Introduction To The Analog Mobile Phone System (
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Introduction to the Advanced Mobile Phone System (AMPS) EE421 Fall 2001 Dr. Julie Dickerson
Information Sources • Theodore Rappaport, Wireless Communications: Principles & Practice, Prentice-Hall, 1996 • Gallagher & Snyder, Mobile Telecommunications Networking with IS-41, McGraw-Hill • Tutorial on Mobile Communications from the International Engineering Consortium - on course web site
10/14/08
EE421 Fall 2001
2
History • 1946 First mobile phones deployed, FM 120KHz, half-duplex • 1950 FM 60KHz, better RF filters available • Mid 1960’s: 30KHz, full-duplex, IMTS (Improved Mobile Telephone Service), limited capacity: 12 channels, 543 paying customers in New York City (1976) 10/14/08
EE421 Fall 2001
3
Early Mobile Systems
10/14/08
EE421 Fall 2001
4
Cellular Communications • 1968 – Cell concept proposed by AT&T • 1983 – FCC allocates 40 MHz of spectrum in 800 MHz band, 30 kHz for each one-way channel (for full-duplex) analog • Channels split between two carriers • More frequencies allocated, 1.8-1.9 GHz, etc. over time • Bandwidth limited 10/14/08
EE421 Fall 2001
5
Cellular Concept
10/14/08
EE421 Fall 2001
6
Digital Communications • Early 1990’s digital systems begin with the goal of increasing the number of users, DAMPS • TDMA • FDMA • CDMA
10/14/08
EE421 Fall 2001
7
F r e q u e n c y
FDMA-Frequency Division Multiple Access
Time -Requires no synchronization or central timing, channels independent. 10/14/08
EE421 Fall 2001
8
TDMA-Time Division Multiple Access: Fixed Slots F r e q u e n c y
Time
10/14/08
EE421 Fall 2001
9
CDMA-Code Division Multiple Access: Frequency Hopping F r e q u e n c y
Time
10/14/08
EE421 Fall 2001
10
10/14/08
EE421 Fall 2001
11
Terminology • Base Station – Fixed station used for radio communication with mobiles. Located at the center or edge of coverage region. Consists of radio channels, transmit/receive antennas.
• Control Channels – Radio channels used for transmission of call setup, request, initiation and other control purposes
• Full Duplex – Communication system that allows simultaneous two-way communication, transmission reception usually on two different frequencies (FDD) 10/14/08
EE421 Fall 2001
12
• Forward Channel – Radio channel for transmission from base station to mobile
• Reverse channel – Radio channel for transmission from mobile to base station
• Handoff – Process of transferring a mobile from one channel or base station to another
• Mobile Switching Center – Switching center that coordinates call routing in a large service area. MSC connects cellular base stations and the mobiles to the PSTN (also called Mobile Telephone switching office (MTSO) 10/14/08
EE421 Fall 2001
13
Wireless System Basics
Reverse Link Forward Link Control or Setup Channels Mobile Unit
Base Station 10/14/08
EE421 Fall 2001
14
Cellular System
10/14/08
EE421 Fall 2001
15
Making calls from a cellphone 1. Mobile sends call initiation request, its ID number to base station on reverse control channel 2. Base station receives and relays to the MSC 3. MSC validates user, instructs base station to move mobile to voice channels 4. Base station pages mobile with instructions 5. Mobile moves to voice channel 6. MSC connects mobile to PSTN 7. Voice transmission/reception between mobile and base station begins 10/14/08
EE421 Fall 2001
16
Making calls to a cellphone 1. 2. 3. 4. 5. 6.
