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NA-TDMA (IS-136)

NA-TDMA (IS-136). Original by: George Palafox, Ai Wen Liang, Gee Yee, Johnny Kuok, EL604, Fall 2001; Modified by Prof. M. Veeraraghavan. Introduction Why North-American TDMA (NA-TDMA) was created Started as IS-54; additions made to create IS136 Frequency allocation and FDD/TDD Channels

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NA-TDMA (IS-136)

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  1. NA-TDMA (IS-136) Original by: George Palafox, Ai Wen Liang, Gee Yee, Johnny Kuok, EL604, Fall 2001; Modified by Prof. M. Veeraraghavan • Introduction • Why North-American TDMA (NA-TDMA) was created • Started as IS-54; additions made to create IS136 • Frequency allocation and FDD/TDD • Channels • Messages • Handoff

  2. Upgrade to NA-TDMA • Three ways to expand (as number of cellular users grew) • move into new spectrum bands (FCC said there was no more available spectrum) • split existing cells into smaller cells (cannot be pushed beyond a point) • introduce new technology that uses the existing spectrum more efficiently • Needed better security • Allow mobile units to have their own source of power (portable phones vs. car-installed phones) • In 1987, FCC allowed cellular licensees to introduce new technology in the cellular band: 824 –849MHz and 869-894MHz • Dual-mode phones: AMPS and NA-TDMA; cells with only AMPS cell sites or phones with only AMPS capability allowed; gradual upgrade

  3. A hybrid FDMA/TDMA scheme • NA-TDMA is a hybrid FDMA/TDMA scheme • Therefore each frequency will have time slots that are shared by multiple calls • Typical: three calls share one frequency • NA-TDMA is three times as efficient

  4. Is NA-TDMA system FDD or TDD? • Answer: FDD • because different frequencies are used for the two directions of voice transmission • from mobile to BS • from BS to mobile

  5. Original AMPS frequency band for dual-mode NA-TDMA/AMPS opeation Reverse Channel 824 825 835 845 847 849 A A B A B 869 870 880 890 892 894 Forward Channel 25 Mhz Frequency spectrum Another allocation: around 1.9Ghz for PCS (Personal Communication Systems) In all bands, carriers are spaced 30Khz apart

  6. 1 2 3 The TDMA aspect: frames and time slots • Every frame is 40ms long and consists of 6 time slots • 1.9ms offset: allows a terminal to perform full-duplex communications without transmitting and receiving simultaneously • done to avoid a duplexing filter that separates strong transmit signal from weak receive signal base station to mobile 6 1 2 3 4 5 6 4 45 Mhz or 80 Mhz 1.9ms mobile to base station 6 1 2 3 4 5 6 1 2 3 4 5 40ms

  7. Data rate of a carrier (frequency) • What is the data rate of a carrier (frequency) • Each time slot carries 324 bits • Data rate per carrier (frequency)

  8. What is a channel in NA-TDMA? • Four types of channels • A full-rate channel occupies two time slots per frame • data rate: 16.2kb/s • can have three times as many calls as in AMPS • per frame: 1, 2, 3, 1, 2, 3, 1, 2, 3,.... • A half-rate channel (8.1kbps) occupies one time slot per frame • A double full-rate channel (32.4kbps) occupies four time slots per frame • A triple full-rate channel (48.6kbps) occupies an entire carrier

  9. Channels per base station(Service Provider A) Total full-rate channels = 1,248 channels Reuse Factor = 7 Channel/Cell = Channels/N 1,248/7 = 178 Channels in 5 Cells + 179 Channels in 2 Cell

  10. Spectrum efficiency conversations/cell/MHz • Reuse factor most commonly used N = 7 (same as AMPS) • An all-digital network that owns half the AMPS band has 416 carriers (832/2) • Since each carrier can support three full-rate channels, number of channels is • Unlike in AMPS, there is no fixed assignment of physical channels for control • Assume 21 control channels (corresponding to 21 sectors in 7 cells) Spectrum efficiency

  11. Speech coding • A Vector Sum Linear Excited Linear Prediction (VSELP) speech coder is used • bit rate is 7.95kbps • Including channel coding (error detection), the encoded speech rate becomes 13kbps

  12. Logical channels • Term to refer to a part of a time slot or other time base unit for specific functions • Digital Traffic Channels (DTCH) • Carry voice data bits • Some control information • Digital Control Channels (DCCH) • Reverse direction: RACH (Random Access Channel) • Random access MAC protocol used to obtain a channel assignment (fixed) for the voice call • Forward direction: many logical channels (some broadcast)

  13. Digital traffic channels (DTCH) DATA user information FACCH fast associated control channel SYNC DVCC digital verification color code SACCH slow associated control channel CDL coded digital control channel locator

