Hệ thống CMDA
™ NỘI DUNG:
) Thuận lợi của CDMA
) Các Frequency channels trong CDMA
) Các tốc độ mã hoá thoại
) CDMA Call Processing
) Điều khiển công suất
) Chuyển giao mềm (Soft handoff)

Nguồn tham khảo: univ.zte.com.cn

Bài giảng Mạng TT di động

Slice 1

Thuận lợi của CDMA

Bài giảng Mạng TT di động

Slice 2


Vùng phủ sóng rộng
large coverage
almost 2 times than GSM, save money for
operator

Example:cover 1000 km2： GSM need
200 BTS ，CDMA only need 50 BTS
Attention: exact result need
“Link Budget ”

Bài giảng Mạng TT di động

Slice 3

Dung lượng lớn
High spectrum capacity，8--10 times than AMPS,4—6 times
than GSM
FDMA
FDMA---Different user use different
frequency

Power
T im

TACS、AMPS
TDMA---Different user use different
time slot of one frequency

y
nc

TDMA
Power

GSM、DAMPS
CDMA---Different user use same
frequency at the same time,but with
different spreading code


e

ue
eq
Fr

Ti m
e

e
qu
Fr e

y
nc

CDMA
Power
Tim

e
Bài giảng Mạng TT di động

ue
eq
Fr

y
nc


Slice 4


Độ bảo mật cao
¾High privacy, hard to wiretapping

Each user is below
the noise deeply

Spread signal
Information
signal

Demodulated
signal
TX

Spread code

RX

Spread code
Bài giảng Mạng TT di động

Slice 5

Chuyển giao mềm → tin cậy
Use soft handoff, decrease drop-call rate

z CDMA：”make before break”---soft handoff

z Other systems: “make after break”---hard handoff
Bài giảng Mạng TT di động

Slice 6


Chất lượng thoại cao
Good voice quality, use 8k,13K(QCELP,EVRC)voice
coding—the best coding method in the world.

Voice quality
(MOS)

64k
PCM

13k
GSM

8k
13k
CDMA CDMA

8kEVRC
CDMA

Bài giảng Mạng TT di động

Slice 7


Ít ảnh hưởng sức khoẻ
Perfect Power Control and voice activation make the MS
Power low, healthy for body—green mobile phone.

Mean Power

Max Power

GSM:

125mW

2W

CDMA:

2mW

200mW

Bài giảng Mạng TT di động

Slice 8


Các công thức tính tần số sóng mang
CDMA Frequency Calculation：
450MHz
BS receiver(Uplink): 450.00+0.025(N-1)
BS sender(downlink): 460.00+0.025(N-1)

800MHz
BS receiver(Uplink): 825.00+0.03N
BS sender(downlink):870.00+0.03N
1900MHz
BS receiver(Uplink): 1850.00+0.05N
BS sender(downlink):1930.00+0.05N

Bài giảng Mạng TT di động

Slice 9

Frequency channels: tần số sóng mang

B Band

799

A Band

716
715

667
666

334
333

991


1
1023

A”

A’

B’

Channel
Numbers

1019

8

37

78

119

160

201

242

283


384

425

466

507

548

589

630

691

7

6

5

4

3

2

1


1

2

3

4

5

6

7

9

*
CDMA A-Band Carriers

CDMA B-Band Carriers

*

736

777

9

8


**

*

Requires frequency coordination with
non-cellular interferers

** Requires frequency coordination with
A-band carrier

A Band Primary Channel
A Band Secondary Channel

Channel number?

Center
frequency

283
691

B Band Primary Channel
B Band Secondary Channel

384
777

Up link: 825.00MHz+0.03MHz*N
Down link: 870.00MHz+0.03MHz*N

Bài giảng Mạng TT di động

Slice 10


Frequency channels: tần số sóng mang
CDMA 800 MHz Cellular Spectrum Assignment
Primary Channel

Secondary channel

A段

B段

825MHz

A’

