Cellular Communication

Fundamentals

By
Tanveer Hasan
Assistant Professor
EES, University Polytechnic
AMU, Aligarh

1
Wireless Communication
Transmitting/receiving voice and data using electromagnetic waves in open space.
• The information from sender to receiver is carried over a well-defined frequency
band.
• Each channel has a fixed frequency bandwidth and Capacity (bit-rate).
• Different channels can be used to transmit information in parallel and
independently.

2
Types of Wireless Communication
• Mobile (moving at high speed e.g. in a car)
• Cellular Phones (GSM / cdma2000)
• Portable (Laptop connected to Wireless LAN)
• IEEE 802.11b (WiFi)
• IEEE 802.15.3 (UWB)
• Fixed
• IEEE 802.16 (Wireless MAN)

3
Terminology
• Mobile Station (MS): A mobile station or simply mobile is a radio terminal that
may be attached to a high speed mobile platform (e.g. a cell phone in a fast
moving vehicle).
• Portable: It is a radio terminal that can be hand held and can be used by
someone at walking speed (e.g. a cordless phone).
• Subscriber: A mobile or a portable user.
• Base Station (BS): Base stations are fixed antenna units with which the
subscribers communicate. Base stations are connected to a commercial power
source and a backbone network.

4
Terminology…
Cell: The area of coverage is divided into cells. Each cell has a base station usually
located at its center or at the edge.
Control Channel: Radio channels used for transmission of call setup, call request
and call initiation.
Forward Channel (downlink): Radio channel used for transmission of information
from the base station to the mobile.
Reverse Channel (uplink): Radio channel used for transmission of information from
the mobile to the base station.

5
Terminology…
Full Duplex System: A full duplex system is one in which simultaneously two way
communications can take place. The transmission and reception takes place on two
different channels.
Half Duplex System: Two way communication done by using same radio channel
for both transmission and reception. At any given time the user can either transmit
or receive.
Handoff: The process of transferring the mobile station from one channel or base
station to another.
Page: Page is a brief message that is broadcast over the entire service area by
many base stations at the same time primarily to locate where the mobile station
is.

6
Terminology…

Mobile Switching Centre (MSC): Mobile switching centre plays central role in
the cellular switching network. All of the base stations are connected to
mobile switching centre and MSC itself is connected to the public switched
telephone network (PSTN).
Transceiver: A device capable of transmitting and receiving radio signals.
Forward Voice Channel (FVC): Used for voice transmission from BS to MS.
Reverse Voice Channel (RVC): Used for voice transmission from MS to BS.

7
Terminology…

Forward Control Channel (FCC): Used for initiating a call from BS to MS.
Reverse Control Channel (RCC): Used for initiating a call from MS to BS.

8
Basic Concepts

Multiple Access: Multiple Access schemes are used to allow many mobile
users to share a finite amount of radio spectrum.

9
Frequency Division Multiple Access (FDMA)

• In FDMA available spectrum is

divided into smaller frequency
bands called channels. Different
users are given different channels.
• Guard bands are provided between
adjacent channels.

10
Time Division Multiple Access (TDMA)

• In TDMA whole available spectrum

is used by any user.
• The time is divided into slots and
each time slot constitutes a channel.
Different users are allocated
different time slots.
• Time guard band is provided
between adjacent time slots.

11
Time Division Multiplexing…

• TDMA can be used only in digital systems.

• Synchronization is very important in TDMA otherwise one user may listen in
some other time slot.

12
Code Division Multiple Access (CDMA)

• User may use whole available

spectrum and may transmit all the
time.
• The users use different codes.

13
Full Duplex Systems
• Full duplex is provided either by FDD (frequency division duplex) or TDD (time
division duplex).
• Here full duplex refers to the communication between the mobile and the base
station. It is for single user.
• So a mobile system may use a combination of a multiple access scheme and a
duplex system. For example we can have an FDMA/FDD or a TDMA/FDD and
CDMA/FDD.

14
Full Duplex Systems
FDD: In FDD one frequency band is used for communication from base station to
mobile and another frequency band is used for communication from mobile to
base station.
TDD: In TDD one time slot is used for communication from base station to mobile
and another time slot is used for communication from mobile to base station.

