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)
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
Radio & Telecommunications Systems 4
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
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
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
Radio & Telecommunications Systems 8
Fig.2 Mobile radio transmission
Direct path
Reflected path1
2
30 -
100m
2km or further
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)
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Fig. 3 Typical fading signal
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• 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
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Fig. 4 Multi-path fading
~100
Radio & Telecommunications Systems 13
Frequency Re-Use
• Reuse channels
• Reuse distance
• Cochannel interference
• Cell splitting
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)
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
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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
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
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
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• 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)
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1
3
2
3
2
2
4
3
1
4
4
1
K=4
Fig. 2a 4-cell reuse pattern
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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
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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
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
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
Radio & Telecommunications Systems 25
Operation of Cellular Systems
•Call Process
•Handoff Procedure
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
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
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
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• 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
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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
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Fig 1 Occurrence of handoff
CS: Cell Site
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• 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
Radio & Telecommunications Systems 33
Mobile and Base Stations
Structure of a Mobile Station
Structure of a Base Station
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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
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Structure of a Mobile Station
Radio & Telecommunications Systems 36
Fig 1 Block diagram of a mobile station
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
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• 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
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• 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
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• 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
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• 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
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Structure of a Base Station
• The general structure of a base station consists of
• base station control function (BCF)
• one to sixteen transceivers (TRX)
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Fig 2 Block diagram of a base station
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
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
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
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
Radio & Telecommunications Systems 48
Multiple Access Methods
•Frequency Division Multiple Access
•Time Division Multiple Access
•Code Division Multiple Access
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
Radio & Telecommunications Systems 50
Fig.1 Basic Concept of FDMA system - spectrumS
pect
rum
chl 1 chl Nchl 3chl 2
FrequencyBandwidt
h
guardspace
Systembandwidth
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Fig. 2 Call initiation and holding model
User 1
User 5
User 4
User 3
User 2
time
Holdingtime
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
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
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
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
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Fig 6 Call initiation and holding model in TDMA
User 1
User 5
User 4
User 3
User 2
time
Holdingtime
Radio & Telecommunications Systems 57
Fig 7 Slot assignment in TDMA
frequ
ency
time
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)
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
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
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• 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
Radio & Telecommunications Systems 62
Fig 8 Call initiation and holding model
User 1
User 5
User 4
User 3
User 2
time
Holdingtime
Radio & Telecommunications Systems 63
Fig 9 Channel assignment in CDMA
User 1
User 2
User 3
User 4
User 5
frequ
ency
time
Radio & Telecommunications Systems 64
Radio & Telecommunications Systems 65
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• 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