1. Capacity and Throughput Optimization in Multi-cell 3G WCDMA Networks
Son Nguyen, B.S.
Advisor: Dr. Akl
Dr. Brazile
Dr. Tate
Department of Computer Science and Engineering

2. Outline Mobile Phone Systems History CDMA and WCDMA Overview
User and Interference Modeling Using 2-D Gaussian Function
Maximization of WCDMA Capacity
Optimization of WCDMA Call Admission Control and Throughput
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3. Mobile Phone Systems History
1st Generation
First commercial cellular telephone system began operation in Tokyo in 1979
AMPS (Advance Mobile Phone System)
Available in Chicago by Ameritech in 1983
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4. Mobile Phone Systems History (cont)
2nd Generation
TDMA Interim Standard 54 (TDMA IS-54) in 1991
TDMA IS-136 (updated version)
GSM (Global System for Mobile Communications)
In 1987, standard created with hybrid of FDMA and TDMA technologies
Accepted in the United States in 1995
Operated in 1996
Major carriers of GSM 1900: Omnipoint, Pacific Bell, BellSouth, Sprint Spectrum, Microcell, Western Wireless, Powertel and Aerial
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5. Mobile Phone Systems History (cont)
CDMA IS-95 (Code Division Multiple Access)
Developing by Qualcomm corporation in late 1980s
Operated in 1996
Major US carriers using CDMA: Air Touch, Bell Atlantic/Nynex, GTE, Primeco, and Sprint PCS
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6. Latest Global Cellular Statistics (End of 2004)
Global Mobile Users: 1.57 billion
GSM: 1.25 billion
CDMA: 202m
TDMA: 120m
Facts
#1 Mobile Country: China (300m)
Total European users: m
US Mobile users: 140m
Total African users: 53m
1.87 billion mobile users by 2007 (27.4% of the world’s population)
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7. CDMA and WCDMA Overview
Introduction to CDMA
Power Control
Frequency Reuse
Voice Activity Detection
Soft Handoff
WCDMA Overview
Spectrum Allocation in Different Countries
Main differences between WCDMA and IS-95 air interfaces
Development to all-IP
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8. Code Division Multiple Access (CDMA)
Introduced by Qualcomm in 1989
2nd Generation (2G) of mobile phone networks
Distinguishes different calls by unique codes.
Capacity of a CDMA network is limited by interference
FDMA
TDMA
CDMA
Frequency
Frequency
Frequency
Call 3
Call 5
Call 6
Call 2
Call 3
Call 4
Call 1
Call 1
Call 2
Time
Time
Time
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Code

9. Spread Spectrum
Spreading the information signal over a wider bandwidth to make jamming and interception more difficult
Three types of SS: Frequency Hopping, Time Hopping, and Direct Sequence (DS)
Frequency Hopping
Time Hopping
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10. Direct Sequence 30 kbps 3.84 Mcps 3.84 Mcps 15 kbps 3.84 Mcps
Spreading
3.84 Mcps
15 kbps
3.84 Mcps
Spreading
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11. Power Control Aims to reduce interference Near-far problem
Reduces power consumption in the MS
Methodologies
Open-loop
Sum of transmit power and the received power is kept constant
Closed-loop
Signifies the other party to increase or decrease transmit power by a pre- defined power step c2
Pt2
Pr2
Base Station
Pt1
Pr1 c1 d2 d1
Distance
Pt1: Power transmitted from c1
Pt2: Power transmitted from c2
Pr1: Power received at base station from c1
Pr2: Power received at base station from c2
Pr1 = Pr2
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12. Frequency Reuse Reuse frequency Smaller = higher network capacity
frequency reuse factor of 4, 7, or 12 in TDMA and FDMA
CDMA cellular systems: universal frequency reuse or frequency reuse factor of 1 6 2 7 4 1 6 2 7 5 3 1 6 4 5 3 1 2 7 4 5 6 2 7 4 3 1 6 2 5 3 1 6 7 4 5 3 2 7 4 5 1 6 2 7 3 1 6 2 4 5 3 1 7 4 5 3
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13. Voice Activity Detection
Reducing multiple access interference
Human speech: 42%
 results in a capacity gain
FDMA and TDMA cellular systems
Frequencies are permanently assigned
 Capacity in FDMA and TDMA systems is fixed and primarily bandwidth limited.
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14. New Requirements of 3G systems
Data communication speeds up to 2 Mbps.
Variable bit data rates on demand.
Mix of services on a single connection
Meet delay requirement constraints
Quality of service (QoS),
Frame error rate
Bit error rate
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15. New Requirements of 3G systems (cont)
Coexistence with second generation systems
Support asymmetric uplink and downlink traffic
Higher spectrum efficiency
Coexistence of FDD and TDD modes.
