Carrier-less amplitude / phase modulation
Carrierless Amplitude / Phase Modulation (CAP) is a line code and non-standardized variation of quadrature amplitude modulation (QAM) for the transmission in the baseband of digital signals on wired transmission links such as subscriber lines .
function
If the bandwidth of a signal to be transmitted is in the order of magnitude of the carrier frequency , which is typically the case with transmission in the baseband, it is possible to avoid the multiplicative mixer stage for frequency conversion from the baseband to the carrier frequency range, which is essential for quadrature amplitude modulation (QAM) . Instead, the band-pass signal is generated in the baseband position in the pulse-shaping filter extended by CAP , in which the carrier component is integrated into the filter transfer function. Since there is no carrier for frequency conversion , this modulation method is called carrierless .
A CAP transmitter, as shown in the simplified sketch opposite, is formed by the two pulse shaping filters for the complex baseband signal sequence ( A k and B k ) for the I and Q components. The two supplied baseband signal sequences are also referred to in the English-language literature as I for English in-phase component and Q for English quadrature component . The two pulse shaping filters consist of the part of the transfer function g (t) for pulse shaping, usually a root-raised cosine filter to ensure minimal intersymbol interference , and the orthogonally superimposed carrier components, each expressed by a cos (•) and sin (•) Term in the filter transfer function. The carrier frequency ω c occurring in this case is of the order of magnitude of the baseband signal and is only generated when the filter coefficients are calculated.
The transmission from the transmitter to the receiver takes place in the baseband position over the transmission channel , which limits CAP applications to wired transmission links, and is split into the two I and Q components in the receiver shown on the right with two optimal filters . The discrete sampling of the two signal components then takes place by means of a sample-and-hold circuit with the symbol rate T in order to obtain the discrete-time complex reception sequence A ' k and B' k at the output.
As with QAM, the data to be transmitted are assigned to appropriate "symbols" and agreed between the transmitter and receiver, which consist of different phase positions and amplitudes of the baseband signal and can be displayed in a constellation diagram. For example, with the CAP16 with 16 different symbols per symbol, an information content of 4 bits can be transmitted at the same time.
Applications
In the early days of ADSL , CAP was used for broadband access via subscriber lines. For this purpose, the frequency band is divided into three parts: one between 0 kHz and 4 kHz for the analog transmission of voice telephony ( POTS ), one between 25 kHz and 160 kHz for the upstream and the range between 240 kHz and 1.5 MHz for the downstream . The modulation technology of ADSL has been replaced by the standardized discrete multitone (DMT) modulation method that is common today . CAP continues to be used in the area of ATM on the bit transmission layer, with mostly comparably low bit rates below around 100 Mbit / s.
literature
- Frank Sjöberg: Research report: A VDSL Tutorial . Lulea Tekniska Universitet: Institutions för Systemteknik, 2000, Chapter 5: Line Codes ( PDF ).
- JJ Werner: Tutorial on Carrierless AM / PM - Part I and Part II . AT&T Bell Laboratories, company publication, Austin, Texas 1993 (Contribution to ANSI X3T9.5 TP / PMD Working Group).