CDMA Radio Resource Management

7/29/2019 CDMA Radio Resource Management

1/83

12 th Virginia Tech/MPRG Symposium on

WIRELESSPERSONALCOMMUNICATIONSJune 5-7, 2002

VIRGINIA POLYTECHNIC INSTITUTE & STATE UNIVERSITY

MOBILE & PORTABLE RADIO RESEARCH GROUPMPRG

VIRGINIA POLYTECHNIC INSTITUTEAND STATE UNIVERSITY

TechVirginia

1 8 7 2

Radio Resource Managementin 3G CDMA

Dr. R. Michael Buehrer [email protected]


2/83




TechVirginia

1 8 7 2

Overview

Introduction What is Radio Resource Management? RRM in CDMA Voice Networks (IS-95/ cdma2000 )

Base station assignment (including handoff) Power control Admission control Load control

RRM for Packet Data Services IS-95B Data cdma2000 (3G1x RTT) Data Services 3G1xEV-DO (High Data Rate or HDR)


3/83




TechVirginia

1 8 7 2

Overview



RRM for Packet Data Services IS-95B Data cdma2000 (3G1X RTT) Data Services 3G1X-EV/DO (High Data Rate or HDR)


4/83




TechVirginia

1 8 7 2

What is RRM?

Service Provider goal: Maximize number of users for fixedresources

User goal: Maximize QoS for least amount of money Wireless Network Design

Placing access points to maximize coverage/capacity for least number of base stations for fixed QoS

Radio Resource Management Given access points how should power, spectrum, channels be allocated

in order to meet QoS requirements for largest number of users as theymove about the system ?

This is the general RRM problem which is applicable to all wirelesssystems. We will look specifically today at CDMA systems


5/83




TechVirginia

1 8 7 2

Radio Resource Management inCDMA Voice Networks

In CDMA voice networks systems (i.e., 2G), RRM is primarilya task of interference management. Capacity is directly dependent on the interference caused by one signal to

another

Maximizing capacity requires minimizing interference while maintainingrequired Frame Error Rate, probability of blocking, and probability of dropped call (i.e., the main QoS metrics in 2G)

All signals have similar data rate, delay and FER requirements

Interference management is done via

Power control Base station assignment (Soft handoff) Admission control Load control


6/83




TechVirginia

1 8 7 2

Radio Resource Management in DataNetworks

In CDMA which support data services, RRM has the additional burdens of managing data connections (including packet access)

Data services have varying Data rate requirements

FER requirements Jitter requirements Delay requirements

These additional requirements are accomplished via burst allocation in mixed voice/data systems packet scheduling in packet data systems

in addition to traditional interference management


7/83




TechVirginia

1 8 7 2

Overview



RRM for Packet Data Services IS-95B Data cdma2000 (3G1X RTT) Data Services 3G1X-EV/DO (HDR)


8/83




TechVirginia

1 8 7 2

Overview


Power control Base station assignment (including handoff) Admission control Load control

RRM for Packet Data Services IS-95B Data cdma2000 (3G1X RTT) Data Services 3G1X-EV/DO (HDR)


9/83




TechVirginia

1 8 7 2

Power Control

The key RRM technique in voice-centric CDMA systems is power control

All channels (single carrier system) in all cells use the samefrequency band. The resulting Multiple Access Interferencelimits system performance.

The basic system resource ( i.e., that which limits capacity) inCDMA is Received interference power at the base station for uplink Total transmit power at the base station for the downlink

In order to maximize capacity, we need to minimize theinterference power caused by each mobile to the system on theuplink and the transmit power required by each channel on thedownlink while maintaining a desired quality of service (FER).

This is accomplished with power control


10/83




TechVirginia

1 8 7 2

Power Control

Definitions Open Loop Power Control

Transmitter attempts to minimize transmit power using average receivedsignal strength as indication of path loss

Long time constant

Capable of adapting to large scale propagation effects: path loss andshadowing

Closed Loop Power Control Uses feedback from the receiver to adjust the transmit power Slow power control (typically 50Hz or slower)

Feeds back Frame Error Rate (FER) information

Adapts to large scale propagation effects Helps maintain target performance level

Fast power control (typically 800Hz or faster) Receiver measures E b /I o and compares to threshold. If measured value less than

threshold, requests increase in transmit power (often called the inner loop) Receiver threshold is adjusted to maintain a target FER performance level (often

called the outer loop)


11/83




TechVirginia

1 8 7 2

Open Loop Power Control

When a mobile first attempts to access the CDMA network ituses Open Loop Power Control to assure that it achieves a goodtrade-off between Interference caused to system Access time

The interference caused to other users is inversely proportionalto mobile transmit power while the probability of network access for a given attempt is directly proportional to transmit

power. In Open Loop Power Control, the mobile measures the pilot

strength which is related to path loss. The transmit power isthen set inversely to the measured pilot strength Weak pilot large path loss high mobile transmit power Strong pilot low path loss low mobile transmit power

This Open Loop Control can continue throughout the call


12/83




TechVirginia

1 8 7 2

Closed Loop (Slow) Power Control

For more accurate power control, mobile feedback is required. This feedback is referred to as closed loop power control. One metric which can be fed back to the transmitter is a frame

error rate (FER) measurement or frame error indicator. The transmitter adjusts power levels in order to keep FER at

desired level. This loop is slow (typically on the order of 50Hz). The fastest it

can feedback information is once per frame (frame error

indication). If an FER measurement is taken, the feedback rateis even slower. The forward link of second generation CDMA systems rely on

this type of power control.


13/83




TechVirginia

1 8 7 2

Closed Loop (Fast) Power Control

Previous power control methods are slow and can compensateonly for path loss and long-term shadowing.

