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.
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.
Modern-day applications of fifth-generation (5G) and sixth-generation (6G) systems require fast, efficient, and robust transmission of multimedia information over wireless communication medium for both mobile and fixed users. The hybrid amalgamation of massive multiple input multiple output (mMIMO) and orthogonal frequency division multiplexing (OFDM) proves to be an impressive methodology for fulfilling the needs of 5G and 6G users. In this paper, the performance of the hybrid combination of massive MIMO and OFDM schemes augmented with fast Fourier transform (FFT), fractional Fourier transform (FrFT) or discrete wavelet transform (DWT) is evaluated to study their potential for reliable image communication. The analysis is carried over the Rayleigh fading channels and M-ary phase-shift keying (M-PSK) modulation schemes. The parameters used in our analysis to assess the outcome of proposed versions of OFDM-mMIMO include signal-to-noise ratio (SNR) vs. peak signal-to-noise ratio (PSNR) and SNR vs. structural similarity index measure (SSIM) at the receiver. Our results indicate that massive MIMO systems incorporating FrFT and DWT can lead to higher PSNR and SSIM values for a given SNR and number of users, when compared with in contrast to FFT-based massive MIMO-OFDM systems under the same conditions.
Non-orthogonal multiple access (NOMA) is expected to be used in beyond fifth-generation (B5G) and sixth-generation (6G) mobile networks to support ultra-massive connectivity. Since the two preceding mobile networks generations used orthogonal frequency division multiplexing (OFDM), NOMA is expected to be combined with OFDM. Unfortunately, the OFDM signal suffers from a high peak to average power ratio (PAPR) that limits its performance as it passes through the nonlinear high power amplifier (HPA). In literature, few works have studied the effect of nonlinear distortion on OFDM-NOMA. Furthermore, the HPA models used in previous works to describe the impact of nonlinear distortion on OFDM-NOMA in downlink (DL) were impractical or inaccurate. In contrast, this work uses the well-known soft limiter (SL) model with input back-off (IBO) as a practical controlling parameter. Also, instead of investigating the effect of nonlinear distortion on OFDM-NOMA in DL only, this work investigated this effect in both DL and uplink (UL). In particular, during this work, the performance of the OFDM-NOMA system in the presence of nonlinear distortion in both UL and DL is investigated in terms of users' achievable data rate, sum rate capacity, system fairness, and the bit error rate (BER) of each user. Results showed that, in DL, the NU is the most affected by the nonlinear distortion, while, in UL, the nonlinear distortion caused by the NU's HPA is more severe than the nonlinear distortion caused by other users.
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