OVER VIEW OF CDMA(Code Division Multiple Access)

CDMA(Code Division Multiple Access)

Multiple users occupying the same band by having different codes is known as CDMA - Code Division Multiple Access system.

Channels in CDMA

Forward Link Channels Reverse Link Channels

Forward Link Channels

Forward Pilot Channel (F-PICH)– Shared by the mobiles– Provide timing and phase information of the cell– Used for cell acquisition and handoff by the mobile

Forward Sync Channel (F-SYNC)– Used by the mobiles to acquire initial time synchronization

Forward Paging Channel (F-PCH)– Used to send control information and paging messages

Forward Fundamental Channel (F-FCH)– Traffic channel carrying voice and data

Forward Supplemental Channel (F-SCH)– Used (but not limited) to carrying high speed data

Air Interface Forward Link

Base

Station

MobileStation

Pilot (F-PICH)

Paging (F-PCH)

Sync (F-SYNC)

Fundamental (F-FCH)

Supplemental (F-SCH)

Quick Paging Channel (F-QPCH)

Reverse Link Channels

Reverse Access Channel (R-ACH) and Reverse Common Control Channel (R-CCCH)– Used for communication of layer 3 and MAC messages from

the mobile to the base station Reverse Pilot Channel (R-PICH)

– Time tracking– Power control measurements

Reverse Dedicated Control Channel (R-DCCH)– Used for call set up

Reverse Fundamental Channel (R-FCH)– Voice and data traffic channel

Reverse Supplemental Channel (R-SCH)– Used for data calls– MSM5105 supports up to 78.6 kbps R-SCH rate

Air Interface Reverse Link

Base

Station MobileStation

Pilot (R-PICH)

Access (R-ACH) or R-CCCH

Dedicated Control (R-DCCH)

Fundamental (R-FCH)

Supplemental (R-SCH)

CDMA Handoffs

The process whereby a Mobile Station moves to acquiring a new traffic channel

CDMA implements various types of handoff schemes to reduce call drop rate– Soft Handoff– Softer Handoff– Soft-softer Handoff– Hard handoff

Type of CDMA Handoffs Softer Handoff: Mobile Station communicates with sectors within

the cell. Soft Handoff: “Make before Break”. Mobile communicates with two

cells. Soft-softer Handoff: Mobile Station communicates with two sectors

within a cell and another sector or cell. Hard handoff ;A MS is disconnected from one BTS and is switched to

another ,Break-before-make

Power Control in CDMA

CDMA goal is to maximize the number of simultaneous users Capacity is maximized by maintaining the signal to interference ratio

at the minimum acceptable Power transmitted by mobile station must be therefore controlled

• Transmit power enough to achieve target BER: no less no more

Types of Power Control Reverse Link Power Control

– BTS instructs Mobile Station to raise or reduce power level based on received signal quality

Forward Link Power Control– BTS raise or reduce its transmit power level based on FER

reports from Mobile Stations

Reverse Link Open Loop Power Control Mobile station adjusts its transmit power level based on received

signal quality. Mobile transmit power based on Open Loop Power Control

Tx = -Rx-K+(NOM.PWR-16*NOM.PWR.EXT)+Sum of Access Probe Corrections (dBm)

Rx: Mean input receive powerNOM.PWR: nominal power (dB), part of system parameterNOM.PWR.EXT: nominal power for extended handoff, part of

system parameterK: 76 for cellular and 73 for PCS

Reverse Link Closed Loop Power Control

Closed loop power control attempts to compensate for multipath fading losses

BTS instructs Mobile to adjust its transmit power in a 1dB step size: “0” = -1 dB,

“1”=+1dB Response time: 1.25ms

Forward Link Power Control BTS collect FER report from the Mobile Station and makes decision

on adjusting its transmit power Prevents excessive interference to other cells while maintaining a

desired Frame Error Rate

Advantage in CDMA over GSM Reduce call drop rate Improve capacity

– Path diversity allows mobile to reduce transmit power to the level only the closest cell needs

– Less power means less interference– Less interference means more mobiles can access the network