Life Cycle of Cellular networks

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11/20/2020
Ph.D. in Telecommunication
Hussein Al Haj Hassan
Mobile Networks
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• Fundamental Concepts in Cellular Networks
• Life Cycle of Cellular networks
• GSM
• 3G
• 4G
• 5G
What do you find in this course?
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Methodology of Teaching
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• Lecture will be used predominantly
• Use handouts and problem sets
• Readings from research papers
• Home works and Quizzes
• Each student (or group) will be assigned a
small project, which will be carried out
throughout the semester
• Stay connected! Any where any times
• Mobility
• Instantaneous communication
• Communication will reach where wiring is
infeasible or costly
Why wireless communication?
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• Goals of a Cellular System
– High capacity
– Large coverage area
– Efficient use of limited spectrum
• Large coverage area – Bell system in New York City had early
mobile radio
– Single Tx, high power, and tall tower
– Low cost
– Large coverage area – Bell system in New York City had 12
simultaneous channels for 1000 square miles
– Small # users
– Poor spectrum utilization
• What are possible ways we could increase the number of
channels available in a cellular system?
Why wireless communication?
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• The cellular concept is a system level idea which calls for replacing
a single, high power transmitter (large cell) with many low power
transmitters (small cells), each providing coverage to only a small
portion of the service area.
• A cell site or cell tower is a cellular telephone site where
antennae and electronic communications equipment
• It is called BTS in case of GSM, node B in case of 3G and eNB in
case LTE.
The cellular concept
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– Frequency reuse pattern
The cellular concept
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• Cells labeled with the same letter use the same
group of channels.
• Cell Cluster: group of N cells using complete set of
available channels
• Many base stations, lower power, and shorter
towers
• Each cell allocated a % of the total number of
available channels
• Nearby (adjacent) cells assigned different channel
groups to prevent interference between
neighboring base stations and mobile users
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The cellular concept
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• Same frequency channels may be reused by cells a
“reasonable” distance away
– reused many times as long as interference between same
channel (co-channel) cells is < acceptable level
• As frequency reuse↑ → # possible simultaneous
users↑→ # subscribers ↑→ but system cost ↑ (more
towers)
• To increase number of users without increasing radio
frequency allocation, reduce cell sizes (more base
stations) ↑→ # possible simultaneous users ↑
• The cellular concept allows all mobiles to be
manufactured to use the same set of frequencies
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The cellular concept
 The working range of a cell site (the range which mobile devices connects reliably
to the cell site) is not a fixed figure. It will depend on a number of factors, including,
but not limited to:
 Height of antenna over surrounding terrain (Line-of-sight propagation)
 The frequency of signal in use
 Timing limitations in some technologies (e.g., GSM is limited to 35 Km)
 The transmitter’s rated power
 The required uplink/downlink data rate of the subscriber’s device
 The directional characteristic of the site antenna array
 Reflection and absorption of radio energy by buildings
 It may also be limited by local geographical or regulatory factors and weather conditions.
 Generally, in areas where there are enough cell sites to cover a wide area, the
range of each one will be set to:
 Ensure there is enough overlap for “handover” to/from other sites (moving the signal for a mobile
device from one cell site to another, for those technologies that can handle it – e.g., making a GSM
phone call while in a car or train).
 Ensure that the overlap area is not too large, to minimize interference problems with other sites.
The cellular concept
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 Tessellation or tiling in two dimensions is the branch of mathematics that studies
how shapes can be arranged to fill a plane without any gaps, according to a given
set of rules.
 Common one is that all corners should meet and that no corner of one tile can lie along the edge of
another. There are only three shapes that can form such regular tessellations: the equilateral
triangle, square, and regular hexagon. Any one of these three shapes can be duplicated infinitely to
fill a plane with no gaps at all.
Tessellation
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• A cell must be designed to serve the weakest mobiles within the footprint (edge)
• When using hexagons to model coverage areas, base station transmitters are
depicted as either being in the center of the cell (center-excited cells) or on three
of the six cell vertices (edge-excited cells).
