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Page 1: Radio & Telecommunications Systems 1 Basic cellular system Propagation Cellular Radio Principle.

Radio & Telecommunications Systems 1

• Basic cellular system

• Propagation

Cellular Radio Principle

Page 2: Radio & Telecommunications Systems 1 Basic cellular system Propagation Cellular Radio Principle.

Radio & Telecommunications Systems 2

• Basic Cellular System• consists of three parts: (Fig. 1)

• mobile unit

• cell site

• mobile telephone switching office (MTSO)

Page 3: Radio & Telecommunications Systems 1 Basic cellular system Propagation Cellular Radio Principle.

Radio & Telecommunications Systems 3

Fig. 1 Cellular System

Switches andprocessor

Voice

circuits

Data link

Voice link

Dedicated

voice grade

circuit

Cell sites (radio

base station sites)

Mobile

telephone

switching

office

Local telephone network

Page 4: Radio & Telecommunications Systems 1 Basic cellular system Propagation Cellular Radio Principle.

Radio & Telecommunications Systems 4

Page 5: Radio & Telecommunications Systems 1 Basic cellular system Propagation Cellular Radio Principle.

Radio & Telecommunications Systems 5

• Mobile units unit contains

• control unit

• transceiver

• antenna system

• Cell site provides interface between the MTSO and the mobile units, it has

• control unit

• radio cabinets

• antennas

• power plant

Page 6: Radio & Telecommunications Systems 1 Basic cellular system Propagation Cellular Radio Principle.

Radio & Telecommunications Systems 6

• MTSO is the switching office, coordinating element for all cell sites, contains

• cellular processor

• cellular switch

• interfaces with telephone company zone offices

• controls call processing

• handles billing activities

• each mobile unit can only use one channel at a time for its communication link; channel is not fixed, can lie any one in the entire band assigned by the serving area

Page 7: Radio & Telecommunications Systems 1 Basic cellular system Propagation Cellular Radio Principle.

Radio & Telecommunications Systems 7

Propagation

• incident angle of the direct wave is 1 and the incident angle of the reflected wave is 2, 1 is also called the elevation angle (Fig. 2)

• C is inversely proportional to R4

C R -4 = R -4

• where

C = received carrier power

R = distance measured from the transmitter to the receiver

= is a constant

Page 8: Radio & Telecommunications Systems 1 Basic cellular system Propagation Cellular Radio Principle.

Radio & Telecommunications Systems 8

Fig.2 Mobile radio transmission

Direct path

Reflected path1

2

30 -

100m

2km or further

Page 9: Radio & Telecommunications Systems 1 Basic cellular system Propagation Cellular Radio Principle.

Radio & Telecommunications Systems 9

Fading

• antenna of the mobile unit is lower than its typical surroundings

• multi-path waves are generated

• at the mobile unit

• the sum of the multi-path waves causes a signal-fading phenomenon (Fig. 3)

Page 10: Radio & Telecommunications Systems 1 Basic cellular system Propagation Cellular Radio Principle.

Radio & Telecommunications Systems 10

Fig. 3 Typical fading signal

Page 11: Radio & Telecommunications Systems 1 Basic cellular system Propagation Cellular Radio Principle.

Radio & Telecommunications Systems 11

• signal fluctuates about 40 dB (10 dB above and 30 dB below the average signal)

• if the mobile unit moves fast, the rate of fluctuation is fast

• Rayleigh fading is also called multi-path fading in the mobile radio environment (Fig. 4)

• multi-path waves bounce back and forth due to the buildings and houses

• summed together and become an irregular way fading structure

Page 12: Radio & Telecommunications Systems 1 Basic cellular system Propagation Cellular Radio Principle.

Radio & Telecommunications Systems 12

Fig. 4 Multi-path fading

~100

Page 13: Radio & Telecommunications Systems 1 Basic cellular system Propagation Cellular Radio Principle.

Radio & Telecommunications Systems 13

Frequency Re-Use

• Reuse channels

• Reuse distance

• Cochannel interference

• Cell splitting

Page 14: Radio & Telecommunications Systems 1 Basic cellular system Propagation Cellular Radio Principle.

Radio & Telecommunications Systems 14

Concept of Frequency Reuse Channels

• radio channel consists of a pair of frequencies

• one for each direction of transmission

• for full-duplex operation

• a particular radio channel F1

• used in one geographic zone to call a cell C1

• coverage radius R

• can be used in another cell

• with the same coverage radius at a distance D away (Fig. 1)

Page 15: Radio & Telecommunications Systems 1 Basic cellular system Propagation Cellular Radio Principle.