They are not sufficiently fast to track multipath-induced fading. To track this fluctuation, fast closed-loop power control must be

used. In IS-95/ cdma2000 the BS receiver measures the received signal

strength every 1.25ms and sends a power control command tothe transmitter. That command tells the transmitter to either increase or decrease the power by a predetermined step size

(e.g., 1dB). Due measurement and reporting delays, as well as a fixed stepsize, fast power control, while significantly faster than openloop power control, still invert the channel fading in sufficientlyfast fading conditions.


14/83




TechVirginia

1 8 7 2


AWGNchannel Tx and rx

power variationdue to finitestep size(0.5dB)

Unit average tx power


15/83




TechVirginia

1 8 7 2


Slow Fading5 Hz fading800Hz PC0.5dB step

Fast power controlcan track slowmultipath fadingchannel Receiver performanceimproved due to nearlyconstant receive power However, we paysome penalty at thetransmitter in higher average tx power

Power rise


16/83




TechVirginia

1 8 7 2


Fast fading150 Hz fading800 Hz PC

0.5dB step Power controlcannot track extremely fast fading Receive power

varies wildly Little increase intransmit power Must rely ondiversity


17/83




TechVirginia

1 8 7 2

Outer Loop Power Control

There are two parts to fast, closed loop power control: Inner loop power control Outer loop power control

The inner loop works on a smaller time scale (e.g., 1.25ms) andinstructs the transmitter to change its transmit power in order toequalize the received power.

The outer loop works on a slightly longer time scale (e.g., 20ms)and adjusts the inner loop target in order to achieve a

performance specification.


18/83




TechVirginia

1 8 7 2


Outer Loop Power Control adjusts the channel quality target (typically E b/Io)to obtain a target Frame Error Rate (FER)

This is accomplished by the following logic:

n = 0FER_Target = 0.01; % could be any reasonable target EbIo_Target( n) = EbIo_Target_Init;For n >= 0 do

if frame( n) is in error

EbIo_Target( n+1) = EbIo_Target( n) + OL_Step_Size;else

EbIo_Target( n+1) =

EbIo_Target( n) OL_Step_Size*FER_Target/(1-FER_Target)

n = n + 1 ;

Frame errors aretypically detected byusing a CRC check onthe information bits.

Frame errors cause anincrease in the

threshold, whilecorrect frames cause adecrease in thethreshold.


19/83




TechVirginia

1 8 7 2


( )1

a OuterLoopStepSizea FER

a FER

=

=

Setpoint leaks

down whenframes are good

After loop converges, every aincrease in the setpoint will beaccompanied by 100( FER )-1 a dBdecreases in the setpoint.

Setpoint jumpswhen frame error detected


20/83




TechVirginia

1 8 7 2

Power Control 2G vs. 3G

IS-95 versus cdma2000 In IS-95 only the reverse link uses the inner loop of closed-loop power

control The forward link uses outer-loop power control only. The mobile simply

reports frame errors and the base station adjusts transmit power accordingly

It was originally believed that The reverse link was the bottleneck The forward link did not need fast power control

It turns out that the downlink tended to be the bottleneck in CDMAsystems and that the forward link can benefit from fast power control

Thus, in cdma2000 fast inner-loop power control was added to theforward link


21/83




TechVirginia

1 8 7 2

Power Control Gains Forward Link

Outer loop power control only (50Hz)

Inner and outer loop power control (800Hz)

Required transmit power (as a fractionof the total basestation power) is

plotted versus mobile

speed for cdma2000 . Red curves are for asingle transmitantenna Blue curves are for transmit diversity

Fast inner loop power control benefits cdma2000 atlow speeds where itis most needed

[Nicoloso00]

~6dB

Temporal diversity benefit at high speeds


22/83




TechVirginia

1 8 7 2

Power Control Gains Reverse Link

If we monitor the received signal we find that for slow to moderate fading, power control improves the received signal quality significantly. Thus, thegains from a received E b /I o perspective are large.

If we monitor the transmitter we find that the improvement in received signalquality did not come for free. The penalty paid is a larger average transmit

power. However, regardless of which perspective we choose, we find that power

control is absolutely necessary from a system perspective. From a link performance perspective, we find that inner-loop power control provides benefits at slow to moderate fading rates, but can actually slightly degrade performance at high speeds.

-0.8dB-0.5dBITU Veh A

(50km/hr)

1.0dB1.8dBITU Veh A (3km/hr)

3.6dB5.8dBITU Ped A (3km/hr)

Gains in transmit power

Gains in receiveE b/Io

UMTS

[Holma00]


23/83




TechVirginia

1 8 7 2

Factors which impact power controlperformance

Mobile Speed Due to feedback delay power control becomes less effective as the fading rate increases

Power control errors Power control bits are not coded to reduce delay. Power control bit errors will cause

increase in required E b /I o.

Diversity If diversity exists (either in space or time) the gains from power control will be reduced

since fading is mitigated and worst case is improved.

Soft Handoff Power drift: When the mobile is in handoff, multiple BSs receive and independently detect

a single power control command. This causes the two BSs to drift apart. The data received at the mobile from different BSs can be combined to improve quality,

however power control bits are different thus the reliability of the power control bits on thedownlink is degraded

Estimation of channel quality If SNR measurements are inaccurate, power control will be less effective

Outer loop step size Smaller step size on the outer loop reduces error in slow fading conditions, but may not be

able to adequately track fast fading.


24/83




TechVirginia

1 8 7 2

Overview


Power control Base station assignment (soft handoff) Admission control Load control



25/83




TechVirginia

1 8 7 2

Base Station Assignment (Handoff)

Due to user mobility, base station assignment must changeduring a call (or session). Changing base station assignment istermed handoff.

