When you talk into a mobile telephone it converts the sound of your voice to radiofrequency energy (radio waves). The radio waves are trmitted through the air to a nearby base station. The base station then sends the call through the telephone network until it reaches the person you are calling. When you receive a call on your mobile phone the message travels through the telephone network until it reaches a base station near to you. The base station sends out radio waves, which are detected by your telephone and converted back to speech. Depending on the equipment and the operator, the frequency that each operator utilises is 900MHz, 1800MHz or 2100MHz.
The mobile phone network operates on the basis of a series of cells. Each cell requires a radio base station to enable it to function.
There are three types of base station and each has a particular purpose:
Each base station can only cope with a certain number of calls at any one time. So if demand exceeds the capacity of a base station an additional base station is needed.
An integrated circuit (IC), also called a microchip, is an electronic circuit etched onto a silicon chip. Their main advantages are low cost, low power, high performance, and very small size.
Multiplexing (known as muxing) is a term used to refer to a processwhere multiple analog message signals or digital data streams are combined into one signal over a shared medium. The aim is to share an expensive resource. For example, in telecommunications, several phone calls may be trferred using one wire.
Crosstalk is a form of interference caused by signals in nearby conductors. The most common example is hearing an unwanted conversation on the telephone. Crosstalk can also occur in radios, televisions, networking equipment, and even electric guitars.
There is no difference, just language use, which differs from country to country, so in Britain it is called a mobile, and in USA and South Africa and other places a cell phone. Even in Europe the name differs. The Germ call it a “handy”, which in English has completely another meaning as an adjective, meaning useful. In Italy it is called a telofonino or “little phone”.
This difference in British and American English is also evident in many other things we use every day, like lifts and elevators, nappies and diapers, pickups and trucks. The list goes on and on, any student of English has to decide which he or she will use, as the default setting.
These are the two different me of mobile communication being presently used worldwide. The basic difference lies in the Multiplexing method used in the aerial communication i.e. from Mobile Tower to your mobile and vice versa. CDMA uses Code Division Multiple Access as the name itself indicates, for example you are in a hall occupied with number of people speaking different language.
You will find that the one language you know will be heard by you and the others will be treated like noise. In the same manner each CDMA mobile communication takes place with a “code” communicating between them and the other end if one is knowing that code then only it can listen to the data being trmitted i.e. the communication is in the coded form.
On the other hand GSM (Global System for Mobile Communications)uses narrowband TDMA, which allows eight simultaneous calls on the same radio frequency. TDMA works by dividing a radio frequency into time slots and then allocating slots to multiple calls. In this way, a single frequency can support multiple, simultaneous data channels.
Radio frequency (RF) is a frequency or rate of oscillation within the range of about 3 Hz to 300 GHz. This range corresponds to frequency of alternating current electrical signals used to produce and detect radio waves. Since most of this range is beyond the vibration rate that most mechanical systems can respond to, RF usually refers to oscillations in electrical circuits or electromagnetic radiation.
A stopband is a band of frequencies, between specified limits, in which a circuit, such as a filter or telephone circuit, does not let signals through, or the attenuation is above the required stopband attenuation level.
AMAmplitude modulation is a type of modulation where the amplitude of the carrier signal is varied in accordance with the information bearing signal. FMFrequency modulation is a type of modulation where the frequency of the carrier signal is varied in accordance with the information bearing signal.
It states that, while taking the samples of a continuous signal, it has to be taken care that the sampling rate is equal to or greater than twice the cut off frequency and the minimum sampling rate is known as the Nyquist rate.
Base station is a radio receiver/trmitter that serves as the hub of the local wireless network, and may also be the gateway between a wired network and the wireless network
A trducer is a device, usually electrical, electronic, electromechanical, electromagnetic, photonic, or photovoltaic that converts one type of energy or physical attribute to another for various purposes including measurement or information trfer.
In telecommunication, the term trponder (shortforTrmitterresponder and sometimes abbreviated to XPDR, XPNDR, TPDR or TP) has the following meanings:
Passband is the range of frequencies or wavelengths that can pass through a filter without being attenuated.
A rectifier changes alternating current into direct current. This process is called rectification. The three main types of rectifier are the halfwave, fullwave, and bridge. A rectifier is the opposite of an inverter, which changes direct current into alternating current. HWR The simplest type is the halfwave rectifier, which can be made with just one diode. When the voltage of the alternating current is positive, the diode becomes forwardbiased and current flows through it. When the voltage is negative, the diode is reversebiased and the current stops.
The result is a clipped copy of the alternating current waveform with only positive voltage, and an average voltage that is one third of the peak input voltage. This pulsating direct current is adequate for some components, but others require a more steady current. This requires a fullwave rectifier that can convert both parts of the cycle to positive voltage.
FWR The fullwave rectifier is essentially two halfwave rectifiers, and can be made with two diodes and an earthed centre tap on the trformer. The positive voltage half of the cycle flows through one diode, and the negative half flows through the other. The centre tap allows the circuit to be completed because current cannot flow through the other diode. The result is still a pulsating direct current but with just over half the input peak voltage, and double the frequency.