  14. Digital Traffic Channel (DTCH) G – guard time R – ramp time DL – Digital Control Channel Locator RSVD – Reserved for future use One Frame Within One Time Slot – Reverse (Terminal  Base) Within One Time Slot – Forward (Base  Terminal)

  15. Data fields of DTCH • Of the 324 bits per time slot, only 260 used to carry actual data (voice) • The speech rate used in NA-TDMA system with three full rate users sharing a carrier • Remaining 16.2-13=3.2kbps used for other fields in DTCH

  16. DCCH • Any physical carrier can be designated to be a DCCH • Unlike AMPS where a set of frequencies were set aside in the middle of the band as control channels

  17. Digital Control Channel (DCCH) Frame Within One Time Slot – Reverse (Terminal  Base) Within One Time Slot – Forward (Base  Terminal)

  18. How is a channel assignment obtained? • Random-access MAC protocol used in reverse direction on the RACH • SCF (Shared Channel Feedback) bits of the forward DCCH carry information related to this random-access MAC

  19. Forward direction information • Shared Channel Feedback (SCF) in the forward DCCH • Busy/reserved/idle (BRI) • Informs terminals of whether the current slot is being used by a random access channel • Received/not-received (R/N) • Informs terminals of whether the BS successfully decoded the information transmitted in a time slot on the reverse DCCH • Code partial echo (CPE) • ACKs receipt of information on the reverse DCCH (carries part of MIN)

  20. Random-access MAC protocolused on RACH • Purpose: to obtain channels for a voice call • Terminal that needs to send request waits for IDLE indication in BRI of a forward DCCH • Terminal sends request in an appropriate time slot of RACH • BS replies in a time slot that occurs 120ms (three frames) after the slot with the IDLE indication that caused the terminal to send its request

  21. Random-access MAC contd. • If successful: BRI = Busy, R/N = Received; CPE = last 7 bits of MIN • If failed: terminal waits a random time and tries again • Continue until successful or number of attempts exceeds limit specified in the Access Parameters message broadcast on forward channel • RACH also supports a reserved mode (polling using BRI bits of SCF)

  22. NSZTR = 0 NBUSY = 1 busy/idle = 0? NBUSY = 0 Send originate Continue RACH access protocol Monitor yes no Too many failures Abandon NBUSY = NBUSY+1 NBUSY < MAXBUSY no yes random delay

  23. RACH access protocol (cont’d) Monitor Continue BRI = Busy CPE= last 7 bits of MIN R/N = Received If equal If not equal NSZTR= NSZTR+1 Too many failures Abandon NSZTR < MAXSZTR yes no yes Apparent success; wait for response random delay

  24. Messages • Messages on AMPS logical channels • Messages on FACCH and SACCH (on DTCH) • Messages on DCCH

  25. Messages on AMPS logical channels • IS136 retains AMPS messages (like origination, page, etc.) • IS136 adds extra messages: • control NA-TDMA authentication procedures – enhanced relative to AMPS security • direct dual-mode terminals to DTCHs • inform BS and switch of the capabilities of a mobile terminal

  26. Messages on associated control channels of DTCHs • Call management messages • Authentication messages • Radio resources management messages • User information transport message • OA&M (Operations, Administration and Maintenance) messages

  27. Example set: radio resource management messages Forward SACCH and FACCH Reverse SACCH and FACCH Measurement Order Stop Measurement Order Handoff Physical Layer Control Channel Quality

  28. Messages carried on DCCH • DCCH: comparable to the forward and reverse control channels in AMPS • Initialization messages • Call management messages • Authentication messages • User information transport messages • Mobility management messages (e.g. registration) • Radio resources management messages • Special services messages (SMS: Short Message Service) • OA&M messages

  29. IS136 handoffs: Mobile Assisted Hand-Offs (MAHO) • Four types of handoffs • digital-to-digital, digital-to-analog, analog-to-analog, and digital-to-analog • The mobile station measures quality of the forward voice channel from neighboring cells during idle time slots • Bit Error Rate (BER) • Radio Signal Strength Indicator (RSSI) • Measurement results are sent back to the base station via the SACCH (Slow Associated Control Channel) on DTCH • Voice channel quality is used as a criteria for handoff decisions

  30. Advantages of using MAHO • Can handle signal quality problems at the terminal • Quality is measured at the MS as well as at the BS • Fast response to signal quality problems • Quality of neighboring cells is readily available • BER is used in addition to RSSI • Can handle excessive interference on traffic channels • Reduces signaling and information processing requirement on the MSC

  31. Reference • David Goodman, “Wireless Personal Communication Systems,” Prentice Hall, ISBN 0-201-63470-8, 1997.

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