lecture2 (cellular networks)

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Cellular Wireless Networks William Stallings chapter 14 in Data and Computer Communications and Chapter 10 in Wireless Communications and Networks

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Cellular Networks Miss Rabia NoorSir Syed University of engineering and technologySSUET

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Page 1: Lecture2 (Cellular Networks)

Cellular Wireless NetworksWilliam Stallings chapter 14 in Data and Computer

Communications and Chapter 10 in Wireless Communications and Networks

Page 2: Lecture2 (Cellular Networks)

Principles of Cellular Networks• Cellular technology is the underlying

technology for mobile phones, personal communication systems, wireless networking etc.

• Developed for mobile radio telephone—Replace high power transmitter/receiver systems

• Typical support for 25 channels over 80km

—Use lower power, shorter range, more transmitters

Page 3: Lecture2 (Cellular Networks)

Cellular Network Organization• Cellular network use multiple low power

transmitters , on the order of 100W or less• Range of such a transmitter is small,

Therefore area is divided into cells—Each cell served by its own antenna—Each with own range of frequencies—Served by base station

• Transmitter, receiver, control unit

—Adjacent cells are assigned different frequencies to avoid interference or crosstalk

—However, cells sufficiently distant from each other can use the same frequency band

Page 4: Lecture2 (Cellular Networks)

Cellular Systems Terms• Mobile Unit (MU)• Base Station (BS)• Mobile telecommunications switching office

(MTSO)

Page 5: Lecture2 (Cellular Networks)

Operation of Cellular Systems

• Mobile Unit (MU). —Mostly Hand held device used to communicate at the user end.

• Base station (BS) —Placed at centre of each cell—Controller handles call process between the MU and the rest of

the network—Number of mobile user units may be active and moving around

within the cell, communicating with the BS—Both MU and BS contains Antenna, controller, transceivers

• MTSO:—Assigns voice channel, Performs handoffs, Monitors calls

(billing)—Connects calls between mobile units and from mobile to fixed

telecommunications network—One MTSO serves multiple BS—MTSO to BS link by wire or wireless

Page 6: Lecture2 (Cellular Networks)

Channels• Two types of channels are available between

the mobile unit and the base station (BS): • Control channels

—Setting up and maintaining calls—Establish relationship between mobile unit and

nearest BS

• Traffic channels—Carry voice and data connection between the

users

Page 7: Lecture2 (Cellular Networks)

Typical Call in Single MTSO Area

• Mobile unit initialization—Scan and select strongest set up control channel—Automatically select BS antenna of cell

• Usually but not always nearest (propagation irregularity)—Handshake to identify user and register location—Scan repeated to allow for movement

• Change of cell can happen—Mobile unit monitors for pages (see below)

• Mobile originated call—Check if set up channel is free

• Monitors the forward channel (from BS) and wait for idle state—Send number on pre-selected channel

• Paging—MTSO attempts to connect to mobile unit—Paging message sent to BSs depending on called mobile

number—Paging signal transmitted on set up channel

Page 8: Lecture2 (Cellular Networks)

Typical Call in Single MTSO Area• Call accepted

—Mobile unit recognizes number on set up channel—Responds to BS which sends response to MTSO—MTSO sets up circuit between calling and called BSs—MTSO selects available traffic channel within cells and

notifies BSs—BSs notify mobile unit of channel

• Ongoing call—Voice/data exchanged through respective BSs and MTSO

• Handoff—Mobile unit moves out of range of cell into range of another

cell—Traffic channel changes to one assigned to new BS

• Without interruption of service to user

Page 9: Lecture2 (Cellular Networks)

Call Stages

Page 10: Lecture2 (Cellular Networks)

Other Functions• Call blocking

—During mobile-initiated call stage, if all traffic channels busy, mobile tries again

—After number of fails, busy tone returned• Call termination

—User hangs up—MTSO informed—Traffic channels at two BSs released

• Call drop—BS cannot maintain required signal strength—Traffic channel dropped and MTSO informed

