CDMA KEY TECHNOLOGY

Copyright 2003, ZTE CORPORATION

CDMA KEY TECHNOLOGY

2004.10.3


Objectives

-- Power control in CDMA why power control how to realize -- Handoff in CDMA what is handoff CDMA’s soft handoff soft handoff procedure

Upon completion of this lesson, the student will be able to master:


Why 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

• Each user is a small voice in a roaring crowd -- but with a uniquely recoverable code.

• Transmit power on all users must be tightly controlled so their signals reach the base station at the same signal level and 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 dBCDMA: Eb/No ~6to7 dB

Figure of Merit: Ec/Io, Eb/No

(energy per chip [bit] /interference [noise] spectral density)

CDMA: Ec/Io -17 to -2 dBCDMA: Eb/No ~6to7 dB


Reverse Open Loop Power Control

• The mobile station makes a coarse initial estimation of the required transmit power, based upon the total received power.

• 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

Mobile BTS

Reverse Open LoopPower Control


Estimated Reverse Open LoopOutput Power

mean output power (dBm) = - mean power input (dBm)+ K+ NOM_PWR - 16 x NOM_PWR_EXT+ INIT_PWR

Power output level for the initial probe during open loop probing on the Access Channel (with closed loop correction inactive):

Subsequent probes in the sequence are sent at increased power levels(each probe is incremented by a value equal to the parameter PWR_STEP)

The “turn around constant” K is calculated assuminga nominal cell Effective Radiation Power (ERP) of 5 W

and a nominal cell loading of 50%.

Its value is -73 for cellular systems and -76 for PCS systems


Estimated Reverse Open LoopOutput Power

Power output level for the initial transmission on the Reverse Traffic Channel:Power output level for the initial transmission on the Reverse Traffic Channel:

mean output power (dBm) = - mean power input (dBm)+ K+ NOM_PWR - 16 x NOM_PWR_EXT+ INIT_PWR+ the sum of all access probe corrections (dB)


Reverse Closed Loop Power Control

• Compensates for asymmetries between the forward and reverse paths

• 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

• Each command requests a 1dB increase or decrease of the mobile station transmit power

• Transmitted 800 times per second, always at full power

• Allows to compensate for the effects of fast fading

Mobile BTS

Signal StrengthMeasurement

Setpoint

or

Reverse Closed LoopPower Control


Reverse Outer Loop Power Control

• Most gradual form of reverse link power control– Setpoint is varied according to the FER 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

FER

Mobile BTS BSC

Reverse Outer Loop Power

Control


Setpoint

or



Forward Traffic Channel Power Control

• The base station slowly decreases power to each mobile station.• As the FER (determined at the mobile station) increases, the mobile

station requests a Forward Traffic Channel power increase.

FER

Mobile BTS BSC

Adjust Fwd.power

Forward Link Power Control


Summary of All Power Control Mechanisms

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.

FER FER

Mobile BTS BSC


Setpoint

or

Adjust Fwd.power

Reverse Outer Loop Power

Control


Forward Link Power Control

Reverse Open LoopPower Control


Handoffs

• 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.

• Handoffs 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


CDMA Handoffs

• CDMA Handoffs– Make-before-break – Directed by the mobile not the base station– Undetectable by user– Improves call quality

• Handoffs consist of the following phases:– Initiation (trigger), Target Selection, and Completion (execution)

Cell SiteB

Cell SiteA

Cell SiteA

Cell SiteB

CDMA


CDMA Handoffs (cont.)

Duringa Call

Idle Handoff

Soft Handoff

Softer Handoff

CDMA-to-CDMA Handoff

Inter-System Soft Handoff

CDMA-to-Analog Handoff

While in the Idle State


Soft Handoff

• Soft Handoff: the mobile station starts communications with a target base station without interrupting communications with the current serving base station.

