第4章 蜂窝的概念：系统设计基础

Chapter 4: The Cellular Concept– System Design Fundamentals

Introduction The cellular concept was a major breakthrough in solving the problem of spectral congestion and user capacity. It offered very high capacity in a limited spectrum allocation without any major technological changes. It enables a fixed number of channels to serve an arbitrarily large number of subscribers by reusing the channels throughout the coverage region.

Frequency Reuse Concept One of the major problem in cellular is to support a large number of users with a limited frequency spectrum. In the 1970s, the Bell mobile system in New York could only support 12 simultaneous calls over a thousand square miles. The cellular concept can solve this problem to increase the system capacity. Frequency Reuse is a major cellular concept. Two fundamental ideas:– Cellular Topology: A large region cell is divided into small regions called cells.– Reuse the frequency spectrum.

Frequency Reuse Concept A large service area is divided into many small regions called cells with hexagonal shape. Each cell is one BS with a low power transmitter instead of high power transmitter. Each cell is assigned a set of frequency channels. Neighboring cells are assigned a different set of channels to avoid co-channel interference which is the interference due to the common use of same channel. The same set of channels can be assigned to different cells that are separated large enough to limit co-channel interference to a tolerable level. The minimum distance between two co-channel cells (cells using the same channel) is called reuse distance.

Example

Cellular concept Cellular topology is a special case of an infrastructure multi-BS network configuration that exploits the frequency reuse concept. The cellular concept is a system-level idea– which replacing a single, high power transmitter (large cell), with many low power transmitters (small cells), each providing coverage to only a small portion of the service area. The available channels are distributed throughout the geographic region and may be reused as many times as necessary so long as the interference between cochannel stations is kept below acceptable levels.

Cellular concept The fundamental principle of the cellular concept is to divide the coverage area into a number of contiguous smaller areas which are each served by its own radio base station.– Each of these smaller areas is called a cell.– Cells are grouped into clusters.– Each cluster utilizes the entire available radio spectrum.– The number of cells in a cluster is called cluster size or frequency reuse factor. The spatial distribution of chunks of radio spectrum (which are called sub-bands) within a cluster has to be done in a manner such that the desired performance can be obtained. This forms an important part of network planning in cellular radio.

The Cellular Concept

Example: Im

portance of Cellular Topology We want to provide a radio communication service to a city. The total bandwidth available is 25 MHz, and each user requires 30KHz of bandwidth for voice communication. If we use one antenna to cover the entire town, we can only support 25MHz/30 KHz= 833 simultaneous users. Now let us employ a cellular topology where 20 lower power antennas are opportunistically located to minimize both kinds of interference.– We divide our frequency band into four sets and assign one set to each cell.– Each cell has a spectrum of 25 MHz/4= 6.25 MHz allocated to it.– We have a cluster of four cells in this example.– The number of simultaneous users supported per cell is 6.25 MHz/30 KHz= 208.– The number of users per cluster is 4 x 208= 832.– The total number of simultaneous users is now 832 x 5= 4,160 because we have five clusters of four cells each. The new capacity is roughly five times the capacity with a single antenna.

Capacity of the network The number of simultaneous users is given by: n= m (W/N)/ B= (m/N) (W/B)– If W is the total available spectrum,– B is the bandwidth needed per user,– N is the frequency reuse factor,– m is the number of cells required to cover an area, The capacity of the network can be increased by– increasing m,– decreasing the frequency reuse factor N

Capacity of the network To implement frequency reuse:– N cells are grouped together and called cluster. N is called a frequency reuse factor or cluster size.– Each cluster uses the all available, S channels.– Each cell in a cluster is allocated S/N channels if using uniform fixed channel assignment.– The whole service area is divided into M clusters.– The total number of channels, n, in the service area is n= M× N× S/ N= MS= (m/N)× (W/B)– With hexagonal cellular geometry, the possible values of N are given N= i2+ ij+ j2– which are N=1, 3, 4, 7, 9, 12, 13

Cell reuse system example

蜂窝网 全向天线辐射的覆盖区大体上是圆。 为了实现无缝隙覆盖，一个个天线辐射源产生的覆盖圆形必然会重叠。 重叠区就是干扰区，什么样的多边形最佳？ 主要考虑以下几个因素： 相邻小区的中心间隔； 单位小区的面积； 重叠区宽度； 重叠区面积； 所需要的最少频率个数。

蜂窝小区的形状

不同形状小区参数的比较项目重叠区与小区面积比所需最少频率个数

正三角形

正方形

正六角形

1.41 6

0.57 4

0.21 3

可以看出：正六角形小区的中心间隔最大，覆盖面积最大，重叠区面积最小，所以采用正六角形构成小区所需的小区数最少。因此，它是最经济的组网方式。