ECE 443: Selected Topics (1)
LEC01: INTRODUCTION TO
MOBILE COMMUNICATIONS
Lecturer: Dr. Shimaa salama
Teaching staff
Title
Selected Topics
Lecturers
Dr. Adly Tag Eldeen
Dr. Shimaa Salama
Lecturers webpages
http://www.bu.edu.eg/staff/shimaasalama3
Lecturer e-mail
shimaa.salama@feng.bu.edu.eg
Teaching assistant (TA)
Eng. Bassant
References
Multiple references will be used
Course Content
Introduction to Mobile Communications
Cellular Concepts
2G (GSM) Systems
2.5G (GPRS) Systems
2.75G (EDGE) Systems
3G (UMTS) Systems
4G (LTE Advanced)
References
▪Behrouz A. Forouzan “Data Communication and Networking” (5th
Edition), McGraw Hill, 2015.
▪Martin Sauter “Communication Systems for the Mobile Information
Society” (1st Edition), Wiley, 2006.
▪ Theodore S. Rappaport “Wireless Communications Principles and
Practice” (2nd edition), Prentice Hall,2002.
Why wireless?
Wireless telephone history
Wireless telephone history
Wireless telephone history
Cellular telephony
Cellular telephony is designed to provide communications between two moving
units, called mobile stations (MSs), or between one mobile unit and one
stationary unit, often called a land unit.
A service provider must be able to
Locate and track a caller.
Assign a channel to the call.
Transfer the channel from base station to base station as the caller moves out
of range.
Cellular telephony
❑To make this tracking, each cellular service area is divided into small regions
called cells.
❑Each cell contains an antenna and is controlled by a solar or AC powered
network station called the base station (BS).
❑Each base station is controlled by a switching office, called a mobile switching
center (MSC).
MSC [ mobile switching center]
▪Coordinates communication between all the base stations and the telephone
central office.
▪It is a computerized center that is responsible for connecting calls, recording call
information, and billing.
Cell
✓Cell size is not fixed and can be increased or decreased
depending on the population of the area.
❑High-density areas require, smaller cells to meet traffic
demands than do low-density areas.
❑Cell size is optimized to prevent the interference of
adjacent cell signals.
❑ The transmission power of each cell is kept low to
prevent its signal from interfering with those of other cells.
Transmitting
1. The caller enters a phone number and presses the send button.
2. The mobile station MS scans the band, seeking a setup channel with a strong
signal then sends the data (phone number) to the closest BS using that
channel.
3. The base station BS relays the data to the MSC.
4. The MSC sends the data on to the telephone central office.
5. If the called party is available, a connection is made and the result is relayed
back to the MSC.
6. At this point, the MSC assigns an unused voice channel to the call, and a
connection is established.
7. The MS automatically adjusts its tuning to the new channel, and
communication can begin.
Receiving
❑When a mobile phone is called, the telephone central office sends the number to
the MSC.
❑The MSC searches for the location of the mobile station by sending query
signals to each cell in a process called paging.
❑ Once the mobile station is found, the MSC transmits a ringing signal.
❑ When the mobile station answers, assigns a voice channel to the call, allowing
voice communication to begin.
Overlapping regions
Cell Shape
The actual radio coverage of a cell is known as the footprint.
✓ Footprint is determined from field measurements or
propagation prediction models.
◼ The hexagonal cell shape is a simplistic model of the radio
coverage for each base station.
◼ For hexagonal cell shape, base station transmitters are
depicted as either being in the center of the cell (centerexcited cells) or on three of the six cell vertices (cornerexcited cells).
◼ Omni-directional antennas are used in center-excited cells.
◼ Sectored directional antennas are used in corner-excited cells
Types of Cells
o Macrocells are large cells for remote and sparsely populated areas.
oMicrocells are small cells used for densely populated areas.
o Femtocell is a small low power cellular base station called access point base
station designed to improve cellular reception inside a home or office building.
oUmbrella cell
▪ Very small cells produces a number of handovers, in order to solve this
problem, the concept of umbrella cells is introduced.
▪covers several microcells and if the speed of the mobile is too high, the
mobile is handed off to the umbrella cell.
Forward Communication and Reverse
Communication
➢The channel used to carry traffic from the mobile user to a base station is called
the reverse channel (Uplink).
➢ The channel used to convey traffic to the mobile user from a base station is
called the forward channel (Downlink).
