Wireless Networks

Instructor: Fatima Naseem

Computer Engineering Department, University of Engineering and Technology, Taxila

Lecture # 11

Cellular Wireless Networks

Introduction Cellular technology is foundation of mobile

wireless communications and supports users in locations which are not served easily with wired networks

Underlying technology for Mobile telephones Personal communications systems Wireless internet Wireless web applications

Principles of Cellular Networks Cellular radio technology developed to increase

capacity available, for mobile radio telephone service.

Before this mobile radio telephone service was only provided by high power tx/rx

Such systems supported only 25 channels with an effective radius of about 80km.

Way to increase capacity of system is to use lower power systems with shortened radius and use numerous tx/rx.

Cellular Systems-Basic Concepts High capacity is achieved by limiting the

coverage of each base station to a small geographic region called a cell.

Same frequencies/timeslots/codes are reused by spatially separated base stations.

A switching technique called handoff enables a call to proceed uninterrupted when one user moves from one cell to another.

Resolves problem of limited radio spectrum.

Cellular Systems-Basic Concept Neighboring base stations are assigned

different group of channels so as to minimize the interference.

By systematically spacing base stations and the channel groups may be reused as many number of times as necessary.

As demand increase, the number of base stations may be increased thereby providing additional capacity.

Cellular Network Organization Use multiple low-power transmitters (100 W or less) Range of tx is small Areas divided into cells

Each served by its own antenna Served by base station consisting of transmitter, receiver,

and control unit Each cell is allocated a Band of frequencies Adjacent cells are assigned different frequencies to avoid

interference or crosstalk. Cells sufficiently distant from each other can use same

frequency band Cells set up such that antennas of all neighbors are

equidistant (hexagonal pattern)

Cellular System Overview

Figure depicts principal elements of cellular system

Operation of Cellular System Base Station (BS) – in the approximate center of each cell is a BS.

Includes an antenna, a controller, and a number of transceivers. Controller: used handle the call process b/w mobile unit and rest of

network. At any time a number os mobile user units may be active and move within a cell and communicate with the BS.

Mobile telecommunications switching office (MTSO) Each BS is connected to a MTSO with one MTSO serving multiple

BSs. The link b/w an MTSO and a BS might be a wired link such as a fiber

optic cable link, or a wireless link.. Connects calls between mobile units MTSO is also connected to the public telecommunications network and

can make a connection b/w a fixed subscriber to the public telephone MTSO assigns voice channel, performs handoffs, monitors call for

billing information.

Operation of Cellular System Two types of channels are available b/w the

mobile unit and BS. Control channel: used to exchange

information required for setting up and maintaining a call and establishing a relationship b/w a mobile unit and nearest BS.

Traffic channel: carry a voice or data connection b/w users.

Forward and Reverse Channels Forward Voice Channels (FVC): used for voice

transmission from BS to MS. Reverse Voice Channels (RVC): Used for voice

transmission from MS to BS. Forward Control Channels (FCC): Used for initiating

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

a call from MS to BS. The FCC and RCC are also called setup channels.

Steps in an MTSO Controlled Call between Mobile Users Mobile unit initialization

When mobile unit is turned on, it scans and selects the strongest setup control channel used for system.

Cells with different frequency bands repetitively broadcast on different setup channels.

The receiver selects the strongest setup channel and monitors that channel.

With this the mobile station has automatically selected the BS antenna of the cell within which it will operate.

Then handshake takes place b/w the mobile unit and MTSO controlling this cell through the BS in this cell.

Handshake is used to identify the user and register its location. As long as the mobile station is on, scanning is repeated periodically to

account for the motion of the unit. If the unit enters a new cell, then a new BS is selected.

Steps in an MTSO Controlled Call between Mobile Users Mobile-originated call

A mobile unit originates a call by sending the number (Mobile Identification Number, MIN)of the called unit on the preselected setup channel.

The receiver of mobile unit checks if the forward channel (from BS) is idle.

If idle the mobile may transmit over the reverse channel( To base station)

BS sends request to the MTSO. Paging

MTSO attempts to complete connection MTSO sends a paging message to certain BSs depending on called

mobile number. BS sends paging signal on its own assigned setup channel.

Steps in an MTSO Controlled Call between Mobile Users Call accepted

Called mobile unit recognizes its number on the setup channel being monitored and responds to that BS, which sends the response to the MTSO.

MTSO stes up a circuit between calling and called BSs. MTSO selects available traffic channel within each BS’s cell and notifies each

BS, which in turn notifies its mobile unit (a data msg called alert is transmitted over FVC to instruct the mobile to ring).

