Chapter 1

Cellular Wireless Networks

Introduction to Mobile Computing
Mobile Computing is used to describe technologies that enable people to access network services
anyplace, anytime, and anywhere.
Mobile computing can be defined as a computing environment over physical mobility. The user
of the mobile computing environment will be able to access data, information or logical objects
from any device in any network while on move. A computing environment is defined as mobile
if it supports one or more of these characteristics:
1. User mobility: User should be able to move from one physical location to another
location and use same service
2. Network mobility: User should be able to move from one network to another network and
use same service.
3. Device mobility: User should be able to move from one device to another and use same
service
4. Session mobility: A user session should be able to move from one user-agent
environment to another.
5. Service mobility: User should be able to move from one service to another.
6. Host mobility: The user should be either a client or server.
The mobile computing functions can be logically divided into following major segments
1. User with device: The user device, this could be fixed device like desktop computer in
office or a portable device like mobile phone. E.g. Laptop Computers, Desktop
Computers, Fixed Telephones, Mobile Phones, Digital TV with set top box, palmtop
computers, pocket PCs, two way pagers, handheld terminals etc.
2. Network: Whenever a user is mobile, we will be using different networks at different
places at different time. E.g. GSM, CDMA, Ethernet, Wireless LAN, and Bluetooth etc.
3. Gateways: This is required to interface different transport bearers. These gateways
convert one specific transport bearer. These gateways convert one specific transport
bearer to another transport bearer.
Example: From a fixed phone (with voice interface) we access a service by pressing different
keys on the telephone.
 These keys generate DTMF (Dual Tone Multi Frequency) signals.
 These analog signals are converted into digital data by Interactive Voice Response
(IVR) gateway to interface with a computer application
 Other Examples will be wireless application protocol (WAP) Gateway, SMS
Gateway etc.
4. Middle Ware: This is more of a function rather than a separate visible node. In the
present context middleware handles the presentation and rendering of the context on a
particular device. It will also handle the security and personalization for different users.
5. Content: This is the domain where the origin server and content. This could be an
application, system or even an aggregation of systems. The content can be mass market,
personal or corporate content. Origin Server will have some means to accessing the
database and the storage devices.

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Cellular Systems
Wireless communications are especially useful for mobile applications, so wireless systems are
often designed to cover large areas by splitting them into many smaller cells. This cellular
approach introduces many difficulties such as how to avoid interference, or how to hand-over
from one cell to another, while maintaining good service quality. Coverage, capacity,
interference, and spectrum reuse are important concerns of cellular systems.

Cellular Concepts
Providing wireless service over wide areas requires different schemes to efficiently use spectrum
in different locations while avoiding interference.
Cellular Network:-There are many types of cellular services. Cellular network/telephony is a
radio-based technology; radio waves are electromagnetic waves that antennas propagate. Most
signals are in the 850 MHz, 900 MHz, 1800 MHz, and 1900 MHz frequency bands.

Clusters

A cluster is a grouping of cells in which each cell uses different frequencies. A cell’s frequencies
may be reused by other cells in the system, but those cells will be in other clusters and therefore
sufficiently far away not to cause interference.

Frequency Reuse
Frequency Channels are resources which service providers should manage efficiently. It is
necessary that maximum area should be covered or maximum number of users is served with the
limited frequency spectrum. One of the most important techniques to ensure this is Frequency
reuse concept.
The design process of selecting and allocating channel groups for all of the cellular base stations
within a system is called frequency planning. The same set of frequency is reused after a specific
distance to ensure increase in capacity and coverage.

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 Figure 1: Frequency Reuse and cluster formation
In Figure 1, all cells marked as ‘Cell 1’ will be allotted the same group of channels. I.e. cells
which have been given the same number in the diagram have the same group of channels. Cells
which have been allotted the same group of frequency channels are called Co-channel cells.
Cells 1-Cell 7 has unique channels and there are no repetitions. Group of cells in which every
channel is unique is called as a Cluster.
Since co-channel cells use the same set of channels, there is always possibility of interference in
these cells. Interference between the co-channel cells is called as Co-channel interference. There
should be a minimum Distance after which the same channel can be reused with minimum
interference. This distance is called as Minimum safe distance and is given by,

Where N is the Cluster size and R is the Radius of each cell.