Duplex Mode
Frequency Division Duplex (FDD) mode
Time Division Duplex (TDD) mode
➢ A band of spectrum is used for uplink Single block of spectrum is used
and another band is used for downlink for uplink and downlink, but only transmits
separated by a gap.
on the uplink or the downlink at one time.
➢ FDD provides simultaneous radio
transmission channels for the subscriber
and the base station, so that they both
may
constantly
transmit
while
simultaneously receiving signals from
one another.
It is possible to share a single radio channel
in time, so that a portion of the time is used
to transmit from the base station to the
mobile, and the remaining time is used to
transmit from the mobile to the base
station.
Frequency Reuse
▪Frequency reuse or frequency planning is
the process of using the same radio
frequencies on radio transmitter sites within
a geographic area that are separated by
sufficient distance to cause minimal
interference with each other.
▪Additional radio capacity with no additional
increase in radio spectrum can be achieved
by frequency reuse principle.
▪Cells labeled with the same number use the
same group of channels.
Handoff (Handover)
Handover is the process of maintaining a mobile user’s active connections as it
changes its point of attachment (POA).
➢It may happen that, during a conversation, the mobile station moves from one
cell to another., the signal may become weak.
➢To solve this problem, The MSC monitors the level of the signal every few
seconds. If the strength of the signal diminishes, the MSC seeks a new cell.
Handovers can be categorized as imperative (forced) and
alternative (non-forced) handovers.
❑Handovers due to low link quality are imperative; because the handover process
has to be done fast in order to keep on-going connections.
❑Handovers, which are used to provide a user with better performance or to meet
a particular preference, can be considered as alternative handovers. E.g., a user
might require more bandwidth to speed up a data transfer, or need a cheaper
network to reduce the service cost.
Handoff Types
Factor A: Technologies
Involved
Factor B: Number of
Connections Involved
Factor C: Handoff
Control
Factor A: Technologies Involved
❑ Horizontal Handoff
- Handoff process of a mobile station between
cells supporting the same technology. For
example, between two neighboring base stations
of a cellular network.
❑ Vertical Handoff
- Handoff process of a mobile station between
cells supporting different technologies. For
example, between 2G and 3G cells and between
3G and WLAN cells.
Factor B: Number of Connections Involved
Hard Handoff
o Handoff that involves the
mobile station only
communicates with only one POA (base station) at any
given time.
o When mobile user moves from one cell to another,
communication must first be broken with the previous
POA before communication can be established with
the new one.
oHard handoff is known as Break Before Make.
Factor B: Number of Connections Involved
Soft Handoff
❑ Handoff that involves the mobile station can
communicate with two POAs at the same time when
moving between them.
❑ When mobile user moves from one cell to another,
a mobile station may continue with the new POA
before breaking off from the old one.
❑ Soft handoff is known as Make Before Break.
Factor C: Handoff Control
Network-Controlled Handoff (NCHO)
➢Network takes its own signal strength measurements and makes the handoff decision.
Mobile-Controlled Handoff (MCHO)
➢ Mobile station takes its own signal strength measurements and makes the handoff
decision.
Network-Assisted Handoff (NAHO)
➢ The handoff process is distributed. The network makes measurements, the mobile
station makes the decision.
Mobile-Assisted Handoff (MAHO)
➢ The
handoff process is distributed. The mobile station makes measurements, the
network makes the decision.
Handoff Strategies:
Many handoff strategies prioritize handoff requests over call initiation requests
when allocating unused channels in a cell site.
Handofts must be performed successfully and as infrequently as possible.
System designers must specify an optimum signal level at which to initiate a
handoff.
A particular signal level is specified as the minimum usable signal for
acceptable voice quality at the base station receiver, a slightly stronger signal
level is used as a threshold at which a handoff is made.
Handoff Strategies:
Handoff Strategies:
Handoff Strategies:
Improper handoff means that a handoff is not made, so the call is dropped.
This dropped call event can happen when there is an excessive delay by the
MSC in assigning a handoff, or when the threshold is set too small for the
handoff time in the system.
Excessive delays may occur during high traffic conditions at the MSC or due
to the fact that no channels are available on any of the nearby base stations.
Roaming
▪Roaming means, that a user can have access to communication or can be
reached where there is coverage.
▪A service provider usually has limited coverage. When a mobile station enters
another network of neighboring service providers, it can be allowed to use the
services of this service provider through a roaming contract. [NATIONAL
ROAMING]
▪ Roaming gives a service also for users traveling outside the area of their
“home” operator. [INTERNATIONAL ROAMING]