This 237 page report has addressed the end-to-end design of an interstellar communication system taking account of all known impairments from the interstellar medium and induced by the motion of source and observer. Both discovery of the signal and post-discovery communication have been addressed. It has been assumed that the transmitter and receiver designers wish to jointly minimize the cost, and in that context we have considered tradeoffs between resources devoted in the transmitter vs. the receiver. The resources considered in minimizing cost are principally energy consumption for transmitting the radio signal and energy consumption for signal processing in a discovery search.
Note – project Icarus had worked out an even more energy efficient communication system if the sender and receiver are at gravitation lensing points of two stars. “One tenth of a milliwatt is enough to have perfect communication between the Sun and Alpha Cen through two 12-meter FOCAL spacecraft antennas. A similar bridge between the Sun and a Sun-like star inside M31, using the gravitational lenses of both. We’re working here with a distance of 2.5 million light years, but a transmitted power of about 107 watts would do the trick.”
Thus study is applicable to communication with interstellar spacecraft (sometimes called ”starships”) in the future, although it will probably be quite some time before these spacecraft wander far enough to invoke many of the impairments considered here. In the present and nearer term this study if relevant to communication with other civilizations. In the case of communication with other civilizations, we of course will be designing either a transmitter, or a receiver, which in either case has to be interoperable with a similar design by the other civilization. In either case, it is very helpful to consider the end-to-end design and resource tradeoffs, as we have attempted here. In our view, a major shortcoming of existing SETI observation programs (which requires the design of a receiver for discovery) is the lack of sufﬁcient attention to the issues faced by the transmitter in overcoming the large distances and the impairments introduced in interstellar space.
“The gravity of a large galaxy cluster is so strong, it bends, brightens and distorts the light of distant galaxies behind it. The scale has been greatly exaggerated; in reality, the distant galaxy is much further away and much smaller.”
One profound conclusion of this study is that if we assume that the transmitter seeks to minimize its energy consumption, which is related to average transmit power, then the communication design becomes relatively simple. The fundamental limit on power efficiency in interstellar communication has been determined, that limit applying not only to us but to any civilization no matter how advanced. Drawing upon the early literature in power-efﬁcient communication design, five simple but compelling design principles have been identiﬁed. Following these principles has been shown to permit designs that approach the fundamental limit. Although the same fundamental limit does not apply to the discovery process, we have deﬁned an alternative resource-constrained design approach that minimizes processing resources for a given power level, or power level for a given processing resource. Again, application of a subset of the ﬁve principles leads to a design that can achieve dramatic reductions in the transmitter’s energy consumption relative to the type of Cyclops beacons that have been a common target of SETI observation programs, at the expense of a more modest increase in the receiver’s energy consumption through an increased observation time in each location.
The power efﬁciency for narrow-bandwidth interstellar radio communication signals assumed in many current SETI searches has a penalty in power efficiency of four to five orders of magnitude. A set of ﬁve power-efficient design principles can asymptotically approach the fundamental limit, and in practice increase the power efﬁciency by three to four orders (thousand to tens of thousands of times more power efficient interstellar communication) of magnitude. The most fundamental is to trade higher bandwidth for lower average power. In addition to improving the power efficiency, average power can be reduced by lowering the information rate.
Five principles of power-efficient design
* Use energy bundles
* Avoid channel impairments
* Convey information by location rather than amplitude
* Make energy bundles sparse
* Combat scintillation with time-diversity combining
Required average receive power
The state of the art in receiver design on Earth can achieve a receiver internal noise temperature of 5 degrees kelvin. This would be a representative value that a transmitter would have to assume in order to communicate reliably with us. If we were to construct our own transmitter, it might be reasonable to assume a lower value. Added to this is cosmic background radiation at 3 degrees kelvin, applicable to more advanced civilizations as well as ourselves, giving a total noise temperature of T = 8 degrees kelvin. This is the most optimistic case, since it does not take into account star noise. Based on an assumption of 8 degrees kelvin, the fundamental limit for received energy per bit is
7.66 X 10^-23 joules
1 bit per second communication would be 7.66 X 10^-23 watts
Required average transmit power
To transmit 500 light years at the fundamental limit at 5 GHz frequency
by Pat Galea
In a previous article we took a broad look at the problems involved in interstellar communications. In this article, we will take a closer look at one of the ‘exotic’ techniques mentioned: Gravitational Lensing. Einstein published his General Theory of Relativity in 1915. This theory considered gravity to be the result of the curvature of space (or, more precisely, spacetime, though the distinction will not trouble us here). One of the consequences of this is that a massive object such as the Sun will bend the path of a light beam that passes it. Indeed, this effect predicted by Einstein provided one of the first verifications of General Relativity. During a total eclipse of the Sun it is possible to see the small distortion caused by the Sun’s gravity on the apparent location in the sky of distant stars. Once it was realized that gravity could bend light this way it was also noted that massive objects can act as huge lenses, focusing light to a point, just as a glass lens does. The gravitational bending of light is much weaker than you can get from a glass lens, so despite using a huge lens like the Sun, the focus is still very far away. Interestingly there is an important difference between the optical focusing that we are familiar with and gravitational focusing. A normal glass lens has a specific focal distance. If you want to get a sharp image you have to hold the lens at exactly the right distance from your focal plane (which could be film or some electronic image detector). With a gravitational lens there is no specific focal length. Beyond a minimum focal distance at which the distant light rays are brought together, all the points in the line away from the Sun are foci.
