Quadrature Amplitude Modulation (QAM): Principles and Applications in Radio Communications, Summaries of Kinetics of Phase Transformations

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QUADRATURE

AMPLIDUDE

MODULATION

Prepared

by

Deepa.T,

Asst.Prof.

/TCE

INTRODUCTION

QAM

Quadrature Amplitude Modulation or QAM is a form ofmodulation which is widely used for modulating datasignals onto a carrier used for radio communications. It iswidely used because it offers advantages over other formsof data modulation such as PSK, although many forms ofdata modulation operate along side each other.

Quadrature Amplitude Modulation, QAM is a signal inwhich two carriers shifted in phase by 90 degrees aremodulated and the resultant output consists of bothamplitude and phase variations. In view of the fact thatboth amplitude and phase variations are present it mayalso be considered as a mixture of amplitude and phasemodulation.

ANALOG

DIGITAL

QAM

Quadrature amplitude modulation, QAM may exist in what may betermed either analog digital formats. The analog versions of QAM aretypically used to allow multiple analog signals to be carried on a singlecarrier. For example it is used in PAL and NTSC television systems,where the different channels provided by QAM enable it to carry thecomponents of chroma or colour information. In radio applications asystem known as C-QUAM is used for AM stereo radio. Here thedifferent channels enable the two channels required for stereo to becarried on the single carrier.

Digital formats of QAM are often referred to as "Quantised QAM" andthey are being increasingly used for data communications often withinradio communications systems. Radio communications systems rangingfrom cellular technology through wireless systems including WiMAX,and Wi-Fi 802.11 use a variety of forms of QAM, and the use of QAMwill only increase within the field of radio communications.

Advantages and Disadvantages of QAM

Advantages:

QAM appears to increase the efficiency of transmission for radiocommunications systems by utilising both amplitude and phase variations,

Drawbacks

more susceptible to noise because the states are closer together so that a lowerlevel of noise is needed to move the signal to a different decision point.Receivers for use with phase or frequency modulation are both able to uselimiting amplifiers that are able to remove any amplitude noise and therebyimprove the noise reliance. This is not the case with QAM.

The second limitation is also associated with the amplitude component of thesignal. When a phase or frequency modulated signal is amplified in a radiotransmitter, there is no need to use linear amplifiers, whereas when using QAMthat contains an amplitude component, linearity must be maintained.Unfortunately linear amplifiers are less efficient and consume more power, andthis makes them less attractive for mobile applications.

Constellation diagrams for QAM

Quadrature amplitude modulation, QAM, when used for digital transmissionfor radio communications applications is able to carry higher data rates thanordinary amplitude modulated schemes and phase modulated schemes. As withphase 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 formatdescription, e.g. 16QAM uses a 16 point constellation.

When using QAM, the constellation points are normally arranged in a squaregrid with equal vertical and horizontal spacing and as a result the most commonforms of QAM use a constellation with the number of points equal to a powerof 2 i.e. 2, 4, 8, 16....

By using higher order modulation formats, i.e. more points on the constellation,it is possible to transmit more bits per symbol. However the points are closertogether and they are therefore more susceptible to noise and data errors.

To provide an example of how QAM operates, the table below provides the bitsequences, and the associated amplitude and phase states. From this it can beseen that a continuous bit stream may be grouped into threes and represented asa sequence of eight permissible states.

Why

QAM

called

combined

ASK

AND

PSK

Quadrature Amplitude Modulation usesthe phase and amplitude of the carriersignal to encode data. QAM findswidespread use in current and emergingwireless standards, including Wi-Fi,Digital Video Broadcast (DVB), WiMAX,IEEE 802.11n, and HSDPA/HSUPA. - The QAM modulation scheme encodesdata by varying both amplitude and phaseof the carrier signal. Thus, it is sometimesviewed as a combination of ASK and PSKmodulation. A more fundamental way ofviewing QAM thought is that it encodesdata by varying the amplitude of twocarrier signals that are in-quadrature(phase difference of 90). Hence the name“quadrature-amplitude modulation”. Wewill now leverage our understanding of IQdata to understand this idea. As we haveseen, a modulated carrier signal can beexpressed in terms of it’s IQ componentsas: where and are the amplitudes of the in

phase and quadrature ‐phase components respectively. Thus, we can change the amplitude

and phase ()of the carrier signal by varying the

I

and

Q

values.

Constellation diagrams for QAM

Let’s look at the time ‐domain representation of

QAM

signals. Taking

‐QAM

as an example, suppose we wish to transmit the bitstream 100111. We map these to

QAM

symbols representing

The resulting time ‐domain waveform for this bitstream is shown in Figure

Each symbol is represented by National Instruments RF

Communications Handbook, Page

Copyright

National Instruments Corporation

one period of the sine wave and has a unique phase shift. In this respect, 4

QAM

might be considered a special case of

QAM

where the amplitude is the same for all symbols.

Constellation diagrams for QAM

The constellation plot in this Figure shows the phase and amplitude transitions of the carrier signal. The raw

IQ

data is represented by the red trance with the white dots representing those samples of

IQ

data that occur on symbol clock periods and that are mapped back to digital bit patterns based on the

QAM

symbol map. We note that the transitions go through the origin. This causes abrupt amplitude variations between consecutive symbols and causes noise to be injected in the transmitted symbol due to the amplifier turning off and back on abruptly. This problem can be fixed by using offset

QAM.

Refer to the offset

PSK

modulation scheme discussed earlier for more details.