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Published in Philip A. Laplante, Comprehensive Dictionary of Electrical Engineering, 2018
coherence length distance over which the amplitude and phase of a wave can be predicted. coherence time the time over which the effect of communication channel can be assumed constant. Signals of duration less than this can be transmitted without significant distortion. coherent integration where magnitude and phase of received signals are preserved in summation. coherent acousto-optical processor acoustooptical (AO) signal processor where the light is amplitude-modulated by the acoustic wave in the AO device as opposed to intensity or power modulated. coherent detection detection technique in which the signal beam is mixed with a locally generated laser beam at the receiver. This results in improved receiver sensitivity and in improved receiver discrimination between closely spaced carriers. coherent illumination a type of illumination resulting from a point source of light that illuminates the mask with light from only one direction. This is more correctly called "spatially coherent illumination."
Wireless Communication Systems
Published in Keshab K. Parhi, Takao Nishitani, Digital Signal Processing for Multimedia Systems, 2018
To characterize coherence times we consider the degree of Doppler spread which depends on the velocity of the receiver with respect to the transmitter. In mobile communication systems this value is typically in the range of 0 – 120 Km/H. At zero velocity the Doppler spread equals zero, the space-time correlation is a constant, hence the coherence time equals infinity. At a velocity of 120 Km/H and at a carrier frequency of 1 GHz the maximum Doppler spread is ωm=ωcνc≈698 or fm ≈ 110Hz. The coherence time is τc = 3.4ms. All of these values depend on our assumptions of uniformly distributed received power over angle and exponentially distributed delay spread. In practice these assumptions do not hold in all cases and we can expect smaller delay spreads (larger coherence bandwidths) and smaller Doppler spreads (larger coherence times).
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Published in Lal Chand Godara, Handbook of Antennas in Wireless Communications, 2018
where J0(·) is the zero-order Bessel function of the first kind [11], V Δt is distance traversed, and k = 2π/λ is the free-space phase constant (transforming distance to radians of phase). Coherence time can be measured in terms of either time or distance traversed (assuming some fixed velocity). Amoroso [17] described such a measurement using a CW signal and a dense-scatterer channel model. He measured the statistical correlation between the combination of received magnitude and phase sampled at a particular antenna location x0, and the corresponding combination sampled at some displaced location x0 + ζ, with displacement measured in units of wavelength λ. For a displacement ζ of 0.4 λ between two antenna locations, the combined magnitudes and phases of the received CW are statistically uncorrelated. In other words, the signal observation at x0 provides no information about the signal at x0 + ζ. For a given velocity, this displacement is readily transformed into units of time (coherence time).
Exploring pilot assignment methods for pilot contamination mitigation in massive MIMO systems
Published in Cogent Engineering, 2020
Angelina Misso, Mussa Kissaka, Baraka Maiseli
Pilot contamination is a condition that occurs when the precoding matrix adopted at a base station is correlated with the channel of users in other cells because of non-orthogonal pilots allocated to users (Fatema et al., 2018). For the duration of the channel estimation process, the coherence time is limited. (Coherence time defines a time interval within which a system can acquire CSI in an attempt to estimate the channel.) CSI is computed based on the pilots used during transmission and then matched with the received pilot signals.