OFDM and applications

7/30/2019 OFDM and applications

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ORTHOGONAL FREQUENCY-DIVISION MULTIPLEXING (OAND ITS APPLICATION INWIRELESS BROADBAND MULTIMEDIA COMMUNICAT

SYSTEMS (WBMCS)

Presented By:Vipul SaxenaFinal yearElectrical


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Motivation for Wireless Communication

High data rate wireless applications.

High capacity

- More users

- More service

Emc4

Where,

E is evolution of wireless communications,

m is multimedia communications,

and c is consumer electronics, computer technology, communications technology,

and contents.


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Limitations of the Mobile Radio Environment

Path loss

Shadow fading

Frequency-selective fading

All of them result in Interference

Challenges faced by mobile e


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Evolution of OFDM

In a single-carrier system, a single fade or interferer can cause th

link to fail, but in a multicarrier system (OFDM), only a small percethe SCs will be affected.

OFDM is a special case of multicarrier transmission, where a sinstream is transmitted over a number of lower-rate subcarriers (SCs

OFDM increases robustness against frequency-selective fading as

SCs are affected.

Thus, Error-correction coding can then be used to correct for erroneous SCs.


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From FDM to OFDM In a classical parallel-data system (FDM) , the total signal frequen

is divided into N non-overlapping frequency sub-channels.

Each sub-channel is modulated with a separate symbol, and thesub-channels are frequency multiplexed.

It seems good to avoid spectral overlap of channels to eliminachannel interference.

However, this leads to inefficient use of the available spectrum.


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The different carriers are

orthogonal to each other, that is,they are totally independent of

one another.

This is achieved by placing the

carrier exactly at the nulls in the

modulation spectra of each other.


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OFDM Subcarrier Spectra


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Concept of the OFDM signal: (a) conventional multicarrier technique (FD

and (b) orthogonal multicarrier modulation technique.


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Comparison of OFDM over FDM and single-carrier systems.

OFDM is more spectral efficient than FDM, since it utilizes the surface effect

with adjacent tiny streams.


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Lets us have following information bits seriallySERIAL DATA = 1, 1, -1, -1, 1, 1, 1, -1, 1, -1, -1, -1, -1, 1, -1, -1,

Just convert the serials bits to parallel bits

C1 C2 C3 C41 1 -1 -11 1 1 -11 -1 -1 -1-1 1 -1 -1-1 1 1 -1-1 -1 1 1


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Example of OFDM cont..

Modulated signal for C1 Modulated signal f

Modulated signal for C3 Modulated signal fo

Modulate each column with corresponding sub-carrier using BP


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Example of OFDM cont..

Final OFDM Signal = Sum of all signal

Generated OFDM signal, V(t)

V(t)


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Orthogonality :

Sub-CarriersEach sub carrier in an OFDM system is a sinusoid with a frequency that is an intefundamental frequency.


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Inter symbol interference (ISI)

Inter symbol interference (ISI) is a form of distortion of a signal in

one symbol interferes with subsequent symbols.

ISI is usually caused by multipath propagation or the inherent nonfrequency response of a channel causing successive symbols to bltogether.

The presence of ISI in the system introduces error in the decision d

the receiver output.


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Inter Carrier Interference (ICI)

Presence of Doppler shifts and frequency and phase offsets in an O

system causes loss in orthogonality of the sub carriers.

As a result, interference is observed between sub carriers. Thisphenomenon is known as inter carrier interference (ICI) .


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Cyclic Prefix

The Cyclic Prefix is a periodic extension of the last part of an OFDM symbol tha

to the front of the symbol in the transmitter, and is removed at the receiver befdemodulation. The cyclic prefix has to two important benefits

The cyclic prefix acts as a guard interval. It eliminates the inter symbol interfefrom the previous symbol.

