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# Cellular system design fundamentals

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### Cellular system design fundamentals

1. 1. Mobile Communication (ET4 153) 01/05/00 Mobile Communications (ET4 153) 2. Cellular System Design Fundamentals Part 2 Jos Nijhof Delft University of Technology Cellular Systems mc_02 # 1 Cellular Systems – Overview Part 1 • Frequency reuse • Cluster size (N) • Frequency reuse factor (1/N) • Co-channel interference • Co-channel reuse ratio (Q) • Signal-to-Interference ratio (S/I) • Trunking • Grade of Service (GOS) • Erlang-B formula • Cell splitting • Sectoring Cellular Systems mc_02 # 2Cellular Systems 1
2. 2. Mobile Communication (ET4 153) 01/05/00 Cellular Systems – Overview Part 2 • Exercises on cellular planning • Channel assignment strategies • Handoff (handover) strategies • Power control Cellular Systems mc_02 # 3 Example 1.1 - Problem statement (2) (1) (5) Compute: 66.7 30.8 (4) 38.2 (1) (a) The number of channels (3) required in each cell 33.2 (7) (b) The number of subscribers (6) 32.6 48.6 served by the system (c) The average number of 37.8 subscribers per channel Given: (d) The number of calls Total available channels: 395 supported by the system Each subscriber generates 0.03 erlang (e) The subscriber density per Average holding time: 120 s square mile System area: 1200 miles2 Grade of service: 2% (f) The cell radius in miles Cellular Systems mc_02 # 4Cellular Systems 2
3. 3. Mobile Communication (ET4 153) 01/05/00 Example 1.1 - Solution Cell Traffic No. of No. of No. of Channel Number (erlang) Channels Subscribers Calls Utilisation Required Per Cell Per Cell (An) (a) (b) (d) 1 30.8 40 1026.7 924 0.77 2 66.7 78 2223.3 2001 0.86 3 48.6 59 1620.0 1458 0.82 4 33.2 43 1106.7 996 0.77 5 38.2 48 1273.3 1146 0.80 6 37.8 48 1260.0 1134 0.79 7 32.6 42 1086.7 978 0.78 Total 287.9 358 9596.7 8637 From An An erlang (b) x 0.9 0.03 (a) table/chart Cellular Systems mc_02 # 5 Example 1.1 - Solution (a), (b) : see table (c) : avg. number of subscriber s per channel : 9597 358 = 26.8 (d) : number of calls supported by system : A = λh ⇒ 0.03 = λ × 120 = 0.00025 [calls/s] = 0.00025 × 3600 = 0.9 [calls/hr ] 0.03 λ= 120 Cell (1) : Number of calls supported : 1026.7 × 0.9 = 924 (e) : subscriber density per mile 2 : 9597/1200 = 8.0 (f) : cell radius in miles : area/cell = 1200/7 = 171.4 miles 2 ⇒ radius Cellular Systems mc_02 # 6Cellular Systems 3
4. 4. Mobile Communication (ET4 153) 01/05/00 Example 1.3 - Problem statement Compare the spectral efficiency of the digital system with respect to the analogue system using the following data: (a) The total number of channels in the analogue cellular system = 416 (b) The number of control channels = 21 (c) The number of voice channels = 395 (d) The channel bandwidth = 30 kHz. The digital systems has 3 channels per 30 kHz (e) The reuse factor N = 7 (f) The total available bandwidth in each direction = 12.5 MHz (g) The total coverage area = 10,000 km2 (h) The required S/I ratio for the analogue system = 18 dB (63.1) (I) The required S/I ratio for the digital system = 14 dB (25.1) (j) The call blocking (GOS) = 2% Cellular Systems mc_02 # 7 Example 1.3 - Solution Spectral efficiency - η m = ( Total traffic carried by the system ) ( Bandwidth ) × ( Total coverage area) ANALOGUE SYSTEM: No. of voice channels / cell: 395 / 7 = 56.4 ⇒ 56 Offered traffic load: N = 56, B = 0.02 ⇒ A = 459 (from erlang table) . Carried traffic load: C = (1 − B) × A = (1 − 0.02) × 45.9 = 44.98 [erlang / cell] 10,000 10,000 Number of cells: = Acell 2.6 R 2 10,000 44.98 × ⇒ Spectral efficiency = 2.6 R 2 = 1384 . 12.5 × 10,000 R2 Cellular Systems mc_02 # 8Cellular Systems 4
