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CDMA and WLL Networks

Muhammad Ali Raza Anjum

ali.raza.anjum@gmail.com

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Part I

Introduction

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FDMA

Frequency spectrum is divided up into channels and shared

Each channel is used by a single user Least spectrally efficient

Frequency

Time

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TDMA Channels occupy cyclically repeating time intervals or time slots

DAMPS is 6 times more spectrally efficient than FDMA, and GSM is 8

times more so

Frequency

Time

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CDMA

Each channel is assigned a unique code and occupies the same frequency

and time as other users Most prone to interference

Maximum spectral efficiency

Frequency

Time

Same frequency; sametime; different code

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Some Cellular BandsStandard Access Spectrum

(MHz)

Channel

Spacing

AMPS FDD 825-845 t870-890 r

30 kHz

GSM FDMA /

TDMA

890-915 t

935-960 r

200 kHz

EGSM FDMA/TDMA/

FDD

880-915 t925-960 r

200 kHz

DAMPS

IS-136

FDMA/

TDMA/

FDD

824-849 t

869-894 r

30 kHz

CDMAOne /

CDMA2000

CDMA 824-849 t

869-894 r

1.25

MHz

WCDMA CDMA 1920-1980 t

2110-2170 r

5 MHz

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Cellular full duplex channels on FDD

Tx frequency (825.030 MHz) 30 KHz

Duplex spacing (45 MHz)

Rx frequency (870.030 MHz)

AMPS; DAMPS

Tx frequency (890 MHz) 200 KHz

Duplex spacing (45 MHz)

Rx frequency (935 MHz)GSM 900

Tx frequency (1710 MHz) 200 KHz

Duplex spacing (95 MHz)

Rx frequency (1805 MHz)

GSM 1800

Tx frequency (1850 MHz) 200 KHz

Duplex spacing (80 MHz)

Rx frequency (1930 MHz)

PCS

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Erlang B: Blocking System

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Erlang C: Non Blocking System

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BHCA and Traffic per subscriber

BHCA (Busy Hour Call Attempts)

The number of calls that a subscriber attempts in a

system busy hour

MHT (Mean Holding Time)

The time in seconds for which a trunk is seized

Erlang

If a trunk is busy for the entire duration of the

observation time interval (usually I hour or 3600

seconds), then the traffic on the trunk is 1 Erlang

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.BHCA

For cellular, BHCA is taken as 4

MHT is 45 seconds

Observation time interval is 1 hr

Traffic / Subscriber = BHCA x MHT =0.05 Erl

3600

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Grade of service (GoS)

Required grade of service Usually 2% blocking probability during busy hour. This means that

during the busy hour, 2 out of every 100 calls would be blocked dueto congestion

Busy hour may be (1) busy hour at busiest cell or (2) systembusy hour or (3) system average over all hours

Estimated traffic distribution Traffic intensity is measured in Erlang (named after Danish

mathematician A.K. Erlang)

One Erlang = completely occupied channel, eg, a radio channeloccupied for 30 min. per hour carries 0.5 Erl

GoS signifies the likelihood that a call is blocked or isqueued for more than the designed time

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Grade of Service

Erlang B formula is used for non-queuing systems andis given by

where C = number of channels

A = traffic intensity

Pr( blocking ) =AC / C!

Ak / k!k= 0

C

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Queued Trunking System

Erlang C formula is used to find GoS in queuedtrunked systems

A queue is provided to hold blocked calls

Pr (delay) = AC

C-1

AC+C!(1-A/C) Ak/k!k=0

Erlang B and Erlang C formulae are used to determine

important network parameters such as maximumnumber of users for a given GoS and number ofchannels

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A practical exampleFind maximum number of users that can be supported for a 0.5%blocking probability if connected trunks are 100.

Assume each user generates 0.1Erl traffic.

Solution:

Traffic/subs Au = 0.1 Erl; Trunks C = 100; GoS = 0.5%Users U = A/Au; where A is traffic intensity for a given GoS.From graph we can see that A for 0.5 GoS and 100 trunks equals80.9.

http://www.erlang.com/calculator/erlb/

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A practical example

So U = 80.9/0.1 = 809 users.

