Here we present interesting
topical technology and introduce concepts and ideas which could
well shape the future.
you prefer - High Altitude Platforms or geostationary satellites
with Micro-ElectroMechanical Systems (MEMS)
(IP) - back to future ?
Control Protocol (TCP) problems over satellite
computing - emulating humans ...
Brain-computer interface .. read
High Altitude Platform
(HAP) or High Altitude Long Endurance Platform (HALEP)
are stratospheric platforms being investigated for broad-band wireless
telecommunications. The platform are hoisted at altitudes of about
17~22 Kms to set communication links similar to satellites. Within
this height range, wind currents are low and commercial aviation
So why the interest
? The answer is much simpler than the technology - there
is a considerable demand for broad-band wireless communications
and HAPs are a promising medium. They appear at a high elevation
angle compared to terrestrial base stations thereby mitigating the
terrestrial propagation effects, and with a visibility of around
200 Km at 5 Deg elevation, they can replace a large number of terrestrial
base stations; and being considerably closer to the ground than
satellites offer much lower path loss than satellites - better by
~ 34 dB for LEO satellites and ~ 66 dB for GEO satellites. HAPs
have been assigned frequency bands in 47/48 GHz and 28 GHz (ITU
region 3 only), bands where at present spare spectrum is in plenty.
HAP telecommunication systems can be designed to respond dynamically
to traffic demands; they are relatively low cost compared to satellites
(perhaps, $50 million ? ); they can be deployed incrementally and
rapidly when necessary; the platforms and payload are upgradeable;
and they are environmentally friendly using solar power, without
need of launchers and eliminate the need of terrestrial masts. Typical
cell size of HAPs range between 1-10 Km and the communication throughput
can range between 25-155 Mb/s. The coverage is regional, though
it is possible to inter-link HAPs creating a national grid, or alternatively
they can be connected to distant gateways via satellites. They have
been proposed for broad-band fixed wireless access (B-FWA), mobile
communications as base stations, rural telephony, broadcasting,
emergency/disaster applications, military communications, etc.
With such advantages why have they not
been exploited commercially? Well, the
technology has yet to arrive. There are a number of open technical
issues being actively pursued. A number of system issues are under
investigation including - system architecture, frequency planning,
network protocols, resource planning, etc. Propagation characteristics
at 47/48 GHz are not well defined yet; modulation/coding techniques
have to be optimized for such propagation conditions; 48 GHz antenna
technology with multiple spot beams is under development. Other
issues include platform station-keeping, hand-off considerations
even for fixed stations due to platform movement and payload power.
HAPs have similar eclipse problem to satellite with regards to payload
power due to the use of solar cells. Reliable platforms are yet
to be developed. Platforms under investigations include Airship
, Airplane, unmanned aerial vehicle and tethered aerostat which
reach up to 5 Km. Airships use very large semi-rigid helium-filled
containers. One implementation of airplane HAP is the solar powered
plane which flies in a tight approximate circular path.
A number of research and commercial programs
are addressing the problems. They include HeliNet Program, HALO
Project, SkyNet and others (see reference) . The technology is a
few years away from implementation (2001) but entrepreneur are ready
to launch their system today only if the financiers were a bit less
condescending. To be impartial - their innocent question is - when
could we expect a return ? It is believed that HAPs will arrive
when the technology has matured to make them commercially viable.
Due to overall costs and complexity in providing
world wide seamless coverage it is unlikely that HAP will replace
satellite, but synergistic solutions can, in fact, augment traffic
carried by each. Both the books of the author refer to the concept
of communication through HAPs in context of their role in a satellite
1. Tozer T. C, and Grace D (2001). 'High-altitude
platforms for wireless communications', Electronics Communication
Engineering Journal, June, pp. 127-137.
2. Djuknic G. M, Freidenfelds J. and Okunev
Y. (1997). 'Establishing wireless communications via high altitude
aeronautical platforms: a concept whose time has come ?' IEEE communications
magazine, September, pp.128-135.
3. Helinet http://www.helinet.polito.it
4. Advanced Technologies Group http://www.airship.com
5. SkyStation http://www.skystation.com
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systems or MEMS as the name implies are tiny
systems which combine semiconductor technology with micro mechanical
devices such as gears, diaphragms, fluid thrusters, accelerometer,
motors and heat controllers built
typically on silicon thus giving
the aerospace industry the possibility of creating very small or
'pico' satellites weighing less than a kilogram.. MEMS are already
in use as car airbag triggers, low power filters on mobile phones
and in optical fibre routers as switch. MEMS satellites when mass
produced could be used for a vast array of satellite-based applications
with advantages in terms of low launch costs, high resistance to
radiation and vibration (due to their micro size).
