A Study of Orthogonal Frequency Division Multiplexing

Advanced multi carrier modulation methods have been

In the 1870s, Frequency Division Multiplexing (FDM) was developed for the first time. Frequency Division Multiplexing (FDM) was briefly investigated in the 1870s as part of research on telegraph systems capable of transmitting data across several channels. Orthogonal frequency-division multiplexing (OFDM) is a method of transmitting and receiving digital data that was first introduced by Chang in 1966. Saltzberg published his patent for OFDM in 1967. In 1969, FFT was introduced as a method for generating orthogonal signals. It wasn't until 1980 that the concept of a cyclic prefix was established. As early as 1985, OFDM was developed by Bell Labs' Cimini specifically for use in wireless communication. Coded OFDM was first implemented for radio transmission in response to a 1987 presentation by Lassalle & Alard on the need of forward error correction (FEC) with OFDM. Thus, OFDM gained notoriety for military uses and served as the foundation for numerous telecommunications standards, including the discrete multitude low-bit-rate (DSL) standards (DMT). By using DFT, Weinstein was able to get a patent for OFDM signal production and reception in 1971, doing away with the need for large banks of analogue subcarrier oscillators. OFDM was first developed in the 1980s for commercial usage with DAB to create DVB-T. Although research on OFDM in the RF domain spanned four decades, it wasn't until the late 1990s that researchers began focusing on how the technique might be used to optical communication. In 2001, a patent was issued for optical frequency division multiplexing (OFDM) used for wireless optical communications. A proposal for using OFDM in optical fiber communication was made in 2005. As of late 2009, research on using OFDM for long-distance transmission was underway. As a result, for more efficient use of the spectrum and increased receiver sensitivity in

However, tolerance from chromatic and polarization mode dispersion, compatibility with DSP algorithms and high spectral efficiency makes OFDM attractive choice for optical communication systems.[6]

Two numbers, I and j; the fundamental frequency, f; the symbol period, Ts, When using orthogonal frequency division multiplexing (OFDM), the subcarrier spacing, Δf, must be adjusted to 1/NTs in order to preserve orthogonality between overlapping sub-carriers. Subcarrier count N and symbol rate 1/Ts are indicated here.[7] Time-domain and frequency-domain analyses both provide orthogonal results. If the difference between neighboring subcarriers is exactly one cycle in the time domain, then the subcarriers are said to be orthogonal to one another. To be orthogonal in a frequency domain, the subcarrier spacing must be adjusted to 1/NTs, and the subcarrier peak must be at the subcarrier's own centre frequency, while being zero in the centers of other subcarriers. What this means is that, as illustrated in Figure 1.9, the peak of one carrier occurs at the null of the other. There is no interference even when the sub-carriers are tightly spaced or even overlapped. If these conditions are not met, the orthogonality of the two subcarriers is broken.

For orthogonality, the inner product is zero for two orthogonal functions with different symbols i.e. zero inter-symbol interference (ISI) and inner product is also zero for orthogonal functions with different subcarriers i.e. zero inter-carrier interference (ICI). Optical-OFDM In spite of its relative newness to the field of optical communications, orthogonal frequency division data rates. It's a powerful tool against color shifts in glass and other optical materials. Utilizing digital signal processing (DSP) technology, OFDM becomes a powerful method for handling information. This causes transmission rates to rise annually; many Tb/s have been seen experimentally. Linearity between the IFFT input at the transmitter and the FFT output at the receiver is crucial to the proper operation of the OFDM system. Multi-mode fiber systems, which can transmit several optical modes, and single-mode fiber systems, which can only transmit a single optical mode, are the two primary types of optical systems. Intensity modulation is often used to build multi-mode fiber systems, where the many modes of the transmitted signal are linear with respect to intensity. RF-OFDM signals are bipolar (having both positive and negative values), however intensity modulation does not accommodate bipolar signals. [8] All electrical OFDM systems use a bipolar electrical signal to transfer data. Alternatively, information in optical OFDM systems is often modulated in line with the light signal's intensity, which can only be positive (unipolar). Therefore, the OFDM signal must be transformed to a real and positive signal before it can be sent via an IM/DD connection using an optical carrier. Biased orthogonal frequency division multiplexing (B-OFDM) and clipped orthogonal frequency division multiplexing (OFDM) are two solutions that have been developed for this purpose (C-OFDM). In B- OFDM, a non-negative signal may be formed by combining an OFDM signal with a bias component (often a dc bias) of a big enough magnitude. However, the huge PAPR of an OFDM signal causes only a tiny fraction of the negative component to be clipped, which in turn causes distortion. Bias components are included into the OFDM signal in C-OFDM to zero-clip the negative part of the OFDM signal. Linear field modulation, in which the signal is represented by optical field and this form of the signal may also be called as an unclipped-OFDM, on the other hand, can be simply implemented in single mode fiber systems (U-OFDM).[9] Optical-OFDM Signal Processing Five functional blocks make up an optical OFDM system: an OFDM transmitter that generates the OFDM signal from bit streams, an RF to optical up converter that converts an RF (electrical) signal to optical by modulating a light with an RF signal, optical media that carries the optical signal through the channel, an optical to RF down converter that converts an optical signal to RF using a photodiode, and an OFDM receiver that detects the original bit streams from the OFDM signal. Figure 1.10 shows a block diagram of the most fundamental components of an optical OFDM (O-OFDM) system.