Hard handoff TDMA/FDMA systems typically employ hard handoff where the

mobile is only communicating with a single base station at any giventime

Thus, the mobile must terminate communication with one base stationand simultaneously begin communication with a second base station (i.e.,it must make a hard or break before make change)

Soft handoff In CDMA, a mobile station can more easily communicate to multiple base stations simultaneously due to universal frequency reuse

Due to power control, soft handoff is actually necessary in CDMA When the power received from two or more cells (or sectors) exceeds a

predetermined threshold, the mobile will communicate with all of these

cells until one base station becomes dominant


26/83




TechVirginia

1 8 7 2


IS-95 handoff Each cell in a CDMA system transmits a pilot. The pilot is used for

acquisition, searching, demodulation and for performing mobile assistedhandoff.

Pilots are distinguished by transmitting different phases of a singlespreading code. Offsets (phases) are in multiples of 64 chips.

Mobiles assist in soft handoff by performing pilot strength measurementson all pilots in its vicinity

T_ADD: This parameter is stored by the mobile and is used as the pilotdetection threshold. When a measured pilot strength is above T_ADD,the mobile moves that pilot to its candidate set and requests a handoff

T_DROP: This parameter is stored by the mobile and is used for movinga pilot out of the active set. It is lower than T_ADD to provide hysteresisand avoid cells from going in and out of handoff at an excessive rate


27/83




TechVirginia

1 8 7 2


IS-95 handoff Active set: This set contains the pilots associated with the forward traffic

channels assigned to the mobile. Since there are three fingers in the

Rake, three-way handoff is typically the maximum allowed. (Six wayhandoff is allowed by the standard) Candidate set: This set contains pilots that are not in the active set but are

received with sufficient signal strength such that they could be properlydemodulated. This set is typically no more than six pilots.

Neighbor set: This set contains all the neighboring pilots that are notcurrently in the active or candidate sets. They represent pilots which arecandidates for handoff due to physical proximity, but are not currentlystrong enough.


28/83




TechVirginia

1 8 7 2

Handoff

T_ADDT_DROP

1. Pilot strength exceeds T_ADD. Mobile requests a handoff and moves pilot tocandidate set.2. Base station sends message to begin handoff.3. Mobile moves pilot to active set and completes handoff.4. Pilot strength drops below T_DROP and mobile begins handoff drop timer.5. Handoff drop timer expires. Mobile sends message to base station.6. Base station sends handoff message.

7. Mobile terminates connection and moves pilot to neighbor set.

(1) (2)(3) (4) (5)(6) (7)

Measured Pilot Strength

Neighbor Set Active Set

Candidate Set

Neighbor Set


29/83




TechVirginia

1 8 7 2

Soft handoff

Pilot Strength

P1 P2 P3

T_ADD

Active Set:P1

Neighbor Set:P2P3

Others


30/83




TechVirginia

1 8 7 2

Soft handoff

Pilot Strength

P1 P2 P3

T_ADD

Active Set:P1

Neighbor Set:P2P3

Others


31/83




TechVirginia

1 8 7 2

Soft handoff

Pilot Strength

P1 P2 P3

T_ADD

Active Set:P1P2

Neighbor Set:P3

Others


32/83




TechVirginia

1 8 7 2

Soft handoff

Pilot Strength

P1 P2 P3

T_ADD

Active Set:P1P2P3

Neighbor Set:

Others


33/83




TechVirginia

1 8 7 2

Soft handoff

Pilot Strength

P1 P2 P3

T_ADD

Active Set:P2P3

Neighbor Set:P1

Others

T_DROP


34/83




TechVirginia

1 8 7 2

Soft Handoff

Soft handoff is both necessary and a positive feature of CDMA It is necessary due to power control

Due to power control, the mobile should always be communicating withthe base station with the strongest pilot to avoid a positive feedback loop.

Hard handoff cannot guarantee this condition while soft handoff can. Soft handoff improves system performance by

Improving coverage Reducing call dropping probability Reducing the required E b /I o by providing macro-diversity

Soft handoff does require substantially more network resourcessince Multiple channels are being used for a single mobile Base stations must communicate to facilitate the soft handoff procedure


35/83




TechVirginia

1 8 7 2

Softer Handoff

Soft handoff is defined as the condition where a mobile iscommunicating with multiple base stations.

Softer handoff is defined as the condition where a mobile iscommunicating with multiple sectors of a single base station.

Softer handoff differs from soft handoff primarily on the uplink.The performance of the downlink is roughly the same.

On the uplink: In soft handoff, each base station reports frame estimates to the Mobile

Switching Center (MSC) and MSC must choose one of the two frames. In softer handoff the base station can coherently combine multipath from

different sectors using a Rake receiver. Only one frame estimate is thensent to the MSC.


36/83




TechVirginia

1 8 7 2

Gains of Soft Handoff

Soft handoff provides macro diversity which positively impactsthe system performance via Increased coverage Reduced interference on the downlink which increases downlink capacity

Reduced interference to other cells on the uplink which leads to increaseduplink capacity Associated lower blocking probability for same offered load

Disadvantages of Soft handoff Increased network resource usage due to communication between base

stations Increased Walsh code usage on the downlink

Could limit downlink capacity if large number of users are in soft handoff


37/83




TechVirginia

1 8 7 2

Soft handoff CoverageImprovement

Performance of Soft handoff Coverage Improvement Typically target 90%

coverage Three-way handoff

provides ~6dB coverage

improvement for (-7dB pilot allocation) Assumptions:

Path loss exponent = 4

37 cell layoutLog-normal shadowing

(=8dB)Uncorrelated base stationsEc/Io threshold = -20dB

[Kim00]


38/83




TechVirginia

1 8 7 2

Uplink Capacity Improvement

Soft hand-off provides macro-diversity which leads to lower required E b /I o for a target FER.

