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Cross Layer Issues in UMTS-LTE
Bujar Krasniqi
[email protected]
2008-07-03
Mobile Communication Seminar
Outline of Presentation
Introduction to layered system
Cross layer design approach
Layer interaction
Definition of Cross layer design
Cross layer proposals
Cross layer optimization
Conclusions
Bujar Krasniqi
2008-07-03
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Cross Layes Issues in UMTS-LTE
Introduction to layered system
Why do we need Cross Layer Design (CLD) ?
Open System Interconnection (OSI)
Protocols
Quality of Service (QoS)
Types of Services
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Cross Layes Issues in UMTS-LTE
Why do we need CLD ?
Wireless communication today is expected to be mixture of real
time traffic [1]
Different application requires differnet Quality of Service (QoS) [1]
By using Cross Layer Design (CLD) we try to offer dedicated
QoS for dedicated applications
We use CLD in order to have higher througphput, cheaper
networks, more bandwidth, which are some of the goals of LTE
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Cross Layes Issues in UMTS-LTE
OSI model
The International Standardization Organization (ISO) began
to work in 1980s [1]
This work led to design the Open System Interconnection(OSI)
Application layer
Presentation layer
Session layer
Transport layer
Network layer
Data link layer
Physical layer
OSI model
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Cross Layes Issues in UMTS-LTE
Protocols
In this model each layer communicates only with adjacent layer
above and adjacent layer below [1]
Traditional approach has been to treat the layers as different
entities
TCP/IPv4 is the most succesful implementation in OSI model [1]
Application
TCP
UDP
IP
Physical and Datalink
TCP/IP and UDP in a network
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QoS and types of Services
In IPv4 QoS and routing tables were implemented differently [1]
Now QoS is a key design component of any communication
system
Types of services enable the application to tell the network
what type of netwok services the application requires [1]
Types of services are:
Minimum delay
Maximum throughput
Maximum reliability
Minimum cost
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Cross Layes Issues in UMTS-LTE
Cross layer design approach
To design a wireless network, we need to identify each process
or module
Research community is split in different groups because each
process or module has been treated seperately
CLD approach to network architecture is located where the three
groups of research intersect [1]
There is a difference between CLD in wireless and CLD in
wireline
In wireless: fading, scattering and multipath can change channel
state
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Cross Layes Issues in UMTS-LTE
Cross layer design approach
Wireless
Networking
Architecture :connection versus
connectionless
Energy efficient analysis
Scaling laws of large scale
networks
Traffic theory
Protocols
Signal Processing
Cross
Layer
Design
Increasing spectral efficiency
( bits / s / Hz )
Information Theory
Developing capacity limits
Reducing Bit Error Rate ( BER )
Reducing the transmission energy
Detection and estimation algorithms
for multi-acces
Designing efficient source coding
and channel coding algorithms
CLD as an intersection of different disciplines
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Cross Layes Issues in UMTS-LTE
Layer Interaction
Early work done in wireline networks shows that information
exchange between protocol layers is useful, i.e [2]
TCP packet loss information communicated to the application layer
Link-MAC layer tuning transmit power of the PHY based on BER
Layer interaction can be categorized as:
Upper to lower layers (Top-down approach)
Lower to upper layers (Bottom-up approach)
Also PHY layer transmit power and BER communicated to the
Link/MAC layer enables adaptation of error mechanisms
*PHY-Physical Layer, MAC-Medium Access Control, BER- Bit Error Rate
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Cross Layes Issues in UMTS-LTE
Definition of CLD
The layered architecture like OSI defines a hierarchy of services
to be provided by individual layers
Services at layers are realized by designing protocols for
different layers
A higher layer protocol only makes use of services at lower layers
and is not concerned how the service is being implemented
Protocols can be designed by allowing direct communication
between protocols in nonadjacent layers
Such communication between layers is named CLD. However,
this is not the only definition for CLD.
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Cross Layes Issues in UMTS-LTE
Cross layer proposals
Proposals can be divided by:
Creation of new interfaces
Bottom-up
Top-down
Merging adjacent layers (super layers)
Application layer
Application layer
Presentation layer
Presentation layer
Radio Resource Control (RRC)
L3
Session layer
Transport layer
Top-down
Bottom-up
Packet Data Convergence Protocol (PDCP)
Session layer
Transport layer
Network layer
Network layer
Data link layer
Data link layer
Physical layer
Physical layer
Radio Link Control (RLC)
L2
Medium Access Control ( MAC)
Physical layer
Super layer L2
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Cross Layes Issues in UMTS-LTE
L1
Cross layer proposals
The way of exchanging information between layers [3]
Direct communication between layers
A shared database across the layers
Application layer
Radio Resource Control (RRC)
Control
L3
Presentation layer
Packet Data Convergence Protocol (PDCP)
Session layer
Radio Link Control (RLC)
Transport layer
L2
Logical channels
A shared
database
Network layer
Medium Access Control ( MAC)
Data link layer
Transport channels
L1
Physical layer
Physical layer
Direct communication
between different layers
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A shared database
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Cross Layes Issues in UMTS-LTE
Cross layer optimization
Future wireless communication like LTE requires optimization of
parameters in all layers
For optimum performance, parameter setup should take into
account the properties of application
We formulate CLD problem as an optimization problem with
objective to select joint strategy across multiple OSI layers
Joint cross layer optimization strategy is defined as: [4]

