CDMA 2000: 1xEVDO and1xEVDV, An Overview

Vivek P. Mhatre04/13/04

Talk for EE 647

School of Electrical and Computer Engineering

1

Presentation Outline

Objective: To provide an overview of networking-related aspects of

CDMA Data networks

• CDMA Basics

• CDMA-HDR, i.e., IS-856 or 1xEVDO

– Forward link: Rate Control

– Reverse link: Power Control

• 1xEVDV

• Recent Work

• Research Challenges

Will focus on: MAC, Scheduling, Power Control, Rate Control

Will not talk about: Signaling, Modulation techniques, Coding

schemes

2

CDMA Basics

• Orthogonal (or pseudo-orthogonal) spreading code per user

• Transmitter multiplies the signal by the code before transmission

• Correlation receiver: received signal multiplied by the same

spreading code, demodulation

• Chip duration much smaller than symbol duration

• Robust against multi-path fading, interference and jamming

• E.g. codes: Walsh codes

+ − − +

− − + +

− + − +

+ + + +

3

CDMA Basics (contd.)

• Perfect orthogonality not possible due to channel variations

• Interference limited capacity

γi =PiGi

η +∑

j 6=i θPjGj

Ri = W log(1 + φγi)

• All about Power control and Rate control!!!

• Reverse link power control to counter “near-far” effect

BS FN

4

IS-95

• First CDMA system

• Mainly for voice

• One CDMA code per user on forward and reverse link

• Forward and reverse links separated in frequency

• Limited data rate (64 Kbps)

5

1xEVDO or IS-856 or CDMA-HDR

• 1xEVDO stands for 1xRTT (Radio Transmission Technology)

EVolution to Data Only

• Qualcomm Inc. was the driving force

• Need to get ahead of other players ⇒ Quick deployment

• Philosophy:

– Do not tamper with the spectrum allocated to voice traffic of

IS-95

– Use separate spectrum for data traffic

– Use CDMA for data communication as well

– Forward and reverse links separated in frequency

• Resources available to the base station on the forward link

– A set of orthogonal codes (64 per sector)

– Base station transmit power (PT )

6

1xEVDO (contd.)

• FLa as perceived by each user is time-varying

• Question: For a given user, to achieve the same average rate, R̄,

should the BS use power control or rate control on FL?

• Assume linear model for rate vs SINR

Rate Control: R = KP0g

I⇒ P0 =

R̄

K

1

E[

g

I

]

Power Control: P =R̄

K

I

g⇒ P̄ =

R̄

KE

[

I

g

]

⇒P̄

P0= E[X]E[1/X] ≥ 1

P̄ ≥ P0 ⇒ Rate Control better than power control

aFL = Forward Link, RL = Reverse Link

7

1xEVDO (contd.)

• In reality, R varies as log(SINR), not linearly

• Answer: For practical CDMA network settings (noise levels, path

losses etc.) P̄ /P0 ≥ 1

• Hence CDMA systems use a bunch of modulation schemes to adapt

rate to the current channel state

• Conclusion: Rate control is better than power control on FL

• Use channel state feedback about FL from each user for rate control

8

Channel State Feedback: FL

Feedback about FL: FL is rate controlled

• Inner Loop Rate Control

– FL pilot signal sent by BS, monitored by all active data users

– DRC (Data Rate Control) channel on RL (one code per active

user) used by each user to indicate supportable FL rate and best

sector

– Coarse adjustments of rate control

• Outer Loop Rate Control

– During actual packet receptions, measure PER, and request

increase in data rate if PERmeas < PERtarget and vice-versa

– Fine adjustments of rate control

9

Channel State Feedback: RL

Unlike FL, RL is power controlled to counter the “near-far” effect

• Coarse power control

– RA (Reverse Activity) channel of FL used by BS to request user

to ↑ / ↓ its rate depending on the total interference at the BS

– Objective: Keep the total interference level (load) at BS below a

threshold T

∗ Maximum user transmit power, link budget analysis ⇒ cell

coverage area

∗ Fixed margin is alloted to in-cell interference

∗ Hence must ensure that intra-cell interference is below a

threshold

– Slow, time averaged decisions over 128 slots

• On RL, power control dictates rate control

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Channel State Feedback: RL (contd.)

