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ENTC 455

WIRELESS TRANSMISSION SYSTEMS. ENTC 455. Basics of Wireless Networks. Harvey Lehpamer – Transmission Systems Design Handbook for Wireless Networks Chapter 2 – pages 5 to 45. Basics of Wireless Networks. Historical Background – The Cellular Concept Wireless Local Loop (WLL) Cellular Systems

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ENTC 455

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  1. WIRELESS TRANSMISSION SYSTEMS ENTC 455 Electronics & Telecommunications

  2. Basics of Wireless Networks • Harvey Lehpamer – Transmission Systems Design Handbook for Wireless Networks • Chapter 2 – pages 5 to 45 Electronics & Telecommunications

  3. Basics of Wireless Networks • Historical Background – The Cellular Concept • Wireless Local Loop (WLL) • Cellular Systems • Analog (FDMA) – AMPS • Digital: • TDMA: NA-TDMA (IS-136), GSM • CDMA (IS-95) • 1G, 2G, 2.5G, 3G,….. • Satellite Networks • Microwave Systems • Bluetooth Electronics & Telecommunications

  4. Historical Background • 1946 – the first public mobile telephone service was introduced in 25 major American cities • Transmitter tower that serves a metropolitan area (> 50 km) • Mobile users • Several channels available • Half-duplex mode • 120 khz of bandwidth per channel (voice) • 1960’s • Channels • 30 Khz of bandwidth per channel (voice) • IMTS – Improved Mobile Telephone Service • Full duplex, auto-dial, auto-trunking phone systems • Spectrally innefficient • 1976 – New York – 12 channels that could serve only 543 paying customers Electronics & Telecommunications

  5. Cellular Concept (proposed in 1968, implemented late 70’s) • Metropolitan areas were divided into cells • Small geographic area: cells • Low power transmitters • Frequency re-use • Each cells operating on a set of frequencies that differed from the frequencies of adjacent cells. Electronics & Telecommunications

  6. Figure 3: Mobile Telephone System Using a Cellular Architecture Cellular Concept Electronics & Telecommunications

  7. Cellular Concept – Frequency reuse 7 Cell cluster Cluster Size N = 7 6 2 1 5 3 4 Frequency reuse factor = 1/7 Each cell contains one-seventh of the total number of available channels Electronics & Telecommunications

  8. Frequency reuse concept 7 6 2 1 5 3 A cell cluster is replicated over the coverage area 7 4 6 2 7 1 6 2 5 3 1 4 5 3 4 Electronics & Telecommunications

  9. Mobile Switching Center Figure 8: Cellular System Components Cellular Network Down Link Up Link Electronics & Telecommunications

  10. Figure 7: Handoff between Adjacent Cells Handoffs Electronics & Telecommunications

  11. Method of locating co-channel cells in a cellular system Number of cells per cluster N N can only have values that Satisfy the equation: N = i2 + i j + j2 To find the nearest co-channel neighbors of a particular cell: 1) move i cells along any chain of hexagons 2) turn 60 degrees counter-clockwise and move j cells N = 7 i = 2 j = 1 1 1 1 Electronics & Telecommunications

  12. Example – To understand how frequency re-use improves capacity • If a total of 5 MHz of bandwidth is allocated to a particular FDD wireless system which uses 25 KHz simplex channels to provide full duplex voice and control channels, compute the number of channels available per metropolitan area. 25 KHz 5,000 KHz Electronics & Telecommunications

  13. Solution • Total bandwidth = 5 MHz • Channel bandwidth = 25 KHz x 2 simplex channels = 50 KHz/duplex channel • Total available channels = 5,000/50 = 100 channels Reverse Channel Forward Channel 1 2 3 1 2 3 100 100 25 KHz 5,000 KHz Electronics & Telecommunications

  14. Now assume: • Number of channels C =100 • Traffic intensity generated by each user: • μ: call request rate: 2 calls/hour • H: Holding time: 3 minutes/call • Au = μ x H = 2 x (3/60) = 0.1 Erlangs • Grade of Service: 1% • Blocked calls cleared (Erlang-B formula) • How many users can this system support? Electronics & Telecommunications

  15. How many users? • A = 84.1 Erlangs • U = A / Au = 84.1 Erlangs / 0.1 Erlangs/user = = 841 users Electronics & Telecommunications

  16. Example – Cellular System • If a total of 5 MHz of bandwidth is allocated to a particular FDD cellular wireless system which uses 25 KHz simplex channels to provide full duplex voice and control channels, compute the number of channels available per cell if a system uses • 4-cell reuse • 7-cell reuse 25 KHz 5,000 KHz Electronics & Telecommunications

  17. Solution • Total bandwidth = 5 MHz • Channel bandwidth = 25 KHz x 2 simplex channels = 50 KHz/duplex channel • Total available channels = 5,000/50 = 100 channels • A) for N=4 • Total number of channels per cell = 100/4 = 25 channels • B) for N=7 • Total number of channels per cell = 100/7 ~14 channels Electronics & Telecommunications

  18. For N=7: • Number of channels per cell C =14 • Traffic intensity generated by each user: • μ: call request rate: 2 calls/hour • H: Holding time: 3 minutes/call • Au = μ x H = 2 x (3/60) = 0.1 Erlangs • Grade of Service: 1% • Blocked calls cleared (Erlang-B formula) • How many users per cell? Electronics & Telecommunications

  19. For N=7: (cont.) • From the Erlang B chart: • Total carried traffic A = 9 Erlangs • Number of users/cell • U = A/ Au = 9/0.1 = 90 users • If there are 100 cells in a metropolitan area, then the total number of subscribers is 9000. • Exercise: • Do the calculations for N=4 Electronics & Telecommunications

  20. Conclusion C = 100 channels Number of subscribers One antenna – no frequency re-use 841 users Cellular, N=7 90 users/cell Cellular, N=4 Homework: page 8, explain the numbers of second paragraph Electronics & Telecommunications

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