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Modulation Techniques
Published in Z. Ghassemlooy, W. Popoola, S. Rajbhandari, Optical Wireless Communications, 2019
Z. Ghassemlooy, W. Popoola, S. Rajbhandari
A cyclic prefix (CP) is often included in the OFDM signal to combat intersymbol interference and inter-carrier interference that could be an issue in a dispersive optical wireless communication channel. The CP will, however, be omitted for simplicity as it has a negligible impact on both the required electrical signal-to-noise ratio (SNR) and the spectral efficiency. A continuous time-domain optical OFDM signal x(t) is obtained by feeding the discrete signal x(n) into a digital-to-analogue converter (DAC). The intensity of the optical source is then modulated by x(t). For a linear driver, the radiated optical power can thus be expressed as Pt=K(xdc+βx(t))
MIMO for WirelessMAN
Published in Yan Zhang, Hsiao-Hwa Chen, Mobile Wimax, 2007
Xiaopeng Fan, Steven Y. Lai, Yuan Zheng, Jiannong Cao
OFDM has become a popular technique for transmission of signals over wireless channels. It converts a frequency-selective channel into a parallel collection of frequency-flat subchannels, which makes the receiver simpler. The time domain waveforms of the subcarriers are orthogonal, yet the signal spectra corresponding to different subcarriers overlap in frequency. Hence the available bandwidth is used very efficiently. Using adaptive bit loading techniques based on the estimated dynamic properties of the channel, the OFDM transmitter can adapt its signaling to match channel conditions and approach the ideal water-pouring capacity of a frequency-selective channel. The increased symbol duration improves the robustness of OFDM to delay spread. Furthermore, introduction of the cyclic prefix (CP) can completely eliminate intersymbol interference (ISI) as long as the CP duration is longer than the channel delay spread. The CP is typically a repetition of the last samples of data of the block that is appended to the beginning of the data payload.
Non-Orthogonal Multiple Access Schemes for Next-Generation 5G Networks: A Survey
Published in Hrishikesh Venkatarman, Ramona Trestian, 5G Radio Access Networks: Centralized RAN, Cloud-RAN, and Virtualization of Small Cells, 2017
Namadev Bhuvanasundaram, Huan X. Nguyen, Ramona Trestian, Quoc-Tuan Vien
However, the main disadvantages of OFDM and OFDMA are as follows:The peak-to-average rower ratio (PAPR) is relatively high due to parallel transmission of modulated symbols.The limited spectral efficiency due to the cyclic prefix.Highly sensitive to frequency offsets and phase noise.Proper synchronization between transmitter and receiver is a must for better performance.
Low-complexity MU-MIMO-GFDM joint receivers
Published in International Journal of Electronics, 2023
Hsuan-Fu Wang, Fang-Biau Ueng, Yun-Hsuan Sung
It is currently embarking on the beginning of the fifth-generation (5 G) of mobile communications (Bondarenko et al., 2019; Öztürk et al., 2016), and the scope of influence is not only limited to wireless communication devices but also combined with the use of the Internet of Things (IoT; Khurpade et al., 2018). The tactile IoTs (Fettweis, 2014) are an essential part of the IoT, and devices will integrate various interfaces for tactile, visual, and auditory transmission. Application scenarios cover significant industries, such as medical, automotive, logistics, agriculture, etc. In the initial stage, the 5 G mobile communication uses orthogonal frequency-division multiplexing (OFDM) technology (Chang, 1966). However, the required cyclic prefix (CP) scheme limits the spectrum utilisation, i.e., low spectrum efficiency (SE; Lim et al., 2005). Another drawback of OFDM is that the addition of sub-carriers of the same phase will result in a relatively high peak-to-average power ratio (PAPR).
Fractional Wavelet Transform based PAPR Reduction Schemes in Multicarrier Modulation System
Published in IETE Journal of Research, 2022
R. Ayeswarya, N. Amutha Prabha
Orthogonal frequency division multiplexing (OFDM) is one of the most prominent multicarrier modulation (MCM) schemes highlighted in the current wireless transmission scenario. Especially, OFDM has been chosen as a promising candidate for various applications such as DAB, LTE, LTE-A, HDTV and power line communications. High spectral efficiency, robust against inter symbol interferences (ISIs), efficient implementation and multipath transmission are some advantages of OFDM [1]. A simultaneous transmission of data on parallel and closely spaced orthogonal subcarriers is performed in this multicarrier OFDM system. It utilizes inverse fast Fourier transform (IFFT) and fast Fourier transform (FFT) to multiplex and decode the signal at transmitter and receiver, respectively. The cyclic prefix (CP) is appended to signal before transmitting to the channel in order to reduce the ISI and inter carrier interference. The major drawback with these CP is reduction in spectral containment. This multicarrier system also generates high PAPR that clips the OFDM signal passed through a non-linear high power amplifier (HPA) at the end of the transmitter [2,3]. Thus, the OFDM system results in high bit error rate (BER) and reduces system performance.
Overview of the challenges and solutions for 5G channel coding schemes
Published in Journal of Information and Telecommunication, 2021
Madhavsingh Indoonundon, Tulsi Pawan Fowdur
Precoding is performed on the mapped modulated symbols to maximize the throughput before performing Cyclic-Prefix Orthogonal Frequency Division Multiplexing (CP-OFDM) on the modulated symbols (Toni et al., 2019). Basically, with OFDM, the bandwidth is split into narrower subcarriers. The subcarriers are orthogonal to each other, hence preventing them from interfering with each other without the use of guard bands, thereby increasing the overall spectral efficiency. In CP-OFDM, the same principles are applied but with the addition of a cyclic prefix at the start of each OFDM symbol. The cyclic prefix in a symbol is a copy of the end part of the same symbol. It acts as a guard band that reduces inter-symbol interference which is known to affect the performance of traditional OFDM (Toni et al., 2019).