Orthogonal Frequency Division
Modulation (OFDM)
• OFDM diagram
• Inter Symbol Interference
• Packet detection and synchronization
• Related works
Motivation
• 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
frequency
2.45GHz (Central frequency)
20MHz
Capacity = BW * log(1+SNR)
Basic Concept of OFDM
Orthogonal Frequency Division Multiplex (OFDM) Tutorial
1
Intuitive Guide to Principles of Communications
www.complextoreal.com
Orthogonal Frequency Division Multiplexing (OFDM)
Modulation - a mapping of the information on changes in the carrier phase, frequency or
amplitude or combination.
Multiplexing - method of sharing a bandwidth with other independent data channels.
OFDM is a combination of modulation and multiplexing. Multiplexing generally refers to
independent signals, those produced by different sources. So it is a question of how to share the
spectrum with these users. In OFDM the question of multiplexing is applied to independent
signals but these independent signals are a sub-set of the one main signal. In OFDM the signal
itself is first split into independent channels, modulated by data and then re-multiplexed to create
the OFDM carrier.
OFDM is a special case of Frequency Division Multiplex (FDM). As an analogy, a FDM channel
is like water flow out of a faucet, in contrast the OFDM signal is like a shower. In a faucet all
water comes in one big stream and cannot be sub-divided. OFDM shower is made up of a lot of
little streams.
(a) (b)
Fig. 1 – (a) A Regular-FDM single carrier – A whole bunch of water coming all in one stream. (b)
Orthogonal-FDM – Same amount of water coming from a lot of small streams.
Think about what the advantage might be of one over the other? One obvious one is that if I put
my thumb over the faucet hole, I can stop the water flow but I cannot do the same for the shower.
So although both do the same thing, they respond differently to interference.
Fig. 2 – All cargo on one truck vs. splitting the shipment into more than one.
Another way to see this intuitively is to use the analogy of making a shipment via a truck.
We have two options, one hire a big truck or a bunch of smaller ones. Both methods carry the
exact same amount of data. But in case of an accident, only 1/4 of data on the OFDM trucking
will suffer.
Copyright 2004 Charan Langton www.complextoreal.com
Orthogonal Frequency Division Multiplex (OFDM) Tutorial
1
Intuitive Guide to Principles of Communications
www.complextoreal.com
Orthogonal Frequency Division Multiplexing (OFDM)
Modulation - a mapping of the information on changes in the carrier phase, frequency or
amplitude or combination.
Multiplexing - method of sharing a bandwidth with other independent data channels.
OFDM is a combination of modulation and multiplexing. Multiplexing generally refers to
independent signals, those produced by different sources. So it is a question of how to share the
spectrum with these users. In OFDM the question of multiplexing is applied to independent
signals but these independent signals are a sub-set of the one main signal. In OFDM the signal
itself is first split into independent channels, modulated by data and then re-multiplexed to create
the OFDM carrier.
OFDM is a special case of Frequency Division Multiplex (FDM). As an analogy, a FDM channel
is like water flow out of a faucet, in contrast the OFDM signal is like a shower. In a faucet all
water comes in one big stream and cannot be sub-divided. OFDM shower is made up of a lot of
little streams.
(a) (b)
Fig. 1 – (a) A Regular-FDM single carrier – A whole bunch of water coming all in one stream. (b)
Orthogonal-FDM – Same amount of water coming from a lot of small streams.
Think about what the advantage might be of one over the other? One obvious one is that if I put
my thumb over the faucet hole, I can stop the water flow but I cannot do the same for the shower.
So although both do the same thing, they respond differently to interference.
Fig. 2 – All cargo on one truck vs. splitting the shipment into more than one.
Another way to see this intuitively is to use the analogy of making a shipment via a truck.
We have two options, one hire a big truck or a bunch of smaller ones. Both methods carry the
exact same amount of data. But in case of an accident, only 1/4 of data on the OFDM trucking
will suffer.
