A: OFDM is a broadband
multicarrier modulation method that offers superior performance and benefits
over older, more traditional single-carrier modulation methods because it is a
better fit with today’s high-speed data requirements and operation in the UHF
and microwave spectrum.
Q: Is OFDM a new technique?
A: No. Conceptually, it
has been known since at least the 1960s and 1970s. Originally known as
multicarrier modulation, as opposed to the traditional single-carrier
modulation, OFDM was extremely difficult to implement with the electronic
hardware of the time. So, it remained a research curiosity until semiconductor
and computer technology made it a practical method.
Q: Why has there been all the
interest in OFDM in the past few years?
A: OFDM has been
adopted as the modulation method of choice for practically all the new wireless
technologies being used and developed today. It is perhaps the most spectrally
efficient method discovered so far, and it mitigates the severe problem of multipath
propagation that causes massive data errors and loss of signal in the microwave
and UHF spectrum.
Q: What are some of the wireless technologies that
use OFDM?
A: The list is long and
impressive. First, it is used for digital radio broadcasting—specifically Europe’s DAB and Digital Radio Mondial. It is used in the
U.S.’s
HD Radio. It is used in TV broadcasting like Europe’s
DVB-T and DVB-H. You will also find it in wireless local-area networks (LANs)
like Wi-Fi. The IEEE 802.11a/g/n standards are based on OFDM. The wideband
wireless metro-area network (MAN) technology WiMAX uses OFDM. And, the almost
completed 4G cellular technology standard Long-Term Evolution (LTE) uses OFDM.
The high-speed short-range technology known as Ultra-Wideband (UWB) uses an
OFDM standard set by the WiMedia Alliance. OFDM is also used in wired
communications like power-line networking technology. One of the first
successful and most widespread uses of OFDM was in data modems connected to
telephone lines. ADSL and VDSL used for Internet access use a form of OFDM
known as discrete multi-tone (DMT). And, there are other less well known
examples in the military and satellite worlds.
A: OFDM is based on the
concept of frequency-division multiplexing (FDD), the method of transmitting
multiple data streams over a common broadband medium. That medium could be
radio spectrum, coax cable, twisted pair, or fiber-optic cable. Each data
stream is modulated onto multiple adjacent carriers within the bandwidth of the
medium, and all are transmitted simultaneously. A good example of such a system
is cable TV, which transmits many parallel channels of video and audio over a
single fiber-optic cable and coax cable.
Q: Is that how OFDM works
today?
A: Sort of. The FDD technique
is typically wasteful of bandwidth or spectrum because to keep the parallel
modulated carriers from interfering with one another, you have to space them
with some guard bands or extra space between them. Even then, very selective
filters at the receiving end have to be able to separate the signals from one
another. What researchers discovered is that with digital transmissions, the
carriers could be more closely spaced to one another and still separate. That
meant less spectrum and bandwidth waste.
Q: Given the multiple parallel channels, what is the
actual modulation process?
A: The serial digital
data stream to be transmitted is split into multiple slower data streams, and
each is modulated onto a separate carrier in the allotted spectrum. These
carriers are called subcarriers or tones. The modulation can be any form of
modulation used with digital data, but the most common are binary phase-shift
keying (BPSK), quadrature phase-shift keying (QPSK), and quadrature amplitude
modulation (QAM). The outputs of all the modulators are linearly summed, and
the result is the signal to be transmitted. It could be upconverted and
amplified if needed.
Q: That sounds like a
straightforward approach. Is OFDM really implemented this way?
A: Not really. OFDM works
best, as explained later, if hundreds or even thousands of parallel subcarriers
are used. To implement that with hardware is a challenge even with modern
semiconductor technology. It’s just not done. Instead, the whole process can be
accomplished in computer hardware by using the fast Fourier transform (FFT) or,
more specifically for the transmitter, the inverse FFT (IFFT).
Q: I don’t have time for a
math lesson, so give me a quick overview of the FFT.
A: The FFT is a
variation of the discrete Fourier transform (DFT). Fourier, as you may remember
from your college math days, was the Frenchman who discovered that any complex
signal could be represented by a series of harmonically related sine waves all
added together. He also developed the math to prove it. The math is difficult,
and even early computers couldn’t perform it quickly. Cooley/Tukey developed
the fast Fourier transform in the 1960s as a way to greatly speed up the math
to make Fourier analysis more practical. In general, you can take any analog
signal, digitize it in an analog-to-digital converter (ADC), and then take the
resulting samples and put them through the FFT process. The result is
essentially a digital version of a spectrum analysis of the signal. The FFT
sorts all the signal components out into the individual sine-wave elements of
specific frequencies and amplitudes—a mathematical spectrum analyzer of a sort.
