26-04-2010, 11:55 AM
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Introduction
In older multi-channel systems using FDM, the total available bandwidth is divided into N non-overlapping frequency sub-channels. Each sub-channel is modulated with a separate symbol stream and the N sub-channels are frequency multiplexed. OFDM is a multi-channel modulation system employing Frequency Division Multiplexing (FDM) of orthogonal sub-carriers, each modulating a low bit-rate digital stream. In FDM, the prevention of spectral overlapping of sub-carriers reduces Interchannel Interference, but leads to an inefficient use of spectrum. The guard bands on either side of each sub-channel are a waste of precious bandwidth. To overcome this problem, OFDM uses N overlapping (but orthogonal) sub carriers, each carrying a baud rate of 1/T and spaced 1/T apart. Because of the frequency spacing selected, the sub-carriers are all mathematically orthogonal to each other. This permits the proper demodulation of the symbol streams without the requirement of non overlapping spectra.
Another way of specifying the sub-carrier orthogonality condition is to require that each sub-carrier have exactly integer number of cycles in the interval T. It can be shown that the modulation of these orthogonal sub-carriers can be represented as an Inverse Fourier Transform. Alternatively, one may use a DFT operation followed by low-pass filtering to generate the OFDM signal. It must be noted that OFDM can be used either as a modulation or a multiplexing technique.
OFDM using Inverse DFT
The use of Discrete Fourier Transform (DFT) in the parallel transmission of data using
Frequency Division Multiplexing was investigated in 1971 by Weinstein and Ebert [1].Consider a data sequence d0, d2, ¦, dN-1, where each dn is a complex symbol.(the data sequence could be the output of a complex digital modulator, such as QAM, PSK etc). Suppose we perform an IDFT on the sequence 2dn (the factor 2 is used purely for scaling purposes), we get a result of N complex numbers Sm (m = 0,1¦,N-1) as:
Where, Ts represents the symbol interval of the original symbols. Passing the real part of
the symbol sequence represented by equation (2.1) thorough a low-pass filter with each
symbol separated by a duration of Ts seconds, yields the signal,
OFDM Modulator
Where, T is defined as NTs. The signal y(t) represents the baseband version of the OFDM
Three Subcarriers within an OFDM symbol Spectra of Individual Sub-Carriers
Guard Time and Cyclic Extension
One of the main advantages of OFDM is its effectiveness against the multi-path delay spread frequently encountered in Mobile communication channels. The reduction of the symbol rate by N times, results in a proportional reduction of the relative multi-path delay spread, relative to the symbol time. To completely eliminate even the very small ISI that results, a guard time is introduced for each OFDM symbol. The guard time must be chosen to be larger than the expected delay spread, such that multi-path components from one symbol cannot interfere with the next symbol. If the guard time is left empty, this may lead to inter-carrier interference (ICI), since the carriers are no longer orthogonal to each other. To avoid such a cross talk between sub-carriers, the OFDM symbol is cyclically extended in the guard time. This ensures that the delayed replicas of the OFDM symbols always have an integer number of cycles within the FFT interval as long as the multi-path delay spread is less than the guard time.
Guard time and cyclic extension-Effect of Multipath
OFDM Generation
The generation of OFDM symbol is as follows
¢ First, the N input complex symbols are padded with zeros to get Ns symbols that are used to calculate the IFFT. The output of the IFFT is the basic OFDM symbol
¢ Based on the delay spread of the multi-path channel, a specific guard-time must be chosen (say Tg). A number of samples corresponding to this guard time must be taken from the beginning of the OFDM symbol and appended at the end of the symbol. Likewise, the same number of samples must be taken from the end of the OFDM symbol and must be inserted at the beginning.
¢ The OFDM symbol must be multiplied with the raised cosine window to remove the power of the out-of-band sub-carriers.
¢ The windowed OFDM symbol is then added to the output of the previous OFDM symbol with a delay of Tr, so that there is an overlap region of r T ?between each symbol.
Figure shows the block diagram of an OFDM transmitter and receiver.
OFDM System Block Diagram
The following are the most important design parameters of an OFDM system.
