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Questions and Answers
Which condition indicates frequency selective fading?
Which condition indicates frequency selective fading?
What characterizes flat fading channels compared to fast fading channels?
What characterizes flat fading channels compared to fast fading channels?
What is typically true for a fast fading channel?
What is typically true for a fast fading channel?
In OFDM, what is the desired relationship between coherence bandwidth, symbol rate, and fading rate?
In OFDM, what is the desired relationship between coherence bandwidth, symbol rate, and fading rate?
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Which factors are managed using traditional equalization techniques?
Which factors are managed using traditional equalization techniques?
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What happens when the number of subcarriers $N_C$ is appropriately chosen in OFDM?
What happens when the number of subcarriers $N_C$ is appropriately chosen in OFDM?
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What aspect defines the worst-case fading profiles?
What aspect defines the worst-case fading profiles?
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What condition is necessary to obtain flat fading and slow fading channels in OFDM?
What condition is necessary to obtain flat fading and slow fading channels in OFDM?
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What is the coherence bandwidth $f_0$ calculated in the solution?
What is the coherence bandwidth $f_0$ calculated in the solution?
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What is the minimum number of required subcarriers $(N_c)_{min}$ based on the provided bandwidth?
What is the minimum number of required subcarriers $(N_c)_{min}$ based on the provided bandwidth?
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Which factor determines the maximum number of required subcarriers $(N_c)_{max}$?
Which factor determines the maximum number of required subcarriers $(N_c)_{max}$?
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What is the relationship between the wavelength $ au$ and the velocity $v$ when $T_0 = 10 ms$?
What is the relationship between the wavelength $ au$ and the velocity $v$ when $T_0 = 10 ms$?
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How many bits per second is the coded data rate $R_C$ given a bit rate $R = 1 Mbit/s$ with a code rate of 1/2?
How many bits per second is the coded data rate $R_C$ given a bit rate $R = 1 Mbit/s$ with a code rate of 1/2?
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What is the maximum multipath spread $T_m$ calculated from the rms delay $ au$?
What is the maximum multipath spread $T_m$ calculated from the rms delay $ au$?
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What main advantage does OFDM provide over traditional modulation methods?
What main advantage does OFDM provide over traditional modulation methods?
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Given 8-PSK modulation, what is the symbol rate $R_S$ if the coded bit rate $R_C$ is $2 × 10^6$ bps?
Given 8-PSK modulation, what is the symbol rate $R_S$ if the coded bit rate $R_C$ is $2 × 10^6$ bps?
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What is the purpose of the inverse discrete Fourier transform (IDFT) in the context of OFDM?
What is the purpose of the inverse discrete Fourier transform (IDFT) in the context of OFDM?
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How does the lengthened symbol time in OFDM affect rms smearing?
How does the lengthened symbol time in OFDM affect rms smearing?
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What is the function of the cyclic prefix (CP) in OFDM systems?
What is the function of the cyclic prefix (CP) in OFDM systems?
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What effect does the introduction of guard periods have in an OFDM system?
What effect does the introduction of guard periods have in an OFDM system?
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How does OFDM handle multipath spread compared to non-OFDM systems?
How does OFDM handle multipath spread compared to non-OFDM systems?
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What is a defining characteristic of the channel impulse response in an OFDM system?
What is a defining characteristic of the channel impulse response in an OFDM system?
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When comparing high-rate signals segmented into low-rate subchannels in OFDM, what is a result that occurs?
When comparing high-rate signals segmented into low-rate subchannels in OFDM, what is a result that occurs?
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What happens to the maximum multipath spread when OFDM is used?
What happens to the maximum multipath spread when OFDM is used?
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In BPSK modulation, what phasor is assigned to the bit 0?
In BPSK modulation, what phasor is assigned to the bit 0?
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What does the parameter $m$ represent in the context of 64-QAM?
What does the parameter $m$ represent in the context of 64-QAM?
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What is the significance of the IDFT in the context of constellation mapping?
What is the significance of the IDFT in the context of constellation mapping?
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What condition must the subcarriers satisfy in an OFDM system?
What condition must the subcarriers satisfy in an OFDM system?
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What does the formula $\Delta f = R_s = 1/T_s$ describe?
What does the formula $\Delta f = R_s = 1/T_s$ describe?
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What type of envelope do the output sinusoids from the IDFT have?
What type of envelope do the output sinusoids from the IDFT have?
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What role does the cyclic prefix play in an OFDM symbol?
What role does the cyclic prefix play in an OFDM symbol?
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How many bits do the phasors in a QPSK system represent?
How many bits do the phasors in a QPSK system represent?
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What is the total number of samples per OFDM symbol in the 802.11a standard?
What is the total number of samples per OFDM symbol in the 802.11a standard?
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How is the symbol rate of an OFDM system calculated?
How is the symbol rate of an OFDM system calculated?
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What is the effect of increasing the transform size N in an OFDM system?
What is the effect of increasing the transform size N in an OFDM system?
