1. Polytechnic University1 NA-TDMA (IS-136) George Palafox Ai Wen Liang Gee Yee Johnny Kuok EL604: Wireless & Mobile Networking

2. Polytechnic University2 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. Polytechnic University3 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. Polytechnic University4 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. Polytechnic University5 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. Polytechnic University6 Frequency spectrum Reverse Channel 824 825 835 845 847 849 A A B A B 869 870 880 890 892 894 Forward Channel 25 Mhz Original AMPS frequency band for dual-mode NA-TDMA/AMPS opeation Another allocation: around 1.9Ghz for PCS (Personal Communication Systems) In all bands, carriers are spaced 30Khz apart

7. Polytechnic University7 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 61652341234 61652341234 1.9ms 5 40ms 45 Mhz or 80 Mhz base station to mobile mobile to base station

8. Polytechnic University8 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. Polytechnic University9 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. Polytechnic University10 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. Polytechnic University11 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. Polytechnic University12 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. Polytechnic University13 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. Polytechnic University14 Digital traffic channels (DTCH) DATA user information FACCH fast associated control channel SYNCDVCC digital verification color code SACCH slow associated control channel CDL coded digital control channel locator

15. Polytechnic University15 Digital Traffic Channel (DTCH) Within One Time Slot – Reverse (Terminal  Base) Within One Time Slot – Forward (Base  Terminal) One Frame G – guard time R – ramp time DL – Digital Control Channel Locator RSVD – Reserved for future use

16. Polytechnic University16 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. Polytechnic University17 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. Polytechnic University18 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. Polytechnic University19 Digital Control Channel (DCCH) Frame Within One Time Slot – Reverse (Terminal  Base) Within One Time Slot – Forward (Base  Terminal)

20. Polytechnic University20 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. Polytechnic University21 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. Polytechnic University22 Random-access MAC protocol used 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. Polytechnic University23 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. Polytechnic University24 RACH access protocol busy/idle = 0? NBUSY = NBUSY+1 yesno NBUSY = 0 Send originate Continue NSZTR = 0 NBUSY = 1 Too many failures Abandon no yes NBUSY < MAXBUSY random delay Monitor

25. Polytechnic University25 RACH access protocol (cont’d) Continue If not equal NSZTR= NSZTR+1 yes NSZTR < MAXSZTR random delay Too many failures Abandon no Apparent success; wait for response yes Monitor BRI = Busy CPE= last 7 bits of MIN R/N = Received If equal

26. Polytechnic University26 Messages Messages on AMPS logical channels Messages on FACCH and SACCH (on DTCH) Messages on DCCH

27. Polytechnic University27 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. Polytechnic University28 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. Polytechnic University29 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. Polytechnic University30 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. Polytechnic University31 An example IS-136 procedure: handoff A MAHO scheme Verifying Digital-to-digital handing off

32. Polytechnic University32 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. Polytechnic University33 Measurement Order Measurement Order ACK BS_A

34. Polytechnic University34 Channel Quality MSC Channel Quality BS_A BS_C BS_B Measurements on the FOCC

35. Polytechnic University35 Stop Measurement Order Mobile ACK BS_A

36. Polytechnic University36 Hand Off Request MSC Conversation BS_A BS_C Hand off request

37. Polytechnic University37 Verification MSC Conversation BS_A BS_C Verification Request Result Message Verification of idle channels

38. Polytechnic University38 Channel Allocation MSC BS_C BS_A 1. MSC Orders BS to allocate the channel and Time slot Conversation ACK

39. Polytechnic University39 Available Idle Channels MSC BS_C BS_A 2. Idle channel availability Conversation

40. Polytechnic University40 Hand Off Order MSC BS_C BS_A Conversation 3. Hand-off Order Hand-off Order Fwd ACK

41. Polytechnic University41 SYNC Message MSC BS_C BS_A Conversation 4. SYNC Message

42. Polytechnic University42 Channel Assignment MSC BS_C BS_A 5. Mobile to new Traffic Channel

43. Polytechnic University43 Confirmation MSC BS_C BS_A Conversation 6. Base station confirms success ACK

44. Polytechnic University44 Release MSC BS_C BS_A 7. Idle Traffic Channel Conversation

45. Polytechnic University45 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. Polytechnic University46 Reference David Goodman, “Wireless Personal Communication Systems,” Prentice Hall, ISBN 0-201-63470-8, 1997.