®
White Paper
Implementing OFDM Using Altera Intellectual Property
With the high integration of Altera’s programmable logic devices (PLDs), designers can significantly reduce
time-to-market by instantiating parameterizable signal processing intellectual property (IP) functions in
APEX ™ 20KE devices—creating a system-on-a-programmable-chip (SOPC) solution. Signal processing IP
MegaCore® functions can be used to build orthogonal frequency division multiplexing (OFDM) communication
systems for digital video broadcast transmitters, multichannel multipoint distribution service (MMDS) base-stations,
and wireless LAN modems.
Using the OpenCore feature, designers can instantiate and simulate designs prior to licensing. To download Altera®
MegaCore functions, go to the IP MegaStore on Altera’s web site (http://www.altera/IPmegastore.com).
This white paper describes OFDM, OFDM applications, a complete OFDM system using APEX architecture, and the
implementation of an OFDM solution using Altera devices and IP MegaCore functions.
OFDM Basics
OFDM is a multi-carrier modulation scheme that encodes data onto a radio frequency (RF) signal. Unlike
conventional single-carrier modulation schemes—such as AM/FM (amplitude or frequency modulation)—that send
only one signal at a time using one radio frequency, OFDM sends multiple high-speed signals concurrently on
specially computed, orthogonal carrier frequencies. The result is much more efficient use of bandwidth as well as
robust communications during noise and other interferences.
Frequency division multiplexing (FDM) theory states that aggregate bandwidth is divided into several subchannels,
spaced with guard bands to reduce interference, each of which is transmitted simultaneously. Examples of FDM
systems include cable television and analog radio broadcast, i.e., the receiver must tune-in to the proper station. See
Figure 1.
M-WP-IPOFDM-01
March 2001, ver. 1.0
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Altera Corporation
Implementing COFDM Using Altera IP White Paper
Figure 1. Frequency Division Multiplexing (FDM)
−ω 1
ω1
Magnitude
−ω 2
ω2
−ω3
−ω3
ω3
−ω1 −ω2
ω2
ω1
ω3
Frequency
OFDM systems require significantly less bandwidth than traditional FDM systems. Through the use of special noninterfering orthogonal carriers, guard bands are not required between individual subchannels—allowing the available
spectrum to be used more efficiently. In addition, OFDM multi-carrier modulation allows dynamic allocation of bits
to subchannels. To maximize throughput, the multi-carrier modulator can intelligently assign more bits to
subchannels experiencing less channel noise.
Using OFDM to combat inter-symbol interference (ISI) and inter-channel interference (ICI) is not new. However,
practical implementation of OFDM has been historically limited to the speed and efficiency of the fast Fourier
transform (FFT) function. High-performance PLDs have enabled modern OFDM systems.
Figure 2 shows OFDM modulation as the superposition of several independently modulated subchannels, each
modulated with a single-carrier modulation scheme.
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Altera Corporation
Implementing COFDM Using Altera IP White Paper
Energy
Figure 2. OFDM Modulation for European DVB-T
64 QAM
64 QAM
64 QAM
64 QAM
64 QAM
No
Data
64 QAM
64 QAM
Subband
0 MHz
1 MHz
Frequency
Modern single-carrier modulation methods—such as quadrature amplitude modulation (QAM) or quadrature phase
shift keying (QPSK)—combine basic amplitude modulation, phase modulation, and frequency modulation techniques
to offer higher noise immunity and better system throughput. Utilizing increasingly complex modulation techniques
requires high-performance digital logic, but also allows system architects to achieve higher SNR and spectral
efficiency approaching the Shannon limit. See Figure 3.
Figure 3. Modern Modulation Schemes
100
Bits/Sec Per Hz
10
1024-QAM
Shannon Capacity
64-QAM
256-QAM
16-QAM
8-PSK
QPSK
1
0.1
-5
0
5
10
15
20
25
30
35
Signal-to-Noise Ratio Per Bit in dB
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Altera Corporation
Implementing COFDM Using Altera IP White Paper
OFDM Applications
Recently, OFDM has been adopted into several European wireless communications applications such as the digital
audio broadcast (DAB) and terrestrial digital video broadcast (DVB-T) systems. In the United States, OFDM has
been adopted in multipoint multichannel distribution services (MMDS). Both wireless LAN applications—using
standards such as IEEE 802.11a—and the new European Telecommunications Standard Institute’s (ETSI)
HiperLAN/2 specification have also installed OFDM as the modulation scheme. Wired applications have already
implemented OFDM-based systems as discrete multitone (DMT) systems in xDSL and cable modem applications.
