BLUETOOTH

Bluetooth allows two devices to be connected to each other wirelessly. The most common use of Bluetooth technology is in hands-free devices such as headsets used with mobile phones. Bluetooth technology can also be used to transfer data between two electronics devices without using wires.

Features

• Bluetooth technology uses radio waves to send information between two devices that are close to each other. Unlike traditional radio waves, Bluetooth waves typically can only travel 33 feet or less.

Types

• Bluetooth is used is mobile phones, headsets, headphones, MP3 players, computers, boom boxes, laptops, computer mice, GPS units, and car stereos. Almost any Bluetooth device can be paired with another Bluetooth device in order to exchange information.

Indentification

• You will typically be able to identify if a device is Bluetooth capable by whether the phrase “Bluetooth Capable or Compatible” is printed on the device or its packaging, or by the presence of the Bluetooth logo. Every Bluetooth device has its own unique pairing code that is used to identify it and connect it to other Bluetooth-capable devices.

Benefits

• Bluetooth technology enables two devices to be connected together wirelessly, eliminating the clutter and confusion typically associated with wires. Bluetooth can be used to allow users to make hands-free calls using their mobile phone and headset or speakerphone in their car. Bluetooth can also be used to connect a computer mouse to home computers, headphones to MP3 players, digital cameras to printers, and it can wirelessly send data from your computer to another device.

Satellite communications basics

When used for communications, a satellite acts as a repeater. Its height above the Earth means that signals can be transmitted over distances that are very much greater than the line of sight. An earth station transmits the signal up to the satellite. This is called the up-link and is transmitted on one frequency. The satellite receives the signal and retransmits it on what is termed the down link which is on another frequency.

Using a satellite for long distance communications

The circuitry in the satellite that acts as the receiver, frequency changer, and transmitter is called a transponder. This basically consists of a low noise amplifier, a frequency changer consisting a mixer and local oscillator, and then a high power amplifier. The filter on the input is used to make sure that any out of band signals such as the transponder output are reduced to acceptable levels so that the amplifier is not overloaded. Similarly the output from the amplifiers is filtered to make sure that spurious signals are reduced to acceptable levels. Figures used in here are the same as those mentioned earlier, and are only given as an example. The signal is received and amplified to a suitable level. It is then applied to the mixer to change the frequency in the same way that occurs in a superheterodyne radio receiver. As a result the communications satellite receives in one band of frequencies and transmits in another.

In view of the fact that the receiver and transmitter are operating at the same time and in close proximity, care has to be taken in the design of the satellite that the transmitter does not interfere with the receiver. This might result from spurious signals arising from the transmitter, or the receiver may become de-sensitised by the strong signal being received from the transmitter. The filters already mentioned are used to reduce these effects.

Block diagram of a basic satellite transponder

Signals transmitted to satellites usually consist of a large number of signals multiplexed onto a main transmission. In this way one transmission from the ground can carry a large number of telephone circuits or even a number of television signals. This approach is operationally far more effective than having a large number of individual transmitters.

Obviously one satellite will be unable to carry all the traffic across the Atlantic. Further capacity can be achieved using several satellites on different bands, or by physically separating them apart from one another. In this way the bandwidth of the antenna can be used to distinguish between different satellites. Normally antennas with very high gains are used, and these have very narrow bandwidths, allowing satellites to be separated by just a few degrees.