Modulation Techniques

Modulation techniques used in 56K modem, Asymmetrical Digital Subscriber Line, and Wi-Fi.

With modulation being a technique that is used in order to transmit information, either digital or analog by encoding the information onto analog carrier signals. Modulation has been used in modems in order to be able to transmit digital data signals over an analog transmission line such as the public telephone network. Modems at the end of the communication link perform the digital-to-analog and analog-to-digital conversion. The information signal is than modulated onto an analog signal with a specific frequency that is called the carrier.

Asymmetrical digital subscriber line (ADSL) supports 8Mbps in the down link direction and up to 1 Mbps in the up link direction. The line code that is used for the ADSL interface is the Discrete Multitone (DMT) code. The DMT line code had been designed to provide a match to the twisted-wire pair channel. One of the characteristics of the twisted-wire pair channel is the variation of the transmission functions versus the frequency within range of interest. Overcoming the variation the Discrete Fourier Transform (DFT) techniques have been used in order to decompose the modulated signal into various orthogonal signals, which are located in accordance with ITU-Y Rec. G. 992.1 standard, with downstream signals decomposed into 256 discrete subcarriers, and the upstream signals decomposed into 32 subcarriers. The subcarriers modulation methods are a form of quadrature amplitude modulation (QAM). With the high bandwidths using QAM, the subcarriers are able to use their own frequency band, making it possible to process the subcarriers independently of other subcarriers, in order to compensate in an optimal way for group delay, as well as intersymbol interference (ISI).

Wi-Fi stands for Wireless Fidelity and was developed on IEEE 802.11 standards and is used in today's technology of wireless communication. Wi-Fi provides wireless internet access of data across a radio network. Wi-Fi gives the sender and receiver a number of ways to connect together and receive Direct Sequence Spread Spectrum's (DSSS), Frequency Hopping Spread Spectrum (FHSS), Infrared & OFDM. Modulation techniques for Wi-Fi are known as Direct Sequence Spread Spectrum's (DSSS). The technique utilizes a group of frequencies that are 22MHz wide, also known as channels. The devices that utilize other modulation techniques, such as the Frequency Hopping Spread Spectrum (FHSS) uses frequencies that are 1MHz wide, but they "hop" according to the predefined sequences across different channels. Mingling the DSSS system with the FHSS system isn't a perfect solution because it produces less interference than two DSSS systems in the same physical space.