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Frequency division multiplexing (FDM) is a technology that transmits multiple signals simultaneously over a single transmission path, such as a cable or wireless system. Each signal travels within its own unique frequency range (carrier), which is modulated by the data (text, voice, video, etc.).
Orthogonal FDM's (OFDM) spread spectrum technique distributes the data over a large number of carriers that are spaced apart at precise frequencies. This spacing provides the "orthogonality" in this technique which prevents the demodulators from seeing frequencies other than their own.
The benefits of OFDM are high spectral efficiency, resiliency to RF interference, and lower multi-path distortion. This is useful because in a typical terrestrial broadcasting scenario there are multipath-channels (i.e. the transmitted signal arrives at the receiver using various paths of different length). Since multiple versions of the signal interfere with each other (inter symbol interference (ISI)) it becomes very hard to extract the original information.
A single-carrier system modulates information onto a single carrier using one of the most common techniques used in signal processing i.e., by varying frequency, phase, or amplitude of carrier. As digital signals are quantized in the form of bits and symbols, and these bits and symbols are in greater number so digital signals require higher bandwidth. Consequently the time period of these bits and symbols become shorter and shorter which causes interference between these bits or symbols. This type of interference is commonly known as Intersymbol Interface (ISI).
The time variations of the channel during one OFDM frame destroy the orthogonality of different subcarriers and results in power leakage among the subcarriers, known as Intercarrier Interference (ICI).
In some respects, OFDM is similar to conventional frequency-division multiplexing (FDM). The difference lies in the way in which the signals are modulated and demodulated. Priority is given to minimizing the interference, or crosstalk, among the channels and symbols comprising the data stream. Less importance is placed on perfecting individual channels.
In my view OFDM use different amounts of carriers for different modulated packets of data which are transmitted over sub carriers & each subcarrier transmit signal at less speed and receive it in the original speed at receiver. In OFDM multiple high speed digital signals are send in parallel on orthogonal carrier frequencies.
OFDM systems are implemented using a combination of fast Fourier Transform (FFT) and inverse fast Fourier Transform (IFFT) blocks.
In implementing the OFDM uses several steps which are
the incoming data is first converted from a serial stream to parallel sets of data. Each set of data contains one symbol, representing each subcarrier. source symbols are used as the inputs to an IDFT block ,the Orthogonal Sinusoidal symbols are used as input functions for IDFT each having a different frequency and they are Complex. The IDFT output is the summation of all N sinusoids. Thus, the IDFT block provides a simple way to modulate data onto N orthogonal subcarriers.
A DFT block at receiver, is used to process the received signal and bring it into the frequency domain. DFT output will be the original symbols sent.
Before performing the Discrete Fourier Transform (DFT), the incoming analog OFDM data is first converted into digital data stream, then this digital OFDM data stream shifts from serial into parallel sets of data, making easier to change it into frequency domain.
1. INTRODUCTION :
Frequency Division Multiplexing (FDM):
Frequency Division Multiplexing (FDM) is a technique that transmits multiple signals simultaneously over a single transmission path, such as a cable or wireless system. Each signal travels within its own unique frequency range (carrier), which is modulated by the data (text, voice, video, etc.). 
Conventional FDM Spectrum
The basic idea behind (FDM) is to divide the available bandwidth into different number of sub channels (carriers), each of which is spaced with suitable guard band to reduce interference, whereas each sub channel is transmitted simultaneously.
Orthogonal Frequency Division Multiplexing (OFDM) :
Orthogonal Frequency Division Multiplexing (OFDM) is a digital transmission technique that uses a large number of carriers spaced apart at slightly different frequencies. OFDM carries used only one data stream broken up into multiple signals. Hundreds or thousands of carriers, known as "sub carriers", are used for a single data channel. 
The basic idea behind (OFDM) is to occupy significantly lesser bandwidth than (FDM) technique, efficiently by using non overlapping (In Frequency only) orthogonal carriers each of a different frequency, The frequency spacing of the carriers is chosen in such a way that the modulated carriers are orthogonal and do not interfere with one another. Therefore guard bands are not required between individual sub channels (carriers), providing the facility for using the available bandwidth in a more efficient manner.