The fact that we don’t have to be at a specific distance from the gravitational lens in order to use it has an interesting consequence for astronomy, We can use a distant galaxy as a lens and observe the bending of the light that it causes in the image. For example, in this image of Einstein’s Cross we are seeing the same distant quasar four times, brought to a focus by an intervening galaxy which is acting as a lens. The quasar is 8 billion light years from Earth, while the galaxy is only 400 million light years away. So we know that this isn’t just some theoretical quirk of General Relativity with no practical effect. We can observe this lensing in reality. But can we exploit the Sun in a controlled way to focus on specific targets? First of all, we need to know how far away from the Sun our camera needs to be in order for the distant light to be focused. It turns out that if you consider the Sun to be a perfect sphere, then you need to get at least 550 AU from the Sun. (1 Astronomical Unit (AU) is the distance from the Earth to the Sun.) However, there is a complication. The Sun is not a perfect sphere. There is a corona on the outside surface that tends to deflect light away from the Sun. This is working against the gravitational lensing effect, and serves to push the focus even further away from the Sun. The result is that we actually need to place our camera about 700 AU from the Sun.
So we fly our camera out to 700 AU, and start snapping pictures of the planets around Alpha Centauri. Is it that easy? No, of course not! First of all, 700 AU is an enormous distance. By comparison, Voyager 1 was launched in 1977 to take a tour of the gas giants of our solar system. This is one of the fastest craft mankind has ever produced, and yet it is (at time of writing) only 113 AU from Earth. There are options for getting craft out to these vast distances relatively quickly, but they don’t need to concern us here. Let’s stipulate that we can get our camera to 700 AU.
We can now use our camera to spy on the comings-and-goings on the surface of a planet at Alpha Centauri. (That’s no idle boast; the resolving power of a gravitational lens telescope is phenomenal.) However, we’ll leave such activities for the astronomers and xenobiologists. Here we are concerned with exploiting the gravitational lens for interstellar communications. Well, it’s really quite simple… in principle! We have our receiver craft sitting at the Sun’s focus, and the distant probe at some point in interstellar space or around another star. We use the Sun to focus the transmissions from the distant probe onto the receiver. This gives us an enormous antenna gain compared to what we would be able to achieve if we were trying to receive the signals directly, without using the Sun. What this means is that we can use much lower transmitter power on the probe without impacting the bandwidth that we can transmit. And this means that we do not have such hefty power supply requirements on a probe that may have been flying for a hundred years before reaching its destination. These seemingly mundane resource constraints should not be underestimated. Like the old military saying goes: “amateurs study strategy; professionals study logistics.” It’s all very well having a gleaming state-of-the-art fusion drive to get your probe to the target, but it’s all for nothing if you don’t have the power to get any data back to Earth.
So we’ve got our receiver out to the focus, and the distant probe is beaming data back to us via the Sun’s gravitational lens. Have we solved all the problems? Unfortunately, no. The big problem remaining here is that the receiver has to stay very closely aligned with the transmissions from the probe to a phenomenal degree of accuracy. We are talking about a tolerance on the order of tens of metres, for a craft which is over 700 AU from the Sun. There is plenty more work to be done in this area, and the Icarus team is studying some ideas to see if they might help to make this a practical system in the near future. Let’s assume that we have solved the positioning accuracy problem, and we can keep the receiver exactly in line with the distant probe. Is there anything else we can do with the system to improve communications even further? Indeed there is. We can exploit two gravitational lenses: the Sun, and the distant star. So we have our receiver craft at the Sun’s gravitational focus, and the distant probe at the focus of the target star. If we can keep both the probe and the receiver exactly in line with each other, then the antenna gain we achieve is beyond enormous; it’s simply phenomenal. Using trivial amounts of power, we can achieve perfect communications between the Sun and (say) Alpha Centauri. (Read the Centauri Dreams article referenced below for the fascinating details.)
