Why psk is better than fsk?
This page on ASK vs. FSK vs. PSK provides difference between ASK, FSK, PSK modulation types. All these are digital modulation techniques. Unlike Analog modulation, here input is in digital binary form. The other input is the RF carrier. Input binary data is referred as modulating signal and output is referred as modulated signal.
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ASK
The short form of Amplitude Shift Keying is referred as ASK. It is the digital modulation technique. In this technique, amplitude of the RF carrier is varied in accordance with baseband digital input signal. The figure depicts operation of ASK modulation. As shown in the figure, binary 1 will be represented by carrier signal with some amplitude while binary 0 will be represented by carrier of zero amplitude(i.e. no carrier).
Fig.1 ASK Modulation
ASK modulation can be represented by following equation:
s(t) = A2* cos(2*π*fc*t) for Binary Logic-1
s(t) = A1* cos(2*π*fc*t) for Binary Logic-0
Here A2>A1
Signaling used is ON-OFF signaling.
Bandwidth requirement for ASK is:
BW = 2/Tb = 2*Rb
Often in ASK modulation, binary-1 is represented by carrier with amplitude-A2 and binary-0 is represented by carrier with amplitude-A1. Here A2 is greater in magnitude compare to A1. The form of ASK where in no carrier is transmitted during the transmission of logic zero is known as OOK modulation (On Off Keying modulation). This is shown in the figure-1. Refer OOK vs ASK modulation >> which compares OOK vs. ASK and depicts difference between OOK and ASK modulation types with signal diagrams.
FSK
The short form of Frequency Shift Keying is referred as FSK. It is also digital modulation technique. In this technique, frequency of the RF carrier is varied in accordance with baseband digital input. The figure depicts the FSK modulation. As shown, binary 1 and 0 is represented by two different carrier frequencies. Figure depicts that binary 1 is represented by high frequency ‘f1’ and binary 0 is represented by low frequency ‘f2’.
Fig.2 FSK
Binary FSK can be represented by following equation:
s(t) = A* cos(2*π*f1*t) for Binary 1
s(t) = A* cos(2*π*f2*t) for Binary 0
In FSK modulation, NRZ signaling method is used. Bandwidth requirement in case of FSK is:
BW = 2*Rb + (f1-f2)
In order to overcome drawbacks of BFSK (Two level Binary FSK) , multiple FSK modulation techniques with more than two frequencies have been developed. In MFSK (Multiple FSK), more than one bits are represented by each signal elements.
Refer 2FSK and 4FSK Modulation types.
PSK
The short form of Phase Shift Keying is referred as PSK. It is digital modulation technique where in phase of the RF carrier is changed based on digital input. Figure depicts Binary Phase Shift Keying modulation type of PSK. As shown in the figure, Binary 1 is represented by 180 degree phase of the carrier and binary 0 is represented by 0 degree phase of the RF carrier.
Fig.3 PSK
Binary PSK can be represented by following equation :
If s(t) = A*cos(2*π*fc*t) for Binary 1 than
s(t) = A*cos(2*π*fc*t + π) for Binary 0
In PSK modulation, NRZ signaling is used. Bandwidth requirement for PSK is:
BW = 2 * Rb = 2 * Bit rate
Advantages and Disadvantages of ASK, FSK and PSK
ASK Advantages | ASK Disadvantages | Amplitude Shift Keying
This page covers advantages and disadvantages of ASK.ASK stands for Amplitude Shift Keying. Both ASK advantages and ASK disadvantages are covered.
Following are the silent features of ASK modulation.
Figure-1: ASK Modulation
ASK Advantages
Following points summarizes ASK advantages:
➨It offers high bandwidth efficiency.
➨It has simple receiver design.
➨ASK modulation can be used to transmit digital data over optical fiber.
➨ASK modulation and ASK demodulation processes are comparatively inexpensive.
➨Its variant OOK is used at radio frequencies to transmit more codes.
ASK Disadvantages
Following points summarizes ASK disadvantages:
➨It offers lower power efficiency.
➨ASK modulation is very susceptible to noise interference. This is due to the fact that noise affects the amplitude. Hence another alternative modulation technique such as BPSK which is less susceptible to error than ASK is used.
FSK Advantages | FSK Disadvantages | Frequency Shift Keying
This page covers advantages and disadvantages of FSK. It mentions FSK advantages or benefits and FSK disadvantages or drawbacks. FSK stands for Frequency Shift Keying.