15
Cellular Networks Evolution
• First Generation (1G)
• Launched in mid-1980s
• Analog Systems
• Analog modulation, mostly FM
• Supported voice traffic only
• Multiple access technique used was FDMA/FDD
• Confined to national boundaries
• Example: AMPS (Advanced Mobile Phone Services)

16
Cellular Networks Evolution
• Second Generation (2G)
• Developed for voice communication
• Digital system, digital modulation (e.g. GMSK in GSM)
• TDMA/FDD and CDMA/FDD multiple access
• Even though they were designed for voice they had provision for data rates of
the order of ~9.6 kbps
• After some time need of data traffic was felt but a lot of GSM phone had been
sold by that time (66 % of the mobile phones in 2002 were GSM phones). So it
was not possible to change the standard immediately. So 2.5G was developed.

17
Cellular Networks Evolution
• Examples of 2G Systems
• Global System for Mobile Communication (GSM) (uses TDMA/FDD)
• Personal Digital Communication (Popular in Japan)
• IS-95 (CDMA, US/South Korea)

18
Cellular Networks Evolution
• Limitation of 2G Systems
• Developed for voice communication (not suitable for data traffic)
• Average data rate of the order of tens of kbps
• Not suitable for Internet (packet switched network)
• Multiple standards (no true global coverage)

19
Cellular Networks Evolution
• 2.5 G
• The efforts to remove the limitations of 2G resulted in 2.5G
• Digital System
• Voice + low-data rate
• Internet access through GPRS (General Packet Radio Service, provides data-
rates of the order of 50 kbps)
• Enhanced Data Rate Through Global Evolution (EDGE): provided better data
rates through the use of better modulation. Data-rate of the order of
200kbps.

20
Cellular Networks Evolution
• 3G
• Digital modulation
• Simultaneous voice + high-data rate
• Multi-megabit Internet access
• Voice-activated calls
• Multimedia transmission
• 3G puts constraints on how fast you go and how fast you can download the
traffic.
• 3G is truly a world standard
• Examples are W-CDMA which is the wide band CDMA and the other is a
CDMA 2000 standard (data-rate 384 kbps), improved version HSPA: provides
data rate of the order of 5-30 Mbps.
21
Cellular Networks Evolution
• 4G
• Based on an all-IP packet switched network
• The ability to offer high quality of service for next generation multimedia
support.
• Use multiple antenna system and MIMO to provide high spectral efficiency
• Use Orthogonal Frequency Division Multiplexing (OFDM)
• WiMAX and LTE provide data rates in the range 100-200 Mbps

22
Cellular Mobile System Concept
1. The early mobile radio systems achieved a large coverage area by using a
single, high powered transmitter with an antenna mounted on a tall tower.
2. This approach achieved very good coverage, but it was impossible to reuse the
frequencies.
3. The cellular concept offered very high capacity in a limited spectrum by reusing
frequencies.
4. In cellular system the coverage area is divided into (conceptually hexagonal)
cells. A low power transmitter provides coverage to a cell. The actual radio
coverage of a cell is known as the footprint and is amorphous in nature.

23
Cellular Mobile System Concept
5. Each base station is allocated a portion of the total number of channels
available to the entire system. Nearby base stations are assigned different
groups of channels to reduce interference. All the available channels are
assigned to a small number (called cluster size) of neighboring base stations.
6. The channel groups may be reused as many times as necessary, as long as the
interference between co-channel stations is kept below acceptable levels.
7. As the demand for service increases (i.e., as more channels are needed within a
particular market), the number of base stations may be increased (along with a
corresponding decrease in transmitter power to avoid added interference),
thereby providing additional radio capacity without additional radio spectrum.

24
Frequency Reuse
1. In a cellular system, by limiting the
coverage area to within the
boundaries of a cell, the same group
of channels may be used to cover
different cells that are separated from
one another by large enough
distances to keep interference levels
within tolerable limits.
2. The design process of selecting and
allocating channel groups for all of
the cellular base stations within a
system is called frequency reuse or
frequency planning. Figure 1

25
Frequency Reuse
3. Figure 1 illustrates the concept of cellular frequency reuse, where cells labelled
with the same letter use the same group of channels. They are called co-
channel cells.
4. 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).
5. Normally, omni-directional antennas are used in center-excited cells and
sectored directional antennas are used in corner-excited cells.

