Orthogonal Frequency Division Multiplexing (OFDM) is a technique for transmitting large amounts of digital data over a radio wave The technology works by splitting the radio signal into multiple smaller
sub-signals that are then transmitted simultaneously at different
frequencies to the receiver. OFDM reduces the amount of crosstalk in
When we talk about
broadband wireless access, most of the action
takes place at the Media Access Control (MAC) layer (layer 2) and the
Physical (PHY) layer (layer 1 or the Air Interface).
There are numerous types of broadband wireless air interfaces
including single carrier, Orthogonal Frequency Division Multiplexing (OFDM),
and Orthogonal Frequency Division Multiple Access (OFDMA). Others are
Wideband Code Division Multiple Access (WCDMA) a cellular 3G technology,
and Universal Mobile Telecommunications System (UMTS) also cellular 3G.
With the advent of
WiMAX, the terms OFDM and OFMDA, scalable OFDMA (sOFDMA),
and Flarion's alternative version
of OFDMA, Flash OFDM, have all become buzzwords.and subject to the
standards process. Other terms such as Fast Fourier Transform (FFT),
Time Division duplex (TDD), and Frequency Division Duplex (FDD) modes
play a part in the various flavors of this modulation scheme.
The marketplace today seems to have decided that OFDM (or OFDMA)
offers real advantages for broadband wireless transport. The
WiMAX Forum has clearly focused
on these technologies. The topics are, of course, very complex and in this
article we will only be able to provide an overview. If one any
particular aspect of this discussion is relevant to you, we recommend
that you conduct further research.
Before we delve into the arcane minutia of what are essentially
subtle differences in OFDMA, let's go over the history of the topics
Key Terms To
Short for Orthogonal Frequency Division Multiplexing, an FDM
modulation technique for transmitting large amounts of digital data
over a radio wave. OFDM works by splitting the radio signal into
multiple smaller sub-signals that are then transmitted
simultaneously at different frequencies to the receiver.
Commonly referred to as WiMAX or less commonly as WirelessMAN. or
the Air Interface Standard, IEEE 802.16 is a specification for fixed
broadband wireless metropolitan access networks (MANs) that use a
The Media Access Control Layer is one of two sublayers that make up
the Data Link Layer of the OSI model. The MAC layer is responsible
for moving data packets to and from one Network Interface Card (NIC)
to another across a shared channel.
We are all used to the term
Wi-Fi, which generally refers to the
802.11a/b/g/n family of standards. However, 802.11 standards were
written for indoor wireless networks. Many vendors built proprietary MAC
and PHY systems that extended these capabilities to outdoor networks.
Some of these systems used a single carrier. Several leveraged OFDM
capabilities. Others chose WCDMA or
UMTS approaches. But the idea was to
create effective outdoor networks. This was and is a very fragmented
Enter the 802.16 movement, which sought to define a proper
metropolitan area network (MAN) standard for broadband wireless or
This standard has evolved into two standards: One delivers fixed
broadband wireless (802.16-2004) and another delivers mobile broadband
wireless (802.16e). Interestingly, both support multiple PHY modes, none
of which include WCDMA or UMTS.
|OFDM 256 FFT
||OFDM 256 FFT
|OFDMA 2048 FFT
||OFDM 2048 FFT
||sOFDMA 1024 FFT
||sOFDMA 512 FFT
||sOFDMA 128 FFT
The WiMAX Forum chose the OFDM 256 FFT mode for the first fixed WiMAX
product profile. The first product profiles for mobile WiMAX have yet to
be chosen as the standard is not yet ratified. However, it appears some
version of OFDMA will get the nod, which brings us to why it makes sense
to understand a bit about OFDMA.
There is a third flavor of OFDMA competitive to WiMAX called Flash
OFDM that Flarion uses which is also very similar, but more on that
There could ultimately be WiMAX product profiles that have the same
PHY mode for both fixed and mobile. For example, some vendors believe
there will ultimately also be an OFDM 256 FFT mode for 802.16e. The rule
is that three vendors must agree on the product profile for the Forum to
define a product profile for interoperability testing. Many mobility
proponents seem to prefer an OFDMA version. In any event, product
profiles with different modes will not be interoperable. Also, profiles
of modes at a given FFT size (512 for example) will not interoperate
with the different fixed FFT size mode of 2048.
