Free Space Optical (FSO) Communications Towards the Speeds of Wireline Networks
Free Space Optical (FSO) Communications: Towards the Speeds of Wireline Networks FSO Basic Principle • Connects using narrow beams two optical wireless transceivers in line-of-sight. • Light is transmitted from an optical source (laser or LED) trough the atmosphere and received by a lens. • Provides full-duplex (bi-directional) capability. • 3 “optical windows” : 850 nm, 1300 nm, & 1550 nm. • WDM can be used => 10 Gb/s (4x2.5 Gb/s) over 1 Km & 1.28 Tb/s (32x40 Gb/s) over 210 m.
Free Space Optical (FSO) Communications: Towards the Speeds of Wireline Networks Why FSO ? • License-free • Cost-effective • Behind windows • Fast turn-around time • Suitable for brown-field • Very high bandwidth (similar to fiber) • Narrow beam-widths (point-to-point) - Energy efficient - Immune to interference - High level of security
Free Space Optical (FSO) Communications: Towards the Speeds of Wireline Networks FSO Applications • Initially used for secure military as well as space applications • Commercial use: Last mile solution, optical fiber back-up, high data rate temporary links, cellular communication backhaul, etc …
Free Space Optical (FSO) Communications: Towards the Speeds of Wireline Networks FSO Challenges & Solutions • Additive noise (photo-detector) and background radiation (direct, scattered, and reflected sun light) => sensitive detectors + filters + heterodyne detection • Free space path loss => limited range • Atmospheric losses (rain, snow, fog, aerosol gases, smoke, low cloud, sand storms, etc … ) => power control + mesh architecture + hybrid RF/FSO • Atmospheric turbulences => space diversity • Buildings swaying, motion, and vibrations => tracking systems
Free Space Optical (FSO) Communications: Towards the Speeds of Wireline Networks Commercial Deployment Vendor Wavelength Data Rate Range MIMO Hybrid Price (@ 10 dB/km) RF/FSO Range (USD) fSONA 1550nm Full Duplex 1 km No Yes 8-12K (Canada) with 2.5 RF: 150 Mbps Gbps (60 – 70 GHz) LightPointe 850nm Full Duplex 1.6 kms Yes Yes 11-19K (USA) 1550nm with 1.25 (2 X 2) RF: 250 Mbps Gbps (4 X 4) (5.4 – 5.8 GHz) RedLine 850nm Full Duplex 0.9 kms Yes Yes 15-24K (South- with 1.25 (4 X 4) RF: 250 Mbps Africa) Gbps (4.9 – 5.8 GHz)
Free Space Optical (FSO) Communications: Towards the Speeds of Wireline Networks Deployment Example: Lasers for High-Speed Traders (CNN)
Free Space Optical (FSO) Communications: Towards the Speeds of Wireline Networks Characterization of the Scintillations • Frequency flat fading channel • Channel coherence time: 10 μs and 100 ms • Turbulence strength depends on Rytov variance/number (i.e. distance and index of refraction structure) • Turbulence regimes: – Rytov number << 1 => Weak turbulence regime – Rytov number >> 1 => Strong turbulence regime • Statistical models: – Weak turbulence: Rice-Lognormal or Gamma-Gamma (Generalized K) – Strong turbulence: Exponential or Gamma-Gamma (Generalized K) – More generalized models: Double Gamma-Gamma or Malaga
Free Space Optical (FSO) Communications: Towards the Speeds of Wireline Networks Pointing Errors • Definition: Thermal expansion, dynamic wind loads, and weak earthquakes result in the building sway phenomenon that causes vibration of the transmitter and the receiver known as pointing error. • Effect on Communication ( ξ ): These pointing errors may lead to an additional performance degradation and are a serious issue in urban areas, where the FSO equipments are placed on high-rise buildings. • Model: The pointing error model developed and parameterized by ξ which is the ratio between the equivalent beam radius and the pointing error jitter can be: - With Pointing Error: ξ is any number between 0 through 7 - Without Pointing Error: ξ → ∞
Free Space Optical (FSO) Communications: Towards the Speeds of Wireline Networks Generalized Pointing Errors Model • The general model reduces to special cases as follows Rayleigh No misalignment Single sided Gaussian Hoyt Rician
Free Space Optical (FSO) Communications: Towards the Speeds of Wireline Networks Generalized Pointing Errors Model • The fraction of collected power at the receiver can be approximated by [Farid and Harilovic, IEEE/OSA JLT, 2007] x 2 + y 2 with r = | r| = is random