• Can involve up to three cells simultaneously and use all signals

– Mobile station combines the frames from each cell

Cell SiteB

Cell SiteA

PSTN

MSC

BSC


Softer Handoff

• Handoff is between sectors of the same cell

• Communications are maintained across both sectors until the mobile station transition has completed

• May happen frequently• MSC is aware but does not

participate• All activities are managed by

the cell site• Signals received at both sectors

can be combined for improved quality

alpha

beta

gamma


Inter-System Soft Handoffs (ISSHO)

• Mobile Station starts communications with a new cell controlled by a different BSC while still communicating with the cell controlled by the source BSC

• Soft Handoffs over Hard Handoffs• Fewer border cell

MSC

SVBS

HIRS E1 Links

MSC

SVBS

HIRS

Af1

Af1

Df1

Df1

Bf1

Bf1

Ef1

Ef1

Cf1

Cf1

Ff1

Ff1

Uf1

Uf1

Xf1

Xf1

Vf1

Vf1

Yf1

Yf1

Wf1

Wf1

Zf1

Zf1

CDMA Soft Handoff ISSHO CDMA Hard Handoff

E1 LinksTIA/EIA-41D


CDMA-to-CDMA Hard Handoff

• Between cells operating on different frequencies

• Between cells that could be on the same frequency, but which are subordinated to different MSC

A(ƒ1)

PSTN

MSC

BSC

B(ƒ2)

A

PSTN

MSC

BSC

B

MSC

BSC

T1 or E1 LinksTIA/EIA-41D


What’s In a Handset?

ReceiverRF SectionIF, Detector

TransmitterRF Section

Vocoder

Digital Rake Receiver

Traffic CorrelatorPN xxx Walsh xx



Pilot SearcherPN xxx Walsh 0

ViterbiDecoder

CPUDuplexer

TransmitterDigital Section

Long Code Gen.

Op

en L

oo

p Transmit Gain Adjust

Messages

Messages

Audio

Audio

Bit Packets

Symbols

SymbolsChips

RF

RF

AGC

Bit Packets


Soft Handoff Key:

Ec/

Io

Short PN Code Offsets

Neighbor Set

Active Set

Candidate Set

Remaining Set

Rake Fingers 1 2 3Pilots Above Searched In This Order:

A A A C N

R

A A A C A A A C A A A C A A A C A A A C A A A CN N N N N N

A A A C N A A A C A A A C A A A C A A A C A A A C A A A CN N N N N

A A A CN A A A C A A A C A A A C A A A C A A A C A A A CN N N N N N

N A A A C A A A C A A A CN N N R A A A C N A A A C A A A C A A AN N

C A A A C A A A CN N N

R

A A A C N A A A C A A A C A A AN N C A A AN

C A A A CN N Until all are scanned…then repeat

Understand the searcher’s effect in different state --- MS power up state --- MS idle and calling state


Pilot Sets

• Pilot sets:– 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 least 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

• These sets can be updated during handoff by the base station

Pilot Sets

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


Pilot Set Initialization(While in the Idle or in an Active Call)

Candidate

Remaining

Active

Neighbor

Pilot whose Paging Channel is being monitored or

associated with the assigned Forward Traffic Channel

Pilots specified in the most recently received Extended

Neighbor List Message(AGE set at NGHBR_MAX_AGE)

Empty

Any other possible pilotin the system


Pilot Set Maintenance(While in an Active Call)

Candidate

Remaining

Active

Neighbor

Pilots listed the most recently received Extended

Handoff Direction Msg

Pilots specified in the most recently received Neighbor

List Update Message (or whose AGE is still less than NGHBR_MAX_AGE)

Pilots whose strength exceeds T_ADD

Any other possible pilotin the system


Pilot Strength MeasurementMessage (PSMM)

• The Pilot Strength Measurement Message is used by the mobile station to direct the base station in the handoff process.

• Mobile station reports the strength of the pilots associated with forward traffic channels currently being demodulated (and whether it would like to continue to receive traffic from them), as well as pilots from the neighbor and remaining list which are being received with sufficient strength so that traffic could be demodulated from them successfully.