It acts as a repetition of the end of the symbol thus allowing the linear convolutfrequency selective multipath channel to be modeled as circular convolution turn maybe transformed to the frequency domain using a discrete fourier trans


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Modulation in OFDM In an OFDM system, the high data rate information is divided into small packets

which are placed orthogonal to each other.

This is achieved by modulating the data by a desirable modulation technique ( s

After this, IFFT is performed on the modulated signal which is further processepassing through a parallel to serial converter. In order to avoid ISI we providprefix to the signal.


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Demodulation

The received data is first made to pass through a low pass filter an

cyclic prefix is removed.

FFT of the signal is done after it is made to pass through a serialparallel converter.

A demodulator is used, to get back the original signal.


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Peak-to-Average Power Ratio(PAPR) A Limitation

The major problem one faces while implementing OFDM is the h

to average power ratio (PAPR) of this system.

A large PAPR causes:

Increase in the complexity of the analog to digital and digitaanalog converters.

Reduction in the efficiency of the radio frequency (RF) power aas the transmitter power amplifier is no longer confined to linear rwhich it should operate. This has a harmful effect on the battery lif


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PAPR Reduction Techniques

PAPR reduction techniques vary according to the needs of the sys

are dependent on various factors like PAPR reduction capacity, incpower in transmit signal, loss in data rate, complexity of computat

Some of the popular and efficient techniques of reduction of PAPR

AMPLITUDE CLIPPING AND FILTERING

SELECTED MAPPING

PARTIAL TRANSMIT SEQUENCE


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Contd

AMPLITUDE CLIPPING AND FILTERING: A threshold value

amplitude is set in this process and any sub-carrier having amplituthan that value is clipped or that sub-carrier is filtered to bring outPAPR value.

SELECTED MAPPING: In this a set of sufficiently different datrepresenting the information same as the original data bloselected. Selection of data blocks with low PAPR value makes it

for transmission.

PARTIAL TRANSMIT SEQUENCE: Data is divided into disjoint Suand the Sub-blocks are combined so as to have low PAPR value asent for transmission. It is based on optimization techniques & thmost effective.


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Unmodified OFDM signal

OFDM Signal After Amplitude ClippingEffect of Amplitude

Performance curves of Selected Mapping (SLM )andPartial Transmit Sequence(PTS) Techniques.


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Advantages of OFDM

Multipath delay spread tolerance Immunity to frequency selective fading channels: High transmission bit rates.


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Disadvantages of OFDM

Large peak-to-mean power ratio :Due to the superposition subcarrier signals, this can become a distortion problem.

More complex than single-carrier Modulation.

Requires a more linear power amplifier.


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WBMCS

Applications

Multi-carrierMultiple Access

Techniques

OFDM-CDMA

OFDMA

Flash-OFDM etc..

AdvancedCommunications

Techniques

DAB

DVB

HIPERLAN etc.

APPLICATIONS OF OFDM


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CDMA (Code Division Multiple Access) In CDMA, one channel carries all transmissions simultaneously.

CDMA simply means communication with different codes.

In code-division multiple access (CDMA), the stations use differentto achieve multiple access.

CDMA is based on coding theory and uses sequences of numbers cchips.


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Properties of CDMA Codes

1. Each sequence is made ofN elements, where N is the number of stations.

2. If we multiply a sequence by a number, every element in the sequence is multithat element. This is called multiplication of a sequence by a scalar. For example

2. [+1 +1-1-1]=[+2+2-2-2]

3. If we multiply two equal sequences, element by element, and add the results, wwhere N is the number of elements in the each sequence.

4. If we multiply two different sequences, element by element, and add the result

0.. For example,

[+1 +1 -1 -1] [+1 +1 +1 +1] = 1 + 1 - 1 - 1 = 0

5. Adding two sequences means adding the corresponding elements. The result issequence. For example,

[+1+1-1-1]+[+1+1+1+1]=[+2+2 0 0]


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OFDM-CDMA

In OFDM-CDMA, user data is spread over several subcarriers and/or OFDM sym

spreading codes, and combined with signal from other users. We combine the benefits of OFDM and Code Division Multiple Access (CDMA).