Radio & Telecommunications Systems 15

Fig. 1 D/R ratio

f1 f2

R R

C 1 C 2

DP o P o

C/I= C/I=

q=D/R

Where q is called the cochannel interference reduction factor

Page 16: Radio & Telecommunications Systems 1 Basic cellular system Propagation Cellular Radio Principle.

Radio & Telecommunications Systems 16

Frequency reuse system

• users in different geographic locations (different cells)

• simultaneously use the same frequency channel

• increase the spectrum efficiency

• if the system is not properly designed

• serious interference may occur

• interference due to the common use of the same channel

• called co-channel interference

Page 17: Radio & Telecommunications Systems 1 Basic cellular system Propagation Cellular Radio Principle.

Radio & Telecommunications Systems 17

Frequency reuse distance

• minimum distance which allows the same frequency to be reused depend on

• number of co-channel cells in the vicinity of the center cell

• type of geographic terrain contour

• antenna height

• transmitted power at each cell site

Page 18: Radio & Telecommunications Systems 1 Basic cellular system Propagation Cellular Radio Principle.

Radio & Telecommunications Systems 18

• the frequency reuse distance D can be determined from

D = (3K)1/2R

• where K is the frequency reuse pattern

•R is the radius of cell

• assume all the cell sites transmit the same power

• if K increases

• the frequency reuse distance D increases

• increases D => reduces cochannel interference

Page 19: Radio & Telecommunications Systems 1 Basic cellular system Propagation Cellular Radio Principle.

Radio & Telecommunications Systems 19

• a large K is desired, however

• the total number of allocated channels is fixed

• when K is too large

• number of channels assigned to each of K cells becomes small

• smallest value of K is 3, obtained by setting

• i = 1, j = 1 then

• K = i2 + ij +j2

(see Fig. 2a-2d)

Page 20: Radio & Telecommunications Systems 1 Basic cellular system Propagation Cellular Radio Principle.

Radio & Telecommunications Systems 20

1

3

2

3

2

2

4

3

1

4

4

1

K=4

Fig. 2a 4-cell reuse pattern

Page 21: Radio & Telecommunications Systems 1 Basic cellular system Propagation Cellular Radio Principle.

Radio & Telecommunications Systems 21

Fig. 2b 7-cell reuse pattern

1

6 7

3 4 5

2

1

1

1

1

1

1

R D

q=D/R=4.6K=7

Page 22: Radio & Telecommunications Systems 1 Basic cellular system Propagation Cellular Radio Principle.

Radio & Telecommunications Systems 22

Fig. 2c 12-cell reuse pattern

2

3

12

11

10

9

8

7

6

5

4

1

Cell no.12

8

7

6

9

8

11

10

3

4

10

5

4 6

5

12

11

9

D/R=6K=12

Page 23: Radio & Telecommunications Systems 1 Basic cellular system Propagation Cellular Radio Principle.

Radio & Telecommunications Systems 23

Fig. 2d 19-cell reuse pattern

AA

A

A

AA

A

K=i 2+ij+j 2

split parameter i=3, j=2

Page 24: Radio & Telecommunications Systems 1 Basic cellular system Propagation Cellular Radio Principle.

Radio & Telecommunications Systems 24

Cochannel Interference Reduction Factor

• to find the minimum frequency reuse distance in order to reduce cochannel interference

• co-channel interference is a function of a parameter q defined as:

q = D / R

• where q is the cochannel interference reduction factor

• the ratio q increases, cochannel interference decreases

Page 25: Radio & Telecommunications Systems 1 Basic cellular system Propagation Cellular Radio Principle.

Radio & Telecommunications Systems 25

Operation of Cellular Systems

•Call Process

•Handoff Procedure

Page 26: Radio & Telecommunications Systems 1 Basic cellular system Propagation Cellular Radio Principle.

Radio & Telecommunications Systems 26

Call Processing

• Mobile unit initialization: power ON• user activates the receiver of the mobile unit• the receiver scans set-up channels• selects the strongest and locks on for a certain time• each site is assigned a different set-up channel

• Mobile originated call• user places the called number into an originating

register in the mobile unit• request for service is sent on a selected set-up channel

obtained from a self-location scheme

Page 27: Radio & Telecommunications Systems 1 Basic cellular system Propagation Cellular Radio Principle.

Radio & Telecommunications Systems 27

• cell site receives it, and in directional cell sites, selects the best directive antenna for the voice channel to use

• cell site sends a request to the mobile telephone switching office (MTSO)

• MTSO selects an appropriate voice channel for the call

• the cell site acts on it through the best directive antenna to link the mobile unit

• the MTSO also connects the wire-line party through the telephone company zone office

Page 28: Radio & Telecommunications Systems 1 Basic cellular system Propagation Cellular Radio Principle.

