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Communications, Signal Processing, and Data Handling
Published in Douglas O. J. deSá, Instrumentation Fundamentals for Process Control, 2019
Amplitude modulation is another method of transmitting data, and to see how this works, let us consider a signal of fixed relatively high frequency and amplitude on which we superimpose another varying signal. The signal of fixed amplitude and frequency is called the carrier frequency. The effect of the imposition of the varying signal on the carrier frequency will be to alter the amplitude of each wave in unison with the varying signal, giving the modulated carrier as in Figure 21.6. On arrival at the receiver, a unit called a demodulator extracts the modulation signal (envelope) from the modified carrier and so obtains the transmitted data. Theoretically, the carrier frequency can be eliminated and just the sideband(s) transmitted, for it does not itself contain any data; however, in practice, a pilot carrier is usually transmitted to facilitate signal recovery at the receiver by synchronizing, i.e., frequency-locking the demodulator. By multiplexing the modulation signal content and/or modulating onto subcarriers multiple signals can be sent. Figure 21.6 may help visualize the idea.
Outdoor OWC Links with Diversity Techniques
Published in Z. Ghassemlooy, W. Popoola, S. Rajbhandari, Optical Wireless Communications, 2019
Z. Ghassemlooy, W. Popoola, S. Rajbhandari
In the previous sections, the use of spatial diversity to lessen the effect of atmospheric turbulence has been discussed. And as highlighted in the spatial diversity results, the best gains are achieved when the detectors are physically separated by a distance greater than the turbulence coherence length. The channel coherence length depends on the turbulence strength and is typically in the order of centimeters. Similarly, the use of aperture averaging requires the receiver aperture to be larger than the turbulence coherence length. This not only makes the system bulky, it is also cumbersome and not always feasible. The use of the turbo product code (TPC) as the channel coding scheme with interleaving has been shown to offer good resistance to burst errors and no error floor due to channel fading in FSO links [67]. This section will examine the subcarrier time delay diversity (STDD) as an alternative or complementary means of mitigating channel fading in SIM-FSO links. The conventional use of multiple subcarriers is to increase throughput/capacity via subcarrier multiplexing. But in this scheme, different subcarriers at different frequencies are used to transmit the delayed copies of the original data. The proposed subcarrier STDD scheme has the advantage of simplicity and low cost for achieving a reasonable diversity gain compared to schemes such as adaptive optics or forward error correction. Moreover, the reduction in throughput associated with temporal diversity can be compensated for through subcarrier multiplexing.
New Technology and the Future
Published in Ervan Garrison, A History of Engineering and Technology Artful Methods, 2018
The code is impressed on the carrier by modulation of a transmitter. Demodulation by the receiver deciphers the information from the carrier. The most common modulation system in telemetry is frequency modulation (FM) where the frequency of the carrier is caused to vary with the modulated signal. FM is much less susceptible to interference because variations in amplitude of the received signal caused by interference can be removed can be removed at the receiver before demodulations. In biotelemetry, the physiological signal is sometimes used to modulate a low-frequency carrier, or subcarrier, often in the audiofrequency range. When transmitting several physiological signals, each is placed on a subcarrier of a different frequency and then all are combined to simultaneously modulate the RF carrier. The multichannel technique is termed frequency multiplexing and is economical in that a multiplexed signal can use only one transmitter and receiver for the various subcarriers.
Deep Learning Techniques for OFDM Systems
Published in IETE Journal of Research, 2021
M. Meenalakshmi, Saurabh Chaturvedi, Vivek K. Dwivedi
In OFDM, the multiple carriers (known as subcarriers) carry the information stream and these subcarriers are closely spaced and orthogonal. Many broadband wireless communication standards, including IEEE 802.11, IEEE 802.16, terrestrial digital video broadcasting, ultra-wideband communication systems, and digital audio broadcasting use OFDM standard [3] by considering its advantages such as easy equalization, high spectral efficiency, and robustness to frequency selective fading channel. However, OFDM systems exhibit drawbacks relating to peak-to-average power ratio (PAPR) [4], frequency synchronization, timing synchronization, and intercarrier interference.