26
Frequency Reuse
6. Consider a cellular system which has a total of S duplex channels available for
use. If each cell is allocated a group of k channels (k < S), and if the S channels
are divided among N cells, the total number of available radio channels can be
expressed as
𝑆 = 𝑘𝑁 (1)
7. The N cells which collectively use the complete set of available frequencies is
called a cluster and N is called cluster size. If a cluster is replicated M times
within the system, the total number of duplex channels, C, can be used as a
measure of capacity and is given
𝐶 = 𝑀𝑘𝑁 = 𝑀𝑆 (2)

27
Frequency Reuse
8. The capacity of a cellular system is directly proportional to the number of times
a cluster is replicated in the service area.
9. If the cluster size N is reduced while keeping the cell size constant, more
clusters are required to cover a given area and hence more capacity (a larger
value of C) is achieved. But a small cluster size indicates that co-channel cells
are located closer which means larger co-channel interference.
10. If the cluster size N is increased while keeping the cell size constant, lesser
number of clusters is required to cover a given area and hence less capacity (a
smaller value of C) is achieved. But a larger cluster size indicates that co-
channel cells are located at larger distance which means smaller co-channel
interference.

28
Frequency Reuse
11. Hence the value for N is a function of how much interference a mobile or base
station can tolerate while maintaining a sufficient quality of communications.
12. The frequency reuse factor of a cellular system is given by 1/N, since each cell
within a cluster is only assigned 1/N of the total available channels in the system.
13. In hexagonal geometry, there are only certain cluster sizes and cell layouts
which are possible. N can only have values which satisfy
𝑁 = 𝑖 2 + 𝑖𝑗 + 𝑗 2 (3)
where i and j are non-negative integers.

29
Frequency Reuse
14. To find the nearest co-channel
neighbors of a particular cell, one
must do the following:
i. move i cells along any chain of
hexagons and then
ii. turn 60 degrees counter-
clockwise and move j cells.
This is illustrated in Figure 2 for i = 3
Figure 2
and j = 2 (N = 19).

30
Channel Assignment Strategies
1. Channel assignment strategies can be either fixed or dynamic. The choice of
channel assignment strategy affects the performance of the system, when a
mobile user is handed off from one cell to another.
2. In a fixed channel assignment strategy; each cell is allocated a predetermined
set of voice channels. If all the channels in that cell are occupied, the call is
blocked and the subscriber does not receive service.
3. Several variations of the fixed assignment strategy exist. In one approach,
called the borrowing strategy, a cell is allowed to borrow channels from a
neighboring cell if all of its own channels are already occupied. The mobile
switching center (MSC) supervises such borrowing procedures and ensures that
the borrowing of a channel does not disrupt or interfere with any of the calls in
progress in the donor cell.
31
Channel Assignment Strategies
4. In a dynamic channel assignment strategy, voice channels are not allocated to
different cells permanently. Instead, each time a call request is made, the
serving base station requests a channel from the MSC. The switch then
allocates a channel to the requested cell following an algorithm that takes into
account the likelihood of future blocking within the cell, the frequency of use
of the candidate channel, the reuse distance of the channel, and other cost
functions. Accordingly, the MSC only allocates a given frequency if that
frequency is not presently in use in the cell or any other cell which falls within
the minimum restricted distance of frequency reuse to avoid co-channel
interference.
5. Dynamic channel assignment reduces the likelihood of blocking, which
increases the trunking capacity of the system, since all the available channels in
a market are accessible to all the cells.
32
Channel Assignment Strategies
6. Dynamic channel assignment strategies require the MSC to collect real-time
data on channel occupancy, traffic distribution, and radio signal strength
indications (RSSI) of all channels on a continuous basis. This increases the
storage and computational load on the system but provides the advantage of
increased channel utilization and decreased probability of a blocked call.

