Performance Analysis of Coded Optical OFDM Over free Space Links

Transcription

1 Performance Analysis of Coded Optical OFDM over Free Space Links Performance Analysis of Coded Optical OFDM Over free Space Links 1 A. Sangeetha, 2 Binitha Merin Varghese and 3 Sibaram Khara School of Electronics Engineering, VIT University, Vellore , T. N., India s: 1 asangeetha@vit.ac.in, 2 binithamerin@gmail.com, 3 sianba@rediffmail.com Abstract: OFDM is a multicarrier transmission technique where a data stream is carried with many lower-rate subcarrier tones. The benefits of OFDM are high spectral efficiency, resiliency to RF interference and lower multi-path distortion. Due to the high complexity associated with coherent detection current free space optical (FSO) systems employ intensity modulation with direct detection (IM/DD).Such systems use point-to point communication between the two optical transceivers along a line of sight. FSO proves to be a cost effective solution for high-rate image, voice and data communication. But atmospheric turbulence can degrade the performance of FSO links, particularly over ranges of the order of 1 km or longer. Inhomogeneities in the temperature and pressure of the atmosphere lead to variations of the refractive index along the transmission path. These index inhomogeneities can deteriorate the quality can cause fluctuations in both the intensity and the phase of the received signal. These fluctuations can lead to an increase in the link error probability, limiting 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. LDPC coded OFDM is compared with Convolutional Coded OFDM. Keywords: Optical Communications; Optical Orthogonal frequency division multiplexing; Atmospheric turbulence; FSO; Modulation 1. INTRODUCTION Fiber-optics, RF and copper/coaxial lines are the main state-of-the-art technologies to address the high bandwidth requirements. The incompatibility of RF/microwave and optical communication technologies due to large bandwidth mismatch between RF and optical channels may prove to be a limiting factor in efforts to further increase the transport capabilities in the future. Due to this problem RF/microwave-optical interface solutions that will enable aggregating multiple RF/ microwave channels into an optical channel are becoming increasingly important. Free-space optical (FSO) links are considered as a feasible solution for the last mile bottleneck problem because of the following properties: (i) highdirectivity of the optical beam provides high power efficiency and spatial isolation from other potential interferers, a property not inherent in RF/ microwave communications, (ii) the free-space optical transmission operates in unlicensed bandwidth (iii) large fractional-bandwidth coupled with high optical gain using moderate powers permits very high-data rate transmission, (iv) the state-of-the art fiber-optics-communications employ intensity modulation with direct detection (IM/DD), and the components for IM/DD are widely available, and (v) the FSO links are relatively easy to install and easily accessible for reposition when necessary. The FSO communication is considered as an enabling technology to bring different technologies together and easy to integrate with variety of interfaces and network elements. However, an optical wave propagating through the air experiences fluctuations in amplitude and phase due to atmospheric turbulence. The intensity fluctuation, also known as the scintillation, is one of the most important factors that degrade the performance of an FSO communication link, even under the clear sky condition. The key requirement of FSO is LOS. The LOS problem also exists for most microwave radios but is not as stringent when techniques such as OFDM and MIMO are used to help mitigate the multipath problems that typically crop up. Hence the proposition to integrate optical OFDM with free space optics came up. International Journal of Wireless Communications and Networking, 3(2) Dec

2 OFDM has only been recognized in telecommunications industry in recent times, but it had a long history of existence. The concept of using parallel data transmission by means of frequency division multiplexing (FDM) was published in mid 60 s. Some early development can be traced back in the 50s.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)[5].He obtained the first US patent on OFDM in1970 and this was the first official publication on multicarrier modulation. The proposition to generate the orthogonal signals using an FFT came in 1969.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 [6]. 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 wq3w[7]. 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. A. Sangeetha, Binitha Merin Varghese & Sibaram Khara In 1985 Cimini proposed the use of OFDM in mobile communication [14], 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 1987 considering the use of OFDM for radio broadcasting and the importance of combining forward error correction (FEC) [15]. The use of OFDM for wireline communication was initiated by Cioffi and others at Stanford who demonstrated its potential as a modulation technique for digital subscriber loop (DSL) applications [9]. 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 Veryhigh-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 DAB was proposed in 1992 and the standard was formulated in The development of Digital Video Broadcasting (DVB) was started in DVB along with High-Definition Television (HDTV) terrestrial broadcasting standard was published in The work by Telatar and Foschini on multiple antenna systems stimulated new fields of research in OFDM [11]. 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 In 1999, the IEEE committee on wireless LANs released the a standard for OFDM operation in 5GHz UNI band. The IEEE committee released an OFDMbased standard for wireless broadband access for metropolitan area networks under revision a in In 2003; the IEEE committee released the g standard for operation in the 2.4GHz band. 2. OPTICAL OFDM To enhance the utilization of optical bandwidth more efficiently, new improved transmission 144 International Journal of Wireless Communications and Networking, 3(2) Dec. 2011