• Calls to/from fixed and remote mobile subscriber—MTSO connects to PSTN—MTSO can connect mobile user and fixed subscriber via PSTN—MTSO can connect to remote MTSO via PSTN or via

dedicated lines —Can connect mobile user in its area and remote mobile user

Page 11: Lecture2 (Cellular Networks)

Shape of Cells• Square

—The first design decision to make is the shape of cells to cover an area

—A matrix of square cells would be simplest layout to define

—This geometry is not ideal. —Width of the square cell is d, then cell has

four neighbours at distance d and four at distance

—Better if all adjacent antennas equidistant• Simplifies choosing and switching to new antenna

d2

Page 12: Lecture2 (Cellular Networks)

Shape of Cells• Hexagon

—Hexagonal pattern provides equidistant antennas

—Radius defined as radius of circum-circle• Distance from centre to vertex equals length of side

—For a cell radius R, the distance between the cell centre and each adjacent cell centre is d= R

—Precise hexagonal pattern is not used• Topographical limitations• Local signal propagation conditions• Practical limitation on sitting antennas

3

Page 13: Lecture2 (Cellular Networks)

Cellular Geometries

Page 14: Lecture2 (Cellular Networks)

Frequency Reuse• In a cellular system, each cell has a base

transceiver.• Transmission power is carefully controlled to

allow communication within the cell using a given frequency while limiting the power that escapes the cell into adjacent ones.

• The objective is to use the same frequency in other nearby cells thus,—Allowing the frequency to be used for multiple

simultaneous conversations—Generally 10 to 50 frequencies are assigned to

each cell depending upon the traffic expected

Page 15: Lecture2 (Cellular Networks)

Frequency Reuse• The essential issue is to determine how many cells must

intervene between two cells using the same frequency • So that the two cells do not interfere with each other• Various patterns of frequency reuse are possible

—The pattern consist of N Cells and each cell is assigned the same number of frequencies. Each cell can have K/N frequencies, where

—K is the total number of frequencies used allotted to the system

—For Advanced Mobile Phone Service (AMPS) K=395, N=7 giving 57 is the smallest pattern that can provide sufficient isolation between two uses of same frequency

—It means at most 57 frequencies per cell on average

Page 16: Lecture2 (Cellular Networks)

Frequency Reuse Patterns

Page 17: Lecture2 (Cellular Networks)

Characterizing Frequency Reuse• In characterizing frequency reuse, the following parameters

are commonly used:• D = minimum distance between centers of cells that use the

same band of frequencies (called cochannels)• R = radius of a cell• d = distance between centers of adjacent cells • N = number of cells in repetitious pattern

— Reuse factor— Each cell in pattern uses unique band of frequencies

• Hexagonal cell pattern, following values of N is possible—  N = I2 + J2 + (I x J), I, J = 0, 1, 2, 3, …

•  Possible values of N are 1, 3, 4, 7, 9, 12…. • …….13, 16, 19, 21, …• D/R=• D/d =

N3N

Rd 3

Page 18: Lecture2 (Cellular Networks)

Characterizing Frequency Reuse• As long as the cell size is fixed,

cochannel interference will be independent of the transmitted power of each cell

• The cochannel interference is a function of q where q= D/R

• An increase in q reduces co-channel interference and also the traffic capacity of the cellular system

Page 19: Lecture2 (Cellular Networks)

Mobile Radio Propagation Effects• Signal strength

—Strength of signal between BS and mobile unit should be strong enough to maintain signal quality at the receiver, But strong enough can create too much co-channel interference

—Noise can effect signal ,and it varies • Automobile ignition noise greater in city than in

suburban • Other signal sources vary • Signal strength varies as function of distance from BS • Signal strength varies dynamically as mobile unit moves

• Fading—Even if signal strength is in effective range, signal

propagation effects may disrupt the signal

Page 20: Lecture2 (Cellular Networks)

