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NA-TDMA (IS-136)

NA-TDMA (IS-136). George Palafox Ai Wen Liang Gee Yee Johnny Kuok. EL604: Wireless & Mobile Networking. Outline. 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.

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NA-TDMA (IS-136)

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  1. NA-TDMA (IS-136) George Palafox Ai Wen Liang Gee Yee Johnny Kuok EL604: Wireless & Mobile Networking

  2. Outline • 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

  3. Why the 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 • In 1987, FCC allowed cellular licensees to introduce new technology in the cellular band: 824 –849MHz and 869-894MHz • A hybrid TDMA/FDMA scheme was adopted • Dual-mode phones: AMPS and NA-TDMA; cells with only AMPS cell sites or phones with only AMPS capability allowed; gradual upgrade • Needed better security • Allow mobile units to have their own source of power (portable phones vs. car-installed phones)

  4. 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

  5. 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

  6. 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

  7. 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 frm 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

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

  9. 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

  10. 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

  11. 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

  12. 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 speech rate becomes 13kbps

  13. Logical channels • Term to refer to a part of a time slot or other time base unit for specific functions • Digital Traffic Channels (DTCH) • already seen – specifically to understand how the user plane works, i.e., how are voice data bits carried • 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)

  14. 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

  15. 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)

  16. 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

  17. 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

  18. Frame structure used on the DCCH • Hyper frame • 1.28 seconds • 2 super frames • Super frame • 32 blocks (a block is half a frame) • 16 frames • Frame • 6 time slots

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

  20. 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

  21. Forward direction information • Shared Channel Feedback (SCF) if 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) • Information 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)

  22. Random-access MAC protocolused on RACH • Purpose: to obtain a channel assignment for 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

  23. 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)

  24. 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

  25. 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

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

  27. 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

  28. 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

  29. 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

  30. 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

  31. An example IS-136 procedure: handoff • A MAHO scheme • Verifying • Digital-to-digital handing off

  32. 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

  33. Measurement Order ACK Measurement Order Measurement Order BS_A

  34. Channel Quality Measurements on the FOCC Channel Quality BS_A BS_B BS_C MSC

  35. Mobile ACK Stop Measurement Order Stop Measurement Order BS_A

  36. Hand off request Hand Off Request BS_A Conversation BS_C MSC

  37. Verification of idle channels Verification Request Result Message Verification BS_A Conversation BS_C MSC

  38. 1. MSC Orders BS to allocate the channel and Time slot ACK Channel Allocation BS_A Conversation BS_C MSC

  39. Available Idle Channels BS_A Conversation 2. Idle channel availability BS_C MSC

  40. Hand-off Order Fwd 3. Hand-off Order ACK Hand Off Order BS_A Conversation BS_C MSC

  41. 4. SYNC Message SYNC Message BS_A Conversation BS_C MSC

  42. Channel Assignment BS_A 5. Mobile to new Traffic Channel BS_C MSC

  43. 6. Base station confirms success ACK Confirmation BS_A Conversation BS_C MSC

  44. Release BS_A Conversation 7. Idle Traffic Channel BS_C MSC

  45. 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 traffic channels • Reduce signaling and information processing requirement on the MSC

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

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