The number of cells after which a frequency channel can be reused is called as the Frequency
reuse factor (R.F). It is given by R.F=1/N, Where N is the cluster size.

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If D is the minimum safe distance and R is the radius of each cell, then the ratio of D/R is termed
as Reuse factor Q and is given by

The Reuse Factor Q has a very important significance in deciding the capacity improvement
techniques.

Determination of Co-channel Cells Graphically

The hexagonal geometry of cells has exactly six equidistant neighbors. In order to tessellate—to
connect without gaps between adjacent cells—the geometry of hexagons is such that the number
of cells per cluster, N, can only have values which satisfy the equation N=i2+ij+j2, where i and j
are non-negative integers. Taking various values of i and j, standard cluster sizes are found to be.
3, 4, 7, 12, 19 and so on. Out of all these values, 4 is a cluster size which does not follow the
empirical formula and still used. This is due to the fact that service providers were already using
4 as a cluster size with good performance of the system before this formula was proposed by AT
and T labs. Cluster size of 4 was also allowed by FCC so that the existing infrastructure of some
service providers was not disturbed.
To find the nearest co-channel neighbors of a particular cell, one must do the following:

 Move i cells along any chain of hexagons and then

 Turn 60 degrees counter-clockwise and move j cells.

This is illustrated in Figure 2 for i = 3 and j = 2, (Cluster size N = 19).

Figure 2: Graphical Method of Determination of Co-channel cells

Effect of Cluster Size on Capacity

Capacity can be understood as number of calls that can be handled by a service provider at a
particular time. Consider that a service operator has a total of 700 duplex channels available for
use. If he allots each cell 100 unique channels, then distribution to 7 different cells will be

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possible. Now, suppose the service provider reuses his channels at least 50 different times, the
capacity can be expressed as 50×100×7=3500050×100×7=35000 .users or calls.
Let S be the total number of channels available, k be number of channels allotted to each cell and
N is the cluster size. Then, the relation between the three parameters is given by the equation,

If the cluster is replicated M number of times, the Capacity can be expressed as

Thus, the capacity of a cellular system is directly proportional to the number of times a cluster is
replicated in a fixed service area. 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 (a larger
value of C) is achieved. A large cluster size indicates that the ratio between the cell radius and
the distance between co-channel cells is small. Conversely, a small cluster size indicates that co-
channel cells are located much closer together. The value for N is a function of how much
interference a mobile or base station can tolerate while maintaining a sufficient quality of
communication. From a design viewpoint, the smallest possible value of N is desirable in order
to maximize capacity over a given coverage area.
Numerical:
1) Out of 7, 14 and 19, which of them is not a valid cluster size?
N=i2+j2+ij for valued cluster.
7=22+12+2.1
19=32+22+3.2
There is no value for {i, j} such that i2+j2+ij=14
Hence, we conclude that 14 is not a valid cluster size.
Channel assignment strategies
Channel assignment means to allocate the available channels to the cells in a cellular system.
When a user wants to make a call request then by using channel allocation strategies their
requests are fulfilled. Channel Allocation Strategies are designed in such a way that there is
efficient use of frequencies, time slots and bandwidth.
 Channel Assignment strategies types are:
 Fixed Channel Assignment(FCA)
 Dynamic Channel Assignment (DCA)
 Borrowing Channel Assignment (BCA)
 Hybrid Channel Assignment (HCA):
Fixed Channel Assignment (FCA)
 Each cell is allocated a predetermined set of voice channel
 Any new call attempt can only be served by the unused
channels in the cell.
 The call will be blocked if all channels in that cell are
occupied
 Borrowing strategy is a type of fixed channel assignment
strategy.

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  In this the cell is allowed to borrow channels from
neighboring cell if all of its own channels are already
occupied.
 The MSC (Mobile switching Centre) supervises such
borrowing procedures and ensures that borrowing of a
channel does not disrupt or interfere with any of the calls in
progress in the donor cell.
 Maximize Frequency reuse.