5. 5. Mobile Communication (ET4 153) 01/05/00 Example 1.3 - Solution Total traffic carried by the system Spectral efficiency = ηm = (Bandwidth ) × (Total coverage area ) DIGITAL SYSTEM : No. of channels per 30 kHz = 3 Number of voice channels per cell = 56 × 3 = 168 Offered traffic load : N = 168,B = 0.02 ⇒ A = 154.5 (from erlang table) Carried traffic load : C = (1 − B ) × A = (1 − 0.02 ) × 154.5 [erlang/cel l] 1 1  S 4  S 2 Qdigital 2 25.1 Q = 6  ⇒ Q = 6  ⇒ 2 = = 0.6307  I  I Qanaloge 2 63.1 151.4 7.386 ⇒ Spectral efficiency = = erlang/MHz /km 2 12.5 × 2.6 R × 0.6307 2 R 2 ηm (digital ) 7.386 ⇒ = = 5.34 ηm (analogue ) 1.384 Cellular Systems mc_02 # 9 GSM system architecture (1) PLMN & International OMC ISC PSTN ISDN PDN MS BTS BSC GMSC BSC MSC BTS MS EIR MS AUC BTS HLR VLR Cellular Systems mc_02 # 10Cellular Systems 5
6. 6. Mobile Communication (ET4 153) 01/05/00 GSM system architecture (2) BTS Base Transceiving Station BSC Base Station Controller MSC Mobile Switching Center GMSC Gateway MSC ISC International Switching Center MS Mobile Station HLR Home Location Register VLR Visitor Location Register EIR Equipment Identity Register AUC Authentication Center OMC Operation and Maintenance Center Cellular Systems mc_02 # 11 A mobile radio environment Multipath fading Ra dio pa th Medium Pro pa g los ation s Base station Mobile station Cellular Systems mc_02 # 12Cellular Systems 6
7. 7. Mobile Communication (ET4 153) 01/05/00 The mobile radio channel: fading 10 Shadowing 0 Signal Level (dB) -10 -20 -30 Rayleigh fading -40 (multipath reception) λ -50 2 -60 0 1 2 3 4 5 6 7 time Cellular Systems mc_02 # 13 GSM: Carrier frequencies, duplexing, and TDMA frames 960 MHz 959.8 MHz 124 123 Downlink 25 MHz ⋅⋅⋅ 1 2 3 4 5 6 7 8 200 kHz ⋅⋅⋅ 2 935.2 MHz 1 935 MHz Data burst, 156.25 bit periods = 15/26 ms ≈ 576.9 µs 915 MHz 914.8 MHz 124 45 MHz 123 Delay separation 1 2 3 4 5 6 7 8 ⋅⋅⋅ 200 kHz ⋅⋅⋅ Uplink 2 890.2 MHz 1 890 MHz Cellular Systems mc_02 # 14Cellular Systems 7
8. 8. Mobile Communication (ET4 153) 01/05/00 Channel Assignment Strategies (1) • Fixed channel allocation (FCA) – fixed assignment of frequencies to cell clusters and cells. – not very efficient if traffic load varies – simple to use, but requires careful traffic analysis before installation – used in the GSM system • Variation: Borrowing channel allocation (BCA) – heavy loaded cell can “borrow” channels from a light loaded neighboring cell – problem: interference Cellular Systems mc_02 # 15 Channel Assignment Strategies (2) • Dynamic channel allocation (DCA) – each time a call request is made, the base station requests a channel from the MSC – MSC takes into account: • probability of future blocking within the cell • frequency of use of the channel • frequency reuse distance – Advantages: • lower probability of blocking, increases trunking capacity of the system – Disadvantages: • increased storage and computational load Cellular Systems mc_02 # 16Cellular Systems 8
9. 9. Mobile Communication (ET4 153) 01/05/00 The handover process Handover: Changing physical channels, radio channels of fixed network connections involved in a call, while maintaining the call Two phases: 1. MONITORING PHASE • measurement of the quality of the current and possible candidate radio links • initiation of a handover when necessary 2. HANOVER HANDLING PHASE • determination of a new point of attachment (PoA) • setting up of new links, release of old links • initiation of a possible re-routing procedure Cellular Systems mc_02 # 17 Two basic reasons for a handover • MS moves out of the range of a BTS – signal level becomes too low – error rate becomes too high • Load balancing – traffic in one cell is too high ⇒ shift some MSs to other cells with a lower load The GSM standard identifies about 40 reasons for a handover! Cellular Systems mc_02 # 18Cellular Systems 9