• Fine power control

– RL pilot signal sent by user on RL, monitored by BS

– RPC (Reverse Power Control) channel of FL used by BS to

request user to ↑ / ↓ its power level (target PER 1%)

– Neighboring BSs also have a say

– Fast, every slot

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Reverse Link Structure

RL

Traffic ACK RL Pilot DRC

• One CDMA code per channel per user

• Four CDMA codes per user on RL ⇒ maximum 16 data users per

sector (64 codes per sector)

• Plenty of overheads on RL just to optimize FL by providing fast

and accurate channel state feedback!!

• 1xEVDV rectifies this (probably), since RL capacity of 1xEVDV is

10 times that of 1xEVDO

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So far....

• For FL: Rate control better than Power control

• For RL: Power control

– “Near-far” problem

– Maximum RL interference governed by link budget analysis

• Need channel feedback to tune FL rate and RL power

• Two loops of feedback (for FL rate, RL power)

• Considerable overheads of feedback on RL

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1xEVDO FL in details

• Question: How to serve multiple data users on FL?

• Answer: Opportunistic scheduling

• Philosophy: Serve the user with the best channel

• Since channel is time-varying for everyone, some user will be in

good state with high probability

• Since we only serve “good users”, system throughput improves

• Everyone will be in good state some time or the other, so long term

fairness issue solved

• Short term fairness can also be dealt with through appropriate

modifications

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FL Opportunistic Scheduling, A ToyProblem

Total bandwidth B User A User B

Channel state Good, p 0.8 0.8

Channel state Bad, q 0.2 0.2

Prop Fair Sharing Opp. Scheduling

User throughput (0.5B)0.8 (0.8)(0.2)(B) + (0.8)(0.8)(0.5)(B)

System throughput 0.8B 0.96B

(in general) pB (1 + q)pB

• Conclusion: Choose the best user, and serve him at full rate

• Gains of opportunistic scheduling are better with more users, since

higher probability of finding a “good user”

• In reality, channel (and hence the sustainable rate) varies over a

range, not just “good” or “bad”

Ri = W log(1 + φγi)

• Remarkably, the conclusion of the above toy problem still holds!∗

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Opportunistic Scheduling (contd.)

General Problem:

• M users, measured SINR is γi

• Allocate fraction χi of power and fraction µi of codes to user i

• Determine corresponding rate Ri

• Choose χi and µi in each time slot to maximize the system

throughput

• Optimal solution (No user differentiation): Choose single user

agrmax(γi), and set χi = µi = 1

• Conclusion: Choose the best user, and serve him at full rate and

power

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Opportunistic Scheduling (contd.)

• Hence in CDMA-HDR, on FL, BS serves the “best” user with

maximum power and maximum bandwidth (use all 64 codes)

• Essentially, FL is TDM!!

• No need to account for fading power margin as in IS-95, since only

one data user served at a time, no voice users

P (max)TX

P (max)TX

Pilo

t Cha

nnel

Pilot Channel

Paging ChannelSync Channel

Total Traffic

Unused margin

Tot

al T

raff

ic

Con

trol

Cha

nnel

MA

C C

hann

el

Sector Tx Power Sector Tx Power

IS−856 FLIS−95 FL time fraction of time

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Performance of 1xEVDO

• FL maximum bandwidth = 2.5 Mbps

• RL maximum bandwidth = 180 Kbps per user

• Maximum number of data users = 16

• Soft handoff of data users possible

EOF

18

1xEVDV

DataData

Voice

Movable boundary

1xEVDV1xEVDO

frequency

• 1xEVDV is not the next version of 1xEVDO

• Motorola and Ericsson are the key drivers

• Promises up to 3 Mbps on FL and 1.5 Mbps on RL

• Fundamentally different from 1xEVDO since voice and data

integrated (same carrier)

• Upto 88% of channels (58 out of 64) for data

• Claim: Better adaptability to changing load (traffic and voice)

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1xEVDV (contd.)