That makes the FFT a good way to separate out all the carriers of an OFDM
signal.
Q: Then how does the IFFT
work?
A: The IFFT just
reverses the FFT process. All the individual carriers with modulation are in
digital form and then subjected to an IFFT mathematical process, creating a
single composite signal that can be transmitted. The FFT at the receiver sorts
all the signals to recreate the original data stream.
Q: Just how does the FFT
process keep the individual modulated carriers from interfering with one
another?
A: This is where the
term “orthogonal” comes in. Orthogonal really means at a right angle to. The
signals are created so they are orthogonal to one another, thereby producing
little or no interference to one another despite the close spacing. In more
practical terms, it means that if you space the subcarriers from one another by
any amount equal to the reciprocal of the symbol period of the data signals,
the resulting sinc (sin x/x) frequency response curve of the signals is such
that the first nulls occur at the subcarrier frequencies on the adjacent
channels. Orthogonal subcarriers all have an integer number of cycles within
the symbol period. With this arrangement, the modulation on one channel won’t
produce intersymbol interference (ISI) in the adjacent channels.
Q: How is OFDM implemented in
the real world?
A: OFDM is accomplished
with digital signal processing (DSP). You can program the IFFT and FFT math
functions on any fast PC, but it is usually done with a DSP IC or an
appropriately programmed FPGA or some hardwired digital logic. With today’s
super-fast chips, even complex math routines like FFT are relatively easy to
implement. In brief, you can put it all on a single chip.
Q: What are the benefits of
using OFDM?
A: The first reason is
spectral efficiency, also called bandwidth efficiency. What that term really
means is that you can transmit more data faster in a given bandwidth in the
presence of noise. The measure of spectral efficiency is bits per second per
Hertz, or bps/Hz. For a given chunk of spectrum space, different modulation
methods will give you widely varying maximum data rates for a given bit error
rate (BER) and noise level. Simple digital modulation methods like amplitude
shift keying (ASK) and frequency shift keying (FSK) are only fair but simple.
BPSK and QPSK are much better. QAM is very good but more subject to noise and
low signal levels. Code division multiple access (CDMA) methods are even
better. But none is better than OFDM when it comes to getting the maximum data
capacity out of a given channel. It comes close to the so called Shannon limit that defines channel capacity C in bits per
second (bps) as
C = B x log2(1 +
S/N)Here, B is the bandwidth of the channel in hertz, and S/N is the power
signal-to-noise ratio. With spectrum scarce or just plain expensive, spectral
efficiency has become the holy grail in wireless.
Q: What else makes OFDM so
good?
A: OFDM is highly
resistant to the multipath problem in high-frequency wireless. Very
short-wavelength signals normally travel in a straight line (line of sight, or
LOS) from the transmit antenna to the receive antenna. Yet trees, buildings,
cars, planes, hills, water towers, and even people will reflect some of the
radiated signal. These reflections are copies of the original signal that also
go to the receive antenna. If the time delays of the reflections are in the
same range as the bit or symbol periods of the data signal, then the reflected
signals will add to the direct signal and create cancellations or other
anomalies. The result is what we usually call Raleigh fading.
Q: How does OFDM deal with this?
A: The high-speed
serial data to be transmitted is divided up into many much lower-speed serial
data signals. Then OFDM sends these lower-data-rate signals over multiple
channels. This makes the bit or symbol periods longer, so multipath time delays
have less of an effect. The more subcarriers used over a wider bandwidth, the
more resistant the overall signal is to the multipath phenomenon. This means
you can use the higher frequencies with fewer multipath effects to worry about.
But the really good news is that you can use them in mobile situations where
either the transmitter or receiver or both are moving and undergoing changing
environmental conditions with good signal reliability.
Q: What are the downsides to
OFDM?
A: Like anything else,
OFDM is not perfect. It is very complex, making it more expensive to implement.
However, modern semiconductor technology makes it pretty easy. OFDM is also
sensitive to carrier frequency variations. To overcome this problem, OFDM
systems transmit pilot carriers along with the subcarriers for synchronization
at the receiver. Another disadvantage is that an OFDM signal has a high peak to
average power ratio. As a result, the complex OFDM signal requires linear
amplification. That means greater inefficiency in the RF power amplifiers and
more power consumption.
