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Wireless & Mobile Networking: Cellular Concept Azizol Bin Abdullah [email protected] upm. edu. my (A 2. 04)
Chapter 2: Cellular Concept 1) 2) 3) 4) 5) 6) 7) Cell Structure Cell Splitting VS Cell Sectoring Frequency Reuse Cochannel Interference Handoff Region Some Computations
Cellular Concept: Basics - Early mobile telephony systems were not cellular. Coverage over a large area was provided by a high powered transmitter mounted on a tall tower. Frequency reuse was not employed. That resulted in very low capacity. - The cellular concept arose in the 1970 s from the need to restructure the radio telephone system with the increase in demand. The increase in demand could not be satisfied just by additional spectrum allocations.
Infrastructure Cellular System: Back then - Early cellular system had a high-power transmitter to cover whole service area.
Infrastructure Cellular System: These Days - Then, cellular system replaced a large zone with a number of smaller cells, with as single BS covering a fraction of the area. Neighboring base stations (BS) are assigned different sets of channels. Capacity can be increased by additional partitions.
Cellular Concepts: Basics
o This is a modern tower with three different cell-phone providers riding on the same structure. If you look at the base of the tower, you can see that each provider has its own equipment, and you can also see how little equipment is involved today (older towers often have small buildings at the base)
Cellular System: Definition A cellular mobile communication system uses a large number of low-power wireless transmitters to create cells- the basic geographic service area of a wireless communications system.
Cellular System: Cells - In each cell area, users are served by a single BS(Base Station). - Mobile devices in this cell area are known as MSs(Mobile Stations). - Ideally, cell should be in circular shape. -For convenience, the cells are shown with a hex pattern. A hex pattern is the simplest pattern that can translate into an area. -In practice, cells are not hexagonal and BS are not exactly in the center of the cell. R Circular R BS BS R = Radius Hexagon
Cellular System: Cells (2) - The most important factor in cellular system is the size and the shape of cell. - Factors that cause reflections and refractions of the signal: -Elevation of the terrain -Presence of a hill or tall building -Presence of particles in the air. R Circular R BS BS R = Radius Hexagon
Cellular System: Clusters A cluster is a group of cells. No channels are reused within a cluster. Figure 1 illustrates a seven -cell cluster.
Cell Splitting vs Cell Sectoring - Using Cell-Splitting and Sectoring to Improve Capacity/Coverage of Cellular Systems -Cell splitting is achieved by installing smaller cells (microcells) in saturated macrocellular regions. More channels become available to users that appear in the saturated region (based on demand economic consideration) As a service area becomes full of users, this approach is used to split a single area into smaller ones. In this way, urban centers can be split into as many areas as necessary to provide acceptable/sound service levels, while larger, less expensive cells can be used to cover remote rural regions
Cell Splitting o o subdivide a congested cell into smaller cells each with its own base station, reduction in antenna and transmitter power more cells -> more clusters-> higher capacity achieves capacity improvement by essentially rescaling the system.
Cell splitting from radius R to R/2 and R/4
Cell Sectoring -Sectoring improves capacity by using sectorized antennas (120 degrees, 60 degrees) that reduce the co-channel interference. Reduction in co-channel interference means that the cluster size can be reduced which in turn leads to more channels per cell.
Cell Sectoring (cont’d) o o In basic form, antennas are omnidirectional Replacing a single omni-directional antenna at base station with several directional antennas, each radiating within a specified sector.
Cell Sectoring (cont’d) o o o achieves capacity improvement by essentially rescaling the system. less co-channel interference, number of cells in a cluster can be reduced Larger frequency reuse factor, larger capacity
Micro Cell Zone Concept o Large control base station is replaced by several lower powered transmitters on the edge of the cell. o The mobile retains the same channel and the base station simply switches the channel to a different zone site and the mobile moves from zone to zone. o Since a given channel is active only in a particular zone in which mobile is traveling, base station radiation is localized and interference is reduced.
