ACS Dr' Sandra Dudley Notes http:eent3'lsbu'ac'ukstaffdudleymsACS'htm Please print off the notes 4 6

1
ACS Dr. Sandra DudleyNotes http//eent3.lsbu.ac.u
k/staff/dudleyms/ACS.htmPlease print off the
notes (4 6 Slides a Sheet)

• 3). Satellite Communication Systems Lecture 1
  
• Covers basic principles of satellite
  communications. It looks at geostationary
  satellites (GEOs), low-earth orbiting satellites
  (LEOs), and medium-earth orbiting satellites
  (MEOs).
• The key design issue of capacity allocation is
  examined. Next-generation satellite networks will
  be discussed with emphasis on bandwidth
  allocation protocols.
• Learning outcomes
• Know satellite parameters and configurations.
• Understand the capacity allocation FDMA, TDMA
  and more advanced schemes.

2

• 5). High Speed Networks
  3
• This part provides a survey of high-speed
  networks including QoS provision and congestion
  control issues. It covers a survey treatment of
  the widely used ATM technology and looks at the
  services provided by ATM and the use of the AAL
  to implement those services. This topic also
  provides an introduction to high-speed wireless
  LANs.
• Learning outcome
• Understand traffic management, QoS provision and
  congestion control issues in high-speed networks
• Understand technologies and applications involved
  with ATM,
• Understand advanced TCP congestion control
  mechanisms and IP MPLS
• Understand WLANs, IEEE 802.11 standard and
  Bluetooth.

3

• 6). Advanced Data Networks
  2
• This part will introduce Gigabit Ethernet, VLANs
  (Virtual Local Area Networks) and VPNs (Virtual
  Private Networks). These two newer network
  technologies are quickly finding their way into
  businesses because of the benefits they provide
  in terms of cost, flexibility, and security. This
  part will present students with the basic
  concepts and issues involved in security,
  preparing them to address these issues as they
  begin to design networks.
• Learning outcome
• Know the principle of VLANs and VPN, the basic
  concepts and issues involved in network security.

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Outline

• Satellite systems
• advantages over terrestrial systems
• classification of satellite systems
• problems with satellite communication
• Multiple Access and Medium Access Control (MAC)
  in satellite networks
• Link-budget analysis for satellite links

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Satellite Systems
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Satellite Systems
Advantages over terrestrial systems

• They can cover large areas
• Inherent broadcast capability
• Easy deployment and configuration
• High-speed network access
• Inherent capability of by-passing the whole
  terrestrial network

spot beams of the satellite
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Satellite Systems
Orbital classification

• Low Earth Orbit (LEO)
• Medium Earth Orbit (MEO)
• Geosynchronous Orbit (GEO)
• Highly Elliptical Orbit (HEO)

8
Satellite Systems
Low Earth Orbit (LEO)
Advantages

• short propagation delays (10-15 msec)
• low transmission power required
• low price for satellite and equipment

Disadvantages

• small coverage spot
• they have to be in rotation to preserve their low
  altitude (90 mins period)
• a network of at least 6 LEO satellites is
  required to cover a region continuously
• high system complexity due the need for handovers
  and satellite tracking

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Satellite Systems
Medium Earth Orbit (MEO)
Advantages

• slightly longer propagation delays (40 msec)
• slightly higher transmission power required
• more expensive than LEOs but cheaper than GEOs

Disadvantages

• coverage spot greater than a LEO, but still less
  than a GEO
• still the need to be in rotation to preserve
  their low altitude
• multiple MEO satellites are still needed to cover
  a region continuously
• handovers and satellite tracking are still
  needed, hence, high complexity

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Satellite Systems
Geosynchronous Orbit (GEO)
Advantages

• large area coverage, stay where they are at
  22,000miles above the Earth
• satellite rotation is synchronous to earth
• three satellites can cover the whole globe
• low system complexity

Disadvantages

• long propagation delay (125 msec)
• high transmission power is required

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Satellite Systems
High Elliptical Orbit (HEO)
Advantages