These relaxed requirements allow more users per cell, i.e.,greater capacity.

This is especially true for the uplink since no additional radioresources are required (unlike the downlink) Capacity is inversely proportional to the normalized interference

seen by the base station K (1+ f ) where f is the averageinterference from other cells.

Hard Handoff Soft Handoff ( N =2) Soft Handoff ( N =3) Soft Handoff ( N =4) f 2.38 0.77 0.57 0.55

CapacityImprovement

1 1.91 2.15 2.18

[Viterbi94]

Path Loss exp. = 4Shadow fading , =8dB


39/83




TechVirginia

1 8 7 2

Overview





40/83




TechVirginia

1 8 7 2

Admission Control

Unlike TDMA Systems CDMA systems have a soft capacity limit. That is,they are not necessarily hard limited to a fixed number of channels (i.e.,frequency or time slots)

CDMA Systems are limited by the interference that can be tolerated by thesystem.

Higher FER requirement Lower E b /I o requirement Lower E b/No requirement Higher interference levels tolerated Higher interference levels Higher number channels can be supported

Thus, we must have some method of determining when the system load hasreached a critical level

Admission Control accepts or rejects a request to establish a radioconnection. Metrics for characterizing the system load

Wideband power Throughput Number of connections


41/83




TechVirginia

1 8 7 2

Admission Control

If the number of connections are limited (either by mapping to system performance or by a channel element limit at the base station), thesystem capacity essentially is hard-limited.

If wideband power (either transmit or receive) is monitored and used tocontrol the load, the capacity is more directly related to the interference

environment and the capacity is truly soft. Reverse Link Admission Control

Limit A (typically 60% of pole capacity) new calls are blocked, but handoffsaccepted

Limit B (typically 85% of pole capacity) both new calls and handoffs blocked

Forward Link Admission Control

Limit A (typically 60% of tx power) new calls are blocked, but handoffsaccepted

Limit B (typically 85% of tx power) both new calls and handoffs blocked

Note: ~10% of tx power is dedicated to pilot and overhead channelsTwo tier admission processallows system to trade off

blocking probability and

dropping probability


42/83




TechVirginia

1 8 7 2

Admission Control

total in cell other cell N I I I P = + +

NoiseRise - 11

NoiseRise N

ULtotal

P I

= =

( )1

1(1 )

1/

N

UL j

b o j j j

f W

E N R

=

= ++

Total received interference at the base station

Uplink load as a function of the increase in background noise or noise rise is defined as

NoiseRise total N

I P

=

Bit rate

Voice activityChip Rate

Ratio of other-to-own

cell interference

Uplink load can also be defined as:


43/83




TechVirginia

1 8 7 2

Admission Control

1

1/

b o

LW

v E N R

=+

1-total I I L

Interference increase can be calculated from load increase as

where uplink load due to new user can be calculated as:


44/83




TechVirginia

1 8 7 2

Admission Control

Interference Limit B

Current load

Estimated load Noise rise dueto new user Interference Limit A

Uplink


45/83




TechVirginia

1 8 7 2

Admission Control

Downlink Load Estimation Downlink load can be determined from transmit power

Admission control is simply based on the increase in transmit power

P total depends on the initial power estimate obtained from open loop power control

max

total DL

P P

=

_ total old total threshold P P P +


46/83




TechVirginia

1 8 7 2

Overview





47/83




TechVirginia

1 8 7 2

Load Control

Even with admission control, occasionally overload can occur. When overload occurs, load control must be exercised to avoid system

instability (and amplifier overload). There are several methods of load control

Downlink: Deny power-up commands from the mobile

Uplink: Reduce the E b /I o target at base station Drop calls in a controlled fashion Throttle Packet traffic Handover to another carrier Apply Amplifier Overload Control (AOC)

Amplifier Overload Control The output power of the entire sector is reduced (including the pilot). This has the effect of reducing the cell range, causing mobiles on the periphery to

be picked up by neighboring cells which are hopefully under more lightly loadedconditions

Takes advantage of cell breathing phenomenon


48/83




TechVirginia

1 8 7 2

Load Control

Application Amplifier Overload Control (AOC) Cells are defined by received pilot strength Reducing transmit pilot power protects against overload and

effectively shrinks the cell

Cell of interest

Cell of interest

Before AOC After AOC

By reducing the pilot strength,mobiles naturallywill handover tosurrounding cells


49/83




TechVirginia

1 8 7 2

Overview





50/83




TechVirginia

1 8 7 2

Data Services in CDMA

IS-95 , the original CDMA cellular service, allowed for dataservice at 9.6kbps or 14.4kbps (the equivalent of one voicechannel)

IS-95B allowed for higher data rates by allowing a singlemobile to use up to 7 additional supplemental channels for amaximum burst data rate of 8*9600=76.8kbps or 8*14400=115.2kbps

cdma2000 3G1X allows for multiple supplemental channels(SCCs) each of which can achieve 9.6kbps-307kbps (assumingthe system load allows it) by varying the code length (spreadinggain) and coding. Multiple SCCs allows multimedia (i.e.,simultaneous voice and data services)

cdma2000 3G1xEV-DO achieves even higher throughput than3G1X by providing packet-based data service only.


51/83




TechVirginia

1 8 7 2

Data Services in CDMA

CDMA is ideally suited to voice communications where allusers require the same data rate and introduce roughly the sameinterference to the system.

