cdma & wll
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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
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