What is FSK?
Introduction:
It is a digital modulation technique which shifts the frequency of the carrier with respect to binary data signal. FSK stands for Frequency Shift Keying. The FSK modulation technique uses two different carrier frequencies to represent binary 1 and binary 0.
As shown in the figure-1, carrier frequency f1 represents binary data one and carrier frequency f2 represents binary data zero. Here amplitude and phase of the carrier remain constant while carrier frequency is changed. Binary FSK (BFSK) can be represented by following mathematical equation:
s(t) = A* cos(2*π*f1*t) for Binary 1
s(t) = A* cos(2*π*f2*t) for Binary 0
In this equation, f2 and f2 are offset from carrier frequency (Fc) by equal but opposite amounts.
Following are the typical applications of FSK modulation.
Benefits or advantages of FSK
Following are the benefits or advantages of FSK:
➨It has lower probability of error (Pe).
➨It provides high SNR (Signal to Noise Ratio).
➨It has higher immunity to noise due to constant envelope. Hence it is robust against variation in attenuation through channel.
➨FSK transmitter and FSK receiver implementations are simple for low data rate application.
Drawbacks or disadvantages of FSK
Following are the disadvantages of FSK:
➨It uses larger bandwidth compare to other modulation techniques such as ASK and PSK. Hence it is not bandwidth efficient.
➨The BER (Bit Error Rate) performance in AWGN channel is worse compare to PSK modulation.
In order to overcome drawbacks of BFSK, multiple FSK modulation techniques with more than two frequencies have been developed. In MFSK (Multiple FSK), more than one bits are represented by each signal elements.
PSK Advantages | PSK Disadvantages | Phase Shift Keying
This page covers advantages and disadvantages of PSK. It mentions PSK advantages or benefits and PSK disadvantages or drawbacks. PSK stands for Phase Shift Keying.
What is PSK?
Introduction:
It is a digital modulation technique which uses phase of the analog carrier to represent digital binary data. Phase of the carrier wave is changed according to the binary inputs (1 or 0). In two level PSK, difference of 180 phase shift is used between binary 1 and binary 0.
There are many different types of modulation techniques which utilizes this concept to transmit digital binary data. It include two level PSK (i.e. BPSK), Four level PSK (i.e. QPSK) etc. Some techniques employ both amplitude and phase variation to represent binary data such as 16-QAM, 64-QAM, 256-QAM etc. Two level PSK represents single bit by each signaling elements while four level PSK represents two bits by each signaling elements and so on. 8-PSK represents three bits by each signaling elements.
Following are the equations used to represent BPSK.
➨s(t) = A*cos(2*π*fc*t) for Binary 1 than
➨s(t) = A*cos(2*π*fc*t + π) for Binary 0
As mentioned there are many variants of PSK modulation. Each of these PSK types have different advantages and disadvantages. We will have a look at common advantages and disadvantages of PSK techniques.
Benefits or advantages of PSK
Following are the benefits or advantages of PSK:
➨It carries data over RF signal more efficiently compare to other modulation types. Hence it is more power efficient modulation technique compare to ASK and FSK.
➨It is less susceptible to errors compare to ASK modulation and occupies same bandwidth as ASK.
➨Higher data rate of transmission can be achieved using high level of PSK modulations such as QPSK (represents 2 bits per constellation), 16-QAM (represents 4 bits per constellation) etc.
Drawbacks or disadvantages of PSK
Following are the disadvantages of PSK:
➨It has lower bandwidth efficiency.
➨The binary data is decoded by estimation of phase states of the signal. These detection and recovery algorithms are very complex.
➨Multi-level PSK modulation schemes (QPSK, 16QAM etc.) are more sensitive to phase variations.
➨It is also one form of FSK and hence it also offers lower bandwidth efficiency compare to ASK modulation type.
Digital Phase Modulation: BPSK, QPSK, DQPSK
Digital phase modulation is a versatile and widely used method of wirelessly transferring digital
data.
In the previous page, we saw that we can use discrete variations in a carrier’s amplitude or frequency as a way of representing ones and zeros. It should come as no surprise that we can also represent digital data using phase; this technique is called phase shift keying (PSK).
