1) Problem statement - what is the problem? why do you think it is a problem? what is the importance of the problem? and why does it happen?

2) What is the state of the art (related work) - What others have done to solve the above problem? why do you think other solutions are not good enough?

3) What would you achieve when you solve the problem in terms of advantages and merits?

4) What is your proposed solution? and why do you think your solution is unique and good?

5) How do you compare your solution with the prior art (previous similar techniques and methods)? what is your evaluation tool? what are your metrics?

1) Why is it described as a low complexity modulation scheme? Because of two reasons: a) It requires half the number of subcarriers (which means half the IFFT size) that would be needed by OFDM to achieve the same data rate; b) The detection process of the bits conveyed by the power difference is thresholding-based, which is much less complex than ML detectors. Note that the performance enhancement is not due to using the detector, but rather due to the way the data is modulated at the transmitter. Also, you can always use ML detector to detect the data, but such detector is not desirable in practice due to its high complexity that can drain the battery of the device quickly aside from causing latency; that is why we proposed the use of a lower complexity detector that can give a very good performance when the optimal threshold values are found and used.

2) What about its peak to average power ratio (PAPR)? Since half the number of subcarriers is used to achieve the same data rate as that of OFDM, then the PAPR, which depends on the number of subcarriers, is half that of OFDM. This will have direct impact on the coverage area, and that is why it can be very useful for mMTC/IoT scenarios.

3) What about its out of band emission (OOBE)? Since half the number of subcarriers is used to achieve the same data rate as that of OFDM, then the OOBE, which depends on the number of subcarriers, is half that of OFDM.

4) What about its transmission time and latency? Again for the same above mentioned reason, it is half of that required by OFDM for achieving similar data rates. Read the appendices of the following paper for more details https://www.researchgate.net/publication/339350562_Orthogonal_Frequency_Division_Multiplexing_With_Subcarrier_Power_Modulation_for_Doubling_the_Spectral_Efficiency_of_6G_and_Beyond_Networks

5) If we compare OFDM-SPM with QPSK to that of OFDM with 16-QAM assuming both use the same amount of transmit power, what would be the gain? The gain of OFDM-SPM-QPSK would be 3 dB for achieving the same BER as that of OFDM-16QAM. For more details on this specific result, you can check the article at this link https://rs-ojict.pubpub.org/pub/ofdm-spm/release/5

6) So, How does it double the spectral efficiency? If we want the PAPR, OOBE, transmission latency, and transmission power of OFDM-SPM to be exactly the same as that of OFDM, then the gain would be sending twice the data as that of OFDM, due to improving the spectral efficiency by double.

7) Does it have any relation with Index modulation? No, OFDM-SPM does not use indices or positions or numbers to convey data at all, and this what makes this technique capable of doubling the spectral efficiency through utilizing all the subcarriers to convey additional information bits equal to that transmitted by conventional modulation schemes (without causing much complexity).

8) So, what is the effect of mobility on the proposed scheme and how to handle it? The effect of time-varying channels and mobility issues are going to be the same as conventional OFDM, which means that most of the techniques used for OFDM can be still applicable to OFDM-SPM. This ensures having backward compatibility with OFDM, so you don't need to make radical changes in the transceiver design to implement OFDM-SPM, which is something highly desirable by operators (i.e., performance enhancement with not much changes or cost).

9) How is it related to 6G and beyond? One of the main challenges the existing standardized 5G designs for certain use-cases (e.g., URLLC) suffers from is the fact that the subcarrier spacing parameter, which corresponds to certain numerology in the OFDM waveform, has to be increased and become large enough to obtain short symbol intervals that are capable of meeting the low latency transmission requirements of URLLC services. However, such large subcarrier spacing used by high-order numerologies will inevitably reduce the amount of data rate that can be delivered within a single time slot, aside from the fact that the signaling overhead would increase in the case of using short time slots, resulting in significant reduction in the data rate/throughput in an outrageous manner. This specific shortcoming can make current 5G designs incapable of fulfilling and satisfying the requirements of new emerging applications such as extended reality (XR) services (including augmented, mixed, and virtual reality (AR/MR/VR)), haptics (tactile Internet), real-time gaming, telemedicine, brain-computer interfaces, flying vehicles, and connected autonomous systems. This is so because such applications require to be simultaneously supported not only by low latency with good reliability, but also with high throughput through increasing the overall spectral efficiency per device. Motivated by this observation, OFDM-SPM has been introduced as a novel, low-complexity, and low-latency modulation scheme, which is capable of adding a third data-carrying dimension to double the spectral efficiency per device, is developed and proposed for meeting the future requirements of 6G and beyond networks.