Radio & Telecommunications Systems 28

• Network originated call

• land-line party dials a mobile unit number

• telephone company forwards the call to the MTSO

• MTSO sends a paging message to certain cell sites based on the mobile unit number

• each cell site transmits the page on its own set-up channel

• mobile unit recognizes its own identification on a strong set-up channel, locks onto it, and responds to the cell site

• mobile unit also follows the instruction to tune to an assigned voice channel and initiate user alert

Page 29: Radio & Telecommunications Systems 1 Basic cellular system Propagation Cellular Radio Principle.

Radio & Telecommunications Systems 29

• Call termination

• mobile user terminates the call

• a particular signal transmits to the cell site

• both sides free the voice channel

• mobile unit resumes monitoring pages through the strongest set-up channel

Page 30: Radio & Telecommunications Systems 1 Basic cellular system Propagation Cellular Radio Principle.

Radio & Telecommunications Systems 30

Handoff procedure

• during the call, two parties are on a voice channel

• mobile unit moves out of the coverage area of a particular cell site, the reception becomes weak (Fig. 1)

• present cell site requests a handoff

• the system switches the call to a new channel in a new cell site without either interrupting the call or alerting the user

• call continues as long as the user is talking

Page 31: Radio & Telecommunications Systems 1 Basic cellular system Propagation Cellular Radio Principle.

Radio & Telecommunications Systems 31

Fig 1 Occurrence of handoff

CS: Cell Site

Page 32: Radio & Telecommunications Systems 1 Basic cellular system Propagation Cellular Radio Principle.

Radio & Telecommunications Systems 32

• Mobile Assisted Handoff (MAHO)

• the mobile receiver is capable of monitoring the signal strength of the setup channels of the neighboring cells while serving a call

• Soft Handoff• applied to CDMA systems• all cells use the same radio carrier, change from one

code to another code

Page 33: Radio & Telecommunications Systems 1 Basic cellular system Propagation Cellular Radio Principle.

Radio & Telecommunications Systems 33

Mobile and Base Stations

Structure of a Mobile Station

Structure of a Base Station

Page 34: Radio & Telecommunications Systems 1 Basic cellular system Propagation Cellular Radio Principle.

Radio & Telecommunications Systems 34

Structure of a Mobile Station

• Most portable phones are divided into two parts: (Fig. 1)

• RF part

• handles the receiving, transmitting, and modulation tasks

• digital part

• takes care of the data processing, control, and signaling functions

Page 35: Radio & Telecommunications Systems 1 Basic cellular system Propagation Cellular Radio Principle.

Radio & Telecommunications Systems 35

Structure of a Mobile Station

Page 36: Radio & Telecommunications Systems 1 Basic cellular system Propagation Cellular Radio Principle.

Radio & Telecommunications Systems 36

Fig 1 Block diagram of a mobile station

Page 37: Radio & Telecommunications Systems 1 Basic cellular system Propagation Cellular Radio Principle.

Radio & Telecommunications Systems 37

• antenna combiner

• couples the receiving and the transmitting paths onto the single antenna connector or a fixed antenna

• receiver

• contains the front end, a receiving filter network, and a mixer to down-convert the input signal onto an IF that is eventually converted into the data domain by the ADC

• equalizer

• due to multipath propagation and other reflections, the signals arriving at the receiver are distorted

• the equalizer compensate distortions

Page 38: Radio & Telecommunications Systems 1 Basic cellular system Propagation Cellular Radio Principle.

Radio & Telecommunications Systems 38

• demodulator

• extracts the bit stream from the IF

• demultiplexer

• sorts the received information from the different time slots and frames

• channel codec

• Channel coding is necessary to reduce probability of errors

Page 39: Radio & Telecommunications Systems 1 Basic cellular system Propagation Cellular Radio Principle.

Radio & Telecommunications Systems 39

• speech codec

• compresses digitized speech coming from the ADC before being encoded

• minimizes bandwidth requirement

• control and signaling unit

• performs all the control functions of the mobile station

Page 40: Radio & Telecommunications Systems 1 Basic cellular system Propagation Cellular Radio Principle.

Radio & Telecommunications Systems 40

• multiplexer

• assigns each individual burst to a time slot within a numbered frame

• modulator

• imparts information onto the IF carrier

• transmitter

• a mixer up-converts the modulated IF signals

• an amplifier increases the level of the signal

• output filters limit the bandwidth of the output to its assigned channel

Page 41: Radio & Telecommunications Systems 1 Basic cellular system Propagation Cellular Radio Principle.