A number of MEMS devices are under development
- switches, gyros, thrusters, thermal control systems, propulsion
system in institutions like the Jet Propulsion Laboratory (JPL),
Pasadena, and Aerospace Corporation in El Segundo. Several MEMS
satellites have been flown for testing components. The first application
of a MEM satellite may be a tiny satellite inspector which will
be used to inspect faults housed within a mother spacecraft to fly
around it to inspect in case of malfunction thereby saving considerable
diagnostic efforts on the ground. It is anticipated that the technology
is at least one to two decades away.
1. Crass S (2001). 'MEMS in space', IEEE
Spectrum, July, pp.56-61.
2. IEEE Spectrum (1998). 'How to model and
simulate microgyroscopic systems', June, pp. 66-75.
3. IEEE Spectrum (2001).'Large jobs for little
devices', January pp. 72-73.
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suite or IP was developed in 1970s as a part
of research funded by DARPA (Defence Research Projects Agency) for
the experimental network ARPANET and in 2001 there is evidence that
the protocol will form a backbone transport protocol of future telecommunication
networks. It is a remarkable achievement considering the unprecedented
changes in telecommunications technology in recent years.
Why is IP so resilient ? In
a nutshell, the protocol is general and extendable.
an end to end protocol which operates without specific knowledge
of the network and therefore passes on the error recovery to the
end system as it does not expect cent percent network reliability.
This implies that protocol can traverse a mix of networks during
transport - e.g. Ethernet, X.25, ISDN, satellite network, etc.
two types of end-to-end transport services - TCP, which has features
to reduce network congestion and errors using a handshake between
end services and UDP, which is a basic transport mechanism without
the handshake feature of TCP. This way IP can support a mix of services
such as video, voice, data streaming.
The 32 bit address field in
conjunction with a decision to maintain a global address registry,
allowing each IP network to keep its own address repertoire is a
key to the extension of the Internet as a global network.
The IP protocol suite is modular,
thereby allowing independent enhancements to each component of the
stack. For example numerous modifications to the routing protocols
have allowed scaling up the number of computers without affecting
any other layer. Similarly, TCP suite has been continually refined
over the years to improve network congestion and user feedback mechanism
without affecting other layers while retaining backward compatibility
with earlier TCP generations.
Acceptability of IP protocol
has been aided by the ready availability of the protocol specifications
and reference implementation; openness in its formulation; and changes
to the suite with a general consensus - a model which continues
to be practice in the Internet Engineering Task Force (IETF).
However, despite the universal
acceptability of the protocol, a number of improvements are necessary
to usher the paradigm to the next generation Internet and other
telecommunication networks which intend to incorporate IP as a transport
Routing and traffic management
continue to present challenges as Internet growth continues and
there appears to be a need for better management of traffic to avoid
traffic congestion by adaptive routing. IP operates with the notion
of identity and location within its address which is not amiable
to mobile communications. Identity and hence authentication aspects
are inadequate, due to its support of a distributed architecture
based on trust. The suite is not amicable for multicast, an application
which is growing in demand. Another area in need of enhancement
is the support of quality of service despite several alternative
and often propriety solutions. TCP mechanism within the suite is
inefficient in wireless environment, and in particular, on satellite
routes. The 32 bit address space of IP suite is already beginning
to reach its limits.
IPv6 has been developed to improve
several IPv4 features including the address limitations by increasing
the address space to 128. However, its introduction to the Internet
is slow. In the meanwhile users continue to introduce interim solutions
to skirt existing limitations.
A note addressing TCP
limitations over satellites is available in 'Innovations'.
1. Huston G, 'The IP scorecard',
(2001). Satellite broad-band, August. pp. 20-22.
2. Richharia M, ' Mobile satellite
communications, Principles and trends', (2001). Addison and Wesley,
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Transmission Control Protocol
(TCP) is a commonly used transport
protocol for non real-time application over IP
networks. The protocol was developed for
fixed links assuming that there are negligible errors or transmission
delay in the link and packets are lost only due to congestion. The
assumptions do not apply to satellite links with long delays and
transmission errors therefore TCP/IP throughput over satellite channels
can degrade. Problems are caused by a number of reasons : (a) slow
start up of TCP protocol; (b) transmission errors; (c) link asymmetry;
(d) attempts to transfer real-time data over TCP.
Slow start up problem occurs because TCP
ramps up the transmission rate gradually while assessing network
capacity. The capacity is probed by awaiting acknowledgment of received
packets. The congestion window is initially set to 1 and increased
on each successful receipt of acknowledgment. In geostationary,
medium earth orbit and low earth orbit systems, operating at 1 Mb/s
the full capacity can be reached respectively in about 3.91, 1.49
and 0.18 seconds when using 1 Kbyte packets.
