Communication Networks/Cellular Networks

Signal Overlapping

Signals need to be separated in either time, space, or frequency to prevent multiple transmissions from overlapping and interfering with one another. FDMA and TDMA techniques attempt to separate transmissions into different frequency bands and time slices, respectively. These systems allow multiple users in a single area to communicate without data collisions.

However, networks can also be physically separated by space to prevent data collisions. In such cases, users can communicate at the same time on the same channel, so long as they are in different networks in different places.

Cellular networks are a method for breaking large networks into smaller groups called "cells". Each cell has different frequency characteristics. This means that frequencies can be reused by non-adjacent cells without causing interference.

Example: Cellular Phones

Cellular phones, or mobile phones, are a very common sight today. Cellular phones connect wirelessly to a local base station, which receives the phone signal and transmits it into the phone network.

Roaming occurs when the mobile phone is moving from one cell to another. Cellular phones, in addition to transmitting voice data, also transmit and receive control data. The control data tells the phone how far it is from the base station. If a phone is moving from one cell to another, the distance from the new base station becomes smaller than the distance to the old base station. In this case, the new base station begins to handle the call and the old base station stops communicating with the phone.

Modeling Cells

Cells are typically modeled as regular hexagons. Regular hexagons have equidistant center between all adjacent cells. To avoid frequencies being used by adjacent cells, all cells don't share the same frequencies. If each cell used unique frequencies, then there wouldn't be enough frequencies to implement a large network. To get around this, frequency reuse is used to group cells into a pattern that within their group they don't share frequencies. This pattern is tessellated to fill out the area of service. The number of cells in a group is called the reuse factor. Common reuse factors include: 1, 3, 4, 7, 9, 12, 13, 16, 19, and 21.

In these hexagons, only four frequency bands are required to provide non-overlapping service to the entire network.

Network Expansion

As the demand increases, there are multiple ways in which the capacity of the network can be expanded.

Sub-Cells

Sub-cell techniques involve dividing an existing hexagonal cell into 7 smaller sub-cells. Smaller cells means that smaller base stations can be used, less transmit power is required, and more frequencies can be reused in a smaller area. Additionally, giving an entire frequency range to a smaller geographical area means that more people can be serviced in that area, and data throughput for the entire network can be increased.

Sectoring

Sectoring is similar to the sub-cell concept, except that instead of breaking a cell into smaller cells, a cell is broken up radially into "pie slices" called sectors. Each sector in a cell can reuse frequency ranges. Cells can be broken into 3 sector (120° divisions) and 6 sector (60° divisions) architectures.

Implementations

The reality of cellular networks is far different from the theoretical conception of them. In reality, base stations are not equidistant, and cells are not uniform size or shape. Because of the irregular size, shape, and placement of these cells, frequency orthogonality is more important, and networks often need to make use of many frequency ranges, instead of the theoretical minimum of three.