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Fiber optic communication has revolutionized the telecommunication industry. It has enabled telecommunication links to larger distances with higher data rate and lower loss in transmission medium while free-space systems provide high data-rate communication links between satellites at geosynchronous distances.
Free Space Optics (FSO) is an optical communication line of sight technology that employs light propagating in free space to transmit data between two points. The technology is useful where deployment of fibre optic cables is impractical, due to high costs or other considerations.
Orthogonal frequency division multiplexing (OFDM) is a modulation and multiplexing technique. It consists of a number of subcarriers that are made orthogonal to each other by appropriately choosing the frequency spacing between them. Each subcarrier carriers a portion of the user information that is transmitted along the communication channel.
OFDM forms the basis of Digital Audio Broadcasting (DAB) standard and Global ADSL(Asymmetric Digital Subscriber Line) standard. OFDM is also now used in LAN and MAN application and Wireless Personal Area Network (PAN).The application of OFDM to the field of optical communication occurred very recently. It has been studied for its application in light wave hybrid AM/OFDM cable systems and in radio over fiber based networks.
It is an effective solution to subchannel and intersymbol interference (ISI) caused by a dispersive channel. It provides high spectral efficiency and requires no equalization.These features together with its immunity to burst errors due to intensity fluctuations proves OFDM suitable for free space optical communication.
Channel coding plays a key role in OFDM systems performance. It assures that the channel is robust against all random errors. Also interleaving assures frequency diversity. Coded OFDM is known as COFDM.
HISTORY AND EVOLUTION OF OFDM
OFDM has only been recognized in telecommunications industry in recent times, but it had a long history of existence. OFDM based systems were in use since the Second World War. The OFDM technique was used by US military in several high frequency military systems such as KINEPLEX, ANDEFT and KATHRYN. In December 1966, Robert W. Chang showed a theoretical way to transmit simultaneous data stream through linear band limited channel without reducing data rate and preventing Inter Symbol Interference (ISI) and Inter Carrier Interference (ICI). He obtained the first US patent on OFDM in1970 and this was the first official publication on multicarrier modulation.
A major breakthrough in the history of OFDM occurred in 1971 when Weinstein and Ebert demonstrated that Discrete Fourier Transform (DFT) can be used to perform multicarrier modulation enabling efficient processing. The requisite for a large number of subcarrier oscillators to perform parallel modulations and demodulations were eliminated paving the way for an efficient implementation of the system.
The proposed schemes until this time used guard spaces in frequency domain and a raised cosine windowing in time domain to eliminate ISI and ICI. Another landmark in the history of OFDM was in 1980, when Peled and Ruiz introduced Cyclic Prefix (CP) or cyclic extension. This novel idea ensured maintaining orthogonal characteristics of the transmitted signals at severe transmission conditions. The idea conveyed was to use cyclic extension of OFDM symbols instead of using empty guard spaces in frequency domain. This effectively turns the channel as performing cyclic convolution, which provides orthogonality over dispersive channels when CP is longer than the channel impulse response. The advantage of mitigating ICI by introducing a CP overlooks the cons of the same.
In 1985 Cimini proposed the use of OFDM in mobile communication,The application of FFT and CP in OFDM system and substantial advancements in Digital Signal Processing (DSP) technology made OFDM an integral part of telecommunication. Alard and Lasallae described Coded OFDM(COFDM) in 1985. In the 1990s, OFDM was used for wideband data communications over mobile radio FM channels, High-bit-rate Digital Subscriber Lines (HDSL at 1.6Mbps), Asymmetric Digital Subscriber Lines (ADSL up to 6Mbps) and Very-high-speed Digital Subscriber Lines (VDSL at 100Mbps). The OFDM technology was first commercially utilized in Digital Audio Broadcasting(DAB). The development of DAB started in 1987. DAB was proposed in 1992 and the standard was formulated in 1994. The development of Digital Video Broadcasting (DVB) was started in 1993. DVB along with High-Definition Television (HDTV) terrestrial broadcasting standard was published in 1995.
By the 20th century, several Wireless Local Area Network (WLAN) standards implemented OFDM on their physical layers. Development of European WLAN standard HiperLAN began in 1995.In 1999,the IEEE 802.11 committee on wireless LANs released the 802.11a standard for OFDM operation in 5GHz UNI band.The IEEE 802.16 committee released an OFDM-based standard for wireless broadband access for metropolitan area networks under revision 802.16a in 2002.In 2003, the IEEE 802.11 committee released the 802.11g standard for operation in the 2.4GHz band.
1.2 PROBLEM DESCRIPTION
Optical Free-Space Transmission is a line of sight technology (LOS) which transmit information with lowest transmit power and at highest data rates.
