Report

Optical Wireless Communications Prof. Brandt-Pearce Lecture 8 Deep-Space Optical Communications 1 Outline Deep-Space Optical Communications Introduction Channel Model System Performance Optical Deep-Space Network RF/FSO Hybrid System 2 3 Deep-Space Communications Sending and receiving data from space crafts has been a challenging problem since 1950s Communication over deep-space distances is extremely difficult, much more difficult than satellite communications Communications beams spread as the square of the distance between the transmitter and the receiver 4 Deep-Space Optical Communications The distance from Earth to Neptune or Pluto can be on the order of 4,000,000,000 km. After propagating over such a distance, the communications beam from a spacecraft will spread to an area 10 billion times (100 dB) larger in area than if the beam from the same system traveled from just the GEO distance (40,000 km). A system capable of transmitting 10 Gbps from GEO to the ground would only achieve 1 bps from Pluto/Neptune distances. 5 Deep-Space Optical Communications Optical communications has lower divergence compared to RF Comparison of RF and optical beam spreads from Saturn. 6 Deep-Space Optical Communications An important factor for a high data-rate deep-space optical link is the laser transmitter Lasers are required to have High output power Low divergence 7 Deep-Space Optical Communications Another key technology component is a thermally stable and lightweight optical spacecraft telescope. Similar to satellite communications, for a small beam divergence, tracking and pointing plays an important role in the reliability of deep-space optical links This pointing must be accomplished in the presence of attitude changes of the host spacecraft that are perhaps a thousand times larger than the laser beam divergence. 8 Growth of the Deep-Space Comm. Capacity 9 Hamid Hemmati, “Deep Space Optical Communications”, Jet Propulsion Laboratory, California Institute of Technology, 2005 Deep-Space Communications Optical deep-space communications can be implemented in two ways: Direct optical link: A direct optical link is set up between the earth station and space-craft Atmosphere disperses and attenuates the transmitted and received signals High power transmitter and large receivers can be used Indirect optical link: the optical signal is sent from a satellite outside the atmosphere Atmosphere effect is mitigated Transmitter and receiver sizes are limited 10 METOL MARS-EARTH Terahertz Optical Link 5 W 1.54 micron Laser 1 - 10 Gbps 5W 26 GHz 100 Mbps (RF) Small Lander UHF: 128 kbps (150 Mb in 20 minutes) 11 Channel Model Cloud opacity is an atmospheric physical phenomenon that jeopardizes optical links from deep space to any single ground station Clearly, when clouds are in the line-of-sight, the link is blocked Ground receiving telescopes need to be located in sites where cloud coverage is low and statistically predictable To guarantee continuity of data delivery from deep space to ground, while the Earth is rotating, a global network of telescopes is necessary The selection of the sites for telescopes belonging to an optical deep space network (ODSN) is driven by considerations based, among other factors, on cloud-cover statistics 12 Channel Model: Atmospheric Transmittance Main Gases composing the Earth Atmosphere 13 Channel Model: Atmospheric Transmittance Earth atmospheric number density profiles for individual species 14 Channel Model: Atmospheric Transmittance Transmittance spectrum at sea level with zenith angle of zero. 15 Channel Model: Sun Irradiance 16 Channel Model: Sky Irradiance Sky radiance spectrum experienced at an observation point at sea level for 23 km of visibility and Sun zenith angle of 45 deg while observer zenith angle varies as 10, 40, and 70 deg 17 Deep Space Optical Communications Merits of five deep-space communication link wavelengths. 18 Hamid Hemmati, “Deep Space Optical Communications”, Jet Propulsion Laboratory, California Institute of Technology, 2005 Deep Space Optical Communications Data of a NASA optical link between Earth and Mars Modulation scheme: 256-ary PPM BER: 10-3 Bit-rate: 1 Mbps Range: 3.59 × 108 km 19 Hamid Hemmati, “Deep Space Optical Communications”, Jet Propulsion Laboratory, California Institute of Technology, 2005 Optical Deep Space Network To support deep space missions aimed to the exploration of the universe for the last four decades, NASA has designed and operated a global network of radio-frequency ground stations termed the Deep Space Network A similar network can be used for optical communications called optical deep-space network (ODSN) Today NASA’s DSN only requires three radio-telescope hubs to successfully operate the network. The DSN stations (located at approximately 120 deg of separation around the Earth: Goldstone, California; Madrid, Spain; and Canberra, Australia) allow continuous coverage of deep space from Earth 20 Optical Deep Space Network Since the laser transmitter beam width from space covers a limited area on Earth it is necessary that the ODSN consists of a number of ground stations located around the Earth as a linear distributed optical subnet (LDOS) The idea behind LDOS is to have the spacecraft always pointing at a visible station belonging to the LDOS When either the line of sight is too low on the horizon (20 deg of elevation) or is blocked by atmospheric conditions (e.g., clouds or low transmittance), the spacecraft beam is switched