What Is Orthogonal Frequency Division Multiplexing (OFDM)?
OFDM is one of the most recently introduced digital modulation scheme that modulates digital data onto a radio frequency (RF) signal, then divides the data in different sub parts. OFDM efficiently uses different number of carriers for different number of modulated data packets, each sub carrier can transmit a lower-speed signal, all of which are converted back in the receiver into the original higher-speed signal. All of the data packets are transmitted simultaneously over these sub carriers.
Each packet of data is transmitted over a carrier that is spaced apart at precise frequency from other carrier carrying another packet of modulated data. This spacing provides the "orthogonality" in this technique which prevents the demodulators from recovering the signals with frequencies other than their predefined frequency. Therefore in OFDM multiple high speed digital signals are send in parallel on orthogonal carrier frequencies.
The result is much more efficient use of bandwidth as well as robust communications during noise and other interferences. 
A single-carrier system modulates information onto a single carrier using one of the most common techniques used in signal processing i.e., by varying frequency, phase, or amplitude of carrier. As digital signals are quantized in the form of bits and symbols, and these bits and symbols are in greater number so digital signals require higher bandwidth. Consequently the time period of these bits and symbols become shorter and shorter which causes interference between these bits or symbols. This type of interference is commonly known as Intersymbol Interface (ISI), when the reciprocal of the system rate is significantly smaller than the time dispersion. Actually ISI is caused by multi-path delays. This is the result of receiving not one, but several copies of the signal, due to multiple reflections (i.e., Multipath), say, off buildings, airplanes, etc., of the transmitted signal. In such a case, a channel equalizer is employed at the receiver to compensate for the channel distortions. The figure below shows a simple representation of a Single-carrier modulation system.
In telecommunication systems, intersymbol interference (ISI) is a form of distortion of a signal in which one symbol interferes with subsequent symbols.
In this way all the symbols have similar effect as noise. Therefore system becomes more susceptible to loss of information from impulse noise, harmonic noise, timing jitter, frequency offset, and signal reflections. All of these noise factors can make it difficult for the receiver to recover all the symbols in their original form as they were sent by the transmitter.
Frequency division multiplexing (FDM) extends the concept of single carrier modulation by using multiple subcarriers within the same single channel. The total data rate to be sent in the channel is divided between the various subcarriers. The data do not have to be divided evenly nor do they have to originate from the same information source. FDM systems usually require a guard band between modulated subcarriers to prevent the spectrum of one subcarrier from interfering with another. These guard bands lower the system's effective information rate when compared to a single carrier system with similar modulation. 
In order to increase system's effective information rate, which was not maintained by the FDM, an updated modulation system known as Orthogonal Frequency Division Multiplexing (OFDM) is introduced. OFDM uses a collection of orthogonal subcarriers, and the guardbands that were necessary to demodulate individual carriers in an FDM system would no longer be necessary in OFDM. The use of orthogonal subcarriers allows the subcarriers to overlap, without loosing their spectral information and energy, and as long as this orthogonality is maintained, it would be possible to recover the individual subcarriers signals despite their overlapping spectrums. The figure below shows a simple representation of an OFDM system.
2. HISTORY :
In the early-1960s the idea was to use parallel data transmission carriers and was named as Frequency Division Multiplexing (FDM). OFDM; sometimes called multi-carrier or discrete multi-tone modulation was the idea proposed in the late 1960s, according to which each subcarrier overlapped to its adjacent subcarriers with minimum frequency spacing in between, and this spacing was carefully designed so that each subcarrier is orthogonal to the other subcarriers. The OFDM technique was used in several high frequency military systems in the early days of its invention, named as under.
In 1971, Weinstein and Ebert applied the Discrete Fourier Transform (DFT) to parallel data transmission systems as part of modulation and demodulation process. 
- FFT-based OFDM.
In the 1980s, OFDM was studied for high-speed modems digital mobile communication, and high-density recording. 
- Pilot tone is used to stabilize carrier and frequency control.
- Trellis code is implemented.
