# What is the index modulation in OFDM and Subcarrier Index modulation?

I'm new here, I'd like to ask about the OFDM-IM (OFDM - Index modulation), what is it and how it works? Is it the same as SIM-OFDM?

I checked white papers, I see it's using the index of sub-carriers for transmitting bit too. But what does it mean by the index of sub-carriers?

Could you please explain it in clear way? or share any article where I can understand it well.

• You probably mean index modulation, right? (singular: index, plural: indices) Which white papers do you refer to? "Index Modulation" means different things depending on which author you ask; most are simply other names for Spatial Modulation, other actually describe a multitone FSK. – Marcus Müller Nov 15 '18 at 8:57
• Actually I got many papers, now I'm reading this one trying to understand it. ieeexplore.ieee.org/document/5449882 – Gze Nov 15 '18 at 9:09
• ok, that paper is pretty clear. Can you elaborate on what you do not understand? – Marcus Müller Nov 15 '18 at 11:16
• OK .. that paper is just an example for the Index modulation. I'm trying to search on different papers. what I didn't understand exactly, how does it use the index of subcarrier to transmit bits ? what does it means by index of subcarriers ?? that's point I couldn't understand it well, if someone can explain those two points clearly, that will be clearer for me. – Gze Nov 15 '18 at 11:35
• an index is just the number with which you count the things. "how does it use the index": That's what the paper is about. You need to be more specific, please. – Marcus Müller Nov 15 '18 at 11:41

Yes OFDM-IM is somehow new and it's very hot topic of searching. That article you have shares is the basic of OFDM-IM. So if you are at the beginning to understand it, that's the right paper to to read and understand.

OK let me explain that paper for you in easier way,

consider we have block with length 32 bits you want to transmit it, in the traditional OFDM, using 4-$$QAM$$ modulation, we supposed to have $$32/2 = 16$$ FFT size (which represents the number of sub-carriers). According to that figure 01 in the article you've shared, all sub-carriers will be active to convey data to receiver following the common steps of OFDM.

Now, let's look at OFDM-IM, using the block whose length is $$32-bits$$ and 4-$$QAM$$. In OFDM-IM, we start by dividing our bits into two parts. each with length of $$16-bits$$ The first part will be handled similar to traditional OFDM, but the second part will be handled using OOK and using also that module added in Figure 01 called (sub-carrier index modulation). How??

That second part, we start count how many zeros and ones we have in that block (it's called subblobk). suppose we have 10 ones and 6 zeros similar to that example in your article,That's ok, but the issue is how can we know the location of zeros and locations of ones? That's what the subcarrier index means!!! It means that the index of sub-carrier will indicate that location.

For example, in that example, you've 16 FFT size representing the number of sub-carriers, so we use each index to point that location of zeros. In other words, we use the subblock in 4-$$QAM$$ to show the location of zeros in subblock OOK. It means, we look at first bit in OOK, if it's 0 we let the sub-carriers to be empty, if it's 1 we upload the bits of $$QAM$$ into sub-carrier and so on. So, it's called OOK fashion.

That's what the idea of paper you've shared, I will let the other parts of paper to you to analyze and understand. then you notice easily the gain of using OFDM-IM

Hope that will help

• Thank you .. that's very clear now. but what's about the N(maj)) , how did they calculate that? – Gze Nov 16 '18 at 8:40
• well, as I told you,, you should understand the rest of details in that article by yourself. However, all other part are taking about that idea I gave you above. regaring that N majority, it's just if you have ones more than zeros. that's all, they compare it to length of FFT = 16 which is nothing but half of block length 32-bits. remember we are using 4-QAM .. by the way, using PSK is more efficient than QAM but I'm explaining according to your example. – Zeyad_Zeyad Nov 16 '18 at 8:45