# Phase-shift keying

Phase-shift keying (PSK) is a digital modulation process which conveys data by changing (modulating) the phase of a constant frequency reference signal (the carrier wave). The modulation is accomplished by varying the sine and cosine inputs at a precise time. It is widely used for wireless LANs, RFID and Bluetooth communication.

Any digital modulation scheme uses a finite number of distinct signals to represent digital data. PSK uses a finite number of phases, each assigned a unique pattern of binary digits. Usually, each phase encodes an equal number of bits. Each pattern of bits forms the symbol that is represented by the particular phase. The demodulator, which is designed specifically for the symbol-set used by the modulator, determines the phase of the received signal and maps it back to the symbol it represents, thus recovering the original data. This requires the receiver to be able to compare the phase of the received signal to a reference signal – such a system is termed coherent (and referred to as CPSK).

CPSK requires a complicated demodulator, because it must extract the reference wave from the received signal and keep track of it, to compare each sample to. Alternatively, the phase shift of each symbol sent can be measured with respect to the phase of the previous symbol sent. Because the symbols are encoded in the difference in phase between successive samples, this is called differential phase-shift keying (DPSK). DPSK can be significantly simpler to implement than ordinary PSK, as it is a 'non-coherent' scheme, i.e. there is no need for the demodulator to keep track of a reference wave. A trade-off is that it has more demodulation errors.

## Introduction

There are three major classes of digital modulation techniques used for transmission of digitally represented data:

All convey data by changing some aspect of a base signal, the carrier wave (usually a sinusoid), in response to a data signal. In the case of PSK, the phase is changed to represent the data signal. There are two fundamental ways of utilizing the phase of a signal in this way:

• By viewing the phase itself as conveying the information, in which case the demodulator must have a reference signal to compare the received signal's phase against; or
• By viewing the change in the phase as conveying information – differential schemes, some of which do not need a reference carrier (to a certain extent).

A convenient method to represent PSK schemes is on a constellation diagram. This shows the points in the complex plane where, in this context, the real and imaginary axes are termed the in-phase and quadrature axes respectively due to their 90° separation. Such a representation on perpendicular axes lends itself to straightforward implementation. The amplitude of each point along the in-phase axis is used to modulate a cosine (or sine) wave and the amplitude along the quadrature axis to modulate a sine (or cosine) wave. By convention, in-phase modulates cosine and quadrature modulates sine.

In PSK, the constellation points chosen are usually positioned with uniform angular spacing around a circle. This gives maximum phase-separation between adjacent points and thus the best immunity to corruption. They are positioned on a circle so that they can all be transmitted with the same energy. In this way, the moduli of the complex numbers they represent will be the same and thus so will the amplitudes needed for the cosine and sine waves. Two common examples are "binary phase-shift keying" (BPSK) which uses two phases, and "quadrature phase-shift keying" (QPSK) which uses four phases, although any number of phases may be used. Since the data to be conveyed are usually binary, the PSK scheme is usually designed with the number of constellation points being a power of two.

### Definitions

For determining error-rates mathematically, some definitions will be needed:

• ${\displaystyle E_{b}}$, energy per bit
• ${\displaystyle E_{s}=nE_{b}}$, energy per symbol with n bits
• ${\displaystyle T_{b}}$, bit duration
• ${\displaystyle T_{s}}$, symbol duration
• ${\displaystyle {\frac {1}{2}}N_{0}}$, noise power spectral density (W/Hz)
• ${\displaystyle P_{b}}$, probability of bit-error
• ${\displaystyle P_{s}}$, probability of symbol-error

${\displaystyle Q(x)}$ will give the probability that a single sample taken from a random process with zero-mean and unit-variance Gaussian probability density function will be greater or equal to ${\displaystyle x}$. It is a scaled form of the complementary Gaussian error function:

${\displaystyle Q(x)={\frac {1}{\sqrt {2\pi }}}\int _{x}^{\infty }e^{-{\frac {1}{2}}t^{2}}\,dt={\frac {1}{2}}\operatorname {erfc} \left({\frac {x}{\sqrt {2}}}\right),\ x\geq 0}$.

The error rates quoted here are those in additive white Gaussian noise (AWGN). These error rates are lower than those computed in fading channels, hence, are a good theoretical benchmark to compare with.

## Applications

Owing to PSK's simplicity, particularly when compared with its competitor quadrature amplitude modulation, it is widely used in existing technologies.

The wireless LAN standard, IEEE 802.11b-1999,[1][2] uses a variety of different PSKs depending on the data rate required. At the basic rate of 1 Mbit/s, it uses DBPSK (differential BPSK). To provide the extended rate of 2 Mbit/s, DQPSK is used. In reaching 5.5 Mbit/s and the full rate of 11 Mbit/s, QPSK is employed, but has to be coupled with complementary code keying. The higher-speed wireless LAN standard, IEEE 802.11g-2003,[1][3] has eight data rates: 6, 9, 12, 18, 24, 36, 48 and 54 Mbit/s. The 6 and 9 Mbit/s modes use OFDM modulation where each sub-carrier is BPSK modulated. The 12 and 18 Mbit/s modes use OFDM with QPSK. The fastest four modes use OFDM with forms of quadrature amplitude modulation.

Because of its simplicity, BPSK is appropriate for low-cost passive transmitters, and is used in RFID standards such as ISO/IEC 14443 which has been adopted for biometric passports, credit cards such as American Express's ExpressPay, and many other applications.[4]

Bluetooth 2 will use ${\displaystyle \pi /4}$-DQPSK at its lower rate (2 Mbit/s) and 8-DPSK at its higher rate (3 Mbit/s) when the link between the two devices is sufficiently robust. Bluetooth 1 modulates with Gaussian minimum-shift keying, a binary scheme, so either modulation choice in version 2 will yield a higher data-rate. A similar technology, IEEE 802.15.4 (the wireless standard used by ZigBee) also relies on PSK using two frequency bands: 868–915 MHz with BPSK and at 2.4 GHz with OQPSK.

