1. NA-TDMA (IS-136) Introduction Frequency allocation and FDD/TDD
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
Original by: George Palafox, Ai Wen Liang,
Gee Yee, Johnny Kuok, EL604, Fall 2001;
Modified by Prof. M. Veeraraghavan

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 MHz
Dual-mode phones: AMPS and NA-TDMA; cells with only AMPS cell sites or phones with only AMPS capability allowed; gradual upgrade
3 ways to expand in the cellular industry 1) further divide existing cells

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. In all bands, carriers are spaced 30Khz apart
Frequency spectrum
Reverse Channel
824
825
835
845
847
849 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

6. The TDMA aspect: frames and time slots
base station to mobile 6 1 2 3 4 5 6 1 2 3 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
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

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
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
conversations/cell/MHz

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 =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 protocol used 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. RACH access protocol Monitor NSZTR = 0 NBUSY = 1 busy/idle = 0? yes no
Send originate
Continue
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 not equal
If equal
NSZTR= NSZTR+1
Too many failures
Abandon
NSZTR <
MAXSZTR
yes
yes no
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
Reverse SACCH and FACCH
Forward 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 , 1997.