Orthogonal Frequency Division Multiplexing
Course Presenter
Prof fred harris
fred harris (sic) teaches at San Diego State University where occupies the
CUBIC Signal Processing Chair. His teaching and research areas include, Digital
Signal Processing, Multirate Signal processing, Communication Systems, Source
Coding and Modem Design. He has extensive practical experience in communication
systems, high performance modems, sonar and advanced radar systems and high
performance laboratory instrumentation. He holds a number of patents on
Multirate Signal Processing for Satellite and Cable Modems and lectures
throughout the world on DSP applications. He consults for organizations
requiring high performance, cost effective DSP solutions and has contributed to
a number of textbooks and handbooks on various aspects of signal processing.
Mode of Delivery
TBA
Course Summary
Orthogonal Frequency Division Multiplexing, also called Discrete Multi-tone, is
a modulation process that delivers digital data to a channel as a parallel set
of low rate, low bandwidth, and extended-duration time waveforms. By virtue of
the extended time duration, the composite signal is insensitive to short time
duration channel impairments as well as to channel dispersion. Consequently
OFDM has become a prime contender for delivery of high data rate signals
through dispersive channels such as the mobile wireless, and copper channels.
Europe has embraced OFDM for digital Audio Broadcast (DAD), and Digital Video
Broadcasting (DVD_T), DMT has become the choice of the Regional Bell Operating
Systems for high-speed copper loop transmission, and OFDM is the modulation
selected by a number of high data rate Wireless LANs.
Consider the following perspective. In a standard QAM communication system, the
bit stream is delivered sequentially by a series of translated and overlapping
but mutually orthogonal Nyquist Pulses. The Nyquist pulses occupy a common
rectangular bandwidth but maintain mutual orthogonality by residing on each
other's zeros crossings. An OFDM system simply reverses the roles of time and
frequency. The bit stream is delivered in parallel by many simultaneous tone
bursts occupying a common rectangular time interval, which when observed in the
frequency domain are seen to be translated Nyquist shaped spectra residing on
each other's zero crossings. The many QAM modulated sinusoids are presented to
the channel as a parallel set of narrowband, long duration tone bursts which
are less sensitive to channel dispersion.
This presentation introduces participants to the essential elements of an OFDM
communication system. The OFDM signal is synthesized at the transmitter by an
Inverse Discrete Fourier Transform (IDFT) and is analyzed at the receiver by a
Forward Discrete Fourier Transform (DFT). We examine the OFDM modulation and
demodulation processes as well as standard timing and carrier acquisition
processes. We also examine the and address techniques to combat the affects of
non linear distortion due to amplifiers, of linear distortion such as timing
and carrier offsets, and implementation effects such as I-Q gain and phase
mismatches. We also examine a number of techniques to aid the receiver and to
combat the channel. These include guard intervals, cyclic extensions,
preambles, pilot probes, interleaving, channel coding, and frequency and time
diversity.