Contents
Terms we shall use here
Mobile station
This is responsible for communicating information with the user and also modifies this information to the transmission protocol of the air and the user. The mobile station has two modules, that is The mobile equipment and the Subscriber identity module as explained in details below.
Mobile Equipment ME
This is just a piece of hardware a customer/Cellular user purchase from the equipment manufacturer and it contains all the components needed to implement the transmission protocols with both the user and the air interface to the base station.
Subscriber identity Module
This is just a smart card issued at the time of subscription, which identifies the user and the type of service. It carries every user personal information, stores contacts and as well as stores SMS. Also, calls are directed to the SIM card rather than the terminal.
Base station
While a telephone is only capable of transmitting the user’s data and reception, the base station does more than that. it is capable of handling several subscribers simultaneously.
Base transceiver station
This lies between the base station and the mobile phone and is responsible for handling user data transmission between the telephone and the base station. It has the ability to receive and transmitting.
How a phone works
Cellular telephony is sometimes referred to as mobile telephony and the first commercial telephone was approved in 1983 by the Federal Communications Commission (FCC)
It was designed to provide communication between two moving mobile stations (MS), and also between a stationary mobile station and a moving mobile station.
This is a type of shortwave, which is either analog or digital, whereby a subscriber has a wireless connection from a mobile phone to a relatively nearby transmitter.
Transmitters have cells, which are spans or ranges within which signals can be accessed. Call it an area of coverage.
The telephone will be effectively passed on to the local cell transmitter as the telephone user moves from one cell to the next cell coverage.
In this process, it will need a service provider to be able to track the caller, assign a channel of communication to the caller and transfer this channel of communication from the base station to the base station while the caller moves in and out of range from the original base station.
How is this tracking possible?
Each cellular service area is divided into small regions called cells (Range/Span) and each cell has an antenna that is supported by a solar, or AC powered network station called the base station. In return, each base station is controlled by a switching office called Mobile Switching Center (MSC)
MSC is responsible for controlling communication between all base stations and the telephone central office. Billing, recording of call information and automatic connection of calls is done by a computerized center.
The span/cell has no fixed size, that is: its size largely depends on the size of the population in the area. High-density area shall require smaller cells than in lower density areas.
Why do we care about the span?
This is necessary due to the need to control interference between adjacent cell signals.
The transmission power of each cell should be kept low for the same reason. Controlling interference
Why we need Frequency reuse principle
As a result of two adjacent cells using the same frequency set, interference may arise for the user that are located near the boundaries of the cells. Since the sets of frequencies available are limited, there arises a need for frequency reuse.
Frequency reuse in play
The frequency reuse pattern is a configuration of N cells, where N is a reuse factor, and each cell uses a unique pattern or set of frequencies. When this unique pattern is repeated, it means that the set of frequency can be reused.
Note: The number in the cell defines the pattern to use, that is, cells with the same number in the pattern shall use the same frequency set (These are called reusing cells)
For example: in the reusing factor of 4, only one cell shall separate the cells that are using the same frequency set while in that of reusing factor of 7, only 3 cells separate the reusing cells.
Transmitting and Receiving
How a call is made?
To place a call, the mobile station enters a phone number which is either made up of 7 digits or 10 and presses the send button.
The mobile station continues to scan the band and looks for a setup channel with strong signals. It will then send the data to the closest base station using the chosen channel.
The mobile station then relays this data to the Mobile switching center which in return shall send this data to the telephone central office.
If the called party is available, a connection shall be made, and the data relayed back to the mobile switching center.
The mobile switching center then assigns an unused channel to this communication, and a connection is established. The mobile station will automatically adjust its tuning to the new channel and the communication will begin.
How receiving a call Work?
When a mobile phone is called, the telephone central officer shall send the number to the mobile station and the mobile switching center shall search for the location of the calling mobile station.
This is done by sending a query signal to each cell in a process known as paging. Once the calling mobile station is found, the MSC transmits the ringing signals, and once the called MS answers; the MSC assigns a voice channel to the call allowing voice communication to begin.
Two problems
Handoffs
Handoffs happen as a result of a moving mobile station travels across numerous cells during a conversation under signals that may be weaker
Solution
The MSC monitors the level of the signals every second. In case the strength of the signals decreases, it seeks new cells that can better accommodate the communication and in the same background; it changes the channel carrying the call to a new channel.
Hard handoff
Used in the past, a mobile station only communicated with one base station. And while it moves across the cell, the communication must first be broken from the previous station before being established with the new one. This usually created a rough transition.
Soft Handoffs
Used in the new systems. A mobile station is in communication with two base stations. This means that during a handoff, a mobile station may continue with the new base station before breaking off from the old one.
How does Roaming Work?
Since service providers always have limited coverage, yet a user may always require coverage to keep his/her communication ongoing, neighboring service providers may provide extended coverage through a roaming contract. This is how roaming works.