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Cellular Principles

Cellular Principles. Cellular Hierarchy. System Management. Link Quality Measurement Forward and reverse links are continually monitored Parameters: received signal quality and the bit error rates Cell Selection Choice of operator User preferences Available Networks MS capabilities

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Cellular Principles

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  1. Cellular Principles

  2. Cellular Hierarchy Cellular Principles

  3. System Management • Link Quality Measurement • Forward and reverse links are continually monitored • Parameters: received signal quality and the bit error rates • Cell Selection • Choice of operator • User preferences • Available Networks • MS capabilities • Network capabilities • MS mobility • Service requirements Cellular Principles

  4. System Management • Cell reselection • Unsuitability of current cell due to interference or output power requirements • Radio link failure • Network request • Traffic load considerations • User request • Channel Selection/Assignment • Channel assignment algorithms usually take into account the following: • System load • Traffic patterns • Service types • Service priorities • Interference situations Cellular Principles

  5. System Management • Handover (Handoff) • “The change of Physical Channel(s) involved in a call whilst maintaining the call” • Handovers may take place in several conditions: • within the cell: Intracell handover • between cells in the same cell layer: Intercell handover • between cells of different layers: Interlayer handover • between cells of different networks: Internetwork handover • Hard handover • In FDMA and TDMA wireless network • Soft-type handover • Soft handover (boundary of the cell) • Softer handover (boundary of the coverage area of the sector) • Soft-softer handover (both) • In CDMA wireless network Cellular Principles

  6. System Management • The following criteria may be used to initiate a handover for radio transmission reasons: • Signal strength measurements • Signal-to-interference ratio • Bit error rates • Distance between MS and BS • MS speed • MS Mobility trends • Others Cellular Principles

  7. System Management • Mobility Support • Logon-logoff • Location Updating Cellular Principles

  8. System Performance • Interference Control • Diversity Strategies • Diversity strategies are used to combat fading • Space • Frequency • Time • Variable Data Rate Control • Direct support of variable data rates over the air interface • Variation of the number of bearer channel • Packet access Cellular Principles

  9. System Performance • Capacity Improvement Techniques • Slow frequency hopping • Dynamic power control • Dynamic channel allocation • Discontinuous transmission for voice, including voice activity detection • Nonvoice services • Battery-Saving Techniques • Output power control • Discontinuous reception • Discontinuous transmission Cellular Principles

  10. Cellular Reuse Pattern • Co-cells: Cells using the same carrier frequency • Cluster: A group of cells among which the whole spectrum is shared and within which no frequency reuse exists • The number of cells per cluster defines the reuse pattern and this is a function of the cellular geometry Cellular Principles

  11. Macro cellular Reuse Pattern • Circles x Regular Polygons (Equilateral triangles, squares, and hexagons) • Hexagonal cellular geometry • Propagation symmetry • Low-capacity systems Cellular Principles

  12. Macro cellular Reuse Pattern Cellular Principles

  13. Macro cellular Reuse Pattern • R = Cell radius • d = The distance between the center of two cells. • D = Reuse distance, that is, the distance between two co-cells. • A =Area of the hexagonal cluster. • a = Area of the hexagonal cell. • N = Reuse Factor (Number of cells per cluster) Cellular Principles

  14. Macro cellular Reuse Pattern Cellular Principles

  15. Macrocellular Reuse Pattern Cellular Principles

  16. Macro cellular Reuse Pattern • Co-channel Reuse Ratio • The reuse ratio gives a qualitative measure of the signal quality (carrier-to-interference ratio) as a function of the cluster size. • Positioning of the Co-Cells • There are 6n co-cells on the nth tier Cellular Principles

  17. Micro cellular Reuse Pattern • Square cellular geometry • High traffic demand in dense urban regions • Low mobility • The propagation direction of the radio waves is greatly influenced by the environment • Inherent asymmetry • A much greater number of BS • The per-subscriber cost is determinant • The interference is dependent not only on the distance between transmitter and receiver but also, and mainly, on the LOS Cellular Principles

  18. Micro cellular Reuse Pattern • Reuse distance • Reuse Factor (Number of Cells per Cluster) • Reuse Ratio Cellular Principles