• Normally, omni-directional antennas are used in center-excited cells and sectoraldirectional antennas are used in corner-excited cells
Tessellation
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Real cell radio coverage
• Non-hexagonal cell shape:
Tessellation
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• Consider a cellular system which has a total of S duplex channels available for use.
• k : number of channels for each cell (k < S)
• The N cells which collectively use the complete set of available frequencies is called
a cluster
S = k.N
• If a cluster is replicated M times within the system, the total number of duplex
channels, C, can be expressed as a measure of capacity and is given by:
S=M.K.N=M.S
• As we can see, the capacity of a cellular system is directly proportional to the
number of times a cluster is replicated in a fixed service area. The factor N is called
the cluster size and is typically equal to 4, 7, or 12.
• For a fixed total coverage area Atotal and the coverage area of each cell Acell, the
number of cells in the system is:
System Capacity
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• If the cluster size N is reduced while the cell size is kept
constant, more clusters are required to cover a given area,
and hence more capacity (larger C) is achieved.
• The smallest possible value of N is desirable in order to
maximize the capacity over a given coverage area.
However, a small value for N may lead to large interference.
This is another factor that needs to be considered when
determining the value of N.
• The frequency reuse factor of a cellular system is given by
1/N
System Capacity
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• Due to the fact that the hexagonal geometry has
exactly six equidistant neighbors and that the lines
joining the centers of any cell and each of its
neighbors are separated by multiples of 60 degrees,
there are only certain cluster sizes and cell layouts
which are possible.
• Valid clusters are those that result in 6 cells with the
same frequency of a particular cell located at equal
distance from it.
Nearest Co-channel interference
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Nearest Co-channel interference
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Nearest Co-channel interference
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Nearest Co-channel interference
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Example
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• Handoffs must be performed successfully and as infrequently as possible, and be
imperceptible to the users
• System designers must specify an optimum signal level at which to initiate a
handoff.
• Once a particular signal level is specified as the minimum usable, a slightly
stronger signal level is used as a threshold at which handoff is made.
• This margin, given by Δ= Pr,handoff – Pr,minimum usable
• Δ cannot be too large or too small.
• If is too largeunnecessary handoff. If is too smallinsufficient time to
complete a handoff before a call is lost due to weak signal conditions.
Handoff
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• Types of Handoff:
– Hard Handoff: channel in source cell is released and channel in target cell is engaged. The
connection is broken before or as the connection to the target is made. We call these handoffs
breakbefore- make. When the mobile is between base stations, the mobile will switch with any of
these base stations, so the base station will bounce the link with the mobile back and forth. This is
called ping-ponging.
– Soft Handoff: when the channel in the source cell is retained and used for a while in parallel with
the channel in the target cell. It is referred to as make-before-break. The interval, during which the
two connections are used in parallel, may be brief or substantial. For this reason the soft
handover is perceived by network engineers as a state of the call, rather than a brief event.
• Prioritizing handoff:
– Guard Channel Concept: each cell of the system reserves few channels for handoffs that are never
used for initiating a call. However, if all the channels of the cell are fully occupied including the
reserved channel, the handoff fails and the call is dropped. The reservation of few channels for
handoff reduces the system
– Queuing of Handoff requests: Once the received signal power drops below the handoff threshold,
a handoff request is initiated. If an available channel exists, the handoff is completed. If there are
no available channels, the call is queued waiting for a channel of the system to become free.
Handoff
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• 1st Generation Cellular (Analog FM → AMPS)
– Received signal strength (RSS) measured at base station & monitored by
central entity (MSC) you will see it in Advance Networking!!!!
– A spare Rx in base station (locator Rx) monitors RSS of RVC’s in neighboring
cells
– Locator Rx can see if signal to this base station is significantly better than to
the host base station
– MSC monitors RSS from all base stations & decides on handoff
• 2nd Generation Cellular (GSM,)
– Mobile Assisted HandOffs (MAHO)
• The mobile measures the RSS from adjacent base stations & reports back
to serving base station
• if Rx power from new base station > Rx power from serving (current) base
station by pre-determined margin for a long enough time period →
handoff initiated by MSC
Handoff
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• Propagation measurements in a mobile radio channel show that the average
received signal strength at any point decays as a power law of the distance of
separation between a transmitter and receiver.