Both QPSK and 8PSK are widely used in satellite broadcasting. QPSK is still widely used in the streaming of SD satellite channels and some HD channels. High definition programming is delivered almost exclusively in 8PSK due to the higher bitrates of HD video and the high cost of satellite bandwidth.[5] The DVB-S2 standard requires support for both QPSK and 8PSK. The chipsets used in new satellite set top boxes, such as Broadcom's 7000 series support 8PSK and are backward compatible with the older standard.[6]

Historically, voice-band synchronous modems such as the Bell 201, 208, and 209 and the CCITT V.26, V.27, V.29, V.32, and V.34 used PSK.[7]

## Binary phase-shift keying (BPSK)

Constellation diagram example for BPSK

BPSK (also sometimes called PRK, phase reversal keying, or 2PSK) is the simplest form of phase shift keying (PSK). It uses two phases which are separated by 180° and so can also be termed 2-PSK. It does not particularly matter exactly where the constellation points are positioned, and in this figure they are shown on the real axis, at 0° and 180°. Therefore, it handles the highest noise level or distortion before the demodulator reaches an incorrect decision. That makes it the most robust of all the PSKs. It is, however, only able to modulate at 1 bit/symbol (as seen in the figure) and so is unsuitable for high data-rate applications.

In the presence of an arbitrary phase-shift introduced by the communications channel, the demodulator (see, e.g. Costas loop) is unable to tell which constellation point is which. As a result, the data is often differentially encoded prior to modulation.

BPSK is functionally equivalent to 2-QAM modulation.

### Implementation

The general form for BPSK follows the equation:

${\displaystyle s_{n}(t)={\sqrt {\frac {2E_{b}}{T_{b}}}}\cos(2\pi ft+\pi (1-n)),\quad n=0,1.}$

This yields two phases, 0 and π. In the specific form, binary data is often conveyed with the following signals:

${\displaystyle s_{0}(t)={\sqrt {\frac {2E_{b}}{T_{b}}}}\cos(2\pi ft+\pi )=-{\sqrt {\frac {2E_{b}}{T_{b}}}}\cos(2\pi ft)}$ for binary "0"
${\displaystyle s_{1}(t)={\sqrt {\frac {2E_{b}}{T_{b}}}}\cos(2\pi ft)}$ for binary "1"

where f is the frequency of the base band.

Hence, the signal space can be represented by the single basis function

${\displaystyle \phi (t)={\sqrt {\frac {2}{T_{b}}}}\cos(2\pi ft)}$

where 1 is represented by ${\displaystyle {\sqrt {E_{b}}}\phi (t)}$ and 0 is represented by ${\displaystyle -{\sqrt {E_{b}}}\phi (t)}$. This assignment is, of course, arbitrary.

This use of this basis function is shown at the end of the next section in a signal timing diagram. The topmost signal is a BPSK-modulated cosine wave that the BPSK modulator would produce. The bit-stream that causes this output is shown above the signal (the other parts of this figure are relevant only to QPSK). After modulation, the base band signal will be moved to the high frequency band by multiplying ${\displaystyle \cos(2\pi f_{c}t)}$.

### Bit error rate

The bit error rate (BER) of BPSK under additive white Gaussian noise (AWGN) can be calculated as:[8]

${\displaystyle P_{b}=Q\left({\sqrt {\frac {2E_{b}}{N_{0}}}}\right)}$ or ${\displaystyle P_{e}={\frac {1}{2}}\operatorname {erfc} \left({\sqrt {\frac {E_{b}}{N_{0}}}}\right)}$

Since there is only one bit per symbol, this is also the symbol error rate.

Constellation diagram for QPSK with Gray coding. Each adjacent symbol only differs by one bit.

Sometimes this is known as quadriphase PSK, 4-PSK, or 4-QAM. (Although the root concepts of QPSK and 4-QAM are different, the resulting modulated radio waves are exactly the same.) QPSK uses four points on the constellation diagram, equispaced around a circle. With four phases, QPSK can encode two bits per symbol, shown in the diagram with Gray coding to minimize the bit error rate (BER) – sometimes misperceived as twice the BER of BPSK.

The mathematical analysis shows that QPSK can be used either to double the data rate compared with a BPSK system while maintaining the same bandwidth of the signal, or to maintain the data-rate of BPSK but halving the bandwidth needed. In this latter case, the BER of QPSK is exactly the same as the BER of BPSK – and deciding differently is a common confusion when considering or describing QPSK. The transmitted carrier can undergo numbers of phase changes.

Given that radio communication channels are allocated by agencies such as the Federal Communication Commission giving a prescribed (maximum) bandwidth, the advantage of QPSK over BPSK becomes evident: QPSK transmits twice the data rate in a given bandwidth compared to BPSK - at the same BER. The engineering penalty that is paid is that QPSK transmitters and receivers are more complicated than the ones for BPSK. However, with modern electronics technology, the penalty in cost is very moderate.

As with BPSK, there are phase ambiguity problems at the receiving end, and differentially encoded QPSK is often used in practice.

### Implementation

The implementation of QPSK is more general than that of BPSK and also indicates the implementation of higher-order PSK. Writing the symbols in the constellation diagram in terms of the sine and cosine waves used to transmit them:

${\displaystyle s_{n}(t)={\sqrt {\frac {2E_{s}}{T_{s}}}}\cos \left(2\pi f_{c}t+(2n-1){\frac {\pi }{4}}\right),\quad n=1,2,3,4.}$

This yields the four phases π/4, 3π/4, 5π/4 and 7π/4 as needed.

This results in a two-dimensional signal space with unit basis functions

{\displaystyle {\begin{aligned}\phi _{1}(t)&={\sqrt {\frac {2}{T_{s}}}}\cos \left(2\pi f_{c}t\right)\\\phi _{2}(t)&={\sqrt {\frac {2}{T_{s}}}}\sin \left(2\pi f_{c}t\right)\end{aligned}}}

The first basis function is used as the in-phase component of the signal and the second as the quadrature component of the signal.