In cell A 20 Channels or Voice channels are allocated. If all channels are occupied and user
makes a call then the call is blocked. Borrowing Channels handles this type of problem. These
cells borrow channels from other cells.
Advantages of FCA:
 Simple to implement and manage.
 Does not require complex equipment or algorithms
Disadvantages of FCA:
 Limited channel utilization as unused channels remains unused.
 Susceptible to interference and congestion (extremely crowded and
blocked with traffic).
Dynamic Channel Assignment (DCA)
 Channels are not allocated to cells permanently.
 Mobile Switching Centre (MSC) allocates channels based
on request.
 Reduce the likelihood of blocking, increase capacity.
 This requires the MSC to collect real time data on
channel occupancy, traffic distribution & Radio Signal
strength Indications (RSSI) of all channels on a
continuous basis.
Advantages of DCA:
 Efficient use of available bandwidth.
 Reduces call blocking and improves call quality.
 Allows for dynamic allocation of resources.
Disadvantages of DCA:
 Requires more complex equipment and algorithms.
 May result in call drops or poor quality if resources are not available
Borrowing Channel Assignment (BCA):

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  When a cell experiences high traffic demand and all of its channels
are occupied, it can borrow channels from neighboring cells that
are not being used at that time.
 Assigned to the busy cell and are used to support the additional
traffic demand. Once the demand subsides, the borrowed channels
are released and returned to their home cell.
 It can be implemented manually or automatically using algorithms
or policies but main disadvantage is that if the borrowed channel is
reclaimed by the original cell the call drop may occur.
Advantages of BCA:
 Efficient use of available bandwidth.
 Reduces call blocking and improves call quality.
Disadvantages of BCA:
 Increases interference between cells.
 Can cause call drops if borrowed channels are reclaimed by the home cell.
Hybrid Channel Assignment (HCA):
 Combination of both Fixed Channel Allocation (FCA) and
Dynamic Channel Allocation (DCA).
 The total numbers of channels or voice channels are divided into
fixed and dynamic set.
 When a user make a call then first fixed set of channels are
utilized but if all the fixed sets are busy then dynamic sets are
used. The main purpose of HCA is to work efficiently under
heavy traffic.
Advantages of HCA:
 Provides the benefits of both FCA and DCA.
 Allows for dynamic allocation of resources while maintaining predictable
call quality and reliability.
Disadvantages of HCA:
 Requires more complex equipment and algorithms than FCA.
 May not provide the same level of efficiency as pure DCA.

Structure of Wireless Communication Link

The Structure of a wireless communication link involves several key components that work
together and maintains the connection between the transmitter and the receiver. Every
component plays an important role in the proper transmission of data from the transmitter to the
receiver. The following block diagram demonstrates the complete structure of the wireless
communication link.

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Transmitter Side
Information Source: It is a device that contains the information, or we can say it produces
information and that information must be processed before transmitting through the propagation
channel.
Source Encoder: It is a device that compresses the data provided by the information source in
order to reduce the amount of data to be transmitted by removing the redundancies in the data.
Channel Coder: It is a device that is used to increase the reliability of the system by adding the
redundant bit (parity bit) to the coded message to protect against errors that may occur during the
transmission.
Modulator: It is a device that converts the coded message into a signal so that it can be
transmitted through the communication channel. It converts digital data into analog signals.
Multiplexer: It is a device that allows multiple signals to share a single transmission line by
combining them to a single composite signal.

Propagation Channel
It is a physical medium that carries the modulated signal. It is s the medium through which the
signal travels. It can be air, water, or any other medium that allows the signal to propagate.
Receiver Side Diversity Combiner: It is a device that combines multiple versions of the same
signal that have been transmitted through different paths or channels. It Combines all the best-
arising signals and by combining them produces the high power signal.
Equalizer: It is a device that compensates for the distortion introduced by the propagation
channel by adjusting the amplitude and phase of the received signal.
Demodulator: A device that extracts the original coded message from the modulated signal
received from the propagation channel. It is a device that converts the analog signal to digital
data.
Channel Decoder: It is a device that corrects the errors in the coded message transmitted by the
information source. It uses various techniques in order to correct the errors that occur due to
noise and various other causes.
Source Decoder: It is a device that performs the opposite of the work performed by the source
encoder; it decompresses the coded message back to its original format in order to recover the
original data.
Information Sink: A device that receives and processes the information transmitted by the
information source.
SDMA-TDMA-FDMA-CDMA