Copyright 2004 Charan Langton www.complextoreal.com
Orthogonal Frequency Division Multiplex (OFDM) Tutorial
1
Intuitive Guide to Principles of Communications
www.complextoreal.com
Orthogonal Frequency Division Multiplexing (OFDM)
Modulation - a mapping of the information on changes in the carrier phase, frequency or
amplitude or combination.
Multiplexing - method of sharing a bandwidth with other independent data channels.
OFDM is a combination of modulation and multiplexing. Multiplexing generally refers to
independent signals, those produced by different sources. So it is a question of how to share the
spectrum with these users. In OFDM the question of multiplexing is applied to independent
signals but these independent signals are a sub-set of the one main signal. In OFDM the signal
itself is first split into independent channels, modulated by data and then re-multiplexed to create
the OFDM carrier.
OFDM is a special case of Frequency Division Multiplex (FDM). As an analogy, a FDM channel
is like water flow out of a faucet, in contrast the OFDM signal is like a shower. In a faucet all
water comes in one big stream and cannot be sub-divided. OFDM shower is made up of a lot of
little streams.
(a) (b)
Fig. 1 – (a) A Regular-FDM single carrier – A whole bunch of water coming all in one stream. (b)
Orthogonal-FDM – Same amount of water coming from a lot of small streams.
Think about what the advantage might be of one over the other? One obvious one is that if I put
my thumb over the faucet hole, I can stop the water flow but I cannot do the same for the shower.
So although both do the same thing, they respond differently to interference.
Fig. 2 – All cargo on one truck vs. splitting the shipment into more than one.
Another way to see this intuitively is to use the analogy of making a shipment via a truck.
We have two options, one hire a big truck or a bunch of smaller ones. Both methods carry the
exact same amount of data. But in case of an accident, only 1/4 of data on the OFDM trucking
will suffer.
Copyright 2004 Charan Langton www.complextoreal.com
Orthogonal Frequency Division Multiplex (OFDM) Tutorial
1
Intuitive Guide to Principles of Communications
www.complextoreal.com
Orthogonal Frequency Division Multiplexing (OFDM)
Modulation - a mapping of the information on changes in the carrier phase, frequency or
amplitude or combination.
Multiplexing - method of sharing a bandwidth with other independent data channels.
OFDM is a combination of modulation and multiplexing. Multiplexing generally refers to
independent signals, those produced by different sources. So it is a question of how to share the
spectrum with these users. In OFDM the question of multiplexing is applied to independent
signals but these independent signals are a sub-set of the one main signal. In OFDM the signal
itself is first split into independent channels, modulated by data and then re-multiplexed to create
the OFDM carrier.
OFDM is a special case of Frequency Division Multiplex (FDM). As an analogy, a FDM channel
is like water flow out of a faucet, in contrast the OFDM signal is like a shower. In a faucet all
water comes in one big stream and cannot be sub-divided. OFDM shower is made up of a lot of
little streams.
(a) (b)
Fig. 1 – (a) A Regular-FDM single carrier – A whole bunch of water coming all in one stream. (b)
Orthogonal-FDM – Same amount of water coming from a lot of small streams.
Think about what the advantage might be of one over the other? One obvious one is that if I put
my thumb over the faucet hole, I can stop the water flow but I cannot do the same for the shower.
So although both do the same thing, they respond differently to interference.
Fig. 2 – All cargo on one truck vs. splitting the shipment into more than one.
Another way to see this intuitively is to use the analogy of making a shipment via a truck.
We have two options, one hire a big truck or a bunch of smaller ones. Both methods carry the
exact same amount of data. But in case of an accident, only 1/4 of data on the OFDM trucking
will suffer.
Copyright 2004 Charan Langton www.complextoreal.com
Send a sample using
the entire band
Send samples concurrently using
multiple orthogonal sub-channels
Wide-band channel
Multiple narrow-band channels
Why OFDM is better?
• 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
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Importance of Orthogonality
• Why not just use FDM (frequency division
multiplexing)
! Not orthogonal
• Need guard bands between adjacent frequency
bands ! extra overhead and lower throughput
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