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Final wc
- 1. Chapter 3 The Cellular Concept - System Design Fundamentals
- 2. Introduction cell system Frequency Reuse Channel Assignment Strategies Handoff Strategies Interference and System Capacity Improving Capacity In Cellular Systems
- 3. Older System • Achieve a large coverage area by using a simple, high powered transmitter. • Put BS on top of mountains or tall towers, so that it could provide coverage for a large area. • The next BS was so far away that interference was not an issue. • Severely limit the number of users that could communicate simultaneously. • Noise-limited system with few users.
- 4. • The Bell mobile system in New York City in the 1970s could only support a maximum of twelve simultaneous calls over a thousand square miles. • The number of simultaneous calls a mobile wireless system can accommodate is essentially determined by the total spectral allocation for that system and the bandwidth required for transmitting signals used in handling a call. Example • Using a typical analog system, each channel needs to have a bandwidth of around 25 kHz to enable sufficient audio quality to be carried, as well as allowing for a guard band between adjacent signals to ensure there are no undue levels of interference. Using this concept, it is possible to accommodate only forty users in a frequency band 1-MHz wide. Even if 100 MHz were allocated to the system, this would enable only 4000 users to have access to the system.
- 6. older (Early) mobile radio systems • A single high powered transmitter (single cell) • Large coverage area • Low frequency resource utility • Low user capacity
- 7. The cellular concept • A major breakthrough in solving the problem of spectral congestion and user capacity • Many low power transmitters (small cells) • Each cell covers only a small portion of the service area. • Each base station is allocated a portion of the total number of Channels • Nearby base stations are assigned different groups of channels so that the interference between base stations is minimized
- 8. Today cellular systems have millions of subscribers, and therefore a far more efficient method of using the available spectrum is needed.
- 9. The concept of cells was 1st proposed as early as 1947 by Bell Laboratories in the US, with a detailed proposal for a “High-Capacity Mobile Telephone System" incorporating the cellular concept submitted by Bell Laboratories to the FCC in 1971. The 1st AMPS system was deployed in Chicago in 1983.
- 10. Cell Shapes Geometric shapes covering an entire region without overlap and with equal area. By using the hexagon, the fewest number of cells can cover a geographic region, and the hexagon closely approximates a circular radiation pattern which would occur for an omni-directional antenna.
- 11. Center-excited cell Base station transmitter is in the center of the cell. Omni-directional antennas are used. Edge-excited cell Base station transmitters are on three of the six cell vertices. Sectored directional antennas are used.
- 12. • We use hexagonal cell shape as a simplistic model of the radio coverage for each base station. • Universally adopted since the hexagon permits easy and manageable analysis of a cellular system. • The actual radio coverage of a cell is known as the footprint and is determined from field measurement • In reality, it is not possible to define exactly the edge of a cell. The signal strength gradually reduces, and towards the edge of the cell performance falls.
- 13. • As the mobiles themselves also have different levels of sensitivity, this adds a further sensitivity of the edge of the cell. • It is therefore impossible to have a sharp cut-o between cells. In some areas they may overlap, whereas in others there will be a “hole" in coverage. • Although the real footprint is amorphous in nature, a regular cell shape is needed for systematic system design and adaptation for future growth.nts or propagation prediction models.
- 14. Why hexagon? • Adjacent circles cannot be overlaid upon a map without leaving gaps or creating overlapping regions. • When considering geometric shapes which cover an entire region without overlap and with equal area, there are three sensible choices: a square, an equilateral triangle, and a hexagon. • A cell must be designed to serve the weakest mobiles within the footprint, and these are typically located at the edge of the cell.
- 15. For a given distance between the center of a polygon and its farthest perimeter points, the hexagon has the largest area of the three. • By using the hexagon geometry, the fewest number of cells can cover a geographic region Closely approximate a circular radiation pattern which would occur for an omnidirectional base station antenna and free space propagation. • Permit easy and manageable analysis of a cellular system.
- 17. Types of cells • macrocell – their coverage is large (aprox. 6 miles in diameter); used in remote areas, high-power transmitters and receivers are used • microcell – their coverage is small (half a mile in diameter) and are used in urban zones; low-powered transmitters and receivers are used to avoid interference with cells in another clusters
- 18. Cellular systems generations • 1G (first generation) – voice-oriented systems based on analog technology; ex.: Advanced Mobile Phone Systems (AMPS) and cordless systems • 2G (second generation) - voice-oriented systems based on digital technology; more efficient and used less spectrum than 1G; ex.: Global System for Mobile (GSM) and US Time Division Multiple Access (US-TDMA) • 3G (third generation) – high-speed voice-oriented systems integrated with data services; ex.: General Packet Radio Service (GPRS), Code Division Multiple Access (CDMA) • 4G (fourth generation) – WiMax, LTE, based on Internet protocol networks and will provide voice, data and multimedia service to subscribers
- 19. Frequency Reuse • A service area is split into small geographic areas, called cells. • Each cellular base station is allocated a group of radio channels. • Base stations in adjacent cells are assigned different channel groups. • By limiting the coverage area of a base station, the same group of channels may be reused by different cells far away. • The design process of selecting and allocating channel groups for all of the cellular base stations within a system is called frequency reuse or frequency planning.