Hence, the signal constellation consists of the signal-space 4 points

${\displaystyle \left(\pm {\sqrt {\frac {E_{s}}{2}}},\pm {\sqrt {\frac {E_{s}}{2}}}\right).}$

The factors of 1/2 indicate that the total power is split equally between the two carriers.

Comparing these basis functions with that for BPSK shows clearly how QPSK can be viewed as two independent BPSK signals. Note that the signal-space points for BPSK do not need to split the symbol (bit) energy over the two carriers in the scheme shown in the BPSK constellation diagram.

QPSK systems can be implemented in a number of ways. An illustration of the major components of the transmitter and receiver structure are shown below.

Conceptual transmitter structure for QPSK. The binary data stream is split into the in-phase and quadrature-phase components. These are then separately modulated onto two orthogonal basis functions. In this implementation, two sinusoids are used. Afterwards, the two signals are superimposed, and the resulting signal is the QPSK signal. Note the use of polar non-return-to-zero encoding. These encoders can be placed before for binary data source, but have been placed after to illustrate the conceptual difference between digital and analog signals involved with digital modulation.
Receiver structure for QPSK. The matched filters can be replaced with correlators. Each detection device uses a reference threshold value to determine whether a 1 or 0 is detected.

### Bit error rate

Although QPSK can be viewed as a quaternary modulation, it is easier to see it as two independently modulated quadrature carriers. With this interpretation, the even (or odd) bits are used to modulate the in-phase component of the carrier, while the odd (or even) bits are used to modulate the quadrature-phase component of the carrier. BPSK is used on both carriers and they can be independently demodulated.

As a result, the probability of bit-error for QPSK is the same as for BPSK:

${\displaystyle P_{b}=Q\left({\sqrt {\frac {2E_{b}}{N_{0}}}}\right)}$

However, in order to achieve the same bit-error probability as BPSK, QPSK uses twice the power (since two bits are transmitted simultaneously).

The symbol error rate is given by:

{\displaystyle {\begin{aligned}P_{s}&=1-\left(1-P_{b}\right)^{2}\\&=2Q\left({\sqrt {\frac {E_{s}}{N_{0}}}}\right)-\left[Q\left({\sqrt {\frac {E_{s}}{N_{0}}}}\right)\right]^{2}.\end{aligned}}}

If the signal-to-noise ratio is high (as is necessary for practical QPSK systems) the probability of symbol error may be approximated:

${\displaystyle P_{s}\approx 2Q\left({\sqrt {\frac {E_{s}}{N_{0}}}}\right)}$

The modulated signal is shown below for a short segment of a random binary data-stream. The two carrier waves are a cosine wave and a sine wave, as indicated by the signal-space analysis above. Here, the odd-numbered bits have been assigned to the in-phase component and the even-numbered bits to the quadrature component (taking the first bit as number 1). The total signal – the sum of the two components – is shown at the bottom. Jumps in phase can be seen as the PSK changes the phase on each component at the start of each bit-period. The topmost waveform alone matches the description given for BPSK above.

Timing diagram for QPSK. The binary data stream is shown beneath the time axis. The two signal components with their bit assignments are shown at the top, and the total combined signal at the bottom. Note the abrupt changes in phase at some of the bit-period boundaries.

The binary data that is conveyed by this waveform is: 11000110.

• The odd bits, highlighted here, contribute to the in-phase component: 11000110
• The even bits, highlighted here, contribute to the quadrature-phase component: 11000110

### Variants

#### Offset QPSK (OQPSK)

Signal doesn't pass through the origin, because only one bit of the symbol is changed at a time.

Offset quadrature phase-shift keying (OQPSK) is a variant of phase-shift keying modulation using four different values of the phase to transmit. It is sometimes called staggered quadrature phase-shift keying (SQPSK).

Difference of the phase between QPSK and OQPSK

Taking four values of the phase (two bits) at a time to construct a QPSK symbol can allow the phase of the signal to jump by as much as 180° at a time. When the signal is low-pass filtered (as is typical in a transmitter), these phase-shifts result in large amplitude fluctuations, an undesirable quality in communication systems. By offsetting the timing of the odd and even bits by one bit-period, or half a symbol-period, the in-phase and quadrature components will never change at the same time. In the constellation diagram shown on the right, it can be seen that this will limit the phase-shift to no more than 90° at a time. This yields much lower amplitude fluctuations than non-offset QPSK and is sometimes preferred in practice.

The picture on the right shows the difference in the behavior of the phase between ordinary QPSK and OQPSK. It can be seen that in the first plot the phase can change by 180° at once, while in OQPSK the changes are never greater than 90°.

The modulated signal is shown below for a short segment of a random binary data-stream. Note the half symbol-period offset between the two component waves. The sudden phase-shifts occur about twice as often as for QPSK (since the signals no longer change together), but they are less severe. In other words, the magnitude of jumps is smaller in OQPSK when compared to QPSK.

Timing diagram for offset-QPSK. The binary data stream is shown beneath the time axis. The two signal components with their bit assignments are shown the top and the total, combined signal at the bottom. Note the half-period offset between the two signal components.

#### π/4-QPSK

Dual constellation diagram for π/4-QPSK. This shows the two separate constellations with identical Gray coding but rotated by 45° with respect to each other.
Transition scheme of the modulation symbols of the π/4-QPSK (signal constellation). No zero crossings.

This variant of QPSK uses two identical constellations which are rotated by 45° (${\displaystyle \pi /4}$ radians, hence the name) with respect to one another. Usually, either the even or odd symbols are used to select points from one of the constellations and the other symbols select points from the other constellation. This also reduces the phase-shifts from a maximum of 180°, but only to a maximum of 135° and so the amplitude fluctuations of ${\displaystyle \pi /4}$-QPSK are between OQPSK and non-offset QPSK.

One property this modulation scheme possesses is that if the modulated signal is represented in the complex domain, transitions between symbols never pass through 0. In other words, the signal does not pass through the origin. This lowers the dynamical range of fluctuations in the signal which is desirable when engineering communications signals.

