the cyclic prefixer block as we get a ...
2
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2answers
134 views

What is the advantages of BPSK+OFDM over usual BPSK (with no OFDM Modulation)?

The other days I read a useful article (on BER v.s. Eb/N0 curve of ofdm) whose link is as bellow. While watching the plot inside, a question was arisen in my mind. I wonder if the BER v.s. Eb/N0 curve ...
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0answers
60 views

OFDM Rayleigh Channel 16-QAM

Is there a difference in the BER of a 16-QAM modulated signal that is sent through a rayleigh channel using OFDM versus not using OFDM? I know for BPSK, there is barely any difference in the BER.
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1answer
76 views

Really basic question about OFDM

I am new to signal processing and am currently doing research into the subject. I am learning about Orthogonal Frequency Division Multiplexing and have a question. Most definitions of OFDM mention its ...
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2answers
55 views

How do you perform OFDM modulation?

I'm new to OFDM modulation. During OFDM modulation in the frequency domain, are the negative frequencies modulated with a conjugate version of the data of the positive frequencies, or do the negative ...
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3answers
70 views

comparison between frequency offset estimators

I have been working frequency offset estimation in OFDM. The objective was to compare different frequency offset estimation techniques. By using MATLAB, I have simulated three different estimation ...
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0answers
47 views
0
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1answer
50 views

How to calculate BW ofdm?

HI I have these specification for an OFDM program I was wondering, what is the bandwidth and the sampling frequency? I'm not a student of Digital signal processing, hence I am so confused. ...
0
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1answer
159 views

Simulate a simple multipath channel for OFDM Modulation test. MATLAB [closed]

Hi I need to simulate a simple multipath channel in matlab through which I can send my OFDM transmission signal, I want to test my OFDM receiver design. So far I have: ...
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1answer
78 views

What type of correlation is this equation?

In a book [1], gives an equation (equation 5.31 in the book) for estimating frequency offset in OFDM systems proposed by Classen. the procedure is: first, two OFDM symbols, $y_l [n]$ and $y_{l+D} ...
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1answer
219 views

IFFT and OFDM upconversion

I am trying to understand some basic concepts with IFFT as represented here OFDM Modem page 50-53, and 108-114. Some basics that are represented - The base sampling rate is 8khz. The symbol period ...
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2answers
307 views

generating pilot sub-carriers in matlab

I wanted to simulate OFDM system in MATLAB. I'm using Fixed WiMAX OFDM parameters in my simulation, where the number of data sub-carriers are 192, number of Pilot sub-carriers are 8, number Null ...
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1answer
137 views

Using Pilot tones to estimate carrier frequency offset in ofdm

I have been studying carrier frequency offset estimation techniques in OFDM systems. I have understood several of them. when using Pilot tones, as stated in paper (1), the process involves two methods ...
0
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1answer
166 views

Oversampling in OFDM

Why do we need an oversampling in OFDM signal? and how to do it? If around half of the Sub-carriers are only used to send data, then do we need to perform oversampling to the signal? Is it necessary ...
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1answer
140 views

Carrier frequency offset (CFO) estimation based on Symbols

In (1), it's said that: If two identical training symbols are transmitted consecutively, the corresponding signals with CFO of $\epsilon$ are related with each other as follows: ...
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1answer
95 views

Choosing clock frequency and number of bits of an OFDM transceiver [closed]

I want to implement an OFDM transceiver chip and my question is how to choose the clock frequency of the IFFT block? The number of output bits of the IFFT block? Also, how has the clock frequency and ...
2
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2answers
502 views

Carrier frequency offset (CFO) estimation using cyclic prefix

I understand the cyclic prefix, but in (1) it's said that: "If we consider that the channel effect is minimal and can be neglected. then the phase difference of the CP and the OFDM symbol which the ...
2
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1answer
83 views

Software-based sound card OFDM

I'm trying to implement an sound card-based OFDM modem. A paper says: ...A serial to parallel converter is applied and the IFFT operation is performed on the parallel complex data. Say I have ...
3
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1answer
58 views

OFDM DA conversion problem

I'm working on a sound card based OFDM system and ran into some problems: How can I perform DAC on the real and imaginary parts of the IFFT? I think they are real numbers instead of 0 and 1. ...
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2answers
1k views

How to Show A OFDM Symbol's PSD in Matlab

I'm trying to suppress the sidelobe of OFDM but having troubles showing its spectrum in Matlab. After IFFT, I get a time-domain signal, but in which way I can get its spectrum(or PSD)? I want to ...
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0answers
70 views

Hilbert Transform Filter for OFDM Signals

I am trying to design a Hilbert Transform Filter for OFDM signals. The specifications for the ofdm system are as follow: ...
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2answers
1k views

Coherence bandwidth and coherence time in an OFDM system

While studying an OFDM system, I have encountered statements like coherence bandwidth is the reciprocal of delay spread and coherence time is the reciprocal of the Doppler spectrum. Can somebody help ...
4
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2answers
123 views

What's the rationale behind this pulse shaping function?

Section 13.2 of the HD Radio standard for AM defines a pulse shaping function that appears to be some strange variant on a raised cosine filter: More specifically, H(Ī¾) is a raised cosine filter ...
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1answer
172 views

OFDM with beamforming

I'm trying to compute the the interference that one OFDM user with beamforming causes to another user. The two systems are fully overlapped in frequency and time. I know how to generate an OFDM signal ...
4
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1answer
1k 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 ...
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2answers
490 views

what does the output of ifft in ofdm look like?

i have a problem in ofdm modulation .... there is a stream of bits that is converted from serial to parallel and is applied to an IFFT block. based on the definition of IFFT the output should be an ...
3
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1answer
331 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 ...
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2answers
1k 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. ...
2
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0answers
290 views

Ricean Multipath channel model implementation

I'm trying to implement a multipath ricean channel model. I had found several resources but I'm kind of lost with too much information. What I have from now is the following step: ...
1
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1answer
357 views

Windowing a time domain OFDM signal

Windowing is a popular method of reducing the spectral sidelobes of OFDM. A popular window usually used for this purpose in the raised cosine (RC) window, because of its tapered and smooth edges. I ...
2
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3answers
210 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 ...
1
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1answer
1k 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 ...
3
votes
2answers
170 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 ...
2
votes
1answer
967 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 ...
2
votes
1answer
658 views

Spectrum of OFDM with raised cosine window

I'm having trouble implementing OFDM with a raised cosine (RC) window in Matlab. I know how to generate an OFDM signal and how to show its spectrum, I just don't know how to generate the window ...
3
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3answers
2k 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 ...
3
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4answers
3k 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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3answers
3k views

IFFT Concept in OFDM

can any one explain how actually IFFT generating sub carriers in OFDM.... when we are simulating OFDM transmitter in matlab, we are taking input data stream, encoding it and modulating the encoded ...
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1answer
952 views

OFDM transmitter data rate

i am simulating OFDM transmitter in matlab....As u all know OFDM consists of different sub systems such as FEC coding, Bit interlever , Modulator , IFFT block and Adding cyclic prefix.... i have ...