10) Last question, is there any future research direction that you recommend regarding this promising scheme? I would recommend investigating and quantifying the performance gains that can result from its integration and combination with other technologies, techniques and for different scenarios. Most importantly, I would strongly suggest finding new use cases for the proposed scheme. For example, we all know that with the current 5G NR standard, users can be assigned only one service at a time (URLLC, or mMTC, or eMBB) due to the difficulty of using two different numerologies simultaneously at the user equipment. So, the question that can be asked here is that, can OFDM-SPM be a solution for this problem where you serve mMTC application with the data stream conveyed by the power difference, whereas you serve URLLC application with the data stream conveyed by phase modulation for instance? The short answer is yes, but you need to explain this and prove it by simulations and math. Another interesting use case is to utilize OFDM-SPM as a multi-accessing technology to serve different users with different requirements and compare its results with the power domain NOMA (that was rejected by 3GPP standard) and see the gains that can be obtained. A good starting point is to read this article first https://rs-ojict.pubpub.org/pub/ofdm-spm/release/5 and go over the other articles found in the recourses given below.

====== Resources Related to OFDM-SPM

https://rs-ojict.pubpub.org/pub/ofdm-spm/ (https://assets.pubpub.org/9uzj2h9o/61590251911346.pdf )

https://rs-ojict.pubpub.org/pub/ofdm-spm-dpsk-non-coherent-modulation-schemes/

https://www.researchgate.net/publication/339350562_Orthogonal_Frequency_Division_Multiplexing_With_Subcarrier_Power_Modulation_for_Doubling_the_Spectral_Efficiency_of_6G_and_Beyond_Networks

https://onlinelibrary.wiley.com/doi/abs/10.1002/ett.3921

https://www.researchgate.net/publication/339858521_Orthogonal_frequency_division_multiplexing_with_subcarrier_power_modulation_for_doubling_the_spectral_efficiency_of_6G_and_beyond_networks

===== Crash Course on OFDM-SPM

https://researcherstore.com/courses/crash-course-ofdm-spm/

===== Implementation Codes of OFDM-SPM (Simulations in MATLAB)

https://researcherstore.com/product/simulations-of-ofdm-spm-with-qpsk-modulation/

===== For licensing matters

If your company is interested in licensing the technology described above for commercial use, you may contact the administration of RS-OJICT journal at this email rs-ojict@researcherstore.com

• 1) Advanced waveform designs: The purpose here is to create new improved waveforms (other than the conventional OFDM waveform) to enhance the system performance metrics in terms of PAPR, OOBE, ACI, INI, synchronicity, complexity,  spectral efficiency, reliability, and robustness to ICI, Doppler, and phase noise, etc. Examples of waveforms in this domain include GFDM, FBMC, DFT-s-OFDM, ZT-DFT-s-OFDM, W-OFDM, F-OFDM, edge filtered OFDM, OTFS, OCDM, etc. 

• 2) New modulation techniques: The goal here is to come up with novel modulation schemes (other than the conventional M-ary QAM/PSK modulation schemes) to improve the system performance metrics in terms of spectral efficiency, energy efficiency and reliability.  Examples of techniques in this domain include OFDM-SIM, OFDM-ESIM, OFDM-IM, OFDM-GIM, OFDM-DMIM, OFDM-MMIM, OFDM-SNM, OFDM-SPM, MIMO-SM, MIMO-SSK, etc. 