The conceptual diagram of the O-OFDM system, including all the functional blocks, is shown in Figure 1.11. Using orthogonal frequency division multiplexing (OFDM) modulation, high bit rate data streams can be transmitted by dividing them into multiple low rate streams, each of which is modulated using a different modulation format, such as binary phase shift keying (BPSK), quadrature phase shift keying (QPSK), or quadrature amplitude modulation (QAM) (4QAM, 16QAM, 64QAM, etc.). OFDM Basics The unique class of Multiplexing and Multicarrier Modulation (MCM) method that OFDM employs is well-known. OFDM's othogonality between subcarriers is what sets it apart from other types of multicarrier transmission systems. Multi-user applications that need bidirectional communication in OFDM have seen less development than their one-way counterparts, such as DAB and DVB-T, which only transmit data from a base station to end users. Some examples of where this technology has been put to use include in wireless modems, WLANs, WLLs, mobile phones, and HSDPA-enabled mobile internet. The purpose of this thesis is to examine the use of orthogonal frequency division multiplexing (OFDM) in such contexts, and to assess the benefits and drawbacks that emerge as a consequence. As a broadcasting and multiuser system, OFDM may be greatly enhanced by the methods presented in this thesis. A multiuser OFDM system is shown, along with adaptive modulation and user allocation algorithms. All three of these aspects of signal transmission—spectral efficiency, frequency offset, and signal quality—are enhanced by these methods. The public and commercial sectors are both expected to increase their need for a wireless multimedia broadband system, making multimedia essentially an infrastructure technology. As a result, this thesis also explores strategies for facilitating multimodal exchanges while on the go. Frequency division multiplexing is a method of transmitting a signal in which the signal is divided into many parts, or "subcarriers," each of which operates at a different frequency (FDM). In order to get an accurate FDM signal, it is necessary to modulate these subcarriers using a certain modulation structure. In FDM transmission, the receiver is able to separate and reassemble each subcarrier. Spectrum overlap may increase spectral efficiency by making these subcarrier signals orthogonal to one another. Consequently, we refer to this method as "orthogonal frequency division multiplexing" (OFDM).[10]

Fig. 1.15 depicts a simplified model of an OFDM modulation system. A total of N parallel channels, each operating at a different frequency, transport data at bitrate R, The total bit rate shared by all channels is split proportionally based on the ratio of the bit rate to the number of parallel channels, denoted as (R/N). Here, information signals are represented by data maps, and those maps are multiplied by their associated frequencies in each channel. These simultaneous information symbols combine to produce an OFDM symbol, hence the duration of an OFDM symbol is denoted in terms of symbol time., Ts= N/R. Block System Representation of OFDM

Optical amplifiers' "Amplified Spontaneous Emission" (ASE) noise, In 2002, we set a record for capacity with 40 Gb/s/channel across 64 channels and 4000 metres with a BER < 10-12. The next generation of light wave systems, using distributed photon counting and amplitude modulation (DPSKAs), is on the cusp of development, so the current operating wavelength window, the conventional wavelength window known as C-band, which is from 1530nm-1565nm, is not providing better results at the user end. Thus, the primary focus has been on the L-band (1570nm-1610nm) and S-band (1.6-2.8 GHz) extensions of the wavelength range (1485nm-1520nm). Second, efforts have been undertaken to boost the data rate of each channel. However, the advent of OFDM and other forms of multicarrier modulation has been a

information across both wireless and wired networks. Research into minimising interference between closely spaced channels led to the first notion of OFDM technology being explored in the 1960s and 1970s. There was a rise in the number of research that stressed the need of achieving error-free data transmission despite interference and selective propagation. At first, OFDM's high processing needs made it impractical for widespread use. Digital terrestrial broadcasting was an early adopter of OFDM due to the reliability it offered in data transfer under varying signal path circumstances. One such example is DAVB-T digital radio, which was initially launched in Europe and other regions on June 1, 1995, with the Norwegian Broadcasting Corporation NRK launching the first terrestrial service. The digital television industry also made use of OFDM. As a consequence, OFDM was taken into consideration for the 4G mobile communications systems that began to be implemented around 2009 due to the increase in processing power afforded by the increasing integration levels. OFDM is now used in many wireless communications systems, including Wi-Fi. OFDM is a kind of multicarrier modulation. Overlapping frequency division multiplexing is characterised by a large number of modulated carriers with small intervals between them. Sidebands radiate outward from a carrier when it is modulated in any way, whether by speech, data, or anything else. In order to demodulate information from a transmission, a receiver has to pick up the complete signal. Thus, a guard band and enough spacing are required when transmitting signals in close proximity to one another so that the receiver may distinguish between them using a filter. Unfortunately, OFDM is not like this at all. Due to their orthogonality, the overlapping sidebands from each carrier may be received without creating interference. The multiplexing and modulation techniques in which it figures prominently as one of the most promising developments have broad practical implications. Understanding how OFDM works requires a look at the receiving end. It's like a sea of demodulators, bringing every carrier down to direct current. Data from a carrier is recovered by integrating the resultant signal across its symbol period. This is because all of the carriers may be demodulated by the same demodulator. When the carrier spacing is a whole number of cycles less than the symbol period, there is no interference since the total of the carrier contributions is zero.

In various experimental and simulated situations, OFDM over optical power has attained a feasible data rate in the region of 100 Gbps across 100's of kilometers of fiber length; nonetheless, practical development still has a long way to go. The writers of this study have taken a number of difficulties and real-world considerations into account while creating the suggested system. Because an MMF employs a light-

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Research Scholar, Shri Krishna University, Chhatarpur M.P.