5g ve ötesi için indis modülasyonlu OFDM

Orthogonal frequency division multiplexing (OFDM) is a multi-carrier transmission scheme that has been used for the most of recent wireless communications systems, even it has been decided to be used as the modulation technique with multi-numerologies structure in 5th Generation (5G). This is due to the advantages of OFDM, such as efficient spectrum utilization by varying number of subcarriers, power of subcarriers, and pulse-shaping filters; flexible adjustment to different requirements by changing OFDM symbol block size, cyclic prefix size, and subcarrier spacing; well-suitability to multiple antennas systems; frequency domain equalization; permission to flexible scheduling mechanism for multiple users. On the other hand, it has two main disadvantages that has been tried to be solved up till now: high peak-to-power average ratio (PAPR) that causes distortion of the signal due to non-linearities of power amplifiers (PA) and high out-of-band emission (OOBE) that makes spectrum granularity difficult. Since OFDM has been selected as the core waveform of 5G, researches about these drawbacks will continue to find better solutions. As being an enhancement modulation technique to OFDM, orthogonal frequency division multiplexing with index modulation (OFDM-IM) has been proposed for spectrum- and energy-efficient next generation wireless communication systems, and it has been attracted by many researchers recently. Unlike conventional OFDM, OFDM-IM employs both the indices of active subcarriers and data symbols at the active subcarriers to convey information. In OFDM-IM, the incoming bit sequences are divided into two groups. First group determines the active subcarriers according to look-up tables or special algorithms, then second group determines the data symbols at the active subcarriers. OFDM-IM offers attractive advantages in comparison to OFDM thanks to the information bits conveyed by IM, such as better bit error rate (BER) performance at low-to-mid level spectral efficiency values, higher robustness to inter-carrier interference (ICI), better ergodic achievable rate, and being more flexible based on different channel conditions and system requirements. However, OFDM-IM has also the drawback of high PAPR similar to OFDM, and this important problem has not been studied well in the literature. Active constellation extension (ACE), which is one of the well-known PAPR reduction methods for OFDM, can be used to solve this drawback of OFDM-IM. Owing to the fact that this PAPR reduction method is less effective for OFDM-IM due to the inactive subcarriers that do not convey data symbols, we propose the extension of the constellation over inactive subcarriers through adding clipped signals over them. These subcarriers have a signal power limited by an upper bound, and this causes a slight degradation in the bit error rate (BER) performance. Computer simulation results demonstrate that our proposed method has a better PAPR reduction performance than the ACE method for OFDM and OFDM-IM while being more energy efficient with a very slight degradation in BER performance when a proper clipping threshold level is selected. Additionally, it is shown that the proposed method and ACE can be further combined, and this provides an improved PAPR reduction. In order to decrease the computational complexity of the PAPR reduction method to the linear-logarithmic level, smart gradient projection (SGP) is employed. 5G of wireless networking is coming with diverse use cases, such as enhanced-Mobile BroadBand (eMBB), Ultra Reliable and Low Latency Communications (URLLC), and massive Machine Type Communications (mMTC). eMBB services require very high data rates and wider bandwidths. URLLC services require very low latency, very high reliability and availability. And, mMTC services require massive connectivity, low bandwidth, low power consumption, and enhanced coverage. As a result, 5G wireless networks require flexible physical layer solutions through new radio access technologies (RATs). At this point, OFDM-IM appears a flexible solution to satisfy the diverse user demands. Considering the strict requirements of mMTC services, such as low throughput, low power consumption, and low cost design, we propose fading-aligned OFDM-IM for more spectrum- and energy-efficient communication. In the proposed method, inactive subcarriers in OFDM-IM are cleverly utilized to avoid deep fading sub-channels, because the deep fading of the active subcarriers decreases bit error rate (BER) performance significantly. Computer simulation results demonstrate that more than 10 dB gain is obtained for a reference BER value of 10^(-4) at the same spectral efficiency with conventional OFDM. Moreover, the proposed method is compared with convolutional coded (CC) OFDM at the same spectral efficiency, and it is shown that the proposed scheme performs better in terms of BER performance. Furthermore, theoretical error performance of the proposed method is investigated to support our computer simulations.