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Radio Frequency Communications (components of radio system (Filters (Type…
Radio Frequency Communications
components of radio system
Filters
Type of filters
Low-pass filter : maximun frequency is set and all signals below that value are allowed
High-pass filter : minimun frequency is set and all signals above that level are allowed
Bandpass filter : sets a range called a passband and signals that fall within the passband are allowed
-Filter: removes unwanted RF signals
--RF filter either passes or rejects a signal based on frequency
Filters are also found in transmitter and are used to eliminate some unwanted frequencies called harmonic oscillations
Mixers
Mixers combine two radio frequency inputs to create a single output
-
output
is in the range of the highest sum and the lowest difference of the two frequencies
-sum and differences are known as the sidebands of the frequencies
mixers are used to convert an input frequency to a specific desired output frequency
Amplifiers
Amplifiers = increase the amplitude of an RF signals
it is an active device so it must be supplied with electricity.Electricity is used to increase a signal's intensity or strength
Antennas
Antennas = transmit or receive an RF signals
Design of radio system
Multiple User Access
~only a limited number of frequencies
are available for radio transmission
~Conserving frequencies
share a frequency among multiple users
methods that allow multiple access
Frequencies Division Multiple access(FDMA)
Time Division Multiple Access(TDMA)
Code Division Multiple Access(CDMA)
(FDMA)
divides the bandwidth of a channel into several smaller frequencies bands
-most often used with analog transmission - cable television is transmitted using FDMA
Drawback of FDMA : crosstalk
-causes interference on the other frequency and may distrupt the transmission
(TDMA)
divides the transmission time into several slots
-each user is assigned the entire frequency for the transmission
-Advantages
-uses the bandwidth more efficiently
-allows both data and voice transmission to be mixed using the same frequency
(CDMA)
used primarily for cellular telephones communications
used direct sequence spread spectrum(DSSS)
the longer the code is the more users will be able to share the same channel
number of chips in the code deteremine the amount of spreading the bandwidth
-Advantages
~can carry up to three times the data as TDMA ~transmission are much harder to eavesdrop on
~a would-be eavesdrop must also know the exact chip in which the transmission start
Transmission Direction
simplex transmission
-occurs in only one direction
-rarely used in wireless communication today
Half-duplex transmission
-sends data in both directions but only one way at a time
-used in consumer devices such as citizens band(CB) radios or walkie-talkies
Full-duplex transmission
-allows data to flow directions simultaneously
-a filter can be used to handle it if the same antennas is used for wireless transmission and reception
Switching
-involves moving the signals from one wire or frequency to another
-circuit switching
~ type of switching used by telephone system
~A dedicated and direct physical connection is made between the caller and the recipient
-packet switching
~used by data networks
~data transmission are broken into packets
~each packet is sent independently
- Advantages
-allows better utilization of the network
-allows multiple computers to share the same line or frequency
-if a transmission error occur
-it usually affects only one or a few packets
-only packets affected must be resent, not entire message
Signal strength
Strength of the signal
-it must be sufficient for the signal to reach its destination
Electronic interference(EMI)
-affects radio signal strength
-also called noise
signal-to-noise ratio
-compares signal strength with background noises
-when strength of a signal is well above the noise the interference can be filtered out
-to reduce the interference of the noises
~boost the strength of the signal
~use of filters on the receiving end
Attenuation = loss of signal strength
multipath distortion
as a radio signal is transmitted,the electromagnetic waves will spread out
-waves travel different path between transmitter and receiver
Directional antenna
-used to minimize multipath distortion
-radiates electromagnetic waves in one direction only
other methods to reduce multipath distortion
~use an amplifier in front of receiver to increase SNR
-transmit the same signal on separate frequencies