¢ Guard Time
¢ Symbol Duration
¢ Number of Sub-carriers
Advantages of OFDM
OFDM possesses some inherent advantages for Wireless Communications. This section glances on few of the most important reasons on why OFDM is becoming more popular in the Wireless Industry today.
¢ Multi-path Delay Spread Tolerance
¢ Effectiveness against Channel Distortion
¢ Throughput Maximization (Transmission at Capacity)
¢ Robustness against Impulse Noise
Synchronization in OFDM Systems
Synchronization in OFDM system is achieved by the following methods
¢ Synchronization using Cyclic Extension
¢ Synchronization using Training Sequences
Synchronization using Cyclic Extension
Since a Cyclic extension is added to every OFDM symbol, the first Tg seconds of the OFDM symbol is identical to the last part. This property can be exploited for both timing and frequency synchronization using a scheme depicted in figure.
Synchronization using cyclic extension
This scheme correlates Tg seconds of the OFDM symbol with a part that is T seconds delayed (T “ being the symbol time, less the guard period Tg). The output of the
correlator can be written as:
The symbol timing is estimated from the correlation peaks at the output of the correlator.
The characteristics of the correlation peaks are better if the correlation is
performed over a large number of independent samples. Since the number of independent
samples is proportional to the number of sub-carriers, this cyclic extension correlation
method is efficient only if a large number of sub-carriers are present (more than 100). In
the case of less number of sub-carriers, the side-lobe to peak ratio of the correlator output
will be high and sometimes this might lead to wrong timing.
Once the timing is established using the correlation output, the frequency offset can be directly estimated. The phase of the correlator output is equal to the phase drift between samples that are T seconds apart. Hence the frequency offset can be estimated as the correlation phase divided by T p 2 .
Multi-Carrier CDMA
Recently a new proposal for a system based on a combination of CDMA and OFDM has gained increasing attention in the research community. This system is called the Multi-Carrier CDMA (MC-CDMA) system and it combines the advantages offered by both OFDM and CDMA.
System Model
A MC-CDMA transmitter spreads the data signal using a given spreading code in the frequency domain. In other words, each chip of the signal is transmitted over a separate sub-carrier. The block diagram of a basic OFDM transmitter is shown in figure (trans). In the MC-CDMA transmitter, the input data stream is first converted into a parallel symbol stream (of width P), using a serial to parallel converter. Each data symbol is spread using a spreading code K. All the data in total ( K P ´ ), are now transmitted in
parallel using sub-carrier modulation (OFDM).
Multi-carrier CDMA Transmitter Multi-carrier CDMA receiver
In the MC-CDMA receiver, after down-conversion, the K sub-carrier components
corresponding to the received users data is first coherently detected with the DFT and
combined (using various diversity combining strategies) to yield the received data.
Advantages of MC-CDMA
Combining OFDM with CDMA has a lot of advantages when compared to using DSCDMA alone. Some of them are discussed in this section:
¢ _ The transmitted symbol duration is much larger than the chip duration of DS-CDMA,this makes the job of synchronization much easier.
¢ _ Provided there is an adequate guard interval provided, the multi-path correction in the form of RAKE combining is not necessary.
¢ The OFDM-CDMA system provides inherent frequency diversity, since a single
symbol is spread over a wide range of frequencies that may fade independently
and a diversity combiner can be used to improve the fading performance of the
system.
¢ _ Finally, it must be noted that all these advantages are in addition, to what is alreadyoffered by CDMA.
Applications of OFDM
A lot of applications that use OFDM technology have spawned over the last few years. In this section, one such application will be described in detail, while a introduction to the other applications will be provided.
Digital Video Broadcasting (DVB)
Digital Video Broadcasting (DVB) is a standard for broadcasting Digital Television over satellites, cables and thorough terrestrial (wireless) transmission. DVB was standardized by the ETSI in 1997 [9]. The following are some important parameters of DVB: _ DVB has two modes of operation: the 2k mode with 1705 sub-carriers and the 8k modes with 6817 sub-carriers.