33
Handoff Strategies
1. When a mobile moves into a different cell while a conversation is in progress,
the MSC automatically transfers the call to a new channel belonging to the new
base station. This process called handoff requires that the voice and control
signals be allocated to channels associated with the new base station.
2. Processing handoffs is an important task in any cellular radio system. Many
handoff strategies prioritize handoff requests over call initiation requests when
allocating unused channels in a cell site.
3. The system designers must specify an optimum signal level at which to initiate
a handoff. A slightly stronger signal level is used as a threshold at which a
handoff is made. This margin, given by Δ=Prhandoff -Prminimumusable should not be
too large or too small.

34
35
Handoff Strategies
4. If Δ is too large, unnecessary handoffs will occur and will burden the MSC. If Δ
is too small, there may be insufficient time to complete a handoff before a call
is lost due to weak signal conditions.
5. It is important to ensure that the drop in the measured signal level is not due to
momentary fading and that the mobile is actually moving away from the
serving base station. To ensure this the base station monitors the signal level
for a certain period of time before a handoff is initiated.
6. The length of time needed to decide if a handoff is necessary depends on the
speed at which the vehicle is moving. If the slope of the short-term average
received signal level in a given time interval is steep, the handoff should be
made quickly.

36
Handoff Strategies
7. In first generation analog cellular systems, signal strength measurements are
made by the base stations and supervised by the MSC. Each base station
constantly monitors the signal strengths of all of its reverse voice channels to
determine the relative location of each mobile user with respect to the base
station tower.
8. In second generation systems that use digital TDMA technology, handoff
decisions are mobile assisted. In mobile assisted handoff (MAHO), every mobile
station measures the received power from surrounding base stations and
continually reports the results of these measurements to the serving base
station. A handoff is initiated when the power received from the base station of
a neighboring cell begins to exceed the power received from the current base
station by a certain level or for a certain period of time.
9. The MAHO method enables the call to be handed over between base stations
at a much faster rate.
37
Handoff Strategies
10. During the course of a call, if a mobile moves from one cellular system to a
different cellular system controlled by a different MSC, an intersystem handoff
becomes necessary. An MSC engages in an intersystem handoff when a mobile
signal becomes weak in a given cell and the MSC cannot find another cell
within its system to which it can transfer the call in progress.

38
Practical Handoff Considerations
1. In practical cellular systems, several problems arise when designing for a wide
range of mobile velocities. High speed vehicles pass through the coverage
region of a cell within a matter of seconds, whereas pedestrian users may
never need a handoff during a call.
2. Several schemes have been devised to handle the simultaneous traffic of high
speed and low speed users while minimizing the handoff intervention from the
MSC.
3. Another practical limitation is the ability to obtain new cell sites. Often it is
more convenient to install additional channels and base stations at the same
physical location of an existing cell, rather than find new site locations.

39
Practical Handoff Considerations
4. By using different antenna heights (often on the same building or tower) and
different power levels, it is possible to provide "large" and "small" cells which
are co-located at a single location. This technique is called the umbrella cell
approach and is used to provide large area coverage to high speed users while
providing small area coverage to users traveling at low speeds.
5. The umbrella cell approach ensures that the number of handoffs is minimized
for high speed users and provides additional microcell channels for pedestrian
users.
6. If a high speed user in the large umbrella cell is approaching the base station,
and its velocity is rapidly decreasing, the base station may decide to hand the
user into the co-located microcell, without MSC intervention.

40
Practical Handoff Considerations
7. Another practical handoff problem in microcell systems is known as cell
dragging. Cell dragging occurs when there is a line-of-sight (LOS) radio path
between the subscriber and the base station. The user enters the neighboring
cell without handoff. This creates a potential interference and traffic
management problem.
8. To solve the cell dragging problem, handoff thresholds and radio coverage
parameters must be adjusted carefully.
9. The IS-95 code division multiple access (CDMA) spread spectrum cellular
system the mobiles share the same channel in every cell. Thus handoff does
not mean a physical change in the assigned channel, but rather that a different
base station handles the radio communication task.

41
Practical Handoff Considerations
10. By simultaneously evaluating the received signals from a single subscriber at
several neighboring base stations, the MSC may actually decide which version
of the user's signal is best at any moment in time.
11. The ability to select between the instantaneous received signals from a variety
of base stations is called soft handoff.

42
Interference
1. 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 non-cellular system which leaks energy into the cellular
frequency band.
2. Interference on voice channels causes cross talk, where the subscriber hears
interference in the background due to an undesired transmission.
3. On control channels, interference leads to missed and blocked calls due to
errors in the digital signalling.
4. Interference is more severe in urban areas.

