Difference between FDM and OFDM f guard band. Frequency division multiplexing. Figure 9: Frequency Division Multiplexing. Orthogonal sub-carriers in OFDM. PDF | Orthogonal Frequency Division Multiplexing (OFDM) has become the modulation technique used in several present and future communication standards. PDF | 35+ minutes read | Orthogonal Frequency Division Multiplexing (OFDM) is a multi-carrier modulation scheme that provides efficient bandwidth utilization.
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PDF | Orthogonal Frequency Division Multiplexing (OFDM) is a multi-carrier modulation technique which is very much popular in new wireless networks of IEEE. communication systems OFDM is becoming the chosen modulation technique Key words: Orthogonal frequency division multiplexing (OFDM), fast Fourier . MATLAB kaz-news.info~mai/tutorial/OFDM/kaz-news.info, Abstract: Orthogonal Frequency Division Multiplexing (OFDM) is a multi-carrier modulation technique which is very much popular in new wireless networks of.
Frequency subcarrier interleaving increases resistance to frequency-selective channel conditions such as fading. For example, when a part of the channel bandwidth fades, frequency interleaving ensures that the bit errors that would result from those subcarriers in the faded part of the bandwidth are spread out in the bit-stream rather than being concentrated.
Similarly, time interleaving ensures that bits that are originally close together in the bit-stream are transmitted far apart in time, thus mitigating against severe fading as would happen when travelling at high speed. However, time interleaving is of little benefit in slowly fading channels, such as for stationary reception, and frequency interleaving offers little to no benefit for narrowband channels that suffer from flat-fading where the whole channel bandwidth fades at the same time.
The reason why interleaving is used on OFDM is to attempt to spread the errors out in the bit-stream that is presented to the error correction decoder, because when such decoders are presented with a high concentration of errors the decoder is unable to correct all the bit errors, and a burst of uncorrected errors occurs.
A similar design of audio data encoding makes compact disc CD playback robust.
A classical type of error correction coding used with OFDM-based systems is convolutional coding , often concatenated with Reed-Solomon coding. Usually, additional interleaving on top of the time and frequency interleaving mentioned above in between the two layers of coding is implemented. The choice for Reed-Solomon coding as the outer error correction code is based on the observation that the Viterbi decoder used for inner convolutional decoding produces short error bursts when there is a high concentration of errors, and Reed-Solomon codes are inherently well suited to correcting bursts of errors.
Capacity limit for faster-than-Nyquist non-orthogonal frequency-division multiplexing signaling
Newer systems, however, usually now adopt near-optimal types of error correction codes that use the turbo decoding principle, where the decoder iterates towards the desired solution. Some systems that have implemented these codes have concatenated them with either Reed-Solomon for example on the MediaFLO system or BCH codes on the DVB-S2 system to improve upon an error floor inherent to these codes at high signal-to-noise ratios.
Based on this feedback information, adaptive modulation , channel coding and power allocation may be applied across all subcarriers, or individually to each subcarrier. In the latter case, if a particular range of frequencies suffers from interference or attenuation, the carriers within that range can be disabled or made to run slower by applying more robust modulation or error coding to those subcarriers.
The term discrete multitone modulation DMT denotes OFDM-based communication systems that adapt the transmission to the channel conditions individually for each subcarrier, by means of so-called bit-loading. The upstream and downstream speeds can be varied by allocating either more or fewer carriers for each purpose.
Some forms of rate-adaptive DSL use this feature in real time, so that the bitrate is adapted to the co-channel interference and bandwidth is allocated to whichever subscriber needs it most. OFDM extended with multiple access[ edit ] Main article: Orthogonal frequency-division multiple access OFDM in its primary form is considered as a digital modulation technique, and not a multi-user channel access method , since it is used for transferring one bit stream over one communication channel using one sequence of OFDM symbols.
However, OFDM can be combined with multiple access using time, frequency or coding separation of the users. OFDMA supports differentiated quality of service by assigning different number of subcarriers to different users in a similar fashion as in CDMA , and thus complex packet scheduling or Media Access Control schemes can be avoided. Abstract Optical wireless communication is an energy-efficient and cost-effective solution for high-speed and highly-secure wireless connections.
The peak power constraint, bandwidth limit of the light source, turbulence fading underwater channel, and the channel estimation error are taken into account. To maximize the achievable data propagation distance, we propose to optimize the modulation index that controls the signal magnitude, and a bitloading algorithm is applied.
This optimization process trades off the clipping distortion caused by the peak power constraint and the signal to noise ratio SNR. Keywords: optical wireless communications, underwater communications, OFDM, peak power constraint, propagation distance, BER, band-limited channel, turbulence fading 1.
Introduction In recent years, underwater wireless communications UWC has attracted more and more research attention due to the increasing civilian and military demands [ 1 , 2 , 3 ]. For example, underwater vehicles need high-speed and long-distance wireless connections between each other.
The underwater sensor network needs a reliable link to transmit the collected data back to the ground. Radio-frequency RF waves, acoustic waves, and optical waves are three possible transmission media for UWC.
Due to the high security and transmission speed, underwater optical wireless communication UOWC becomes a more and more promising technique, supporting highly-reliable wireless underwater connections [ 4 ].
Compared with optical wireless communications, underwater acoustic communication has a longer application history due to its low attenuation level and long propagation distance [ 1 ]. However, the slow data rate and severe delay are the drawbacks for underwater acoustic systems.
Although RF communication can provide a much higher data rate than acoustic systems, the extremely limited propagation range due to the high attenuation of radio waves in the water limits its application.
UOWC is an emerging technique for underwater systems with a high-speed transmission for moderate distance communication. Although UOWC can provide a longer propagation distance than underwater RF systems, an extremely long-range underwater communication should currently rely on acoustic systems. Therefore, in UOWC, most of the research focuses on improving the transmission data rate and propagation distance [ 5 , 6 , 7 ].
A light-emitting diode LED working as a high power-efficient light source has attracted more attention recently and shows a big potential in underwater optical wireless communication systems [ 8 , 9 , 10 , 11 ].
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Single-carrier frequency division multiple access SC-FDMA can achieve a lower peak to average power ratio than OFDM, which has the potential to be beneficial for underwater optical wireless communications as well [ 12 , 13 , 14 , 15 ]. Orthogonal frequency division multiplexing OFDM is the key technique for 4G wireless communications, which is also widely used in many applications.Also, it is indicated that the demodulation most part, Orthogonal Frequency Division Multiplexing method has many advantages over conventional ones, such as the powerful anti-noise ability, the small transfer delay, and Manuscript received November 10, The FTN binary sinc-pulse scheme compresses the time period to obtain the transmission rate faster than Nyquist rate.
At the receiver, an FFT block is used to process the received signal and bring it into the frequency domain which is used to recover the original data bits. Fundamentals, vol.
Simple A similar design of audio data encoding makes compact disc CD playback robust. The like a phone trunk between two telephone central offices.
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