A: The A stands for
access. It means that OFDM is not only a great modulation method, it also can
provide multiple access to a common bandwidth or channel to multiple users. You
are probably familiar with multiple access methods like frequency-division
multiplexing (FDM) and time division multiplexing (TDM). CDMA, the widely used
cellular technology, digitally codes each digital signal to be transmitted and
then transmits them all in the same spectrum. Because of their random nature,
they just appear as low-level noise to one another. The digital coding lets the
receiver sort the individual signal out later. OFDMA permits multiple users to
share a common bandwidth with essentially the same benefits.
Q: How is OFDMA accomplished?
A: The OFDM system
assigns subgroups of subcarriers to each user. With thousands of subcarriers,
each user would get a small percentage of the carriers. In a modern system like
the 4G LTE cellular system, each user could be assigned from one to many
subcarriers. In LTE, subcarrier spacing is 15 kHz. Using a 10-MHz band, the
total possible number of subcarriers would be 666. In practice, a smaller
number like 512 would be used. If each subscriber is given six subcarriers, you
could put 85 users in the band. The number of subcarriers assigned will depend
on the user’s bandwidth and speed needs.
Q: Is
there anything better than OFDM?
A: Not right now. What
makes OFDM even better is MIMO, the multiple-input multiple-output antenna
technology. It is currently used in 802.11n Wi-Fi and the forthcoming LTE. Look
for MIMO in another FAQ Tutorial.
OFDM or OFDMA?
IEEE 802.16d (fixed
service) uses Orthogonal Frequency Division Multiplexing (OFDM). IEEE 802.16e
(mobile) uses Orthogonal Frequency Division Multiple Access (OFDMA). So, what’s
the difference between the two, and why is there a difference?
OFDM is sometimes
referred to as discrete multi-tone modulation because, instead of a single
carrier being modulated, a large number of evenly spaced subcarriers are
modulated using some m-ary of QAM. This is a spread-spectrum technique that
increases the efficiency of data communications by increasing data throughput
because there are more carriers to modulate. In addition, problems with
multi-path signal cancellation and spectral interference are greatly reduced by
selectively modulating the “clear” carriers or ignoring carriers with high
bit-rate errors.
The OFDM
spread-spectrum scheme is used for many broadly used applications, including
digital TV broadcasting in Australia,
Japan and Europe; digital
audio broadcasting in Europe; Asynchronous
Digital Subscriber Line (ADSL) modems and wireless networking worldwide (IEEE
802.11a/g).
OFDM allows only one
user on the channel at any given time. To accommodate multiple users, a
strictly OFDM system must employ Time Division Multiple Access (TDMA) (separate
time frames) or Frequency Division Multiple Access (FDMA) (separate channels).
Neither of these techniques is time or frequency efficient: TDMA is a time hog
and FDMA is a bandwidth hog.
OFDMA is a multi-user
OFDM that allows multiple access on the same channel (a channel being a group
of evenly spaced subcarriers, as discussed above). WiMAX uses OFDMA, extended
OFDM, to accommodate many users in the same channel at the same time.
OFDMA distributes
subcarriers among users so all users can transmit and receive at the same time
within a single channel on what are called subchannels. What’s more, subcarrier-group
subchannels can be matched to each user to provide the best performance,
meaning the least problems with fading and interference based on the location
and propagation characteristics of each user.
The WiMAX forum
established that, initially, OFDM-256 will be used for fixed-service 802.16d
(2004). It is referred to as the OFDM 256 FFT Mode, which means there are 256
subcarriers available for use in a single channel. Multiple access on one
channel is accomplished using TDMA. Alternatively, FDMA may be used.
On the other hand,
OFDMA 128/512/1024/2048 FFT Modes have been proposed for IEEE 802.16e (mobile
service). OFDMA 1024 FFT matches that of Korea’s WiBRO. OFDM 256 also is
supported for compatibility with IEEE 802.16d (fixed, 2004). The final IEEE
802.16e standard is expected to be completed and published in December of this
year.
The bottom line is
that, most likely, the finalized selection for the OFDMA mode will be 1024 FFT,
to be compatible with WiBRO. However, it will not be compatible with the OFDM
256 FFT Mode initially specified for WiMAX fixed service. Perhaps service
providers will simply abandon 802.16d in favor of 802.16e for both fixed and
mobile services.
is the central frequency of the mth
sub-channel and
is the corresponding transmitted symbol.
The signals 
