26 CDMA Capacity Theory

The information contained in the file is solely property of ZTE corporation. Any kind of disclosing without permission is prohibited.ZTE University univ.zte.com.cnuniv.zte.com.cnuniv.zte.com.cn

CDMA RF Planning Unit6


Main Content

Basic Elements and Calculation of Traffic Capacity

Recognized Busy Hour Methodologies

Determine Erl B Table and GOS

Determine the Call Mix and It’s Effects

Calculating Um Interface Capacity


What is “ capacity”?

Capacity more typically refers to the amount of activity that a particular device or group of devices can facilitate support forwithout experiencing a failure or fault.

In telephone switching system, demand for the server from the source is called traffic, whereas it is called traffic load from the perspective of the server. The definition is as follows: the traffic (or traffic load)produced (or shouldered) by a source (or a server) during the period T is the total of the lasted time for each of all services during this period

Note: Grade of Service (GOS) is defined as the probability that a random call will be delayed, or receive a busy signal, under a given traffic load.


Components of a Typical Cellular System

Two major components that effect traffic:• Access components• Network components

MSC and BSC BTS

PSTN Antenna

E1

Network Access

“Capaciy” can be seen everywhere！


Units for Capacity

• Centi-call second (CCS)– The sum of the number of busy circuits, providing the busy

trunks were observed every 100 seconds (36 observations in 1 hour)

• Erlangs– Most common measurement of traffic– One circuit continuously used for one hour

• Observed once every 100 seconds– One Erlang equals 36 CCSs


Minutes of Use

1 Erlang = 60 MOU = 36 CCS

1 MOU = .16 Erlang = .6 CCS

1 CCS = .028 Erlang = 1.67 MOU


Conversion Triangle

ErlangsCCS/

36

Erlan

gsx 3

6

CCS/.6 min x .6

Erlangs x 60 min/60

CCS MOU


Capacity Flow DefinitionTraffic flow through an office is defined as the product of the number of calls during a period of time and their average length, called the holding time.

A = ACHT x BHCA/3600BHCA designates the number of calls originated during a period of one hour ACHT is the average holding time, Typically, between 60 seconds and 120 seconds.A is the traffic flow in Erl

For example: 200 calls of an average duration of two minutes are generated during a period of one hour, then the traffic flow equals:200(BHCA) x 120(ACHT)/3600 = 6.67Erl(traffic flow)

Traffic flow expressed in hour-calls is referred to as traffic intensity. In the example, the traffic intensity equals: 6.67Erl


Capacity Intensity

Traffic intensity is the ratio of the time during which a facility is occupied continuously to the time this facility is available.

A traffic intensity of one traffic unit (one Erlang) means continuous occupancy of a facility during the time period under consideration, regardless of whether or not information is transmitted.

In one day，the capacity intensity is different in different hour.So we usually use “busy hour” as capacity intensity in planning.

For example: in China the capacity intensity model for one user is0.025Erl/sub


Main Content







Busy Hour Methodologies

Network elements should be engineered to provide an acceptable level of service during an average busy hour of the day, during the busiest seasons of the year.

Busy hour methodologies are based on measurement of call traffic intensity for discrete periods, carried on over an extended period of time.

These periods of measurement can vary based on hour of day, day of week, and season.

All above we should take care, that our planning should satisfy all the time requirement.


Daily Traffic VariationsTr

affic

07:0

0

08:0

0

09:0

0

10:0

0

11:0

0

12:0

0

13:0

0

14:0

0

15:0

0

16:0

0

17:0

0

18:0

0

19:0

0

Exploded view of measured hour

peak trafficmeasured traffic

(=average traffic)

Actual busiest 60-minute period

Busiest hour as measured

Time of Day

Traf

fic07

:00

08:0

0

09:0

0

10:0

0

11:0

0

12:0

0

13:0

0

14:0

0

15:0

0

16:0

0

17:0

0

18:0

0

19:0

0

Exploded view of measured hour

peak trafficmeasured traffic

(=average traffic)

Time of DayKey:Key:

Actual busiest 60-minute period

Busiest hour as measured


Hourly Traffic Variation

0

500

1000

1500

2000

2500

8:00 A

M10

:00 A

M12

noon2:0

0 PM

4:00 P

M6:0

0 PM

8:00 P

M

Hourly traffic variation is usually selected for traffic characterization over a day because an hour is a convenient frame of reference.


