Università degli Studi di Firenze
08 July 2004
COST 289 - 5th MCM - Budapest, Hungary
Cross-layer design for Multiple access
techniques in wireless communications
Daniele Tarchi
University of Florence, Italy
E-mail: [email protected]
1
Outline
08 July 2004
COST 289 - 5th MCM - Budapest, Hungary
• Link Adaptation
 Adaptive Modulation and Coding
 VSF-CDMA
• TETRA Release 2
• Ad-Hoc Networks
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2
Link Adaptation
08 July 2004
COST 289 - 5th MCM - Budapest, Hungary
• The received signal can be received affected by
several type of interference
 Wireless channel is often affected by multipathfading effect
 A CDMA signal is affected by Multiple Access
Interference
• This means that the useful signal strength
varies during the transmission due to the user
mobility or number of users within the cell
• The Link Adaptation concept aims to adapt
some transmission parameter to the actual
channel state with the goal of respect some
transmission constraint such as the QoS or the
Bit Error rate
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3
Channel capacity
08 July 2004
COST 289 - 5th MCM - Budapest, Hungary
 E S R C
C  log2 1 
N0  I

 Base Station



Noise
Attenuation
UE
Doppler Effect
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MAI
Multiple Reflections
4
Wireless Nets Scenarios
08 July 2004
COST 289 - 5th MCM - Budapest, Hungary
• Wireless Nets
 Centralized
 Distributed
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Centralized Systems
Distributed Systems
5
Ad Hoc Networks
08 July 2004
COST 289 - 5th MCM - Budapest, Hungary
• The Ad Hoc Networks
are:
 Distributed
 Wireless
 With peer-to-peer
connections
 Without fixed
infrastructure
• Advantages:
 Absence of infrastructures
 Low cost
 Flexibility and adaptability
• Disadvantages
 Distributed systems
 Routing and access
techniques
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6
Wireless MAC
08 July 2004
COST 289 - 5th MCM - Budapest, Hungary
• Radio Channel
characteristics:
Network
LLC
MAC
Physical
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 Half-duplex
 Time variant
 Bursty nature
• Access techniques:
 CSMA/CA
 Control
Handshaking
 Acknowledgements
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Drawbacks of MAC for
distributed networks
08 July 2004
COST 289 - 5th MCM - Budapest, Hungary
 Hidden Node
 Exposed
Node
Hidden
Node
Capture
Effect
 Capture
Effect
Exposed Node
A
C
B
D
D
B
A
A
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C C
B
D
8
Standard IEEE 802.11
08 July 2004
COST 289 - 5th MCM - Budapest, Hungary
It is foreseen a CSMA/CA technique
with a 4-way handshaking
A
B
RTS
D
A
B
CTS
RTS
Ack
Data
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C
CTS
Data
Ack
9
CDMA in AdHoc
Networks
08 July 2004
COST 289 - 5th MCM - Budapest, Hungary
• In literature there are four CDMA approaches in
Ad-Hoc networks:
 Common Code All the nodes have a common code.
 Receiver-Based Code Each node has its own
Receiver-Based Code.
 Transmitter-Based Code Each node has its own
Transmitter-Based Code.
 Pairwise-Based Code Each pair of nodes has
assigned a unique code.
• We have selected herein the Receiver-based
Code technique
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10
Proposed Protocol for
CDMA based AdHoc Nws
08 July 2004
COST 289 - 5th MCM - Budapest, Hungary
• In the proposed MAC protocol it is foreseen to adapt the
spreading factor for the RTS/CTS packets to the
network load.
• Starting from the smallest spreading factor (SF=8),
whenever a collision occurs SF is doubled; this leads to:
 Higher time occupation by RTS/CTS packets, i.e., higher
channel occupancy by each terminal during the contention
phase;
 Lower MAI with data packet, i.e., higher number of transmitting
users.
• The proposed protocol adapts the SF in order to
maximize the net throughput taking into account the
channel state in terms of interference from other
terminals.
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11
Proposed Protocol
(Cont’d)
08 July 2004
COST 289 - 5th MCM - Budapest, Hungary
T1
C (RTS – CTS)
R1
RTS
Cr1 (Data)
CTS
Data
T1
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R1
Ack
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Proposed Protocol
(Cont’d)
08 July 2004
COST 289 - 5th MCM - Budapest, Hungary
Rx RTS
Send CTS
Idle
no data received
data message arrived
Rx ACK
Rx CTS
Send RTS
Tx/Rx
Data
no CTS received
Rx NACK
Backoff
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13
Simulation Parameters
08 July 2004
COST 289 - 5th MCM - Budapest, Hungary
•
•
•
•
Chip Time, Tc=2.5¢10-07 s
RTS and CTS packets length , Lc=20 Byte
Data packet length Ld=1460 Byte
Truncated Pareto traffic model
• Mean number of packets per message, N=25
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14
Collision Probability
08 July 2004
Collision Probabilty
COST 289 - 5th MCM - Budapest, Hungary
0,04
0,03
Low Traffic
Condition
0,02
Collision
Avoidance
0,01
0,00
0
10
20
30
40
50
60
70
80
90
100
Connectivity [%]
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15
Throughput
08 July 2004
COST 289 - 5th MCM - Budapest, Hungary
1,00
3%
Throughput
0,95
0,90
0,85
0,80
40%
0,75
SF 8
SF 16
SF 32
Variable SF
0,70
0,65
0,60
0
0,1
0,2
0,3
0,4
0,5
Arrival Rate [msg/s]
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16
Delay
08 July 2004
Delay [s]
COST 289 - 5th MCM - Budapest, Hungary
0,50
0,45
0,40
0,35
0,30
0,25
0,20
0,15
0,10
0,05
0,00
SF 8
SF 16
SF 32
Variable SF
0
0,1
0,2
0,3
0,4
0,5
Arrival Rate [msg/s]
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17
Collision Probability
08 July 2004
Collision Probability
COST 289 - 5th MCM - Budapest, Hungary
0,50
0,45
0,40
0,35
0,30
0,25
0,20
0,15
0,10
0,05
0,00
SF 8
SF 16
SF 32
Variable SF
0
0,1
0,2
0,3
0,4
0,5
Arrival Rate [msg/s]
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18
Throughput
Comparison with
IEEE802.11
1,00
0,90
0,80
0,70
0,60
0,50
0,40
0,30
0,20
0,10
0,00
08 July 2004
COST 289 - 5th MCM - Budapest, Hungary
15%
>50%
IEEE 802.11
Proposed MAC
Protocol
0
0,1
0,2
0,3
0,4
0,5
0,6
0,7
Arrival Rate
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19
Conclusion & Future
Developments
08 July 2004
COST 289 - 5th MCM - Budapest, Hungary
• Conclusions
 It has been proposed a MAC protocol that exploits the CSMA/CA
technique and the CDMA in order to allow multiple
communications at the same time
 The Spreading Sequences length is selected in an a adaptive
way following the network loading condition
 The proposed protocol allows a higher throughput of about 3%
for low message arrival rate and about 40% of higher arrival
rate for a target throughput
 Also in comparison to the classical IEEE 802.11 MAC technique
higher performance are allowed
• Future Developments
 User mobility and power control
 User priority and type of traffic priority in order to respect QoS
constraints
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