Capacity and Throughput Optimization in Multi-cell 3G WCDMA Networks

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

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

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

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

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: 342.43m • US Mobile users: 140m • Total African users: 53m • 1.87 billion mobile users by 2007 (27.4% of the world’s population)

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

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 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

10. 30 kbps 3.84 Mcps Spreading Spreading 3.84 Mcps Direct Sequence 3.84 Mcps 15 kbps

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

12. Frequency Reuse • Reuse frequency • Smaller = higher network capacity • frequency reuse factor of 4, 7, or 12 in TDMA and FDMA • 6 5 2 3 7 1 4 • 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

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.

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

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.

16. WCDMA Overview • 3rd Generation (3G) of mobile phone networks • Designed for multimedia communication • Cover both FDD and TDD operations

17. Main differences between WCDMA and IS-95 air interfaces

18. Development to all-IP

19. Cell i Cell j CDMA with One Class of Users Relative average interference at cell i caused by nj users in cell j where is the standard deviation of the attenuation for the shadow fading m is the path loss exponent

20. WCDMA with Multiple Classes of Users • Inter-cell Interference at cell i caused by njusers 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

21. Total Inter-cell Interference Density in WCDMA is the total number of cells in the network M G total number of services W is the bandwidth of the system

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 interferencedensity caused by all users in cell i

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

24. Simultaneous Users in WCDMA Must Satisfy the Following Inequality Constraints The capacity in a WCDMA network is defined as the maximum where is the minimum signal-to-noise ratio is the maximum signal power the number of users in BS i for given service g number of simultaneous users for all services . This is for perfect power control (PPC).

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

26. Multi-Cell WCDMA SimulationUniform User Distribution 2-D Gaussian approximation of users uniformly distributed in cells. 1= 2 = 12000, μ1 = μ2 = 0. The maximum number of users is 548. Simulated network capacity where users are uniformly distributed in the cells. The maximum number of users is 554.

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.

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.

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.

30. Optimized Capacity Calculation(Outline) • Relationship between Spreading and Scrambling • Spreading Factor • Orthogonal Variable Spreading Factor (OVSF) codes • Imperfect Power Control (IPC) • Numerical Results

31. Relationship between Spreading and Scrambling • Channelization codes: separate communication from a single source • Scrambling codes: separate MSs and BSs from each other

32. Main differences between WCDMA and IS-95 air interfaces

33. Spreading Factor

34. Orthogonal Variable Spreading Factor (OVSF) codes

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

36. Numerical Results

37. Numerical Results

38. Numerical Results

39. Numerical Results

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.

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

42. Feasible States where satisfying the above equations is A set of callsn = 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.

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 cellj : Probability that a call with service g progress in cell i after completing its dwell time does to cellj : Set of cells adjacent to cell j : Total offered traffic to cell j for service g

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

45. Maximization of Throughput max subject to for i = 1, …, M : vector of blocking probabilities : matrix of call arrival rates

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

47. Three Mobility Models No 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

48. WCDMA Network Throughput Optimization with SF = 256

49. WCDMA Network Throughput Optimization with SF = 64

50. WCDMA Network Throughput Optimization with SF = 16