840MHz

835MHz

B’ 段

845MHz

849MHz

846.5MHz


N=

37

78

119

160

201

242

283

384

425

466

507

548

ƒ Total 10M

ƒ Total 5M


ƒ 7 Frequency Carries

ƒ 3 Frequency Carries

A Band Primary Channel
B Band Primary Channel

283
384

589

630

691

736

777

Not allocation

A Band Secondary Channel
B Band Secondary Channel
Bài giảng Mạng TT di động

691
777
Slice 11


Frequency channels: tần số sóng mang
CDMA 1900 MHz Cellular Spectrum Assignment

N=

925

950

975

925

1900M

1895M
（Uplink：1895-1900）

950

975

1975M

1980M
（Downlink：1975-1980）

Bài giảng Mạng TT di động


Slice 12


Mã hoá thoại với tốc độ thay đổi Variable Rate Vocoding
bits

Rate Set 1 Frame Sizes

bits

Rate Set 2 Frame Sizes

192

Full Rate Frame

288

Full Rate Frame

96 1/2 Rate Frame
48

144 1/2 Rate Frame

1/4 Rt.

72

24 1/8


1/4 Rt.

36 1/8

Rate set1(8KQCELP & EVRC):

Rate set2 (13KQCELP ):

Full rate：9.6Kbps

Full rate ：14.4Kbps

Half rate：4.8Kbps

Half rate ：7.2Kbps

1/4 rate：2.4Kbps

1/4 rate ：3.6Kbps

1/8 rate ：1.2Kbps

1/8 rate ：1.8Kbps
Bài giảng Mạng TT di động

Slice 13

CDMA Call Processing
Power-Up


Initialization
Mobile station
has fully acquired
system timing

Mobile station is in idle handoff
with NGHBR_CONFG equal to
‘011’ or is unable to receive
Paging Channel Message
Mobile station ends use
of the Traffic Channel

Idle
Mobile station receives a Paging
Channel message requiring ACK
or response, originates a call, or
performs registration

Mobile station receives an ACK to
an Access Channel transmission
other than an Origination Message
or a Page Response Message

System
Access
Mobile station is directed
to a Traffic Channel

Traffic

Bài giảng Mạng TT di động

Slice 14


Điều khiển công suất
Power control
™

All CDMA users occupy the same
frequency at the same time! Frequency
and time are not used as discriminators.

™

CDMA operates by using CODES to
discriminate between users.

™

CDMA interference comes mainly from
nearby users

™

Transmit power on all users must be
tightly controlled so their signals reach
the base station
---at the same signal level
--- at the absolute minimum power level

necessary to ensure acceptable service
quality

Figure of Merit: Ec/Io, Eb/No
(energy per chip [bit] /
interference [noise] spectral density)

CDMA: Ec/Io -17 to -2 dB
CDMA: Eb/No ~6 to7 dB
Bài giảng Mạng TT di động

Slice 15

Phân loại điều khiển công suất
Reverse Power Control
Open-loop Power Control
Closed-loop Power Control (Forward traffic Channel)
9Outer-loop Power Control
9Inner-loop Power Control

Forward Power Control
IS-95 Power Control
IS-2000 Fast Power Control (Reverse Pilot Channel)
9Outer-loop Power Control
9Inner-loop Power Control
Bài giảng Mạng TT di động

Slice 16



Reverse Open-loop Power Control
Reverse Open Loop
Power Control

Mobile

BTS

‰

Reverse open loop power is mobile station controlling its transmit power

‰

Reverse open loop power control consists of :
ƒ
ƒ

‰

estimating how strong the mobile station should transmit based on a
coarse measurement of how much power it is receiving from the base
station
some correcting parameters delivered in the access parameters message

The Reverse open loop method of power control provides a quick response to
changes in signal conditions.
Bài giảng Mạng TT di động

Slice 17


Reverse Open-loop Power Control
‰ Problems with Reverse Open Loop Power Control
) Assumes same exact path loss in both directions; therefore,
cannot account for asymmetrical path loss
) Estimates are based on total power received; therefore the
power received from other cell sites by mobile station
introduces inaccuracies

Bài giảng Mạng TT di động

Slice 18


Reverse Closed Loop Power Control

Mobile

BTS
or
Reverse Closed Loop
Power Control

Signal Strength
Measurement

Setpoint

ƒ Compensates for asymmetries between the forward and reverse paths
ƒ Consists of power up (0) & power down (1) commands sent to the mobile

stations, based upon their signal strength measured at the Base Station and
compared to a specified threshold (setpoint)
ƒ Each command requests a 1dB increase or decrease of the mobile station
transmit power
ƒ Transmitted 800 times per second, always at full power
ƒ Allows to compensate for the effects of fast fading

