1. Lecture 4. US Systems

2. Advanced Mobile Phone System Analog Signal Processing at the sender side Compress Pre Emphasize Limit Low Pass Filter + Frequency Modulator Audio Input Amplify & Transmit SAT

3. 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.

4. 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!

5. 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

6. 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.

7. 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.

8. 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

9. Block Code (n,k,d) code Usually d is odd (d = 2m+1) Can correct m errors Or can detect 2m errors

10. 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.

11. Sample bit pattern Bit sync Word sync Five copies of coded data 1011 40

12. Message Structure Lacks uniformity in formats –Different messages have different length

13. Modes of operation Power on InitIdle ConversationAccess

14. 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)

15. 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

16. 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.

17. Conversation aka. Mobile station control on the voice channel Transmits SAT to let tower know that it has tuned to voice channel Starts conversation

18. Mobile User Calls MTSO Base Station Mobile User Broadcast FOCC message Scan control channels Lock on FOCC Record sys. Params. Display roaming status INIT FOCC

19. Mobile User Calls MTSO Base Station Mobile User Access channel info idle FOCC Record channel info Page commands PAGE message Press SEND button

20. Mobile User Calls MTSO Base Station Mobile User Select voice channel access RECC ORIGINATE Service request INITIAL VOICE CHANNEL Transmit SAT

21. Mobile User Calls MTSO Base Station Mobile User conversation Transmit SAT Confirm VC connection Tune to voice channel Detect SAT Conversation Complete call Press END Transmit ST Call release Turnoff transmitter Release call in network

22. Call to a mobile user MTSO Base Station Mobile User Access channel info idle FOCC Page commands to cells PAGE Detect MIN Record channel info. Call arrives from network Send ringing tone to caller

23. Call to a mobile user MTSO Base Station Mobile User Select voice channel access RECC PAGE RESPONSE Service request INITIAL VOICE CHANNEL Transmit SAT

24. MTSO Base Station Mobile User conversation Transmit SAT Confirm VC connection Tune to voice channel Detect SAT Conversation Transmit ST Turnoff transmitter ALERT Generate audible beep Transmit ST Detect STUser answers Stop transmit ST Remove audible ring to caller Complete connection Stop ALERT Stop audible beep Detect absence of ST Detect hangup RELEACE release indication

25. MTSOOriginal BSTNew BSTCaller 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

26. MTSOOriginal BSTNew BSTCaller Transmit new SAT Detect SAT Confirm VC connection Transfer conversation To new BST conversation