¢ _ DVB uses QPSK, 16-QAM or 64-QAM sub-carrier modulation.
¢ _ DVB uses a Reed-Solomon outer code (204,188,t=8) and a inner convolutional code with generator polynomials (177,133 octal) combined with two layers of interleaving for error-control.
¢ _ Pilot Sub-carriers are used to obtain reference amplitudes and phases for coherent
demodulation. Two-dimensional channel estimation is performed using the pilot
sub carriers, which aids in the reception of the OFDM signal.
Wireless LANs
Wireless LANs are one of the most important applications of OFDM. A lot of standards have been proposed for Wireless LANs during the past decade, most of then based on spread-spectrum schemes. In July 1998, IEEE Wireless LAN standardization group IEEE 802.11 standardized a scheme based on OFDM operating in the 5-GHz band. It is interesting to note that this standard is one of the first packet-based one to use OFDM. The parameters of this WLAN standard are given in table
WLAN-OFDM Parameters
One of the main reasons for using OFDM for Wireless LANs is relatively small amount of delay spread encountered in such applications. In the case of indoor environments, the delay spread is still much less and the efficiency of OFDM in such environments is very high. In outdoor-environments however, directional antennas need to be employed if the same guard interval were used
OFDMA
Finally, it is also possible to use OFDM for multiple-access too. This technique is called OFDMA and is implemented by providing each user with a small number of sub-carriers. Even though this technique is similar to FDMA, it avoids the use of large guard bands that are used to prevent adjacent channel interference.
Conclusion
OFDM has several interesting properties that suit its use over Wireless channels and hence many Wireless standards have started to use OFDM for modulation and multiple access. The various methods of generation and demodulation of OFDM and specific issues such as linearity and synchronization were analyzed. Application of OFDM such MC-CDMA, DAB, DVB , WLAN etc, were also discussed in detail.
please read
http://studentbank.in/report-orthogonal-...ull-report
http://studentbank.in/report-wideband-ofdm-full-report
http://studentbank.in/report-ofdm-on-mob...munication
for getting more technical information about orthogonal frequency division multiplexing OFDM technology
Introduction
In older multi-channel systems using FDM, the total available bandwidth is divided into N non-overlapping frequency sub-channels. Each sub-channel is modulated with a separate symbol stream and the N sub-channels are frequency multiplexed. OFDM is a multi-channel modulation system employing Frequency Division Multiplexing (FDM) of orthogonal sub-carriers, each modulating a low bit-rate digital stream. In FDM, the prevention of spectral overlapping of sub-carriers reduces Interchannel Interference, but leads to an inefficient use of spectrum. The guard bands on either side of each sub-channel are a waste of precious bandwidth. To overcome this problem, OFDM uses N overlapping (but orthogonal) sub carriers, each carrying a baud rate of 1/T and spaced 1/T apart. Because of the frequency spacing selected, the sub-carriers are all mathematically orthogonal to each other. This permits the proper demodulation of the symbol streams without the requirement of non overlapping spectra.
Another way of specifying the sub-carrier orthogonality condition is to require that each sub-carrier have exactly integer number of cycles in the interval T. It can be shown that the modulation of these orthogonal sub-carriers can be represented as an Inverse Fourier Transform. Alternatively, one may use a DFT operation followed by low-pass filtering to generate the OFDM signal. It must be noted that OFDM can be used either as a modulation or a multiplexing technique.
