Phase modulation

Phase modulation is a method by which an analog or a digital signal is transmitted over a communication channel. Phase modulation is closely related with the frequency modulation. Both modulations belong to the group of angle modulation methods.

Analog phase modulation

The modulated transmit signal may be represented generally by a transmitting frequency for the phase modulation, the frequency of which only varies to a certain extent, when the useful signal to be transmitted varies with time. Due to this change in frequency, a phase shift of the broadcast signal is achieved to the original transmit frequency. Is fs time constant, the transmit frequency is output. Mathematically, this relationship follows with arbitrary real constant k describe:

Is a factor which indicates how much the phase of the transmission signal is set to change in dependence on the useful signal, and is a kind of phase modulation index. The term describes the time derivative of the useful signal to be transmitted. The modulated transmit signal is then calculated as:

The second term can be thought of intuitively as follows: the instantaneous values ​​at certain times of the useful signal adjust quasi phase angle of the cosine function, which is also the name of this type of modulation is derived.

Practical Applications

In contrast to the analog frequency modulation, the analog phase modulation obtained hardly significant practical applications. The reason is that the receiver must keep to receive the correct phase position of the original transmitted signal, so as with digital methods at the beginning of a transmission, a kind of synchronization and equality of the reference phase is necessary. Keeping this reference phase at the receiver constant is complex and not particularly stable analog circuit technology, while at the frequency modulation simply the error signal of a PLL can be used as a demodulator at the receiver without much effort. Therefore, the phase modulation obtained when only the digital transmission method, where the synchronization and demodulation by means of a Costas loop can be achieved, essential for practical application.

Digital phase modulation

The designations for digital modulations originate from their properties at the sampling instants at the receiver side. Here, the property is named, respectively, which may have changed from sampling to sampling in the received signal: phase, frequency, amplitude, or combinations thereof.

Keying means (re) key ( "Key" is also the name for the Morse key. ) And points out that the digital modulation is considered only at the sampling instants. Therefore, even casually spoken of it at this modulation method, the amplitude "jumps" or the phase " jumps ".

For digital signals, it is called phase shift keying, Eng. Phase Shift Keying. The phase of a sine wave (carrier ) is modulated by phase shift. One speaks of binary phase modulation, BPSK (also 2 -PSK ) when switched between two phase angles ( keyed ). Typically, the phase positions 0 ° and 180 ° correspond to the binary states "0" and "1".

In a multistage phase modulation, a symbol representing a sequence of several bits. For 4- PSK (or QPSK) are transmitted per symbol 2 Bit, 8- PSK symbol per 3 bits, etc.. 4 -PSK is used for example for the transmission of facsimiles via the telephone network. In the GSM extension EDGE ( Enhanced Data Rates for GSM Evolution ), for example, 8 -PSK is used, and thus tripling the gross transmission rate compared with a binary modulation.

Shown as a pair of spectral lines in two side complex frequency spectrum ( see Figure ), the phase modulation causes a rotation of the carrier around the frequency axis. In PSK you can ask the carrier by selecting one of the two phase angles (0 ° and 180 ° ) point to two points. With 4- PSK are the four points (45 °, 135 °, 225 ° and 315 ° ) and 8- PSK eight. The points are evenly distributed on a circle.

Is combined with phase modulation, amplitude modulation, then quadrature arises amplitude modulation (QAM ), which is a combination of ASK ( amplitude shift keying or amplitude shift keying ), and PSK. In the frequency range corresponding to an additional modulation of the length of the spectral lines.

Phase modulation can occur simultaneously on multiple frequencies (COFDM, Discrete Multitone ).

With the phase difference modulation, Differential Phase Shift Keying ( DPSK ), the bits encoded by the change in phase. For example, change of the phase around 0 ° encoding 0, changing the phase by 180 ° first encoding

Symmetrical with differential phase shift keying ( SDPSK ) the phase shift is symmetric. A positive phase of 90 ° corresponds to bit 1, a negative phase of 90 ° to the bit 0

Examples

Fax tone (see text)? / I

The musical is the answer a fax when it is called. The first signal is a pure sine wave, the more times a popping noise is superimposed. It is a phase shift of 180 °, see Fig. You can transfer information from exactly one bit. Therefore it as binary phase shift (binary phase shift keying) is called.

For a phase shift of 90 °, 4 different states can be encoded: 0 °, 90 °, -90 °, and 180 ° ( quadrature phase - shift keying or quaternary phase - shift keying or QPSK). At multiples of 45 °, there are 8 states and 3 bits ( octal phase - shift keying or QPSK ). Generally we speak of multiple phase - shift keying or MPSK.