On the other hand, ${\displaystyle \pi /4}$-QPSK lends itself to easy demodulation and has been adopted for use in, for example, TDMA cellular telephone systems.

The modulated signal is shown below for a short segment of a random binary data-stream. The construction is the same as above for ordinary QPSK. Successive symbols are taken from the two constellations shown in the diagram. Thus, the first symbol (1 1) is taken from the "blue" constellation and the second symbol (0 0) is taken from the "green" constellation. Note that magnitudes of the two component waves change as they switch between constellations, but the total signal's magnitude remains constant (constant envelope). The phase-shifts are between those of the two previous timing-diagrams.

Timing diagram for π/4-QPSK. The binary data stream is shown beneath the time axis. The two signal components with their bit assignments are shown the top and the total, combined signal at the bottom. Note that successive symbols are taken alternately from the two constellations, starting with the "blue" one.

#### SOQPSK

The license-free shaped-offset QPSK (SOQPSK) is interoperable with Feher-patented QPSK (FQPSK), in the sense that an integrate-and-dump offset QPSK detector produces the same output no matter which kind of transmitter is used.[9]

These modulations carefully shape the I and Q waveforms such that they change very smoothly, and the signal stays constant-amplitude even during signal transitions. (Rather than traveling instantly from one symbol to another, or even linearly, it travels smoothly around the constant-amplitude circle from one symbol to the next.)

The standard description of SOQPSK-TG involves ternary symbols.

#### DPQPSK

Dual-polarization quadrature phase shift keying (DPQPSK) or dual-polarization QPSK - involves the polarization multiplexing of two different QPSK signals, thus improving the spectral efficiency by a factor of 2. This is a cost-effective alternative to utilizing 16-PSK, instead of QPSK to double the spectral efficiency.

## Higher-order PSK

Constellation diagram for 8-PSK with Gray coding

Any number of phases may be used to construct a PSK constellation but 8-PSK is usually the highest order PSK constellation deployed. With more than 8 phases, the error-rate becomes too high and there are better, though more complex, modulations available such as quadrature amplitude modulation (QAM). Although any number of phases may be used, the fact that the constellation must usually deal with binary data means that the number of symbols is usually a power of 2 to allow an integer number of bits per symbol.

### Bit error rate

For the general M-PSK there is no simple expression for the symbol-error probability if ${\displaystyle M>4}$. Unfortunately, it can only be obtained from

${\displaystyle P_{s}=1-\int _{-\pi /M}^{\pi /M}p_{\theta _{r}}\left(\theta _{r}\right)d\theta _{r},}$

where

${\displaystyle p_{\theta _{r}}(\theta _{r})={\frac {1}{2\pi }}e^{-2\gamma _{s}\sin ^{2}\theta _{r}}\int _{0}^{\infty }Ve^{-{\frac {1}{2}}\left(V-2{\sqrt {\gamma _{s}}}\cos \theta _{r}\right)^{2}}\,dV,}$
${\displaystyle V={\sqrt {r_{1}^{2}+r_{2}^{2}}},}$
${\displaystyle \theta _{r}=\tan ^{-1}\left({\frac {r_{2}}{r_{1}}}\right),}$
${\displaystyle \gamma _{s}={\frac {E_{s}}{N_{0}}}}$ and
${\displaystyle r_{1}\sim N\left({\sqrt {E_{s}}},{\frac {1}{2}}N_{0}\right)}$ and ${\displaystyle r_{2}\sim N\left(0,{\frac {1}{2}}N_{0}\right)}$ are jointly Gaussian random variables.
Bit-error rate curves for BPSK, QPSK, 8-PSK and 16-PSK, additive white Gaussian noise channel

This may be approximated for high ${\displaystyle M}$ and high ${\displaystyle E_{b}/N_{0}}$ by:

${\displaystyle P_{s}\approx 2Q\left({\sqrt {2\gamma _{s}}}\sin {\frac {\pi }{M}}\right).}$

The bit-error probability for ${\displaystyle M}$-PSK can only be determined exactly once the bit-mapping is known. However, when Gray coding is used, the most probable error from one symbol to the next produces only a single bit-error and

${\displaystyle P_{b}\approx {\frac {1}{k}}P_{s}.}$

(Using Gray coding allows us to approximate the Lee distance of the errors as the Hamming distance of the errors in the decoded bitstream, which is easier to implement in hardware.)

The graph on the left compares the bit-error rates of BPSK, QPSK (which are the same, as noted above), 8-PSK and 16-PSK. It is seen that higher-order modulations exhibit higher error-rates; in exchange however they deliver a higher raw data-rate.

Bounds on the error rates of various digital modulation schemes can be computed with application of the union bound to the signal constellation.

## Differential phase-shift keying (DPSK)

### Differential encoding

Differential phase shift keying (DPSK) is a common form of phase modulation that conveys data by changing the phase of the carrier wave. As mentioned for BPSK and QPSK there is an ambiguity of phase if the constellation is rotated by some effect in the communications channel through which the signal passes. This problem can be overcome by using the data to change rather than set the phase.

For example, in differentially encoded BPSK a binary "1" may be transmitted by adding 180° to the current phase and a binary "0" by adding 0° to the current phase. Another variant of DPSK is Symmetric Differential Phase Shift keying, SDPSK, where encoding would be +90° for a "1" and −90° for a "0".

In differentially encoded QPSK (DQPSK), the phase-shifts are 0°, 90°, 180°, −90° corresponding to data "00", "01", "11", "10". This kind of encoding may be demodulated in the same way as for non-differential PSK but the phase ambiguities can be ignored. Thus, each received symbol is demodulated to one of the ${\displaystyle M}$ points in the constellation and a comparator then computes the difference in phase between this received signal and the preceding one. The difference encodes the data as described above. Symmetric Differential Quadrature Phase Shift Keying (SDQPSK) is like DQPSK, but encoding is symmetric, using phase shift values of −135°, −45°, +45° and +135°.

The modulated signal is shown below for both DBPSK and DQPSK as described above. In the figure, it is assumed that the signal starts with zero phase, and so there is a phase shift in both signals at ${\displaystyle t=0}$.