A resistor is a twoterminal electronic component that opposes anelectric current by producing a voltage drop between its terminals in proportion to the current, that is, in accordance with Ohm’s law: V= IR
The process of obtaining a set of samples from a continuous function of time x(t) is referred to as sampling.
Modulation is the process of varying some characteristic of a periodic wave with an external signals. Radio communication superimposes this information bearing signal onto a carrier signal. These high frequency carrier signals can be trmitted over the air easily and are capable of travelling long distances. The characteristics (amplitude, frequency, or phase) of the carrier signal are varied in accordance with the information bearing signal. Modulation is utilized to send an information bearing signal over long distances.
An inductor is a passive electrical device employed in electrical circuits for its property of inductance. An inductor can take many forms.
For Analog modulation–AM, SSB, FM, PM and SM .
Digital modulation–OOK, FSK, ASK, Psk, QAM, MSK,CPM, PPM, TCM, OFDM.
An electronic device or electrical circuit that is used to boost (amplify) the power, voltage or current of an applied signal.
Electronics work on DC and with a voltage range of 48vDC to +48vDC. If the electronic device is plugged into a standard wall outlet, there will be a trformer inside which will convert the AC voltage you are supplying to the required DC voltage needed by the device.
Examples: Computer, radio, T.V, etc…
Electric devices use line voltage (120vAC, 240vAC, etc…).Electric devices can also be designed to operate on DC sources, but will be at DC voltages above 48v.
Examples: incandescent lights, heaters, fridge, stove, etc…
Full duplex refers to the trmission of data in two directions simultaneously. For example, a telephone is a fullduplex devicebecause both parties can talk at once. In contrast, a walkietalkie is ahalfduplex device because only one party can trmit at a time. Most modems have a switch that lets you choose between fullduplex and halfduplex modes. The choice depends on whichcommunications program you are running.
In fullduplex mode, data you trmit does not appear on yourscreen until it has been received and sent back by the other party. This enables you to validate that the data has been accurately trmitted. If your display screen shows two of each character, it probably me that your modem is set to halfduplex mode when it should be in fullduplex mode.
Code division multiple access (CDMA) is a channel access methodutilized by various radio communication technologies. CDMA employsspreadspectrum technology and a special coding scheme (where each trmitter is assigned a code) to allow multiple users to be multiplexed over the same physical channel. By contrast, time division multiple access (TDMA) divides access by time, whilefrequencydivision multiple access (FDMA) divides it byfrequency.
An analogy to the problem of multiple access is a room (channel) in which people wish to communicate with each other. To avoid confusion, people could take turns speaking (time division), speak at different pitches (frequency division), or speak in different directions (spatial division).
In CDMA, they would speak different languages. People speaking the same language can understand each other, but not other people. Similarly, in radio CDMA, each group of users is given a shared code. Many codes occupy the same channel, but only users associated with a particular code can understand each other.
Barkhausen criteria, without which you will not know which conditions, are to be satisfied for oscillations.
“Oscillations will not be sustained if, at the oscillator frequency, the magnitude of the product of the trfer gain of the amplifier and the magnitude of the feedback factor of the feedback network ( the magnitude of the loop gain ) are less than unity”.
The condition of unity loop gain Aβ = 1 is called the Barkhausencriterion. This condition implies that | Aβ|= 1and that the phase of – Aβ is zero.
In electronics, a tristor is a semiconductor device commonly used to amplify or switch electronic signals. The tristor is the fundamental building block of computers, and all other modernelectronic devices. Some tristors are packaged individually but most are found in integrated circuits.
A capacitor is an electrical/electronic device that can store energyin the electric field between a pair of conductors (called “plates”). The process of storing energy in the capacitor is known as “charging”, and involves electric charges of equal magnitude, but opposite polarity, building up on each plate.
Capacitors are often used in electric and electronic circuits asenergystorage devices. They can also be used to differentiate between highfrequency and lowfrequency signals. This property makes them useful in electronic filters.
Capacitors are occasionally referred to as condensers. This term is considered archaic in English, but most other languages use acognate of condenser to refer to a capacitor.
In electronics, a diode is a twoterminal device. Diodes have two active electrodes between which the signal of interest may flow, and most are used for their unidirectional current property.
Demodulation is the act of removing the modulation from an analog signal to get the original baseband signal back. Demodulating is necessary because the receiver system receives a modulated signal with specific characteristics and it needs to turn it to baseband.
A repeater is an electronic device that receives a signal and retrmits it at a higher level and/or higher power, or onto the other side of an obstruction, so that the signal can cover longer distances without degradation.
An oscillator is a circuit that creates a waveform output from a direct current input. The two main types of oscillator are harmonic and relaxation. The harmonic oscillators have smooth curved waveforms, while relaxation oscillators have waveforms with sharp changes.
A substance, body, or device that readily conducts heat, electricity, sound, etc. Copper is a good conductor of electricity.
Much attention has been given by researchers to negative feedback processes, because negative feedback processes lead systems towards equilibrium states. Positive feedback reinforces a given tendency of a system and can lead a system away from equilibrium states, possibly causing quite unexpected results.
3 satellites are required to cover the entire earth, which is placed at 120 degree to each other. The life span of the satellite is about 15 years.