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Orthogonal Frequency-Division Multiplexing (OFDM)

Used in DSL, WLAN, DAB, WIMAX, 4G

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Definition

Orthogonal frequency division multiplexing (OFDM) is a communications technique that divides a communications channel into a number of equally spaced frequency bands.

A subcarrier carrying a portion of the user information is transmitted in each band.

Each subcarrier is orthogonal (independent of each other) with every other subcarrier.

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Overview

OFDM is a modulation (multitone modulation) and multiple-access technique that has been explored for more than 20 years. Only recently has it been finding its way into commercial communications systems.

Used for a variant of digital subscriber line (DSL) European

digital broadcast television standard digital radio in North America fixed wireless systems and wireless local-area

network (LAN) products

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Characteristics and Evolution of DataCommunications for a Mobile Environment

Data traffic differs from voice in at least three important ways

data traffic is much more unpredictable (as bursty)

data has very different requirements in terms of reliability

data traffic encompasses a much different and wider range of services than voice

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Overview of the Wireless Environment

The wireless channel is distinct and much more unpredictable than the wire-line channel because of factors such as multipath and shadow fading, Doppler spread, and time dispersion or delay spread.

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Multipath

a transmitted signal is reflected by objects in the environment between the base station and a user

The reflected signals arrive at the receiver with random phase offsets

The result is random signal fades

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Time dispersion

distortion to the signal and is manifested by the spreading in time of the modulation symbols

Occurs when the coherence bandwidth of the channel is smaller than the modulation bandwidth

leads to inter-symbol interference (ISI) It also leads to fading

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Doppler spread

the random changes in the channel introduced as a result of a user's mobility and the relative motion of objects in the channel.

shifting, or spreading, the frequency components of a signal.

fast fading: When the rate of change of the channel is higher than the modulated symbol rate,

Slow fading

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Fading models

Rayleigh fading: no line of sight (LOS) component present in the received signal

Ricean distribution: there is a LOS component present

More frequently no direct LOS path to a mobile

a large dynamic range of signals (as much as 70 dB

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Overview of Techniques used in Mobile Wireless Systems

equalization, channel or error-correction coding, spread spectrum, interleaving, and diversity.

Diversity has long been used to help mitigate the multipath-induced fading that results from users' mobility.

spatial diversity: two or more receive antennae at a base station that are separated by some distance

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frequency diversity: Spread spectrum (signal is spread over a much larger bandwidth than is needed for transmission) frequency hopping (FH) code division multiple access (CDMA)

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OFDM for Mobile Communications

It is a technique that divides the spectrum into a number of equally spaced tones and carries a portion of a user's information on each tone.

each tone is orthogonal with every other tone

OFDM allows the spectrum of each tone to overlap

the overall amount of spectrum required is reduced

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OFDM Tones

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OFDM is a modulation technique in that it enables user data to be modulated onto the tones.

can greatly reduce, or even eliminate, ISI OFDM can also be considered a multiple-

access technique, the system called OFDMA

OFDM can be combined with frequency hopping to create a spread spectrum system to minimise losses due to frequency selective fading

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• Data stream at R bps

• Total bandwidth: Nfb

• Central frequency:f0

• N substreams, R/N bps

• Subcarriers spaced fb

• Base frequency, fb

• Other Subcarriers, 2fb, 3fb, 4fb, … Nfb

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Distribute date over multiple carriers at precise frequencies.

The precise relationship among the subcarriers is referred to as orthogonality

The peaks of the power spectral density of each subcarrier occurs at a point at which the power of others is zero.

The subcarriers can be packed tightly together because there is minimal interference between adjacent subcarriers.

The set of OFDM subcarriers is further modulated to a higher frequency band.

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Advantages of OFDM

frequency selective fading only affects some subchannels and not the whole signal. If the data stream is protected by a forward error-correcting code, this type of fading is easily handled.

More importantly, OFDM overcome intersymbol interference (ISI) in a multipath environment.

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OFDM256

OFDM256 air interface 256 subcarriers, 192 used for user data, 56 for guard bands 8 used for pilot tones Cyclic prefix can be from 8- 32 depending on

environment (channel delay spread) there is also OFDM2048 and scalableOFDMA