19-11-2012, 06:27 PM
Carriers and Modulation CS442
modulation.ppt (Size: 719.5 KB / Downloads: 117)
Baseband Transmission
Digital transmission is the transmission of electrical pulses. Digital information is binary in nature in that it has only two possible states 1 or 0. Sequences of bits encode data (e.g., text characters).
Digital signals are commonly referred to as baseband signals.
In order to successfully send and receive a message, both the sender and receiver have to agree how often the sender can transmit data (data rate).
Data rate often called bandwidth – but there is a different definition of bandwidth referring to the frequency range of a signal!
ANALOG TRANSMISSION OF DIGITAL DATA
Analog Transmission occurs when the signal sent over the transmission media continuously varies from one state to another in a wave-like pattern.
e.g. telephone networks, originally built for human speech rather than data.
Advantage for long distance communications: much less attenuation for analog carrier than digital
Sine Wave
Peak Amplitude (A)
maximum strength of signal
volts
Frequency (f)
Rate of change of signal
Hertz (Hz) or cycles per second
Period = time for one repetition (T)
T = 1/f
Phase ()
Relative position in time, from 0-2*pi
General Sine wave
Data Rate and Bandwidth
Any transmission system has a limited band of frequencies
This limits the data rate that can be carried
Spectrum
range of frequencies contained in signal
Absolute bandwidth
width of spectrum
Effective bandwidth
Often just bandwidth
Narrow band of frequencies containing most of the energy