MSK output to OFDM modulator input

[Dr. Dhiman Kakati]

Hi all, I am new to OFDM design, attaching a part of the system where I want to connect the output of MSK sequence generator to the input of OFDM modulation block, but unable to connect the two as the MSK sequence generator output is not in M-ary form and the input of OFDM is in M-ary form so need to convert the output of MSK sequence generator to M-ary form. When I try to connect the M-ary pulse generator because its input and output type are opposite as per my requirement. so how can I convert the electrical signal to M-ary form. Thanks in advance.

Regards,
Dhiman

[Dr. Dhiman Kakati]

I am also not finding the decoder for MSK, i.e. MSK decoder in the components library. Do I need to design it manually, I need your suggestion.T Thanks.

Regards

[alistu]

Hi Dhiman,

The OFDM inputs signal format should be M-ary. MSK changes the M-ary output signal of the PSK coder into an electrical signal. So why not using the PSK before the OFDM instead of MSK? Any particular characteristic of MSK that you are suggesting might be useful when being used in OFDM scheme?

Regards

[Dr. Dhiman Kakati]

Alistu, I am actually asking for if there is some provision or way to connect the output of MSK sequence generator to the input of OFDM modulator. Thanks for the reply.

Regards,
Dhiman

[alistu]

I am aware of this, and I gave the above suggestion for the same purpose. Instead of converting electrical signal to M-ary signal, or in other words negating what pulse generators do in MSK layout (which you can see in MSK help section), not using pulse generators in the first place. Plus it is interesting for me to know based on what reasoning MSK has been used instead of PSK if you don’t mind.

Regards

[Dr. Dhiman Kakati]

Thanks Alistu I will look into this.

Regards,
Dhiman

[Dr. Dhiman Kakati]

Hi Alistu,
I looked into the internal diagram from the help section of MSK sequence generator that is a combination of PSK, M-ary pulse generator multiplied with sine wave and a electrical time delay as in the lower arm to generate the MSK sequence at the output. the output o the MSK sequence generator is in electrical form because of the electrical multiplier, so can you tell me how the sine wave is helping in transforming the PSK sequence to the MSK sequence (I am not getting that portion). furthermore think as if I remove the sine wave it will not be a MSK sequence at the output. So how to make the blocks such that the output is MSK sequence and in M-ary form. Thanks.

Regards,
Dhiman

[Dr. Dhiman Kakati]

The internal blocks attached. please anyone help.

Regards

[alistu]

Hi Dhiman,

Minimum Shift Keying is a continuous phase modulation, meaning that sudden shifts from one level to the other (which cause frequency lobes that degrade bandwidth efficiency) are not there in it. So the output of the MSK must be an electrical signal with and not rectangular like M-ary pulses.

Regards

[Dr. Dhiman Kakati]

Hi Alistu, I agree with you as you have said din the above reply that the output will be a electrical signal rather than an Ma-ary pulse. I am facing difficulty in connecting the OFDM modulation block input to the output of the M-ary sequence generator. i looking am for a way to connect the both, I am worried how i will bw able to do that. Is it like that the MSk signal can not be use in the OFDM technique???? there should be some way to convert the electrical signal to M-ary rectangular pulse. Thanks for the reply.

Regards,
Dhiman

[alistu]

If you mention the reasoning behind using MSK instead of PSK, I might be able to provide better help. If you are doing this because of better bandwidth efficiency due to phase continuity, OFDM will change everything by taking IFFT. BTW, electrical to M-ary can be done by threshold detector component.

Regards

[Dr. Dhiman Kakati]

Hi Alistu, The reason behind going for MSK is that spectral efficiency as you have mentioned taht in turn we can increase the bit rate and you know today’s wireless transmission in case of mobile communication uses GMSK that is a gaussian filtered minimum shift keying. There are several reasons for going towards MSK instead of PSK. Thanks.

Regards,
Dhiman

[alistu]

Thank you for your further explanation Dhiman. I know how MSK can be beneficial in systems you have mentioned and I don’t say it may not be in your case, but I cannot see how. In OFDM component, after the IFFT stage, the signal shape will be completely changed (the frequency domain waveform changes to the time domain waveform of the signal). So the time signal whose frequency has better bandwidth efficiency is converted to another waveform and its spectrum is changed as a result. Am I right?

[Dr. Dhiman Kakati]

Hi Alistu, I can understand that you have explained in case of OFDM after the IFFT the signal shape will be completely changed i.e. time domain conversion conversion. In case of MSK we can see comparatively less power as that of QPSK, QAM spectrum, this will help in multiplexing more no of channels for a specific band of frequency, i.e. we can put lesser channel spacing. so if we can multiplex more no of channels this will lead to higher bit rate for the same bandwidth. that is the reason for going towards MSK in comparison to other modulation scheme. Thanks.

Regards,
Dhiman

[alistu]

Thank you for the explanation. This actually happens to be something vastly done in electrical communications and even in optical systems (you can see the results of just one in the paper http://ieeexplore.ieee.org/xpl/login.jsp?tp=&arnumber=6334404&url=http%3A%2F%2Fieeexplore.ieee.org%2Fiel5%2F50%2F4357488%2F06334404.pdf%3Farnumber%3D6334404 as an example). Unfortunately, I don’t know how M-ary threshold detector can here be of any help, since as I said it just might negate the whole phase continuity. Although I think using Matlab you may be able to convert the signal to M-ary without having to quantize it, but there must be some easier way. i hope you will get help soon.

Regards

[Dr. Dhiman Kakati]

Thanks Alistu for providing with the paper, I will look into this. There are lots of advantages in case of minimum shift keying aside from these that I have mentioned in the previous comments you can find googling over other modulations.

Regards,
Dhiman

[alistu]

You’re welcome. Even though MSK has some advantages over PSK (including what I mentioned in reply #28338), I think the reason for PSK being used instead of MSK in many cases is the complexity factor, which is a very important aspect especially in passive optical networks and virtually determines their feasibility.

Regards

[Dr. Dhiman Kakati]

Hi Alistu I am still unable to solve the question in this issue what asked in the main question.

Regards

[Dr. Dhiman Kakati]

I am attaching herewith a design file, while simulating it it stops responding at the M-ary pulse generator 2. I am unable to solve this issue.
please have a look. Thanks in advance.

Regards,
Dhiman

[Dr. Dhiman Kakati]

attached file missed.

Regards

[Dr. Dhiman Kakati]

Please have a look into the design attached in reply no.28343

regards

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