polytechnic university1 na-tdma (is-136) george palafox ai wen liang gee yee johnny kuok el604:...
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1 Polytechnic University
NA-TDMA (IS-136)
George Palafox
Ai Wen Liang
Gee Yee
Johnny Kuok
EL604: Wireless & Mobile Networking
2 Polytechnic University
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. car-installed 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
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
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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
6 1 652 3 4 1 2 3 4
6 1 652 3 4 1 2 3 4
1.9ms
5
40ms
45 Mhz or
80 Mhz
base station to mobile
mobile to base station
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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)
skbframems
frametimeslotstimeslotbits/6.48
/40
/6/324
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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 channelsReuse Factor = 7Channel/Cell = Channels/N1,248/7 = 178 Channels in 5 Cells+179 Channels in 2 Cell
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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)
12484163
01.7257
)211248(
ESpectrum efficiency
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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)
DATAuser
information
FACCHfast
associated
control
channel
SYNC DVCCdigital
verification
color code
SACCHslow
associated
control
channel
CDLcoded digital
control
channel locator
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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
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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
skbframemsframetimeslotstimeslotbits /13/040.0/2/260
• 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 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
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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
busy/idle = 0?
NBUSY = NBUSY+1
yes no
NBUSY = 0
Send originate
Continue
NSZTR = 0NBUSY = 1
Too many failures
Abandonnoyes NBUSY <
MAXBUSY
random delay
Monitor
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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 = BusyCPE= last 7 bits of MINR/N = Received
If equal
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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
Forward SACCH and FACCH Reverse 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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Channel Quality
MSC
Channel Quality
BS_A
BS_C
BS_B
Measurements on the FOCC
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Verification
MSC
Conversation
BS_A
BS_C
Verification Request
Result Message
Verification of idle channels
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Channel Allocation
MSC
BS_C
BS_A
1. MSC Orders BS to allocate the channel and Time slot
Conversation
ACK
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Available Idle Channels
MSC
BS_C
BS_A
2. Idle channel availability
Conversation
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Hand Off Order
MSC
BS_C
BS_A
Conversation
3. Hand-off Order
Hand-off Order Fwd
ACK
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Confirmation
MSC
BS_C
BS_A
Conversation
6. Base station confirms success
ACK
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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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