NA-TDMA (IS-136)
George Palafox
Ai Wen Liang
Gee Yee
Johnny Kuok
EL604: Wireless & Mobile Networking
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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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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. carinstalled phones)
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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
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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
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Frequency spectrum
Original AMPS frequency band for dual-mode NA-TDMA/AMPS opeation
Reverse Channel
824 825
A
A
869 870
Forward Channel
835
845
847
B
A
B
880
890
892
25 Mhz
Another allocation: around 1.9Ghz for PCS (Personal Communication Systems)
In all bands, carriers are spaced 30Khz apart
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849
894
The TDMA aspect:
frames and time slots
base station to mobile
6
1
2
3
4
5
1.9ms
6
1
2
3
6
1
2
3
4
45 Mhz
or
80 Mhz
mobile to base station
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 frm weak
receive signal
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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)
324bits / timeslot  6timeslots / frame
 48.6kb / s
40ms / frame
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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
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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
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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 3 416  1248
• 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
E
(1248  21)
 7.01 conversations/cell/MHz
7  25
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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
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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)
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Digital traffic channels (DTCH)
DATA
FACCH
user
information
SYNC
DVCC
SACCH
CDL
fast
digital
slow
coded digital
associated
verification
associated
control
control
color code
control
channel locator
channel
channel
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Digital Traffic Channel
(DTCH)
G – guard time
One Frame
R – ramp time
DL – Digital Control
Channel Locator
Within One Time Slot – Reverse (Terminal  Base)
RSVD – Reserved for
future use
Within One Time Slot – Forward (Base  Terminal)
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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
260bits / timeslot  2timeslots / frame  0.040ms / frame  13kb / s
• Remaining 16.2-13=3.2kbps used for other
fields in DTCH
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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
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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
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Digital Control Channel (DCCH)
Frame
Within One Time Slot – Reverse (Terminal  Base)
Within One Time Slot – Forward (Base  Terminal)
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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
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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)
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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
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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)
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RACH access protocol
NSZTR = 0
NBUSY = 1
Monitor
yes
no
busy/idle = 0?
NBUSY = 0
NBUSY = NBUSY+1
yes
NBUSY <
MAXBUSY
no
Too many
failures
Abandon
Send
originate
Continue
random delay
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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
yes
NSZTR <
MAXSZTR
no
Too many
failures
Abandon
yes
Apparent success;
wait for response
random delay
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Messages
• Messages on AMPS logical channels
• Messages on FACCH and SACCH (on
DTCH)
• Messages on DCCH
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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
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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
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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
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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
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An example IS-136 procedure:
handoff
• A MAHO scheme
• Verifying
• Digital-to-digital handing off
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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
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Measurement Order
BS_A
Measurement Order ACK
Measurement Order
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Channel Quality
BS_A
BS_B
Channel Quality
BS_C
Measurements on the
FOCC
MSC
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Stop Measurement Order
BS_A
Mobile ACK
Stop Measurement Order
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Hand Off Request
BS_A
Conversation
Hand off request
BS_C
MSC
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Verification
BS_A
Conversation
Verification of idle
channels
BS_C
Verification Request
MSC
Result Message
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Channel Allocation
BS_A
Conversation
BS_C
1. MSC Orders BS to allocate the channel and Time slot
MSC
ACK
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Available Idle Channels
BS_A
Conversation
2. Idle channel availability
BS_C
MSC
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Hand Off Order
BS_A
Conversation
Hand-off Order
Fwd
ACK
BS_C
3. Hand-off Order
MSC
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SYNC Message
BS_A
Conversation
BS_C
4. SYNC Message
MSC
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Channel Assignment
BS_A
5. Mobile to new Traffic Channel
BS_C
MSC
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Confirmation
BS_A
Conversation
BS_C
6. Base station confirms success
MSC
ACK
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Release
BS_A
Conversation
7. Idle Traffic Channel
BS_C
MSC
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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
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Reference
• David Goodman, “Wireless Personal Communication
Systems,” Prentice Hall, ISBN 0-201-63470-8, 1997.
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