3 Performance Analysis of Coded Optical OFDM over Free Space Links technologies are intensively studied. High-speed optical transmission systems are impaired by interchannel and intrachannel nonlinearities, polarization mode dispersion and chromatic dispersion, and require precise dispersion compensation techniques. In order to enhance the transmission capacity, narrower channel spacing is pursued and novel modulation formats are investigated [10]. Optical OFDM (OOFDM) system built of standard optical and RF components was proposed for long haul transmission in regard with its ability to deal with large pulse spreads due to chromatic dispersion. In the above proposition all operations are performed in the RF domain. This proves to be advantageous because microwave devices are much more advanced than their corresponding optical devices and because the frequency selectivity of microwave filters and the frequency stability of microwave oscillators are significantly better than that of optical devices. Additionally the phase noise levels of microwave oscillators is much lower than that of DFBs, implying that RF coherent detection is easier to implement than optical coherent detection. This allowed the system architect to be applied directly the most advanced modulation formats already developed for wireless RF communications. The proposed methodology a for long-haul transmission system provided a number of advantages: (i) increasing of the transmission distance, (ii) improvement of the spectral efficiency, and (iii) simplification of the dispersion compensation engineering. Orthogonal frequency division multiplexing (OFDM) is proposed as a promising technology for optical communications [3]. 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 directdetection 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 a laser at the receiver to generate the carrier locally, and is more sensitive to phase noise. 3. OPTICAL OFDM OVER FSO LINKS 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. Unitless Rytov variance is used to quantify the strength of the turbulence /6 11/6 σ = C K L (1) 1.23 n where k = 2π/λ is the optical wave number λ is the wavelength, L is the propagation distance, and 2 C is the refractive index structure n parameter, which is assumed to be constant for horizontal paths. 2 The refractive index structure parameter C n varies from about m- 2/3 for very weak turbulence to about m -2/3 for strong turbulence. The two major communication techniques to alleviate turbulence are spatial-domain techniques and temporal domain techniques [3]. 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 optimizes the decision threshold according to the maximum likelihood criterion. When the receiver has knowledge of the joint International Journal of Wireless Communications and Networking, 3(2) Dec

4 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[2]. OFDM combined with error control coding is considered as a very good modulation format for FSO IM/DD systems Coded OFDM 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. The performance of OFDM under different digital modulation techniques (BPSK, QPSK, 8QAM, 16QAM) combined with concatenated interleaved FEC scheme was evaluated for an audio signal in [8]. The proposed OFDM system incorporated RS encoder (255, 239, 8) with convolutional encoder of rate 1/2, 2/3 and 1/3 rated codes in FEC coding system. It concluded the performance of BPSK with Reed Solomon Code with Convolution code rated 3/4 outperformed all the above mentioned modulation schemes. In [1] 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. The design and demodulation of LDPC codes is studied and implemented in [4]. It describes the large-girth binary LDPC design, the min-sum-with correction term, decoding algorithm and its FPGA implementation. A. Sangeetha, Binitha Merin Varghese & Sibaram Khara Transmitter Configuration The data streams are encoded using an LDPC encoder and then modulation is performed to combine OFDM. The LDPC encoded data stream is then parsed into groups of B bits. The B bits in each group (frame) are subdivided into K subgroups with the i th subgroup containing b i bits, B = Σ bi. The bi bits from the i th subgroup are mapped into a complex-valued signal from a 2 bi -point signal constellation such as QAM. The complex-valued signal points from all K subchannels are considered as the values of the discrete Fourier transform (DFT) of a multi-carrier OFDM signal. After D/A conversion and RF up-conversion, the OFDM signal drives a Mach-Zehnder modulator (MZM) for transmission over the FSO link. Fig shows the transmitter configuration. Figure 1: Transmitter Configuration of the Proposed Methodology Receiver Configuration At the receiver, an optical system collects the light, and focuses it onto a detector, which delivers an electrical signal proportional to the incoming optical power. After the RF down-conversion, carrier suppression, A/D conversion and cyclic extension removal, the transmitted signal is demodulated using the FFT algorithm. The soft outputs of the FFT demodulator are used to estimate the symbol reliabilities, which are converted to bit reliabilities, and provided as input to an LDPC iterative decoder Proposed Methodology The bandwidth of the OFDM signal is set to 2.5 GHz. The number of OFDM sub-channels is set to 64. The OFDM sequence is zero-padded and the FFT is calculated using 128 points and a cyclic prefix of 32 samples are also added. Figure 2: Receiver Configuration of the Proposed Methodology 146 International Journal of Wireless Communications and Networking, 3(2) Dec. 2011