Daily Traffic Variation

0

500

1000

1500

2000

2500

3000

3500

4000

Mon Tues Wed Thurs Fri Sat SunOringinating Busy Hour Calls

Falling off traffic occurs during the course of the week. Higher average intensities occur on business days with lower activity during weekends and holidays.


Seasonal Traffic Variations

2700

2900

3100

3300

3500

3700

Jan. . . . . . . . . .

Aug. . .

Oct

. . . . .

Within a busy season, each system experiences weekly and daily traffic variations. Due to conditions peculiar to the area served by the system, someweeks have more traffic than others.


Main Content







Capacity Efficiency

• The efficiency or capacity of how a facilityhandles traffic is effected by the number ofchannels or trunks.

• If the number of channels double the efficiencyor capacity more than doubles.

• For example, if given: CCS = 36 x Erlang and GOS = 2%then:1 channel = 0 to 1 CCS

2 channels = 0 to 8 CCS3 channels = 0 to 22 CCS4 channels = 0 to 39 CCS5 channels = 0 to 60 CCS


Erlang B Table

N P.01 P.012 P.015 P.02 P.03 P.05 P.07 P.101 0.010

10.0121

0.0152

0.0204

0.0309

0.0526

0.0753

0.1112 0.153 0.168 0.19 0.223 0.282 0.381 0.47 0.5953 0.455 0.489 0.535 0.602 0.715 0.899 1.06 1.274 0.869 0.922 0.992 1.09 1.26 1.52 1.75 2.055 1.36 1.43 1.52 1.66 1.88 2.22 2.5 2.886 1.91 2 2.11 2.28 2.54 2.96 3.3 3.761 channel = 0 to 1 CCS or 36 x .0204 = 0.734 CCS2 channels = 0 to 8 CCS or 36 x .223 = 8.028 CCS3 channels = 0 to 22 CCS or 36 x .602 = 21.672 CCS4 channels = 0 to 39 CCS or 36 x 1.09 = 39.24 CCS5 channels = 0 to 60 CCS or 36 x 1.66 = 59.76 CCS


Erlang Tables and Poisson Table

Three types of table used to determine carried traffic

• Erlang B table• Most often used in cellular• Calls blocked – subscriber tries again later

• Erlang C table• Calls not blocked but are queued up to wait for service

• Poisson table• Calls are in the system for one holding time• If call is blocked then put into queue• Need to overestimate number of channels needed compared to

Erlang


Same Number of Trunksin Erlang and Poisson Tables


Same Traffic Capacity in Erlang and Poisson Tables

No. ofTrunks

POISSONTraffic in ErlangsP=1% P=2% P=5%

ERLANG-BTraffic in ErlangsP=1% P=2% P=5%

ERLANG-CTraffic in ErlangsP=1% P=2% P=5%

1 0.01 0.02 0.05 0.01 0.02 0.06 0.01 0.02 0.05

2 0.15 0.21 0.35 0.15 0.23 0.38 0.15 0.21 0.34

3 0.43 0.57 0.82 0.46 0.60 0.90 0.43 0.55 0.79

4 0.82 1.01 1.37 0.87 1.09 1.53 0.81 0.99 1.32

5 1.28 1.53 1.97 1.36 1.66 2.22 1.26 1.50 1.916 1.78 2.09 2.61 1.91 2.28 2.96 1.76 2.05 2.53

31 19.53 20.66 22.45

32 20.32 21.48 23.30

33 21.12 22.30 24.15

34 21.92 23.12 25.01

35 22.72 23.94 25.87

36 23.53 24.77 26.73

37 24.33 25.60 27.59

21.19 22.83 25.77

22.05 23.73 26.74

22.91 24.63 27.72

23.77 25.53 28.70

24.64 26.44 29.68

25.51 27.34 30.66

26.38 28.25 31.64

19.37 20.45 22.09

20.16 21.26 22.93

20.95 22.07 23.78

21.75 22.89 24.63

22.55 23.71 25.48

23.35 24.53 26.33

24.15 25.36 27.19


Grade of Service

How do we relate traffic, grade of service (GoS), and Erlang/Poisson tables to provision the proper number of channels/trunks to support traffic?

Let’s begin by examining the Erlang/Poisson Tables.


