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What is QAM

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  What is QAM - Quadrature Amplitude Modulation- overview, information and tutorial about the basics of what is QAM, Quadrature AmplitudeModulation, a form of modulation used for radio communications applications.Quadrature Amplitude Modulation or QAM is a form of modulation which is widely used for modulatingdata signals onto a carrier used for radio communications. It is widely used because it offers advantagesover other forms of data modulation such as PSK, although many forms of data modulation operatealong side each other.Quadrature Amplitude Modulation, QAM is a signal in which two carriers shifted in phase by 90 degreesare modulated and the resultant output consists of both amplitude and phase variations. In view of thefact that both amplitude and phase variations are present it may also be considered as a mixture of amplitude and phase modulation.Analogue and digital QAMQuadrature amplitude modulation, QAM may exist in what may be termed either analogue or digitalformats. The analogue versions of QAM are typically used to allow multiple analogue signals to becarried on a single carrier. For example it is used in PAL and NTSC television systems, where the differentchannels provided by QAM enable it to carry the components of chroma or colour information. In radioapplications a system known as C-QUAM is used for AM stereo radio. Here the different channels enablethe two channels required for stereo to be carried on the single carrier.Digital formats of QAM are often referred to as Quantised QAM and they are being increasingly usedfor data communications often within radio communications systems. Radio communications systemsranging from cellular technology through wireless systems including WiMAX, and Wi-Fi 802.11 use avariety of forms of QAM, and the use of QAM will only increase within the field of radiocommunications.Digital / Quantised QAM basicsQuadrature amplitude modulation, QAM, when used for digital transmission for radio communicationsapplications is able to carry higher data rates than ordinary amplitude modulated schemes and phasemodulated schemes. As with phase shift keying, etc, the number of points at which the signal can rest,i.e. the number of points on the constellation is indicated in the modulation format description, e.g.16QAM uses a 16 point constellation.  When using QAM, the constellation points are normally arranged in a square grid with equal vertical andhorizontal spacing and as a result the most common forms of QAM use a constellation with the numberof points equal to a power of 2 i.e. 2, 4, 8, 16 . . . .By using higher order modulation formats, i.e. more points on the constellation, it is possible to transmitmore bits per symbol. However the points are closer together and they are therefore more susceptibleto noise and data errors.To provide an example of how QAM operates, the table below provides the bit sequences, and theassociated amplitude and phase states. From this it can be seen that a continuous bit stream may begrouped into threes and represented as a sequence of eight permissible states.BIT SEQUENCE AMPLITUDEPHASE (DEGREES)000 1/2 0 (0°)000 1 0 (0°)010 1/2 π/2 (90°)  011 1 πi/2 (90°)  100 1/2 π (180°)  101 1 π (180°)  110 1/2 3πi/2 (270°)111 1 3π/2 (270°)   Bit sequences, amplitudes and phases for 8-QAM  Phase modulation can be considered as a special form of QAM where the amplitude remains constantand only the phase is changed. By doing this the number of possible combinations is halved.QAM advantages and disadvantagesAlthough QAM appears to increase the efficiency of transmission for radio communications systems byutilising both amplitude and phase variations, it has a number of drawbacks. The first is that it is more  susceptible to noise because the states are closer together so that a lower level of noise is needed tomove the signal to a different decision point. Receivers for use with phase or frequency modulation areboth able to use limiting amplifiers that are able to remove any amplitude noise and thereby improvethe noise reliance. This is not the case with QAM.The second limitation is also associated with the amplitude component of the signal. When a phase orfrequency modulated signal is amplified in a radio transmitter, there is no need to use linear amplifiers,whereas when using QAM that contains an amplitude component, linearity must be maintained.Unfortunately linear amplifiers are less efficient and consume more power, and this makes them lessattractive for mobile applications.QAM comparison with other modesAs there are advantages and disadvantages of using QAM it is necessary to compare QAM with othermodes before making a decision about the optimum mode. Some radio communications systemsdynamically change the modulation scheme dependent upon the link conditions and requirements -signal level, noise, data rate required, etc.The table below compares various forms of modulation:MODULATIONBITS PERSYMBOLERROR MARGINCOMPLEXITYOOK 1 1/2 0.5 LowBPSK 1 1 1 MediumQPSK 2 1 / √2 0.71 Medium16 QAM 4 √2 / 6 0.23 High64QAM 6 √2 / 14 0.1 High Summary of types of modulation with data capacities  

Brochure Sarawak

Nov 12, 2017

BCI88108L

Nov 12, 2017
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