Increasing Capacity in Cellular Systems o o As demand for wireless services increases, the number of channels assigned to a cell is not enough to support the required number of users. Solution ? ? ? n increase channels per unit coverage area.
Approaches to Increasing Capacity o o Frequency borrowing – frequencies are taken from adjacent cells by congested cells Cell splitting – cells in areas of high usage can be split into smaller cells Cell sectoring – cells are divided into a number of wedge-shaped sectors, each with their own set of channels Microcells – antennas move to buildings, hills, and lamp posts
Cellular Concept: Cell & Co-channel Interference -Cell A and B of a conventional, analog system are using the same frequency. The area of overlap, area C, has a frequency conflict and interference. -This is similar to what you experience when you are driving between the broadcast zones of two radio stations transmitting at the same frequency. co-channel interference: Interference resulting from two or more simultaneous transmissions on the same channel. Cell Sectoring is said able to reduce this type of interference.
Cellular Concept: Frequency Reuse -Because only a small number of radio channel frequencies were available for mobile systems, engineers had to find a way to reuse radio channels to carry more than one conversation at a time. The solution the industry adopted was called frequency planning or frequency reuse. -The concept of frequency reuse is based on assigning to each cell a group of radio channels used within a small geographic area. Cells are assigned a group of channels that is completely different from neighboring cells. The coverage area of cells is called the footprint. This footprint is limited by a boundary so that the same group of channels can be used in different cells that are far enough away from each other so that their frequencies do not interfere -Cells with the same number have the same set of frequencies. Here, because the number of available frequencies is 7, the frequency reuse factor is 1/7. That is, each cell is using 1/7 of available cellular channels.
Cellular Concept: Handoff -Handoff is the process of transferring a call from one base station to another when a user's radio signal becomes weaker at the first and stronger at the second base station. "Weaker" and "stronger" is quantified by a signal threshold level, which is above the minimum signal level for acceptable voice communications. Selecting this threshold level is critical to 1) ensure unnecessary handoffs do not occur; and 2) call dropping does not occur. - In early cellular, signal strength measurements were made by base stations and relayed to MSC. The MSC decided on handoff and performed the necessary communication between base stations to allow for a successful handoff. In modern cellular, handoffs are mobile assisted, called mobile-assisted handoffs (MAHO). MAHO 1) leads to faster handoff; and 2) allows handoff decisions to be based on metrics other than signal strength, e. g. , SIR.
Capacity Computations q Assume there are N cells, each allocated k different frequency channels. These N cells are said to form a cluster. Total number of channels per cluster is given by S = k. N q Total capacity associated with M clusters: C = Mk. N = MS q A cluster may be replicated more times in a given area if the cells are made smaller (note that power needs to be reduced accordingly).
Capacity Computations q Total number of users where: W = total bandwidth N = frequency reuse factor B = channel bandwidth m = number of cells required to cover an area
E 1 A total of 33 MHz are allocated to a system which uses 2 x 25 k. Hz for full duplex (i. e. , each channel is 50 k. Hz). What is the number of channels per cell? Number of channels per system Since 4 and 7 are popular number of cells per cluster/system a. For reuse N = 4: b. For reuse N = 7:
E A total of 33 MHz are allocated to a system which uses 2 x 25 k. Hz for full duplex (i. e. , each channel is 50 k. Hz). What is the number of channels per cell? Number of channels per system a. For reuse N = 4: For reuse N = 7:
E 2 Now assume 1 MHz of the 33 MHz is allocated to control channels. Each control channel is still 50 k. Hz Total number of voice (traffic) channels is now excluding control traffic Again, we take N=4 and N=7: For N = 4 => For N = 7 =>
E 2 Now assume 1 MHz of the 33 MHz is allocated to control channels. Each control channel is still 50 k. Hz Total number of voice (traffic) channels is now For N = 4 => 640/4 = 160 voice channels + control For N = 7 => 640/7 = 91 channels + control. channels.