Non-orthogonal multiple access (NOMA) is an attractive candidate for 6G networks to support ultra-massive machine-type communications (umMTC). Power domain NOMA (PD-NOMA) is the simplest type of NOMA, which assigns a different power level to each user. Power allocation in PD-NOMA can be classified into fixed/dynamic power allocation (FPA/DPA). FPA is simple, but DPA is more suitable for the mobile environment than FPA. However, finding optimum power per each user in DPA is extremely complex. Fortunately, many DPA strategic design methods were introduced in literature as simple suboptimal solutions of DPA. Therefore, DPA strategic design methods and FPA techniques are simple approaches to implement PD-NOMA in 6G and beyond. In literature, no previous work had compared the performances of all DPA strategic design methods, nor FPA techniques, to ease the selection of a simple strategy for PD-NOMA in 6G. Motivated by that, this work compares performances of all DPA strategic design methods as well as FPA techniques, in terms of sum-rate capacity, fairness, and bit error rate (BER). Results showed that the best DPA strategic design method and the best FPA technique have comparable performance.

Orthogonal frequency division multiplexing (OFDM) is used in high data rate applications due to its ability to cope with frequencyselective channels. However, OFDM suffers from the high peak-toaverage power ratio (PAPR) problem, which reduces the power amplifier (PA) efficiency or otherwise degrades bit error rate (BER) and increases out-of-band (OOB) radiation. In the literature, there are various PAPR reduction techniques. Among them companding techniques have small computational complexity, which make them attractive to be used in mobile stations (MS). Generally, companding techniques expand small signals while compressing large signals or compress large signals without affecting small signals. In this paper, different PAPR reduction companding transforms are compared. Results showed that companding transforms that compress large signals without affecting small signals (such as, Log companding and Tanh companding) are better than the others from a BER point of view. Results also showed that the Log companding transform is better than the Tanh transform, in terms of PAPR reduction gain and OOB radiation reduction. So the Log companding transform can be considered as the best practical companding transform among others. ARTICLE HISTORY

Orthogonal frequency division multiplexing (OFDM) is an attractive technique for wireless communication systems due to its ability to mitigate frequency selectivity. However, OFDM suffers from high peak‐to‐average power ratio (PAPR) problem, which reduces the power amplifier (PA) efficiency or worse it degrades bit error rate (BER) performance and increases out‐of‐band radiation. In literature, there are different PAPR reduction techniques. Among them PAPR reduction by precoding matrices has small computational complexity along with high PAPR reduction gain. In this paper the six precoding matrices found in the literature are compared. Results showed that, all precoding matrices, except the one that based on square‐root raised cosine function (SRC), are not effective in terms of BER performance in presence of nonlinear PA, especially in high modulation order schemes (eg, 16‐QAM and 64‐QAM). However, precoding technique based on SRC matrix requires high data rate loss, to enhance the BER performance in presence of nonlinear PA. So, it can be said that, precoding technique worsens the problem especially in high modulation order schemes, except the SRC matrix‐based precoding technique, which cannot be used without high data rate loss.

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Several high-speed wireless systems use Orthogonal Frequency Division Multiplexing (OFDM) due to its advantages. 5G has adopted OFDM and is expected to be considered beyond 5G (B5G). Meanwhile, OFDM has a high Peak-to-Average Power Ratio (PAPR) problem. Hybridization between two PAPR reduction techniques gains the two techniques’ advantages. Hybrid precoding-companding techniques are attractive as they require small computational complexity to achieve high PAPR reduction gain. Many precoding-companding techniques were introduced to increasing the PAPR reduction gain. However, reducing Bit Error Rate (BER) and out-of-band (OOB) radiation are more significant than increasing PAPR reduction gain. This paper proposes a new precoding-companding technique to better reduce the BER and OOB radiation than previous precoding-companding techniques. Results showed that the proposed technique outperforms all previous precoding-companding techniques in BER enhancement and OOB radiation reduction. The proposed technique reduces the Error Vector Magnitude (EVM) by 15 dB compared with 10 dB for the best previous technique. Additionally, the proposed technique increases high power amplifier efficiency (HPA) by 11.4%, while the best previous technique increased HPA efficiency by 9.8%. Moreover, our proposal achieves PAPR reduction gain better than the most known powerful PAPR reduction technique with a 99% reduction in required computational complexity.