Free Space Optical (FSO) Communications: Towards the Speeds of Wireline Networks On-Going Research Directions • Unified performance analysis accounting for type of detection, weak/strong scintillations, and pointing errors. • Computation of ergodic capacity over generalized FSO fading channels – High SNR and low SNR bounds and approximations – Bounds and exact results for the capacity of diversity systems – Accurate approximations • Average probability of error computations over generalized FSO fading channels – Differentially coherent vs. coherent system performance – Asymptotic results (coding and diversity gains)
Free Space Optical (FSO) Communications: Towards the Speeds of Wireline Networks On-Going Research Directions: Ergodic Capacity Computation • High SNR and Low SNR Results over FSO channels. • Bounds on the Capacity of Selection Diversity Systems • Exact Capacity Results for MRC and EGC Diversity Systems • Approximate results using PDF approximation
Free Space Optical (FSO) Communications: Towards the Speeds of Wireline Networks On-Going Research Directions: Asymptotic Analysis of Ergodic Capacity Unified SNR Statistics • Heterodyne Detection • IM/DD • Unified with irradiance I = I a I p
Free Space Optical (FSO) Communications: Towards the Speeds of Wireline Networks On-Going Research Directions: Ergodic Capacity Calculations under the Impact of Pointing Errors Asymptotic Ergodic Capacity Recall that the irradiance I = I a I p and SNR g is proportional to I r • • The asymptotic ergodic capacity can be obtained as [Yilmaz and Alouini, SPAWC2012] • We need to find the moments of I a and then compute derivatives. ,
Free Space Optical (FSO) Communications: Towards the Speeds of Wireline Networks On-Going Research Directions: Asymptotic Analysis of Ergodic Capacity Exact Closed-Form Moments • I= I a I p = I R I L I p where I R , I L , and I P are independent random processes • Unified Rician Moments
Free Space Optical (FSO) Communications: Towards the Speeds of Wireline Networks On-Going Research Directions: Asymptotic Analysis of Ergodic Capacity Asymptotic Results • High SNR • Low SNR
Free Space Optical (FSO) Communications: Towards the Speeds of Wireline Networks On-Going Research Directions: Asymptotic Analysis of Ergodic Capacity Asymptotic Results Figure: Ergodic capacity results for IM/DD technique and varying k at high SNR regime for RLN turbulence
Free Space Optical (FSO) Communications: Towards the Speeds of Wireline Networks On-Going Research Directions: Ergodic Capacity Calculations under the Impact of Pointing Errors Generalized Pointing Errors Model • The fraction of collected power at the receiver can be approximated by [Farid and Harilovic, IEEE/OSA JLT, 2007] x 2 + y 2 • Such that r = | r| = is Beckmann distributed RV So
Free Space Optical (FSO) Communications: Towards the Speeds of Wireline Networks On-Going Research Directions: Ergodic Capacity Calculations under the Impact of Pointing Errors Generalized Pointing Errors Model • The general model reduces to special cases as follows Rayleigh No misalignment Single sided Gaussian Hoyt Rician
Free Space Optical (FSO) Communications: Towards the Speeds of Wireline Networks On-Going Research Directions: Ergodic Capacity Calculations under the Impact of Pointing Errors Asymptotic Ergodic Capacity • The asymptotic ergodic capacity can be obtained as • The moments of I a are known for both lognormal (LN) and Gamma- Gamma (ΓΓ) . Then, the asymptotic capacity can be written as ,
Free Space Optical (FSO) Communications: Towards the Speeds of Wireline Networks On-Going Research Directions: Ergodic Capacity Calculations under the impact of pointing errors Asymptotic Ergodic Capacity Figure: The ergodic capacity for composite log-normal channel (LN). (a) ξ x = 6.7 and ξ y = 5.1 ( b) ξ x = 6.7 and ξ y = 0.9 (c) ξ x = 0.8 and ξ y = 0.9 Reference: H. Al-Quwaiee, H.- C. Yang, and M. -S. Alouini, “ On the Asymptotic Ergodic Capacity of FSO Links with Generalized Pointing Error Model ”, Submitted to ICC’15.
Free Space Optical (FSO) Communications: Towards the Speeds of Wireline Networks On-Going Research Directions: Average Probability of Error Computations SER Performance of MPSK and MDPSK • Symbol error rate performance of MPSK and MDPSK over AWGN are given by [Pawula , TCOM’1999] and with
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