43
Interference
5. Interference is a major bottleneck in increasing capacity and is often
responsible for dropped calls.
6. The two major types of system generated cellular interference are co-channel
interference and adjacent channel interference.
7. Even though interfering signals are often generated within the cellular system,
they are difficult to control in practice (due to random propagation effects).

44
Co-channel Interference and System Capacity
1. Due to frequency reuse there are several cells that use the same set of
frequencies. These cells are called co-channel cells, and the interference
between signals from these cells is called co-channel interference.
2. Co-channel interference cannot be reduced by simply increasing power of a
transmitter. An increase in transmit power also increases the interference to
neighboring co-channel cells.
3. To reduce co-channel interference, co-channel cells must be physically
separated by a minimum distance.
4. When the size of each cell is same and the base stations transmit the same
power, the co-channel interference is independent of the transmitted power
and becomes a function of the radius of the cell (R) and the distance between
centers of the nearest co-channel cells (D).
45
Co-channel Interference and System Capacity
5. By increasing the ratio of D/R, interference is reduced from the co-channel cell.
The parameter Q = D/R, is called co-channel reuse ratio. For hexagonal
geometry
𝑄 = 𝐷Τ𝑅 = 3𝑁 (1)
A small value of Q provides larger capacity since the cluster size N is small,
whereas a large value of Q improves the transmission quality, due to a smaller co-
channel interference. A trade-off must be made between these two objectives in
cellular design.

46
Co-channel Interference and System Capacity
6. Let i0 be the number of co-channel interfering cells. Then, the signal-to-
interference ratio (S/I or SIR) for a mobile receiver which monitors a forward
channel can be expressed as
𝑆 𝑆
= 𝑖0 (2)
𝐼 σ𝑖=𝑖 𝐼𝑖
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.

47
Co-channel Interference and System Capacity
7. In a mobile radio channel, the average received power P at a distance d from the
transmitting antenna is approximated by
−𝑛
𝑑
𝑃𝑟 = 𝑃0 (3)
𝑑0
𝑑
or 𝑃𝑟 dBm = 𝑃0 dBm − 10𝑛log (4)
𝑑0
where P0 is the power received at a close reference point at a small distance d0
from the transmitting antenna, and n is called path loss exponent.
8. Now consider the forward link where the desired signal is the serving base
station and interference is due to co-channel base stations. If Di is the distance of
the ith interferer from the mobile, the received power at a given mobile due to
the ith interfering cell will be proportional to (Di)-n 48
Co-channel Interference and System Capacity
9. When the transmit power of each base station is equal and the path loss
exponent is the same throughout the coverage area, S/I for a mobile can be
approximated as
𝑆 𝑅−𝑛
= 𝑖0 (5)
𝐼 σ𝑖=𝑖 𝐷𝑖 −𝑛

49
Co-channel Interference and System Capacity
10. Considering only the first layer of interfering cells, if all the interfering base
stations are equidistant from the desired base station and if this distance is
equal to the distance D between cell centers, then equation (5) simplifies to
𝑛 𝑛
𝑆 𝐷 Τ𝑅 3𝑁
= = (6)
𝐼 𝑖0 𝑖0
11. The co-channel interference determines link performance, which in turn
dictates the frequency reuse plan and the overall capacity of cellular systems.

50
Co-channel Interference and System Capacity
Example: If a signal to interference ratio of 15 dB is required for satisfactory
forward channel performance of a cellular system, what is the frequency reuse
factor and cluster size that should be used for maximum capacity if the path loss
exponent is (a) n = 4 , (b) n = 3? Assume that there are 6 co-channels cells in the
first tier, and all of them are at the same distance from the mobile. Use suitable
approximations.
Solution: n = 4, First, let us consider a 7-cell reuse pattern. The co-channel reuse
ratio D/R = 4.583. The signal-to-noise interference ratio is given by S/I = (1/6) x
(4.583)4 = 75.3 = 18.66 dB. Since this is greater than the minimum required S/I, N =
7 can be used.
51
Co-channel Interference and System Capacity
n = 3, First, let us consider a 7-cell reuse pattern. The signal-to-interference ratio is
given by S/I = (l/6) x (4.583)3 = 16.04 = 12.05 dB.
Since this is less than the minimum required S/I, we need to use a larger N. For
N=12, the corresponding co-channel ratio is given by equation (2.4) D/R = 6.0. The
signal-to-interference ratio is given by S/I = (1/6) x 63 = 36 = 15.56 dB. Since this is
greater than the minimum required S/I, N = 12 can be used.

