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WCDMA Air Interface Training Part 3 CDMA Capacity Considerations

WCDMA Air Interface Training Part 3 CDMA Capacity Considerations. Cell Planning. FDMA systems Frequency Reuse Planning SNR Link Budget TDMA systems Frequency Reuse Planning Timeslot Allocation SNR Link Budget CDMA systems Code Reuse Planning SNR Link Budget Interference Link Budget.

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WCDMA Air Interface Training Part 3 CDMA Capacity Considerations

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  1. WCDMA Air Interface TrainingPart 3CDMA Capacity Considerations

  2. Cell Planning • FDMA systems • Frequency Reuse Planning • SNR Link Budget • TDMA systems • Frequency Reuse Planning • Timeslot Allocation • SNR Link Budget • CDMA systems • Code Reuse Planning • SNR Link Budget • Interference Link Budget

  3. F2 F2 F2 F2 F2 F2 F2 F7 F7 F7 F7 F7 F7 F7 F3 F3 F3 F3 F3 F3 F3 F1 F1 F1 F1 F1 F1 F1 F6 F6 F6 F6 F6 F6 F6 F4 F4 F4 F4 F4 F4 F4 F5 F5 F5 F5 F5 F5 F5 Frequency Reuse • Basic Reuse Pattern of 7

  4. F2 F2 F2 F2 F2 F2 F2 F2 F2 F2 F2 F2 F2 F2 F2 F2 F2 F3 F3 F3 F3 F3 F3 F3 F3 F3 F3 F4 F4 F4 F4 F4 F4 F4 F3 F3 F3 F3 F3 F3 F3 F1 F1 F1 F1 F1 F1 F1 F1 F1 F1 F1 F1 F1 F1 F1 F1 F1 Frequency Reuse • Reuse Patterns of 4 , 3 Reuse = 4 Reuse = 3

  5. 11 11 11 11 11 11 11 17 14 20 14 14 20 17 17 14 17 17 17 17 14 14 14 20 20 20 20 20 2 2 5 2 8 8 8 2 2 5 5 8 5 8 5 8 5 2 5 8 2 10 10 10 10 10 10 10 13 19 13 19 13 16 13 13 13 16 16 16 13 16 16 16 19 19 19 19 19 1 1 1 4 4 1 1 7 1 4 4 4 1 4 4 7 7 7 7 7 7 15 21 21 21 18 15 21 15 15 18 21 15 15 15 21 21 18 18 18 18 18 3 9 3 9 9 3 6 3 3 9 3 9 6 9 9 6 3 6 6 6 6 12 12 12 12 12 12 12 Frequency Reuse Reuse Pattern7/21 • Cell Sectorization

  6. CDMA Efficiency vs. FDMA, TDMA Notes: [1] GSM also uses frequency reuse of 4, frequency hopping, and other techniques to optimize cell planning efficiencies. [2] Users per cell is calculated in a 5 MHz carrier, assuming a frequency reuse of 7 for FDMA, TDMA; 1 for CDMA [3] All FDMA, TDMA, and CDMA systems can make proportional capacity gains by using cell sectorization patternssuch as 7/21, 4/12, etc. [4] Theoretical capacity of cdma2000 and WCDMA depends on Orthogonal Code allocations, and is therefore dependent on the mix of users and data rates. Practical capacity will depend on these and many other factors.

  7. PN2 PN2 PN2 PN2 PN2 PN2 PN2 PN7 PN7 PN7 PN7 PN7 PN7 PN7 PN3 PN3 PN3 PN3 PN3 PN3 PN3 PN1 PN1 PN1 PN1 PN1 PN1 PN1 PN6 PN6 PN6 PN6 PN6 PN6 PN6 PN4 PN4 PN4 PN4 PN4 PN4 PN4 PN5 PN5 PN5 PN5 PN5 PN5 PN5 CDMA Code Planning CDMA Frequency Reuse: 1 PN Code Reuse Factor: 7 Sectorization: 3-Sector (7/21) Reuse Codes available for code planning: IS-95: 512* cdma2000: 512* WCDMA: 512 * IS-95 and cdma2000 use 512 discrete time offsets of the same PN code to facilitate cell planning. In practice, IS-95 consecutive codes are avoided in order prevent potential time overlap of transmissions from widely spaced cells. The maximum number of available codes is therefore 256.

  8. CDMA Capacity • What determines the capacity of an FDMA system? • What determines the capacity of a TDMA system? • What determines the capacity of a CDMA system?

  9. CDMA Capacity • Factors influencing CDMA capacity • Eb/N0 (Energy per bit - to - Noise ratio) • Limited by transmit power; • All base station transmissions share a single transmit power budget • Eb/I0 (Energy per bit - to Interference Ratio) • Uplink: SSMA interference from mobiles in same cell, mobiles in other cells • Downlink: SSMA interference from surrounding base stations, distant base stations • Excessive interference associated with imperfect power control • Spread Spectrum Processing Gain is reduced at higher data rates • Multipath reflections, doppler shift, near-far ratio, obstructions, etc.