• maximum time spent over populated northern
  hemisphere
• large area coverage over the northern hemisphere

Disadvantages

• still the need to be in rotation to preserve
  their orbit
• still no continuous coverage of a region with one
  HEO satellite
• handovers and satellite tracking are still
  needed, hence, high complexity

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Satellite Systems
LEO
MEO
GEO
Propagation Delay
HIGHER
LOWER
Covered Area
LOWER
HIGHER
Transmission Power Requirement
LOWER
HIGHER
Network System Complexity
HIGHER
LOWER
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Satellite Systems
Bent-pipe and On-board Processing classification
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Bent Pipe (BP)
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On Board Processing (OBP)
ISL- Inter-Satellite Link
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Satellite Systems
Problems with satellite communication

• satellite uplink (return link) is a shared medium
• satellite links are highly erroneous
• long propagation delays in GEO
• limited available bandwidth
• very limited on-board space, power, and
  complexity

Highly efficient and low-complexity medium access
control protocols with QoS guarantee for
real-time and interactive applications are needed
!
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Satellite Systems
Satellite frequency bands
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Multiple Access in Satellite Networks
Basic multiple access schemes

• Frequency Division (FDMA) Bandwidth ? Carrier
  Frequency
• allows smaller size antennas
• not flexible, not suitable for dynamic bandwidth
  allocation

• Time Division (TDMA) Bandwidth ? Time Slot
• requires high transmission power and large
  antenna
• highly flexible, suitable for dynamic bandwidth
  allocation
• fits well in all-digital network management
• synchronisation is a problem
• better use of downlink power

• Code Division (CDMA) Bandwidth ? Channel Code
• synchronisation is only an end-to-end problem
• popular in military applications due its
  resistance against jamming
• inherently inferior to TDMA and FDMA in
  bandwidth efficiency
• resistance against multi-path and
  frequency-selective fading
• popular in cellular networks

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Multiple Access in Satellite Networks
Hybrid multiple access schemes
Multi-Frequency Time Division (MF-TDMA) Bandwidth
? Carrier frequency and time slot

• each terminal can use one carrier frequency AT A
  TIME
• very popular in future broadband satellite
  network designs
• flexibility of TDMA low transmission power and
  small antenna size requirements of FDMA

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Medium Access Control (MAC) in Satellite Networks
MAC Protocols regulate the access to the
satellite uplink by earth stations that are
scattered over a large geographic area.
Due to large size of the areas covered by a spot
beam carrier-sensing is not possible in a
satellite uplink !! (Forget about Ethernet)
Remember the long propagation delays. In a GEO
satellite it takes 250 msec to get an ACK from
the satellite for each packet you send !
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Medium Access Control (MAC) in Satellite Networks
Basic MAC Protocols

• Random Access
• Fixed-assignment
• Demand-assignment (reservation protocol)
• Fixed-rate Demand-assignment
• Variable-rate Demand-assignment
• Free-assignment

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Medium Access Control (MAC) in Satellite Networks
Basic MAC Protocols

• Random Access
• Fixed-assignment
• Demand-assignment
• Fixed-rate Demand-assignment
• Variable-rate Demand-assignment
• Free-assignment

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Medium Access Control (MAC) in Satellite Networks
More on Random Access SLOTTED ALOHA

• Time is divided into equal size slots ( packet
  trans. time)
• Node with new arriving packet transmit at
  beginning of next slot
• If collision retransmit pkt in future slots with
  probability p, until successful.

Success (S), Collision (C), Empty (E) slots
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Medium Access Control (MAC) in Satellite Networks
More on Random Access SLOTTED ALOHA

• Assume N stations have packets to send
• each transmits in a slot with probability p
• prob. successful transmission S(p) is
• - S Throughput
• - G Mean Transmission attempts per frame
  time for old and new combined (Assume Poisson
  Distribution)
• Results For Pure ALOHA S(p) Gexp-2G
• For Slotted ALOHA S(p) Gexp-G
• Slotted ALOHA is TWICE AS EFFICIENT!!