  19. Micro cellular Reuse Pattern Cellular Principles

  20. Micro cellular Reuse Pattern Cellular Principles

  21. Interference in Narrowband (NB) and Wideband (WB) Systems • NB and WB systems are affected differently by interference • NB System: • Interference is caused by a small number of high-power signals • There are different interference patterns between Macrocellular and Microcellular networks • Macrocellular systems: • Uplinks and downlinks present approximately the same interference performance (Note: regardless of the system, the uplink performance is always worse) • The larger the reuse pattern (N), the better the interference performance • Microcellular systems: • Interference Performance of uplinks and downlinks are very dissimilar • In general, the larger the reuse pattern (N), the better the interference performance Cellular Principles

  22. Interference in Narrowband (NB) and Wideband (WB) Systems • WB System: • Interference is caused by a large number of low-power signals • Traffic profile and channel activity have great influence on interference performance • Uplinks and downlinks have different performances • The interference performance analysis of a Cellular System is performed in terms of: • carrier-to-interference ratio (C/I) • efficiency of frequency reuse (f) Cellular Principles

  23. Interference in Narrowband Macrocellular Systems • The propagation is characterized by an NLOS (non line-of-sight) condition • The Mean Power (P) received at a distance (d) from the transmitter is: • K is a proportionality constant that depends on several parameters, such as: f, Base Station (BS) antenna height and gain, Mobile Station (MS) antenna height and gain, environment, etc. •  is the propagation path loss coefficient and usually ranges between 2 and 6 Cellular Principles

  24. Interference in Narrowband Macrocellular Systems • Subsequent calculations assume that: • K and  remain constant • MS is positioned for the worst-case condition, that is, at the border of the serving cell (distance R from the BS) • C/I ratio for the downlink is calculated at the MS: • C is the signal power received from the serving BS • I is the sum of the signal powers received from the interfering BS’s (co-cells) Cellular Principles

  25. Interference in Narrowband Macrocellular Systems • C/I ratio for the uplink is calculated at the BS: • C is the signal power received from the wanted MS • I is the sumof the signal powers received from the interfering MS’s (from the various co-cells) • Macrocellular network: • In this network, it is convenient to investigate the effects of interference by using: • omnidirectional antennas: 6n interferers for the nth tier (all possible) • directional antennas: reduction to  6n/s interferers, where ´s´ is the number of sectors used in the cell Cellular Principles

  26. good approximation Interference in Narrowband Macrocellular Systems • Downlink Interference - Omnidirectional Antenna • For the worst-case condition, the MS is positioned at a distance R from the BS. It is assumed that the 6n interfering BS’s in the nth ring are  at a distance of nD. Therefore: • (x) is the Riemann function: (1)=, (2)=2/6, (3)=1.2021, and (4)=2/6. Cellular Principles

  27. good approximation Interference in Narrowband Macrocellular Systems • Consider  = 4 and N = 7: • Exact C/I = 61.14 = 19.9 dB • Approximate C/I = 73.5 = 18.7 dB • Uplink Interference - Omnidirectional Antenna • For the worst-case condition, the MS is positioned at a distance R from the BS. It is assumed that the 6n interfering MS’s in the nth ring are  at a distance of (nD - R), which is the closest distance that the MS can be with respect to the interfered BS. Therefore: Cellular Principles

  28. Interference in Narrowband Macrocellular Systems • Consider  = 4 and N = 7: • Exact C/I = 25.27 = 14.0 dB • Approximate C/I = 27.45 = 14.38 dB • Downlink Interference - Directional Antenna • Following the same procedure above: • Consider  = 4, N = 7 and s = 3 (Three-sector cell): • Exact C/I = 183.42 = 22.6 dB • Approximate C/I = 220.5 = 23.4 dB • Uplink Interference - Directional Antenna Cellular Principles

  29. Interference in Narrowband Macrocellular Systems • Consider  = 4, N = 7 and s = 3 (Three-sector cell): • Exact C/I = 75.81 = 18.8 dB • Approximate C/I = 82.35 = 19.16 dB • Examples: • The table below gives some examples of C/I figures for  = 4 and for several reuse patterns, with omnidirectional and directional (1200 antennas, or three-sectored cells) antennas Cellular Principles