For practice repeat the above example for

C=20; GoS = 2%, Au = 0.2 Erl C=50, GoS = 0.1%, Au = 0.2 Erl

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CDMA Full Duplex Channel (450 MHz band)

Tx frequency (453.625 MHz) 1.25 MHz

Duplex spacing (10 MHz)

Rx frequency (463.625 MHz)

Channel 146 in CDMA 450 MHz band

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Frequency Allocation For GSM Operators in 900 MHz

band

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Frequency Allocation For GSM

Operators in 1800 MHz Band

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Frequency Allocation For WLL Operators

F All ti F WLL

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Frequency Allocation For WLL

1.9 GHz band

Each band of 5 MHz can have 3 carriers of 1.5 MHz each

S-333 configuration is allowed, which increases cell capacity upto

three times

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Companies in 450 MHz WLL Band

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Companies in 1.9 GHz WLL Band

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Evolution Paths

IS-41 Core Network

GSM Core Network

2G 2.5G 2.75G 3G

cdmaOne

IS-95A

TDMA

Cdma20001xEV-DO

Cdma2000 1x

cdmaOne

IS 95B

Cdma2000

1xEV-DV

GSM

GPRS

EDGE WCDMA

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Part II

CDMA Technology

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Walsh Codes and Channel Elements

Air interface uses Walsh codes in downlink toseparate individual traffic channels

Hardware dealing with Walsh Codes is ChannelElement between BTS and BSC

Channel elements are less than Walsh Codes and provided by specific chips

designed by Qualcomm copyright protection

depend upon Erlang calculations inside a BTS

usually 1:2 for downlink and uplink respectively

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.Walsh Codes & Channel Elements

Standard for WLL traffic is ~22 Erl per sector, GoS=1%

Channels = (Traffic + 4)/0.8 = 33 CDMA is interference limited, so 33 simultaneous

conversations amounts to 32 interfering signals within the cell

This means that in each sector, there should be at least 32channel elements

These 32 channel elements correspond to 32 Walsh Codes thatthe system assigns in the downlink direction

In the uplink, the same channel elements are utilised, butWalsh codes are used for Orthogonal Modulation

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CDMA Spread Spectrum

)X-OR

Data Signal

PN Code

Output

O/P =Digital Signal PNCode+

Chip

Time

PN Code and how it spreads data for a spreading factor of 5

Data Bit Time

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CDMA - Codes and Orthogonality

Orthogonal or Walsh Codes Used to separate users. Each forward channel is assigned a distinct WalshCode

Orthogonal Spreading and Despreading XOR twice and retrieve original signal

each encoded symbol is XORed with 64 Walsh Code chips eg 1 0000111111110000111100000000111111110000000011110000111111110000

= 1111000000001111000011111111000000001111111100001111000000001111

After XORing, pattern is transmitted as a 64 bit representation and at Rx, it

is again XORed with the 64 bit Walsh code which gives original symbol. Tx

signal 64 bit Walsh Code = 1

The second type of code used in CDMA systems is the

Psuedorandom Noise (PN) code

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Codes and OrthogonalityAn ExampleSignal from user A

Signal 00 code 0101

Signal from user C11 0000

Signal from user B10 0011

Composite SignalA + B + C

Spreading Despreading

Composite SignalA + B + C

User A Walsh Code0101

Product

3 users and 3 orthogonal codes

Signals are spread and thensummed up for transmission

At As receiver, composite signal ismultiplied by As Walsh code 0101

Result is averaged over symbol time

Average voltage over symbol time = 1

so bit transmitted = 0

+1

- 1

+1

- 1

+1

- 1

+1

- 3

+1

- 3

+1

- 1

+3

- 1

Symbol Period

Construction Principle of Walsh Code Matrix

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1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

2 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1

3 0 0 1 1 0 0 1 1 0 0 1 1 0 0 1 1 0 0 1 1 0 0 1 1 0 0 1 1 0 0 1 1

4 0 1 1 0 0 1 1 0 0 1 1 0 0 1 1 0 0 1 1 0 0 1 1 0 0 1 1 0 0 1 1 0

5 0 0 0 0 1 1 1 1 0 0 0 0 1 1 1 1 0 0 0 0 1 1 1 1 0 0 0 0 1 1 1 1

6 0 1 0 1 1 0 1 0 0 1 0 1 1 0 1 0 0 1 0 1 1 0 1 0 0 1 0 1 1 0 1 0

7 0 0 1 1 1 1 0 0 0 0 1 1 1 1 0 0 0 0 1 1 1 1 0 0 0 0 1 1 1 1 0 0

8 0 1 1 0 1 0 0 1 0 1 1 0 1 0 0 1 0 1 1 0 1 0 0 1 0 1 1 0 1 0 0 1

9 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1

10 0 1 0 1 0 1 0 1 1 0 1 0 1 0 1 0 0 1 0 1 0 1 0 1 1 0 1 0 1 0 1 0

11 0 0 1 1 0 0 1 1 1 1 0 0 1 1 0 0 0 0 1 1 0 0 1 1 1 1 0 0 1 1 0 0

12 0 1 1 0 0 1 1 0 1 0 0 1 1 0 0 1 0 1 1 0 0 1 1 0 1 0 0 1 1 0 0 1

13 0 0 0 0 1 1 1 1 1 1 1 1 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 0 0 0 0

