The Orthogonal Frequency Division Multiplexing

Published Date: 02 Nov 2017

Abstract

Orthogonal frequency division multiplexing is the recent developments across the telecommunications and there is lot of advantages with OFDM implementation. The detailed discussion to the technical standards of OFDM implementations and they are discussed in this report. There are some key advantages and some limitations with OFMD are also and they are also discussed in this report. There is lot of applications of OFDM and they are discussed in this report and a detailed conclusion to the observations is done in this report and they are as given below.

Contents

Introduction to OFDM

According to the views and opinions of Van Nee.et.al (2000) there is lot of research and development done towards the OFDM (Orthogonal Frequency Division Multiplexing) since many decades and still the work is under progress. The rage of technologies used at this level is complex and sophisticated in nature and OFDM was accepted as the new wireless technology by IEEE standards 802.11a, mobile multimedia access and high performance LAN type 2 as well. The main advantage with OFDM is that it can be used across the high end mobile broadband services and access. The main aim of this report is to discuss the range of technical specification against implementing the OFDM and also list out the key advantages and limitations of OFDM and is as given below

Technical details of OFDM

Schmidl.et.al (1997) stated that the data rate standards of OFDM are sufficient for the broadband communication and thus it has wide range of usage across the broadband mobile services and the bit rate is almost equal to 54 Mbps. OFDM can be considered can be either a multiplexing or modulation technique the main reason for OFDM to increased its frequency against robustness is that it has narrowband interference and selective fading channels across its operations. The main limitation with single carrier is system or single fading channel is that the chances of link failures are more and thus multiple carrier systems are always preferable and thus OFDM has gained lot of popularity across telecommunication world. Error correction coding is another key implementation across OFDM and it is used for reducing the errors across few erroneous subcarriers. The actual concept of parallel data transmission is introduced around the year 1960 and later it has gained wide range of popularity and across the parallel data transmission is divided into N non overlapping frequency channels. All the N sub channels are frequency multiplexed and all the sub-channels will be forwarded with the individual symbols towards the implementation of OFDM. Spectral overlap of the channels is completely eliminated over the OFDM implementation and thus the overall inter channel interference is completely avoided (Yang.et.al, 2000).

In views of Mody & Stuber (2003) there are some cased where this situation leads to insufficient usage of the available spectrum and thus to overcome this situation parallel data and FDM techniques can be used and this idea was proposed around mid of 1960’s. Signaling rate b is used for against frequency and the use of high speed equalization is completely eliminated and the multipath distortion and combat impulsive noise are also achieved against using the complete bandwidth available. Almost of 50% of bandwidth can be saved by using conventional non overlapping multicarrier technique and there are some chances due to this implementation that the crosstalk can be increased and thus orthogonality is required in this context between the modulated carrier signals. In general the terms Orthogonal indicates the mathematical relationship across frequencies of different carries in the system.

When the case with simple frequency division multiplexing is considered all the carriers are spaced apart and the required signals can be received using the demodulators and conventional filters as well. The concept of guard bands are introduced across these receivers across different frequencies and the carriers and thus the overall spectrum efficiency is reduced a lot. While the case with OFDM is considered to arrange the carrier’s individual carriers and the sidebands are overlapped and thus the signals receive without adjacent carrier interference. Thus to achieve this all the signals should be orthogonal in nature and now the receiver acts as the storage place of demodulators and all the carriers are translated to DC and the resulting signals are integrated over symbol period and the overall raw data is recovered (Mody & Stube,2001).

Y Li (2002) revealed that f the rest of the carriers are down to the estimated frequencies the overall time domain has whole number of cycles across the symbol period say T and a zero contribution is achieved towards integration process from the rest of the carriers. Thus all the carriers are linearly independent and orthogonal in nature and the required carrier spacing is multiple of 1/T factor and the corresponding figure is as shown below

Figure: Signal Spectra of OFDM

Source: Van Nee & Prasad (2000)

A wide range of research is done towards the high efficient multicarrier transmission system that was based on orthogonal frequency carriers and it is discussed in this section. As per the research done by Ebert in the year 1971, it is clear that discrete Fourier transform (DFT) is applied for parallel data transmission system across the modulation and demodulation process. OFDM signals are multiplexed across the individual spectra using a frequency spacing b and this is equal to transmission speed of the sub carriers and thus the overall crosstalk from each and every carrier is reduced a lot.

