Spectrum Congestion

1
Spectrum Congestion
2
Radio Link Environment

• The Shannons Law assumes an isolated
  communication channel
• no fading,
• no inter-symbol interference,
• no shadowing
• no Co- or Adjacent Channel interference
• No radio link can be isolated as radio waves
  cannot be confined to any specific volume.

3
Degree of Isolation

• Any radio link interacts with its EM environment
• The Shannons model of communication channel is
  to be modified to include interaction with
  environment
• The degree of this interaction can be described
  by Isolation Index

4
Isolation index
Isolated link N gtgt I, a ? 1

• Definition
• a N / (I N)
• Environmental noise
• Intrinsic noise power

Link coupled to its environment I gtgt N, a ? 0
5
Isolation Depends on

• System characteristics
• Modulation, Coding, Signal processing,
• Equipment characteristics
• Power, Frequency, Polarisation,
• Spatial deployment
• Distance, Direction, Polarization,
• Environment
• Propagation effects
• Reflections, Terrain shadowing,

6
Capacity Degradation
R/Blog2(1aq) q S/N
With constant bandwidth, the amount of bits
transmitted per second per Hz (R) increases with
the isolation index (a) and signal-to-noise ratio
(q).
7
Amount of Information
Isolated radio link
Radio link coupled to environment
t
Log(1q)
Log(1aq)
t
B
B
8
Congestion ? Loss of Capacity

• A cluster of mutually coupled radio links.
• Capacity of each radio link is reduced due to
  the impact of other links.
• The worst case
• Co capacity of an isolated link
• k number of other links (identical)

C/Co log21 (S/N)/ 1 k(Io/N)
9
Summary

• The congestion increases as new radio users
  appear, the capacity of radio links deteriorates,
  and the existing users suffer
• A reduced amount of information can be
  transmitted in a given time and bandwidth
• A wider bandwidth is required to transmit a given
  amount of information within a given time
• A longer time is required to transmit a given
  amount of information within a given bandwidth
• It means increased transmission costs
• The concepts of isolation and capacity loss due
  to congestion might serve
• As objective measures of spectrum congestion
  and/or as a criterion in comparing various
  trade-offs that may be difficult to compare
  otherwise (in system design and in spectrum
  management)
• As a criterion in optimisation projects seeking
  min. capacity loss of a specific radio link
  (local optimum), a specific set of radio links
  (local optimum extended), or all radio links in
  question (global optimum)
• Was proposed recently - has not been generally
  accepted yet

10
References

• Struzak R Spectrum Congestion and Capacity of
  Radio Links, Annals of Operations Research 107,
  339-347, 2001-2002, Cluwer Academic Publishers