What is the bandwidth of the FM signal if the frequency sensitivity of themodulator is 25 KHz per volt? Derive expression for the AM wave and draw its spectrum. Explain their Bandwidth requirements. Derive and explain phase deviation, Modulation index, frequency deviation and percent modulation. Calculate the maximum phase deviation when a modulating signal of 10 V is applied? Determine :. Describe its Bandwidth Considerations. Describe its Bandwidth considerations.
Draw its phasor and constellation diagram. Discuss how carrier recovery is achieved by the squaring loop and Costas loop circuits. Determine the :. For QPSK modulator, construct the truth table, phasor diagram and constellation diagram. Explain the Costas loop method of carrier recovery. Determine the Nyquist sample rate for a maximum analog input frequency of. Explain in detail the Nyquist criterion for distortionless transmission of baseband PAM signal.
Explain the applications of eye pattern to detect ISI. The information signal operates at Hz. Explain the operation of the sample and hold circuit. Draw the block diagram and describe the operation of a delta modulator. What are its advantages and disadvantages compared to a PCM system? What is companding? Explain analog companding process with the help of block diagram.
Block Diagram of Communication System with Detailed Explanation
How does delta modulation differ from PCM? Explain delta modulation transmitter with the help of a block diagram. Determine Processing Gain. Explain the two common multiple access technique for wireless communication.
Differentiate direct sequence and frequency hop spread spectrum technique. With the help of block diagram explain how DSSS can be implemented. Draw the input and output waveforms. What are the properties of Pseudo noise sequence?
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Define modulation index of an AM signal A transmitter radiates 9 kW without modulation and Determine depth of modulation. Define the transmission efficiency of AM signal.Communication is the process of establishing connection or link between two points for information exchange. Communication is simply the basic process of exchanging information.
The electronics equipements which are used for communication purpose, are called communication equipments. Different communication equipments when assembled together form a communication system. Typical example of communication system are line telephony and line telegraphy, radio telephony and radio telegraphy, radio broadcasting, point-to-point communication and mobile communication, computer communication, radar communication, television broadcasting, radio telemetry, radio aids to navigation, radio aids to aircraft landing etc.
In the most fundamental sense, communication involves the transmission of information from one point to another through a succession of process as listed below :.
Quadrature Amplitude Modulation (QAM)
Please subscribe to Electronics Post Channel if you like my tutorials. As we know, a communication system serves to communicate a message or information.
This information originates in the information source. In general, there can be various messages in the form of words, group of words, code, symbols, sound signal etc.
However, out of these messages, only the desired message is selected and communicated. Therefore, we can say that the function of information source is to produce required message which has to be transmitted. A transducer is a device which converts one form of energy into another form. The message from the information source may or may not be electrical in nature.
In a case when the message produced by the information source is not electrical in nature, an input transducer is used to convert it into a time-varying electrical signal. For example, in case of radio-broadcasting, a microphone converts the information or massage which is in the form of sound waves into corresponding electrical signal. The function of the transmitter is to process the electrical signal from different aspects. For example in radio broadcasting the electrical signal obtained from sound signal, is processed to restrict its range of audio frequencies upto 5 kHz in amplitude modulation radio broadcast and is often amplified.
In wire telephony, no real processing is needed. However, in long-distance radio communication, signal amplification is necessary before modulation. Modulation is the main function of the transmitter.Post a comment. It is also a kind of phase shift keying but it is different from the binary phase shift keying. We will also see the waveform of QPSK in this post and how it is formed.
Here you will also get an idea about the benefits of QPSK over other digital modulation techniques. Actually each discrete state of the carrier is called as symbol.
It means that if we talk about ASK, then we have two states symbols only of carrier wave's amplitude either 'No transmission of the carrier' or 'Transmission of the carrier'. In ASK, no carrier wave is transmitted, when we want to transmit binary '0' and when we want to transmit binary 1 then a continuous carrier wave is transmitted.
The same thing is with BPSK, here binary '0' and binary '1' corresponds to phase shift of '0 degree' or ' degree', so there are two phases in binary phase shift keying, that's why it is known as binary phase shift scheme.
Now look at the image given below. This image shows two waveforms. The first waveform is a complete cycle of a sinusoidal wave, showing angles in degrees on the x-axis.
While the second waveform is the QPSK waveform. This waveform shows only the phases angles that are used in quadrature phase shift keying. We know that quadrature phase shift keying has the following phases. So this QPSK waveform shows only these four phases. Note- Watch carefully both the waveforms and observe these four phases angles used in QPSK; in the first waveform of sinusoidal wave.
By observing the first waveform, you will clearly understand how we have phase shifted in QPSK. No comments:. Newer Post Older Post Home. Subscribe to: Post Comments Atom.If there is a good margin, higher orders of QAM can be used to gain a faster data rate, but if the link deteriorates, lower orders are used to preserve the noise margin and ensure that a low bit error rate is preserved.
As the QAM order increases, so the distance between the different points on the constellation diagram decreases and there is a higher possibility of data errors being introduced. Accordingly there is a balance to be made between the data rate and QAM modulation order, power and the acceptable bit error rate.