Access methods are multiplexing techniques that provide communications services to multiple
users in a single-bandwidth wired or wireless medium. Communications channels, whether
they’re wireless spectrum segments or cable connections, are expensive. Communications
services providers must engage multiple paid users over limited resources to make a profit.
Access methods allow many users to share these limited channels to provide the economy of
scale necessary for a successful communications business. There are five basic access or
multiplexing methods: frequency division multiple access (FDMA), time division multiple
access (TDMA), code division multiple access (CDMA), and spatial division multiple access
(SDMA).

The multiplexing methods for FDMA, TDMA, CDMA, and SDMA are all generally similar.

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FDMA

FDMA is the process of dividing one channel or bandwidth into multiple individual bands, each
for use by a single user (Fig. below). Each individual band or channel is wide enough to
accommodate the signal spectra of the transmissions to be propagated. The data to be transmitted
is modulated on to each subcarrier, and all of them are linearly mixed together.

FDMA divides the shared medium bandwidth into individual channels. Subcarriers modulated by
the information to be transmitted occupy each sub channel.
The best example of this is the cable television system. The medium is a single coax cable that is
used to broadcast hundreds of channels of video/audio programming to homes. The coax cable
has a useful bandwidth from about 4 MHz to 1 GHz. This bandwidth is divided up into 6-MHz
wide channels. Initially, one TV station or channel used a single 6-MHz band. But with digital
techniques, multiple TV channels may share a single band today thanks to compression and
multiplexing techniques used in each channel.

This technique is also used in fiber optic communications systems. A single fiber optic cable has
enormous bandwidth that can be subdivided to provide FDMA. Different data or information
sources are each assigned a different light frequency for transmission. Light generally isn’t
referred to by frequency but by its wavelength (λ). As a result, fiber optic FDMA is called
wavelength division multiple access (WDMA) or just wavelength division multiplexing (WDM).

One of the older FDMA systems is the original analog telephone system, which used a hierarchy
of frequency multiplex techniques to put multiple telephone calls on single line. The analog 300-
Hz to 3400-Hz voice signals were used to modulate subcarriers in 12 channels from 60 kHz to
108 kHz. Modulator/mixers created single sideband (SSB) signals, both upper and lower
sidebands. These subcarriers were then further frequency multiplexed on subcarriers in the 312-
kHz to 552-kHz range using the same modulation methods. At the receiving end of the system,
the signals were sorted out and recovered with filters and demodulators.

Original aerospace telemetry systems used an FDMA system to accommodate multiple sensor
data on a single radio channel. Early satellite systems shared individual 36-MHz bandwidth
transponders in the 4-GHz to 6-GHz range with multiple voice, video, or data signals via FDMA.
Today, all of these applications use TDMA digital techniques.

TDMA

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TDMA is a digital technique that divides a single channel or band into time slots. Each time slot
is used to transmit one byte or another digital segment of each signal in sequential serial data
format. This technique works well with slow voice data signals, but it’s also useful for
compressed video and other high-speed data.

A good example is the widely used T1 transmission system, which has been used for years in the
telecom industry. T1 lines carry up to 24 individual voice telephone calls on a single line (Fig.
below). Each voice signal usually covers 300 Hz to 3000 Hz and is digitized at an 8-kHz rate,
which is just a bit more than the minimal Nyquist rate of two times the highest-frequency
component needed to retain all the analog content.

This T1
digital telephony frame illustrates TDM and TDMA. Each time slot is allocated to one user. The
high data rate makes the user unaware of the lack of simultaneity.

