FAQ about OFDM-SPM
This post is intended to answer some of the questions raised regarding the modulation scheme named OFDM-SPM:
Before we list the frequently asked questions and provide answers to them, let's first imagine the following scenario: Let's assume that we are required to provide an extra reliable OFDM communication link for a low complexity IoT device, what would we do? We would need to choose a lower order modulation scheme such as BPSK modulation for OFDM subcarriers as it provides the lowest bit error rates (BER) compared to other higher order modulation schemes. Now, imagine that we are asked to double the data rate of this communication link, while using the same bandwidth, and same time interval/symbol duration that a conventional OFDM-BPSK would use, then what would you use in this case to meet such a requirement? of course, you cannot say that we can just use QPSK to double the data rate because the time interval of one QPSK symbol is twice that of BPSK, it consumes double the power of that of BPSK to provide similar BER to that of BPSK, and it utilizes the quadrature component of the RF carrier, which greatly increases complexity at the IoT device and makes it subject to I/Q imbalance problem. So, here where OFDM-SPM comes into the picture to solve such a problem through using the power of subcarriers in an OFDM system as an additional dimension to convey extra data bits, besides those transmitted by conventional BPSK modulation, thus resulting in doubling the data rate while using the same bandwidth and symbol duration of a BPSK symbol.
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
===== Crash Course on OFDM-SPM
===== Implementation Codes of OFDM-SPM (Simulations in MATLAB)
===== For licensing matters
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