OFDM using Inverse DFT
The use of Discrete Fourier Transform (DFT) in the parallel transmission of data using
Frequency Division Multiplexing was investigated in 1971 by Weinstein and Ebert [1].Consider a data sequence d0, d2, ¦, dN-1, where each dn is a complex symbol.(the data sequence could be the output of a complex digital modulator, such as QAM, PSK etc). Suppose we perform an IDFT on the sequence 2dn (the factor 2 is used purely for scaling purposes), we get a result of N complex numbers Sm (m = 0,1¦,N-1) as:
Where, Ts represents the symbol interval of the original symbols. Passing the real part of
the symbol sequence represented by equation (2.1) thorough a low-pass filter with each
symbol separated by a duration of Ts seconds, yields the signal,
OFDM Modulator
Where, T is defined as NTs. The signal y(t) represents the baseband version of the OFDM
Three Subcarriers within an OFDM symbol Spectra of Individual Sub-Carriers
Guard Time and Cyclic Extension
One of the main advantages of OFDM is its effectiveness against the multi-path delay spread frequently encountered in Mobile communication channels. The reduction of the symbol rate by N times, results in a proportional reduction of the relative multi-path delay spread, relative to the symbol time. To completely eliminate even the very small ISI that results, a guard time is introduced for each OFDM symbol. The guard time must be chosen to be larger than the expected delay spread, such that multi-path components from one symbol cannot interfere with the next symbol. If the guard time is left empty, this may lead to inter-carrier interference (ICI), since the carriers are no longer orthogonal to each other. To avoid such a cross talk between sub-carriers, the OFDM symbol is cyclically extended in the guard time. This ensures that the delayed replicas of the OFDM symbols always have an integer number of cycles within the FFT interval as long as the multi-path delay spread is less than the guard time.
Guard time and cyclic extension-Effect of Multipath
OFDM Generation
The generation of OFDM symbol is as follows
¢ First, the N input complex symbols are padded with zeros to get Ns symbols that are used to calculate the IFFT. The output of the IFFT is the basic OFDM symbol
¢ Based on the delay spread of the multi-path channel, a specific guard-time must be chosen (say Tg). A number of samples corresponding to this guard time must be taken from the beginning of the OFDM symbol and appended at the end of the symbol. Likewise, the same number of samples must be taken from the end of the OFDM symbol and must be inserted at the beginning.
¢ The OFDM symbol must be multiplied with the raised cosine window to remove the power of the out-of-band sub-carriers.
¢ The windowed OFDM symbol is then added to the output of the previous OFDM symbol with a delay of Tr, so that there is an overlap region of r T ?between each symbol.
Figure shows the block diagram of an OFDM transmitter and receiver.
OFDM System Block Diagram
The following are the most important design parameters of an OFDM system.
¢ Guard Time
¢ Symbol Duration
¢ Number of Sub-carriers
Advantages of OFDM
OFDM possesses some inherent advantages for Wireless Communications. This section glances on few of the most important reasons on why OFDM is becoming more popular in the Wireless Industry today.
¢ Multi-path Delay Spread Tolerance
¢ Effectiveness against Channel Distortion
¢ Throughput Maximization (Transmission at Capacity)
¢ Robustness against Impulse Noise
Synchronization in OFDM Systems
Synchronization in OFDM system is achieved by the following methods
¢ Synchronization using Cyclic Extension
¢ Synchronization using Training Sequences
Synchronization using Cyclic Extension
Since a Cyclic extension is added to every OFDM symbol, the first Tg seconds of the OFDM symbol is identical to the last part. This property can be exploited for both timing and frequency synchronization using a scheme depicted in figure.
Synchronization using cyclic extension
This scheme correlates Tg seconds of the OFDM symbol with a part that is T seconds delayed (T “ being the symbol time, less the guard period Tg). The output of the
correlator can be written as:
The symbol timing is estimated from the correlation peaks at the output of the correlator.
The characteristics of the correlation peaks are better if the correlation is
performed over a large number of independent samples. Since the number of independent
samples is proportional to the number of sub-carriers, this cyclic extension correlation
method is efficient only if a large number of sub-carriers are present (more than 100). In
the case of less number of sub-carriers, the side-lobe to peak ratio of the correlator output
will be high and sometimes this might lead to wrong timing.
Once the timing is established using the correlation output, the frequency offset can be directly estimated. The phase of the correlator output is equal to the phase drift between samples that are T seconds apart. Hence the frequency offset can be estimated as the correlation phase divided by T p 2 .
Multi-Carrier CDMA
Recently a new proposal for a system based on a combination of CDMA and OFDM has gained increasing attention in the research community. This system is called the Multi-Carrier CDMA (MC-CDMA) system and it combines the advantages offered by both OFDM and CDMA.
