Lecture 4. US Systems
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Lecture 4. US Systems
Advanced Mobile Phone System
Analog Signal Processing at the sender side
CompressPre
EmphasizeLimit
Low PassFilter
+ FrequencyModulator
Audio Input
Amplify &Transmit
SAT
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
Bit sync
Word sync Five copies of coded data
10 11 40
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
MTSOBaseStation
Mobile User
Broadcast FOCC message
Scan control channelsLock on FOCCRecord sys. Params.Display roaming status
INIT FOCC
Mobile User Calls
MTSOBaseStation
Mobile User
Access channel infoidle FOCC
Record channel info
Page commands PAGE message
Press SEND button
Mobile User Calls
MTSOBaseStation
Mobile User
Select voice channel
acce
ss R
EC
C
ORIGINATE
Service request
INITIAL VOICE CHANNEL
Transmit SAT
Mobile User Calls
MTSOBaseStation
Mobile User
conv
ersa
tion Transmit SAT
Confirm VC connection
Tune to voice channelDetect SAT
Detect SAT
ConversationComplete call
Press ENDTransmit ST
Call release Turnoff transmitter
Release call in network
Call to a mobile user
MTSOBaseStation
Mobile User
Access channel infoidle FOCC
Page commands to cells PAGE
Detect MIN
Record channel info.
Call arrives from networkSend ringing tone to caller
Call to a mobile user
MTSOBaseStation
Mobile User
Select voice channel
acce
ss R
EC
C
PAGE RESPONSE
Service request
INITIAL VOICE CHANNEL
Transmit SAT
MTSOBaseStation
Mobile Userco
nver
sati
on Transmit SAT
Confirm VC connection
Tune to voice channelDetect SAT
Detect SAT
Conversation
Transmit STTurnoff transmitter
ALERT
Generate audible beepTransmit STDetect ST User answers
Stop transmit ST
Remove audible ring to callerComplete connection Stop ALERT Stop audible beep
Detect absence of ST
Detect hangupRELEACE release indication
MTSO Original BST New BST Caller
Detect weak signalHandoff request
Measurement requests at many cells
Measurement reports from cells
Select new BST and VCHandoff command
Handoff indicationHANDOFF
Send ST for 50ms
Turnoff transmitterTune to new VCTurn on transmitter
MTSO Original BST New BST Caller
Transmit new SAT
Detect SAT
Confirm VC connection
Transfer conversationTo new BST
conversation
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