Thanks for your great videos. In this one, you are assuming passband signal is double side band taking RF bandwidth twice the bandwidth of one subcarrier. In I/Q transmitters, the transmit signal at RF is single side band. Therefore if you apply a complex tone at 3f1 (corresponding to subcarriers +3), you only get an RF tone at fc+3f1.
Single sideband can only be done for real valued baseband signals - not for complex baseband. It relies on the fact that real valued signals have a conjugate-symmetric Fourier transform. That's not the case for complex baseband signals.
*Summary* * *(**0:00**)* *OFDM Subcarriers and Frequency Selective Channels:* This explanation focuses on how a "notch" in a frequency selective channel (which blocks certain frequencies) impacts OFDM (Orthogonal Frequency-Division Multiplexing) signals. * *(**0:07**)* *Baseband vs. Passband:* The video emphasizes the importance of considering the *passband* (the actual frequencies being transmitted) rather than just the *baseband* (a simplified representation centered around zero frequency) when analyzing the effect of the channel. * *(**2:03**)* *Real and Imaginary Components:* An OFDM signal has both real and imaginary components. While the baseband representation might show energy only in the positive frequencies, both positive and negative frequencies are involved in the actual transmitted signal. * *(**4:11**)* *Impact of a Notch:* A notch in the frequency selective channel affects *both* the positive and negative frequency components of a subcarrier. * *(**6:40**)* *Key Takeaway: A single notch does not completely eliminate a subcarrier.* While it impacts the signal strength (and potentially introduces phase changes), the subcarrier still carries information. This is because the notch typically only affects a portion of the frequency spectrum occupied by that subcarrier. This summary explains the core concept discussed in the video without requiring you to watch it. The focus is on dispelling the common misconception that a single notch can completely wipe out an OFDM subcarrier. I used Google Gemini 1.5 Pro exp 0801 to summarize the transcript. Cost (if I didn't use the free tier): $0.0307 Time: 17.77 seconds Input tokens: 6985 Output tokens: 591
There is a video on how to make your video using a clothe hanger device. But I cannot find it. Coud you provide the link please ! Thank you very much !
It seems youre feeding identical signal to both i and q modulators. But qam or any phase modulated signal would feed different basebad signal to i vs q modulator. If channel is clobbered, you would lose half the constellation, not just half the energy.
At the 3:29 min mark of the video I show the two seperate I and Q waveforms (the bottom two plots). They are clearly not the same waveforms. The top plot shows the magnitude of the (complex valued) constellation points in each subcarrier (in this case there is only data in a single subcarrier).
In case of mimo ofdm all subcarriers at same frequency index at each antenna will be faded in the same fashion. Any effect and physical interpretation of it ?
No, that's not the case. If the antenna elements are more than half a wavelength apart, and if the channel has lots of multi-path, then the antenna elements will experience independent fading.
Are you suggesting that a QAM-OFDM signal can be succesfully received even if a sub-carrier is completely blocked? That seems wrong. It would only work with a real baseband signal, but QAM is not real in the baseband.
The very last spectrum you show, is the spectrum of only the real (or imaginary) part of the time signal, right? The spectrum of the whole signal, expressed as a sum og the imaginary and real parts, would be zero for all frequencies, after passing the channel. The negative frequency componets sum to zero, as you show in matlab around 4:30, and the positive frequency componets are blocked by the channel. (or vise versa) The resultat is that nothing passes the channel.
@@iain_explainsYes. It can be blocked, if the signal is a complex baseband signal (i.e modulated in an I/Q modulator) Your claim holds only for double sideband signals, but you use a single sideband signal in your matlab example.
Not so. They only cancel each other out in the baseband. The passband signals are generated by multiplying the "real" signal by the cos carrier, and the "imaginary" signal by the sin carrier. Then they don't cancel each other out in the passband in the same way.
Thanks for your great videos. In this one, you are assuming passband signal is double side band taking RF bandwidth twice the bandwidth of one subcarrier. In I/Q transmitters, the transmit signal at RF is single side band. Therefore if you apply a complex tone at 3f1 (corresponding to subcarriers +3), you only get an RF tone at fc+3f1.