• In the log model, the average received power Pr at a distance d from the
transmitting antenna in the log model is approximated by:
where P
0 is the power received at a close-in reference point in the far field region of
the antenna at a small distance d
0 from the transmitting antenna, and n is the path
loss exponent.
Power Log model
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Example
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Example
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• Goal is to minimize interference & maximize use of capacity
– lower interference allows smaller N to be used → greater frequency
reuse → larger C
• Two main strategies: Fixed or Dynamic
• Fixed
– each cell allocated a pre-determined set of voice channels
• calls within cell only served by unused cell channels
• all channels used → blocked call → no service
– several variations
• MSC allows cell to borrow a channel from an adjacent cell
• donor cell must have an available channel to give
Channel Assignment Strategies
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• Dynamic
– channels NOT allocated permanently
– call request → goes to serving base station → goes to
MSC
– MSC allocates channel “on the fly”
• allocation strategy considers:
– likelihood of future call blocking in the cell
– reuse distance (interference potential with other cells that are
using the same frequency)
– channel frequency
– All frequencies in a market are available to be used
Channel Assignment Strategies
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• Advantage: reduces call blocking (that is to say, it
increases the trunking capacity), and increases
voice quality
• Disadvantage: increases storage & computational
load @ MSC (central entity)
– requires real-time data from entire network related to:
• channel occupancy
• traffic distribution
• Radio Signal Strength Indications (RSSI’s) from all channels!!!
Channel Assignment Strategies
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• Interference is the major limiting factor in the performance
of cellular radio systems.
• Sources of interference include another mobile in the same
cell, a call in progress in a neighboring cell, other base
stations operating in the same frequency band, or any
other noncellular system which inadvertently leaks energy
into the cellular frequency band.
• The two major types of system-generated cellular
interference are
– Co-channel interference
– adjacent channel interference
Interference and System Capacity
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• Frequency reuse implies that in a given coverage area there are several cells that
use the same set of frequencies (co-channel cells)
• Unlike thermal noise which can be overcome by increasing the SNR, co-channel
interference cannot be combated by simply increasing the carrier power of a
transmitter.
• To reduce co-channel interference, co-channel cells must be physically separated
by a minimum distance to provide sufficient isolation due to propagation.
• Co-channel interference depends on:
– R : cell radius
– D : distance to base station of nearest co-channel cell
• if D / R ↑ then spatial separation relative to cell coverage area ↑
– improved isolation from co-channel RF energy
• Q = D / R : co-channel reuse ratio
– hexagonal cells → Q = D/R =
Interference and System Capacity
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3N
• Fundamental tradeoff in cellular system design:
– small Q → small cluster size → more frequency reuse → larger
system capacity great
– But also: small Q → small cell separation → increased cochannel interference (CCI) → reduced voice quality → not so
great
– Tradeoff: Capacity vs. Voice Quality
Interference and System Capacity
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• If i
0 is the number of co-channel interfering cells, then the signal to interference
ratio SIR:
• Where S is the desired signal power from the desired base station and Ii is the
interference power caused by the ith interfering co-channel cell base station. If the
signal levels of co-channel cells are known, then the
• S/I ratio for the forward link can be found using equation
• D
i is the distance of the ith interferer from the mobile, n is the path loss exponent:
Interference and System Capacity
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• Considering only the first layer of interfering cells, if all the interfering base stations
are equidistant from the desired base stations and if this distance is equal to the
distance D between cell centers, then:
Interference and System Capacity
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Example
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Adjacent Channel Interference
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• imperfect receiver filters which allows nearby frequencies leak into the passband
• The near–far problem is a condition in which a receiver captures a strong signal and
thereby makes it impossible for the receiver to detect a weaker signal.
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Trunking
 Cellular radio systems rely on trunking to accommodate a
large number of users in a limited radio spectrum.