Goule & Leib (2003) opined that thus DFT can be used at the receiver end and the correlation values can be calculated using the center of frequency of each sub carrier and the required transmission data can be recovered with negligible crosstalk across the data transmission process. Using the DFT multicarrier technique frequency division multiplexing can be achieved by band pass filtering and baseband processing techniques as well. To eliminate the corresponding banks of subcarrier oscillators and coherent demodulators frequency division multiplexing is required and thus the required digital implementations can be used by taking the additional hardware into consideration across fast Fourier transformation (FFT) which is considered to be a better implementation than DFT.

As per the recent developments in VLSI large size and high speed FFT chips are made to be available for the commercial usage and thus the receiver and transmitter are using the recent FFT techniques and the number of operations is reduced from NXN to N log N. At the initial periods of 1980’s OFDM’s are purely used for digital mobile communications, high speed modems and high density recording and later these techniques are used for multiplexed QAM using the DFT techniques where the carriers are stabilized using the clock frequency control along with the implementation of trellis coding standards as well. Later telephone networks are developed using these high speed modems and in the year 1990 OFDM usages are expanded towards the wideband data communications (Guillaud & Slock, 2003)

Advantages with OFDM

As per the views of Liu Xin.et.al (2002) the usage range of mobile communications is increased across the FM radio channels, high bit rate digital subscriber lines and these lines are asymmetric in nature and the main usage of these standards can be observed across broadband communications and high definition television terrestrial broadcasting. Following are the key advantages with OFDM transmission scheme

Efficient use of spectrum across overlap can be achieved with OFDM transmission

ISI and IFI can be eliminated and the use of cyclic prefix can also be eliminated

OFDM is more resistant towards the frequency selective fading when compared to single carrier systems

Lost symbols can be recovered using the interleaving and adequate channel coding across OFDM

Channel equalization can be made more easily by using the adaptive channel equalization techniques with the single carrier systems

OFDM is computationally efficient as it used FTF techniques and thus the modulation and demodulation bas become very simple

Sample timing offsets are reduced a lot when compared to single carrier systems

Implementation of channel estimator is completely eliminated across OFDM

Co-channel interferences is reduced a lot with the OFDM transmission impulsive parasitic noise is also reduced a lot (Gama,2003)

Thus from the above discussion it is clear that the overall advantages are high with the OFDM transmission and still there are some listed advantages apart from the above and they are as given below

High transmission bit rates can be achieved and the overall packet loss is also reduced in this context

There is always a chance with OFDM implementation to cancel the required channels if they are affected with fading

N narrowband transmissions are done with OFDM and it is always easy to accomplish all the channels and thus the overall transmission is not affected with this high number of channels.

Inter symbol interference (ISI) and Inter channel interference (ICI) are introduced across multipath channel coding against the OFDM implementation

The duration of guard period is increased across the OFDM transmission

OFDM is free from multipath propagation and thus always a high data rate and bit rate can be achieved at this level

As the data stream is divided into multiple parallel stream prior to the actual data transmission the OFDM symbol rate is reduced a lot

Pilot subcarriers that were used across channel estimation are easily allowed across the OFDM transmission process (Y. Liu, M. P. Fitz,2001)

Frequency selective burst errors are reduced against the implementation of OFDM

Usage of FFT and IFFT is made very easy across the OFDM transmission process

If there is any excess delay across the data transmission degradation is always possible in this context

Computational complexity is reduced across the equalization and bandwidth stabilization processes

Adaptive modulation and coding of frequency is allowed across the OFDM implementation

Thus from the overall advantages with OFDM it is clear that the usage of this technology across high speed communication systems and mobile broadband communications is proved to be successful. Apart from these noted advantages there are some disadvantages as well and they are discussed as below