In 1980, Hirosaki suggested an equalization algorithm in order to suppress both intersymbol and intercarrier interference caused by the channel impulse response or timing and frequency errors. In the late 1980, Hirosaki also introduced the DFT-based implementation of Saltzburg's O-QAM OFDM system. 
In the 1990s, OFDM was exploited for wideband data communications.
- Mobile radio FM channels.
- Fix-wire network.
- High-bit-rate digital subscriber line. (HDSL)
- Asymmetric digital subscriber line. (ADSL)
- Very-high-speed digital subscriber line. (VDSL)
- Digital audio broadcasting. (DAB)
- Digital video broadcasting. (DVB)
- High-definition television (HDTV) terrestrial broadcasting.
There exist three mechanisms about the digital terrestrial television broadcasting system in European (COFDM), North America (8-VSB), and Japan (BST-OFDM). 
- Wireless LAN
- HIPERLAN2 (European)
- IEEE 802.11a (U.S.A)
- IEEE 802.11g (U.S.A)
3. DESCRIPTION :
Earlier the approach was to design a bandwidth-efficient communication system in the presence of channel distortion i.e., to subdivide the available channel bandwidth into a number of equal-bandwidth sub channels, where the bandwidth of each sub channel is sufficiently narrow so that the frequency response characteristics of the sub channels are nearly ideal. Such a subdivision of the overall bandwidth into smaller sub channels is illustrated in Figure below. Thus, we create K = W/∆f sub channels, where different information symbols can be transmitted simultaneously in the K sub channels. Consequently, the data is transmitted by frequency-division multiplexing (FDM). 
With each subchannel, we associate a carrier:
Xk (t) = sin 2∏ fk t,k= 0, 1 …… k-1
With an OFDM system having K subchannels, the symbol rate on each subcarrier is reduced by a factor of N relative to the symbol rate on a single carrier system that employs the entire bandwidth W and transmits data at the same rate as OFDM. Hence, the symbol interval in the OFDM system is T= KTs, where Ts is the symbol interval in the single-carrier system. By selecting K to be sufficiently large, the symbol interval T can be made significantly larger than the time duration of the channel-time dispersion. Thus, intersymbol interference can be made arbitrarily small by selection of K. In other words, each subchannel appears to have a fixed frequency response C (fk), k = 0, 1…... K - 1. 
As long as the orthogonality among the subcarriers is kept constant, then by using the OFDM system, different number of bits/symbol can be modulated on each subcarrier. Hence, subcarriers that yield a higher SNR due to a lower attenuation can be modulated to carry more bits/symbol than subchannels that yield a lower SNR (high attenuation). As QAM, PSK, QPSK, FSK and other modulation schemes with different constellation sizes can be used in an OFDM system.
4. IMPLEMENTATION :
OFDM is a special form of multicarrier modulation. An efficient way to implement OFDM by means of a Discrete-time Fourier Transform (DFT) was found by Weinstein in 1971. The computational complexity could be further reduced by a Fast Fourier Transform (FFT). However, OFDM was not popular at that time because the implementation of large-size FFTs was still too expensive. Recent advances in VLSI technologies have enabled cheap and fast implementation of FFTs and IFFTs. 
In practice, OFDM systems are implemented using a combination of fast Fourier Transform (FFT) and inverse fast Fourier Transform (IFFT) blocks that are mathematically equivalent versions of the DFT and IDFT. 
The complete block diagram of an OFDM system implemented using Discrete-time Fourier Transform (DFT) and Fast Fourier Transform (FFT) techniques, is shown in figure below.
Serial to Parallel, And Parallel to Serial Buffer, D/A converter:
Before performing the Inverse Discrete Fourier Transform (IDFT), the incoming data is first converted from a serial stream to parallel sets of data, making it easier for IDFT block in time domain conversion. Each set of data contains one symbol, representing each subcarrier. The signal samples Xn generated after computing the IDFT are passed through a serial to parallel converter, in which parallel sets of data are converted into serial stream again. The serial stream of OFDM data is then passed from a digital-to-analog (D/A) converter, whose output, ideally is the OFDM signal waveform X(t).