Clearly the positioning accuracy problem is greater again when we are trying to keep two craft, separated by over four light years, exactly in line with each other. However, we also get another great bonus: we are now able to send decent amounts of data from Earth back to the probe. Remember that the probe may have left Earth up to a hundred years earlier. While it has been en route, our algorithms for data processing may have improved, and we may have obtained fresh data by other means that the probe could usefully take advantage of. It would be incredibly productive to be able to update the probe with the new information. If we can make this system work, especially with the double-lens communications, we will have taken the first steps in creating an interstellar internet. Imagine the possibilities that a network of transceivers around all the local stars will open up for the next generation of probes. They will no longer need to send signals directly back to Earth. They can just “log on” to their local node which will relay data back to Earth for them. With the establishment of an interstellar communications infrastructure, humanity will truly be a starfaring civilization.
“Error-correcting block code, first invented by Claude Shannon in the 1940’s, has been discovered embedded WITHIN the equations of superstring theory”
EMBEDDED ERROR-CHECKING ALGORITHM found in STRING THEORY
BIT ERROR RATE
The Gravitational Lens and Communications
by Paul Gilster / November 6, 2009
If we can get the right kind of equipment to the Sun’s gravitational focus, remarkable astronomical observations should follow. We’ve looked at the possibilities of using this tremendous natural lens to get close-up images of nearby exoplanets and other targets, but in a paper delivered at the International Astronautical Congress in Daejeon, South Korea in October, Claudio Maccone took the lensing mission a step further. For in addition to imaging, we can also use the lens for communications. The communications problem is thorny, and when I talked to JPL’s James Lesh about it in terms of a Centauri probe, he told me that a laser-based design he had worked up would require a three-meter telescope slightly larger than Hubble to serve as the transmitting aperture. Laser communications in such a setup are workable, but getting a payload-starved probe to incorporate a system this large would only add to our propulsion frustrations. The gravitational lens, on the other hand, could serve up a far more practical solution.
Keeping Bit Error Rate Low
Maccone goes to work on Bit Error Rate, a crucial measure of signal quality, in assessing the possibilities. Bit error rate charts the number of erroneous bits received divided by the total number of bits transmitted. Working out the numbers, Maccone posits a human probe in Centauri space trying to communicate with a typical NASA Deep Space Network antenna (70 meter dish), using a 12-meter antenna aboard the spacecraft (probably inflatable). Using a link frequency in the Ka band (32 GHz), a bit rate of 32 kbps, and forty watts of transmitting power (and juggling the other parameters reasonably), the math is devastating: we get a 50 percent probability of errors. So much for data integrity as we operate within conventional systems. But if we send Maccone’s FOCAL probe to the Sun’s gravitational lens at 550 AU, we now tap the tremendous magnification of the lens, which brings us a huge new gain. Using the same forty watts of power, we derive a completely acceptable bit rate. In fact, Maccone’s figures show that the bit error rate does not begin to become remotely problematic until we reach a distance of nine light years, when the increase in BER begins slowly increasing.
The Bit Error Rate (BER) (upper, blue curve) tends immediately to the 50% value (BER = 0.5) even at moderate distances from the Sun (0 to 0.1 light years) for a 40 watt transmission from a DSN antenna that is a DIRECT transmission, i.e. without using the Sun’s Magnifying Lens. On the contrary (lower red curve) the BER keeps staying at zero value (perfect communications!) if the FOCAL space mission is made, so as the Sun’s magnifying action is made to work. Credit: Claudio Maccone.
Building a Radio Bridge
Now this is interesting stuff because it demonstrates that when we do achieve the ability to create a human presence around a nearby star, we will have ways to establish regular, reliable communications. A second FOCAL mission, one established at the gravitational lens of the target star, benefits us even more. We could, for instance, create a Sun-Alpha Centauri bridge. The bit error rate becomes less and less of a factor:
…the surprise is that… for the Sun-Alpha Cen direct radio bridge exploiting both the two gravitational lenses, this minimum transmitted power is incredibly… small! Actually it just equals less than 10-4 watts, i.e. one tenth of a milliwatt is enough to have perfect communication between the Sun and Alpha Cen through two 12-meter FOCAL spacecraft antennas.