The two mobile units tune to their respective channels. Ongoing call

While connection is maintained, two mobile stations exchange voice or data, through BSs and MTSO.

Handoff If a mobile unit moves from range of one cell to another the traffic channel has

to change . System makes this change without either interrupting the call or alerting the

user.

Additional Functions in an MTSO Controlled Call Call blocking: If all traffic channels are busy even after

multiple attempts a busy tone is returned. Call termination: When one of the users hangs up,

MTSO is informed and the traffic channels are released Call drop: during a connection if because of interference

or weak signal spots, the BS cant maintain the minimum required signal strength for a certain period of time the traffic channel is dropped and MTSO is informed.

Calls to/from fixed and remote mobile subscriber: MTSO connects to the public switched telephone network. Thus can setup calls b/w mobile user in its area, fixed subscriber via telephone network, remote MTSO.

Frequency Reuse : The Need Fixed telephone network runs wires to every

household Suppose we give every household their own

allocation of radio spectrum for analog speech of 4 kHz bandwidth

E.g. 12.5 million households each with 4 kHz=50 GHz

Clearly impractical No other services possible using radio transmission Most of the spectrum unused most of the time

Frequency Reuse Cellular radio systems rely on intelligent

allocation and reuse of channels throughout the coverage area.

Each base station is allocated a group of radio channels to be used within the small geographic area of its cell.

Neighboring base stations are given different channel allocation from ach other.

Frequency Reuse By design of antennas the coverage area is

limited within the cell and the same group of frequencies is reused to cover another cell separated by a large enough distance to keep co-channel interference within limits.

The design procedure of allocating channel groups for all of the cellular BS within a system is called Frequency reuse or Frequency Planning

Frequency Reuse Adjacent cells assigned different frequencies to

avoid interference or crosstalk Objective is to reuse frequency in nearby cells

10 to 50 frequencies assigned to each cell Transmission power controlled to limit power at that

frequency escaping to adjacent cells

Example of Frequency Reuse

The Cell Shape Circular Triangular Square Hexagonal

The Cell Shape Hexagonal cells are conceptual. For most theoretical treatment, hexagonal

model of cells is universally adopted because:

Hexagons are geometric shape that approximates a circle (for omni-directional radiations)

Using a hexagon geometry, fewest number of cells can cover the entire geographical region

Geometry of hexagons

Cochannel Cell Location The issue is to determine how many cells must intervene

between two cells using the same frequency so that the two cells don’t interfere with each other. If pattern consists of N cells and each cell has same no of

frequencies Ktotal no of frequencies Then each cell can have K/N frequencies E.g. in AMPS K=395, N-7, 395/7=57 freq for each cell Dmin dist b/w centers of cells that use same freq (co channel) Rradius of cell ddistance b/w adjacent cells Nno of cells in a repetitious pattern, such that each cell in the

pattern uses a unique band of frequencies (reuse factor)

Cochannel Cell Location In a hexagonal pattern only following values

of N are possible N = I2+J2+(I*J), I, J = 0,1,2,3,… Hence possible values of N are 1,3,4,7, 9, 12,

….etc. Following relationship holds D/R= (3N)1/2

D/d = (N)1/2

Approaches to Cope with Increasing Capacity As more customers use the system , traffic may build up so that

there are not enough frequencies assigned to a cell to handle its calls. Following are a few approaches to cope with this situation: Adding new channels: when a system is set up , not all the channels are

used. Growth and expansion can be handled in an orderly fashion by adding new channels.

Frequency borrowing – frequencies are taken from adjacent cells by congested cells. Frequencies can also dynamically be assigned

Cell splitting – cells in areas of high usage can be split into smaller cells Cell sectoring – cells are divided into a number of wedge-shaped

sectors, each with their own set of channels Microcells – antennas move to buildings, hills, and lamp posts

Cell Splitting Distribution of traffic is not uniform Cells in areas of high usage can be split into

smaller cells Originally the cells are about 6.5 to 13 km in

size. Min size can be upto 1.5 km To use a smaller cell the power level used

must be reduced to keep the signal within the cell

Cell Splitting Handoff: as MS moves from one cell to another it requires

transferring of the call from one base transceiver to another. This is called handoff.

As the cells become smaller the handoffs become more frequent A radius reduction by factor of F reduces the coverage area and increases the required number of base stations by a factor of F2

as cells become smaller, antennas move from top of tall buildings top of small buildings and lamp posts, wherethey form microcells. Microcells are useful in city streets, in congested areas, inside large public buildings.