Digital Signal Processing Reference
In-Depth Information
users' mobility. A terminal remains connected to a single network for seconds (a
phone call, while driving) to several hours (WLAN in the home).
When dynamics are slow, the system can be observed through events. For net-
work dynamics, such an event would for instance be a terminal joining the network.
Between events, a system can be approximated as being static.
In Chap. 7 it will be illustrated how to adapt the network parameters to improve
coexistence with other networks of the same type. In Chap. 5 the coexistence prob-
lem of IEEE 802.15.4 networks with dynamic IEEE 802.11 networks will be con-
sidered.
3.3.3 The Sharing Challenge
Throughout this topic, the sharing problem is a key driver for the control challenge
and, hence, the need for a systematic flow. It is not sufficient to solely consider the
impact of a local decision, e.g., decreasing the transmission rate, to evaluate local
performance. Thus, the results of the proposed framework in its global environment
are considered: a single network consisting of multiple users, different networks
sharing a same channel, different heterogeneous sharing the ISM band and finally
the case where primary and secondary users coexist.
In this section we briefly discuss three challenges related to sharing in a wireless
context: the spectrum policy, multi-user interaction and fairness.
3.3.3.1 The Spectrum Policy
Like water, air or oil, the wireless spectrum is a shared resource. This however does
not imply that it should be free! Indeed, the base of all economics is scarcity and we
have already shown above that wireless spectrum appears to be very scarce today
and is, hence, expensive.
As mentioned in Chap. 1, the apparent spectrum scarcity and the success of the
ISM bands has coerced regulatory bodies into defining a roadmap towards more
local and organic policies for sharing the wireless spectrum. Today, policies cover
large areas and remain static over years. The end goal of the regulatory roadmap is
to increase the policy dynamism to days, hours or even minutes. This implies that
future terminals will operate in a diverse environment, where they can act amongst
equal, superior and inferior terminals from a policy point of view.
3.3.3.2 Multi-user Interaction
Wireless devices and networks operating in the same frequency bands interact. Sim-
ilar to channel dynamics, multi-user interaction or interference has an impact on the
channel quality. However, unlike channel dynamics, interference dynamics are cre-
ated by the actions of radiating objects in the vicinity (microwave ovens, wireless