S  PHY1 ,..., PHYN P , MAC1 ,..., MAC N M , APP1 ,..., APPN A

there are N  NP  NM  N A possible joint strategies
*APP-application layer
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Cross Layes Issues in UMTS-LTE
Cross layer optimization
denote the number of adaptation and protection
strategies available in PHY, MAC and APP
NP , NM , N A
Modulation, channel coding schemes existing for a UMTS or LTE
are contained on this set
PHYi , i   1 ,..., N P

Packetization, ARQ, scheduling, admission control and FEC are
contained on this set
MACi , i   1 ,..., N M

Adaptation of video compression, traffic prioritization are
contained on this set
APPi , i   1 ,..., N A

*ARQ-Automatic Repeat Request, FEC-Forward Error Coding
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Cross Layes Issues in UMTS-LTE
Joint cross layer optimization
The joint CLO strategy is to find the best multimedia quality
S opt ( x)  argmax Q( S ( x))
S
where
Delay ( S ( x))  Dmax
Power ( S ( x))  Pmax
Finding the optimal solution is hard because the set contains
a lot of parameters
Joint CLO means optimization utility for given constraints
The objective function can be a utility function which is often
chosen to be concave and strictly increasing [5]
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Cross Layes Issues in UMTS-LTE
Joint cross layer optimization
The conceptual scheme for joint CLO is: [4]
Input media
Different
layer
parameters
Optimize utility
given constraints
Constraints
(delay, power, etc.)
Output
( Cross layer adaptation strategy )
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Cross Layes Issues in UMTS-LTE
Utility function
The graph of choosen utility function is
U (x)
x
We use this function to explain NUM by using mathematical
decomposition techniques for CLD [6]
*NUM-Network Utility Maximization
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Cross Layes Issues in UMTS-LTE
Network utility maximization
The simplest optimization problem is when the objective function
contain just a parameter
maximize
U(x )
 x  C , l, x  0
s
s
subject to
sS(l)
s
l
A higher throughput can be obtained on a link if tradeoff is made
between data rate and reliability
For example: with lower decoding reliability we have higher data
rate and with higher reliability we have lower data rate
- xs source data rate
- R reliability
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Cross Layes Issues in UMTS-LTE
Network utility maximization
Reliability (log)
The graphical representation between data rate and reliability
is:
Rate (log)
Now the utility function U(.) will be defined as a function of rate
and reliability
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Cross Layes Issues in UMTS-LTE
Differentiated dynamic reliability
The optimization problem that represents tradeoff between rate
and reliability for differentiated dynamic reliability case is: [6]
maximize
U (x , R )
R  1   E (r
s
s
s
s
subject to
s
lL(s)
l
l,s
), s
xs
 cl,s , l, s  S(l)
rl,s
 cl,s  Clmax , l
sS(l)
min
s
min
s
x
 xs  xsmax , s
R  Rs  1, s
0  rl,s  1, l, s  S(l)
0  cl,s  Clmax , l, s  S(l)
- E error probability
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Cross Layes Issues in UMTS-LTE
Differentiated dynamic reliability
The network diagram for that optimization problem is:
xs
Rs
Network
( xs )sS (l )
Sources
Linkl
R 1
s
E (r
l L ( s )
l
l,s
Network
)
(cl , s )sS (l )
(rl , s )sS (l )
Network diagram contain just primal variables
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Cross Layes Issues in UMTS-LTE
Differentiated dynamic reliability
We use primal-dual mathematical decomposition techniques to
solve the optimization problem for rate-reliability tradeoff
The Lagrangian of the constrained optimization problem is
 U x , R    x
L ( x ' , R , r , c,  ,  ) 
'
s
'
s
s
s
'
s