ExampleConsider four different cellular systems with the cluster of cells N= 4,7,12

and 19 are duplicated 16 times. each cell has a radius of 1.6 Km. Supposing an FDMA system with frequency bands being used are 825 to 845MHz for mobile unit transmission ,and 870 to 890 MHz for Base station transmission ,A duplex circuit consists of one 30 KHz channel in each direction Find for each system

1. The total area covered 2. The total number of frequencies used3. Number of channels per cell4. The total capacity. (Total Number of simultaneous calls)

Page 21: Lecture2 (Cellular Networks)

ExampleConsider four different cellular systems with the cluster of cells N= 4,7,12

and 19 are duplicated 16 times. each cell has a radius of 1.6 Km. Supposing an FDMA system with frequency bands being used are 825 to 845MHz for mobile unit transmission ,and 870 to 890 MHz for Base station transmission ,A duplex circuit consists of one 30 KHz channel in each direction Find for each system

1. The total area covered 2. The total number of frequencies used3. Number of channels per cell4. The total capacity. (Total Number of simultaneous calls)

Page 22: Lecture2 (Cellular Networks)

Increasing Capacity• As the number of customers increases in the system,

more traffic may build up that require enough frequencies assigned to a cell at a time to handle all calls.

• A number of approaches have been used to cope with this situation, including the following:

1.Adding new channels—When a system is setup in a region, not all of the

channels are used to start with. Growth and expansion can be managed in an orderly fashion by adding new channels

2.Frequency borrowing—Frequencies are taken from adjacent cells by

congested cells—The frequencies can also be assigned to cells

dynamically

Page 23: Lecture2 (Cellular Networks)

Increasing Capacity3. Cell splitting

—The distribution of traffic and topographic features is not uniform, and this presents opportunities for capacity increases.

—Cells in areas of high usage can be split into smaller cells

—The original cells are about 6.5 to 13 km in size• 1.5 km limit in general

—Increases Number of Base stations and handoffs• A radius Reduction by a factor of F reduces the coverage

area and increases the required number of base stations by a factor of F2

• To use a smaller cell, the mobile units move, they pass from cell to cell, which requires transferring the call from one base transceiver to another. This process is called a handoff.

• As the cell get smaller…… more frequent handoff

Page 24: Lecture2 (Cellular Networks)

Cell Splitting

Page 25: Lecture2 (Cellular Networks)

Increasing Capacity4. Cell Sectoring

—Cell divided into wedge shaped sectors—3 – 6 sectors per cell—Each with own channel set

• Subsets of cell’s channels

—Require Directional antennas

Sectoring improves S/N

Page 26: Lecture2 (Cellular Networks)

Increasing Capacity5. Microcells

—As cell become smaller, move antennas from tops of hills and large buildings to tops of small buildings and sides of large buildings

—Form microcells—Reduced power—Good for city streets, congested areas, along

roads and inside large buildings

Page 27: Lecture2 (Cellular Networks)

Typical parameters for Macrocells and Microcells

Cell radius

MACRO CELL

1 to 20 Km

MICRO CELL

0.1 to 1 Km

Transmission power

1 to 10 Watt 0.1 to 1 Watt

Average delay spread(Multipath delay)

0.1 to 10 microsec

10 to 100 nsec

Maximum bit rate 0.3Mbps 1Mbps

Page 28: Lecture2 (Cellular Networks)

Design FactorsIn design of a cellular layout, the communication

engineer must take account of these various propagation effects

• The factors that will determine the size of individual cell—Maximum transmit power level at BS and MUs—Typical height of mobile unit antenna—Available height of the BS antenna

• Unfortunately, the propagation effects are dynamic and difficult to predict

• The best that can be done is to come up with a model based on empirical data and to apply that model to a given environment to develop guidelines for cell size

Page 29: Lecture2 (Cellular Networks)