Radio & Telecommunications Systems 41

• synthesizer

• provides the internal timing references for the bit and frame clock as well as for the RF sources in the transmitter and the receiver

• voltage controlled oscillator (VCO) provides a stable operating frequency

Page 42: Radio & Telecommunications Systems 1 Basic cellular system Propagation Cellular Radio Principle.

Radio & Telecommunications Systems 42

Structure of a Base Station

• The general structure of a base station consists of

• base station control function (BCF)

• one to sixteen transceivers (TRX)

Page 43: Radio & Telecommunications Systems 1 Basic cellular system Propagation Cellular Radio Principle.

Radio & Telecommunications Systems 43

Fig 2 Block diagram of a base station

Page 44: Radio & Telecommunications Systems 1 Basic cellular system Propagation Cellular Radio Principle.

Radio & Telecommunications Systems 44

• receiver

• contains the receiving filter which blocks frequencies other than the desired receiving band

• the signals are down-converted to an IF or directly to the baseband frequency, where the signals are sampled and quantized with an ADC

• equalizer

• compensates for the influences coming from the mobiles

• demodulator

• extracts the bit stream from the equalized signal and passes it to the demultiplexer

Page 45: Radio & Telecommunications Systems 1 Basic cellular system Propagation Cellular Radio Principle.

Radio & Telecommunications Systems 45

• channel codec

• detect errors that have been introduced into the RF path and correct them

• speech codec

• compresses digitized speech to minimize bandwidth requirement

• signaling unit

• the logical interface for the control messages between the network and the mobile stations

Page 46: Radio & Telecommunications Systems 1 Basic cellular system Propagation Cellular Radio Principle.

Radio & Telecommunications Systems 46

• control unit

• performs all the internal control tasks of the base station

• multiplexer

• maps the single bursts onto the single time slots bound for the individual mobile stations

• modulator

• modulates the digital signals onto the radio frequency carrier

Page 47: Radio & Telecommunications Systems 1 Basic cellular system Propagation Cellular Radio Principle.

Radio & Telecommunications Systems 47

• transmitter

• contains the output filters to band-limit the signals

• controls the output level depending on the base station's power class

• synthesizer

• provides the necessary frequencies for the different entities in the BTS (Base Transceiver Station)

• usually synchronized with the clock from the BSC (Base Station Controller)

• alternatively, it is possible to have a local clock reference in each base station

• poor system performance (high hand-off failure rate) & low reliability due to sync problem

Page 48: Radio & Telecommunications Systems 1 Basic cellular system Propagation Cellular Radio Principle.

Radio & Telecommunications Systems 48

Multiple Access Methods

•Frequency Division Multiple Access

•Time Division Multiple Access

•Code Division Multiple Access

Page 49: Radio & Telecommunications Systems 1 Basic cellular system Propagation Cellular Radio Principle.

Radio & Telecommunications Systems 49

Frequency Division Multiple Access• multiple access scheme for land mobile communication

systems (Fig. 1 - 3)

• analog cellular systems use FDMA

• assigned system bandwidth is divided into bands with its bandwidth of Wch

• guard space to prevent spectrum overlapping

• each user sends a call request to the BS

• BS assigns one of the unused channels to the user

• channel is used exclusively by that user during a call

• when the call is terminated, the channel is reassigned to a different user

Page 50: Radio & Telecommunications Systems 1 Basic cellular system Propagation Cellular Radio Principle.

Radio & Telecommunications Systems 50

Fig.1 Basic Concept of FDMA system - spectrumS

pect

rum

chl 1 chl Nchl 3chl 2

FrequencyBandwidt

h

guardspace

Systembandwidth

Page 51: Radio & Telecommunications Systems 1 Basic cellular system Propagation Cellular Radio Principle.

Radio & Telecommunications Systems 51

Fig. 2 Call initiation and holding model

User 1

User 5

User 4

User 3

User 2

time

Holdingtime

Page 52: Radio & Telecommunications Systems 1 Basic cellular system Propagation Cellular Radio Principle.

Radio & Telecommunications Systems 52

Fig. 3 FDMA channel assignment

chl 1

chl 3

chl 2

time

user 1

user 2

user 3

user 4

user 5

frequency

Page 53: Radio & Telecommunications Systems 1 Basic cellular system Propagation Cellular Radio Principle.

