a.
Define Cellular System
A cellular system is a communication network that divides a large geographical area into
smaller areas called cells. Each cell has its own base station that communicates with mobile
users inside that cell. This system allows efficient use of limited radio frequencies by reusing
them in different cells that are far enough apart.
b. What is a Cluster?
A cluster is a group of adjacent cells in a cellular system where each cell uses a unique set of
frequencies to avoid interference. Clusters repeat their frequency sets throughout the
system to maximize frequency reuse while minimizing interference.
c. What is the Need for Diversity?
Diversity is used in wireless communication to improve signal quality and reliability. The
need arises because the wireless signal can be affected by fading, interference, and
obstacles. Diversity means using multiple antennas, paths, or frequencies to send/receive
the same signal, so if one path is bad, another may be good, reducing errors and signal loss.
d. What is Channel Modeling?
Channel modeling refers to creating mathematical models to represent how a wireless
communication channel affects the transmitted signals. It helps predict the behavior of radio
waves as they travel, considering factors like reflection, diffraction, scattering, and fading.
This is important for designing and testing communication systems.
e. What is Pure ALOHA?
Pure ALOHA is a simple random access protocol used in communication networks where
devices transmit data whenever they have data to send. If the data collides with another
transmission, the sender waits a random time and retransmits. It’s easy but inefficient, as
many collisions happen, leading to low throughput.
f. Define SDMA
SDMA (Space Division Multiple Access) is a technique where multiple users share the same
frequency channel but are separated by their physical location or space. Using directional
antennas or beamforming, SDMA allows multiple communications simultaneously in the
same frequency band without interfering.
g. List GSM Services
GSM (Global System for Mobile communications) provides the following main services:
Telephony services: Voice calls
Data services: SMS (Short Message Service), MMS (Multimedia Messaging Service), and
Internet access
Supplementary services: Call forwarding, call waiting, call barring, and caller ID
Emergency services
Roaming services (use network in different countries)
Security services: Authentication and encryption
h. What is IMSI and TMSI?
IMSI (International Mobile Subscriber Identity): A unique number assigned to each mobile
subscriber stored in the SIM card, used to identify the user on the network.
TMSI (Temporary Mobile Subscriber Identity): A temporary identifier assigned to a mobile
user by the network to protect the IMSI from being exposed frequently, enhancing privacy.
i. Define Reduction Factor
Reduction factor refers to the factor by which a system reduces the number of channels or
resources needed. In cellular communication, it often means how much the frequency reuse
and other techniques reduce the total number of frequencies required for coverage.
j. List Disadvantages of Cell Splitting (Explained Simply)
Cell splitting is a technique to increase capacity by dividing a large cell into smaller cells, each
with its own base station.
Disadvantages:
Increased Infrastructure Cost: More base stations mean more equipment, more
maintenance, and higher cost.
Complex Frequency Management: Smaller cells mean more cells to manage frequencies for,
increasing the complexity of avoiding interference.
Increased Handoff Rate: Mobile users moving between many small cells require frequent
handoffs, which can cause call drops if not managed well.
Interference Issues: With cells closer together, interference between neighboring cells can
increase if frequency planning is not done carefully.
Power Control Problems: Smaller cells need lower power transmitters, but managing power
levels precisely becomes harder, which affects signal quality.
a) How much is increased in spectrum efficiency offered by 2G compared to 1G?
Spectrum efficiency means how well a communication system uses the available frequency
spectrum.
1G (First Generation) systems were analog and had low spectrum efficiency.
2G (Second Generation) systems use digital techniques like TDMA and CDMA, which can
pack more users into the same bandwidth.
Typically, 2G offers about 3 to 5 times better spectrum efficiency than 1G. This means 2G
can support 3 to 5 times more users in the same frequency band compared to 1G.
b) If the cluster size N is reduced while the cell size is kept constant then what is the effect
on capacity?
Cluster size (N) is the number of cells in a cluster, each using unique frequencies to avoid
interference.
If you reduce the cluster size, it means frequencies are reused more often.
Since the cell size stays the same, the frequency reuse distance decreases, so you can fit
more cells using the same frequencies in the area.
Effect on capacity: The system capacity increases because more users can be accommodated
due to more frequent reuse of frequencies. However, reducing cluster size too much can
increase interference.
c) Define Dwell Time
Dwell time is the amount of time a mobile station stays connected or "dwells" on a
particular channel or cell before moving to another cell or channel.
In mobile communication, dwell time can also refer to the time spent on a frequency in
frequency hopping systems or the time during which a user stays in a particular coverage
area.
d) How will you define PN sequences used in CDMA?
PN sequences (Pseudo Noise sequences) are special binary sequences used in CDMA (Code
Division Multiple Access) systems.
They look like random noise but are deterministic and repeatable.
These sequences help spread the signal over a wide frequency band and allow multiple users
to share the same channel simultaneously by assigning different PN codes.
The receiver uses the same PN sequence to decode the intended signal, while other signals
appear as noise.
e) What is Near-Far Problem?
The near-far problem occurs in CDMA systems when a mobile device close to the base
station transmits a strong signal, while a device farther away transmits a weaker signal.
The strong signal can overpower or mask the weaker signal, making it difficult for the base
station to detect the weaker user's signal correctly.
This causes interference and reduces communication quality for users far from the base
station.
