OFDM



orthogonal frequency division multiplexing
Frequency division multiplexing (FDM) is a technology that transmits multiple signals simultaneously over a single transmission path, such as a cable or wireless system. Each signal travels within its own unique frequency range (carrier), which is modulated by the data (text, voice, video, etc.).
MIMO-OFDM
Multiple Input, Multiple Output Orthogonal Frequency Division Multiplexing is a technology developed by Iospan Wireless that uses multiple antennas to transmit and receive radio signals. MIMO-OFDM will allow service providers to deploy a Broadband Wireless Access (BWA) system that has Non-Line-of-Sight (NLOS) functionality. Specifically, MIMO-OFDM takes advantage of the multipath properties of environments using base station antennas that do not have LOS. According to Iospan,
“In this environment, radio signals bounce off buildings, trees and other objects as they travel between the two antennas. This bouncing effect produces multiple “echoes” or “images” of the signal. As a result, the original signal and the individual echoes each arrive at the receiver antenna at slightly different times causing the echoes to interfere with one another thus degrading signal quality.
The MIMO system uses multiple antennas to simultaneously transmit data, in small pieces to the receiver, which can process the data flows and put them back together. This process, called spatial multiplexing, proportionally boosts the data-transmission speed by a factor equal to the number of transmitting antennas. In addition, since all data is transmitted both in the same frequency band and with separate spatial signatures, this technique utilizes spectrum very efficiently.
Orthogonal FDM’s (OFDM) spread spectrum technique distributes the data over a large number of carriers that are spaced apart at precise frequencies. This spacing provides the “orthogonality” in this technique which prevents the demodulators from seeing frequencies other than their own. The benefits of OFDM are high spectral efficiency, resiliency to RF interference, and lower multi-path distortion. This is useful because in a typical terrestrial broadcasting scenario there are multipath-channels (i.e. the transmitted signal arrives at the receiver using various paths of different length). Since multiple versions of the signal interfere with each other (inter symbol interference (ISI)) it becomes very hard to extract the original information.

BBC / Proceeding of 20th International Television Symposium 1997 Explaining
some of the magic of COFDM Coded Orthogonal Frequency Division
Multiplexing (COFDM) [1, 2] has been specified for digital broadcasting
systems for both audio — Digital Audio Broadcasting (DAB) [3] and
(terrestrial) television — Digital Video Broadcasting (DVB-T) [4, 5, 6].
COFDM is particularly well matched to these applications, since it is very
tolerant of the effects of multipath (provided a suitable guard interval
is used).
Cisco Systems Overcoming
Multipath in Non-Line-of-Sight High-Speed Microwave Communication Links
Since the beginning of development of microwave wireless transmission
equipment, manufacturers and operators have tried to mitigate the
effects of reflected signals associated with signal propagation. These
reflections are called multipath. In real world situations, microwave
systems involve careful design in order to overcome the effects of
multipath. Most existing multipath mitigation approaches fall well short
of the full reliable information rate potential of many wireless
communications systems. This paper discusses how to create a digital
microwave transmission system that can not only tolerate multipath
signals, but can actually take advantage of them.
CSD Enabling Fast
Wireless Networks with OFDM Spread-spectrum technology gives
respectable data rates for many WLAN types, but for media-rich data, OFDM
could provide a better solution. Spread spectrum modulation has been the
basis for many proprietary and research 802.11-based WLANs. Through the use of
frequency hopping and direct sequence, these WLANs provide data rates from
1 to 11 Mbps. In addition, new activity within the research 802.11 committee
is considering a 22-Mbps version of direct sequence. Regardless of these
relatively high data rates, the demand for wireless broadband LANs and
MANs, is pushing the envelope on spread spectrum technologies. Because of
relatively inefficient use of bandwidth, spread spectrum systems will
probably not satisfy the even higher data rates that multimedia
applications require. In addition, multimedia applications operating
outdoors or within industrial environments require a wireless network
capable of operating more effectively in “RF hostile” areas.
Michael Speth OFDM
Receivers for Broadband-Transmission OFDM and the orthogonality
principle, The general problem: Data transmission over multipath channels,
Single carrier approach, Multi carrier approach, Orthogonal Frequency
Division Multiplexing, An OFDM receiver for DVB-T, Tasks of the inner
receiver and receiver structure, Channel estimation for OFDM, Performance
of the complete receiver…
Andrew McCormick
University of Edinburgh		 Interactive OFDM
Tutorial An introduction to Orthogonal Frequency Division
Multiplexing.
WAVE Report OFDM Tutorial
Frequency division multiplexing (FDM) is a technology that transmits
multiple signals simultaneously over a single transmission path, such as a
cable or wireless system. Each signal travels within its own unique
frequency range (carrier), which is modulated by the data. Orthogonal
FDM’s (OFDM) spread spectrum technique distributes the data over a large
number of carriers that are spaced apart at precise frequencies. This
spacing provides the “orthogonality” in this technique which
prevents the demodulators from seeing frequencies other than their own.
The benefits of OFDM are high spectral efficiency, resiliency to RF
interference, and lower multi-path distortion.
Wi-LAN OFDM Technology
Introduction, Messages, Carrier Waves, Modulation
research 802.16 Working Group on
Broadband Wireless Access Standards (wirelessman.org)		 Coded
Orthogonal Frequency Division Multiple Access (PDF) Coded Orthogonal
Frequency Division Multiple Access (COFDMA) technique. The tutorial
contains explanation on the OFDM basics and coverage options, including
Single Frequency Network (SFN) and its possible application for fixed and
mobile applications. Coding and multiplexing implementations, including
scenarios for multiple access and bandwidth on demand allocations.
Multiple access allocation using Reed Solomon series, which allows a
better Carrier Allocation.