 Trunking allows a large no of users to share a relatively small
number of channels in a cell by providing access to each user,
on demand, from a pool of available channels.
 In a trunked radio system (TRS) each user is allocated a
channel on a per call basis, upon termination of the call, the
previously occupied channel is immediately returned to the
pool of available channels.
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Trunking/ Definitions:
 Setup Time: Time required to allocate a radio channel to a
requesting user
 Blocked Call: Call which cannot be completed at the time of request,
due to congestion(lost call)
 Holding Time: Average duration of a typical call. Denoted by H(in
seconds)
 Request Rate: The average number of calls requests per unit time( λ)
 Traffic Intensity: Measure of channel time utilization or the average
channel occupancy measured in Erlangs
 Load: Traffic intensity across the entire TRS (Erlangs)
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 The fundamentals of trunking theory were developed by Erlang, a Danish
mathematician, the unit bears his name.
 An Erlang is a unit of telecommunications traffic measurement.
 Erlang represents the continuous use of one voice path.
 It is used to describe the total traffic volume of one hour
 A channel kept busy for one hour is defined as having a load of one Erlang
 For example, a radio channel that is occupied for thirty minutes during an
hour carries 0.5 Erlangs of traffic
 For 1 channel
 Min load=0 Erlang (0% time utilization)
 Max load=1 Erlang (100% time utilization)
Trunking/ Definitions:
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Example:
 Example: a group of 100 users made 30 calls in one hour, and each call had
an average call duration (holding time) of 5 minutes, then the number of
Erlangs this represents is worked out as follows:
Minutes of traffic in the hour = 30 x 5 = 150
Hours of traffic in the hour = 150 / 60 = 2.5
Traffic Intensity= 2.5 Erlangs
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Traffic concepts
 Traffic Intensity offered by each user(Au) is the average call arrival rate
multiplied by the holding time(service time)
Au=λH(Erlangs)
 Total Offered Traffic Intensity for a system of U users (A):
A =U*Au(Erlangs)
 Traffic Intensity per channel, in a C channel trunked system
 Ac=U*Au/C(Erlangs)
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Grade of Service
 In a TRS, when a particular user requests service and all the available radio
channels are already in use , the user is blocked or denied access to the
system. In some systems a queue may be used to hold the requesting users
until a channel becomes available.
 Trunking systems must be designed carefully in order to ensure that there is
a low likelihood that a user will be blocked or denied access.
 The likelihood that a call is blocked, or the likelihood that a call experiences
a delay greater than a certain queuing time is called “Grade of Service”
(GOS)’’.
 In a TRS, when a particular user requests service and all the available radio
channels are already in use , the user is blocked or denied access to the system. In
some systems a queue may be used to hold the requesting users until a channel
becomes available.
 Trunking systems must be designed carefully in order to ensure that there is a low
likelihood that a user will be blocked or denied access.
 The likelihood that a call is blocked, or the likelihood that a call experiences a delay
greater than a certain queuing time is called “Grade of Service” (GOS)’’.
 Grade of Service (GOS): Measure of ability of a user to access a trunked system
during the busiest hour. Measure of the congestion which is specified as a
probability.
 The probability of a call being blocked : Blocked calls cleared(BCC) or Lost Call
Cleared(LCC) or Erlang B systems
 The probability of a call being delayed beyond a certain amount of time before
being granted access: Blocked call delayed or Lost Call Delayed(LCD) or Erlang C
systems
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Types of Truncked Systems
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Modelling of block call cleared system
 The Erlang B model is based on following assumptions :
 Calls are assumed to arrive with a Poisson distribution
X is the number of event, λ is the average number of intervals per event, e=2.718….
 There are nearly an infinite number of users
 Call requests are memory less ,implying that all users, including blocked users, may
request a channel at any time
 All free channels are fully available for servicing calls until all channels are occupied
 The probability of a user occupying a channel(called service time) is exponentially
distributed.
F(x)= μe-μx
μ is the rate parameter, x is the call duration, e=2.718
 There are a finite number of channels available in the trunking pool.