INVERSE DISCRETE FOURIER TRANSFORM (IDFT) :
An OFDM system treats the source symbols (e.g., the QPSK or QAM symbols that would be present in a single carrier system) at the transmitter as though they are in the frequency-domain. These symbols are used as the inputs to an IDFT block that brings the signal into the time domain. The IDFT takes in N symbols at a time where N is the number of subcarriers in the system. Each of these N input symbols has a symbol period of T seconds. Recall that the basis functions for an IDFT are N orthogonal sinusoids. These sinusoids each have a different frequency and the lowest frequency is DC. Each input symbol acts like a complex weight for the corresponding sinusoidal basis function. Since the input symbols are complex, the value of the symbol determines both the amplitude and phase of the sinusoid for that subcarrier. The IDFT output is the summation of all N sinusoids. Thus, the IDFT block provides a simple way to modulate data onto N orthogonal subcarriers. The blocks of N output samples from the IDFT make up a single OFDM symbol. After some additional processing, the time-domain signal that results from the IDFT is transmitted across the channel. 
DISCRETE FOURIER TRANSFORM (DFT) :
At the receiver, a DFT block is used to process the received signal and bring it into the frequency domain. Ideally, the DFT output will be the original symbols that were sent to the IDFT at the transmitter. When plotted in the complex plane, the DFT output samples will form a constellation, such as 16-QAM. However, there is no notion of a constellation for the time-domain signal. When plotted on the complex plane, the time-domain signal forms a scatter plot with no regular shape. Thus, any receiver processing that uses the concept of a constellation (such as symbol slicing) must occur in the frequency domain. The block diagram in Figure below illustrates the switch between frequency-domain and time domain in an OFDM system. 
A/D Converter, Serial to Parallel, And Parallel To Serial Buffer :
Before performing the Discrete Fourier Transform (DFT), the incoming analog OFDM data is first converted into digital data stream, then this digital OFDM data stream splits from a serial stream into parallel sets of data, making it easier for DFT block in frequency domain conversion. The Discrete Fourier Transform (DFT) converts the time domain samples back into a frequency domain representation. The magnitudes of the frequency components correspond to the original data. Finally, the parallel to serial block converts these parallel sets of data into a serial stream, hence recovering the original input data.
CYCLIC PREFIX :
The time period of each symbol in OFDM modulation scheme is long. Therefore to remove Intersymbol Interference (ISI), its necessary to insert cyclic prefix to each block of N signal samples. The cyclic prefix for the block of samples consists of the samples XN - m , XN - m + 1, …..., XN - 1. These samples are appended to the beginning of the block, thus, creating a signal sequence of length N + m samples.  Cyclic prefix creates a guard band around individual OFDM symbols, which greatly minimizes the effects of Intersymbol Interference (ISI), and maintaining the orthogonality in between the symbols.
At the receiver side the cyclic prefix between the individual orthogonal signals is removed then converted into time domain, using DFT.
5. APPLICATIONS :
Most of the standards developed for wireless high rate data transmission in the recent years have been based on multi-carrier modulation. i.e., Orthogonal Frequency Division Multiplexing (OFDM). The future success of the wireless broadband communications revolution strongly depends on increasing the data rate available to the mobile user. This will enable value added services that are not possible with current state of the art mobile wireless radio systems. While an increase in data rate could be achieved by combining techniques in the form of multi mode terminals (e.g. UMTS and wireless LAN). 
The Wireless World Research Forum (WWRF) and IEEE have chosen OFDM best suited modulation scheme for various current and future communications system projects, and researches all over the world. OFDM is well suited for systems in which the channel characteristics make it difficult to maintain adequate communications link performance. OFDM is especially suitable for high-speed communication due to its resistance to Inter Symbol Interference (ISI). OFDM has its wide acceptance in wireless as well as wired communications as an appropriate broadband modulation scheme
WIRED APPLICATIONS :
Early advances made in OFDM opened a whole new era of bandwidth efficient systems by providing the benefits of OFDM in data transferring over phone lines, modems, networks, digital radio, and digital televisions.