Timing diagram for DBPSK and DQPSK. The binary data stream is above the DBPSK signal. The individual bits of the DBPSK signal are grouped into pairs for the DQPSK signal, which only changes every Ts = 2Tb.

Analysis shows that differential encoding approximately doubles the error rate compared to ordinary ${\displaystyle M}$-PSK but this may be overcome by only a small increase in ${\displaystyle E_{b}/N_{0}}$. Furthermore, this analysis (and the graphical results below) are based on a system in which the only corruption is additive white Gaussian noise (AWGN). However, there will also be a physical channel between the transmitter and receiver in the communication system. This channel will, in general, introduce an unknown phase-shift to the PSK signal; in these cases the differential schemes can yield a better error-rate than the ordinary schemes which rely on precise phase information.

One of the most popular applications of DPSK is the Bluetooth standard where ${\displaystyle \pi /4}$-DQPSK and 8-DPSK were implemented.

### Demodulation

BER comparison between DBPSK, DQPSK and their non-differential forms using Gray coding and operating in white noise

For a signal that has been differentially encoded, there is an obvious alternative method of demodulation. Instead of demodulating as usual and ignoring carrier-phase ambiguity, the phase between two successive received symbols is compared and used to determine what the data must have been. When differential encoding is used in this manner, the scheme is known as differential phase-shift keying (DPSK). Note that this is subtly different from just differentially encoded PSK since, upon reception, the received symbols are not decoded one-by-one to constellation points but are instead compared directly to one another.

Call the received symbol in the ${\displaystyle k}$th timeslot ${\displaystyle r_{k}}$ and let it have phase ${\displaystyle \phi _{k}}$. Assume without loss of generality that the phase of the carrier wave is zero. Denote the additive white Gaussian noise (AWGN) term as ${\displaystyle n_{k}}$. Then

${\displaystyle r_{k}={\sqrt {E_{s}}}e^{j\phi _{k}}+n_{k}.}$

The decision variable for the ${\displaystyle k-1}$th symbol and the ${\displaystyle k}$th symbol is the phase difference between ${\displaystyle r_{k}}$ and ${\displaystyle r_{k-1}}$. That is, if ${\displaystyle r_{k}}$ is projected onto ${\displaystyle r_{k-1}}$, the decision is taken on the phase of the resultant complex number:

${\displaystyle r_{k}r_{k-1}^{*}=E_{s}e^{j\left(\varphi _{k}-\varphi _{k-1}\right)}+{\sqrt {E_{s}}}e^{j\varphi _{k}}n_{k-1}^{*}+{\sqrt {E_{s}}}e^{-j\varphi _{k-1}}n_{k}+n_{k}n_{k-1}^{*}}$

where superscript * denotes complex conjugation. In the absence of noise, the phase of this is ${\displaystyle \phi _{k}-\phi _{k-1}}$, the phase-shift between the two received signals which can be used to determine the data transmitted.

The probability of error for DPSK is difficult to calculate in general, but, in the case of DBPSK it is:

${\displaystyle P_{b}={\frac {1}{2}}e^{-{\frac {E_{b}}{N_{0}}}},}$[10]

which, when numerically evaluated, is only slightly worse than ordinary BPSK, particularly at higher ${\displaystyle E_{b}/N_{0}}$ values.

Using DPSK avoids the need for possibly complex carrier-recovery schemes to provide an accurate phase estimate and can be an attractive alternative to ordinary PSK.

In optical communications, the data can be modulated onto the phase of a laser in a differential way. The modulation is a laser which emits a continuous wave, and a Mach–Zehnder modulator which receives electrical binary data. For the case of BPSK, the laser transmits the field unchanged for binary '1', and with reverse polarity for '0'. The demodulator consists of a delay line interferometer which delays one bit, so two bits can be compared at one time. In further processing, a photodiode is used to transform the optical field into an electric current, so the information is changed back into its original state.

The bit-error rates of DBPSK and DQPSK are compared to their non-differential counterparts in the graph to the right. The loss for using DBPSK is small enough compared to the complexity reduction that it is often used in communications systems that would otherwise use BPSK. For DQPSK though, the loss in performance compared to ordinary QPSK is larger and the system designer must balance this against the reduction in complexity.

### Example: Differentially encoded BPSK

Differential encoding/decoding system diagram

At the ${\displaystyle k^{\textrm {th}}}$ time-slot call the bit to be modulated ${\displaystyle b_{k}}$, the differentially encoded bit ${\displaystyle e_{k}}$ and the resulting modulated signal ${\displaystyle m_{k}(t)}$. Assume that the constellation diagram positions the symbols at ±1 (which is BPSK). The differential encoder produces:

${\displaystyle \,e_{k}=e_{k-1}\oplus b_{k}}$

where ${\displaystyle \oplus {}}$ indicates binary or modulo-2 addition.

BER comparison between BPSK and differentially encoded BPSK with Gray coding operating in white noise

So ${\displaystyle e_{k}}$ only changes state (from binary "0" to binary "1" or from binary "1" to binary "0") if ${\displaystyle b_{k}}$ is a binary "1". Otherwise it remains in its previous state. This is the description of differentially encoded BPSK given above.

The received signal is demodulated to yield ${\displaystyle e_{k}=}$±1 and then the differential decoder reverses the encoding procedure and produces

${\displaystyle b_{k}=e_{k}\oplus e_{k-1},}$

since binary subtraction is the same as binary addition.

Therefore, ${\displaystyle b_{k}=1}$ if ${\displaystyle e_{k}}$ and ${\displaystyle e_{k-1}}$ differ and ${\displaystyle b_{k}=0}$ if they are the same. Hence, if both ${\displaystyle e_{k}}$ and ${\displaystyle e_{k-1}}$ are inverted, ${\displaystyle b_{k}}$ will still be decoded correctly. Thus, the 180° phase ambiguity does not matter.

