NA-TDMA (IS-136)

1 Polytechnic University

NA-TDMA (IS-136)

Original by: George Palafox, Ai Wen Liang, Gee Yee, Johnny Kuok, EL604, Fall 2001;Modified by Prof. M. Veeraraghavan

• 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


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 869-894MHz• Dual-mode phones: AMPS and NA-TDMA; cells with only AMPS

cell sites or phones with only AMPS capability allowed; gradual upgrade


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


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


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


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 from 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


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)

skbframems

frametimeslotstimeslotbits/6.48

/40

/6/324


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


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


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


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


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)


Digital traffic channels (DTCH)

DATAuser

information

FACCHfast

associated

control

channel

SYNC DVCCdigital

verification

color code

SACCHslow

associated

control

channel

CDLcoded digital

control

channel locator


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


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


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


Digital Control Channel (DCCH)

Frame

Within One Time Slot – Reverse (Terminal Base)

Within One Time Slot – Forward (Base Terminal)


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


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)


Random-access MAC protocolused 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


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)


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


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


Messages

• Messages on AMPS logical channels

• Messages on FACCH and SACCH (on DTCH)

• Messages on DCCH


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


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


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


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


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


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


Reference

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

Documents

NA-TDMA (IS-136)