INTRODUCTION TO OFDM
Orthogonal frequency-division multiplexing is a method of data transmission where a
single information stream is split among several closely spaced narrowband sub channel
frequencies instead of a single Wideband channel frequency. It is mostly used in
wireless data transmission but may be employed in wired and fiber optic communication
as well.
In OFDM, several bits can be sent in parallel, or at the same time, in separate
substream channels. This enables each substream's data rate to be lower than would
be required by a single stream of similar bandwidth. This makes the system less
susceptible to interference and enables more efficient data bandwidth.
Motivati
on
• Signal over wireless channel
⯈ y[n] = Hx[n]
• Work only for narrow-band channels,
but not for wide-band channels
⯈ e.g., 20 MHz for 802.11
20MHz
frequenc
2.45GHz (Central y
Capacity = BW *
frequency)
log(1+SNR)
Basic Concept of
OFDM
Wide-band Multiple narrow-band
channel channels
Send a sample Send samples concurrently
using the using multiple orthogonal
entire band sub-channels
Why OFDM is
better? t
t
0
1
0
f 1 0
1 f
0 0 1 1 0 0 0 1 …........
• Wide-band
‐ Narrow-band
‐
• Multiple sub-channels (sub-carriers) carry
samples sent at a lower rate
• ⯈ Almost same bandwidth with wide-band channel
• Only some of the sub-channels are affected
by interferers or multi-path effect
Importance of
Orthogonality
• Why not just use FDM (frequency
division multiplexing)
⯈Not Individual sub-channel
‐
orthogonal
Leakage interference from
adjacent sub-channels
‐ f
guard band
Guard bands protect
leakage
interference f
• Need guard bands between adjacent
frequency bands extra overhead and
lower throughput
Difference between FDM and
OFDM guard band
f
Frequency division
multiplexing
Orthogonal sub-carriers in
OFDM
Don’t need guard bands
Orthogonal Frequency
Division Modulation
* x[1]
IFFT * x[2] transmit
f
* x[3]
t
…
Data coded in frequency domain Transformation to time domain: Channel frequency
each frequency is a sine wave response
In time, all added up
Decode each subcarrier
separately
receive FFT
Time domain signal Frequency domain signal
OFDM Transmitter and
Receiver
Inter Symbol Interference
(ISI)
• The delayed version of a symbol
overlaps with the adjacent symbol
• One simple solution to avoid this
is to introduce a guard-band
Guard band
Block diagram
OFDM advantages and
disadvantages
Orthogonal frequency-division multiplexing has many advantages over a single-
channel data transmission approach. Primarily, OFDM is more resilient to
electromagnetic interference, and it enables more efficient use of total available
bandwidth because the subchannels are closely spaced. It is also more resistant
to interference because several channels are available.
There are two primary disadvantages with OFDM compared to single-channel
systems. OFDM systems must have closely tuned transmitters and receivers.
This requires the timing on signal modulators and demodulators be closely
matched and produced to tight tolerances. It also makes the system more
sensitive to Doppler shift and, therefore, less effective for high-speed moving
vehicles.
OFDM also has several advantages compared to standard frequency-division
multiplexing. The radio frequency receiver is simpler in OFDM because the entire
signal can be received in a single frequency selective filter and separated in software
using a fast Fourier transform, while an FDM system requires a separate RF
bandpass filter for each channel. It also has better overall bandwidth efficiency. There
are some disadvantages in that the higher overall peak-to-average power (PAPR)
ratio requires less efficient linear transmission circuitry.
OFDM applications
•Digital radio, Digital Radio Mondiale, and digital audio broadcasting and
satellite radio.
•Digital television standards, Digital Video
Broadcasting-Terrestrial/Handheld (DVB-T/H), DVB-Cable 2 (DVB-C2). OFDM
is not used in the current U.S. digital television Advanced Television Systems
Committee standard, but it is used in the future 4K/8K-capable ATSC 3.0
standard.
•Wired data transmission, Asymmetric Digital Subscriber Line (ADSL),
Institute of Electrical and Electronics Engineers (IEEE) 1901 powerline
networking, cable internet providers. Fiber optic transmission may use either
OFDM signals or several distinct frequencies as FDM.
•Wireless LAN (WLAN) data transmission. All Wi-Fi systems use OFDM,
including IEEE 802.11a/b/g/n/ac/ax. The addition of OFDMA to the
Wi-Fi 6/802.11ax standard enables more devices to use the same base station
simultaneously. OFDM is also used in metropolitan area network (MAN) IEEE
802.16 Worldwide Interoperability for Microwave Access (WiMAX>) installations.
•Cellular data. Long-Term Evolution (LTE) and 4G cellphone networks use
OFDM. It is also an integral part of 5G NR cellular deployments.