See Figure 4.
Layer 1
LLC
MAC
Physical
Layer
Digital Audio Broadcast (DAB)
Data
Link
Layer
HiperLAN/2
Layer 2
Terrestrial Digital Video Broadcast (DVB-T)
Network
Layer
BWIF
Layer 3
802.16
Application
Layer
802.11a
Layer 7
802.2
Figure 4. OFDM Technologies Mapped to the OSI Model
Rollout of OFDM systems has just started to intensify, and the adoption of OFDM in the PHY layer for several
different wireless standards is eminent. AT&T’s fixed wireless residential broadband service is built around OFDM,
and is projected to serve over 15 million homes by the end of 2002. AT&T and Nortel Networks are considering the
feasibility of fourth-generation wireless networks, with EDGE proposed as the uplink, and OFDM suggested as the
downlink. Motorola recently unveiled new wireless home-networking solutions built around the HomeRF wireless
proposal, which also uses OFDM in the PHY layer.
In the home networking space, working groups such as HomeRF and HomePlug have adopted OFDM multi-carrier
modulation.
Key advantages to the adoption of OFDM in the PHY layer for these applications include simplified equalization for
narrowband channels, high system throughput, and immunity to noise.
APEX Implementation Advantages for OFDM Systems
Altera’s high-performance, high-density APEX 20KE devices are ideally suited to implement OFDM systems. The
abundance of memory within embedded system blocks (ESBs) is a key feature for implementing memory-intensive
functions such as FFT, as well as for buffering intermediate signals within the datapath.
Compared with the time-multiplexed instructions of DSP systems, OFDM systems implemented in APEX 20KE
devices benefit from dedicated hardware resources that perform the forward error correction (FEC) coding,
modulation, and filtering—within a single device. Using APEX 20KE devices, throughput rates can be effectively
multiplied—limited only by the density of the device. See Figure 5.
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Implementing COFDM Using Altera IP White Paper
Figure 5. Time-Shared DSP Operation Vs. Hardware Dedicated PLD Operation
Traditional DSP Processor
fx
DSP
Engine
fx
Programmable Logic Device
fx
fx
fx
fx
fx
fx
fx
fx
fx
fx
fx
fx
fx
fx
fx
fx
fx
fx
fx
fx
fx
fx
fx
fx
fx
fx
Memory
fx
Sequential (Serial) Operation
n Clocks
Parallel Operation
1 Clock
For example, the Altera Reed-Solomon MegaCore function decodes at a rate of 800 Mbps for 8-bit symbols. For
systems requiring higher throughput, MegaCore functions can be instantiated using dedicated hardware in parallel.
For nominal buffering and control overhead, the Reed-Solomon MegaCore function can decode at a rate of over 3
Gbps. By comparison, preliminary Texas Instruments benchmarks of the C64xx DSP processor requires
approximately 1095 cycles to decode one Reed-Solomon code word. At 300 MHz, the C64xx processor is able to
decode approximately 450 Mbps, using 100% of the processing power available in the device.
The flexibility of Altera PLDs allows designers to adapt to new standards quickly and effectively. The APEX device
family and signal processing IP MegaCore functions combine to form an ideal solution compared with the relative
inflexibility of application specific standard products (ASSPs) and the high risk of ASICs. See Figure 6.
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Altera Corporation
Implementing COFDM Using Altera IP White Paper
Figure 6. The Benefits of Altera Signal Processing IP Solutions
Low performance
High performance
Multi-chip solution
Single chip, low power
solution
Integration issues
Highly flexible & scalable
Time-shared hardware
Low risk
Fast time-to-market
Dedicated hardware
DSP
Processor
ASIC migration path
Altera IP Solution
Flexibility
ASSPs
ASICs
Performance
Standard performance
Highest performance
Inflexible
New design each time
Integration issues
Inflexible
High risk
Extended time-to-market
Figure 7 shows the typical elements of an OFDM transmitter. Systems using OFDM modulation utilize channel
coding to combat multipath propagation. Data symbols are then mapped onto an appropriate constellation (i.e.,
QPSK, QAM). The resulting I and Q values are stored in a buffer, and the inverse fast Fourier transform (IFFT) is
applied. The IFFT performs the modulation on orthogonal carriers. The data is then prepared for transmission, i.e.,
serialized and appended with a cyclic prefix for multipath immunity. The resulting data is then sent to an antenna for
transmission.