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What does reducing the spacing between subcarriers (Δ f) affect?
What does reducing the spacing between subcarriers (Δ f) affect?
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What is the approximate modulation bandwidth (Wmod) of the 802.11a system?
What is the approximate modulation bandwidth (Wmod) of the 802.11a system?
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Which of the following is the sample time duration for a 20 MHz OFDM system?
Which of the following is the sample time duration for a 20 MHz OFDM system?
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What is the relationship between the sample rate (fS) and the number of subcarriers (Nc)?
What is the relationship between the sample rate (fS) and the number of subcarriers (Nc)?
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What is the total time duration of an OFDM symbol in the 802.11a standard?
What is the total time duration of an OFDM symbol in the 802.11a standard?
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What does the Cyclic Prefix (CP) do in an OFDM system?
What does the Cyclic Prefix (CP) do in an OFDM system?
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In discrete Fourier transforms, what does spectral multiplication correspond to in time?
In discrete Fourier transforms, what does spectral multiplication correspond to in time?
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Why is it essential for continuous time signals to be band limited?
Why is it essential for continuous time signals to be band limited?
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What key property allows the transformation of linear convolution into circular convolution in OFDM?
What key property allows the transformation of linear convolution into circular convolution in OFDM?
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What is essential for reconstructing correct OFDM subcarriers at the receiver?
What is essential for reconstructing correct OFDM subcarriers at the receiver?
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What happens to the spectrum of a continuous time waveform as its frequency approaches infinity?
What happens to the spectrum of a continuous time waveform as its frequency approaches infinity?
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How is frequency represented in the DFT of a sampled rectangular function?
How is frequency represented in the DFT of a sampled rectangular function?
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What modification occurs to the continuous transmitted waveform in OFDM during the DFT process?
What modification occurs to the continuous transmitted waveform in OFDM during the DFT process?
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Study Notes
Orthogonal Frequency Division Multiplexing (OFDM)
- OFDM is a multi-carrier signaling technique
- It is a combination of modulation and multiplexing
- Carrier frequencies are orthogonal
- Saves bandwidth due to frequency overlap between neighboring signals
- Demultiplexing uses no filtering
- Effective in fading channels, mitigating their degrading effects for high data rates
FDM versus OFDM
-
FDM (Conventional Multi-Channel):
- Adjacent channels do not overlap.
- Guard bands between channels for separation using filters
- Higher bandwidth required compared to OFDM for the same data rate
-
OFDM (Multi-Channel):
- Adjacent channels do partially overlap
- No guard bands; 50% overlap is acceptable, unlike FDM, but orthogonality is maintained
- Lower total bandwidth needed compared to FDM for the same data rate
OFDM in Fading Channels
-
Coherence Bandwidth: fo
- The range of frequencies, treated by the channel in a similar manner.
- Range of a signal's spectral components traversing the channel with the same gain and linear phase.
-
Coherence Time: To
- The range of time during which a signal is treated similarly by the channel.
- Max delay Tm between the longest and shortest signal paths in the multipath channel
- Doppler frequency (shift) during motion can affect coherence bandwidth and time
- Doppler spread is reciprocally related to coherence time
- Frequency Selective Fading: Occurs when fo < W, where W represents the signal bandwidth in Hertz.
- Flat Fading: Fo > W
- Relationship between delay spread and coherence bandwidth: fo ≈ 1/Tm, and To ≈ 1/fd
OFDM Subcarrier Design
- Calculations: Given values of channel coherence time (Tm), coherence bandwidth (fo), transmission bandwidth (W), and carrier frequency. Calculate the minimum and maximum number of subcarriers needed for flat fading and slow fading. Additional calculations may involve maximum allowed velocity, wavelengths, and sampling rates.
Modem Without OFDM and With OFDM
- High data-rate signals are divided into smaller lower-rate orthogonal subchannels by OFDM to mitigate the impacts of frequency selective fading channels.
Cyclic Prefix (CP)
- OFDM uses guard periods (equal to max delay spread) between symbols to reduce ISI
- CP helps maintain orthogonality, allowing simple spectral scaling in equalization
- CP is used to avoid overlap in transmitted symbols and allow circular convolution
OFDM System Block Diagram
- Operations: The system converts a serial data stream into parallel streams, maps bits to constellation points, performs an inverse discrete Fourier transform (IDFT), and adds a cyclic prefix (CP) before transmission. The receiver performs the reverse operations.
DAC (Digital-to-Analog Converter)
- Converts digital data to analog signals to prepare the signal for transmission.
LP Filter (Low-Pass Filter)
- Limits the signal bandwidth of real and imaginary parts of the baseband composite symbol, preventing interference.
- This is required for bandpass modulation
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Description
Test your knowledge on Orthogonal Frequency Division Multiplexing (OFDM) and its comparison with Frequency Division Multiplexing (FDM). This quiz covers the key principles, advantages, and applications of OFDM in communication systems, especially in fading channels. Explore how OFDM utilizes bandwidth more efficiently and its modulation techniques.