Power control is used to solve this problem by adjusting transmission power.
f) Write the Services Provided by GSM
GSM offers various services including:
Telephony services (voice calls)
Data services (SMS, MMS, GPRS for internet)
Supplementary services (call forwarding, call waiting, call barring, caller ID)
Emergency services (emergency calls)
Roaming services (using network abroad)
Security services (authentication, encryption)
Short message service (SMS) for text communication
g) What is the Role of the Power Control Bit in CDMA?
The power control bit helps the base station tell the mobile device to either increase or
decrease its transmission power.
This is important to manage the near-far problem and reduce interference.
Proper power control ensures all users' signals arrive at the base station with similar power,
improving system performance and battery life.
h) List the GSM Channels
GSM channels are divided into two main categories:
Traffic Channels (TCH): Carry voice and user data.
Full rate (TCH/F)
Half rate (TCH/H)
Control Channels (CCH): Used for signaling and control. Examples:
Broadcast Control Channel (BCCH)
Common Control Channel (CCCH)
Paging Channel (PCH)
Random Access Channel (RACH)
Stand-alone Dedicated Control Channel (SDCCH)
i) Define Channel Modelling
Channel modeling is the process of creating a mathematical representation of how the
wireless communication channel affects transmitted signals.
It includes effects like path loss, fading, multipath, and delay.
Channel models help engineers design systems that can handle real-world signal distortions
and improve reliability.
j) What is the Reduction Factor?
The reduction factor is the amount by which the number of required communication
channels is reduced in a system due to using techniques like frequency reuse or multiplexing.
For example, in cellular systems, using frequency reuse and other techniques reduces the
total spectrum needed, effectively increasing capacity without needing new frequencies.
a. Define Frequency Reuse and Frequency Reuse Factor
Frequency reuse: It is the practice of using the same frequency channels in different cells
that are separated by enough distance to avoid interference. This allows efficient use of the
limited radio spectrum.
Frequency reuse factor: It is the fraction of the total available spectrum that is allocated to
each cell in a cluster. It is given by 1N\frac{1}{N}N1 where NNN is the cluster size (number
of cells in the cluster). For example, if N=7N=7N=7, the reuse factor is 1/71/71/7.
b. What is Co-channel Interference?
Co-channel interference (CCI) occurs when the same frequency is used in different cells (due
to frequency reuse), and signals from one cell interfere with signals from another cell using
the same frequency. It is a major limiting factor in cellular system performance.
c. Define Doppler Spread and Coherence Time
Doppler spread: It measures the range of frequency shifts caused by relative movement
between the transmitter and receiver (like a moving phone user). Higher Doppler spread
means the signal’s frequency changes quickly.
Coherence time: It is the time duration over which the channel's characteristics remain
approximately constant. If the Doppler spread is high, coherence time is short.
d. Define Handoff and Its Types
Handoff: The process of transferring an ongoing call or data session from one cell’s base
station to another without interruption as a mobile user moves.
Types of handoff:
Hard handoff: The old connection is broken before a new connection is made. (Break-
before-make)
Soft handoff: The new connection is made before breaking the old one. (Make-before-break)
Horizontal handoff: Between cells of the same technology.
Vertical handoff: Between cells of different technologies (e.g., 4G to Wi-Fi).
e. Define Blocked Call and Call Dropped Rate
Blocked call: A call that cannot be connected because all channels are busy. The user hears a
busy tone.
Call dropped rate: The percentage of ongoing calls that are unintentionally disconnected due
to issues like handoff failure, interference, or signal loss.
f. What is the Role of Mobile Switching Centre (MSC) in Cellular System?
The Mobile Switching Centre (MSC) is the central component in a cellular network
responsible for:
Call routing and switching between mobile users and between mobile and fixed networks.
Managing mobility like handoffs and location tracking.
Billing and subscriber data management.
Connecting calls to the Public Switched Telephone Network (PSTN).
g. Define AMPS and ETACS
AMPS (Advanced Mobile Phone System): An analog cellular system used in the 1st
generation (1G) cellular networks mainly in North America. It uses FDMA for channel
allocation.
ETACS (Extended Total Access Communication System): An extended version of TACS, an
analog system used primarily in Europe, with more channels and larger coverage than the
original.
h. Why We Use Directional Antenna Instead of Omni-directional One in Cell?
Directional antennas focus the radio signal in a specific direction, increasing coverage
distance and reducing interference to/from other cells.
Omni-directional antennas radiate equally in all directions, which can cause more
interference and less efficient coverage in cellular systems.
Directional antennas improve frequency reuse and system capacity.
i. Differentiate Between TDMA and FDMA
TDMA (Time Division Multiple FDMA (Frequency Division Multiple
Feature
Access) Access)
Shares channel by dividing time
Access method Shares channel by dividing frequencies
into slots
Users transmit in different time Users transmit simultaneously on
Multiple users
slots different frequencies
More complex synchronization
Complexity Simpler to implement
required
Bandwidth
Better (more efficient) Less efficient
utilization
Used in GSM, DECT AMPS, ETACS
j. Differentiate Between Bluetooth and Zigbee
Feature Bluetooth Zigbee
Short-range communication for Low power, low data rate sensor
Purpose
devices like headphones, phones networks (home automation, IoT)
~10 meters (Class 2) to ~100 meters Up to 100 meters (depending on
Range
(Class 1) environment)
Data rate Up to 3 Mbps Up to 250 kbps
Power
Higher Very low power consumption
consumption
Network Piconet (star), scatternet Mesh, star, tree
Feature Bluetooth Zigbee
topology
Typical Audio streaming, file transfer,
Home automation, sensor networks
applications peripherals