Design Factors• One of the most widely used models was

developed by Okumura et al and subsequently refined by Hata—Detailed analysis of Tokyo area—Produced path loss information for an urban

environment—Hata's model is an empirical formulation

• Takes into account variety of environments and conditions

—For an urban environment, predicted path loss is

(14.1)

Page 30: Lecture2 (Cellular Networks)

Design Factors• Where

—fc = carrier frequency in MHz from 150 to 1500MHz

—ht = height of transmitting antenna (BS) in m, from 30 to 300m

—hr = height of receiving antenna (mobile station) in m, from 1 to 10 m

—d = propagation distance between antennas in km, from 1 to 20 km

—A(hr) = correction factor for mobile antenna height

Page 31: Lecture2 (Cellular Networks)

Design Factors• For a small or medium-sized city, the

correction factor is given by

• And for a large city is given by

Page 32: Lecture2 (Cellular Networks)

Design Factors (Example)

Page 33: Lecture2 (Cellular Networks)

Design Factors• To estimate the path loss in a suburban area,

the formula for urban path loss in equation above is modified as

• And the path loss in open areas, the formula is modified as

• The Okumura/Hata model is considered to be among the best in terms of accuracy in path loss prediction and provides a practical means of estimating path loss in a wide variety of situations

Page 34: Lecture2 (Cellular Networks)

Fading• The most challenging technical problem

facing communication system engineers in a mobile environment is Fading—Caused by changes in transmission path(s)—In a fixed environment, fading is affected by

changes in atmospheric conditions such as rainfall

—But in mobile environment movement of (mobile unit) antenna

—The relative location of various obstacles changes over time, creating complex transmission effects

Page 35: Lecture2 (Cellular Networks)

Multipath Propagation• Reflection

—Occurs when an electromagnetic signal encounter a surface that is large relative to the wavelength of the signal

—May have phase shift from original—These reflected waves may interfere constructively or

destructively at the receiver

• Diffraction—Occurs at the edge of an impenetrable body that is large

compared to the wavelength of the radio wave—When a radio wave encounters such an edge, waves

propagate in different directions with the edge as the source

—Thus, the signals can be received even when there is no unobstructed LOS from the transmitter

Page 36: Lecture2 (Cellular Networks)

Multipath Propagation• Scattering

—If the size of an obstacle is on the order of the wavelength of the signal or less, scattering occurs

—An incoming signal is scattered into several weaker outgoing signals

—Lamp posts and traffic signs that can cause scattering

• If a mobile unit has a clear LOS to the transmitter , then the diffraction and scattering are generally minor effects, although reflection may have significant impact

• If there is no clear LOS, such as in urban area at street level, then diffraction and scattering are the primary means of signal reception

Page 37: Lecture2 (Cellular Networks)

Reflection, Diffraction, Scattering

Page 38: Lecture2 (Cellular Networks)

Effects of Multipath Propagation• Signals may cancel out due to phase differences• Intersymbol Interference (ISI)

—Sending narrow pulse at given frequency between fixed antenna and mobile unit

—Channel may deliver multiple copies at different times—Delayed pulses act as noise making recovery of bit

information difficult—Timing changes as mobile unit moves

• Harder to design signal processing to filter out multipath effects

• As the mobile antenna moves, the location of various obstacles changes, hence the number, magnitude, and timings of the secondary pulses change

Page 39: Lecture2 (Cellular Networks)

Two Pulses in Time-Variant Multipath

Page 40: Lecture2 (Cellular Networks)

HARD & SOFT HANDOFF

• A traditional handoff is Hard handoff i.e. “break-before-make”.

• In some cases like in CDMA Soft hand off is possible .Since all cells in CDMA use the same frequency, it is possible to make the connection to the new cell before leaving the current cell. This is known as a “make-before-break”

• Soft handoffs require less power, which reduces interference and increases capacity.