  s Rs 
s



    l , s log cl , s  log rl , s    s El rl , s  
l  sS ( l )

  s
s
where
s 

lS ( l )
and
Bujar Krasniqi
l ,s
xs ' log xs
2008-07-03
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Cross Layes Issues in UMTS-LTE
Differentiated dynamic reliability
Based on primal-dual mathematical decompositon
Source rates=Primal variables
Congestion prices=Dual variables
Lagrange multipliers
Dual problem is:
 and 
are dual variables
minimize Pλ, μ 
subject to λ  0
μ0
The Lagrange dual function is:
P( ,  )  max L( x' , R, r , c,  ,  )
Constraints
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Cross Layes Issues in UMTS-LTE
Differentiated dynamic reliability
The diagram for distributed algorithm of the differentiated
dynamic reliability is
xs
Network
s
Rs
( xs )sS (l )
( s )sS (l )
Source s
Linkl
s 

l L ( s )
R 1
s
E (r
l L ( s )
Bujar Krasniqi
l,s
l
l,s
Network
)
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(cl , s )sS (l )
(l , s )sS (l )
(rl , s )sS (l )
Cross Layes Issues in UMTS-LTE
CLO based on layer description
Now we propose a concept of exchanging information between
adjacent layers in terms of layer description [7]
The model is:
Dn 1
Layer (n+1)
yn 1
Dn
Layer (n)
Layer (n-1)
Layer (1)
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yn
Dn 1
D1
Cross Layes Issues in UMTS-LTE
CLO based on layer description
The layer output set is defined as:

yn  f n ( xn , d n1 )  R
N 0 ,n
: xn   n , d n1  Dn1

Layer function is
f n :  n  Dn1  R
N o ,n
 n - set of feasible parameter values,
Dn 1- description of layer n-1
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Conclusions
OSI model can not be a dedicated model for next wireless
systems
By using CLD techniques we need to adapt OSI model for
dedicated wireless system
It is very important to have a communication between nonadjacent
layers
By using CLD we should try to be more energy efficient
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Cross Layes Issues in UMTS-LTE
References
[1] Frank Aune “Cross-Layer Tutorial” NTNU ,2004
[2] V.T. Raisinghani, S.Iyder “ Cross-Layer optimization in wireless protocols” 2003
[3] V. Srivastava, M.Moutani “ Cross-Layer design: A survey and the road ahead” 2005
[4] M. Schaar, D.Shankar “ Cross-Layer wireless multimedia transmission: Challenges,
principles, and new paradigms”, 2005
[5] S.Stanczak, M Wiczanowski and H.Boche “Resource allocation in wireless networks”
Springer 2006
[6] J. Lee, M.Chiang, R. Calderbank “ Network utility maximization and price-based distributed
algorithms for rate-reliability tradeoff” 2006
[7] J.Brehmer and W.Utschick “ Modular cross-layer optimization based on layer descriptions”
[8] S. Boyd and L. Vandenberghe “ Convex Optimization” Cambridge , 2004
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Thank you for your attention !