Lecture 4. US Systems
Advanced Mobile Phone System
Analog Signal Processing at the sender side
SAT
Compress
Audio
Input
Pre
Emphasize
Limit
Low Pass
Filter
+
Frequency
Modulator
Amplify &
Transmit
Components of signal processing
• Compress – reduce the dynamic range of human
speech by a factor of 2 and this is restored at the
other end – Dolby noise reduction!
• Pre-emphasis filter – amplify higher frequency
which tend to be weaker
• Limiter – limits amplitude so that frequency
deviation is within 12kHz on either side.
• Low-pass filter – attenuate signal components at
frequency 3kHz or above – very less energy above
15kHz.
• SAT – helps in identifying the base station and the
strength of the signal.
Logical Channels
• Control channel (to establish a call)
– Forward: FOCC Broadcasted to all users for signaling
purpose
– Reverse: RECC Random access (many try to access
one) for signaling purpose
• Voice channel (during a call)
– Forward: carries system control info from base to the
user
– Reverse: carries system control info from user to the
base
• Traffic channel (during a call)
– Forward and Reverse to process the conversation!
Voice and Traffic Channel
• Carry this in one physical channel
• AMPS use blank-and-burst mode to share the
traffic from these two logical channel
System interrupts the flow of user conversation
and transmit the control message for
approximately 100ms. User hears this as a click!
• This is called in-band signaling
Contending for access
• How does a user gets hold of a reverse
control channel to contact a base station?
– AMPS specifies a random access protocol that
various contending users use to find out who
gets the channel.
Random access protocol
•
•
•
•
Listen to FOCC.
If not busy transmit interest to connect
Observe FOCC for some time duration
If it is busy too soon (< 5.6ms) then stop since
someone else has won
• If not busy within 10.4ms then collision so wait
for a random duration (up to 200ms) and try again
• Otherwise possibility of success. Listen to FOCC!
• Try connecting for a fixed number of times before
you give up.
Coding of control information
• Use BCH block code (n,k,d)
– K bits of information is coded to n bits so that
the distance between two code word is d
• Repeat message 5 or 11 times and take
majority
• Synchronization bits
Block Code
•
•
•
•
(n,k,d) code
Usually d is odd (d = 2m+1)
Can correct m errors
Or can detect 2m errors
Control bits
• RECC and RVC
– (48,36,5) coding, repeats five times
• FOCC
– (40,28,5) coding, repeats five times
• FVC
– (40,28,5) coding, repeats eleven times
– More important since it carries handoff info.
Sample bit pattern
10 11
40
Bit sync
Word sync
Five copies of coded data
Message Structure
• Lacks uniformity in formats
– Different messages have different length
Modes of operation
Power on
Init
Idle
Conversation
Access
Initialization
• Scans 21 control channels in A band and 21
control channels in B band (preference to home
system)
• Locks on to strongest channel (home first)
• No channel is strong then no service
• After locking, performs update overhead
information
–
–
–
–
Identifier of the local system
Info on active paging channels
Roaming or not
Enter idle mode (init takes 5 to 10 sec)
Idle
•
•
•
•
Monitor selected (paging) channel
Monitor for incoming call
Returns to init if the signal on the FOCC is weak
Moves to access mode if
– Call is initiated by pressing send
– Incoming call is detected when its MIN in a page
message
– Registration event, due to mobility of the user
Access Mode
• User attempts to contact base by transmitting
message in RECC format
– Extracts number of attempt parameters from FOCC
transmission and then runs random access protocol to
acquire a channel for RECC
• If successful in getting a channel for RECC
– Call setup - gets a channel for voice transmission
– Register its location – confirms it and returns to INIT
• If not successful returns to INIT
• Sets a limit on staying at access mode. If exceeds
return to INIT.
Conversation
• aka. Mobile station control on the voice
channel
• Transmits SAT to let tower know that it has
tuned to voice channel
• Starts conversation
Mobile User Calls
MTSO
INIT FOCC
Base
Station
Mobile User
Broadcast FOCC message
Scan control channels
Lock on FOCC
Record sys. Params.
Display roaming status
Mobile User Calls
MTSO
Base
Station
idle FOCC
Mobile User
Access channel info
Record channel info
Page commands
PAGE message
Press SEND button
Mobile User Calls
MTSO
Base
Station
Mobile User
access RECC
ORIGINATE
Service request
Select voice channel
INITIAL VOICE CHANNEL
Transmit SAT
Mobile User Calls
conversation
MTSO
Base
Station
Mobile User
Tune to voice channe
Detect SAT
Transmit SAT
Detect SAT
Confirm VC connection
Complete call
Conversation
Press END
Transmit ST
Call release
Release call in network
Turnoff transmitter
Call to a mobile user
MTSO
Base
Station
idle FOCC
Mobile User
Access channel info
Record channel info.
Call arrives from network
Send ringing tone to caller
Page commands to cells
PAGE
Detect MIN
Call to a mobile user
MTSO
Base
Station
Mobile User
access RECC
PAGE RESPONSE
Service request
Select voice channel
INITIAL VOICE CHANNEL
Transmit SAT
conversation
MTSO
Base
Station
Mobile User
Tune to voice channel
Detect SAT
Transmit SAT
Detect SAT
Confirm VC connection
ALERT
Transmit ST Generate audible beep
Detect ST
User answers
Stop transmit ST
Detect absence of ST
Remove audible ring to caller
Complete connection
Detect hangup
release indication
Stop ALERT
Conversation
Stop audible beep
RELEACE
Transmit ST
Turnoff transmitter
MTSO
Original BST
New BST
Caller
Detect weak signal
Handoff request
Measurement requests at many cells
Measurement reports from cells
Select new BST and VC
Handoff command
Handoff indication
HANDOFF
Send ST for 50ms
Turnoff transmitter
Tune to new VC
Turn on transmitter
MTSO
Original BST
New BST
Caller
Transmit new SAT
Detect SAT
Confirm VC connection
Transfer conversation
To new BST
conversation