Digital Communication - V18 - Offset and Shifted Quadrature Phase-Shift Keying (OQPSK)

[@Wislabi]

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[@Wislabi]

As far as I can see, the only disadvantage for OQPSK is the need for extra processing to make changes in the phase shifts according to the data bits values, which eventually increases the overall system complexity as well as the transmission delay. This is based on my understanding and analysis; however, there might be other drawbacks as well. At the end of the day, what determines which modulation to use is the application and use case you want to serve. Particularly, if you want better coverage and you are not limited with processing resources, you can use OQPSK. However, if the application you want to serve cannot afford extra complexity or delay, then you can just use QPSK. I hope that makes sense to you.

[@kakhaval]

Thanks for the video. I imagine Q channel is delayed by half symbol time BEFORE the shaping filter. Is it possible to implement that delay AFTER shaping filter as it is easier in hardware.

[@Wislabi]

Yes, it is possible

[@Wislabi]

You’re right about the filter ringing being a cause of high amplitude transients. Applying the delay after the shaping filter can help with implementation simplicity in hardware, but it doesn’t fully address the transient issues caused by the filter. This is because the filter applies its response without being aware of the symbol timing offset between the I and Q channels, which can lead to residual ringing. When you place the delay before the shaping filter, the filter can smooth the transitions more evenly across the I and Q paths, reducing the high-frequency transients and preventing them from getting amplified by subsequent stages like upsampling filters before the DAC. Delaying after the filter helps, but it’s more of a partial solution, and the more upsampling you have, the more noticeable the transients might become. So, while delaying after the filter is easier in hardware, the best signal quality comes from applying the delay before the shaping filter to reduce ringing at the source. This course researcherstore.com/courses/rf-communication-system-design/ will be of a great help for you in RF radio design.

[@kakhaval]

@@Wislabi Well actually I deleted my previous comment but thanks. My conclusion is that Q offset is irrelevant until mixer stage as each I or Q channels are independent. Filter ringing will be part of filter response and will be same if there Q offset or not. So my final conclusion is that Q offset only makes sense when the channels are mixed and can be applied anywhere before mixer.

[@robertocosentino7616]

Interesting! Thanks. But what is the drawback in using OQPSK compared to QPSK? otherwise in every case, OQPSK would be preferred, isn't it? In OQPSK you have more variations in terms of 90° transition...but still not clear the drawback

[@MrSocialish]

It is more complex to implement

[@raedalmalki5021]

ليتك شارحها بالعربي 💛💛

[@Wislabi]

you can click on the video subtitles, and then click on auto-translate so that you can translate to any language you want including Arabic.