Diapositiva 1

1
QoS-Aware Resource Allocation forSlowly
Time-Varying Channels
InfoCom Department - University of Rome La
Sapienza giancola_at_infocom.uniroma1.it lucadn_at_newyo
rk.ing.uniroma1.it gaby_at_acts.ing.uniroma1.it
Presented at the 58th IEEE Vehicular Technology
Conference, Orlando, U.S.A., October 2003
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Reference Scenario
High level application

• Two basic problems in the design of a MAC
  protocol are
• the efficient management of the resource
• the need for fulfilling QoS requirements despite
  the unpredictable behaviour of the channel

Data (packets)
MAC
Medium Access Control
We propose an analytical approach for resource
allocation at the MAC level. The resulting
algorithm maximizes transmis-sion efficiency by
adapting error protection to both channel status
and required QoS.
Physical Channel
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Model Assumptions
Traffic sources are characterized by two sets of
parameters Tspecs collects parameters
describing source traffic activity Qspecs
defines QoS requirements
Dmax maximum tolerable end-to-end delay F
minimum tolerable percen-tage of packets
delivered within Dmax
p peak rate r mean rate M max packet
size b token buffer
Qspecs
Tspecs
The MAC protocol works with fixed-size MAC
Protocol Data Units (MACPDUs)
payload
header
effective payload
FEC size
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Resource Allocation (1/4)
Two functions are introduced in order to express
in analytical terms the trade-off which exists
between reserved capacity C and the delay D.
SOURCE BUFFER
Maximum number of retransmissions
Round Trip Time
Minimum capacity
Required capacity in bps
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Resource Allocation (2/4)
In order to evaluate the effect of segmentation
on required capacity, the MAC must evaluate the
size of required overhead on each MACPDU.
Effective Capacity
FEC size can be evaluated by taking into account
the QoS parameter F.
FEC
Effective Payload
MACPDUs
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Resource Allocation (3/4)
Target packet loss probability on each MACPDU
Given the required QoS, it depends on NR and on
LEFF
It depends on channel status, i.e. on the BER
value pb
Corrective capability on each MACPDU
The value of k detemines the FEC size
We obtain a new LFEC which can affect the size of
the effective payload LEFF
We propose an iterative algorithm based on
successive approximations. This algorithm is
computationally efficient and returns the FEC
size which is necessary on each MACPDU.
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Resource Allocation (4/4)
Effective Capacity
Required capacity in terms of the number of
MACPDUs per frame which are necessary for the
application.
DF is the frame duration DNARQ is a corrective
term due to the ARQ
Transmission efficiency is maximized by
selecting the NR value leading to the minimum
number of MACPDUs per frame.
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Performance in static channels
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Performance in slowly time-varying channels
Performance of the proposed algorithm was
verified in the case of a slowly time-varying
channel. The Jakes channel model was used for
characterizing multipath propagation in a generic
indoor environment. Performance degradation is
observed when the channel coherence time is
comparable to the maximum end-to-end delay. In a
scenario with high mobility, QoS cannot be
guaranteed for real-time applications only.
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Acknowledgements
special thanks to John Silver for providing the
PDF conversion of the poster.