14 0 1 0 1 1 0 1 0 1 0 1 0 0 1 0 1 0 1 0 1 1 0 1 0 1 0 1 0 0 1 0 1

15 0 0 1 1 1 1 0 0 1 1 0 0 0 0 1 1 0 0 1 1 1 1 0 0 1 1 0 0 0 0 1 1

16 0 1 1 0 1 0 0 1 1 0 0 1 0 1 1 0 0 1 1 0 1 0 0 1 1 0 0 1 0 1 1 0

17 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

18 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0

19 0 0 1 1 0 0 1 1 0 0 1 1 0 0 1 1 1 1 0 0 1 1 0 0 1 1 0 0 1 1 0 0

20 0 1 1 0 0 1 1 0 0 1 1 0 0 1 1 0 1 0 0 1 1 0 0 1 1 0 0 1 1 0 0 1

21 0 0 0 0 1 1 1 1 0 0 0 0 1 1 1 1 1 1 1 1 0 0 0 0 1 1 1 1 0 0 0 0

22 0 1 0 1 1 0 1 0 0 1 0 1 1 0 1 0 1 0 1 0 0 1 0 1 1 0 1 0 0 1 0 1

23 0 0 1 1 1 1 0 0 0 0 1 1 1 1 0 0 1 1 0 0 0 0 1 1 1 1 0 0 0 0 1 1

24 0 1 1 0 1 0 0 1 0 1 1 0 1 0 0 1 1 0 0 1 0 1 1 0 1 0 0 1 0 1 1 0

25 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 0 0 0 0 0 0 0

26 0 1 0 1 0 1 0 1 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 0 1 0 1 0 1 0 1

27 0 0 1 1 0 0 1 1 1 1 0 0 1 1 0 0 1 1 0 0 1 1 0 0 0 0 1 1 0 0 1 1

28 0 1 1 0 0 1 1 0 1 0 0 1 1 0 0 1 1 0 0 1 1 0 0 1 0 1 1 0 0 1 1 0

29 0 0 0 0 1 1 1 1 1 1 1 1 0 0 0 0 1 1 1 1 0 0 0 0 0 0 0 0 1 1 1 1

30 0 1 0 1 1 0 1 0 1 0 1 0 0 1 0 1 1 0 1 0 0 1 0 1 0 1 0 1 1 0 0 1

31 0 0 1 1 1 1 0 0 1 1 0 0 0 0 1 1 1 1 0 0 0 0 1 1 0 0 1 1 1 1 0 032 0 1 1 0 1 0 0 1 1 0 0 1 0 1 1 0 1 0 0 1 0 1 1 0 0 1 1 0 1 0 0 1

Construction Principle of Walsh Code Matrix1/4 of the matrix is shown

PN C d

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PN Codes

2 short codes (215 = 32768)

Unique offsets serve as identifiers for cellsand sectors

Clock rate of 1.2288 Mcps

1 long code (242 ~ 4400 Billion)

Used for spreading and scrambling

clock rate of 1.2288 Mcps

Repeats every 41 days

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PN Codes

Channelisation of users in reverse direction is

accomplished by assigning them different time

shifted versions (masks) of the long PN code,

thus making them uncorrelated with each other

Each cell or sector uses a unique short PN code PN codes are generated by simple mechanisms

that employ shift registers and XOR gates

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Generation of PN codes

Requires shift registers and OR gates

0 0 1

+

1 1 0

+

0 1 1

+

1 0 0

+

1 1 1

+

0 1 0

+

1 0 1

+

0 0 1

1 0 0

0 1 0

1 0 1

1 1 0

1 1 1

0 1 1

7 digitoutput thatrepeatscontinuously1001011

ReadClockwise

0 0 1

+

0 1 1

+ OutMasking

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Traffic carrying capability

CDMA is the most spectrally efficient technology One channel of 1.25 MHz can carry entire traffic

load for one or more base stations

The same channel may be used in adjacent cellsand for split up and sectorised cells to increase

traffic handling capacity

Soft handoff is employed whenever neighbouring

cells use the same frequency as the reference cell

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.. Traffic Carrying Capability