When performing optical links through the atmosphere, the atmospheric turbulence can degrade its performance, particularly over ranges of the order of 1 km or longer. Inhomogenities in the temperature and pressure of the atmosphere due to solar heating and wind causes the formation of eddies of different diameters and refractive indices. Most of the kinetic energy of the turbulent motion is contained in the large scale structures. The energy is transferred from these large turbulent eddies to eddies of smaller size creating a hierarchy of eddies. Eventually this process creates structures that are small enough where viscous dissipation of energy takes place.
These index inhomogenities can deteriorate the quality causing distortions in both the intensity and the phase of the received signal. These fluctuations causes an increase in the link error probability, impairing the performance of the communication system. Low-density parity-check (LDPC) coded OFDM is proposed as an efficient coded modulation technique suitable for FSO transmission
1.3 LITERATURE SURVEY
Orthogonal frequency division multiplexing (OFDM) is a promising technology for optical communications. There are two forms of unipolar OFDM dc-biased optical OFDM (DCO-OFDM) and asymmetrically clipped OFDM (ACO-OFDM). In dc-biased OFDM, a DC bias is added to the signal. In ACO-OFDM the bipolar OFDM signal is clipped at the zero level .ACO-OFDM is proved to be more efficient than DCO-OFDM. The component at the optical carrier frequency is transmitted with the OFDM signal as in direct-detection optical OFDM (DD-OOFDM) or coherent optical OFDM (CO-OFDM). The interference is avoided by inserting a guard band between the optical carrier and the OFDM subcarriers, hence reducing the spectral efficiency. CO-OFDM is highly complex as it requires requires a laser at the receiver to generate the carrier locally, and is more sensitive to phase noise.
The free space optics communication is a recent and growing technology that has found application in many areas of the short- and long-haul communications space from intersatellite links to interbuilding links. Atmospheric turbulence impair the performance of free space optical links. A number of phenomena in the atmosphere, such as absorption, scattering, and turbulence, can affect beam attenuation, but in the case of wavelengths typical of FSO systems operation, only scattering and turbulence are appropriate to be taken into consideration.
The two major communication techniques to alleviate turbulence are spatial-domain techniques and temporal domain techniques. The techniques depend on the statistical properties of turbulence-induced signal intensity fading, as functions of both temporal and spatial coordinates.The former involve diversity detection using multiple receivers, and latter uses one receiver adaptively optimize the decision threshold according to the maximum likelihood criterion. When the receiver has knowledge of the joint temporal distribution of intensity fluctuations, maximum-likelihood sequence detection (MLSD) can be employed. MLSD and sub-optimal implementations of MLSD such as those based on sub-optimal per-survivor processing (PSP) require the electrical signal to noise ratio larger than 20 dB even in the weak turbulence regime making it unacceptably high for many applications hence novel modulation techniques for IM/DD FSO systems are needed. OFDM combined with error control coding is considered as a very good modulation format for FSO IM/DD systems.
In  Low-density parity-check (LDPC) coded optical orthogonal frequency division multiplexing (OFDM) is ensured to outperform standard FEC schemes such as RS and concatenated RS codes over the atmospheric turbulence channel in terms of both coding gain and spectral efficiency. High tolerance to deep fades due to atmospheric turbulence is obtained by using OFDM in combination with interleaving and LDPC codes. A sufficient DC bias is added so that the resulting OFDM signal is nonnegative and is called biased OFDM scheme. B-OFDM has power efficiency.To improve the power efficiency we propose two alternative schemes.The first alternative scheme is clipped OFDM (C-OFDM) scheme, is based on single-sideband (SSB) transmission and clipping of the OFDM signal after adding a bias.
ORTHOGONAL FREQUENCY DIVISION MULTIPLEXING
This thesis describes about Optical OFDM over FSO links. In order to establish the context and need for the work undertaken, it is a prerequisite to discuss the fundamental concepts behind the work.
This chapter is organized as follows. Following this introduction, section 2.2 discusses
Fundamental concepts behind OFDM, section2.3 discusses about coded OFDM.
2.2 Fundamental Concepts Behind OFDM
2.2.1What is OFDM?
Orthogonal frequency division multiplexing (OFDM) is a communications technique in which the data to be transmitted are spread over a large a number of modulated carriers that are closely spaced in frequency and are orthogonal to each other. It is a combination of modulation and multiple access scheme making it possible for a large number of users to share the channel.
2.2.2 Orthogonality in OFDM
The OFDM concept is based on breaking the existing bandwidth W Hz into N subchannels of bandwidth âˆ†W Hz ,so that W=Nâˆ†W.As a result the information symbols are transmitted on the different equally spaced subchannels. TDMA divides the channel according to time while CDMA according to spreading codes whereas OFDM segments are according to frequency. OFDM can be viewed as frequency division multiplexing (FDM).
