Why can several cellular phones be
used at the same time?
Cellular Phones and Base Stations
• When a cellular phone is used, signals containing a user’s message travel between the
antenna of the cellular phone and the antenna of a base station.
• These signals are electro-magnetic waves.
• Antennas of base stations are often mounted on top of buildings or poles along major
traffic routes.
• Those antennas look like grey, rectangular boxes.
• Question:If two or more cellular phones communicate with the same base station,
how can the base station separate those signals?
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Distinguishing Characteristics of Signals
• Signals are functions of time, i.e., changing with time.
• This view emphasizes the time-domain representation of the signal.
• Signals are also characterized by the range of frequencies that they occupy.
• This is called the spectrum or frequency-domain representation of the signal.
• Signals can be separated easily if they do not overlap in either the time-domain or the
frequency-domain.
• The two representations are connected through what is known as a Fourier Transform.
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Signal Separation in the Frequency Domain
• If two signals do not overlap in the frequency domain, then they can easily be separated
with band-pass filters.
• This is exactly what happens in broadcast radio and television.
• Example: An FM-radio station is often identified by a reference to a frequency, e.g.,
99.1, 101.1, or 88.5.
• What this refers to is the center frequency of the signal that is transmitted by that
radio station in Mega Hertz (MHz).
• The signal actually occupies a range of frequencies 200 Kilo Hertz (KHz) to the left
and right of the center frequency.
• The range of frequencies occupied by the signal is called the bandwidth of the signal.
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Frequency Division Multiple Access
• You can think of radio stations signals stacked up next to each other in the frequency
domain.
• This method of allowing several signals to be transmitted simultaneously is called
Frequency Division Multiple Access.
• Acronym: FDMA.
• When you tune your radio to a particular station, you actually tune a bandpass filter
to let only the signal from your favorite radio station pass.
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Analog Modulation: AM and FM
• Conventional radio systems employ analog modulation methods: amplitude modulation (AM) or frequency modulation (FM).
• Both modulation formats use a carrier signal:
– The carrier is a sinusoidal signal,
– it’s frequency determines the center frequency of the modulated signal.
• For AM, the amplitude of the carrier is varied proportional to the message signal.
• For FM, the frequency changes proportional to the message signal.
Message Signal
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FM Signal
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• AM and FM are called analog modulation methods because the modulating signal, i.e.,
message is an analog signal.
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Signals That do not Overlap in the Time-Domain
• When signals do not overlap in the time-domain then one signal stops before another
one begins.
• Signals are easily separated when the receiver only “listens” while the signal of interest
is sent.
• This multiple-access method is called Time Division Multiple Access.
• Acronym: TDMA.
• TDMA can only be employed when digital information is transmitted.
• Example: TDMA is employed in the digital portion of the telephone network.
• Example: TDMA and FDMA together are used in many modern digital cellular
telephone systems.
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Synchronization in TDMA
• Notice that a “clock” is required for TDMA.
• All transmitters and receivers must be aware of this “clock” to schedule their transmissions and receptions.
• We say that transmissions are synchronized.
• In cellular telephone systems a clock signal that indicates the beginning of time-slots
is transmitted by the base stations. From this signals, mobile stations can determine
when their turn comes up.
• In the digital telephone system, the transmitting exchange sends synchronization information together with the conversations.
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TDMA is for Digital Signals
• In TDMA systems, it is assumed that the signal can be stored until the transmitter’s
turn comes up.
• Analog signals cannot be stored very effectively (this requires magnetic tapes or disks)
• Digital signals are easily stored (or buffered) on memory chips.
• To hide that transmission is actually interrupted in TDMA systems, information is
transmitted at a faster rate than it is generated.
• Example: In digital telephony, a conversation generates 64,000 bits every second.
– 8,000 samples per second,
– 8 bits resolution per sample.
• On a so-called T1-line, 24 conversations are transmitted simultaneously on a single
wire pair carrying 1.544 Million bits per second (24*64,000+8,000=1,544,000).
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History–Telephone system
• In the telephone system, it would be too expensive to use wires running between
telephone exchanges to carry only one conversation.
• From early on, communications engineers have been looking for ways to increase the
number of simultaneous connections carried over a single copper pair.
• FDMA was first used in 1918 by “Ma Bell” to carry four simultaneous conversations
per wire between Pittsburgh and Baltimore.
• TDMA is applicable to digital transmissions only and was first introduced in the ’60s.
• Today, coaxial cables can carry tens of thousands of simultaneous conversations simultaneously.
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