This seems remarkable, but gravitational lenses make remarkable things possible. Recall that it was only months ago that the first tentative discovery of an extrasolar planet in the Andromeda galaxy (M31) was made, using gravitational lensing to make the observation.
Into the Galactic Bulge
Maccone goes on to work out the numbers for other interstellar scenarios, such as a similar bridge between the Sun and Barnard’s Star, the Sun and Sirius A, and the Sun and a Sun-like star in the galactic bulge. That third possibility takes us into into blue sky territory, but it’s a fascinating exercise. If somehow we could use the gravitational lens of the star in the galactic bulge as well as our own gravitational lens, we would have a workable bridge at power levels higher than 1000 watts.
Bit Error Rate (BER) for the double-gravitational-lens of the radio bridge between the Sun and Alpha Cen A (orangish curve) plus the same curve for the radio bridge between the Sun and Barnard’s star (reddish curve, just as Barnard’s star is a reddish star) plus the same curve of the radio bridge between the Sun and Sirius A (blue curve, just as Sirius A is a big blue star). In addition, to the far right we now have the pink curve showing the BER for a radio bridge between the Sun and another Sun (identical in mass and size) located inside the Galactic Bulge at a distance of 26,000 light years. The radio bridge between these two Suns works and their two gravitational lenses works perfectly (i.e. BER = 0) if the transmitted power higher than about 1000 watt. Credit: Claudio Maccone.
I’m chuckling as I write this because Maccone concludes the paper by imagining a similar bridge between the Sun and a Sun-like star inside M31, using the gravitational lenses of both. We’re working here with a distance of 2.5 million light years, but a transmitted power of about 107 watts would do the trick. This paper is a dazzling dip into the possibilities the gravitational lens allows us if we can find ways to reach and exploit it.
Extragalactic Internet by black hole gravitational lensing
by Dr. Claudio Maccone
“In two recent papers [1, 2] the author proved that radio communications between any pair of stars within our Galaxy are feasible with modest transmitted powers if the gravitational lenses of both stars are exploited. In this chapter we extend those innovative results to the case of radio communications between nearby galaxies. We show that radio communications between galaxies may become feasible if supermassive black holes, usually located at the center of galaxies, are exploited as gravitational lenses. In other words, a massive black hole may be regarded as a huge focusing device for radio waves being transmitted out of that galaxy and/or being received from another galaxy.”
1. C. Maccone, “Interstellar radio links enhanced by exploiting the Sun as a gravitational lens,” Acta Astronautica, 68 (2011), 76–84. CrossRef
2. C. Maccone, “Interstellar radio links enhanced by exploiting the Sun as a gravitational lens,” paper #IAC-09.D4.1.8 presented at the 60th International Astronautical Congress, Daejeon, Republic of Korea, October 12–16, 2009, and distributed to all participants as a CD-ROM file, but not published in printed form.
3. C. Maccone, Deep Space Flight and Communications: Exploiting the Sun as a Gravitational Lens, Springer/Praxis, Heidelberg, Germany/Chichester, U.K. A 400-page treatise about the FOCAL space mission that embodies and updates all previously published material about FOCAL.
4. C. Maccone, “SETI among galaxies by virtue of black holes,” Acta Astronautica, 2012, in press.
End-to-end interstellar communication system design for power efficiency
by David G. Messerschmitt / 21 May 2013
“Interstellar radio communication accounting for known impairments due to radio propagation in the interstellar medium (attenuation, noise, dispersion, and scattering) and motion is studied. Large propagation losses and large transmitted powers motivate us to maximize the power efficiency, defined as the ratio of information rate to average signal power. The fundamental limit on power efficiency is determined. The power efficiency for narrow-bandwidth signals assumed in many current SETI searches has a penalty in power efficiency of four to five orders of magnitude. A set of five power-efficient design principles can asymptotically approach the fundamental limit, and in practice increase the power efficiency by three to four orders of magnitude. The most fundamental is to trade higher bandwidth for lower average power. In addition to improving the power efficiency, average power can be reduced by lowering the information rate. The resulting low-power signals have characteristics diametrically opposite to those currently sought, with wide bandwidth relative to the information rate and sparse distribution of energy in both time and frequency. The design of information-free beacons power-optimized for a given observation time is also undertaken. Such beacons need not have wide bandwidth, but at low powers their energy is sparsely distributed in time. The discovery of both beacons and information-bearing signals is analyzed, and shown to require a substantial number of observations (growing as power is reduced) to achieve a high probability of success. The “false alarms” in current searches are characteristic signatures of possible power-efficient and power-optimized signals. Although existing SETI searches will fail to discover these signals, they can be discovered using common algorithms with straightforward modification to current search methodologies.”
by Greg Goebel
SETI experiments performed so far have not found anything that resembles an interstellar communications signal. Says Frank Drake of the SETI Institute: “All we know for sure is that the sky is not littered with powerful microwave transmitters.”