  30. Interference in Narrowband Macrocellular Systems • NOTE that the use of directional antennas substantially improves the C/I ratio • The choice of which antenna to use depends on how tolerant the technology is with respect to interference • N = 7 and N = 4 are reuse patterns widely deployed with 1200 antennas (they are referred as 7x21 and 4x12, respectively) Cellular Principles

  31. Interference in Narrowband Microcellular Systems • nL is the distance between the interferers at the co-cell of the L-th layer and at the target cell (reference) normalized with respect to the cell radius. It is then given in number of cell radii. • nL is used to investigate the performance of different microcellular reuse patterns • nL is greatly dependent on the reuse pattern (N). • nL can be obtained by simple visual inspection, but Appendix D shows a general formulation for calculating it. Cellular Principles

  32. Interference in Narrowband Microcellular Systems • The subsequent performance analysis considers a square cellular pattern with BS’s positioned at every other intersection of streets. Then, BS’s are collinear and each micro cell covers a square area comprising four 900 sectors, each sector corresponding to half a block, with the streets running on the diagonals of this square. • In Fig 2.7, the horizontal and vertical lines correspond to the streets, and diagonal lines represent the borders of microcells Cellular Principles

  33. Interference in Narrowband Microcellular Systems • Figure 2.7 Cellular Principles

  34. Interference in Narrowband Microcellular Systems • Figures 2.8 and 2.9 show the complete tessellation for clusters with 5 (Fig 2.8), 8, 9, 10, and 13 (Fig 2.9) microcells, in which the highlighted cluster accommodates the target cell, and the other dark cells correspond to the co-microcells that at certain time may interfere with the BS or MS of interest • In these Fig’s, stars indicate the sites contributing to the C/I of the downlink, whereas the circles indicate the worst-case location of the MS affecting the performance of the uplink Cellular Principles

  35. A C B D E D E A A D E C C B B A D E D E C B A A D E D E C C B B A A D E D E C C B B A A D E C C B B A D E D E C B A A D E D E C C B B A A E D E D C C B B A A A D E D E C C B B A A D E D E C C B B A A D E C C B B A D E D E C B A A D E D E C C B B A A D E D E C C B B A A D E C C B B A D E D E C B A A C C B B D E A Interference in Narrowband Microcellular Systems • Figures 2.8 Cellular Principles

  36. Interference in Narrowband Microcellular Systems • Figures 2.9 (a) Cellular Principles

  37. Interference in Narrowband Microcellular Systems • Figures 2.9 b ( ) b ( ) Cellular Principles

  38. Interference in Narrowband Microcellular Systems • Figures 2.9 (c) Cellular Principles

  39. Interference in Narrowband Microcellular Systems • Figures 2.9 (d) Cellular Principles

  40. Interference in Narrowband Microcellular Systems • Note that distinct situations can affect in different ways the performance of the downlink and the uplink • In general, the set of micro cells affecting the downlink is a subset of those influencing the uplink • Note that the staggered nature of some patterns implies that the closest interferers are either completely obstructed or obstructed for most of the time with a LOS interferer appearing many blocks away Cellular Principles

  41. Interference in Narrowband Microcellular Systems • For clusters constituted by a prime number of cells (Fig 2.8), the interfering BS in the downlink changes as the target MS moves along the street • Propagation • it is characterized by both LOS and NLOS modes • For NLOS mode, the mean power received at distance d from the transmitter is: • Note that this power strength is similar to that one of macrocellular systems • KNLOS is a proportionality constant that depends on frequency, antenna heights, environment, etc Cellular Principles

  42. Interference in Narrowband Microcellular Systems • For LOS condition, and for a transmitting antenna height ht,a receivingantenna height hr, and a wavelength , the received mean power at distance d is approximately: • KLOS is a proportionality constant and depends on frequency, antenna heights, environment, etc • dB is the breakpoint distance (4hthr/ ) • Note that LOS and NLOS propagation modes a rather different • For NLOS condition, the mean signal strength decreases monotonically with the distance Cellular Principles