to a different station (or network node) by pointing to the adjacent optical ground station 21 Optical Deep Space Network Example of LDOS (star = telescope) architecture for an optical deep space network (ODSN) 22 Hamid Hemmati, “Deep Space Optical Communications”, Jet Propulsion Laboratory, California Institute of Technology, 2005 Global Sites for Deep-Space Optical Communications 23 System Model Usually the received photon count is very low PMTs are used to detect signal The operation temperature of the space-craft is low Thermal noise is proportional to the temperature: 2 = 4 Δ Hence, shot noise is the dominating noise Poisson statistics should be used for analysis 24 System Model For OOK: Probability density functions for transmitting “0” and “1” when K =Data average photon count/pulse K =Background average photon count/pulse Then Pr photon count = = 0) = 1 K exp(−K ) ! Pr photon count = = 1) = 1 ! K + K exp(− K + K ) As discussed before, threshold is where the two pdf’s become equal Threshold = K log 1 + K /K 1 2 BER = − 1 2 ℎℎ =0 −1 Pr = 0) − Pr = 1) 1 2 When K =0, Threshold=0 and BER = exp(−K ) 25 Performance of Deep-Space Optical Communication For PPM Symbol error probability is For Poisson distribution where In the absence of background light 1 − = 2 26 Performance Analysis of OOK BER versus signal level for uncoded OOK signaling on a Poisson channel, for various background levels 27 Performance Analysis of PPM BER of uncoded PPM on a Poisson channel, versus Ks 28 Performance Analysis of PPM BER of uncoded PPM on a Poisson channel, versus Pav = Ks /M 29 FEC in Deep-Space Optical Comm. Due to the low received power the BER is high BER is usually 0.001 Forward error correction (FEC) is used to decrease BER down to 10-15 Deep-space optical systems use high order PPM since they have high energy efficiency Reed-Solomon codes are used as FEC High-order PPM modulation (256-PPM) with a high alphabet (8-bit alphabet) RS code Accumulator (product) codes: 30 Outline Deep-Space Optical Communications Introduction Channel Model System Performance Optical Deep-Space Network RF/FSO Hybrid System 31 RF/FSO Hybrid System Radio-Frequency (RF) Communications Low bandwidth Stable Channel Relatively immune to cloud blocking Sometimes affected by heavy rain Free-Space Optical Communications High Data Rate 2.5 Gbps commercially available (Tbps demonstrated) Bursty Channel Must have clear / haze conditions Less degradation than RF in rain 32 Combining RF and FSO System Enables FSO Communications bandwidth without giving up RF reliability and “adverse-weather” performance Improves network availability: Quality of Service (QoS) More options for adapting to weather Common atmospheric path effects and compensation (directional links) Physical Layer diversity improves jam resistance Size, Weight and Power Focus Leverages common power, stabilization, etc. Economical use of platform volume Enables seamless transition of free space optical communications into RF Environment 33 Average Data-Rate of a Hybrid FSO/RF AVERAGE DATA TRANSFER RATE OF HYBRID FSO/RF LINK AVERAGE DATA RATE (Gb/s) 3 FSO 2.5Gb/s 2 1 RF 10Mb/s 0 0 10 20 30 40 50 60 70 80 90 100 FSO LINK AVAILABILITY (%) 34 Applications Short range applications: Mesh networks Cross-divide links (rivers, canyons, etc.) Indoor systems Long-range applications: Air-to-air links Satellite links Wireless basestation connectivity 35 Hybrid RF/FSO Point-to-Point Link Either switching between technologies or simultaneous use Joint modulation/coding across two technologies With channel state information, can optimize throughput Without channel state information, can use variable-length codes (fountain codes) 36 Hybrid FSO/RF Two different modulations are assumed for RF and FSO links with constellation sizes of M1 and M2 The links are assumed to operate synchronously R1 and R2 are the data rates Let C1 and C2 be the capacity of RF and FSO channel respectively (Ci is a function of Ri) From Shannon capacity we have Then the throughput is 37 Optimal Joint Modulation/Coding 38 Short Range Hybrid RF/FSO Network 39 Hybrid RF/FSO Networks Considering that FSO link has a higher cost, only a given number of FSO links can be used in an RF/FSO system Assume that an RF network is given The problem is to find the best choices for replacing RF with an FSO link This depends on the topology, distances between nodes and the availability of FSO link (depends on the weather condition) 40 Hybrid RF/FSO Networks Formulate the problem as follows The problem is to maximizes the following function where Network is modeled with a directed graph G=(N,L) i ∈ N denote the nodes in the network B is the number of demands lij ∈ L denote the directed link from node i to node j. f (b)ij represent the flow of traffic on link lij Dij is an indicator function of an FSO link from node i to node j One unit time is divided into fractions represented by λk, k = 1,2, ..., K 41 Hybrid RF/FSO Networks The maximization is subject to Input and output flow is equal for intermediate nodes Input flow is zero for source nodes Output flow is zero for sink nodes Flow has to be positive Sum of the time fractions is one The maximum number of FSO links is M 42 Hybrid RF/FSO Networks Here RF capacity is CRFij=100 Mb/s and CFSOij represent the capacity of FSO links between nodes i and j This problem can be solved using mixed integer linear programming (MILP) Optimal throughput and bounds for the 16 node grid network and 28-node random. 43