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Advanced Mobile Phone System Analog Signal Processing at the sender side SAT Compress Pre Emphasize Limit Low Pass Filter Frequency Modulator + Audio Input 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 40 10 11 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 Base Station Mobile User MTSO Broadcast FOCC message INIT FOCC Scan control channels Lock on FOCC Record sys. Params. Display roaming status
Mobile User Calls Base Station Mobile User MTSO Access channel info idle FOCC Record channel info PAGE message Page commands Press SEND button
Mobile User Calls Base Station Mobile User MTSO ORIGINATE Service request access RECC Select voice channel INITIAL VOICE CHANNEL Transmit SAT
Mobile User Calls Base Station Mobile User MTSO Tune to voice channel Detect SAT Transmit SAT conversation Detect SAT Confirm VC connection Complete call Conversation Press END Transmit ST Call release Turnoff transmitter Release call in network
Call to a mobile user Base Station Mobile User MTSO Access channel info idle FOCC Record channel info. Call arrives from network Send ringing tone to caller PAGE Page commands to cells Detect MIN
Call to a mobile user Base Station Mobile User MTSO PAGE RESPONSE Service request access RECC Select voice channel INITIAL VOICE CHANNEL Transmit SAT
Base Station Mobile User MTSO Tune to voice channel Detect SAT Transmit SAT Detect SAT conversation Confirm VC connection ALERT Generate audible beep Transmit ST Detect ST User answers Stop transmit ST Detect absence of ST Remove audible ring to caller Complete connection Stop ALERT Stop audible beep Conversation Detect hangup release indication 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