Binary Phase Shift Keying
The most straightforward type of PSK is called binary phase shift keying (BPSK), where “binary” refers to the use of two phase offsets (one for logic high, one for logic low).
We can intuitively recognize that the system will be more robust if there is greater separation between these two phases—of course it would be difficult for a receiver to distinguish between a symbol with a phase offset of 90° and a symbol with a phase offset of 91°. We only have 360° of phase to work with, so the maximum difference between the logic-high and logic-low phases is 180°. But we know that shifting a sinusoid by 180° is the same as inverting it; thus, we can think of BPSK as simply inverting the carrier in response to one logic state and leaving it alone in response to the other logic state.
To take this a step further, we know that multiplying a sinusoid by negative one is the same as inverting it. This leads to the possibility of implementing BPSK using the following basic hardware configuration:
However, this scheme could easily result in high-slope transitions in the carrier waveform: if the transition between logic states occurs when the carrier is at its maximum value, the carrier voltage has to rapidly move to the minimum voltage.
High-slope events such as these are undesirable because they generate higher-frequency energy that could interfere with other RF signals. Also, amplifiers have limited ability to produce high-slope changes in output voltage.
If we refine the above implementation with two additional features, we can ensure smooth transitions between symbols. First, we need to ensure that the digital bit period is equal to one or more complete carrier cycles. Second, we need to synchronize the digital transitions with the carrier waveform. With these improvements, we could design the system such that the 180° phase change occurs when the carrier signal is at (or very near) the zero-crossing.
QPSK
BPSK transfers one bit per symbol, which is what we’re accustomed to so far. Everything we’ve discussed with regard to digital modulation has assumed that the carrier signal is modified according to whether a digital voltage is logic low or logic high, and the receiver constructs digital data by interpreting each symbol as either a 0 or a 1.
Before we discuss quadrature phase shift keying (QPSK), we need to introduce the following important concept: There is no reason why one symbol can transfer only one bit. It’s true that the world of digital electronics is built around circuitry in which the voltage is at one extreme or the other, such that the voltage always represents one digital bit. But RF is not digital; rather, we’re using analog waveforms to transfer digital data, and it is perfectly acceptable to design a system in which the analog waveforms are encoded and interpreted in a way that allows one symbol to represent two (or more) bits.
QPSK is a modulation scheme that allows one symbol to transfer two bits of data. There are four possible two-bit numbers (00, 01, 10, 11), and consequently we need four phase offsets. Again, we want maximum separation between the phase options, which in this case is 90°.
The advantage is higher data rate: if we maintain the same symbol period, we can double the rate at which data is moved from transmitter to receiver. The downside is system complexity. (You might think that QPSK is also significantly more susceptible to bit errors than BPSK, since there is less separation between the possible phase values. This is a reasonable assumption, but if you go through the math it turns out that the error probabilities are actually very similar.)
Variants
QPSK is, overall, an effective modulation scheme. But it can be improved.
Phase Jumps
Standard QPSK guarantees that high-slope symbol-to-symbol transitions will occur; because the phase jumps can be ±90°, we can’t use the approach described for the 180° phase jumps produced by BPSK modulation.
This problem can be mitigated by using one of two QPSK variants. Offset QPSK, which involves adding a delay to one of two digital data streams used in the modulation process, reduces the maximum phase jump to 90°. Another option is π/4-QPSK, which reduces the maximum phase jump to 135°. Offset QPSK is thus superior with respect to reducing phase discontinuities, but π/4-QPSK is advantageous because it is compatible with differential encoding (discussed in the next subsection).
Another way to deal with symbol-to-symbol discontinuities is to implement additional signal processing that creates smoother transitions between symbols. This approach is incorporated into a modulation scheme called minimum shift keying (MSK), and there is also an improvement on MSK known as Gaussian MSK.
Differential Encoding
Another difficulty is that demodulation with PSK waveforms is more difficult than with FSK waveforms. Frequency is “absolute” in the sense that frequency changes can always be interpreted by analyzing the signal variations with respect to time. Phase, however, is relative in the sense that it has no universal reference—the transmitter generates the phase variations with reference to a point in time, and the receiver might interpret the phase variations with reference to a separate point in time.
The practical manifestation of this is the following: If there are differences between the phase (or frequency) of the oscillators used for modulation and demodulation, PSK becomes unreliable. And we have to assume that there will be phase differences (unless the receiver incorporates carrier-recovery circuitry).