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Medium Access Control (MAC) in Satellite Networks
More on Random Access SLOTTED ALOHA
Throughput versus Offered Traffic!
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Link Budget Analysis Why ?

• System performance tied to operation thresholds.
• Operation thresholds Cmin tell the minimum power
  that should be received at the demodulator in
  order for communications to work properly.
• Operation thresholds depend on
• Modulation scheme being used.
• Desired communication quality.
• Coding gain.
• Additional overheads.
• Channel Bandwidth.
• Thermal Noise power.

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Link Budget Analysis Antennae
Reflector Type Antennae
Symmetrical, Front-Fed
Offset, Front-Fed
Offset-Fed, Cassegranian
Offset-Fed, Gregorian
Where Ae is effective aperture area.
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Link Budget Analysis Antennae

• Aperture antennas (horns and reflectors) have a
  physical collecting area that can be easily
  calculated from their dimensions

• Gain can be re-written as

• Typical values of ?
• Reflectors 50-60
• Horns 65-80

• Therefore, using gain definition we can obtain
  the formula for aperture antenna gain as

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Link Budget Analysis Antennae
Peak (i.e. maximum) GAIN
Angle between the 3 dB down points is the
beamwidth of the antenna ?3db
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Link Budget Analysis Antennae
For Reflector Antennae, as a rule of thumb,
Also remember gain,

• The approximation above, together with the
  definition of gain allow a gain approximation
  (for reflectors only)

• Assuming for instance a typical aperture
  efficiency of 0.55 gives

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Link Budget Analysis Transmission
Isotropic Source
Distance R
Pt Watts
Power Flux Density PFD
Surface Area of sphere 4pR2 encloses Pt.
W/m2
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Link Budget Analysis Transmission

• The output power of a transmitter HPA is
• Pout Watts
• Some power is lost before the antenna
• Pt Pout /Lt watts reaches the antenna
• Pt Power into antenna
• The antenna has a gain of
• Gt relative to an isotropic radiator
• This gives an effective isotropic radiated power
  of
• EIRP Pt Gt watts relative to a 1
  watt isotropic radiator

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Link Budget Analysis Received Power

• We can rewrite the power flux density (PFD) now
  considering the transmit antenna gain

• The power available to a receive antenna of
  effective area Ae m2 we get

W

• Inverting the equation given for gain gives

Inverting
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Link Budget Analysis Received Power
Received Power

• The inverse of the term at the right referred to
  as Path Loss, also known as Free Space Loss
  (Lp)

Therefore
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Link Budget Analysis Summary

• Demonstrated formula assumes idealized case.
• Free Space Loss (Lp) represents spherical
  spreading only.
• Other effects need to be accounted for in the
  transmission equation
• La Losses due to attenuation in atmosphere
• Lta Losses associated with transmitting antenna
• Lra Losses associates with receiving antenna
• Lpol Losses due to polarisation mismatch
• Lother (any other known loss - as much detail
  as available)
• Lr additional Losses at receiver (after
  receiving antenna)

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Link Budget Analysis Summary

• Some intermediate variables were also defined
  before
• Pt Pout /Lt EIRP Pt Gt
  
• Where
• Pt Power into antenna
• Lt Loss between power source and antenna
• EIRP effective isotropic radiated power

• Therefore, there are many ways the formula could
  be rewritten. The user has to pick the one most
  suitable to each need.

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Link Budget Analysis Closing the link

• We need to calculate the Link Budget in order to
  verify if we are closing the link.
• Pr gt Cmin ? Link Closed
• Pr lt Cmin ? Link not closed
• Usually, we obtain the Link Margin, which tells
  how tight we are in closing the link
• Margin Pr Cmin
• Equivalently
• Margin gt 0 ? Link Closed
• Margin lt 0 ? Link not closed