Differential schemes for other PSK modulations may be devised along similar lines. The waveforms for DPSK are the same as for differentially encoded PSK given above since the only change between the two schemes is at the receiver.

The BER curve for this example is compared to ordinary BPSK on the right. As mentioned above, whilst the error rate is approximately doubled, the increase needed in ${\displaystyle E_{b}/N_{0}}$ to overcome this is small. The increase in ${\displaystyle E_{b}/N_{0}}$ required to overcome differential modulation in coded systems, however, is larger – typically about 3 dB. The performance degradation is a result of noncoherent transmission – in this case it refers to the fact that tracking of the phase is completely ignored.

## Mutual information with additive white Gaussian noise

Mutual information of PSK over the AWGN channel

The mutual information of PSK can be evaluated in additive Gaussian noise by numerical integration of its definition.[11] The curves of mutual information saturate to the number of bits carried by each symbol in the limit of infinite signal to noise ratio ${\displaystyle E_{s}/N_{0}}$. On the contrary, in the limit of small signal to noise ratios the mutual information approaches the AWGN channel capacity, which is the supremum among all possible choices of symbol statistical distributions.

At intermediate values of signal to noise ratios the mutual information (MI) is well approximated by:[11]

${\displaystyle {\textrm {MI}}\simeq \log _{2}\left({\sqrt {{\frac {4\pi }{e}}{\frac {E_{s}}{N_{0}}}}}\right).}$

The mutual information of PSK over the AWGN channel is generally farther to the AWGN channel capacity than QAM modulation formats.

## Notes

1. ^ a b IEEE Std 802.11-1999: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications – the overarching IEEE 802.11 specification. Archived August 28, 2007, at the Wayback Machine
2. ^ IEEE Std 802.11b-1999 (R2003) – the IEEE 802.11b specification.
3. ^ IEEE Std 802.11g-2003 – the IEEE 802.11g specification.
4. ^ Understanding the Requirements of ISO/IEC 14443 for Type B Proximity Contactless Identification Cards, Application Note, Rev. 2056B–RFID–11/05, 2005, ATMEL.
5. ^ "How Communications Satellites Work". Planet Fox. 2014.
7. ^ "Local and Remote Modems" (PDF). Black Box. Black Box Network Services. Retrieved December 20, 2015.
8. ^ Communications Systems, H. Stern & S. Mahmoud, Pearson Prentice Hall, 2004, p. 283.
9. ^ Tom Nelson, Erik Perrins, and Michael Rice. "Common detectors for Tier 1 modulations" Archived 2012-09-17 at the Wayback Machine. T. Nelson, E. Perrins, M. Rice. "Common detectors for shaped offset QPSK (SOQPSK) and Feher-patented QPSK (FQPSK)" Nelson, T.; Perrins, E.; Rice, M. (2005). "Common detectors for shaped offset QPSK (SOQPSK) and Feher-patented QPSK (FQPSK)". GLOBECOM '05. IEEE Global Telecommunications Conference, 2005. pp. 5 pp. doi:10.1109/GLOCOM.2005.1578470. ISBN 0-7803-9414-3. ISBN 0-7803-9414-3
10. ^ G.L. Stüber, “Soft Decision Direct-Sequence DPSK Receivers,” IEEE Transactions on Vehicular Technology, vol. 37, no. 3, pp. 151–157, August 1988.
11. ^ a b Blahut, R. E. (1988). Principles and Practice of Information Theory. Boston, MA, USA: Addison Wesley Publishing Company. ISBN 0-201-10709-0.

## References

The notation and theoretical results in this article are based on material presented in the following sources:

• Proakis, John G. (1995). Digital Communications. Singapore: McGraw Hill. ISBN 0-07-113814-5.
• Couch, Leon W. II (1997). Digital and Analog Communications. Upper Saddle River, NJ: Prentice-Hall. ISBN 0-13-081223-4.
• Haykin, Simon (1988). Digital Communications. Toronto, Canada: John Wiley & Sons. ISBN 0-471-62947-2.

An adaptive equalizer is an equalizer that automatically adapts to time-varying properties of the communication channel. It is frequently used with coherent modulations such as phase shift keying, mitigating the effects of multipath propagation and Doppler spreading.

Adaptive equalizers are a subclass of adaptive filters. The central idea is altering the filter's coefficients to optimize a filter characteristic. For example, in case of linear discrete-time filters, the following equation can be used:

${\displaystyle \mathbf {w} _{opt}=\mathbf {R} ^{-1}\mathbf {p} }$

where ${\displaystyle \mathbf {w} _{opt}}$ is the vector of the filter's coefficients, ${\displaystyle \mathbf {R} }$ is the received signal covariance matrix and ${\displaystyle \mathbf {p} }$ is the cross-correlation vector between the tap-input vector and the desired response. In practice, the last quantities are not known and, if necessary, must be estimated during the equalization procedure either explicitly or implicitly.

Many adaptation strategies exist. They include, e.g.:

A well-known example is the decision feedback equalizer, a filter that uses feedback of detected symbols in addition to conventional equalization of future symbols. Some systems use predefined training sequences to provide reference points for the adaptation process.

Amplitude and phase-shift keying

Amplitude and phase-shift keying or asymmetric phase-shift keying (APSK) is a digital modulation scheme that conveys data by changing, or modulating, both the amplitude and the phase of a reference signal (the carrier wave). In other words, it combines both amplitude-shift keying (ASK) and phase-shift keying (PSK) to increase the symbol-set. It can be considered as a superclass of quadrature amplitude modulation (QAM). The advantage over conventional QAM, for example 16-QAM, is lower number of possible amplitude levels.

Moreover, a careful design of the constellation geometry can approach the Gaussian capacity as the constellation size grows to infinity. For the regular QAM constellations, a gap of 1.56 dB is observed. The previous solution, where the constellation has a Gaussian shape, is called constellation shaping.

C-MAC

C-MAC is the variant approved by the European Broadcasting Union (EBU) for satellite transmissions. The digital information is modulated using 2-4PSK (phase-shift keying), a variation of quadrature PSK where only two of the phaser angles (±90°) are used.