Differential QPSK (DQPSK) is a variant that is compatible with noncoherent receivers (i.e., receivers that don’t synchronize the demodulation oscillator with the modulation oscillator). Differential QPSK encodes data by producing a certain phase shift relative to the preceding symbol. By using the phase of the preceding symbol in this way, the demodulation circuitry analyzes the phase of a symbol using a reference that is common to the receiver and the transmitter.
Summary
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Source: rfwireless-world and allaboutcircuits
Phase Shift Keying is a process of digital modulation where data is transmitted by regular reversal of phase. It can be used to transmit data using different data communication techniques. This technique uses a reference signal of constant frequency. The transmission is done in an extremely efficient way. It is quite useful in the engineering and telecommunications sector and is also superior as compared to amplitude shift keying or ASK. We effectively use PSK in radio communication. In the case of Phase Shift Keying or PSK, the phase of the transmission signal is continuously altered to get the output. The Modulation is done by continuously varying the cosine and sine functions at regular time intervals. There are different forms of PSK and each of them has its own advantages as well as disadvantages. It is widely used in different types of communication techniques like LAN, Bluetooth communication, etc. Here, we have provided an in-depth analysis of PSK along with its various advantages and disadvantages.
Definition: Phase Shift Keying is a type of digital modulation technique where we transmit the data by modulating the phase of the carrier signal. The modulation is carried out by changing the inputs at regular intervals of time. Here we use finite phases and each of these phases can be represented by a unique pattern of bits. The number of bits used is the same in each case. A demodulator is used to determine the phase of the signal and recover the original data from it. Such a system is referred to as a coherent system.
We can represent the PSK method by using the constellations diagram. Uniform phase separation is offered among nearby points and therefore it is the best protection to corruption.
Digital modulation is a type of modulation technique where discrete signals are used to change the carrier wave. Digital modulation is basically of three types which are amplitude shift keying, phase-shift keying, and phase-shift keying. This kind of modulation can be used in the efficient transmission of the signal.
PSK is of two types:
BPSK is also called phase reversal keying or 2PSK. It is the most simple form of PSK. Two phases are used which has 180 degrees separation and the two phases are represented by 0 and 1 where 0 represents 0 degrees and 1 represents 180 degrees. In this method, certain basic functions are selected. The position of the constellation points on the wave does not matter. They can be placed anywhere. It is the best form of PSK as it can take care of huge noise distortion. However, one major disadvantage of PSK is that it can only modulate one bit per symbol.
QPSK uses quadrants or four different spaces. In this technique, different constellation points are used for noise reduction. The constellation points also reduce the chances of error. In this kind of PSK, four different phases are used which are 0, 90, 180 and 270 degrees. Two bits are used to represent the four different phases.
Some of the most important forms of PSK have been mentioned below:
All these forms of PSK are frequently used in different applications. All these forms of PSK have their own set of uses. Generally, the higher-order forms are used for high rate data transmission within a given bandwidth. However, the high rate of data transmission requires superior sound to noise ratio in order to reduce the rate of error.
Some of the major advantages of PSK are:
Some of the major disadvantages of PSK are:
Some of the major applications of Phase Shift Keying are:
FAQs
1). What is phase shift keying in simple terms?
PSK is basically a data modulation technique in which data is transmitted by continuously varying the pulses of the reference signal.
2). Which is better PSK or FSK?
PSK signals have more bandwidth efficiency as compared to FSK. However, FSK signals are less susceptible to noise and distortion. Hence, it is quite difficult to draw up a comparison chart between PSK and FSK.
3). What is the Shift keying technique?
Shift keying is a data modulation technique. In shift keying, while data is being transmitted, the values which are used for encoding the data continuously switch between two keys having discrete switching values.
4). How many bits is BPSK?
Only 1 bit is used in the case of BPSK.
5). Which is better PSK or FSK?
PSK signals have more bandwidth efficiency as compared to FSK. However, FSK signals are less susceptible to noise and distortion. Hence, it is quite difficult to draw up a comparison chart between PSK and FSK.
Please refer to this link to know more about Phase modulation.
FSK has lower spectral efficiency than QAM and PSK, as it needs more bandwidth to transmit the same amount of information. FSK also has higher BER performance than QAM and PSK at low SNRs, as the frequency separation becomes more difficult to distinguish.