Operating pointfor 1xEVDV

Walsh codes Walsh codesFL

BS

tran

smit

pow

er

FL B

S tr

ansm

it po

wer

MAXMAX

MAX MAX

Overhead channels(static)

Voice calls

DynamicOperatingPoint

Overhead channels(data)

ActualdataChannels

Simultaneous voice and data calls on the same carrier

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1xEVDV (contd.)

• No soft handoff for data users

• Opportunistic scheduling on FL

• One or Two users served on FL simultaneously

• Improved ARQ

• Improved RL rate 1.5 Mbps

• Details to come out soon in Revision D

EOF

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Recent Work

Opportunistic scheduling over FL

• Qualcomm’s HDR algorithm

– Single user chosen for FL serving

– Maximize system utility over each time slot

• Utility function based opportunistic scheduling, [Xin Liu, Chong and

Shroff]

– Single user chosen for FL serving

– Resource sharing constraints

– A Utility function associated with each user

– Maximize long term system utility

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Recent Work (contd.)

Opportunistic scheduling over FL (contd.)

• Opportunistic power scheduling for multiple-server systems,

[Jang-Won Lee, Mazumdar and Shroff]

– Maximize long term system utility

– Determine power allocation to users

– Multiple users could be chosen for FL service

• Opportunistic scheduling over multiple interfaces, [Kulkarni and

Rosenberg]

– Multiple interfaces correspond to e.g. 802.11 interface, 3G link,

satellite link etc.

– Joint scheduling to choose the best user for each interface

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Recent Work (contd.)

RL power control and scheduling

• Power constrained RL scheduling [Kumaran and Qian]

– Some user terminals constrained by maximum transmit power

– Two classes of users: strong and weak

– Original RL power control aims to solve near-far (strong-weak)

problem

– Conclusion: Weak users scheduled simultaneously, and strong

users one at a time

EOF

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Research Challenges

My viewpoint:

• Inter-cell interference issues

– Law of large numbers on RL

– Law of small numbers on FL

– Co-ordination between multiple BSs difficult

• 1xEVDV, several research challenges

– Resource allocation is difficult since voice and data integrated

– “Movable boundary” problem in two dimensions: bandwidth

(codes) and power

– Hard SINR requirements for voice ⇒ power margin to overcome

deep fades (IS-95 like)

• New technology brings new problems with it ⇒ No dearth of

problems!

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Acknowledgments

• Sunil Kulkarni and Aravind Iyer for useful discussions in MSEE 318

• Sunil Kulkarni for providing several pointers, in particular 1xEVDV

tutorial by Ericsson in Globecomm 2004 and [Kumaran and Qian]

paper

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References• Qualcomm Inc. website for white papers (1xEVDO)

• Motorola website for white papers (1xEVDV)

• 1xEVDV tutorial by Ericsson in Globecomm 2003 (provided by Sunil Kulkarni)

• CDMA/HDR: A Bandwidth-Efficient High-Speed Wireless Data Service for

Nomadic Users, Bender et al. (Qualcomm Inc.)

• An Algorithm for Reverse Traffic Channel Rate Control for cdma2000 High

Rate Packet Data Systems, Chakravarty et al. (Qualcomm Inc.)

• Reverse Link Performance of IS-95 Based Cellular Systems, R. Padovani

(Qualcomm Inc.)

• Uplink Scheduling in CDMA Packet Data Systems, Kumaran and Qian

(Infocom 2003)

• Opportunistic Transmission Scheduling with Resource-Sharing Constraints in

Wireless Networks, Sin Liu et al. (JSAC)

• Opportunistic Power Scheduling for Multi-server Wireless Systems with

Minimum Performance Constraints, Jang-Won Lee et al. (Infocom 2004)

• EE 544 (Digital Communications) notes!

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