System Model
A MC-CDMA transmitter spreads the data signal using a given spreading code in the frequency domain. In other words, each chip of the signal is transmitted over a separate sub-carrier. The block diagram of a basic OFDM transmitter is shown in figure (trans). In the MC-CDMA transmitter, the input data stream is first converted into a parallel symbol stream (of width P), using a serial to parallel converter. Each data symbol is spread using a spreading code K. All the data in total ( K P ´ ), are now transmitted in
parallel using sub-carrier modulation (OFDM).
Multi-carrier CDMA Transmitter Multi-carrier CDMA receiver
In the MC-CDMA receiver, after down-conversion, the K sub-carrier components
corresponding to the received users data is first coherently detected with the DFT and
combined (using various diversity combining strategies) to yield the received data.
Advantages of MC-CDMA
Combining OFDM with CDMA has a lot of advantages when compared to using DSCDMA alone. Some of them are discussed in this section:
¢ _ The transmitted symbol duration is much larger than the chip duration of DS-CDMA,this makes the job of synchronization much easier.
¢ _ Provided there is an adequate guard interval provided, the multi-path correction in the form of RAKE combining is not necessary.
¢ The OFDM-CDMA system provides inherent frequency diversity, since a single
symbol is spread over a wide range of frequencies that may fade independently
and a diversity combiner can be used to improve the fading performance of the
system.
¢ _ Finally, it must be noted that all these advantages are in addition, to what is alreadyoffered by CDMA.
Applications of OFDM
A lot of applications that use OFDM technology have spawned over the last few years. In this section, one such application will be described in detail, while a introduction to the other applications will be provided.
Digital Video Broadcasting (DVB)
Digital Video Broadcasting (DVB) is a standard for broadcasting Digital Television over satellites, cables and thorough terrestrial (wireless) transmission. DVB was standardized by the ETSI in 1997 [9]. The following are some important parameters of DVB: _ DVB has two modes of operation: the 2k mode with 1705 sub-carriers and the 8k modes with 6817 sub-carriers.
¢ _ DVB uses QPSK, 16-QAM or 64-QAM sub-carrier modulation.
¢ _ DVB uses a Reed-Solomon outer code (204,188,t=8) and a inner convolutional code with generator polynomials (177,133 octal) combined with two layers of interleaving for error-control.
¢ _ Pilot Sub-carriers are used to obtain reference amplitudes and phases for coherent
demodulation. Two-dimensional channel estimation is performed using the pilot
sub carriers, which aids in the reception of the OFDM signal.
Wireless LANs
Wireless LANs are one of the most important applications of OFDM. A lot of standards have been proposed for Wireless LANs during the past decade, most of then based on spread-spectrum schemes. In July 1998, IEEE Wireless LAN standardization group IEEE 802.11 standardized a scheme based on OFDM operating in the 5-GHz band. It is interesting to note that this standard is one of the first packet-based one to use OFDM. The parameters of this WLAN standard are given in table
WLAN-OFDM Parameters
One of the main reasons for using OFDM for Wireless LANs is relatively small amount of delay spread encountered in such applications. In the case of indoor environments, the delay spread is still much less and the efficiency of OFDM in such environments is very high. In outdoor-environments however, directional antennas need to be employed if the same guard interval were used
OFDMA
Finally, it is also possible to use OFDM for multiple-access too. This technique is called OFDMA and is implemented by providing each user with a small number of sub-carriers. Even though this technique is similar to FDMA, it avoids the use of large guard bands that are used to prevent adjacent channel interference.
Conclusion
OFDM has several interesting properties that suit its use over Wireless channels and hence many Wireless standards have started to use OFDM for modulation and multiple access. The various methods of generation and demodulation of OFDM and specific issues such as linearity and synchronization were analyzed. Application of OFDM such MC-CDMA, DAB, DVB , WLAN etc, were also discussed in detail.
please read
http://studentbank.in/report-orthogonal-...ull-report
http://studentbank.in/report-wideband-ofdm-full-report
http://studentbank.in/report-ofdm-on-mob...munication
for getting more technical information about orthogonal frequency division multiplexing OFDM technology