Disadvantages of OFDM

Lu & Wang (2002) stated that the key advantages with OFDM are discussed in the previous section and still there are some noted disadvantages with OFDM and they are given as below

OFDM is more sensitive towards carrier frequency offset and due to the leakage of DFT there is a drift towards single signal carrier system also

Most of the OFDM signals has noise like amplitude at a large dynamic range and it always in the need of RF power amplifiers and this can be considered as the key limitation against implementing the OFDM systems

A high rate of synchronism accuracy is achieved across the OFDM and thus the overall quality is reduced a lot

Multipath propagation should be avoided else the overall transmission rate is affected with the OFDM implementation

Peak to mean ratio is increased a lot across the OFDM and thus the overall speed in data transmission may be reduced a lot

Distortion problem is raised due to the superposition of all the subcarrier signals

OFDM is sensitive towards the Doppler shift and thus it can be considered as another key limitation in implementation of OFDM

There are lot of frequency synchronization problems with the implementation of OFDM and these need to be reduced a lot in this context

There is a lot of peak to average power ration across OFDM and this leads to linear transmitter circuitry and thus the overall power efficiency is reduced a lot

OFDM always suffers from the time variations across the channels and also the carrier frequency offsets

A loss in subcarrier orthogonality is observed across the OFDM implementation due to the imperfect frequency synchronization

Intermodulation is more across the OFDM due to the raise in noise floor and this need to be reduced a lot in this context (Ha Mody,2002)

From the overall discussion done towards the limitations of OFDM it is clear that the overall power consumption is more and this can be considered as the main limitations and even the distortion problem across the OFDM is a serious limitation and this need to avoided for a better mobile communication in this context. Thus from the overall advantages and limitations with OFDM it is clear that there are some key application areas of communication technology where the role of OFDM is significant and they are discussed in the below section

Application areas of OFDM

According to the opinions of Tariq et al (2002) there are key applications across usage of OFDM and they are discussed in this section. The main applications related to telecommunications, high speed broadband communications and high definition televisions are included across the OFDM applications and there are some potential applications as well and they are listed below

Broadcast television can be considered as the main application for OFDM and the level of technology used across this technology is sophisticated in nature

Digital video broadcasting terrestrial is the main concept involved across broadcast television and multipath tolerance is used across the OFDM implementation

OFDM is a single frequency network and thus it has made the key application range across the Broadcast television

Digital video broadcasting handheld is another potential application across the OFDM implementation and the time sliced concept is used to reduce the battery drain across the system

Resistance to impulse noise ratio is improved and thus it is more interleaving and has made it to use as the application across digital video broadcasting handheld

Digital radio Mondale is another key applications of OFDM where the broadcast radio is wide used across this concept (Witrisal et al, 2000)

Conclusion

A detailed analysis is done towards the technical specifications of OFDM and thus from the overall analysis carried out it is clear that there are many potential advantages and few disadvantages as well across the implementation of OFDM. The advantages of OFDM include Frequency selective burst errors are reduced against the implementation of OFDM and efficient use of spectrum across overlap can be achieved with its transmission. Implementation of channel estimator is completely eliminated across the model and Sample timing offsets are reduced a lot when compared to single carrier systems.

When the case with limitations of OFDM are considered there are some key limitations across the implementation of it and they are a high rate of synchronism accuracy is achieved across that and thus the overall quality is reduced a lot and it is sensitive towards the Doppler shift and thus it can be considered as another key limitation in implementation of OFDM. There is always a chance with its implementation to cancel the required channels if they are affected with fading and Intermodulation is more across the OFDM due to the raise in noise floor and this need to be reduced a lot in this context.

Multipath propagation should be avoided else the overall transmission rate is affected with the its implementation and a loss in subcarrier orthogonality is observed across the OFDM implementation due to the imperfect frequency synchronization and Peak to mean ratio is increased a lot across the model and thus the overall speed in data transmission may be reduced a lot. OFDM is free from multipath propagation and thus always a high data rate and bit rate can be achieved at this level and Frequency selective burst errors are reduced against the implementation of Orthogonal Frequency Division Multiplexing.