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Modulation is the technique of varying aspects of a single, pure frequency called a carrier frequency, to represent different numbers. The aspects of the carrier frequency: its amplitude (volume), frequency (pitch), and/or phase (relative position in time), are changed in fixed steps, called keying in days long past, shifting between fixed choices.
We'll begin with the simplest method, Amplitude Shift Keying, see how FSK is an improvement, PSK is better and QAM used in most modems today combines phase and amplitude shifting to create multiple possible combinations. Each combination is called a signal.
You'll learn that each possible combination can represent a group of bits, the number of bits determined by the number of combinations QAM-16 has sixteen signals, so each signal can represent a different group of 4 bits.
And, no discussion of modems would be complete without pointing out that the baud rate is the number of times the signal is changed per second. The bit rate is the baud rate times the number of bits indicated by each signal. In a QAM-16 system, the bit rate is four times the baud rate.
Telecom 101 Module 16
Detailed Outline
16 Modems and Modulation
.... 16.1 Modulation of Carrier Frequencies
.... 16.2 Amplitude Shift Keying (ASK)
.... 16.3 Frequency Shift Keying (FSK)
.... 16.4 Phase Shift Keying (PSK)
........... 16.4.1 Baud Rate vs. Bit Rate
.... 16.5 Quadrature PSK (QPSK)
.... 16.6 Quadrature Amplitude Modulation (QAM)
.... 16.7 Constraints on Achievable Bit Rate
저자 정보
Eric Coll is an international expert in telecommunications, broadband and networking, and has been actively involved in the telecom industry since 1983. He holds Bachelor of Engineering and Master of Engineering (Electrical) degrees.
Mr. Coll has broad experience, and broad knowledge of telecom developed working as an engineer in the telecommunications industry.
He has used his knowledge of telecom to develop and teach telecommunications technology training seminars to wide acclaim across North America since 1992... and answering questions at seminars for companies and organizations ranging from Bell Labs to the Department of Justice keeps things up to date.
In his spare time, Mr. Coll authors textbooks and online courses based on the latest updates to the seminar courses.
Mr. Coll also provides consulting services as a Subject Matter Expert in telecommunications to government, carriers, and their customers.