10/14/08
MSC receives call from PSTN , sends page to base stations Base stations send page on forward control channel to mobiles Mobile receives page and acknowledges on reverse control channel; sends validation information MSC validates mobile, asks base station to put user on voice channel pair Base station sends voice channel information on FCC, mobile moves to voice channel Voice transmission/reception initiated; MSC connects with PSTN. EE421 Fall 2001
17
Forward/Reverse Channels
849-851 Air Mobile, air cellular 851-869 Private land mobile, public safety frequencies, trunk lines 10/14/08
EE421 Fall 2001
18
Cell Frequency Reuse
10/14/08
EE421 Fall 2001
19
Locating Cells
N=19 (i= 3, j=2)
10/14/08
EE421 Fall 2001
20
Channel Assignment • Need to assign frequencies to users/cell • Fixed – Each cell has predetermined number of channels – If all channels used, cell is “blocked”, no service – Cells can borrow channels from neighboring cells
• Dynamic – No permanent allocation – Frequency band requested from MSC, assigned using reuse distance, blocking probabilities, use of channel, signal strength 10/14/08
EE421 Fall 2001
21
Handoff Scenarios
10/14/08
EE421 Fall 2001
22
Umbrella Cells
10/14/08
EE421 Fall 2001
23
Interference and Capacity • Interference causes – cross-talk, poor quality,(voice channel) – blocking and missed calls (control channel)
• Co-channel interference – Frequency reuse in nearby cells
• Adjacent channel interference – Signal in adjacent frequency band – Signals from other cell companies 10/14/08
EE421 Fall 2001
24
Co-Channel Interference
10/14/08
EE421 Fall 2001
25
Adjacent Channel Interference • Comes from imperfect filters that allow frequency leakage into the band • Serious problem if interferer is nearby, near-far effect – Nearby mobile transmits on a frequency near to that of a weak mobile
• Base station receivers need high-Q filters to reject adjacent channel interference. 10/14/08
EE421 Fall 2001
26
Power Control • Each mobile should use the minimum amount of power to have good quality. • Base station controls power – CDMA power toggles up and down continuously, like delta modulation – TDMA, AMPS adjusted by base station
10/14/08
EE421 Fall 2001
27
Increasing Capacity • Cell Sectoring – Divide cell up into angular sections, typically 3 or 6 – Increases reuse
• Cell Splitting – Subdivide cell into smaller cells, increases the number of channels, keep same structure – Needs power control 10/14/08
EE421 Fall 2001
28
Cell Sectoring
10/14/08
EE421 Fall 2001
29
Cell Splitting
10/14/08
EE421 Fall 2001
30
AMPS Specs
10/14/08
EE421 Fall 2001
31
AMPS Voice Processing
Compander – compresses signal in amplitude, roughly 2:1 ratio Deviation Limiter –assures that the max. deviation is the +/- 12 kHz Postdeviation limiter filter – LPF, attenuated signal to keep in band and avoid interfering with SAT tones 10/14/08
EE421 Fall 2001
32
Other Cell Specs • • • •
IS-54 – Digital cellular in US, DQPSK IS136 PCS IS 95 A – CDMA spread spectrum Global system for mobiles (GSM) – TDMA with channelization
10/14/08
EE421 Fall 2001
33
Information Sources • Theodore Rappaport, Wireless Communications: Principles & Practice, Prentice-Hall, 1996 • Gallagher & Snyder, Mobile Telecommunications Networking with IS-41, McGraw-Hill • Tutorial on Mobile Communications from the International Engineering Consortium - on course web site
10/14/08
EE421 Fall 2001
2
History • 1946 First mobile phones deployed, FM 120KHz, half-duplex • 1950 FM 60KHz, better RF filters available • Mid 1960’s: 30KHz, full-duplex, IMTS (Improved Mobile Telephone Service), limited capacity: 12 channels, 543 paying customers in New York City (1976) 10/14/08
EE421 Fall 2001
3
Early Mobile Systems
10/14/08
EE421 Fall 2001
4
Cellular Communications • 1968 – Cell concept proposed by AT&T • 1983 – FCC allocates 40 MHz of spectrum in 800 MHz band, 30 kHz for each one-way channel (for full-duplex) analog • Channels split between two carriers • More frequencies allocated, 1.8-1.9 GHz, etc. over time • Bandwidth limited 10/14/08
EE421 Fall 2001
5
Cellular Concept
10/14/08
EE421 Fall 2001
6
Digital Communications • Early 1990’s digital systems begin with the goal of increasing the number of users, DAMPS • TDMA • FDMA • CDMA
10/14/08
EE421 Fall 2001
7
F r e q u e n c y
FDMA-Frequency Division Multiple Access
Time -Requires no synchronization or central timing, channels independent. 10/14/08
EE421 Fall 2001
8
TDMA-Time Division Multiple Access: Fixed Slots F r e q u e n c y
Time
10/14/08
EE421 Fall 2001
9
CDMA-Code Division Multiple Access: Frequency Hopping F r e q u e n c y
Time
10/14/08
EE421 Fall 2001
10
10/14/08
EE421 Fall 2001
11
Terminology • Base Station – Fixed station used for radio communication with mobiles. Located at the center or edge of coverage region. Consists of radio channels, transmit/receive antennas.