The digitized voice appears as individual serial bytes that occur at a 64-kHz rate, and 24 of these
bytes are interleaved, producing one T1 frame of data. The frame occurs at a 1.536-MHz rate (24
by 64 kHz) for a total of 192 bits. A single synchronizing bit is added for timing purposes for an
overall data rate of 1.544 Mbits/s. At the receiving end, the individual voice bytes are recovered
at the 64-kHz rate and passed through a digital-to-analog converter (DAC) that reproduces the
analog voice.

The basic GSM (Global System of Mobile Communications) cellular phone system is TDMA-
based. It divides up the radio spectrum into 200-kHz bands and then uses time division
techniques to put eight voice calls into one channel. Figure below shows one frame of a GSM
TDMA signal. The eight time slots can be voice signals or data such as texts or e-mails. The
frame is transmitted at a 270-kbit/s rate using Gaussian minimum shift keying (GMSK), which is
a form of frequency shift keying (FSK) modulation.

This GSM digital cellular method shows how up to eight users can share a 200-kHz channel in
different time slots within a frame of 1248 bits

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CDMA

CDMA is another pure digital technique. It is also known as spread spectrum because it takes the
digitized version of an analog signal and spreads it out over a wider bandwidth at a lower power
level. This method is called direct sequence spread spectrum (DSSS) as well (Fig. below). The
digitized and compressed voice signal in serial data form is spread by processing it in an XOR
circuit along with a chipping signal at a much higher frequency. In the cdma IS-95 standard, a
1.2288-Mbit/s chipping signal spreads the digitized compressed voice at 13 kbits/s.

Spread spectrum is the technique of CDMA. The compressed and digitized voice signal is
processed in an XOR logic circuit along with a higher-frequency coded chipping signal. The
result is that the digital voice is spread over a much wider bandwidth that can be shared with
other users using different codes.

The chipping signal is derived from a pseudorandom code generator that assigns a unique code
to each user of the channel. This code spreads the voice signal over a bandwidth of 1.25 MHz
The resulting signal is at a low power level and appears more like noise. Many such signals can
occupy the same channel simultaneously. For example, using 64 unique chipping codes allows
up to 64 users to occupy the same 1.25-MHz channel at the same time. At the receiver, a
correlating circuit finds and identifies a specific caller’s code and recovers it.

The third generation (3G) cell-phone technology called wideband CDMA (WCDMA) uses a
similar method with compressed voice and 3.84-Mbit/s chipping codes in a 5-MHz channel to
allow multiple users to share the same band.

SDMA

SDMA uses physical separation methods that permit the sharing of wireless channels. For
instance, a single channel may be used simultaneously if the users are spaced far enough from
one another to avoid interference. Known as frequency reuse, the method is widely used in
cellular radio systems. Cell sites are spaced from one another to minimize interference.

In addition to spacing, directional antennas are used to avoid interference. Most cell sites use
three antennas to create 120° sectors that allow frequency sharing (Fig. below a). New

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technologies like smart antennas or adaptive arrays use dynamic beam forming to shrink signals
into narrow beams that can be focused on specific users, excluding all others (Fig. below b).

SDMA separates users on shared frequencies by isolating them with directional antennas. Most
cell sites have three antenna arrays to separate their coverage into isolated 120° sectors (a).
Adaptive arrays use beam forming to pinpoint desired users while ignoring any others on the
same frequency (b).

One unique variation of SDMA, polarization division multiple access (PDMA), separates signals
by using different polarizations of the antennas. Two different signals then can use the same
frequency, one transmitting a vertically polarized signal and the other transmitting a horizontally
polarized signal.

The signals won’t interfere with one another even if they’re on the same frequency because
they’re orthogonal and the antennas won’t respond to the oppositely polarized signal. Separate
vertical and horizontal receiver antennas are used to recover the two orthogonal signals. This
technique is widely used in satellite systems.

Polarization is also used for multiplexing in fiber optic systems. The new 100-Gbit/s systems use
dual polarization quadrature phase shift keying (DP-QPSK) to achieve high speeds on a single
fiber. The high-speed data is divided into two slower data streams, one using vertical light
polarization and the other horizontal light polarization. Polarization filters separate the two
signals at the transmitter and receiver and merge them back into the high-speed stream.

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