Other Resources in OFDM

CSU High-Throughput,
High-Performance OFDM via Pseudo-Orthogonal Carrier Interferometry
Coding (PDF) OFDM is susceptible to poor probability of error
performance in fading channels. To enhance OFDM’s performance,
many architectures utilize channel coding. The addition of coding,
adds both redundancy and frequency diversity, but comes at a cost of
reduced overall throughput (typically by a factor of 2). This paper
introduces a novel carrier interferometry phase coding to enhance
performance in OFDM systems without bandwidth expansion or decreased
throughput. It is shown that at a bit error rate of 10 -3 , this
method gains 14 dB over OFDM, equaling the performance of COFDM. A
system is now available demonstrating the benefits of Coded OFDM,
which maintains the throughput of OFDM. The cost is one of increased
receiver complexity.
research research
802.16 Tutorial Frequency Domain Equalization for 2-11 GHz
Broadband Wireless Systems. In this tutorial we survey recent
advances in frequency domain equalization (FDE) for single carrier
(SC) systems. SC modulation systems have lower peak-to
average-ratios than OFDM, and when combined with FDE, their
performance is at least as good as OFDM systems (in some cases
better); furthermore, they have the same reduced signal processing
complexity enjoyed by OFDM systems.
research
802.16 Tutorials 802.16 MAN documents.
research 802.16 Working Group on
Broadband Wireless Access Standards (wirelessman.org)		 Frequency
Domain Equalization for 2-11 GHz Broadband Wireless Systems (1/2001,
304Kb, PDF) Broadband wireless systems deployed in outdoor
non-line of sight environments may encounter delay spreads of over 5
to 10 us – which can cause potential intersymbol interference over
50 or more data symbol intervals. OFDM (orthogonal frequency
division multiplexing) has been suggested to combat this ISI problem
with reasonable complexity. However OFDM systems generate high
transmitted peak-to-average ratios and are sensitive to phase noise;
this can increase RF subsystem cost and complexity.
mobilecomms-technology T-Mobile
FLASH-OFDM Mobile Broadband Network, Slovakia In October 2005
T-Mobile launched a new and faster mobile broadband internet access
network in Slovakia. The network is Europe’s first commercial mobile
broadband service (also a world first) and uses Flarion
Technologies’ (now acquired by Qualcomm Inc. for $600 million) Fast,
Low-latency Access with Seamless Handoff – Orthogonal Frequency
Division Multiplexing (FLASH-OFDM) network technology.