Bài giảng Mạng TT di động

Slice 19

Reverse Outer Loop Power Control
Mobile

BSC

BTS
or
Reverse Closed Loop
Power Control

Signal Strength
Measurement

Setpoint

Reverse Outer
Loop Power
Control


FER

) Setpoint is varied/dynamic according to the FER (Frame Error Rate)
level, typically 1% on the Reverse Traffic Channel (determined at
the Base Station Controller),
) Sampled at a rate of 50 frames per second (20 ms / frame)
) Setpoint adjusted every 1-2 seconds
Bài giảng Mạng TT di động

Slice 20


Closed-loop Power Control Procedure
Received the Frame
Quality Information
from the Reverse Link

Outer-loop
Algorithm

Outer Loop Correction
Eb/No

Closed-loop Control

Threshold
No

Demodulate and
measure Eb/Nt


Eb/Nt>Eb/No?

Up Power
Control Bit

Yes
Down Power
Control Bit
Base Station

Puncture to
Power Control
Subchannel

Mobile
Power Control
Command
Measurement

Power Control
Command
Measurement

Bài giảng Mạng TT di động

Slice 21

Forward Power Control in IS-95


Mobile

BTS

BSC

Adjust Fwd.
power

FER
Forward Link Power Control

‰

The base station continually and slowly decreases power to each
mobile station(each user’s forward traffic channel)

‰

As the FER (determined at the mobile station) increases, the
mobile station requests a Forward Traffic Channel power increase
Bài giảng Mạng TT di động

Slice 22


Forward Quick Power Control in IS-2000

‰


In CDMA2000, there is a faster
method used for Forward
Power Control operating much
like the IS-95 Reverse Link
Power control described next

‰

In order to implement Forward
Link Quick Power Control,we
use Reverse Pilot channel
which includes power control
subchannel

Bài giảng Mạng TT di động

Slice 23

Forward Quick Power Control Procedure
Received the Frame
Quality Information
from the Forward Link

Outer-loop
Algorithm

Outer Loop Correction
Closed-loop Control
Demodulate and
measure Eb/Nt


Eb/No
No
Eb/Nt>Eb/No?

Up Power
Control Bit

Yes
Down Power
Control Bit
Mobile Station

Puncture to
Power Control
Subchannel

Base Station
Power Control
Command
Measurement

Power Control
Command
Measurement
Bài giảng Mạng TT di động

Slice 24



Summary of Power Control

Reverse Open Loop
Power Control

Mobile

BSC

BTS
or
Reverse Closed Loop
Power Control

FER

Signal Strength
Measurement

Setpoint

Adjust Fwd.
power
Reverse Outer
Loop Power
Control

FER

Forward Link Power Control


ƒ

All types of power control work together to minimizes power
consumption at the mobile stations, and increases the overall
capacity of the system transmit power.
Bài giảng Mạng TT di động

Slice 25

Chuyển giao mềm - Soft handoff
‰ Handoff is the process by which a mobile station maintains
communications with the Mobile Telephone Switching
center (MSC), when traveling from the coverage area of one
base station to that of another
‰ Handoff keep the call established during the following
conditions:
) Subscriber crosses the boundaries of a cell
) Subscriber experiences noise or other interference above a
specified threshold
) A base station component experiences an out-of-service condition
during a call
Bài giảng Mạng TT di động

Slice 26


Phân loại: CDMA Handoffs

While in the

Idle State

Idle Handoff

Soft Handoff
Softer Handoff
Inter-System Soft Handoff

During
a Call

CDMA-to-CDMA Handoff
CDMA-to-Analog Handoff

Bài giảng Mạng TT di động

Slice 27

CDMA Idle Handoff
PN 108
Ec/Io

104 108

A

ng
gi
a
P


ne
an
h
C

n
Sy

s
sg
M
l

ha
cC

sg
lM
e
nn

It’s not soft and hard
handoff!