• Control Channels – Radio channels used for transmission of call setup, request, initiation and other control purposes
• Full Duplex – Communication system that allows simultaneous two-way communication, transmission reception usually on two different frequencies (FDD) 10/14/08
EE421 Fall 2001
12
• Forward Channel – Radio channel for transmission from base station to mobile
• Reverse channel – Radio channel for transmission from mobile to base station
• Handoff – Process of transferring a mobile from one channel or base station to another
• Mobile Switching Center – Switching center that coordinates call routing in a large service area. MSC connects cellular base stations and the mobiles to the PSTN (also called Mobile Telephone switching office (MTSO) 10/14/08
EE421 Fall 2001
13
Wireless System Basics
Reverse Link Forward Link Control or Setup Channels Mobile Unit
Base Station 10/14/08
EE421 Fall 2001
14
Cellular System
10/14/08
EE421 Fall 2001
15
Making calls from a cellphone 1. Mobile sends call initiation request, its ID number to base station on reverse control channel 2. Base station receives and relays to the MSC 3. MSC validates user, instructs base station to move mobile to voice channels 4. Base station pages mobile with instructions 5. Mobile moves to voice channel 6. MSC connects mobile to PSTN 7. Voice transmission/reception between mobile and base station begins 10/14/08
EE421 Fall 2001
16
Making calls to a cellphone 1. 2. 3. 4. 5. 6.
10/14/08
MSC receives call from PSTN , sends page to base stations Base stations send page on forward control channel to mobiles Mobile receives page and acknowledges on reverse control channel; sends validation information MSC validates mobile, asks base station to put user on voice channel pair Base station sends voice channel information on FCC, mobile moves to voice channel Voice transmission/reception initiated; MSC connects with PSTN. EE421 Fall 2001
17
Forward/Reverse Channels
849-851 Air Mobile, air cellular 851-869 Private land mobile, public safety frequencies, trunk lines 10/14/08
EE421 Fall 2001
18
Cell Frequency Reuse
10/14/08
EE421 Fall 2001
19
Locating Cells
N=19 (i= 3, j=2)
10/14/08
EE421 Fall 2001
20
Channel Assignment • Need to assign frequencies to users/cell • Fixed – Each cell has predetermined number of channels – If all channels used, cell is “blocked”, no service – Cells can borrow channels from neighboring cells
• Dynamic – No permanent allocation – Frequency band requested from MSC, assigned using reuse distance, blocking probabilities, use of channel, signal strength 10/14/08
EE421 Fall 2001
21
Handoff Scenarios
10/14/08
EE421 Fall 2001
22
Umbrella Cells
10/14/08
EE421 Fall 2001
23
Interference and Capacity • Interference causes – cross-talk, poor quality,(voice channel) – blocking and missed calls (control channel)
• Co-channel interference – Frequency reuse in nearby cells
• Adjacent channel interference – Signal in adjacent frequency band – Signals from other cell companies 10/14/08
EE421 Fall 2001
24
Co-Channel Interference
10/14/08
EE421 Fall 2001
25
Adjacent Channel Interference • Comes from imperfect filters that allow frequency leakage into the band • Serious problem if interferer is nearby, near-far effect – Nearby mobile transmits on a frequency near to that of a weak mobile
• Base station receivers need high-Q filters to reject adjacent channel interference. 10/14/08
EE421 Fall 2001
26
Power Control • Each mobile should use the minimum amount of power to have good quality. • Base station controls power – CDMA power toggles up and down continuously, like delta modulation – TDMA, AMPS adjusted by base station
10/14/08
EE421 Fall 2001
27
Increasing Capacity • Cell Sectoring – Divide cell up into angular sections, typically 3 or 6 – Increases reuse
• Cell Splitting – Subdivide cell into smaller cells, increases the number of channels, keep same structure – Needs power control 10/14/08
EE421 Fall 2001
28
Cell Sectoring
10/14/08
EE421 Fall 2001
29
Cell Splitting
10/14/08
EE421 Fall 2001
30
AMPS Specs
10/14/08
EE421 Fall 2001
31
AMPS Voice Processing
Compander – compresses signal in amplitude, roughly 2:1 ratio Deviation Limiter –assures that the max. deviation is the +/- 12 kHz Postdeviation limiter filter – LPF, attenuated signal to keep in band and avoid interfering with SAT tones 10/14/08
EE421 Fall 2001
32
Other Cell Specs • • • •
IS-54 – Digital cellular in US, DQPSK IS136 PCS IS 95 A – CDMA spread spectrum Global system for mobiles (GSM) – TDMA with channelization
10/14/08
EE421 Fall 2001
33
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