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Cellular System Capacity

Cellular System Capacity. Cellular System Capacity. TDMA Capacity Error correction Improves system performance when signal power  & BER  Speech coding Lower & lower data rates allow more users/BW Significant capacity improvements result for TDMA as DSP improves

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Cellular System Capacity

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  1. Cellular System Capacity ECE 4730: Lecture #24

  2. Cellular System Capacity • TDMA Capacity • Error correction • Improves system performance when signal power  & BER  • Speech coding • Lower & lower data rates allow more users/BW • Significant capacity improvements result for TDMA as DSP improves • GSM used 13.3 kbps codec (1991) but 6 kbps codec is now available that achieves comparable voice quality double capacity when deployed • Easy to incorporate into existing GSM frame structure  16 half time slots vs. 8 full time slots ECE 4730: Lecture #24

  3. Cellular System Capacity • TDMA Capacity • Mobile Assisted HandOff (MAHO)? • Mobile unit monitors neighboring base stations and chooses best available signal • Always use base with best signal strength • w/o MAHO  current serving base (or MSC) waits till mobile power @ base drops below HO threshold • Other base station could be better provider long before HO threshold • Yields better overall signal power at mobile  larger S larger I tolerated for same S/I smaller cluster size  larger capacity ECE 4730: Lecture #24

  4. Cellular System Capacity • CDMA Capacity • FDMA vs. TDMA  straightforward comparisons • Capacity limited by BW, frequency reuse, & data rates • CDMA systems can reuse entire frequency spectrum in all cells • Potentially large increase in capacity relative to TDMA or FDMA systems • CDMA capacity limited by interference from despread PN noise from other users (same cell + adjacent cell) • Any reductions in average interference levels will increase capacity • Capacity is enhanced in real time unlike capacity improvements for TDMA/FDMA with ACI or CCI reduction ECE 4730: Lecture #24

  5. Cellular System Capacity • CDMA Capacity • Capacity enhancement • Sectorization (3  6 sectors) and/or SDMA • Limit interference spatially to a % of cell • Signal power from fraction of users in cell • Discontinuous transmission (DTX) • Turn Tx off during silent periods of speech • Voice Activity Factor (VAF)  ½ for mobile • User only contributes interference ½ of time! • Theoretical CDMA capacity calculations • Many assumptions must be made in order to calculate capacity • Difficult to determine accuracy of assumptions for real-world system ECE 4730: Lecture #24

  6. Cellular System Capacity • CDMA Capacity • Key issues that are difficult to assess on paper : • Imperfect power control  near/far • All user signals at base not exactly the same • Macroscopic diversity on forward link creating too many (> finger # of RAKE Rx) useable signals (including multipath) • Adjacent cell PN interference • Adjacent BSs cause non-AWGN forward-link interference at mobile Rx • Theoretical maximum capacity of CDMA different than that obtained in practice ECE 4730: Lecture #24

  7. Cellular System Capacity • CDMA vs. TDMA Capacity • Was a “Hot Issue” in late 1990s • Direct 1-to-1 meaningful comparisons difficult • CDMA does have capacity advantage but certainly not overwhelming • Theoretical capacity advantage difficult to fully achieve in actual real-world system • DSP improvements will continue to increase capacity and data rates of both TDMA and CDMA systems ECE 4730: Lecture #24

  8. Cellular System Capacity • Capacity comparisons in papers & textbooks ignore fundamental issue:  **COST** • A 50% capacity advantage is not useful if it costs 3 as much to provide service as a competitor • TDMA (GSM) technology had a substantial advantage in base station cost • More mature technology which has been around longer • More manufacturers, longer product history  reduced base station costs • Sprint PCS deployment of IS-95 CDMA had many more stations per unit area than TDMA • Higher provider cost even if base station costs were same as TDMA • Sprint PCS began service in 1998 and it took >7 years to make a profit!! ECE 4730: Lecture #24

  9. Cellular Standards 1G Obsolete 2G Obsolete ECE 4730: Lecture #24

  10. Cellular 3G Standards 1G 2G 2G+ 2G - Obsolete GSM-GPRS 2G - Obsolete 2G 2.5G 3G EDGE 3G ECE 4730: Lecture #24

  11. Cellular 3G Standards • CDMA and 3G Standards • Real benefit of CDMA is ability to easily accommodate multi-data rate user services • Voice 8-10 kbps • Non-real-time data internet browsing, email, etc. • Variable data rates : 4-100 kbps • Real-time data  video, web, etc. • Very large data rates : 200 kbps up to 2 Mbps!! • PN codes with different spreading factors used to accommodate variable data rates ECE 4730: Lecture #24

  12. Cellular Standards • Advanced Mobile Phone Service (AMPS) – 1G Analog FM • IS-136 (aka USDC, IS-54) – 2G Digital TDMA • Global System for Mobile (GSM) – 2G Digital TDMA • European • IS-95 CDMA – 2G Digital CDMA • U.S. / Qualcomm • CDMA2000 – 3G U.S./Qualcomm • W-CDMA – 3G Europe ECE 4730: Lecture #24

  13. AMPS • Advanced Mobile Phone Service (AMPS) – 1G • System History • Allocated 40 MHz of BW in 1983 in 800 MHz band  new cellular band • Additional 10 MHz of BW in 1989 for a total of 50 MHz • Two providers w/ 25 MHz each in a market  duopoly • “A” provider  non-wireline (e.g. Cellular One) • “B” provider  wireline (e.g. SWB Mobile  Cingular  AT&T) • ETACS in Europe is nearly identical to AMPS standard ECE 4730: Lecture #24

  14. AMPS Characteristics • N = 7 cell reuse w/ 120° sectoring • Smallest cluster size to meet required S/I of 18 dB • 416 channels/provider w/ RF BW = 30 kHz • 21 control channels (CC)  7 cells x 3 sectors • 395 voice channels (VC) • Analog FM modulation • Forward link band = 869894 MHz • Reverse link band = 824849 MHz • 45 MHz separation between FVC and RVC frequencies • Large separation needed for highly selective duplexers in mobile units ECE 4730: Lecture #24

  15. AMPS Characteristics • 1 Forward & Reverse CC per sector (3 per cell for 120°) • Digital FSK data @ 10 kbps on FCC • Mobile units locked or “camped” on FCC to originate/receive calls • Paging message (broadcast by all base stations) on FCC to originate call from PSTN • Mobile unit FVC & RVC frequency allocation (new call or handoff), call initiation, and setup (ID, authentication, etc.) • Maximum of 57 VC per BS • 57?  [ 416  21 (CC) ] / 7  57 • Supervisory Auditory Tone (SAT) & Voice Mobile Attenuation Code (VMAC) • Both transmitted by base and mobile on VC! ECE 4730: Lecture #24

  16. AMPS Characteristics • SAT : continuous transmission of tone at one of three frequencies used by base and mobile to identify each other from co-channel base and mobiles • N=7 w/ 3 sectors yields 2 possible co-channel interferers in first tier • Tones around 6 kHz and above the audio spectrum used by AMPS • VMAC: power control of mobile unit in wideband data signal • Wideband data signal transmitted on VC in "blank and burst" mode • Blank and burst: temporarily omit speech (blank) and transmit data burst • Imperceptible to users ( ~ < 100 msec in duration) • Also used by MSC to initiate mobile handoff • Handoff control done on VC of each mobile; easy implementation ECE 4730: Lecture #24

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