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| converts the input message to an electric waveform(baseband signal |
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| Modifies the baseband signal for efficient transmission: may consist of one of, A/D converter, encoder, and a modulator. |
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| Medium of choice that can convey the electric signals at the transmitter output over a distance ( can be a pair of twister copper wires, coaxial) |
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| reprocesses the signal received from the channel by reversing the signal modifications made at the transmitter and removing the distortions made by the channel, may consist of demodulator, decoder, and D/A converter |
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| when waveforms are distorted because of phsyical phenomena such as frequency dependent gains, multipath effects and doppler shift, these can be partilally corrected at the receiver by an equalizer with gain and phase characteristics completementary to those of the channel. |
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| Constructed with a finite number of symbols. 1) Printed language 2) telegraph |
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| Characterized by values. Very by a continuous range: Speech waveform, temperature in a day. |
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| SNR= signal to noise ratio |
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Determines the recoverability of the transmitted signal
- The higher the SNR means the transmissted signal can use more signal levels, carrying more bits
Signal power/ Noise power
channel output or the receiver |
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| As long as the distortion is within a certain threshold, digital signals can be received perfectly from a noisy transmission |
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TWO STEPS: a continous time signal is sampled in a discrete time signal whose amplitude is quantized into a discrete level signal. Using the sampling theorem the highest frequency in the signal is B(hz), the signal can be reconstructed from its discrete samples, taken uniformly at a rate not less than 2B samples per second( to preserve the information from a continuous time signal we need to transmit only its samples), we are still in AC though,
step 2: our samples are rounded off to the nearest quantized level |
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| How can one decrease quantaziation error |
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| Increasing the of quantization number at the cost of longer files |
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| The bandwidth of a channel |
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The range of frequencies that it can transmit with reasonable fidelity
B=f2-f1 |
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| Convolution in the time domain= |
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| point wise multiplication in the other(frequency) |
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| Shifts the spectrum of the modulated spectrum of the modulated signal to the left and the right by fc so it has a bandwidth of 2B Hz consistant of two portions USB(outside of +- fc) and LSB(inside of +- fc). To demodulate shift the frequency spectrum to the left and to the right by fc and add a low pass filter |
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| Applications of modulation |
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Making it possible for transmissing signals simultaneously over a channel by sharing a frequency band. this is known as frequency band multiplexing
2) for effecting signal transmission, the antennae size must be on the order of wavelengths of the length of the signal
wavelength=c/f |
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| (e^(j*theta)+e^-(j*theta))/2 |
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| (e^(j*theta)-e^-(j*theta))/2 |
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SUM Dn*e^(j*n*2pi*f*t)
Dn=1/To ∫ g(t)e^-j*n*2*piw*t dt |
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Time shifting property
g(t-to)= |
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Frequency shifting
g(t)e^(j2*pi*f*to) |
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