In a FDM system signals can be recovered at the receiver using conventional filters and demodulators.In such receivers FDM requires a guard band between modulated subcarriers to prevent interference. The presence of a guard band lowers the systems information rate. The guard bands can be removed if the FDM system are able to use orthogonal subcarriers.
In OFDM the use of subcarriers that are orthogonal to each other would increase spectral efficiency allowing the subcarriers to overlap. When a sinusoid frequency n is multiplied by sinusoid frequency m/n the area under the product is zero. For integers n and m sinnx, cosnx, sinmx cosmx are all orthogonal to each other. These frequencies are called harmonics.
2.2.3 Use of discrete fourier transform(dft) in OFDM
The conventional transform focuses on continuous signals which are not limited to in either time or frequency domains. The Fourier transform transforms data from time domain to frequency domain. To perform signal processing at a faster rate the signals should be sampled. Then input signals in DFT are sampled in both time and the frequency domains.
In OFDM these transforms can be viewed as mapping the information symbol into orthogonal subcarriers. To convert the frequency domain data to time domain data ,the IDFT correlates the frequency-domain input data with its orthogonal basis functions, which are sinusoids at certain frequencies. The result of maintaining orthogonality is that the OFDM signal can be defined by using Fourier transform procedures.
To preclude a large number of modulators and filters at the modulator and a similar number of complementary filters and demodulators at the receiver, it is desirable to use modern digital signal processing techniques, such as fast Fourier transform (FFT). The FFT performs time domain to frequency domain representation, whereas the reverse process uses the inverse Fast Fourier transform. An OFDM system considers the information symbols at the transmitter as though they are in the frequency-domain. These symbols are used as the inputs to an IFFT block that brings the signal into the time domain.
The IFFT takes in N symbols having a symbol period of T seconds where N defines the number of subcarriers.The basis function for an IFFT are N orthogonal sinusoids each having different frequency and the lowest is DC. The value of the input symbol are complex determining both the amplitude and phase of the sinusoid for a particular subcarrier. FFT takes the input data and multiplies it successively by complex exponentials over the range of frequencies The output of IFFT is the summation of all N sinusoid and plots the result as a function of frequency. The IFFT modulate the data into N orthogonal subcarriers. The FFT block is used at receiver and its output would be the original input data to the IFFT at the transmitter end.
The difficulty to generate a OFDM signal, and the complexity involved in reception of the signal hindered the expansion of OFDM for a long period of time. The ability to generate and to demodulate the signal using a software implementation of FFT algorithm paved way for widespread use of OFDM.
2.2.4 USE OF CYCLIC PREFIX
The signals arriving at the receiver are multiple delayed versions of the original signal due to multipath environment. The effect on a sinusoidal signal through multipath environment is as follows:
A simple multipath channel is modeled in the form:
The sinusoidal to be transmitted in the form:
The received signal :
The received signal through the multipath channel is same as the original input sinusoid with modifications in amplitude and phase.
The consequences of multipath environment are intersymbol interference and intrasymbol interference. Intersymbol interference occurs when the received OFDM symbol is distorted by previously transmitted OFDM symbol.The interference among the different subcarriers in an OFDM results in intrasymbol interference.
An OFDM splits the data among N subcarriers and transmits the subcarriers at a rate of N/R symbols per second in contrast to single carrier modulation system where the data rate is R symbols per second. As the rate of an OFDM symbol is reduced by a factor of 1/R, the total symbol period of an OFDM signal is increased by a factor of R. The OFDM signal becomes relatively longer than the length of the channel. It results in intersymbol interference as the first few symbols overlaps.
To avoid such distortions a guard band is added at the end of each OFDM symbol and transmitted. The guard band contains a section of zeros appended to the front end of the OFDM symbol .Hence the distortions occur at the zero padding and the useful OFDM symbol doesn't get affected preventing intersymbol interference. The guard bands are discarded at the receiver end as it contains redundant information bits .
The use of guard bands do not prevent interference among the OFDM subcarriers. In continuous-time, a convolution in time domain is equivalent to a multiplication in the frequency-domain. This property is applicable in discrete-time only if the signals are of infinite length or if at least one of the signals is periodic over the range of the convolution. It is impractical to attain an infinite-length OFDM symbol, hence the only possible solution is to make the OFDM symbol appear periodic. This periodic form is attained by replacing the guard interval with something known as a cyclic prefix
The cyclic prefix consists of a replica of last few samples of effective OFDM symbol added in the front of the OFDM signal. The receiver rejects the corrupted cyclic prefix due to overlapping of previously transmitted OFDM subcarriers. The major drawback of using cyclic prefix is loss of data rate due to addition of redundant bits. Hence the duration of a cyclic prefix should be less than the expected length of multipath channel.