A number of solutions are used to circumvent
the slow start problem. Common solutions include - increasing the
initial congestion window, TCP spoofing, cascading TCP connections,
fast start and sudden start. Increasing initial window size can
give an improvement of the order of 3 times the round trip delay.
In TCP spoofing technique, an artificial acknowledgment is sent
back to the source by a router near the sender, while the satellite
link is established at the far end, thus the TCP ramps up throughput
rapidly. In fast start TCP transmission rate is derived from recent
history. In sudden start algorithm, dummy packets are sent initially
to gauge the network conditions. At the end of this training phase
the transmission rate is ramped up to the level acceptable by the
Transmission errors compound the slow start
problem, as the packets lost due to errors are assumed lost due
to congestion and therefore throughput is throttled back. In addition,
the lost packets have to be retransmitted.
Proposed solutions include Explicit Congestion
Notification (ECN), Link Corrupt Notification (LCN) and Rapid Recovery.
In ECN scheme, congestion is notified to the sender; and packets
not lost by congestion are assumed lost due to link errors. Transmission
rate is not decreased on encountering transmission losses. In LCN,
the receiving station notifies bad link conditions to the sender
by continually monitoring the link and the sender takes appropriate
measures depending on channel conditions.
Satellite links are asymmetrical in a number
of applications with a low throughput in the return direction. This
causes traffic burstiness and loss of throughput by TCP whenever
the return link is congested.
Proposed solutions to the bandwidth asymmetry
problem include header compression methods, periodic acknowledgment
by the receiver, reduction in acknowledge data flow through a feedback,
ACK filter, sender adaptation, ACK reconstruction and satellite
transport protocol (STP).
None of the solutions are ideal and therefor
considerable development effort is underway in industry and research
1. Allman M et al,'Ongoing TCP research related to
satellites' (2000). RFC 2760, February.
2. Akyildiz I.F, Morabito G and Palazzo S, 'Research
issues for transport protocols in satellite IP networks' (2001).
IEEE Personal Communications, Vol 8, No 3, June, PP 44-48.
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in satellite arena
Imagine having to operate a mobile satellite
communications network where traffic is changing unpredictably,
users are constantly demanding attention for a variety of reasons,
gateways are malfunctioning due to - scintillation, rain fade, software
crash ....... This type of nightmarish operational scenario, transparent
to the users, is routine. In real world, things are not as precise
as machines would like and therefore humans must keep a vigilant
eye on state of affairs as they have the experience and the skill
to react to unforeseen and unexpected events. Conventional 'hard'
computing follows rules and dogma laid out by its creator whereas
high IQ machines using soft computing have an ability to manage
Soft Computing (SC)
is an emerging discipline attempting to provide robustness in presence
of uncertainty, imprecision and partial truth of real world by synergistically
combining an array of computing technologies - fuzzy logic, neurocomputing,
probabilistic computing, chaotic computing and machine learning.
SC algorithms by contrast to conventional or 'hard' computing are
able to advantageously exploit natural phenomena such as intuition
and subjectivity, allowing modeling ambiguity and uncertainty as
a human would.
SC methodologies are being used in a number
of industries being particularly useful when traditional analytical
methods fail. Intelligent systems can be constructed through flexible
knowledge acquisition and processing in conjunction with powerful
knowledge representation. SC has been applied to in aerospace, communication
systems, consumer appliances, electric power systems, manufacturing
automation, robotics, power electronics and motion control, processing
engineering, transportation, etc..
SC have been used or proposed for - orbital
operations of Space shuttle, aerospace and aircraft control systems,
channel assignment in cellular and satellite systems, multimedia
traffic prediction, design of network topologies, processing of
code division multiple access, etc.
1. Proceedings of IEEE, September 2001, Special issue
'Industrial innovations using soft computing'.
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The electrical activity of human
brain captured in eletro-encephalograph (EEG) changes in response
to human thoughts. EEG based brain-computer
interface make use of thought signals to trigger an appropriate
action in a computer. While the technique is being researched for
medical applications - for example, to aid a paralysed patient,
its application can be conceived for other industrial application.
An astronaut could exercise such an interface when repairing a space
vehicle, a motorist could dial a telephone number or tune the radio
In reference 1 the authors demonstrate an EEG-based
computer interface for restoring the hand grasp function of a paralysed
patient with an electronic hand device using mental imagination
of the motor command and reference 2 reports a number of related
1. Pfurtscheller G and Neuper C 'Motor imagery and
direct brain-computer communication' Proceedings of IEEE, July 2001,
2. Proceedings IEEE, July 2001, 'Neural Engineering:
Merging engineering & Neuroscience' .
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