Modulation Techniques

Modulation is the process in which a signal is entered into and transmitted by a signal carrier. There are several ways signals can be modulated and differ based on the equipment and technique utilized. Three common ways that signals are transmitted include a 56K modem, an asymmetrical digital subscriber line (ADSL), and Wireless Fidelity (Wi-Fi). Each of these methods are effective means of transmitting a digital signal, however, the 56K modem is all but phased out in favor of Wi-Fi which has evolved by way of ADSL.
The 56K modem offers signal modulation at 56 Kbps over a telephone network. The maximum available speed with a 56K modem is actually 64 Kbps and requires a nearly flawless digital connection at the transmitting and receiving end which is seldom available. The signal originates from the user’s Internet Service Provider’s office (usually as a digital signal) and is then converted to an analog signal by the telephone company. It is then the job of the user’s modem to convert the analog signal from the telephone company into a digital signal. The modem accomplishes this task by reading the amplitudes of the signal as an 8-bit symbol and uses an 8 kHz clock to time the signal as it is received in order to decode the data (Lawyer, 2007). As telephone companies increased their supply of internet access the demand for faster signal transmission opened the door for ADSL.
Voice transmission over an ADSL only uses about 3 kHz of the over 1 MHz of available bandwidth. The unused bandwidth is where extremely fast data transmission is possible. ADSL uses carrierless-amplitude phase (CAP)to modulate signals over the line. It separates the line into three channels; upstream access, downstream access, and voice. Another method of modulation is performed with discrete multitone (DMT) in which the line is divided into 256 subchannels each having 4.3125 kHz of bandwidth utilizing quadrature amplitude modulation (QAM) technology. DMT offers faster data transmission, more efficient bandwidth allocation, and less power consumption compared to CAP (Hin & Subramaniam, 2012). While ADSL offers high speed data transmission Wi-Fi offers the same but without the hassle of wires.
The most popular method of modulation in Wi-Fi networks is orthogonal frequency division multiplexing (OFDM) which combines modulation and multiplexing techniques. In OFDM the transmission carrier is divided into several subchannels much like with ADSL. The carriers are spaced in a manner so that they are orthogonal (at right angles) so as not to interfere with one another. An inverse Fast Fourier Transform creates the sent signal and a Fast Fourier Transform restores the received signal. Up to 300 Mbps per carrier can be achieved when OFDM is paired with 64QAM techniques (Frenzel, 2012). References

Frenzel , L. E. (2012). Unlock spectral efficiency with digital modulation. Electronic Design, 60(2), 46-53.

Hin, L. T. W., & Subramaniam, R. (2012). Asymmetric digital subscriber line. Retrieved from http://encyclopedia.jrank.org/articles/pages/6554/Asymmetric-Digital-Subscriber-Line.html on December 5, 2012.

Lawyer, D. S. (2007, January). Modem-howto. Retrieved from http://www.tldp.org/HOWTO/Modem-HOWTO-21.html on December 5, 2012.