The data capacity for C-MAC is 3Mbit/s.

C-MAC data has to be sent to the transmitter separately from the vision.

The transmitter switches between FM (vision) and PSK (sound/data) modulation during each television line period.

Ciena Optical Multiservice Edge 6500

The 6500 Packet-Optical Platform (formerly called the Optical Multiservice Edge 6500 or OME 6500 during the product's time at Nortel) in telecommunication, computer networking and optical communications is a Multi-port multi-protocol system designed by Ciena that supports TDM/WDM/GigE/10G/40G and 100G ports.The system supports high bandwidth demands from applications like IPTV, Internet Video, HD programming, and mobile video by increasing the speeds over existing fiber. Normally increasing the speeds from 10G to 40G to 100G typically entails trade-offs such as shortening the distance of each network segment or increasing optical dispersion because of the weakening of optical signals as they travel. Prevention of this signal loss would normally require amplifiers or repeaters, or in some cases new better quality fiber would need to be installed. Nortel prevents this by using a signal modulation technology called dual-polarization quadrature phase shift keying (DPQPSK).

Delay line interferometer

A delay line interferometer (DLI) can be a Mach–Zehnder interferometer or Michelson interferometer based on two-beam interference, in which one beam is time-delayed to the other by a desired interval.

Delay line interferometers are also known as optical DPSK demodulators. They convert a phase-keyed signal into an amplitude-keyed signal. In this application, an incoming differential phase-shift keying (DPSK) optical signal is first split into two equal-intensity beams in two arms of a Mach Zehnder or Michelson interferometer, in which one beam is delayed by an optical path difference corresponding to 1-bit time delay. After recombination, the two beams interfere with each other constructively or destructively. The resultant interference intensity is the intensity-keyed signal.

Differential coding

In digital communications, differential coding is a technique used to provide unambiguous signal reception when using some types of modulation. It makes data to be transmitted to depend not only on the current signal state (or symbol), but also on the previous one.

The common types of modulation that require differential coding include phase shift keying and quadrature amplitude modulation.

High Frequency Internet Protocol

High Frequency Internet Protocol (HFIP or HF-IP) is usually associated with Automatic Link Establishment and HF radio data communications. HFIP provides protocol layers enabling internet file transfer, chat, web and email. HFIP commonly uses ionospheric propagation of radio waves to form a wide area network that can span thousands of kilometers. HF transceivers in HFIP service typically run 20 to 150 Watts for portable or mobile units, up to approximately 2000 Watts transmitter output for high power base stations with HFIP servers.

STANAG 5066 is a common HFIP standard.

An amateur radio HFIP network called HFLINK uses Automatic Link Establishment for initiating data communications, with ARQ 8FSK frequency-shift keying and PSK phase-shift keying signals.

Higher-order modulation

Higher-order modulation is a type of digital modulation usually with an order of 4 or higher. Examples: quadrature phase-shift keying (QPSK), and m-ary quadrature amplitude modulation (m-QAM).

Keying (telecommunications)

Keying is a family of modulation forms where the modulating signal takes one of a specific (predetermined) number of values at all times. The goal of keying is to transmit a digital signal over an analog channel. The name derives from the Morse code key used for telegraph signaling.

Modulation is the general technique of shaping a signal to convey information. When a digital message has to be represented as an analog waveform, the technique and term keying (or digital modulation) is used. Keying is characterized by the fact that the modulating signal will have a limited number of states (or values) at all times, to represent the corresponding digital states (commonly zero and one, although this might depend on the number of symbols used). This is in contrast to analogue modulation, where an analogue signal is transmitted over an analogue channel, and where the modulated analogue signal will have an infinite number of meaningful states.

Furthermore, note that keying or digital modulation applies to transmitting a digital signal over an analogue passband channel. When a digital signal is to be transmitted over an analogue baseband channel, the modulation technique is termed line coding.

Several keying techniques exist, including phase-shift keying, frequency-shift keying and amplitude-shift keying. Bluetooth, for example, uses phase-shift keying to exchange information between devices.

An overview of keying techniques is given on the modulation page.

The following is a list of the modes of radio communication used in the amateur radio hobby.

Local Multipoint Distribution Service

Local Multipoint Distribution Service (LMDS) is a broadband wireless access technology originally designed for digital television transmission (DTV). It was conceived as a fixed wireless, point-to-multipoint technology for utilization in the last mile.

LMDS commonly operates on microwave frequencies across the 26 GHz and 29 GHz bands. In the United States, frequencies from 31.0 through 31.3 GHz are also considered LMDS frequencies.

Throughput capacity and reliable distance of the link depends on common radio link constraints and the modulation method used - either phase-shift keying or amplitude modulation. Distance is typically limited to about 1.5 miles (2.4 km) due to rain fade attenuation constraints. Deployment links of up to 5 miles (8 km) from the base station are possible in some circumstances such as in point-to-point systems that can reach slightly farther distances due to increased antenna gain.

On-off keying

On-off keying (OOK) denotes the simplest form of amplitude-shift keying (ASK) modulation that represents digital data at the presence or absence of a carrier wave. In its simplest form, the presence of a carrier for a specific duration represents a binary one, while its absence for the same duration represents a binary zero. Some more sophisticated schemes vary these durations to convey additional information. It is analogous to unipolar encoding line code.

On-off keying is most commonly used to transmit Morse code over radio frequencies (referred to as CW (continuous wave) operation), although in principle any digital encoding scheme may be used. OOK has been used in the ISM bands to transfer data between computers, for example.

OOK is more spectrally efficient than frequency-shift keying, but more sensitive to noise when using a regenerative receiver or a poorly implemented superheterodyne receiver.

For a given data rate, the bandwidth of a BPSK (Binary Phase Shift keying) signal and the bandwidth of OOK signal are equal.

In addition to RF carrier waves, OOK is also used in optical communication systems (e.g. IrDA).

In aviation, some possibly unmanned airports have equipment that let pilots key their VHF radio a number of times in order to request an Automatic Terminal Information Service broadcast, or turn on runway lights.

