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Outline Frequency Reuse and the Cellular Concept Cellular Architecture Co-Channel Reuse Factor Cell Types Advantages and Disadvantages of Cellular System Spectral Efficiency of a Cellular System Interference System Capacity Capacity enhancement Cell Sectoring Micro cell deployment Examples related to system capacity evaluation 2
Cellular Concept Cellular concept is a major breakthrough in solving problems of Spectralcongestion User Capacity Cellular concept is a system level idea to replace a single high power transmitter with many low power transmitters. Concept of “Frequency reuse” 3
Definitions / Terminology Frequency reuse Frequency reuse factor=1/N, N=cluster size A cell / sector is a logical network element that uses given resources. A cell site (or site or base station) can include multiple of cells/sectors at the same physical location. Cell coverage is defined as geographical area where the cell is likely to serve mobiles. Network coverage refers to sum of individual cell coverage. Actual radio coverage of a cell is called footprint and is determined from the field measurement or propagation prediction models.
Cellular Architecture In practice the cells are not regular hexagons, but instead are distorted and overlapping areas. The hexagon is an ideal choice for representing macrocellular coverage areas, because it closely approximates a circle and offers a wide range of tesellating (a regular tiling of polygons) reuse cluster sizes. A tesellating reuse cluster of size N can be constructed if, where i and j are non-negative integers, and i ≥ j. It follows that the allowable cluster sizes are N=1,3,4,7,9,12,….. 6
Commonly used cellular reusestructures Examples of 3-, 4-, and 7-cell reuse clusters are shown The reuse clusters are tesellated to form a frequency plan. 7
Macrocellular deployment using7-cell reuse pattern A simplified 7-cell frequency reuse plan is shown where similarly marked cells use identical sets of carrier frequencies. 8
19-cell reuse example (N=19) Figure shows method of locating co-channel cells in a cellular system. In this example, N = 19 (i.e., i = 3, j = 2). 9
Co-channel Reuse Factor The co-channel reuse factor D / R (sometimes also referred as Q), is defined as the ratio of the co-channel reuse distance D between cells using the same set of carrier frequencies and the radius of the cells R. For hexagonal cells, the reuse cluster size N and the co-channel reuse factor D / R are related by, 10
Smaller N is greater capacity Co-channel Reuse Ratio: 11
Microcellular Deployment For microcellular systems with lower BS antenna heights, regular hexagons are no longer appropriate for approximating the radio coverage zones. Typical microcell BSs use an antenna height of about 15 m, well below the skyline of any buildings that might be present. For microcells, the choice of cell shape depends greatly upon the particular deployment. 12
Microcellular deployment along ahighway with a 3-cell reuse pattern The linear cells may provide a more accurate model of highway microcells that are deployed along a highway with directional antennas. 13
Microcellular deployment in anurban canyon In an area with urban canyons, the buildings act as wave guides to channel the signal energy along the street corridors. Figure shows a typical Manhattan microcell deployment that is often used to model microcells that are deployed in city centers.Base stations are deployedat every intersection in a denseurban area with a 2-cellreuse pattern 14
Advantages Higher capacity More frequent resource utilization increases the capacity Less transmission power Reduced cell sizes, less power needed to cover the cell area Relaxed power amplifier specs at base stations Longer life-time for mobile station batteries Localized interference Due to smaller service areas of cells, interference is as well localized to a smaller area Robustness In case that one cell is down, overlapping of cells guarantees that a mobile is able to get connected through other base stations No technological challenges in deployment Major problems related to minimizing the implementation and operational expenses of the system Technological challenges related to capacity improvement methods
Disadvantages Massive infrastructure Number of base stations increases, if more capacity is required Lots of infrastructure needed also after radio air interface (location registers, switches, management servers etc.) More complex mobility management Seamless connections throughout the network has to be provided Handovers needed Management of handover procedures might get complicated Resource planning and management Tight resource planning strategy needed (time slots, frequencies) Resource management needed to support resource planning
Spectral Efficiency of a CellularSystem Main factors that determine the spectral efficiency of a cellular system. The cell sizes, The ability of radio links to withstand interference, and The ability of the cellular system to react to variations in traffic. 18
Requirement for High SpectralEfficiency High spectral efficiency require: effective cellular architecture, fast and accurate link quality measurements, rapid control in all types of environments, installation of BSs to provide radio coverage virtually everywhere, and power and bandwidth efficient air interface schemes that can mitigate the harsh effects of the propagation environment and tolerate high levels of noise and interference. 19
Spectrum Allocation In cellular systems, the available spectrum is partitioned among the BSs with two types of channels Voice Channels Control Channels A given frequency is reused at the closest possible distance that the radio link will allow. Smaller cells have a shorter distance between reused frequencies, and this results in an increased spectral efficiency and traffic carrying capacity. Example 3.1 20
Interference and system capacity Interference limits the system capacity and the performance of a single radio link. The higher is the interference level, the lower is the system capacity and poorer is the quality of communication links. Interference can origin from Mobiles under the same cell Mobile under different cell Base stations operating at the same frequency (or in general using simultaneously the same resources) From non-cellular systems Co-channel interference is the most problematic. Also adjacent channel interference affect the system capacity. However, adjacent channel interference is also filtering problem, which can be decreased by assigning channels intelligently.
Interference and system capacity Hence, the target of frequency (or resource) allocation is to plan frequency usage in such a manner that system capacity is maximized (i.e., frequencies should be utilized as frequently as possible). However, simultaneously the interference should be kept at adequate level. For cellular planning point of view, this can be achieved by providing as much isolation as possible between co-channel cells.
SIR Signal strength from the serving cell is denoted as S and co-channel interference as I. The resulting relation is signal-to-interference ratio (SIR) that defines the quality of the signal. where Ij is the co-channel interference received from jth co-channel cell. For simplicity, if transmission powers (P) of all cells is assumed to be the same as well as radio signal attenuation equal from all cells, SIR can be written as
SIR where d0 is the distance from the serving cell and dj from the jth cochannel cell. Propagation exponent (n) varies according to environment (in free space n=2, in dense urban environment n=4) In simple scenario, where R is the distance from the serving cell, and D is the distance from the first tier of interfering co-channel cell, SIR can be expressed
Capacity and qualityenhancement methods System capacity can be improved by decreasing the frequency reuse factor and quality by improving SIR. System capacity or SIR can be improved by Introducing multiple antennas at the base station site sectoring Minimization of out-of-cell interference (base station antenna characteristics) Decreasing antenna height Microcell deployment
Sectoring Sectoring implies to increasing the number of logical cells belonging to a single base station. Hence, one physical location (base station) includes multiple of antennas of the cells. Co-channel interference can be thus reduced. However, omni-directional antennas (1-sectored base station) has to be changed into directional ones. More intra-cell handovers
Sectoring Illustration of reduction of co-channel interference in N=4 using 3- sectored base stations. Example 3.2
Microcell deployment A network having antennas above roof top level is referred to as macrocellular network. Deploying antennas below roof top level reduces the frequency reuse factor microcellular network. Building prevent the signal propagation co-channel interference reduced significantly. Simultaneously, cell coverage is reduced and more cells are needed to cover certain area. Frequency reuse factor can be 5-7. Microcell deployment is expensive solution.