5 Performance Analysis of Coded Optical OFDM over Free Space Links Fig shows the receiver configuration. The proposed methodology uses Block Circulant LDPC codes. The input data stream is of 52*10^2 bits. The available bandwidth of 20 khz is divided into 64 subcarriers. Out of the available 64 subcarriers having indices [ ], the number of used subcarriers is 52. The used subcarriers are having indices from [ ] are used for transmitting information sequence. Some subcarriers are left unused to ensure spectrum rolloff. In the available bandwidth from -10 KHz to +10KHz, only subcarriers from kHz (-26/ 64*20KHz) to KHz (+26/64*20KHz) are used. This means that the signal energy is spread over a bandwidth of khz, whereas noise is spread over bandwidth of 20 khz (-10 KHz to +10KHz). In an OFDM transmission the transmission of cyclic prefix does not carry extra information through the channel. The signal energy is spread over data symbol duration T d and cyclic duration T cp whereas the bit energy is spread over the time T cp i.e. E s (T d + T cp ) = E b T d (2) A cyclic prefix of 32 samples are appended to the end of the OFDM signal and transmitted. The OFDM signal is passed through a turbulent channel. In order to model such a turbulent channel the signal power is divided by noise variance. The cyclic prefix is removed and the desired subcarriers are taken leaving the rest. Then IFFT is performed on the signal and corresponding demodulation is performed. To calculate the BER, the difference between the received bits and the transmitted bits are calculated divided by the total number of bits. The obtained bit error rate is plotted against E b /No. Figure 3: BER of Convolutional Coded OFDM under Strong Turbulence Figure 4: BER of Convolutional Coded OFDM under no fade 4. PERFORMANCE OF CODED OPTICAL OFDM To enhance the BER performance of Optical OFDM,Convolutional coding and LDPC coding were implemented. BER performance under strong and weak turbulence were compared for both the FEC schemes. Figure 5: BER of LDPC Coded OFDM and Uncoded OFDM International Journal of Wireless Communications and Networking, 3(2) Dec

6 5. CONCLUSION The performance of forward error scheme for Optical OFDM over terrestrial links under weak and strong turbulence has been evaluated. To implement Optical OFDM intensity modulation direct detection becomes a prerequisite. The data protection scheme using Convolutional code became worst at lower channel SNR and showed no BER improvement under turbulent condition. However, LDPC codes showed no errors appear at the received data even at the lower channel SNR. The perfect reconstruction of the transmitted image indicates perfect forward error correction (FEC) scheme was possible in turbulent using the LDPC codes. Thus, the performance of LDPC codes is better that the Convolutional code through atmospheric turbulent medium. REFERENCES [1] Ivan B. Djordjevic, Bane Vasic, and Mark A. Neifeld, LDPC coded OFDM for Optical Communication System using Direct Detection, IEEE Journal of Selected Topics in Quantum Electronics, 13(5), [2] I. B. Djordjevic, B. Vasic, and M. A. Neifeld, LDPC Coded Orthogonal Frequency Division Multiplexing over the A. Sangeetha, Binitha Merin Varghese & Sibaram Khara Atmospheric Turbulence Channel, in Proc. CLEO/QELS 2006, Paper CMDD5, Long Beach, CA, May [3] X. Zhu and J. M. Kahn, Free-space Optical Communication through Atmospheric Turbulence Channels, IEEE Trans. Commun., 50(8), , [4] Ivan B. Djordjevic, Murat Arabaci, and Lyubomir L. Minkov, Next Generation FEC for High Capacity Communication in Optical Transport Networks, Journal of Lightwave Technology, 27(16), [5] R. W. Chang, Orthogonal Frequency Multiplex Data Transmission System, USA U.S. Patent 3, 488, 445, [6] J. Salz and S. B. Weinstein, Fourier Transform Communication System, in Proc. ACM Symp. Problems Optim. Data Commun. Syst., Pine Mountain, GA, USA, [7] A. Peled and A. Ruiz, Frequency Domain Data Transmission using Reduced Computational Complexity Algorithms, in Proc. ICASSP 80,Denver, CO, USA, 1980, III, , IEEE. [8] Ierwan Ab. Karim, Abdul Karem Hussein Mohammed Almawgani, Mohd Fadzli Mohd Salleh, Performance Comparison between Low-density Parity-check and Reedsolomon Codes using Wireless Image Transmission System, June [9] J. S. Chow, J. C. Tu, and J. M. Cioffi, A Discrete Multitone Transceiver System for HDSL Applications, IEEE J. Sel. Areas Commun., 9, , [10] W. Shieh and C. Athaudage, Coherent Optical Orthogonal Frequency Division Multiplexing, Electron. Lett., 42, , International Journal of Wireless Communications and Networking, 3(2) Dec. 2011