Whilst further error correction can be introduced to mitigate any deterioration in link quality, this will also decrease the data throughput. QAM is in many radio communications and data delivery applications. However some specific variants of QAM are used in some specific applications and standards. There is a balance between data throughput and signal to noise ratio required.
As the order of the QAM signal is increased, i. However the downside is that a better signal to noise ratio is required to achieve this. For some systems the order of the modulation format is fixed, but in others where there is a two way link, it is possible to adapt the order of the modulation to obtain the best throughput for the given link conditions.
The level of error correction used is also altered. In this way, changing the modulation order, and the error correction, the data speed can be optimised whilst maintaining the required error rate. For domestic broadcast applications for example, 64 QAM and QAM are often used in digital cable television and cable modem applications. The order of the QAM modulation has to be set at the transmitter, because the transmission is only one way, and in addition to this, there are thousands of receivers, making it impossible to have a dynamically adaptive form of modulation.
For the many forms of wireless and cellular technology it is possible to dynamically alter the order of QAM modulation and error correction according to the link conditions between the two ends.Symbol amplitudes: 16 QAM (0003)
As data rates have risen and the demands on spectrum efficiency have increased, so too has the complexity of the link adaptation technology. Data channels are carried on the cellular radio signal to enable fast adaptation of the link to meet the prevailing link quality and ensure the optimum data throughput, balancing transmitter power, QAM order, and forward error correction, etc. The constellation diagrams show the different positions for the states within different forms of QAM, quadrature amplitude modulation.
As the order of the modulation increases, so does the number of points on the QAM constellation diagram. It can be seen from these few QAM constellation diagrams, that as the modulation order increases, so the distance between the points on the constellation decreases.
Accordingly small amounts of noise can cause greater issues. As the level of noise increases due to low signal strengths, so the area covered by a point on the constellation increases. If it becomes too large, then the receiver is unable to determine which position on the constellation the transmitted signal was meant to be, and this results in errors. It is also found that the higher the order of modulation for the QAM signal, the greater the amount of amplitude variation is present on the transmitted signal.
For transmitter RF amplifiers for everything from Wi-Fi to cellular and more, it means that linear amplifiers are required.QAM, quadrature amplitude modulation has been used for some analogue transmissions including AM stereo transmissions, but it is for data applications where it has come into its own.
It is able to provide a highly effective form of modulation for data and as such it is used in everything from cellular phones to Wi-Fi and almost every other form of high speed data communications system. Quadrature Amplitude Modulation, QAM is a signal in which two carriers shifted in phase by 90 degrees i. The resultant overall signal consisting of the combination of both I and Q carriers contains of both amplitude and phase variations.
In view of the fact that both amplitude and phase variations are present it may also be considered as a mixture of amplitude and phase modulation. A motivation for the use of quadrature amplitude modulation comes from the fact that a straight amplitude modulated signal, i. This is very wasteful of the available frequency spectrum. QAM restores the balance by placing two independent double sideband suppressed carrier signals in the same spectrum as one ordinary double sideband supressed carrier signal.
Quadrature amplitude modulation, QAM may exist in what may be termed either analogue or digital formats. The analogue versions of QAM are typically used to allow multiple analogue signals to be carried on a single carrier. Here the different channels enable the 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 used for data communications often within radio communications systems.
Quadrature amplitude modulation, QAM, when used for digital transmission for radio communications applications is able to carry higher data rates than ordinary amplitude modulated schemes and phase modulated schemes.
Basic signals exhibit only two positions which allow the transfer of either a 0 or 1. Using QAM there are many different points that can be used, each having defined values of phase and amplitude. This is known as a constellation diagram. The different positions are assigned different values, and in this way a single signal is able to transfer data at a much higher rate. As shown above, the constellation points are typically arranged in a square grid with equal horizontal and vertical spacing.
Although data is binary the most common forms of QAM, although not all, are where there constellation can form a square with the number of points equal to a power of 2 i. By using higher order modulation formats, i. However the points are closer together and they are therefore more susceptible to noise and data errors. The advantage of moving to the higher order formats is that there are more points within the constellation and therefore it is possible to transmit more bits per symbol.
Quadrature Amplitude Modulation : Working Principle and Its Applications
The downside is that the constellation points are closer together and therefore the link is more susceptible to noise. As a result, higher order versions of QAM are only used when there is a sufficiently high signal to noise ratio. To provide an example of how QAM operates, the constellation diagram below shows the values associated with the different states for a 16QAM signal.
From this it can be seen that a continuous bit stream may be grouped into fours and represented as a sequence. Additionally 8QAM is not widely used. This arises from the rectangular, rather than square shape of the constellation.
Although QAM appears to increase the efficiency of transmission for radio communications systems by utilising 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 to move the signal to a different decision point.