However, there has been some confusion in the literature in terms of naming, where many people think waveform is modulation as can be seen in [1], which is not very true as they are completely different from each other. To understand the difference and see if there is any relationship between them, we will resort back to the most basic, fundamental definition of each as follows.  

• 1) Modulation means changing the properties of a carrier signal, which can be transmitted and received over a medium, according to the incoming information. For instance, when the used carrier signal in a digital communication system is a sinusoidal function, then its properties including amplitude, phase and frequency can be changed according to the incoming information bits, resulting in what is called ASK, PSK, FSK, respectively. This is well-known in the literature as the most basic form of modulation. As a new front-tier, researchers have recently used the properties of a subblock within an OFDM symbol structure as a new type of carrier at the baseband to send extra information bits.  Basically, the subblock properties in terms of the indices and number of subcarriers within the subblock [2] are used to convey extra information bits. 

• 2) Waveform means the collection of pulses that will be used to carry your modulated data symbols over the channel. These pulses determine all the details related to the time duration, bandwidth, and spectrum shape along with its location in the time-frequency grid to be used during transmission over the channel to protect your data from any kind of interference (ISI, ICI, ACI, INI) and/or other types of distortions (power amplifier distortion and phase noise) while meeting the requirements of the targeted application. For instance, OFDM is a collection of Sinc pulses with certain bandwidth and location in the frequency domain (or summed/multiplexed exponential basis functions with different frequencies), carrying the data modulated symbols. Recently, many different waveforms are proposed such as GFDM, FBMC, DFT-s-OFDM, ZT-DFT-s-OFDM, W-OFDM, F-OFDM, edge filtered OFDM, OTFS, etc. to meet the requirments of future emerging applications in terms of latency, interference, synchronicity and immunity to impairments. 

From the above discussion and explanation, the differences between waveform and modulation concepts are clearly stated and concluded. This description is expected to help researchers choose appropriate terms and names to the proposed schemes, methods, techniques, etc. according to their exact functionality to avoid any confusion in the basic understanding of new concepts.

References:

[1] Y. Cai, Z. Qin, F. Cui, G. Y. Li and J. A. McCann, "Modulation and Multiple Access for 5G Networks," in IEEE Communications Surveys & Tutorials, vol. 20, no. 1, pp. 629-646, Firstquarter 2018.

[2] A. M. Jaradat, J. M. Hamamreh and H. Arslan, "Modulation Options for OFDM-Based Waveforms: Classification, Comparison, and Future Directions," in IEEE Access, vol. 7, pp. 17263-17278, 2019.

[3] J. M. Hamamreh and H. Arslan, "Time-frequency characteristics and PAPR reduction of OTDM waveform for 5G and beyond," 2017 10th International Conference on Electrical and Electronics Engineering (ELECO), Bursa, 2017, pp. 681-685.

Author contact: jehad.hamamreh@researcherstore.com

HAMAMREH is the Founder and Director of WISLAB, Board member at Researcherstore and Editor at RS-OJICT journal, and A. Professor with the Electrical and Electronics Engineering Department, Antalya International (Bilim) University. He received the Ph.D. degree in telecommunication engineering and cyber systems from Istanbul Medipol University. Previously, he worked as a Researcher at the Department of Electrical and Computer Engineering in Texas A&M University. He is an inventor of more than 20+ Patents, and has authored more than 90+ peer reviewed scientific papers along with several book chapters. He supervised and graduated tens of master and PhD students. He received the top researcher awards three times. His innovative patented works won the golden, silver and bronze medals by numerous international invention contests and fairs.

His current research interests include wireless physical and MAC layers security, orthogonal frequency-division multiplexing multiple-input multiple-output systems, advanced waveforms design, multidimensional modulation techniques, and orthogonal/non-orthogonal multiple access schemes for future wireless systems. He is a regular investigator and referee for various scientific journals as well as a TPC member for several international conferences. He is an Editor at RS-OJICT and Frontiers in Communications and Networks.

Key Research Topics: Wireless Communication, Wireless Security, Wireless Sensing, 5G/6G, IoT-AI

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