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Cross-layer design for Multiple access techniques in wireless communications

Cross-layer design for Multiple access techniques in wireless communications. Daniele Tarchi University of Florence, Italy E-mail: tarchi@lenst.det.unifi.it. Outline. Link Adaptation Adaptive Modulation and Coding VSF-CDMA TETRA Release 2 Ad-Hoc Networks. Link Adaptation.

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Cross-layer design for Multiple access techniques in wireless communications

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  1. COST 289 - 5th MCM - Budapest, Hungary Cross-layer design for Multiple access techniques in wireless communications Daniele Tarchi University of Florence, Italy E-mail: tarchi@lenst.det.unifi.it

  2. Outline COST 289 - 5th MCM - Budapest, Hungary • Link Adaptation • Adaptive Modulation and Coding • VSF-CDMA • TETRA Release 2 • Ad-Hoc Networks

  3. Link Adaptation COST 289 - 5th MCM - Budapest, Hungary • The received signal can be received affected by several type of interference • Wireless channel is often affected by multipath-fading 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

  4. Channel capacity MAI Attenuation Noise Multiple Reflections Doppler Effect COST 289 - 5th MCM - Budapest, Hungary Base Station UE

  5. Wireless Nets Scenarios COST 289 - 5th MCM - Budapest, Hungary • Wireless Nets • Centralized • Distributed Centralized Systems Distributed Systems

  6. Ad Hoc Networks 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

  7. Wireless MAC Network LLC MAC Physical COST 289 - 5th MCM - Budapest, Hungary • Radio Channel characteristics: • Half-duplex • Time variant • Bursty nature • Access techniques: • CSMA/CA • Control Handshaking • Acknowledgements

  8. Drawbacks of MAC for distributed networks D A C B B D C A COST 289 - 5th MCM - Budapest, Hungary • Hidden Node • Exposed Node • Capture Effect Hidden Node A Capture Effect C B D Exposed Node

  9. Standard IEEE 802.11 D A B C B A RTS Data CTS Ack COST 289 - 5th MCM - Budapest, Hungary It is foreseen a CSMA/CA technique with a 4-way handshaking RTS CTS Data Ack

  10. CDMA in AdHoc Networks 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

  11. Proposed Protocol for CDMA based AdHoc Nws 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.

  12. Proposed Protocol (Cont’d) C (RTS – CTS) RTS Cr1 (Data) CTS Data Ack COST 289 - 5th MCM - Budapest, Hungary T1 R1 T1 R1

  13. Proposed Protocol (Cont’d) COST 289 - 5th MCM - Budapest, Hungary

  14. Simulation Parameters 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

  15. Collision Probability COST 289 - 5th MCM - Budapest, Hungary Low Traffic Condition Collision Avoidance

  16. Throughput COST 289 - 5th MCM - Budapest, Hungary 3% 40%

  17. Delay COST 289 - 5th MCM - Budapest, Hungary

  18. Collision Probability COST 289 - 5th MCM - Budapest, Hungary

  19. Comparison with IEEE802.11 COST 289 - 5th MCM - Budapest, Hungary 15% >50%

  20. Conclusion & Future Developments 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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