  10. CDMA Capacity • Example of Eb/N0 and Eb/Io capacity limitations Cell 2 Cell 1  MS3 MS1 Cell 1 cannot accommodate MS3 because:  Cell 2 cannot accommodate MS2 because:  MS2

  11. Energy per bit (Eb) equals the average signal power (S) divided by the data bit rate (Rb) Noise power density (N0)The total noise power in the signal bandwidth, divided by the signal bandwidth Energy per bit (Eb) - to - Noise RatioThe Signal-to-Noise Ratio (SNR) times the SSMA Processing Gain CDMA Capacity • Digital SNR: Eb/No

  12. CDMA Capacity • Eb/No vs. Probability of Error (BPSK, QPSK) 100 For QPSK Modulation, an Eb/N0 of ~ 7.5 dB is required to achieve a Pe of 10-3 10-2 10-4 Pe (Probability of bit error) 10-6 10-8 10-10 10-12 0 2 4 6 8 10 12 14 Eb/No (dB)

  13. Signal-to-Interference RatioThe signal-to-interference ratio for any single user. (Assumes perfect power control; all users received at the same power) Energy per bit (Eb) - to - Interference RatioThe Signal-to-Interference ratio times the SSMA Processing Gain CDMA Capacity • Uplink Capacity Limit due to SSMA Interference Desired Signal InterferingSignals

  14. Solving for M (The number of users that can be accommodated at a given Eb/I0): Assuming that M >> 1; The number of users equals the SSMA processing Gain divided by the desired Eb/I0 CDMA Capacity • Uplink Capacity Limit due to SSMA Interference Taking into account Voice Activity Factor: Voice Activity Factor ranges from 0.35 to 0.65

  15. CDMA Capacity • Theoretical Uplink Capacity Example (one MS, one cell) Gp = 128 Spread Spectrum Processing Gain; vf = 0.5 Case I: Desired Pe = 10-3 ; Eb/I0 = 7.5 dB = 5.62 Case II: Desired Pe = 10-2 ; Eb/I0 = 5.1 dB = 3.23 Approximately 45 SSMA users can co-existin a single-cell systemwith a Pe of 10-3 if each has a processing gain of 128

  16. CDMA Capacity • Uplink Capacity in a multi-cell network Required Eb/No in a single-cell network: Required Eb/No in a multi-cell network:Interference from other cells during soft handover is given by f; Also Taking into account other cell interference Inter-cell interference factor increases with thestandard deviation of path loss; typical values of (f) range from 0.4 to 2.6 (V. Garg, “IS-95 and cdma2000”, Table 13-1)

  17. CDMA Capacity • Example 1: Uplink Capacity in a multi-cell IS-95 network SSMA Bandwidth (B) 1.25 MHz Intercell Interference f = 0.6 (typical for 3-way soft handover, path loss deviation 8 dB)Voice Activity Factor Vf = 0.5Required Pe 10-3 (Eb/I0 = 7.5 dB = 5.62) for 9.6 kbps 10-4 (Eb/I0 = 8.5 dB = 7.62) for 14.4 kbps Case I: Data rate = 9.6 kbps; Gp = 1.2288 Mcps / 9.6 kbps = 128 Case II: Data rate = 14.4 kbps; Gp = 1.2288 Mcps / 14.4 kbps = 85.33

  18. CDMA Capacity • Example 1: Uplink Capacity in WCDMA SSMA Bandwidth (B) 3.84 MHz (equal to W-CDMA chip rate for this example)Intercell Interference f = 0.6 (typical for 3-way soft handover, path loss deviation 8 dB)Voice/data activity0.5Required Pe 10-3 (Eb/I0 = 5.62) Case I: Data rate = 30 ksps; Gp = 3.84 Mcps / (60 kbps/2) = 128 Case II: Data rate = 960 ksps; Gp = 3.84 Mcps / (1.92 Mbps/2) = 4 Many other factors will affect actual capacity... see next page

  19. CDMA Capacity • Other factors influencing capacity • Other Factors that increase capacity: • Cell Sectorization • Antenna Gain • Antenna Diversity • Soft Handover Macrodiversity Gain • Use of higher-strength error protection (e.g., turbo coding) • Statistical multiplexing of packet data users • Other Factors that decrease capacity: • Imperfect Power Control • Downlink Interference from other Base Stations • Absorption (body, terrain, structural, atmospheric...) • Use of lower strength error protection on high-speed channels • Multipath fading • Frequency-selective fading

  20. CDMA Capacity • Downlink Capacity • Fundamental Capacity Limitation is available RF transmit power • One RF power budget must be split between all Mobile Stations! • Fixed portion of RF power Budget allocated to Pilot, Broadcast, Paging channels • SSMA interference from other Base Stations • Growing problem in Microcellular and Hierarchy topologies • Traffic channel power is allocated based on Mobile Station needs • More power allocated to distant MS’s; less to nearby MS’s • IS-95B provides 20msec downlink power update for 14.4 kbps data rate • WCDMA, cdma2000 use fast power control on the downlink traffic channels

  21. New User requests Access Interference within limits? Block Access No AdmissionControl Yes • Send users to other cells, frequencies, or systems • Reduce non-CBR data rates • Reduce packet data throughput Reduce error protection • Ignore downlink TPC from MS • Selectively drop calls Load(Congestion)Control GrantAccess Interference within limits? No Yes Making enough money? Adjust Interference Limit No Yes Soft Blocking • Soft vs. Hard Blocking • FDMA/TDMA: When all frequencies and/or timeslots are used, calls are blocked • CDMA: When interference levels are too high, calls are blocked

  22. Summary: CDMA Capacity • Summary • Cell capacity not determined by size of Orthogonal Code set! • Uplink capacity is usually interference-limited • Downlink capacity limited by both RF power allocation to traffic channels and interference from other base stations • Experience from IS-95 • Typical per-sector capacity of 13 to 16 mobile stations • Some operators report up to 20 mobiles per sector on a single CDMA carrier • DL capacity sometimes higher, sometimes lower than UL capacity • And in the final analysis...

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