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