Traffic: Offered/Carried/Blocked

Carried TFC = Offered TFC - Blocked TFC

To/FromSubscribers

CarriedTraffic

OfferedTraffic

TheNetwork

Blocked


Blocking probability and GOS

• Blocking probability is the likelihood that a caller is unable to get a circuit when one is requested

• Blocking probability is usually expressed as a percentage, using a type of shorthand notation:

P.02, implying 2% blocking probability

• Blocking probability is often referred to as GOS, and P.02 is a common goal at the air interface.


Grade of Service - GOS

• Defined as service quality component of a system• Indicates the call blocking percentage by

congestion• Design values in planning:

– Trunks for land-based network: 1% GOS– Subscriber unit: 2-5% GOS– Distributed by the system


Main Content







Call Mix

Differences in mobility affects the capacity of the wireless system. Calls can originate and terminate at a variety of locations. This is known as call mix. It is important to know the call mix of your system and develop call models.

• Call transfers between cells generates considerable work in the system.

• Subscriber Features such as Short Message Service• also affect traffic and capacity.

Since call processing behavior changes constantly, it must be measuredagain whenever definitive capacity analysis is done.


H-diagram – Termination of Calls

Mobile

Trunks

Land

Trunks

TANDEM(Call Delivery,Call Forward,

voice mail, etc)

INTRA(Mobile-to-Mobile)

Output( PSTN,

another CCC)

Input(PSTN,

GATEWAY, etc)

MobileOrigination

MobileTermination

MM

ReorderStimeoutDouborig

Invalid attempts(Tones + announcements)

Pagingtimeout

Inactive Mobile(Treatment)


M - L

L - M

LL

Mobile

Trunks

Land

Trunks

TANDEM(Call Delivery,Call Forward,

voice mail, etc)

INTRA(Mobile-to-Mobile)

Output( PSTN,

another CCC)

Input(PSTN,

GATEWAY, etc)

MobileOrigination

MobileTermination

MMMM

ReorderStimeoutDouborig


Pagingtimeout

Inactive Mobile(Treatment)


M - L

L - M

LL


Main Content







Network Model In Reality

First,we shouldKnow one singleCell capactiy?


CDMA Pole Capacity - Isolated Cell

Student Exercise

For an Isolated Cell, Pole Point capacity is defined as:

Pole Point capacity = 1 + Processing GainEb/No

For an Isolated Cell, Pole Point capacity is defined as:

Pole Point capacity = 1 + Processing GainEb/No

Calculated the Pole Point for Rate Set 1 or 9,600 bps, with a spreadbandwidth of 1.2288 MHz.

Note: We know Eb/No as 7dB. Converting this to a numerical ratiowe get 5.

Assumes :Perfect Power ControlNo Voice Activity FactorNo Sectorization Gain


CDMA Reverse CapacityBTS Receiver Noise RiseVoice Activity Factor (VAF)

40% 60%

VOICE VOICENO VOICE NO VOICE

Average TX Poweris lower by VAF

FULL-RATEFRAMES

1/8th RATEFRAMES

Capacity is increased by 1 = 2.5 times0.40


CDMA Reverse CapacityBTS Receiver Noise RiseIn-Cell vs Out-of-Cell Interference

A1-A7 In-Cell Interferers

B1,B2,C1,C2 Out-of-CellInterferers


CDMA Reverse CapacityBTS Receiver Noise RiseIn-Cell vs Out-of-Cell Interference

60%6%

6%6%

6%

6%

6%.2%

.2%

.2%

.2%

.2%.2%

.2%

.2%

.2%.2%

.2%.2%

.03%

.03%

.03%

.03%.03%

.03%.03%

.03%

.03%

.03%

.03%

.03%

.03%

.03%.03%

.03%

.03%

.03%

In-Cell Out-of-Cell

Other

30% 2.4% 0.36% 0.24%

60% 33%

Ratio Out-of-Cell to In-Cell Interference = 33% = 0.5560%


CDMA Reverse Capacity

What happens if we SECTORIZE the BTS?

We have excludedall these interferersfrom the Alpha sector

Interference in thisSector is much lower


CDMA Reverse CapacityWhat happens if we SECTORIZE the BTS?

We have excludedall these interferersfrom the Alpha sector,assuming ‘perfect’sectorization!

We can now bringeach sector backto full Pole Capacity


CDMA Reverse Capacity

What happens if we SECTORIZE the BTS?