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16. WCDMA Overview 3rd Generation (3G) of mobile phone networks
Designed for multimedia communication
Cover both FDD and TDD operations
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17. Main differences between WCDMA and IS-95 air interfaces
Carrier spacing
5 MHz
1.25 MHz
Chip rate
3.84 Mcps
Mcps
Power control frequency
1500 Hz, both uplink and downlink
Uplink: 800 Hz, downlink: slow power control
Base station synchronization
Not needed
Yes, typically obtained via GPS
Inter-frequency handovers
Yes, measurements with slotted mode
Possible, but measurement method not specified
Efficient radio resource management algorithm
Yes, provides required quality of service
Not needed for speech only networks
Packet data
Load-based packet scheduling
Packet data transmitted as short circuit switched calls
Downlink transmit diversity
Supported for improving downlink capacity
Not supported by the standard
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18. Development to all-IP
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19. CDMA with One Class of Users
Cell j
Cell i
Relative average interference at cell i caused by nj users in cell j
Power control attempts to equalize users received signal power at a given cell’s base station, for all users controlled by that base station. But interference also arrives from users controlled by other cells base stations. Its arrives at the given base station with lower power levels.
The propagation loss is generally modeled as the product of the mth power of distance and a log normal component representing shadowing losses.
Shadow loss: depending on the environment and the surroundings, and the location of objects, the received signal strength for the same distance from the transmitter will be different – buildings, walls etc.
Rayleigh fading: fast fluctuations due to movement.
Since the user is communicating with nearest base station, it will also be power-controlled by that base station. The user’s transmitter power gain thus equals the propagation loss for that cell.
ζ: decibel attenuation due to shadowing, and is a Gaussian random variable with zero mean and standard deviation sigma
χ: Rayleigh random variable that represents the fading on the path from this user to cell I.
γ: ln(10)/10
Denominator: propagation loss to the given base station
Numerator: gain adjustment through power control by the nearest base station.
where
is the standard deviation of the attenuation for the shadow fading
m is the path loss exponent
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20. WCDMA with Multiple Classes of Users
Inter-cell Interference at cell i caused by nj users in cell j of class g
w(x,y)
is the user distribution density at (x,y)
is per-user (with service g) relative inter-cell interference factor from cell j to BS i
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21. Total Inter-cell Interference Density in WCDMA
is the total number of
cells in the network
G total number of services W
is the bandwidth of the system
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22. Model User Density with 2D Gaussian Distribution
“means” is a user density normalizing parameter
“variances” of the distribution for every cell
is the total intra-cell interference density caused by all users in cell i
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23. Signal-to-Noise Density in WCDMA
where
is the thermal noise density,
is the bit rate for service g
is the minimum signal-to-noise ratio required
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24. Simultaneous Users in WCDMA Must Satisfy the Following Inequality Constraints
where
is the minimum signal-to-noise ratio
is the maximum signal power
the number of users in BS i for given service g
The capacity in a WCDMA network is defined as the maximum
number of simultaneous users for all services
. This is for perfect power control (PPC).
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25. Simulations Network configuration COST-231 propagation model
Carrier frequency = 1800 MHz
Average base station height = 30 meters
Average mobile height = 1.5 meters
Path loss coefficient, m = 4
Shadow fading standard deviation, σs = 6 dB
Processing gain, W/R = 21.1 dB
Bit energy to interference ratio threshold, τ = 9.2 dB
Interference to background noise ratio, I0/N0 = 10 dB
Activity factor, v = 0.375
European COST (Cooperation in the field of Scientific and Technical Research) wave propagation model
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26. Multi-Cell WCDMA Simulation Uniform User Distribution
Simulated network capacity where users are uniformly distributed in the cells. The maximum number of users is 554.
2-D Gaussian approximation of users uniformly distributed in cells. 1 = 2 = 12000, μ1 = μ2 = 0. The maximum number of users is 548.
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27. Extreme Cases Using Actual Interference Non-Uniform Distribution
Simulated network capacity where users are densely clustered around the BSs causing the least amount of inter-cell interference. The maximum number of users is 1026 in the network.
2-D Gaussian approximation of users densely clustered around the BSs. 1 = 2 = 100, μ1 = μ2 = 0. The maximum number of users is 1026.
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28. Extreme Cases Using Actual Interference Non-Uniform Distribution
Simulated network capacity where users are densely clustered at the boundaries of the cells causing the most amount of inter-cell interference. The maximum number of users is only 108 in the network.
2-D Gaussian approximation of users densely clustered at the boundaries of the cells. The values of 1 = 2 = 300, μ1, and μ2 are different
in the different cells. The maximum number of users is 133.
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29. Results of User And Interference Modeling with 2-D Gaussian Function
Model inter-cell and intra-cell interference for different classes of users in multi-cell WCDMA.