  43. Interference in Narrowband Microcellular Systems • For LOS condition and d < dB, the mean signal strength decreases monotonically with a power law close to the free space condition (  2). However, for d > dB, the power law follows closely that of the plane earth propagation (  4) • For calculation purposes, it is defined r = d/R as the distance of the serving BS to the MS normalized with respect to the cell radius (0  r  1), and k = R/dB as the ratio between the cell radius and the breakpoint distance (K  0) • It is interesting to investigate the C/I performance as the mobile moves away from the serving BS along the radial street. Note: this pattern is different from the macrocellular one, whose interference pattern is approximately maintained throughout the cell Cellular Principles

  44. good approximation Interference in Narrowband Microcellular Systems • Uplink Interference • By using PLOS for both wanted and interfering signals: • Downlink Interference • Following the same procedure as the uplink interference, C/I can be found. However, since this ratio greatly depends on the position of the target MS within the cell, three different interfering conditions may be identified as MS moves along the street: (1) at the vicinity of the serving BS, (2) away from both the vicinity of the serving BS and the cell border, and (3) near the cell border. Cellular Principles

  45. Interference in Narrowband Microcellular Systems • at the vicinity of the serving base station, more specifically at the intersection of the streets (r  normalized distance from the cell site to the beginning of the block), the MS has a good radio path to its serving BS, but it also has radio paths to the interfering BS on both crossing streets. Then: • Away from the vicinity of the serving BS and away from the cell border, which correspond to most of the paths, the MS enters the block and loses LOS to those BS located on the perpendicular street ... Cellular Principles

  46. Interference in Narrowband Microcellular Systems • Then: • At the border of the cell, new interferers appear in the LOS condition. However, this is not the case for all reuse patterns. This phenomenon only happens for clusters with a prime number of cells. For this clusters, considering that the MS is away from its serving BS (1- r  normalized distance from the site to the beginning of the block) and : Cellular Principles

  47. Interference in Narrowband Microcellular Systems • A good approximation for the downlink C/I can be obtained by simply considering L=1 • Examples • C/I performance for clusters with 5, 8, 9, 10, 13 micro cells are illustrated. The performance has been evaluated with the central micro cell as the target cell and with the MS departing from the cell center towards its edge (see arrow in Fig 2.8, which also shows, in gray, the micro-cells that at certain time may interfere with the wanted MS in a LOS condition). • For numerical results, the calculations considered: R=100 m, street width of 15 m, ht=4 m, hr=1.5 m, f=890 MHz ( = 3/8.9 m), and then, K=1.405 (note that R is 40.5% greater than dB). The network was considered to have an infinite number of cells (in practice, 600 layers of interfering cells) Cellular Principles

  48. Interference in Narrowband Microcellular Systems • Figs 2.10 and 2.11 show, respectively, the uplink and downlink performances for N = 5, 8, 9, 10, and 13 as a function of the normalized distance. • In general, the larger the cluster, the better the C/I. However, the five-micro-cell cluster exhibits a remarkable behavior. Its uplink C/I curve coincides with that for N=8 (lower curve in Fig 2.10), and its downlink C/I curve coincides with that for N=10 for most of the path extension (curve below the upper curve in Fig 2.11). In the latter, the separation of the curves occurs at the edge of the micro cell, where 2 interferers appears in a LOS condition. • Note also that in Fig 2.10, the C/I curves for N=9 and N=13 are also coincident • Fig 2.12 compares the performance between 5- and 10- micro cell clusters. Cellular Principles

  49. Interference in Narrowband Microcellular Systems • Fig 2.12 shows how different the performances between uplink and downlink are for an specific N, and how they get progressively smaller and smaller as N increases • Fig 2.13 and 2.14 examine how the number of interfering layers influences on both downlink and uplink performance analyses for N=5- and N=10- clusters, respectively. Both figures provide the performances as functions of the normalized distance to the BS using L=1 and L= • Note that the difference between the C/I ratio for an infinite-cell network and for a one-layer network is NEGLIGIBLE! This conclusion also applies to the other patterns, with the largest difference found in similar analyses for all reuse patterns being less than 0.35 dB Cellular Principles

  50. Interference in Narrowband Microcellular Systems • Therefore, very accurate estimates can be achieved by only considering the closest layer to the target cell Cellular Principles

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