Radio & Telecommunications Systems 53

Time Division Multiple Access

• enables users to access the assigned bandwidth on a time basis (Fig. 4-6)

• each channel occupies the whole system bandwidth

• occupies only a fraction of the time, called slot, on a periodic basis

• one frame consists of Nch slots

• frame length is T second

Page 54: Radio & Telecommunications Systems 1 Basic cellular system Propagation Cellular Radio Principle.

Radio & Telecommunications Systems 54

Fig. 4 Concept of TDMA operation

Base station

Terminal 1

Terminal 2

Terminal 3

slot 0

slot 1

slot 0

slot 1

slot 1

slot 0

slot 1

slot 0

slot 0slot 1

slot 0slot 1

slot 1slot 0

slot 1slot 0

slot 1 slot 0 slot 1 slot 0

slot 0 slot 1 slot 0 slot 1

Page 55: Radio & Telecommunications Systems 1 Basic cellular system Propagation Cellular Radio Principle.

Radio & Telecommunications Systems 55

• Uplink

• each terminal transmits information using an assigned slot in each frame

• each terminal has to transmit its slot exactly in the assigned slot timing to prevent signal collisions

• Downlink• all the slot signals are transmitted by the BS

Page 56: Radio & Telecommunications Systems 1 Basic cellular system Propagation Cellular Radio Principle.

Radio & Telecommunications Systems 56

Fig 6 Call initiation and holding model in TDMA

User 1

User 5

User 4

User 3

User 2

time

Holdingtime

Page 57: Radio & Telecommunications Systems 1 Basic cellular system Propagation Cellular Radio Principle.

Radio & Telecommunications Systems 57

Fig 7 Slot assignment in TDMA

frequ

ency

time

Page 58: Radio & Telecommunications Systems 1 Basic cellular system Propagation Cellular Radio Principle.

Radio & Telecommunications Systems 58

Code Division Multiple Access• all the transmitted signals other than the desired signal are

regarded as cochannel interference (CCI) signals (Fig. 7 - 9)

• at the CDMA receiver

• the desired signa1 can be picked up by

• taking correlation between the received signal and a code used at the transmitter (code#1)

Page 59: Radio & Telecommunications Systems 1 Basic cellular system Propagation Cellular Radio Principle.

Radio & Telecommunications Systems 59

Fig 7 Configuration of CDMA system

R b

R c code #1

Mod

Tc

Tc

Correlator 1

Correlator 2

R c code #1

Correlator N

Path diversioncombiner

Data

desired

signal

Interference

signal

freq

freq

freq

Page 60: Radio & Telecommunications Systems 1 Basic cellular system Propagation Cellular Radio Principle.

Radio & Telecommunications Systems 60

• spreading code sequence for the receiver and that used at the transmitter are synchronized

• the resultant signal spectrum becomes the same as that for the source signal

• signal bandwidth of the interference signals still remains the same bandwidth even after taking correlation

Page 61: Radio & Telecommunications Systems 1 Basic cellular system Propagation Cellular Radio Principle.

Radio & Telecommunications Systems 61

• all the terminals share the whole system bandwidth

• each terminal signal is discriminated by the code

• when each user sends a call request to the BS

• BS assigns one of the spreading codes to the user

Page 62: Radio & Telecommunications Systems 1 Basic cellular system Propagation Cellular Radio Principle.

Radio & Telecommunications Systems 62

Fig 8 Call initiation and holding model

User 1

User 5

User 4

User 3

User 2

time

Holdingtime

Page 63: Radio & Telecommunications Systems 1 Basic cellular system Propagation Cellular Radio Principle.

Radio & Telecommunications Systems 63

Fig 9 Channel assignment in CDMA

User 1

User 2

User 3

User 4

User 5

frequ

ency

time

Page 64: Radio & Telecommunications Systems 1 Basic cellular system Propagation Cellular Radio Principle.

Radio & Telecommunications Systems 64

Page 65: Radio & Telecommunications Systems 1 Basic cellular system Propagation Cellular Radio Principle.

Radio & Telecommunications Systems 65

Page 66: Radio & Telecommunications Systems 1 Basic cellular system Propagation Cellular Radio Principle.

Radio & Telecommunications Systems 66

• Reference

• Seiichi Sampei, “Applications of Digital Wireless Technologies to Global Wireless Communications,” Prentice-Hall, 1997

• Siegmund M. Redl, et. Al., “An Introduction to GSM,” Artech House, 1995• Lee, William C. Y., “Mobile cellular telecommunications

: analog and digital systems,” McGraw-Hill, 1995

• http://www.radiodesign.com/cellwrks.htm


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