- ADSL and VDSL broadband access via POTS copper wiring.
- Power line communication (PLC).
- Multimedia over Coax Alliance (MoCA) home networking.
- Cable modem.
- Multipoint multichannel distribution services. (MMDS) 
WIRELESS APPLICATIONS :
In the early days of OFDM invention the users had limited speed of wireless networking ranging less than 11Mbps. After a lot of research and advancements OFDM is introduced to wireless broadband network, after which the wireless standards like IEEE 802.11a and 802.11g are running the real-world wireless LAN speed ranges to 50Mbps and above.
Typically, OFDM, is a spread-spectrum technology that gives wireless networking a new physical (PHY) layer, is implemented in embedded chipsets made up of radio transceivers, Fast Fourier Transform (FFT) processors, system input/output (I/O), serial to parallel and back again translators and OFDM logic. 
- The wireless LAN radio interfaces IEEE 802.11a, g and HIPERLAN/2.
- The digital radio systems DAB/EUREKA 147, DAB+, Digital Radio Mondiale, HD Radio, T-DMB and ISDB-TSB.
- The terrestrial mobile TV systems DVB-H, T-DMB, ISDB-T and MediaFLO forward link.
- The beyond 3G cellular communication systems Flash-OFDM and 3GPP Long Term Evolution LTE.
- The Wireless MAN / Fixed broadband wireless access (BWA) standards IEEE 802.16 (or WiMAX) and HIPERMAN.
- The Mobile Broadband Wireless Access (MBWA) standards IEEE 802.20, IEEE 802.16e (Mobile WiMAX) and WiBro.
- The wireless Personal Area Network (PAN) Ultra wideband (UWB) IEEE 802.15.3a implementation suggested by WiMedia Alliance. 
- Anti-Jamming, Military applications.
- MIMO-OFDM system for Wireless LAN.
6. ADVANTAGES / DISADVANTAGES:
- Orthogonal frequency-division multiplexing (OFDM) is a bandwidth-efficient signaling scheme for Wideband digital communications. The main difference between frequency division multiplexing (FDM) and OFDM is that in OFDM, the spectrum of the individual carriers mutually overlap.
- The benefits of OFDM are high spectral efficiency, resiliency to RF interference, and lower multi-path distortion.
- OFDM symbols are designed to have a relatively long time duration, but a narrow bandwidth. Hence OFDM is strong to channel multipath dispersion and results in a decrease in the complexity of equalizers.
- Some others are Immunity to delay spread and multipath, Resistance to frequency selective fading, simple equalization, efficient bandwidth usage.
- Increased symbol duration makes an OFDM system more sensitive to the time variations.In particular, the effect of Doppler spreading destroys the orthogonality of the subcarriers, resulting in intercarrier interference (ICI) due to power leakage among OFDM subcarriers.
- In traditional OFDM, each data symbol modulates one out of N subcarriers. Therefore, if a subcarrier experiences a deep fade or strong interference, that information is completely lost and performance degrades.
- The Orthogonal frequency division multiplexing (OFDM) signal is corrupted by a multipath channel and additive white Gaussian noise (AWGN). The OFDM system introduces the guard interval to avoid the inter-symbol interference (ISI).
- Some others are synchronization, need FFT units at transmitter, receiver, sensitive to carrier frequency offset, high peak to average power ratio.
7. TYPES OF OFDM :
The modulation scheme that DAB uses is Coded Orthogonal Frequency Division Multiplexing (COFDM). COFDM uses a very different method of transmission to older digital radio modulation schemes and has been specifically designed to combat the effects of multipath interference for mobile receivers.
COFDM is a modulation scheme that divides a single digital signal across 1,000 or more signal carriers simultaneously. The signals are sent at right angles to each other (hence, orthogonal) so they do not interfere with each other. .
VOFDM : (Vector Orthogonal FDM) A radio frequency technology that enables a wireless connection to provide the same performance as a cable modem. Known to be reliable in areas with a lot of interference, VOFDM is being developed and supported as an open standard for wireless broadband Internet services by Cisco and other communications vendors. .