52
Adjacent Channel Interference
1. Interference resulting from signals which are adjacent in frequency to the
desired signal is called adjacent channel interference. Adjacent channel
interference results from imperfect receiver filters which allow nearby
frequencies to leak into the passband.
2. The problem is serious if an adjacent channel user is transmitting in very close
range to a subscriber's receiver, while the receiver attempts to receive a base
station on the desired channel. This is referred to as the near-far effect in which
nearby transmitter captures the receiver of the subscriber. The near-far effect
also occurs when a mobile close to a base station transmits on a channel close to
one being used by a weak mobile.
3. Adjacent channel interference can be minimized by careful filtering and channel
assignments. 53
Adjacent Channel Interference
1. By sequentially assigning successive channels in the frequency band to different
cells we can separate adjacent channels in a cell by N channel bandwidths,
where N is the cluster size.
2. Large enough channel separation is needed to bring the adjacent channel
interference to an acceptable level, or tighter base station filters are needed. In
practice, each base station receiver is preceded by a high Q cavity filter to reject
adjacent channel interference.

54
Power Control for Reducing Interference
1. In cellular systems the power levels transmitted by every subscriber unit are
under constant control by the serving base stations. This is done to ensure that
each mobile transmits the smallest power necessary to maintain a good quality
link on the reverse channel.
2. Power control not only increases battery life for subscriber unit, but also
significantly reduces the interference on reverse channel in the system.
3. Power control is especially important for CDMA spread spectrum systems that
allow every user in every cell to share the same radio channel.

55
Improving Capacity in Cellular Systems
As the demand for wireless service increases, the number of channels assigned to a
cell becomes insufficient to support the required number of users. At this point,
cellular design techniques are needed to provide more channels per unit coverage
area. Following techniques are used for increasing capacity
a. Cell splitting
b. Sectoring
c. Coverage zone approach

56
Cell Splitting
1. Cell splitting is the process of subdividing a congested cell into smaller cells, each
with its own base station and a corresponding reduction in antenna height and
transmitter power. Capacity increases due to the additional number of channels
per unit area.
2. Assume that the radius of every cell is reduced to half. To cover the service area
with smaller cells, approximately four times as many cells (and clusters) would
be required. That would increase the number of channels, and thus capacity, in
the coverage area.
3. Cell splitting is done without affecting the channel allocation scheme required to
maintain the minimum co-channel reuse ratio Q between co-channel cells.

57
Cell Splitting
4. For the new cells to be smaller in size, the transmit power of these cells must
be reduced. The transmit power of the new cells will be 1/16 of larger call
(assuming n=4). This is necessary to ensure that the frequency reuse plan for
the new microcells behaves exactly as for the original cell.
5. In practice, not all cells are split at the same time. Therefore, different cell sizes
will exist simultaneously. In such situations, special care should to be taken to
keep the distance between co-channel cells at the required minimum, and hence
channel assignments become more complicated.
6. Also, handoff issues must be addressed so that high speed and low speed traffic
can be simultaneously accommodated (the umbrella cell approach is commonly
used).
58
Cell Splitting
7. One can not simply use the original transmit power for all new cells or the new
transmit power for all the original cells. If the larger transmit power is used for
all cells, some channels used by the smaller cells would not be sufficiently
separated from co-channel cells. On the other hand, if the smaller transmit
power is used for all the cells, there would be parts of the larger cells left
unserved. For this reason, channels in the old cell must be broken down into
two channel groups, one that corresponds to the smaller cell reuse
requirements and the other that corresponds to the larger cell reuse
requirements.
8. Antenna down-tilting, which focuses radiated energy from the base station
towards the ground (rather than towards the horizon), is often used to limit the
radio coverage of newly formed microcells. 59