We can now bringeach sector backto full Pole CapacitySite (BTS) Pole

Capacity Increases3 times users

‘Not Quite!’

Why?


These userscontributeInterferenceinto AdjacentSectors of theSite

CDMA Reverse CapacityWhat happens if we SECTORIZE the BTS?

OverlappingZone betweenSectors

REALITY!

For a 3-sector configuration, the sectorzation gain is about SF = 2.2 to 2.7.For a 6-sector configuration, the gain is about SF = 3.5 to 4.5.


Theoretical equation ofcalculating reverse capacity

SfvIE

GMfb

cp ××

⎥⎥⎦

⎤

⎢⎢⎣

⎡

+×××+= ρ

η)1(/

10

Loading factor ρ

Total Received Power-to-Noise Ratio vs. Cell loading


Theoretical equation of calculating reverse capacity

SfvIE

GMfb

cp ××

⎥⎥⎦

⎤

⎢⎢⎣

⎡

+×××+= ρη

)1(/1

0

Gp is Processing Gain (numerical)

Eb/No is numerical 7dB in IS-95; 4.9dB in 1XRtt

f is ratio of out-of-cell to in-cell interference (estimated at 55% or 0.55)

SG is Sectorization Gain (eg: 2.55for a 3-sector, due to handoff boundaries)

Vf is the Voice Activity Factor eg: 45% or 0.45

Nc is non-accurate power control factor 0.8 in IS-95; 0.9 in 1XRtt


CDMA Reverse CapacityStudent Exercise:

Rate Set 1 8kb/s Data Rate 9,600 bpsSpreading Rate 1.2288 Mcps

How many ?

Processing Gain 128 Loading Factor 0.7Eb/No 7dB Sectorzation Gain 2.55Voice Activity Factor 0.4 Non-accurate power

Control 0.8

Interference Factor 0.55 Capacity

IS-

95

Processing Gain 128 Loading Factor 0.6Eb/No 4.9dB Sectorzation Gain 2.55

Voice Activity Factor 0.4 Non-accurate powerControl

0.9

Interference Factor 0.55 Capacity

IXR

tt

How many ?


Traffic Capacity in One BTS?TRI-SECTOR

1BTS(S111)

35Users

35Users

35Users

ErlB Table (GOS:2%)

ErlB Table (GOS:2%)

ErlB Table (GOS:2%)

26.4Erl26.4Erl26.4Erl

26.4Erl*3

=79.2Erl

GOS:2%


1X Capacity Planning Example1 The total subscribers (voice and data) 50000

2 The voice subscribers ratio 100%

3 The data subscribers ratio 25%

4 The busy hour traffic capacity of voice 0.02Erl/Sub

5 GOS 2%

6 The total traffic capacity requirement for voice (Erl) 1000Erl

7 ZXC10-BSS Single sector capacity (Erl) 26.4Erl

8 The sectors number to support voice 1000/26.4=38

9 The total data subscribers 5000*25%=12500

10 The average data throughput of subscriber in voice busy hour (This parameter prediction decided by operator and manufacture together. )

50 bps

11 The uplink and downlink data ratio 1:4

12 The average downlink throughput of subscriber 40 bps

13 The average uplink throughput of subscriber 10 bps

14 The total downlink throughput of subscriber 40bps*125000=500Kbps

15 The total uplink throughput of subscriber 10bps*125000=125Kbps

16 ZXC10-BSS single sector downlink data throughput threshold 450Kbps

17 ZXC10-BSS single sector uplink data throughput threshold 400Kbps

18 The sectors number to support data 500Kbps/450Kbps=2

19 The total sectors 38+2=40

In reality, voice and data are used together , but in planning, we consider them separately for convenience calculation.


Capacity Analysis and Network Optimization

Limited capacityLimited capacityIncreasing

dropped call rateIncreasing

dropped call rate

Difficult accessDifficult access

Degressivevoice quality

Degressivevoice quality


Network Expanding

F1

OMNI 1BTS


F1

TRI-SECTOR

1BTS


Network Expanding

F1 F2+

1BTSOMNI


F1 F2+

1BTS


Date post:	02-Dec-2014
Category:	Documents
Upload:	surender-yadav
View:	325 times
Download:	3 times

26 CDMA Capacity Theory

Documents