PN 104
Bài giảng Mạng TT di động

Slice 28



CDMA Idle Handoff
PN 108

>3dB
Ec/Io

104 108

ng
gi
a
P

e
nn
a
Ch

s
sg
M
l

C
nc
y
S

ha


sg
lM
e
nn

A

It’s easy to meet
Access failure?
PN 104
Bài giảng Mạng TT di động

Slice 29

CDMA Soft Handoff
CDMA
Cell Site
B

‰

Cell Site
B

Cell Site
A

Cell Site
A


Soft Handoff:The mobile station starts communications with a target
base station without interrupting communications with the current
serving base station
) Make-before-break
) Directed by the mobile not the base station,Undetectable by user
) Improves call quality

‰

Can involve up to three cells simultaneously and use all signals
) Mobile station combines the frames from each cell

Bài giảng Mạng TT di động

Slice 30


CDMA Soft Handoff Mechanics

‰ CDMA soft handoff is driven by the handset
ƒ Handset continuously checks available pilots
ƒ Handset tells system pilots it currently sees
ƒ System assigns sectors (up to 6 max.), tells handset
ƒ Handset assigns its fingers accordingly
‰ Each end of the link chooses what works best, on a frame-by-frame basis
ƒ Users are totally unaware of handoff
Bài giảng Mạng TT di động

Slice 31


CDMA Softer Handoff
ƒ Softer Handoff is between sectors of the
same cell, that means multiple sectors of one
BTS simultaneously serve a handset
ƒ Softer handoff occurs in BTS in a single
channel element

alpha

beta

gamma

ƒ Communications are maintained across both
sectors until the mobile station transition has
completed
ƒ MSC is aware but does not participate
ƒ All activities are managed by the cell site
ƒ Signals received at both sectors can be
combined for improved quality
Bài giảng Mạng TT di động

Slice 32


CDMA-to-CDMA Hard Handoff
PSTN
PSTN
MSC


MSC

MSC
T1 or E1 Links
TIA/EIA-41D

BSC

A
(ƒ1)

B
(ƒ2)

BSC

BSC

A

B

ƒ

Between cells operating on different frequencies

ƒ

Between cells that could be on the same frequency, but which are

subordinated to different MSC
Bài giảng Mạng TT di động

Slice 33

Handset Structure Review
Digital
Rake Receiver Symbols
Chips Traffic Correlator
PN xxx

Receiver
RF Section
IF, Detector

RF

PN xxx
PN xxx

Open Loop

Duplexer

Traffic Correlator
Walsh xx

Traffic Correlator

AGC

RF

Walsh xx

Transmitter
RF Section

Walsh xx

Pilot Searcher
PN xxx

Walsh 0

Symbols

Σ

Viterbi
Decoder
Messages

Bit Packets
Audio

CPU

Vocoder

Transmit Gain Adjust


Transmitter
Digital Section
Long Code Gen.

Audio
Messages

Bit Packets
Bài giảng Mạng TT di động

Slice 34


Pilot Sets
Pilot Sets
1. Active
2. Candidate
3. Neighbor
4. Remaining

™

There are 4 memorizer (pilot set) in handset, to save the related PN offset
index.After power off, all memorizer will be cleared automatically.
) Active Set: Pilots associated with the forward traffic channels assigned to the
mobile station (max 6 pilots)
) Candidate Set: Pilots not currently in the Active Set, but received by the mobile
with sufficient strength to indicate that the corresponding Traffic Channels can
be successfully demodulated (max 5 pilots)

) Neighbor Set: Pilots not currently on the Active or Candidate Sets, that are
likely handoff candidates (at most 20 pilots)
) Remaining Set: All other possible pilots in the current system on the current
CDMA frequency assignment

™

All pilots in a set have the same frequency assignment
Bài giảng Mạng TT di động

Slice 35

Rules of Soft Handoff
‰ The MS assists the BS in the handoff process by
measuring and reporting the strengths of received
pilots using a Pilot Strength Measurement Message
‰ Handset sends PSMM to the system whenever:
ƒ It notices a pilot in neighbor or remaining set
exceeds T_ADD
ƒ An active set pilot drops below T_DROP for
T_TDROP time
ƒ A candidate pilot exceeds an active by
T_COMP
Bài giảng Mạng TT di động

Slice 36


IS-95 Soft Handoff Signaling


Bài giảng Mạng TT di động

Slice 37

Power Control during Soft Handoff
™

During soft handoff, the mobile station receives
power control bits from multiple base stations.

™

If two or more power control subchannels are
identical, their power control bits are diversity
combined into one per 1.25 ms time slot.

™

If the mobile station receives different power
control bits from different power control
subchannels, it decreases its mean power output
level by 1 dB.

™

Only if the power control bits from ALL power
control subchannels request a power increase,
the mobile station increases its mean power
output level by 1 dB .
Bài giảng Mạng TT di động


Slice 38