10. 10. Mobile Communication (ET4 153) 01/05/00 Handover types • Intra-cell handover – narrow-band interference ⇒ change carrier frequency – controlled by BSC • Inter-cell, intra-BSC handover – typical handover scenario – BSC performs the handover, assigns new radio channel in the new cell, releases the old one • Inter-BSC, intra-MSC handover – handover between cells controlled by different BSCs – controlled by the MSC • Inter-MSC handover – handover between cells belonging to different MSCs – controlled by both MSCs Cellular Systems mc_02 # 19 Handover types PLMN MSC MSC MSC MSC BSC BSC BSC BSC BSC old handover new handover PoA PoA handover Intra-BSC handover Inter-BSC / intra-MSC Inter-MSC handover handover Cellular Systems mc_02 # 20Cellular Systems 10
11. 11. Mobile Communication (ET4 153) 01/05/00 Intra-MSC handover (mobile assisted) MS BTSold BSCold MSC BSCnew BTSnew measurement measurement report result HO required HO decision ch. activation HO request resource allocation HO request ack ch. activation ack HO command HO command HO command HO access Link establishment HO complete HO complete clear command clear command clear complete clear complete Cellular Systems mc_02 # 21 Handover scenario at cell boundary Level at point A Received signal level Improper Handoff threshold handover situation Minimum acceptable signal level Level at point B Received signal level Level at point B Proper handover situation Level at which handover is made BS1 A B BS2 Cellular Systems mc_02 # 22Cellular Systems 11
12. 12. Mobile Communication (ET4 153) 01/05/00 Handover decision depending on receive level receive level receive level BTSold BTSnew average level HO_MARGIN MS MS BTSold BTSnew Cellular Systems mc_02 # 23 Handover – 1st generation systems • 1st generation systems (analog cellular): – signal strength measurements made by the BSs and supervised by the MSC – the BS constantly monitors the signal strengths of all the voice channels – locator receiver measures signal strength of MSs in neighboring cells – MSC decides if a handover is necessary or not. Cellular Systems mc_02 # 24Cellular Systems 12
13. 13. Mobile Communication (ET4 153) 01/05/00 Handover – 2nd generation systems • 2nd generation systems (digital TDMA): – handover decisions are mobile assisted – every MS measures the received power from surrounding BSs and sends reports to its own BS – handover is initiated when the power received from a neighbor BS begins to exceed the power from the current BS (by a certain level and/or for a certain period) Cellular Systems mc_02 # 25 Avoiding handovers: Umbrella cells Small microcells for low speed traffic Large “umbrella” cell for high speed traffic Cellular Systems mc_02 # 26Cellular Systems 13
14. 14. Mobile Communication (ET4 153) 01/05/00 Power control • Power levels transmitted by every MS are under constant control by the BSC. • Assures that each MS within a BTS coverage area provides the same signal level to the BTS receiver. • Goals: – to reduce interference – to prolong battery life – to combat the near-far problem in CDMA systems Cellular Systems mc_02 # 27Cellular Systems 14