Hand off is the process of handing over the ongoing call from one base station to another without dropping the call while the MU is in move

Page 41: Lecture2 (Cellular Networks)

Handoff Performance Metrics• Handoff blocking probability – probability that a handoff

cannot be successfully completed• Handoff probability – probability that a handoff occurs

before call termination• Rate of handoff – number of handoffs per unit time• Interruption duration – duration of time during a handoff

in which a mobile is not connected to either base station• Handoff delay – distance the mobile moves from the point

at which the handoff should occur to the point at which it does occur

Page 42: Lecture2 (Cellular Networks)

Handoff Strategies Used to Determine Instant of Handoff• Relative signal strength• Relative signal strength with threshold• Relative signal strength with hysteresis• Relative signal strength with hysteresis and threshold• Prediction techniques

Page 43: Lecture2 (Cellular Networks)

Handoffs

Page 44: Lecture2 (Cellular Networks)

Umbrella Cells

Page 45: Lecture2 (Cellular Networks)

Power Control• Design issues making it desirable to include dynamic

power control in a cellular system—Received power must be sufficiently above the

background noise for effective communication

—Desirable to minimize power in the transmitted signal from the mobile

• Reduce cochannel interference, alleviate health concerns, save battery power

—In SS systems using CDMA, it’s desirable to equalize the received power level from all mobile units at the BS

Page 46: Lecture2 (Cellular Networks)

Types of Power Control• Open-loop power control

—Depends solely on mobile unit—No feedback from BS—Not as accurate as closed-loop, but can react quicker to

fluctuations in signal strength

• Closed-loop power control—Adjusts signal strength in reverse channel based on metric

of performance—BS makes power adjustment decision and communicates

to mobile on control channel

Page 47: Lecture2 (Cellular Networks)

GSM Network Architecture

Page 48: Lecture2 (Cellular Networks)

Mobile Station• Mobile station communicates across Um interface

(air interface) with base station transceiver in same cell as mobile unit

• Mobile equipment (ME) – physical terminal, such as a telephone or PCS—ME includes radio transceiver, digital signal processors

and subscriber identity module (SIM)

• GSM subscriber units are generic until SIM is inserted—SIMs roam, not necessarily the subscriber devices

Page 49: Lecture2 (Cellular Networks)

Base Station Subsystem (BSS)• BSS consists of base station controller and one or

more base transceiver stations (BTS)• Each BTS defines a single cell

—Includes radio antenna, radio transceiver and a link to a base station controller (BSC)

• BSC reserves radio frequencies, manages handoff of mobile unit from one cell to another within BSS, and controls paging

Page 50: Lecture2 (Cellular Networks)

Network Subsystem (NS)• NS provides link between cellular network and

public switched telecommunications networks—Controls handoffs between cells in different BSSs

—Authenticates users and validates accounts

—Enables worldwide roaming of mobile users

• Central element of NS is the mobile switching center (MSC)

Page 51: Lecture2 (Cellular Networks)

Mobile Switching Center (MSC) Databases• Home location register (HLR) database – stores

information about each subscriber that belongs to it• Visitor location register (VLR) database –

maintains information about subscribers currently physically in the region

• Authentication center database (AuC) – used for authentication activities, holds encryption keys

• Equipment identity register database (EIR) – keeps track of the type of equipment that exists at the mobile station

Page 52: Lecture2 (Cellular Networks)

GSM Speech Signal Processing

Page 53: Lecture2 (Cellular Networks)

GSM Signaling Protocol Architecture

Page 54: Lecture2 (Cellular Networks)

Functions Provided by Protocols• Protocols above the link layer of the GSM signaling

protocol architecture provide specific functions:—Radio resource management—Mobility management—Connection management—Mobile application part (MAP)—BTS management

Page 55: Lecture2 (Cellular Networks)

Reading Assignment :

• Generation evolvements of Wireless mobile networks : — 1G, 2G, 3G and 4G

• Find out the technical differences between them