Single sideband can only be done for real valued baseband signals - not for complex baseband. It relies on the fact that real valued signals have a conjugate-symmetric Fourier transform. That's not the case for complex baseband signals.
*Summary*
* *(**0:00**)* *OFDM Subcarriers and Frequency Selective Channels:* This explanation focuses on how a "notch" in a frequency selective channel (which blocks certain frequencies) impacts OFDM (Orthogonal Frequency-Division Multiplexing) signals.
* *(**0:07**)* *Baseband vs. Passband:* The video emphasizes the importance of considering the *passband* (the actual frequencies being transmitted) rather than just the *baseband* (a simplified representation centered around zero frequency) when analyzing the effect of the channel.
* *(**2:03**)* *Real and Imaginary Components:* An OFDM signal has both real and imaginary components. While the baseband representation might show energy only in the positive frequencies, both positive and negative frequencies are involved in the actual transmitted signal.
* *(**4:11**)* *Impact of a Notch:* A notch in the frequency selective channel affects *both* the positive and negative frequency components of a subcarrier.
* *(**6:40**)* *Key Takeaway: A single notch does not completely eliminate a subcarrier.* While it impacts the signal strength (and potentially introduces phase changes), the subcarrier still carries information. This is because the notch typically only affects a portion of the frequency spectrum occupied by that subcarrier.
This summary explains the core concept discussed in the video without requiring you to watch it. The focus is on dispelling the common misconception that a single notch can completely wipe out an OFDM subcarrier.
I used Google Gemini 1.5 Pro exp 0801 to summarize the transcript.
Cost (if I didn't use the free tier): $0.0307
Time: 17.77 seconds
Input tokens: 6985
Output tokens: 591
There is a video on how to make your video using a clothe hanger device. But I cannot find it. Coud you provide the link please !
Thank you very much !
The video was "Behind the Scenes: A Solution Based Approach to Learning and Teaching" th-cam.com/video/hA9ZKAtYADs/w-d-xo.html
@@iain_explains thank you so much
It seems youre feeding identical signal to both i and q modulators. But qam or any phase modulated signal would feed different basebad signal to i vs q modulator. If channel is clobbered, you would lose half the constellation, not just half the energy.
At the 3:29 min mark of the video I show the two seperate I and Q waveforms (the bottom two plots). They are clearly not the same waveforms. The top plot shows the magnitude of the (complex valued) constellation points in each subcarrier (in this case there is only data in a single subcarrier).
In case of mimo ofdm all subcarriers at same frequency index at each antenna will be faded in the same fashion. Any effect and physical interpretation of it ?
No, that's not the case. If the antenna elements are more than half a wavelength apart, and if the channel has lots of multi-path, then the antenna elements will experience independent fading.
Hello friend! Im also completed telecomm and work in telecomm company. Have you a plan make a video about CRC coding?
Thanks for the topic suggestion. I've got it on my "to do" list.
Are you suggesting that a QAM-OFDM signal can be succesfully received even if a sub-carrier is completely blocked? That seems wrong. It would only work with a real baseband signal, but QAM is not real in the baseband.
You seem to have missed the point of the video. As I explained, a single subcarrier will never be “completely blocked”.
The very last spectrum you show, is the spectrum of only the real (or imaginary) part of the time signal, right? The spectrum of the whole signal, expressed as a sum og the imaginary and real parts, would be zero for all frequencies, after passing the channel. The negative frequency componets sum to zero, as you show in matlab around 4:30, and the positive frequency componets are blocked by the channel. (or vise versa) The resultat is that nothing passes the channel.
@@iain_explainsYes. It can be blocked, if the signal is a complex baseband signal (i.e modulated in an I/Q modulator)
Your claim holds only for double sideband signals, but you use a single sideband signal in your matlab example.
@@antonrnnedal7797 LTE, 5G signals are OFDM signals, is it double side band or single side band?
Not so. They only cancel each other out in the baseband. The passband signals are generated by multiplying the "real" signal by the cos carrier, and the "imaginary" signal by the sin carrier. Then they don't cancel each other out in the passband in the same way.