OFDM / CI :
Carrier Interferometry (CI) is a type of spread spectrum multiple access typically employed with Orthogonal frequency-division multiplexing (OFDM). CI spreading codes are commonly used to spread data symbols across multiple OFDM subcarriers for diversity benefits and to shape the resulting superposition of coded subcarriers for reducing the Peak-to-Average Power (PAPR) of the transmitted signal. 
Wideband OFDM, developed by Wi-Lan, develops spacing between channels large enough so that any frequency errors between transmitter and receiver have no effect on performance. 
FLASH OFDM :
Flarion (Lucent/Bell Labs spinoff) has developed this technology, also called fast-hopped OFDM, which uses multiple tones and fast hopping to spread signals over a given spectrum band. 
OFDM / QAM :
OFDM/OQAM modulation, where each subcarrier is modulated with a staggered Offset Quadrature , Amplitude Modulation (OQAM).
OFDM/OQAM is now a recognized alternative to conventional OFDM for the transmission of signals over multipath fading channels. Indeed with OFDM/OQAM an appropriatePulse-shaping can be introduced to fight against time and Frequency dispersion. 
8. FUTURE TRENDS :
One building block for next-generation wireless access, MIMO (multiple-input, multiple-output), is an advanced antenna technology that can carry 4 to 5 times more data traffic than today's most advanced UMTS-HSDPA-ready (3G) networks. With MIMO, for example, a ½ megabit picture can be downloaded in a half second or a 30-megabit video in half a minute.
MIMO works by creating multiple parallel data streams between the multiple transmit and receive antennas (see figure below). Using the multi-path phenomenon, it can differentiate the separate signal paths from each MIMO antenna. 
OFDM is more robust, which means that it provides better performance in cluttered areas with many reflections (multipath). It also allows for simpler receivers. Perhaps most important, OFDM is more amenable to MIMO technologies.
A trial conducted in Nortel's Wireless Technology Lab in early 2005 offers an example of this synergistic nature. During the trial, a mobile user had the ability to view two live streaming videos simultaneously while downloading a 264 MB file at 37 Mb/s over a standard 5MHz PCS band. Using OFDM-MIMO, the download was achieved in less than a minute compared to the 90 minutes that would be required with today's networks.
At this time, OFDM-MIMO is not part of the formal evolution path for existing cellular systems based on the 3GPP (UMTS, HSDPA) and 3GPP2 (CDMA 1X, EV-DO) standards; however, standards groups are working to understand its role in providing wireless broadband. 
- OFDM and MIMO together can support up to double subscribers than OFDM alone. This will help service providers to make more efficient use of existing spectrum.
- MIMO (multiple-input, multiple-output) and OFDM will make it possible for wireless subscribers to download large bandwidth consumption applications , large file transfers, streaming video & mobile games at speeds higher than today's commercial 3G networks. 
It is important to note that the OFDM-MIMO evolution being suggested here for 3GPP and 3GPP2 is an evolution of existing 3G networks - not a new network. This means existing sites and network architecture could be re-used, like today's 3G networks. On the other hand, the use of OFDM-MIMO for WiMAX 802.16e, which is progressing very rapidly, would require a new network. The scalability furnished by these technologies becomes especially important once MP3 players, PDAs, portable games devices and other handheld devices are equipped to provide wireless broadband, giving users a broadband connection wherever they go. 
9. CONCLUSION :
OFDM technology is quickly becoming a popular method for advanced communication networks. Only time will tell if this technology will have a major impact on the next generation of wired and wireless communications systems. However one thing for sure is that each of the functional blocks of the OFDM transmitter/receiver can be mapped onto dedicated, parallel hardware resources of a PLD-avoiding the difficult programming and optimization challenges of scheduling time-critical operations through a single DSP device.
.“Ofdm Tutorial”. Jan 2002 : 10-30. May 5, 2008. < www.wave- report.com/tutorials/OFDM.html>
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.Programmable Logic Facilitates the Implementation of OFDM Based Systems.
By: Tapan A. Mehta, Product Marketing Manager, Altera Corporation.
Available at: http://archive.chipcenter.com/pld/pldf092.htm