Receivers for use with phase or frequency modulation are both able to use limiting amplifiers that are able to remove any amplitude noise and thereby improve the 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 or frequency 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 less attractive for mobile applications. When deciding on a form of modulation it is worth comparing AM vs PSK and other modes looking at what they each have to offer. As there are advantages and disadvantages of using QAM it is necessary to compare QAM with other modes before making a decision about the optimum mode. Some radio communications systems dynamically change the modulation scheme dependent upon the link conditions and requirements - signal level, noise, data rate required, etc.
Typically it is found that if data rates above those that can be achieved using 8-PSK are required, it is more usual to use quadrature amplitude modulation.Figure 1 is a simplified block diagram of a QAM system. The transmitter includes a source of QAM symbols, a root-Nyquist pulse-shaping filter and a Quadrature modulator. The receiver has a Quadrature downconverter, root-Nyquist filter, and decision block.
The matched root-Nyquist filters perform the dual tasks of limiting the bandwidth of the transmitted signal while minimizing the interference from other symbols at the sampling instant [1,2].
For our discussion of MER, I have left out the usual key receiver systems of AGC, carrier recovery, clock recovery, and adaptive equalization. For our purposes, we can model the channel at complex baseband , so we can eliminate the Quadrature up and down conversions. These could include linear impairments such as multipath, phase noise, and interference. Figure 3 plots some of the labeled signals from Figure 2 for a QAM system with noise.
At this point, the signal has 4 samples per symbol, so the valid sampling instants are spaced 4 samples apart. For our simplified model, the eye opening aligns exactly with every fourth sample A real-world receiver must compute the correct sample value using a variable interpolator. The eyes are partially closed due to the noise added to the signal. The final outputs I dec and Q dec are computed in the decision block by rounding slicing I down and Q down to the nearest allowed symbol value, reproducing the original transmitter constellation Figure 3d.
Overall block diagram. Decision block. Figure 3. Signals from Figure 2 for QAM modulation with added noise. The Modulation Error Ratio is a way to quantify the constellation noise seen in Figure 3c. The modulation error ratio of this single symbol is defined as:. In words, MER is the ratio of the target symbol power to the error power.
If N is large and all symbols are equally likely, we can calculate MER as the ratio of average target symbol power to average error power :. The maximum MER is limited by impairments of the transmitter and receiver themselves. This occurs because each decision error causes the error power calculation for that symbol to be too low.
When there are many errors, the calculated MER will be too high. Figure 4. QAM constellation points symbolswith target signal vector and error vector.To understand QAM, we have to differentiate between baseband and passband signals.
The PAM signal has its spectral contents around zero and hence it is a baseband signal. For wireless transmission, this information must be transmitted through space as an electromagnetic wave. Also observe that once this spectrum is occupied, no other entity in the surrounding region can share the wireless channel for the purpose of communications.
For these reasons and a few others as wella wireless system is allocated a certain bandwidth around a higher carrier frequency and the generated baseband signal is shifted to that specific portion of the spectrum. Such signals are called passband signals. That is why wireless signals in everyday use such FM radio, WiFi, cellular like 4G, 5G, and Bluetooth are all passband signals which execute their transmissions within their respective frequency bands sharing the same physical medium.
The easiest method to shift the spectrum to a designated carrier frequency is by multiplying, or mixing, it with a sinusoidal waveform due to the following reason. The bandwidth — positive portion of the spectrum — hence becomes double.
That is the birth of Quadrature Amplitude Modulation QAMin which two independent PAM waveforms are communicated through mixing one with a cosine and the other with a sine. Next, we multiply the resulting complex signal with a complex sinusoid at carrier frequency and collect its real part, i. Such representation of the QAM waveform as a sum of amplitude scaled and pulse shaped sinusoids is known as the rectangular form. A relatively lower noise power is then enough to cause a decision error by moving the received symbol over the decision boundary.
This is the cost of increasing data rate by packing more bits in the same symbol. As the name PSK implies, the amplitude remains constant in this configuration while the information is conveyed by different phases. The symbols are detected through the following steps illustrated in Figure below. In conclusion, the downsampled matched filter outputs map back to the Tx symbols in the absence of noise.
If the world was simple, that would have been an end to it! But the world is complicated, and there are layers of issues that happen between the Tx information generation and Rx decision making. Anything we do after this will be to combat a subset of signal distortions and towards recovering the original information. Observe that the system shown in Figure above is a multirate system. Furthermore, there are some hidden assumptions in the QAM detector:. This is because ADC just samples the incoming continuous waveform without any information about the symbol boundaries.
This is a symbol timing synchronization problem. As we will see later, a resampling system is required in the Rx chain that changes the sample rate from the ADC rate to a rate that is an integer multiple of the symbol rate.
Any movement by the Tx, the Rx or within the environment between them also causes a shift in frequency known as Doppler shift that needs to be compensated. Multipath reflections from a wireless channel introduce ISI and distortion in the signal that need to be recovered through an equalizer. The case of scatter plot is a little different.
Looking at the scatter plot, one can readily deduce a lot of features for the particular transmission system. At this stage, however, it is enough to observe that the diameter of these clouds is a rough measure of the noise power corrupting the signal.
For the ideal case of no noise, this diameter is zero and all the optimally timed samples coincide with their respective constellation points. Your email address will not be published.