PSK31

PSK31 or "Phase Shift Keying, 31 Baud", also BPSK31 and QPSK31, is a popular computer-sound card-generated radioteletype mode, used primarily by amateur radio operators to conduct real-time keyboard-to-keyboard chat, most often using frequencies in the high frequency amateur radio bands (near-shortwave). PSK31 is distinguished from other digital modes in that it is specifically tuned to have a data rate close to typing speed, and has an extremely narrow bandwidth, allowing many conversations in the same bandwidth as a single voice channel. This narrow bandwidth also concentrates the RF energy in a very narrow space thus allowing relatively low-power equipment (25 watts) to communicate globally using the same skywave propagation used by shortwave radio stations.

PSK63

PSK63 (meaning Phase Shift Keying at a rate of 63 baud) is a digital radio modulation mode used primarily in the amateur radio field to conduct real-time keyboard-to-keyboard informal text chat between amateur radio operators.

In signal processing:

Quadrature amplitude modulation (QAM), a modulation method of using both an (in-phase) carrier wave and a 'quadrature' carrier wave that is 90° out of phase with the main, or in-phase, carrier

Quadrature phase, oscillations that are said to be in quadrature if they are separated in phase by 90° (π/2, or λ/4)

Quadrature filter, the analytic signal of a real-valued filter

Quadrature phase-shift keying (QPSK), a phase-shift keying of using four quadrate points on the constellation diagram, equispaced around a circleIn mathematics:

Quadrature (mathematics), drawing a square with the same area as a given plane figure (squaring) or computing that area

Gaussian quadrature, a special case of numerical integration

Formerly, a synonym for "integral"

Integral

AntiderivativeIn physics:

In Optical phase space, quadratures are operators which represent the real and imaginary parts of the complex amplitude; see also in-phase and quadrature componentsIn astronomy:

Quadrature (astronomy), the position of a body (moon or planet) such that its elongation is 90° or 270°; i.e. the body-earth-sun angle is 90°Quadrature may also refer to:

In motion control, quadrature encoder is a rotary encoder or linear encoder giving both the relative position and the direction of motion of a shaft

Quadrature amplitude modulation (QAM) is the name of a family of digital modulation methods and a related family of analog modulation methods widely used in modern telecommunications to transmit information. It conveys two analog message signals, or two digital bit streams, by changing (modulating) the amplitudes of two carrier waves, using the amplitude-shift keying (ASK) digital modulation scheme or amplitude modulation (AM) analog modulation scheme. The two carrier waves of the same frequency are out of phase with each other by 90°, a condition known as orthogonality and as quadrature. Being the same frequency, the modulated carriers add together, but can be coherently separated (demodulated) because of their orthogonality property. Another key property is that the modulations are low-frequency/low-bandwidth waveforms compared to the carrier frequency, which is known as the narrowband assumption.

Phase modulation (analog PM) and phase-shift keying (digital PSK) can be regarded as a special case of QAM, where the magnitude of the modulating signal is a constant, but its sign changes between positive and negative. This can also be extended to frequency modulation (FM) and frequency-shift keying (FSK), for these can be regarded as a special case of phase modulation.

QAM is used extensively as a modulation scheme for digital telecommunication systems, such as in 802.11 Wi-Fi standards. Arbitrarily high spectral efficiencies can be achieved with QAM by setting a suitable constellation size, limited only by the noise level and linearity of the communications channel. QAM is being used in optical fiber systems as bit rates increase; QAM16 and QAM64 can be optically emulated with a 3-path interferometer.

Return-to-zero

Return-to-zero (RZ or RTZ) describes a line code used in telecommunications signals in which the signal drops (returns) to zero between each pulse. This takes place even if a number of consecutive 0s or 1s occur in the signal. The signal is self-clocking. This means that a separate clock does not need to be sent alongside the signal, but suffers from using twice the bandwidth to achieve the same data-rate as compared to non-return-to-zero format.

The "zero" between each bit is a neutral or rest condition, such as a zero amplitude in pulse amplitude modulation (PAM), zero phase shift in phase-shift keying (PSK), or mid-frequency in frequency-shift keying (FSK).

That "zero" condition is typically halfway between the significant condition representing a 1 bit and the other significant condition representing a 0 bit.

Although return-to-zero (RZ) contains a provision for synchronization, it still has a DC component resulting in “baseline wander” during long strings of 0 or 1 bits, just like the line code non-return-to-zero.

Ternary signal

In telecommunication, a ternary signal is a signal that can assume, at any given instant, one of three states or significant conditions, such as power level, phase position, pulse duration, or frequency.

Examples of ternary signals are (a) a pulse that can have a positive, zero, or negative voltage value at any given instant (PAM-3), (b) a sine wave that can assume phases of 0°, 120°, or 240° relative to a clock pulse (3-PSK), and (c) a carrier signal that can assume any one of three different frequencies depending on three different modulation signal significant conditions (3-FM).

Some examples of PAM-3 line codes that use ternary signals are:

hybrid ternary code

bipolar encoding

MLT-3 encoding used in 100BASE-TX Ethernet

B3ZS

4B3T used in some ISDN basic rate interface

8B6T used in 100BASE-T4 Ethernet

return-to-zero

SOQPSK-TG uses ternary continuous phase modulation3-PSK can be seen as falling between "binary phase-shift keying" (BPSK), which uses two phases, and "quadrature phase-shift keying" (QPSK), which uses four phases.

Underwater acoustic communication

Underwater acoustic communication is a technique of sending and receiving messages below water. There are several ways of employing such communication but the most common is by using hydrophones. Underwater communication is difficult due to factors such as multi-path propagation, time variations of the channel, small available bandwidth and strong signal attenuation, especially over long ranges. Compared to terrestrial communication, underwater communication has low data rates because it uses acoustic waves instead of electromagnetic waves.

At the beginning of the 20th century, some ships communicated by underwater bells, the system being competitive with the primitive Maritime radionavigation service of the time. The later Fessenden oscillator allowed communication with submarines.

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