We approximate the user distribution by using 2-dimensional Gaussian distributions by determining the means and the standard deviations of the distributions for every cell.
Compared our model with simulation results using actual interference and showed that it is fast and accurate to be used efficiently in the planning process of WCDMA networks.
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30. Optimized Capacity Calculation (Outline)
Relationship between Spreading and Scrambling
Spreading Factor
Orthogonal Variable Spreading Factor (OVSF) codes
Imperfect Power Control (IPC)
Numerical Results
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31. Relationship between Spreading and Scrambling
Channelization codes: separate communication from a single source
Scrambling codes: separate MSs and BSs from each other
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32. Main differences between WCDMA and IS-95 air interfaces
Channelization code
Scrambling code
Usage
Uplink: Separation of physical data (DPDCH) and control channels (DPCCH) from same MS
Downlink: Separation of downlink connections to different MSs within one cell.
Uplink: Separation of MSs
Downlink: Separation of sectors (cells)
Length
Uplink: chips same as SF
Downlink chips same as SF
Uplink: 10 ms = chips
Downlink: 10 ms = chips
Number of codes
Number of codes under one scrambling code = spreading factor
Uplink: Several millions
Downlink: 512
Code family
Orthogonal Variable Spreading Factor
Long 10 ms code: Gold Code
Short code: Extended S(2) code family
Spreading
Yes, increases transmission bandwidth
No, does not affect transmission bandwidth
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33. Spreading Factor
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34. Orthogonal Variable Spreading Factor (OVSF) codes
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35. Imperfect Power Control
Transmitted signals between BSs and MSs are subject to multi-path propagation conditions
The received signals vary according to a log- normal distribution with a standard deviation on the order of 1.5 to 2.5 dB. Thus in each cell for every user with service needs to be replaced
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36. Numerical Results
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37. Numerical Results
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38. Numerical Results
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39. Numerical Results
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40. Results of Optimized Capacity Calculation
The SIR threshold for the received signals is decreased by 0.5 to 1.5 dB due to the imperfect power control.
As expected, we can have many low rate voice users or fewer data users as the data rate increases.
The determined parameters of the 2- dimensional Gaussian model matches well with the traditional method for modeling uniform user distribution.
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41. WCDMA Call Admission Control
Quality of Service (QoS) of current connections in a network
Grade of Service (GoS), i.e., call blocking rate
Global if the decision to admit a call is based on total calls in the network
Local if the algorithm considers only a single cell for making that decision
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42. Feasible States where satisfying the above equations is
A set of calls n =
defined to be in a “feasible state”
Adding a new call with service g to cell i, the call is blocked if
the new state of the network does not belong to any of the
feasible state.
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43. Handoff Rate & Total Offered Traffic
: the handoff rate out of cell j offered to cell i for service g
: Blocking probability for service g in cell j
: Call with arrival rate service g in cell j
: Probability that a call with service g progress in cell i after completing its dwell time does to cell j
: Set of cells adjacent to cell j
: Total offered traffic to cell j for service g
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44. Blocking probability for service g
: Erlang traffic in cell i with service g
Admissible States:
for i = 1, …, M and g = 1, …, G
: the “new” total number of connection with service g in cell i
: the maximum number of calls with service g in cell i
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45. Maximization of Throughput
subject to
for i = 1, …, M
: vector of blocking probabilities
: matrix of call arrival rates
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46. Numerical Analysis Processing gain, W/R
24.08 dB for spreading factor = 256
18.06 dB for spreading factor = 64
12.04 dB for spreading factor = 16
6.02 dB for spreading factor = 4
Bit energy to interference ratio threshold, τ = 7.5 dB
Interference to background noise ratio, I0/N0 = 10 dB
Activity factor, v = 0.375
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47. Three Mobility Models No Mobility Low Mobility High Mobility
probability that a call with service g in progress in cell i remains in cell i after completing its dwell time.
Low Mobility
probability that a call with service g in progress in cell i after completing its dwell time goes to cell j. It’s equaled zero (=0) if cell i and j are not adjacent.
probability that a call with service g in progress in cell i departs from the network.
High Mobility
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48. WCDMA Network Throughput Optimization with SF = 256
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49. WCDMA Network Throughput Optimization with SF = 64
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50. WCDMA Network Throughput Optimization with SF = 16
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51. WCDMA Network Throughput Optimization with SF = 4
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52. Results of Optimized Network Throughput Calculation
Results for the cases with no mobility and high mobility for each blocking probability are identical.
Low mobility case has an equalizing effect on traffic resulting in slightly higher throughput.
As the spreading factor increases, the optimized throughput is better for all the three mobility models due to trunking efficiency.
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53. Thank You!!
Questions?
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