However, there are several challenges to using CDMA for data

services, including: Signal acquisition for packet access using spread spectrum incurs

overhead and delay Bandwidth spreading limits the data rates allowable In a reuse environment, dynamic TDMA tends to be more efficient than

CDMA for high data rate services. Burst allocation in IS-95B overcomes each of these challenges


52/83




TechVirginia

1 8 7 2

IS-95B Packet Data

IS-95B introduced both higher data rates through channelaggregation, as well as a Burst Mode for packet operation

Reverse Link Burst operation: Mobile is assigned a low rate fundamental channel and remains in a

dormant state when there is no data to transmit.

Then data buffer exceeds predetermined threshold, mobile goes intoactive state and sends a supplemental channel request message (SCRM)along with pilot strength measurements

The base station (BS) or mobile switching center (MSC) uses the pilotmeasurements along with reverse channel load measurements to make a

burst control decision (i.e., burst admission control). If burst is admitted,the BS/MSC sends a supplemental channel assignment message (SCAM)on the fundamental channel. The SCAM specifies the burst length,number of SCCs assigned, and start time of burst.

If mobile still has data to send after assignment period, it sends another SCRM.


53/83




TechVirginia

1 8 7 2

IS-95B Data

Forward Link Burst Operation When the data buffer at the network interface exceeds a predetermined threshold,

the Inter-working Function (IWF) sends a burst request to the BS/MSC. The BS/MSC may request pilot strength measurements from the mobile. It

optionally uses these reported measurements along with the downlink power loadto make an assignment of SCCs to the mobile.

The BS/MSC then sends a SCAM to the mobile specifying the burst length, theWalsh channels to be used, and the start time of the burst.

Power control On the reverse link all codes are transmitted with the same power. The power

control is accomplished via the fundamental channel. This is deemed sufficient

since all codes have same data rate and error target. On the forward link only slow, outer loop control is provided.

Handoff Follows IS-95 voice practices for the fundamental channel May or may not be used for SCHs (more in a moment)


54/83




TechVirginia

1 8 7 2

IS-95B Packet Data

Mobile station BS/MSCCall origination: packet data service option

Call origination: packet data service option

Traffic Channel Release

Service Negotiation: max fwd/rev channels

Packet Data RegistrationLink Establishment (PPP, RLP)Inactivity

timer

Packetarrival

Service Negotiation: max fwd/rev channels

Existing Packet Data RegistrationLink Establishment (RLP)

Supplemental Channel Request Message (SCRM)

Pilot strength measurements, data backlogSupplemental Channel AssignmentMessage (SCAM)Burst Length, assigned supplemental channels, burst time

High rate burst transmission

Active State

Dormant State

Active State

Burst Level Admission Control

[Kumar99]


55/83




TechVirginia

1 8 7 2

Burst Admission Control

Burst admission control algorithms are used by the BS/MSC after a burstrequest is received to determine the start time, duration and number of codesassigned to each burst.

On the forward link burst admission is determined by constraining themaximum transmit power at each sector. The BAC algorithm considers all burst requests and the current downlink power

load and determines if the current request will cause the downlink load to exceeda predetermined limit.

If admission will cause the load to go over the limit, fewer channels can beassigned, lower power can be assigned assuming that ARQ will make up for thehigher FER, or the request can be denied (i.e., zero codes assigned)

On the reverse link burst admission is based on limiting the minimum

attenuation from the mobile to the cells with which it could potentiallyinterfere. Pilot measurements are used to determine the potential interference that the

mobile could cause to all neighboring base stations. If access will cause any of the neighboring base stations to exceed their received power limits, the request isdenied.


56/83




TechVirginia

1 8 7 2

RL Burst Admission Control

The burst admission control algorithm uses a pilot strength threshold T burst todetermine whether or not a burst is admitted. The mobile reports pilotstrength measurements for all pilots in the active and candidate sets.

If the pilot strength of the strongest pilot not in the active set is below T burst ,and the user isnt in the process of executing a handoff, then the burst is

admitted. Note that if T burst = T add then bursts are always admitted. As T burst is reduced,

the data coverage area is reduced, but so is the impact on voice capacity. The number of codes assigned and the length of the burst are a function of

the number of voice users already admitted in the cell and its neighbors, andthe pilot strength measurements. The more loaded the cell, the lower the number of channels and/or burst duration The higher the pilot strength measurements, the lower the number of channels

assigned since it will cause more interference to neighboring cells. Increasing the burst duration increases the likelihood that a burst will be

interrupted by a handoff request


57/83




TechVirginia

1 8 7 2

Burst Admission Control

More stringent requirements lead to lower burst admission probabilities but higher voice capacity since bursts whichare closer to cell boundaries (thus causing more interferenceto adjacent cells) are not admitted. [Kumar 99]

Burst given six codechannels or 57.6kbps


58/83




TechVirginia

1 8 7 2

Overview





59/83




TechVirginia

1 8 7 2

cdma2000 Data Channels

cdma2000 provides improvements over IS-95B by Increasing the granularity of possible data rates using variable spreading

gain and coding schemes as opposed to coded aggregation Multiple concurrent data services Improved Link Access Control Improved Medium Access Control


60/83




TechVirginia

1 8 7 2

IS-95 and cdma2000 Layer Structure

Physical Layer

MediumAccess

Control

MAC control

states

Multiplexing QoS control

Best Effort Delivery

Link AccessControl

LAC protocol Null LAC

High Speed Circuit

Network Layer Services

Circuit dataapplicationSignaling

Services

Packet DataApplication

VoiceServices

TCP UDP

IPPPP

OSI 1

OSI 2

OSI3-7

cdma2000 functionality


61/83



VIRGINIA POLYTECHNIC INSTITUTE

AND STATE UNIVERSITY

TechVirginia

1 8 7 2

LAC/MAC Improvements incdma2000

True LAC protocol entity Supports highly reliable point-to-point transmission over the air for signaling

services and (optionally) for circuit data services. May use ARQ retransmission. Allows Null LAC which allows voice to be treated as circuit data application.