 Inter-arrival times of call requests are independent of each other
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Modelling of block call cleared system
 The Erlang B formula is:
Where C is the number of trunked channels offered by a trunked radio system
and A is the total offered traffic.
Pr[blocking]= (AC/C ! )
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Modelling of block call cleared system
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Modelling of block call cleared system
 How many users can be supported for 0.5% blocking probability for the
following number of trunked channels in a BCC system? (a) 5, (b)=20.
Assumed that each user generates 0.1 Erlangs of traffic.
 Solution
• Given C=5, GOS=0.005, Au=0.1, From graph using C=5 and
GOS=0.005A=1.13Total Number of users U=A/Au=1.13/0.1=11 users
• Given C=20, GOS=0.005, Au=0.1, From graph using C=20 and
GOS=0.005A=11.10Total Number of users U=A/Au=11.10/0.1=110
users
• Notice that when we increased the number of channel by 4, the number of
users are increased by 10
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Block call delayed system
 Queues are used to hold call requests that are initially blocked
 When a user attempts a call and a channel is not immediately available, the
call request may be delayed until a channel becomes available
 Mathematical modeling of such systems is done by Erlang C formula
 The Erlang C model is based on following assumptions :
 Similar to those of Erlang B
 Additionally, if offered call cannot be assigned a channel, it is placed in a
queue of infinite length
 Each call is then serviced in the order of its arrival (FIFO)
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Block call delayed system
 Erlang C formula which gives probability of a call not having immediate
access to a channel (all channels are already in use)
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Erlang C
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Block call delayed system
 Probability that any caller is delayed in queue for a wait time greater than t seconds
is given as GOS of a BCD System
 The probability of a call getting delayed for any period of time greater than zero is
P[delayed call is forced to wait > t sec]=P[delayed] x Conditional P[delay is >t sec]
 Mathematically;
Pr[delay>t] = Pr [delay>0] Pr [delay>t| delay>0]
Where P[delay>t| delay>0]= e(-(C-A)t/H)
Pr[delay>t] = Pr [delay>0] e(-(C-A)t/H)
where C = total number of channels, t =delay time of interest, H=average duration
of call, A= total offered traffic
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Example
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Trunking efficiency
 Trunking efficiency is a measure of the number of users which can be offered a
particular GOS with a particular configuration of fixed channels.
 The way in which channels are grouped can substantially alter the number of users
handled by a trunked system.
 Example:
 10 trunked channels at a GOS of 0.01 can support 4.46 Erlangs, where as two
groups of 5 trunked channels can support 2×1.36=2.72 Erlangs of traffic
 10 trunked channels can offer 60% more traffic at a specific GOS than two 5
channel trunks.
 Therefore, if in a certain situation we sub-divide the total channels in a cell into
smaller channel groups then the total carried traffic will reduce with increasing
number of groups
• Cell splitting is the process of subdividing a
congested cell into smaller cells
• increases the number of times that channels
are used
• mobile phone in the uplink communication
• Disadvantages: require the construction of
new towers
Cell Splitting
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• Cell splitting achieves capacity
improvement by essentially rescaling
the system (Q=D/R is kept constant)
• Capacity improvement can be also
achieved by reducing the number of
cells in a cluster and thus increasing the
frequency reuse
– reduce the relative interference without decreasing
the transmit power
Sectoring
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• Sectoring: Dividing cells into sector:
– Reduce capacity since channels are divided into the sectors!
– Handoff from one sector to another!
– Gain in capacity is achieved by reducing the number of interfering channels
– the SIR is increased for the same cluster size. This allows us to reduce the cluster size and
achieve the same original SIR, which directly increases the network capacity.
Sectoring
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Sectoring
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Example
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Example
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• Multiplexing describes how several users can share a
medium with minimum or no interference.
– Space division multiplexing
– Frequency division multiplexing
– Time division multiplexing
– Code division multiplexing
Multiplexing
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Multiplexing
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• Advantages:
– Higher Capacity
– Less Transmission power
– Local interference only
– Robustness
• Disadvantages:
– Infrastructure needed
– Handover
– Frequency planning
Cellular Networks
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