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Altera Corporation
Implementing COFDM Using Altera IP White Paper
Figure 7. Typical OFDM Transmitter Block Diagram
Start
Forward
Error
Correction
Encoder
Constellation
Mapper
Interleaver
Inverse
Fast
Fourier
Transform
Parallel
to
Serial
Cyclic
Prefix
Insertion
Serial to
Parallel
Shaper
FIR
Filter
DAC
Altera IP Solution
Programmable Logic Solution
Non-Altera Solution
Altera MegaCore OFDM Solution
This section explains the main components of the Altera OFDM solution.
Forward Error Correction
Channel coding, using Reed-Solomon or convolutional error correction code is implemented using the Altera
Reed-Solomon MegaCore function or convolutional encoder. OFDM systems using forward error correction (FEC)
techniques are also referred to as coded-OFDM (COFDM) transmitters. To ensure that transmission errors can be
detected and corrected at the receiver level, FEC codes add error correction bits to the data stream. Altera's signal
processing FEC cores include high-performance encoding and decoding for Reed-Solomon, convolutional, Viterbi,
and turbo codes.
Interleaver
An interleaver is used to help reduce burst errors within the data channel. Altera’s symbol interleaver/deinterleaver
MegaCore function provides easy customization and quick instantiation into the design. The interleaved data is then
passed through a serial-to-parallel converter, which maps the symbols onto an IQ constellation specific to the
modulation scheme.
Constellation mapper
Multi-carrier OFDM systems are considered superior to n-many independent subbands, each modulated by a
single-carrier modulation technique. The constellation mapper takes symbols as inputs and maps them to appropriate
constellation points as dictated by the modulation method specified. This process generates I and Q values which are
then filtered and sent to the IFFT for transformation. Altera high-performance PLDs can implement constellation
mapper functionality in user logic.
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Altera Corporation
Implementing COFDM Using Altera IP White Paper
Buffer
A buffer is required to store the I and Q values before they are sent to the IFFT. Functions from the library of
parameterized modules (LPM) include LPM_RAM functions that can easily be parameterized for the specific
application.
IFFT
The FFT is a fast, efficient implementation of the discrete Fourier transform (DFT) function and mathematically
generates the orthogonal carriers required for OFDM transmission. The Altera IFFT/FFT MegaCore function
supports a variable data-width, number of points, as well as flexible memory interfaces.
At the heart of the OFDM transmitter is the inverse fast Fourier transform (IFFT) function. The IFFT modulates each
sub-channel onto a precise orthogonal carrier. The channelized data is then fed through a parallel-to-serial buffer,
easily implemented as custom logic or by using the standard LPM_SHIFTREG function. The resulting serial data is
then passed through a DAC and is ready for transmission.
The advantage of the Altera signal processing OFDM solution is that each of the functional blocks of the OFDM
transmitter can be mapped onto dedicated, parallel hardware resources within the PLD—avoiding the difficult
programming and optimization challenges of scheduling time-critical operations through a single DSP device.
Parallel-to-Serial Convertor
After the data is transformed via the FFT function, it must be serialized before transmission. The LPM_SHIFT_REG
function can be used to serialize the data.
Cyclic Prefix
The addition of a cyclic prefix creates a guard band around individual OFDM symbols, greatly reducing ISI while
trading off a marginal loss in the signal-to-noise ratio. After serialization, user logic can be designed to append the
last part of the OFDM symbol creating a cyclic prefix.
Shaper FIR Filter
Necessary in any wireless or wired digital communications design, digital filters help shape the signal. Altera's
next-generation finite impulse response (FIR) compiler MegaCore function allows a variety of different filters to be
built, and supports variable coefficient filtering, as well as interpolation and decimation.
Summary
The high integration of Altera’s APEX devices and signal processing IP cores offer a complete system solution
including parallel processing, and the flexibility and time-to-market advantages of programmable logic.
®
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Copyright  2001 Altera Corporation. Altera, APEX, APEX 20KE, MegaCore, and specific device designations are trademarks and/or service
marks of Altera Corporation in the United States and other countries. Altera acknowledges the trademarks of other organizations for their
respective products or services mentioned in this document. Altera products are protected under numerous U.S. and foreign patents and
pending applications, maskwork rights, and copyrights. Altera warrants performance of its semiconductor products to current specifications
in accordance with Altera’s standard warranty, but reserves the right to make changes to any products and services at any time without notice.
Altera assumes no responsibility or liability arising out of the application or use of any information, product, or service described herein except
as expressly agreed to in writing by Altera Corporation. Altera customers are advised to obtain the latest version of device specifications before
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