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Published byRoxana Foxwell Modified over 2 years ago

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OFDM (Orthogonal Frequency Division Multiplexing )

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B ASICS OF OFDM Orthogonal Frequency Division Multiplexing (OFDM) is a method that allows to transmit high data rates over extremely hostile channels at a comparable low complexity. Orthogonal FDMs (OFDM) spread spectrum technique distributes the data over a large number of carriers that are spaced apart at precise frequencies. This spacing provides the orthogonality in this technique which prevents the demodulators from seeing frequencies other than their own.

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H OW OFDM WORKS First of all the FDM part - Frequency division multiplexing is a technology that transmits several signals at the same time over a single transmission path, in a medium such as a cable or wireless system. Each signal is transmitted inside its own unique frequency range (the carrier frequency), which is then modulated by the data that is needing to be transmitted.

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Orthogonal FDM's spread spectrum technique spreads the data over a lot of carriers that are spaced apart at precise frequencies. In OFDM modulation, adjacent channels are mathematically orthogonal, having a 90-degree phase shift.

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In traditional FDM the sub-channels arent orthogonal therefore need to be separated by guard bands which obviously wastes much needed spectrum. Hence a guard interval is used, which is larger than the expected delay spread, which is done by artificially extending the symbol time and then removing this extension at the receiver, in this a minimum bandwidth is lost

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It distributes the data over a large number of carriers that are spaced apart at precise frequencies. This spacing provides the "orthogonality" in this technique which prevents the demodulators from seeing frequencies other than their own.

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FDM OFDM Frequency Division Multiplexing OFDM frequency dividing EARN IN SPECTRAL EFFICIENCY

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OFDM BANDWIDTH DIVISION

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Orthogonal Frequency-Divison Multiplexing (OFDM) - transmitter

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OFDM DEFINITION OFDM = Orthogonal FDM Carrier centers are put on orthogonal frequencies ORTHOGONALITY - The peak of each signal coincides with trough of other signals Subcarriers are spaced by 1/Ts

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M ODULATION

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T YPES OF OFDM C-OFDM MIMO-OFDM V-OFDM W-OFDM Flash-OFDM

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D IFFERENCES FROM OTHER MULTIPLEXING TECHNIQUES OFDM versus TDM -number of carriers -synchronization -sensitivity -capacity/efficiency advantages -complexity and cost issues OFDM versus WDM -more flexible -higher bandwidth efficiency

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ADVANTAGES Makes efficient use of the spectrum by allowing overlap. By dividing the channel into narrowband flat fading subchannels, OFDM is more resistant to frequency selective fading than single carrier systems are. Eliminates ISI and IFI through use of a cyclic prefix. Using adequate channel coding and interleaving one can recover symbols lost due to the frequency selectivity of the channel.

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Provides good protection against cochannel interference and impulsive parasitic noise. It is possible to use maximum likelihood decoding with reasonable complexity. OFDM is computationally efficient by using FFT techniques to implement the modulation and demodulation functions. Is less sensitive to sample timing offsets than single carrier systems are.

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DISADVANTAGES The OFDM signal has a noise like amplitude with a very large dynamic range, therefore it requires RF power amplifiers with a high peak to average power ratio. It is more sensitive to carrier frequency offset and drift than single carrier systems are due to leakage of the DFT. High sensitivity inter-channel interference, ICI

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APPLICATIONS Digital Audio Broadcasting (DAB) HDTV-Digital Video Broadcasting (DVB) Wireless LAN Networks Broadband Wireless Access System Power-line Technology Asymmetric Digital Subscriber Line (ADSL)

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HISTORY ABOUT OFDM OFDM was invented more than 40 years ago. The concept of using parallel data transmission by means of frequency division multiplexing (FDM) was published in mid 60s. In the 1980s, OFDM was studied for high-speed modems, digital mobile communications, and high-density recording. In the 1990s, OFDM was exploited for wideband data communications over mobile radio FM channels, high-bit-rate digital subscriber lines (HDSL; 1.6 Mbps), asymmetric digital subscriber lines(ADSL, 1,536 Mb/s),

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very-high-speed digital subscriber lines (VDSL;100 Mbps), digital audio broadcasting (DAB), and high- definitiontelevision (HDTV) terrestrial broadcasting. In 2007,100 Gb/s CO-OFDM transmission over 1000 km standard single-mode fiber (SSMF) with high spectral efficiency of 2 bit/s/Hz has been demonstrated by various groups. Future, as the industry is embracing the imminent commercialrollout of 100 Gb/s Ethernet (100 GbE), the feasibility of 1 Tb/s Ethernet is the next logical step. Also multimode fiber in conjunction with multiple-input multiple- output OFDM (MIMO-OFDM) is proposed as a technology to achieve 100 Tb/s per fiber that takes advantage of mode multiplexing in the optical fiber.

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