A

B

C

Pilot Channel

Timing

Traffic Channels

PSMM


Pilot Search Windows

• A search window is a range of PN offsets (in chips) where the mobile station searches for usable multipath components of the pilots in a set– Usable means that multipath components can be used for

demodulation of an associated traffic channel

Search Window for pilots in the Active and Candidate Set:

Search Window for pilots in the Neighbor Set:

Search Window for pilots in the Remaining Set:

Earliest arriving usable multipath component of the pilot

pilot PN offset

pilot PN offset

SRCH_WIN_N

SRCH_WIN_A

SRCH_WIN_R


PilotStrength

Pilot Strengthexceeds T_ADD

EHOD message received,Pilot Dropped into Neighbor Set,

EHOC message sent,NLU message received

Time

Pilot strength drops below T_DROP,Drop Timer started

T_TDROP

T_ADD

T_DROP

EHOD message received, Pilot Added to Active Set, EHOC message sent, NLU message received

N C NA(1)(2) (3) (4) (5)(6) (7)

Drop Timer expires

PSMM sent (request to add) , Pilot added to Candidate Set

PSMM Sent (request to remove)

Soft Handoff Signaling

• T_ADD: pilot detection threshold (Typically 26 = -13dB)• T_DROP: pilot drop threshold (Typically 30 = -15 dB)• T_TDROP: drop timer value (Typically 3 = 4 sec)

– Prevents unnecessary transmissions of PSMM when a mobile station experiences a fade


System Parameters Message (PCH) MSG_TYPE (‘00000001’)

PILOT_PN

8

9

CONFIG_MSG_SEQ 6

SID 15

NID 16

REG_ZONE 12

TOTAL_ZONES 3

ZONE_TIMER 3

MULT_SIDS 1

MULT_NIDS 1

BASE_ID 16

BASE_CLASS 4

PAGE_CHAN 3

MAX_SLOT_CYCLE_INDEX 3

HOME_REG

FOR_SID_REG

1

1

FOR_NID_REG 1

POWER_UP_REG 1

POWER_DOWN_REG 1

PARAMETER_REG 1

REG_PRD 7

BASE_LAT 22

BASE_LONG 23

REG_DIST 11

SRCH_WIN_A 4

SRCH_WIN_N 4

SRCH_WIN_R 4

NGHBR_MAX_AGE 4

PWR_REP_THRESH

PWR_REP_FRAMES

5

4

PWR_THRESH_ENABLE 1

PWR_PERIOD_ENABLE 1

PWR_REP_DELAY 5

RESCAN 1

T_ADD 6

T_DROP 6

T_COMP 4

T_TDROP 4

EXT_SYS_PARAMETER 1

EXT_NGHBR_LIST 1

GLOBAL_REDIRECT 1

RESERVED 1

FieldLength(bits)


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 .


Soft Handoff New Parameter in 1X

T_ADD

Neighbor Set

Active Set

Neighbor Set

Candidate Set

Time(2) (4) (5) (7) (8)

Pilot Strength

T_DROP

(1) (3)

P2

Candidate Set

(6)

P1P2

P1

10log10 PSii

A

IS-2000princinpleIS-2000princinple


Handoff Parameters

• T_ADD• T_DROP• T_TDROP• T_COMP• SOFT_SLOPE• ADD_INTERCEPT• DROP_INTERCEPT• SRCH_WIN_A• SRCH_WIN_N• SRCH_WIN_R


Prefered Parameters Settingname range Prefered

rangeQualcomm

prefered

T_ADD 0~31 24~28 28

T_DROP 0~31 28~32 32

T_TDROP 0~15 2~4 3

T_COMP 0~15 4~6 5

SRCH_WIN_A 0~15 7~9 8

SRCH_WIN_N 0~15 9~11 10

SRCH_WIN_R 0~15 9~11 10

SOFT_SLOPE 0~63 0~63 18ADD_INTERCEPT 0~63 0~63 6

DROP_INTERCEPT

0~63 0~63 2~6


Search Window SettingSRCH_WIN_val

0

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

Width, Chips

4 (±2)

6 (±3)

8 (±4)

10 (±5)

14 (±7)

20 (±10)

28 (±14)

40 (±20)

60 (±30)

80 (±40)

100 (±50)

130 (±65)

160 (±80)

226 (±113)

330 (±165)

452 (±226)


The End!

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