Digital Modulation and Data Transmission Methods
Modulation is called the transaction by which certain characteristics of a wave called a carrier, be modified in light of another wave called modules, which contains information
The modulation process requires two signals, Modules and carriers, and as a result generates a third signal, called the modulated signal.
The carrier signal parameters that are affected by the information are: MODULATION
FSK
MODULATION DIGITAL-ANALOG
FSK (modulation frequency shift): It changes the carrier frequency depending on the value of bits to transmit, during the bit cell is used to value a frequency f1 and frequency f2 for bits at zero.
It presents the advantage of simple circuitry for demodulating and requirements of low bandwidth
ASK (shift key modulation amplitude): In this technique does not change the carrier frequency but its amplitude. The two binary values are represented by different levels of carrier signal amplitude
Usually one of the amplitudes is zero, ie the binary one is represented by the presence of carrier and zero by the absence of the same
PSK (phase shift keying): The frequency and amplitude remains constant and varying the carrier phase to represent one and zero levels with different phase angles.
With this type of modulation the receiver must maintain a reference carrier signal with which to compare the phase of the received signal
This involves complex demodulation circuits. Also this type of modulation is very susceptible to random phase change in the transmission waveform
In the most general form of PSK modulation, the carrier moves consistently 45 º, 135 º, 225 º or 335 º, in evenly spaced intervals
DPSK (phase shift keying differential): Given the need for the recipient to know the meaning of each phase in PSK, born a smarter alternative.
This new alternative way of phase modulation, phase shifts used on the transmitted signal prior
QAM (quadrature amplitude modulation)
In more sophisticated modems are usually used various combinations of amplitude and phase, this can be achieved, for example, a signal element represents four bits, with a 2400 baud transmission allows a transmission speed of 9600 bps
Any analog signal can always be converted to digital form by running three sequential processes
In the sampling operation, are only preserved the values of the analog signal sampled at instants spaced discrete instants of time. Usually sampled at twice the highest frequency of the analog signal sent
In the operation of quantification, each sampled value is approximately at the level closest in finite sets of discrete levels
In the coding operation, the selected level (for quantification) is represented by a code word consisting of a number of elements belonging to prewritten code.
Multiplexing FDM
In division multiplexing divides the frequency bandwidth of the channel between the multiplexed signals
Each station can transmit simultaneously to the other
In this type of multiplexing, the channel bandwidth must exceed the sum of the bandwidths of the signals that are transmitted, since it is necessary to make safety bands between the frequencies assigned to the various channels.
TDM
Multiplexing
In time division multiplexing, is assigned to the different transmission stations a rotating shift, for a time quantum transmitting station, then the next …
Then each station actually used the channel alternately, not simultaneously. • Obviously, this is the method for transferring digital data