Multiple instances of MAC state machine possible for multiple services

5ms frame structure for dedicated control channels IS-95 MAC

Two states active/dormant Active: traffic channel assigned to the mobile, link layer and PPP connections

established between IWF and mobile Dormant: No traffic channel is assigned to the call, but knowledge of users

registration for data service and PPP connection maintained. Data can only be transmitted in active state, but timeout is long due to the

expected bursty nature of the data. If timeout is short, inefficiencies arise since awhole new link would need to be established for each packet. However, longtimeouts waste resources since a channel is occupied without data to transmit.


62/83





TechVirginia

1 8 7 2

MAC Control States

Active State Dormant State

Long Timeout

Traffic

Active State Dormant State

Timeout

Traffic

ControlHold State

SuspendedState

Timeout Timeout

TrafficTraffic

Traffic, power control andcontrol channelsassigned

No dedicatedchannels No BS, MSCresources PPP state

maintained

Power control andcontrol channelsassigned Very fast trafficchannel reassignment

No dedicatedchannels RLP and PPP statemaintained

Virtual Active Set

IS-95B

cdma2000


63/83





TechVirginia

1 8 7 2

LAC/MAC Improvements incdma2000

cdma200 MAC Additional MAC control states to allow finer control of the physical layer

resources by data services. Control Hold State: Dedicated control channel (with discontinuous

transmission capabilities) maintained between user and the BS. Controlcommands can be transmitted with little to no latency. Power control alsomaintained.

Suspended State: No dedicated channels to or from the user are maintained, but the state information of the RLP is maintained as well as a virtual activeset which permits either the user or the BS to know which BS can best beused in the event that packet traffic arrives.

Multiple MAC state machines allowed

Best effort delivery: MAC provides reliable transmission using RLP butno guarantees Multiplexing and QoS control: Enforcement of negotiated QoS levels by

mediating conflicting requests from competing services and prioritizationof requests.


64/83





TechVirginia

1 8 7 2

cdma2000 Power Control

IS-95B used code aggregation in order to achieve higher datarates. Thus, a single power control loop handling all of thechannels was possible.

However, cdma2000 provides higher data rates through variablespreading factors. This results in potentially different E b/Iovalues for the supplemental and fundamental channels.

cdma2000 provides two power control loops for Forward Link: Primary power control loop at 800Hz 400Hz 200Hz Secondary power control loop at 0Hz 400Hz 600Hz (Total feedback rate is 800Hz)

cdma2000 provides no support for multiple loops on reverselink However, some research shows that a secondary loop would be

beneficial


65/83





TechVirginia

1 8 7 2

Separate Power Control Loops

2 2.5 3 3.5 4 4.5 5 5.5 610

-3

10-2

10-1

100

Eb /No (dB)

P r o

b a

b i l i t y o

f F r a m e

E r r o r

No Power ControlUsing FCH OnlySeparate SCH PC Loop

Reverse Link Performancecdma2000FCH (9.6kbps) E b /I o = 6dB

SCH 460.8kbps (RC6)Rayleigh fading (Ped A pathmodel )

Power Control provides 2.7dB gain Separate Loop provides additional 1-2 dB gain

[Lee99]


66/83





TechVirginia

1 8 7 2

Performance Trade-offs

Forward Link capacity and coverage As data rate increases, transmit power requirements increase for a

constant E b /I o requirement. Thus, large capacity cannot be madeavailable under same coverage guarantees as voice rate.

In order to maximize system throughput, coverage must reduce withincreasing data rates.

Supplemental channels and Soft handoff By placing supplemental channels in soft handoff we can reduce the E b /I o

requirements for a given FER. However, soft handoff consumes both forward link radio resources as

well as network resources. Since SCCs can consume more resourcesthan voice calls, this consumption is greater in data systems.

FER requirements can often be relaxed due to availability of ARQ thusSCCs often not put in soft handoff


67/83





TechVirginia

1 8 7 2


Reverse Link Interference Constraints Higher data rate users on the uplink consume resources in multiple cells since

their transmit power spills over into neighboring cells The greater the distance of the mobile from the primary base station, the more

interference it will cause to other cells. These users should be restricted in datarate. (Mobiles will also be limited in range for high rates due to power limits.)

To maximize cell throughput the burst admission control should reject a large percentage (~15%) of the requests from mobiles near the edge of the cell.

Reverse link power control Uplink E b /I o requirements are reduced by nearly 2dB at low speeds as compared

to high speeds due to fast power control. Since most data users are anticipated to be low mobility, reverse link power control has a large impact on systemthroughput.

At low speeds smaller power control step sizes are allowable. A lower power control step size can reduce E b /I o requirements by nearly 1dB.

By increasing the FER target in the outer loop (and using ARQ) E b /I o can bereduced by another 0.5dB or so.


68/83





TechVirginia

1 8 7 2


Mobility Tracking There is a large infrastructure overhead associated with signaling and

reallocation/deallocation of resources for users going into and out of softhandoff since the burst allocation algorithm must re-evaluate theinterference conditions.

Thus, users which trigger handoffs often should be assigned shorter bursts to avoid handoffs during burst mode transmission.

Burst allocation algorithms ideally should take mobility into account. A running average of handoff events provides a good mobility metric to

include in these algorithms.


69/83





TechVirginia

1 8 7 2

Overview



RRM for Packet Data Services IS-95B Data cdma2000 (3G1X RTT) Data Services 3G1xEV-DO (High Data Rate or HDR)


70/83





TechVirginia

1 8 7 2

High Data Rate 3G1xEV-DO

While cdma2000 3G1x provides for flexible data rates andincreases throughput over IS-95, if data services are expected to

be Internet-like (i.e., tolerant of delays and asymmetric), a newforward link structure can be designed to increase throughput.