OFDM provides a simple method to overcome the Inter-Symbol Interference (ISof the multi-path frequency selective wireless channel.

CDMA provides the frequency diversity and the multi-user access scheme.

In OFDM-CDMA, the flexibility lies in the allocation of all available codes to the

depending on the required data rates.


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OFDMA (Orthogonal Frequency Division Multiple Access)

In OFDM, available sub-carriers are distributed among all the use

transmission at any time instant.

OFDMA allows assigning subset of subcarriers to different users.

The active subcarriers are divided into subset of subcarriers termesub-channel.

Carrier space is divided into NG groups. Each group contains NE ca

and thus NE sub-channels are created.


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Comparison

OFDMA subc

The type of coding and modulation are set separately for each sub-channel.

The subcarriers can be allocated to different users (one sub-channel per one user

channel conditions.

This characteristic is useful for operators who assign to these users most suitabl

leads to efficient use of resources.

OFDMA is perfectly fitting for fast mobile scenarios.


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Flash OFDM

Data communication over legacy cellular voice systems is inefficie

data is fundamentally different from voice.

Flash-OFDM is an OFDMA system where the sub-carriers are distramong the mobile stations.

Spectrum is divided into N tones but different mobiles in the samedifferent tones.

Flash-OFDM uses fast hopping across all tones.

With fast hopping, a user that is assigned one tone does not transmthe same tone every symbol, but uses a hopping pattern to jump tdifferent tone every symbol duration.


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Contd It provides higher data rates when it is close to the base station.

Hence it enables the MAC layer to perform efficient packet switchthe air.


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Digital Audio Broadcasting (DAB)

DAB is an European standard for digital broadcasting th

standardized by the European Telecommunications Institute (1995.

The DAB transmitted data consists of number of audio signals sama rate of 48 kHz with a 22-bit resolution. This audio signal compressed at rates ranging from 32 to 384 kbps, depending udesired signal quality.

The resulting digital data is then divided into frames of 24 ms. Ddifferential QPSK modulation for the sub-carriers.

A null-symbol is used to indicate the start of the frame.


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Digital Audio Broadcasting (OFDM Parameters)

Four Transmission modes of DAB


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Digital Video Broadcasting (DVB)

Digital Video Broadcasting (DVB) is a standard for broadcastin

Television over satellites, cables and terrestrial (wireless) transDVB was standardized by the ETSI in 1997.

DVB has two modes of operation: the 2k mode with 1705 suband the 8k modes with 6817 sub-carriers.

DVB uses QPSK, 16-QAM or 64-QAM sub-carrier modulation.


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2G , 3G were based on TDMA ,FDMA.

4G, refers to all-IP packet-switched networks, mobile ultra-broadband (gigabit speed) access and mult

4G is based on OFDM which is the key enabler of 4G technology. Other technological aspects of 4G a

and smart antennas.

Currently 3G networks still send their data digitally over a single channel, OFDM is designed to send

parallel streams, thus increasing the amount of information that can be sent at a time over traditional C

4G networks also use smart antenna technology, which is used to aim the radio signal in the direction

terminal from the base station. When teamed up with adaptive techniques, multiple antennas

interference while enhancing the signal.


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REFERENCES

Ahmad R.S. Bahai , Burton R. Saltzberg, Mustafa Ergen - Mult

Digital communications Theory and Applications of OFDM , Internatinal Edition, 2005.

Litwin , Louis and Pugel, Michael, The Principles ofOFDM2001.

Ramjee Prasad-OFDM for Wireless Communication,Universal Communications.

Muhammad Imadur Rahman , Suvra Sekhar Das , Frank H.POFDM Based WLAN Systems , Technical report , Aalborg Un2005.


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Documents

OFDM and applications