So what does OFDMA accomplish? In simplified terms, the OFDMA mode
attempts to optimize mobile access by many simultaneous users through
breaking a signal into sub-channels. Some camps believe OFDM can
accomplish this as well as and cheaper than alternatives. Others believe
OFDM is best suited for simple mobility or portability. Sub
channelization was added to OFDM on the uplink and downlink technology
but ultimately rejected by the IEEE 802.16 working group. Whatever the
relative merits for mobility, the two modes are essentially very
Much in common,
among the differences
The OFDM modulation scheme offers many advantages for broadband
wireless transport. It supports high data rates. The design not
only obviates multipath interference (where reflected signals
return slightly out of phase, creating interference at the
receiver) it can actually utilize multipath to increase signal
quality by processing the reflected packets to increase gain.
This technique also improves non-line of sight delivery. It
supports both TDD and FDD, the latter of which provides
symmetrical data delivery.
The good news is that for most practical purposes, the terms
sOFDMA and OFDMA can be used interchangeably as they are so
similar. Both support sub channelization as a key technology.
Flash OFDM is a bit different.
Sub channelization allows all four variations (OFDM, Flash
OFDM, sOFDMA and OFDMA) to split channels up into sub channels,
even into several thousand sub channels. Essentially, a user on
an OFDMA network is assigned a number of sub channels across the
band. A user close to the base station would normally be
assigned a larger number of channels with a high modulation
scheme such as 64 QAM (quadrature amplitude modulation) to
deliver the most data throughput to that user. As the user moves
farther away, the number of sub channels is re-assigned
dynamically to fewer and fewer sub channels. However, the power
allotted to each channel is raised. The modulation scheme could
gradually shift from 16 QAM to Quaternary Phase Shift Keying (QPSK)
(four channels) and even binary phase shift keying (BPSK) (two
channels) at longer ranges. The data throughput drops as the
channel capacity and modulation change, but the link maintains
Cell sizes must not expand or contract. Each user must have a
strong link to their base station until handoff. The tradeoff is
lower throughput at the edge.
In fixed wireless links, which typically use high gain
directional antennas, this technique is less necessary. For
mobile applications, especially with high speed handoffs, it is
necessary. Customer devices typically feature omni-directional
antennas which radiate in all directions but have lower gain
than directional antennas. In this environment, sub
channelization is necessary.
Another technique called scalability was developed, resulting
Two Flavors of WiMAX
Enter scalability. Because channels differ in size in different
countries, the 802.16 standard supports all of the various
channel sizes, ranging from 1.25 MHz to 20 MHz.
For a variety of complex technical reasons, Intel makes the
argument that keeping the sub channel spacing fixed by changing
the FFT size based on channel size or bandwidth provides better
signal quality. One of the simpler arguments is that the Doppler
shift of a moving body (amongst other aspects) affects signal
quality if the sub channel spacing is not maintained at a fixed
The OFDMA 2048 FFT version was conceived as a fixed FFT and
is supported in both 802.16-2004 and 802.16e. One company in the
forefront of this type of OFDMA technology is
The scalable or sOFDMA versions encompass the 128 FFT, 512
FFT, and 1024 FFT as promoted by Intel and others. This last
flavor of OFDMA can actually shift its FFT size based on channel
and bandwidth, thus becoming scalable. So, for example, a user
traveling through a cell might receive signal through 128 FFT or
512 FFT depending on factors such as channel size.
The Korean WiBro standard is basically a subset of the sOFDMA
at 1024 FFT. For a variety of reasons, it appears most likely
that the first product profile chosen by the Forum may be a
sOFDMA one, though that is not certain as of press time for this
The physics of broadband wireless forces the designers of OFDMA
to make choices, and those choices have tradeoffs. For every
advantage engineered, there is always a price. The various
flavors of OFDMA are about the demands of mobility and speed of
handoffs, the size of the cell, spectrum range, channel sizes,
Ultimately, each flavor has been optimized to meet the needs
of a specific marketplace. Luckily, the technologies are
flexible enough to allow growth into other segments.
In the long run, the marketplace will decide which products
are made, and which products are sold.