The great Italian physicist Enrico Fermi suggested in the 1950s that if there was an interstellar civilization, its presence would be obvious once we bothered to look. While faster-than-light, or “superluminal”, flight is ruled out by contemporary physics, no law of physics absolutely rules out interstellar flight at “subluminal” speeds, though the physical requirements are formidable. Assuming that stars are on the average about ten light-years apart; that an interstellar mission can be conducted at a speed of 10% of the speed of light; and that it takes four centuries for an interstellar colony to grow to the point where it can launch a pair of interstellar missions, then the “doubling time” of the interstellar colonies created by this advanced civilization would be 500 years. This would allow colonization of the entire Galaxy in five million years. Even limiting an interstellar mission to 1% of the speed of light and assuming it takes a millennium for a society to get to the point where it can mount two interstellar missions, this still means the Galaxy would be completely populated in 20 million years. That is a very short interval on a cosmic time scale. Given the lack of observable signals, as well as the lack of any persuasive evidence that extra-terrestials have ever visited this planet, Fermi’s argument suggests that there is no such interstellar civilization. This depressing argument is called the “Fermi paradox”.
*The fact that SETI searches have not come up with anything very interesting so far is not cause for despair. As the previous sections of this document show, trying to find another civilization in space is a difficult proposition, and we have only searched a small fraction of the entire “parameter space” of targets, frequencies, power levels, and so on.
The negative results do place limits on the proximity of certain “classes” of alien civilizations, as specified in a scheme proposed by Soviet SETI researcher Nikolai S. Kardashev in the early 1960s and later extended by Carl Sagan. In this scheme, a “Type I” civilization is one capable of using all the sunlight falling on the surface of an Earthlike planet for an interstellar signal; a “Type II” civilization is capable of harnessing the power of an entire star; and a “Type III” civilization is capable of making use of an entire Galaxy. Intermediate civilizations can be numerically defined on a logarithmic scale. Assuming that an alien civilization is actually transmitting a signal that we could pick up, the searches so far rule out a Type I civilization within a spherical radius of 1,000 light-years, though there may be many civilizations comparable to our own within a few hundred light years that have remained undetected. A similar analysis using the same assumption shows that there is no detectable Type II civilization in our Galaxy. In the early days of SETI, researchers assumed that such advanced civilizations were very common in our Galaxy. It is discouraging that this does not seem to be so. However, it is important to emphasize that our SETI hunts have been based on assumptions on communications frequencies and technologies that may be laughable to alien societies, if they have the concept of humor. The lack of results do not say that alien civilizations don’t exist. They only say that if they do, our most optimistic assumptions for getting in touch with them have proven unrealistic.
*There is another issue that hints as to why we don’t see evidence of a large number of alien societies. That issue is time.
Our Sun is not a first-generation star. All first-generation stars are either very small and dim, or have exploded, or have burned out. This first generation synthesized the heavy elements needed to create planets and lifeforms. Later generations of stars, including our Sun, have been born and have died or will die in their turn. Our Galaxy is more than 10 billion years old. Intelligent life and technological societies may have arisen and died out many times during this ten billion years. Assuming that an intelligent species survives for ten million years, that means that only 0.1% of all societies that have arisen during the history of our Galaxy are in existence now.
Science writer Timothy Ferris has suggested that since galactic societies are probably transitory, then if there is in fact an interstellar communications network, it consists mostly of automated systems that store the cumulative knowledge of vanished civilizations and communicate that knowledge through the Galaxy. Ferris calls this the “Interstellar Internet”, with the various automated systems acting as network “servers”. Ferris suspects that if such an Interstellar Internet exists, communications between servers are mostly through narrow-band, highly directional radio or laser links. Intercepting such signals is, as discussed earlier, very difficult. However, the network probably still maintains some broadcast nodes in hopes of making contact with new civilizations. The Interstellar Internet may be out there, waiting for us to figure out how to link up with it.