S0

, Snnn BTS configuration, where values of n are 1,2 or 3

S0is omnidirectional used for low traffic areas

S111 is sectorised, with each sector using one carrier

Variations include S222, S333, S123, S322 etc

Increase in traffic carrying capability is linear Capacity increases in direct proportion with increase in carriers

Usually in rural areas, S0 is used and in urban, Snnn is

employed

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Frequency Planning in CDMA Since cluster size N = 1, frequency planning is not a

big issue

Adjacent base stations may use the same frequency

However limited frequency reuse is required in certainconditions

Interfering cells on the same channel as the servingcell may create interference overload leading todropped calls f1/f2 cell planning (hard handoffs)

Near far effect Breathing cell

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If number of users increase beyond a certain level, theremay be an abrupt increase in dropped calls

More users mean degraded performance

Power levels and thresholds for VC and CC have to bemeticulously designed

Frequency planning in CDMA

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CDMA specific behaviour

f1/f2 cell planning

nearest cells use different radio frequencies

implemented where interference is experienced

used for hard handoffs

Breathing Cell dynamic, time varying, user dependent cell boundaries

Soft handoff

MSC monitors MS from two or more base stations

the strongest channel is automatically allocated to MSwithout a change in frequency

CDMA ifi b h i

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Near Far Effect

precise power control for each user power from each user should be equal at base station. If not,near far effect occurs

generally stronger signal at Rx drowns weaker signals

this is avoided by sending power change commands overthe forward radio link to all mobiles

each MS provides the same signal level to the base stationRx and near far effect is avoided from mobiles within a cell

however, out of cell mobiles may cause near far effect

CDMA specific behaviour

Data Rates in CDMA

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Data Rates in CDMA

RS1

9.6 kb/s

4.8 kb/s

2.4 kb/s

1.2 kb/s

RS2

14.4 kb/s

7.2 kb/s

3.6 kb/s 1.8 kb/s

All four rates are used

Data rates change in real time

System adjusts to user requirement

and adjusts data transfer speeds

Either RS1 or RS2 is used

PTCL WLL is using RC3

RC Data speed Kb/s

RC1 9.6RC2 14.4

RC3 153.6

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Downlink Channel

Conv Encoder&

Repetition

BlockInterleaver

MUX

Long PNCode

ChannelGain

PWRControl bit

DCMT

ChannelGain

Walsh Code1.2288 Mcps

Offset I PN

Offset Q PN

DCMT

9.64.82.41.2

14.47.23.61.8

RS1 or RS2 kb/s

or

R=1/2 for RS1or

R=3/4 for RS2

19.2 ksps

1.2288 Mcps 19.2 ksps

Orthogonal

Spreading

QuadratureSpreading

For uplink, Walsh codes are used for orthogonal modulation

Repetition

0137

PN Codes

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PN Codes

2 short codes (215 = 32768)

Unique offsets serve as identifiers for cellsand sectors

Clock rate of 1.2288 Mcps

1 long code (242 ~ 4400 Billion)

Used for spreading and scrambling

clock rate of 1.2288 Mcps

Repeats every 41 days

Connectivity of Peshawar WLL MSC

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Connectivity of Peshawar WLL MSC

MSC

Peshawar

BSC

Hayatabad

Pshr City

MSC

LahoreMSC

Multan

MSC

H/abadMSC

Quetta

NSS Plane

BSS Plane

Khar Bajaur

Mathani

Bara

Sakhakot

Thana

Mardan

Charsadda

Tank

Bannu

DIKhan

Hangu

Kohat

Parachinar

Sadda

Thall

Rwp CanttAbbottabad

Mansehra

Nathiagali F-8 Iba

Bara Kahu

Westridge

Landi Kotal

Karak

Laki Marwat

64,6

HLR

3,6

PDSNSMC

2,1 5

References

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References

1.Wireless Communications Principles & Practice (2nd

Edition) by

Theodore S. Rappaport

2. IS-95 CDMA & cdma2000 Cellular/PCS Systems Implementation byVijay K. Garg

3. Telecommunications by Warren Hioki

4. M/S Qualcomm cdmaOne and cdma2000 manuals

5. M/S Huawei cdma2000 manuals

6. Management and technical staff of M/S Paktel, Instaphone, Mobilink, Ufone

and Telenor

7. PTA Headquarters

8. PTCL Headquarters

9. The world wide web (www)10. ITU-T & ITU-R Recommendations