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Coding Techniques to Improve Bit Error Rate in Orthogonal Frequency Division Multiplexing System
Published in Rajeshree Raut, Ranjit Sawant, Shriraghavan Madbushi, Cognitive Radio, 2020
Rajeshree Raut, Ranjit Sawant, Shriraghavan Madbushi
The telecommunications industry is in the midst of a veritable explosion in wireless technologies [1]. Once exclusively military, satellite and cellular technologies are now commercially driven by ever more demanding consumers, who are ready for seamless communication from their home to their car, to their office, or even for outdoor activities. With this increased demand comes a growing need to transmit information wirelessly, quickly, and accurately. To address this need, communication engineers have combined technologies suitable for high-rate transmission with forward error correction techniques. The latter are particularly important as wireless communications channels are far more hostile as opposed to wire alternatives, and the need for mobility proves especially challenging for reliable communications. Orthogonal frequency division multiplexing (OFDM) is a multicarrier modulation technique in which a single high rate data stream is divided into multiple low rate data streams and is modulated using subcarriers that are orthogonal to each other.
Wireless Networking Standards (WLAN, WPAN, WMAN, WWAN)
Published in K.R. Rao, Zoran S. Bojkovic, Dragorad A. Milovanovic, Wireless Multimedia Communications, 2018
K.R. Rao, Zoran S. Bojkovic, Dragorad A. Milovanovic
Data for transmission is supplied to the PHY layer in the form of an input PDU train or physical layer convergence procedure (PLCP) protocol data unit frame. This is then passed to a scrambler that prevents long runs of 1s and 0s in the input data. Although both 802.11a and HIPERLAN2 scramble the data with a length 127 pseudorandom sequence, the initialization of the scrambler is different. The scrambled data then passes to a convolutional encoder. The encoder consists of a 1/2 rate mother code and subsequent puncturing. The puncturing schemes facilitate the use of code rates 1/2, 3/4, 9/16 (HIPERLAN2 only), and 2/3 (802.11a only). In the case of 16-QAM, HIPERLAN2 uses rate 9/16 instead of rate 1/2 in order to ensure an integer number of OFDM symbols per PDU train. The rate 2/3 is used only for the case of 64–QAM in 802.11a. Note that there is no equivalent mode for HIPERLAN2. HIPERLAN2 also uses additional puncturing in order to keep an integer number of OFDM symbols with 54-byte PDUs. The coded data is interleaved in order to prevent error bursts from being input to the convolutional decoding process in the receiver. The interleaved data is subsequently mapped to data symbols according to a BPSK, QPSK, 16-QAM, or 64-QAM constellation. OFDM modulation is implemented by means of an inverse fast Fourier transform (FFT); 48 data symbols and four pilots are transmitted in parallel in the form of one OFDM symbol.
Wireless LAN Technology for the Factory Floor: Challenges and Approaches
Published in Richard Zurawski, Industrial Communication Technology Handbook, 2017
In the last decade, there was a strong uptake of OFDM-based transmission technologies [6,66,108], which have been adopted in the LTE [55] technology for 4G mobile broadband, in WiMax (or IEEE 802.16), and also in the newer physical layers adopted for IEEE 802.11 (formerly known as the IEEE 802.11a and g extensions). OFDM stands for orthogonal frequency division multiplexing. In OFDM, the available spectrum is subdivided into a number N of small and closely packed subchannels or subcarriers (the 5 GHz OFDM physical layer of IEEE 802.11 uses 52 subcarriers). The bandwidth of each subcarrier is small enough to obtain a frequency-nonselective channel. Data are transmitted over all subcarriers in parallel; the data rate on each subcarrier (using standard modulation schemes like BPSK, QPSK, and so forth) is small enough so that (due to the additional presence of a guard interval) the delay spread is much smaller than the average symbol duration and intersymbol interference can be avoided. The OFDM scheme used in IEEE 802.15.4 additionally applies error-correction coding such that coding is applied across subcarriers instead of applying it to each subcarrier separately.
ABER of M-QAM and K-PSK modulations over double-GG fading channel
Published in International Journal of Electronics, 2023
Piyush Jain, N. Jayanthi, M. Lakshmanan
Figure 1 shows the Orthogonal Frequency Division Multiple Access (OFDM) RoFSO system. Here, OFDM signal is transmitted by Laser diode and propagate through FSO channel where it gets attenuated due to atmospheric turbulences and received by a photo-detector at the receiver. It is noticeable that OFDM is a special multicarrier modulation scheme in which parallel subcarriers convey the information signal. Specifically, in OFDM system, streams of high data rate are split into the streams of lower data rate and then transmitted over several narrow band subcarriers. Further, different modulation (in our case, Phase shift keying (PSK) or Quadrature amplitude modulation (QAM) modulation) techniques are used to modulate these subcarriers on a high-frequency carrier (H. E. Nistazakis et al., 2015). At the transmitter end, Inverse Fast Fourier Transform (IFFT) and at the receiving end, Fast Fourier Transform (FFT) is done to retrieve data.
Interference Cancellation in Wireless Communications: Past, Present, and Future
Published in IETE Journal of Education, 2022
S. M. Zafaruddin, Pranay Bhardwaj
High bit-rate transmissions over multi-path channels may cause inter-symbol interference (ISI) if the symbol duration becomes comparable or less than the delay spread of the channel. A naive approach may be to lower the data rate transmissions sufficiently to account for the ISI. However, channel equalization is a potential technique to suppress the ISI and render the demand for high data rates in modern communication systems [11]. Channel equalization compensates for the distortion in the transmitted signal due to the frequency-selective channel. There are many equalization techniques for ISI mitigation such as linear equalizer, adaptive equalizer, decision feedback equalizer, and maximum-likelihood sequence estimator [12]. On the other hand, multicarrier modulation divides a wideband channel into multiple low-rate orthogonal sub-channels with negligible ISI [13]. A popular implementation of multicarrier modulation for wireless systems is orthogonal frequency division multiplexing (OFDM). However, OFDM is sensitive to the carrier frequency offset (CFO), which may be caused due to differences in the carrier frequencies of transmitter and receiver.
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.