Third Generation Phase 1 (3G1x) Evolution (EV) Data Only(DO) or 3G1x-EV/DO radically changes the forward link from3G1x. The physical layer interface is incompatible with 3G1xand thus it requires its own 1.25MHz carrier.

The downlink uses time multiplexing rather than code

multiplexing since this is more efficient for delay tolerantservices.

Instead of using power control to maximize resource utility,HDR uses rate control.

VIRGINIAPOLYTECHNICINSTITUTE&STATEUNIVERSITY


71/83





TechVirginia

1 8 7 2

Forward Link Scheduling

The base station time multiplexes users along with pilot burstsat full transmit power using rate control , as opposed to codemultiplexing all users and the pilot and using power control .

Rate control is based on pilot strength measurements (andconsequent E c /N t or E c /I o estimates) at the mobile.

All mobiles estimate E c /N t using pilot strength measurementsand map that estimate to a data rate request. This request istransmitted to the base station along with the ID of the strongest

base station every 1.67ms slot. The MSC uses this informationto schedule packets in order to maximize throughput while

constraining latency using a proportionally fair schedulingalgorithm.

Specifically, the scheduler sends data to the mobile that has thehighest where DRC is data rate request and R is averagerate it has received over a predetermined window.

DRC R



72/83





TechVirginia

1 8 7 2

3G1xEV-DO : Various Data Rates

Data Rate Packet Length Slots FEC Modulation E c/N t38.4kbps 128 bytes 16 1/5 QPSK -12.5dB76.8kbps 128 bytes 8 1/5 QPSK -9.5dB

153.6kbps 128 bytes 4 1/5 QPSK -6.5dB307.2kbps 128 bytes 2 1/5 QPSK -4.0dB614.4kbps 128 bytes 1 1/3 QPSK -1.0dB921.6kbps 384 bytes 2 1/3 QPSK 1.3dB

1228.8kbps 256 bytes 1 2/3 QPSK 3.0dB1843.2kbps 384 bytes 1 2/3 8PSK 7.2dB2457.6kbps 512 bytes 1 2/3 16QAM 9.5dB

[Bender00]

Packet sent during 1.67ms slots(2048 chips)



73/83





TechVirginia

1 8 7 2

3G1xEV-DO: Predicted Data RateProbabilities

76.8 102.4 153.6 204.8 307.2 614.4 921.6 1228.8 1873.0 2457.0Data Rate (kbps)

[Bender00]



74/83





TechVirginia

1 8 7 2

Forward Link Scheduling

An intra-cell packet-based scheduling scheme providinghigh-speed packet data transmission to cellular mobile users

Total throughput is sacrificed in order to preserve fairness Maximum throughput of an individual user in a multi-path

fading environment is achieved when[Bedekar99]

Each base station transmits to its data users one at a time with full power

The optimal total throughput is a constrained optimization

problem subject to different user QoS requirements



75/83





TechVirginia

1 8 7 2

Scheduling Tradeoff

Throughput vs. latency For a general case of N classes and the latency ratio Lmax /Lmin, the maximum

achievable throughput C is [Bender00]

Rn: date rate of class-n packets, and R1C for all n>n o.

Bimodal latency

Each users latency is either Lmax (if RnC ) 1xEV-DO scheduling sacrifices a certain amount of system capacity for fairness among users using latency ratio constraint

1xEV-DO down link supports 12 combinations of data rate and slotallocation, and deviates from the strict bimodal latency allocation due tonumerology considerations

[Bender00]

0

0

0

0

min max1 1

min max1 1

( / )b/s,

/ ( / )( / )

n N

n nn n n

n N

n n n nn n n

P P L LC

P R P R L L

= = +

= = +

+

=

+



76/83




TechVirginia

1 8 7 2

Proportional Fairness (PF)

A vector of rates R=[R 1 , R2 , , R N ] T is proportionally fair if it is feasibleand if for any other feasible vector R , the aggregate of proportionalchanges are zero or negative, i.e.,

Kelly has proven that [Kelly97]

A system optimum is achieved when users choices of charges and network choice of allocated rates are in equilibrium

1xEV-DO uses PF criteria to provide the best possible schedulingalgorithm in the sense that the total percentage decrease suffered by all

the other users is greater than the increase by one or some specific usersunder another scheduling algorithm [Kelly97] Proportionally fair all users are served with a throughput that is

proportional to their C/I. It can be shown that the algorithm will serveall users approximately the same amount of time/power, but at data rates

proportional to their channel conditions [Holtzman00].

'

1

0, N

n n

n n

R R R=



77/83




TechVirginia

1 8 7 2

PF Algorithm

Multi-user diversity is obtained byscheduling transmission to the user having more favorable channelconditions Time constant T c determines the

maximum time duration that a user can be starved

T c represents a tradeoff betweenmaximum tolerable delay and theoverall throughput

PF limitations Does not satisfy the differing multi-

user QoS requirements, but fairness across the board

Unavoidable schedulinginefficiencies when channel ischanging

Pseudocode of PF algorithm

// Definitions

DRC i(t): current requested rate from user i at slot t,i=1, , N.

Ri(t): moving-average data rate of user i at slot t ,i=1, , N.

Rc: current transmission rate of user i, i=1,,N.