2.2.5 ADVANTAGES AND DISADVANTAGES OF OFDM
Efficient use of the spectrum by allowing spectrum overlap.
By dividing the channel into equally spaced narrowband subchannels, OFDM is more immune to frequency selective fading than single carrier systems.
Mitigates ISI and ICI through use of a cyclic prefix.
OFDM is computationally efficient by using FFT and IFFT techniques to implement modulation and demodulation techniques respectively.
It has high peak to average power ratio.
It is more sensitive to carrier frequency offset and drift due to leakage of the DFT compared to single carrier systems .
Interleaving is a communication technique to overcome correlated noise along the channel such as burst error or fading. The interleaver rearranges the input data such that the successive data are spread among different blocks. At the receiver end, the de-interleaver arranges the interleaved data back into the original sequence. As a consequence of interleaving, long burst noise sequence introduced in the transmission channel appears to be statistically independent at the receiver ensuring better error correction.
The amount of error protection based on the length of the burst noise sequence determines the span length or depth of interleaving required. Interleaving is classified as either periodic or pseudo-random. The periodic interleaver rearranges the data in a repeating sequence of bytes whereas pseudo-random interleavers rearrange the data in a pseudo-random sequence. Block interleaving is an example of periodic interleaving. These interleavers accept symbols in blocks and perform identical permutations over each block of data. Periodic interleaving is used mostly because it is more easily accomplished in hardware.
2.3.2FORWARD ERROR CORRECTION
Forward error-correction coding is a communication technique that introduces a known structure into a data sequence prior to transmission to enhance the data reliability. This structure consisting of redundant bits enables the receiving system to detect and correct errors due to corruption from the channel and the receiver and to recover the original data.Coding decreases the information bit error rate while sustaining a fixed transmission rate.
18.104.22.168Reed Solomon Coding
Reed Solomon codes are nonbinary block codes and a subset of BCH codes. A RS code is specified as RS(n,k) for any RS code where n≤ 2m - 1, and n - k ≥ï€ 2t.The parameters are:
m = the number of bits per symbol
n = the block length in symbols
k = the uncoded message length in symbols
(n-k) = the parity check symbols (check bytes)
t = the number of correctable symbol errors
(n-k) = 2t (for n-k even)
(n-k)-1 = 2t (for n-k odd)
An RS decoder works on multi-bit symbols rather than on single bits. Thus, up to eight bit-errors in a symbol can be treated as a single symbol error. Hence burst of errors are handled efficiently.The RS codes with very long block lengths tend to average out the random errors and make block codes capable for use in random error correction. The complexity of the decoder can be reduced as the code block length increases and the redundancy overhead decreases. Hence, RS codes are typically large block length, high code rate, codes.
FREE SPACE OPTICS
FSO is a line-of-sight technology approach that employs invisible beams of light to provide optical bandwidth connections. It facilitates the transmission of data, voice, and video communications at bandwidths upto 1.25 Gbps simultaneously through the air. It enables fiber-optic connectivity without requiring physical fiber-optic cable or spectrum licenses. It permits optical communications at the speed of light as light travels faster in air than in glass.
FSO was originally developed by the military and NASA. It's based on connectivity between FSO-based optical wireless units, each consisting of an optical transceiver with a transmitter and a receiver to provide full-duplex capability. The optical wireless unit composes of an optical source,and a lens or telescope that transmits light through the atmosphere to another lens receiving the information.
Requires no RF spectrum licensing.
High bit rate and low bit error rate
Immune to radio frequency interference or saturation.
Ease of deployment even behind windows, eliminating the need for costly rooftops.
Full duplex operation
Fog: The first and foremost challenge to FSO-based communications is dense fog. Rain and snow doesn't affect much. Fog is vapor composed of water droplets of very small diameter that can alter characteristics of the light or completely obstruct the passage of light through a combination of absorption, scattering, and reflection.
Absorption: Absorption occurs when suspended water molecules in the terrestrial atmosphere deplete the photons. This attenuates the FSO beam and directly affects the FSO based system.
Physical obstructions: Flying birds or construction cranes can temporarily hinder a single-beam FSO system, but this tends to cause only short interruptions.
Scintillation: Heated air rising from the earth or man-made devices such as heating ducts create temperature variations among different air pockets. This can cause fluctuations in signal amplitude .
Beam Wander: Beam wander is intiated by turbulent eddies that are larger than the beam.
Beam Spreading: Beam spreading is of two types: long-term and short-term. It is the spread of an optical beam as it propagates through the atmosphere.
Safety: Safety can be a major concern because the technology uses lasers for transmission. The two major concerns are exposure of eye to light beams and high voltages within the light systems and their power supplies.