T c: time constant of user moving-average data rate

// Scheduling at each new packet transmission

1. decide the highest DRC m(t)/R m(t)=max{DRC i(t)/R i(t), i=1,,N.}

2. Send data to user m

3. randomly break the ties if any

// update average user data rate at each slot

Ri(t+1) = (1-1/T c )R i(t) + 1/T c*Rc , i=1,, N,

where R c= DRC i(t) if user i is receiving data,

Rc= 0 otherwise



78/83




TechVirginia

1 8 7 2

EV-DO Simulation Example

Moving average AccessTerminal data rates over 1000consecutive slots (1.67ms/slot)

Access Terminal SINR estimates

AT0: 8.3m/s, ~1000m

AT1: 2.8m/s, ~850mAT2: 0.8m/s, ~300m

AT3: 8.3m/s, ~1200m

Drops in C/I correspond to drops inindividual throughput, but notnecessarily cell throughput



79/83




TechVirginia

1 8 7 2

EV/DO Simulation Example

Drops in individual C/I correspond todrops in individual throughput, but notnecessarily cell throughput

Moving average AccessTerminals data rates over 1000consecutive slots (1.67ms/slot)

Average sector throughputwith 4 Access Terminals



80/83




TechVirginia

1 8 7 2

1xEV-DO

Bursty, delay-tolerant packet data services are most efficientlyserved via time multiplexing rather than code multiplexing.

1xEV-DO takes advantage of this fact and uses timemultiplexing with rate adaptation achieve very high sector throughputs (hundreds of kbps)

Proportionally fair scheduling algorithm maximizes sector throughput under the constraint of maximum latency ratios.

The above discussion focused on the forward link. The reverselink is essentially the same as cdma2000 . Data systems asenvisioned to be asymmetric with higher forward link requirements.

Standard also specifies the use of demultiplexing of the data into16 parallel streams, each using a separate Walsh code to allowtransmit signal to appear similar to IS-95/ cdma2000 voicesignals. This allows reuse of RF front end.



81/83




TechVirginia

1 8 7 2

Summary

The key to maximizing the efficiency of CDMA-based wirelesssystems is management of the radio resources.

In voice systems this is primarily done through interferencemanagement. Minimizing interference while maintaining error rate performance maximizes capacity.

This is accomplished via Power Control Soft hand-off Admission Control Overload control

Adding data to voice systems adds additional burst admissioncontrol requirements. This is needed to balance data coveragewith voice capacity.

Packet-data-only systems use a combination of rate control, fastcell-site selection and multi-user diversity.



82/83




TechVirginia

1 8 7 2

References

[Garg00] V.K. Garg, IS-95 and cdma2000: Cellular/PCS Systems Implementation,Prentice-Hall, 2000.

[Holma00] H. Holma and A. Toskala, ed., WCDMA for UMTS, John Wiley and Sons,2000.

[Kim00] K.I. Kim, ed., Handbook of CDMA System Design, Engineering, andOptimization , Prentice-Hall, 2000.

[Yang98] S.C. Yang, CDMA RF System Engineering, Artech House Publishers, 1998.[IS95] TIA/EIA/IS-95-A, Mobile Station-Base Station Compatibility Standard for DualMode Wideband Spread Spectrum Cellular System, Telecommunication IndustryAssociation, Washington, DC, May 1995.

[Nicoloso00] S.P. Nicoloso, M. Metke, and R.M. Buehrer, Frame-Quality Based vs.Eb/No based Power Control Methods for the cdma2000 Third Generation Standard,

Proceedings of the Virginia Tech Wireless Symposium, June 2000.[Knisely98] D.N. Knisely, S. Kumar, S. Laha, and S. Nanda, Evolution of Wireless Data

Services: IS-95 to cdma2000, IEEE Communications Magazine, pp. 140-149,October 1998.

[Kumar99] S. Kumar and S. Nanda, High Data-Rate Packet Communications for Cellular Networks Using CDMA: Algorithms and Performance, IEEE Journal onSelected Areas in Communications, vol. 17, no. 3, pp. 472-492, March 1999.

[Holtzman00] J. Holtzman, CDMA Forward Link Waterfilling Power Control, Proceedings of VTC2000 Spring , pp. 1663-1667, May 2000.



83/83


TechVirginia

References

[Bender00] P. Bender, et. al, CDMA/HDR: A Bandwidth-Efficient High-SpeedWireless Data Service for Nomadic Users, IEEE Communications Magazine, pp. 70-77, July 2000.

[Bedekar99] A. Bedekar, et al., Downlink scheduling in CDMA data networks, Proceedings of Globecom99 , pp. 2653-2657.

[Kelly97] F. Kelly, Charging and rate control for elastic traffic, European Transactionson Telecommunications , Vol. 8, 1997, pp. 33-37.

[Perez] J. Perez-Romero, et al., Traffic and physical layer effects on packet schedulingdesign in W-CDMA systems, Electronics Letters , vol. 38, no. 7, pp. 341-342.

[Mot01] Motorola, HSDPA system performance with/without FCS (faded but nomotion), 3GPP TSG RAN WAG1, TSGR1#18(01)0046, Jan, 2001.

[Ejzak97] R.P. Ejzak, et.al., BALI: A Solution for High-Speed CDMA Data, Bell LabsTechnical Journal, vol. 2, no. 3, Summer, 1997, pp. 134-51.

[Lee99] W. Lee and N.P. Secord, Performance of Closed-Loop Power Control for aMultiple-Channel Mobile Station in the cdma2000 System, Proceedings of WCNC99 , pp. 908-912.

[Viterbi94] A.J. Viterbi, et. al., Soft Handoff Extends CDMA Cell Coverage andIncreases Reverse Link Capacity, IEEE Journal on Selected Areas inCommunications, vol. 12, no. 8, pp. 1281-1287, October 1994.

CDMA Radio Resource Management

Documents

Transcript of CDMA Radio Resource Management