Università degli Studi di Firenze 08 July 2004 COST th MCM - Budapest, Hungary 1 Cross-layer design for Multiple access techniques in wireless communications Daniele Tarchi University of Florence, Italy

Università degli Studi di Firenze 08 July 2004 COST th MCM - Budapest, Hungary 2 Outline Link Adaptation Adaptive Modulation and Coding VSF-CDMA TETRA Release 2 Ad-Hoc Networks

Università degli Studi di Firenze 08 July 2004 COST th MCM - Budapest, Hungary 3 Link Adaptation 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

Università degli Studi di Firenze 08 July 2004 COST th MCM - Budapest, Hungary 4 Noise Multiple Reflections Doppler Effect Attenuation MAI Base Station UE Channel capacity

Università degli Studi di Firenze 08 July 2004 COST th MCM - Budapest, Hungary 5 Centralized Systems Distributed Systems Wireless Nets Scenarios Wireless Nets Centralized Distributed

Università degli Studi di Firenze 08 July 2004 COST th MCM - Budapest, Hungary 6 Ad Hoc Networks 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

Università degli Studi di Firenze 08 July 2004 COST th MCM - Budapest, Hungary 7 Wireless MAC Radio Channel characteristics: Half-duplex Time variant Bursty nature Access techniques: CSMA/CA Control Handshaking Acknowledgements Physical MAC LLC Network

Università degli Studi di Firenze 08 July 2004 COST th MCM - Budapest, Hungary 8 Hidden Node Exposed Node Capture Effect C DB A C DB A Hidden Node Exposed Node Capture Effect C D B A Drawbacks of MAC for distributed networks

Università degli Studi di Firenze 08 July 2004 COST th MCM - Budapest, Hungary 9 Ack Data It is foreseen a CSMA/CA technique with a 4-way handshaking AB RTS CTS Data Ack RTS C D B A CTS Standard IEEE

Università degli Studi di Firenze 08 July 2004 COST th MCM - Budapest, Hungary 10 CDMA in AdHoc Networks 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

Università degli Studi di Firenze 08 July 2004 COST th MCM - Budapest, Hungary 11 Proposed Protocol for CDMA based AdHoc Nws 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.

Università degli Studi di Firenze 08 July 2004 COST th MCM - Budapest, Hungary 12 R1T1 RTS CTS Data Ack Proposed Protocol (Contd) T1R1 Cr1 (Data) C (RTS – CTS)

Università degli Studi di Firenze 08 July 2004 COST th MCM - Budapest, Hungary 13 Proposed Protocol (Contd)

Università degli Studi di Firenze 08 July 2004 COST th MCM - Budapest, Hungary 14 Simulation Parameters Chip Time, T c =2.5 ¢ s RTS and CTS packets length, L c =20 Byte Data packet length L d =1460 Byte Truncated Pareto traffic model Mean number of packets per message, N=25

Università degli Studi di Firenze 08 July 2004 COST th MCM - Budapest, Hungary 15 Collision Probability Low Traffic Condition Collision Avoidance

Università degli Studi di Firenze 08 July 2004 COST th MCM - Budapest, Hungary 16 Throughput 3% 40%

Università degli Studi di Firenze 08 July 2004 COST th MCM - Budapest, Hungary 17 Delay

Università degli Studi di Firenze 08 July 2004 COST th MCM - Budapest, Hungary 18 Collision Probability

Università degli Studi di Firenze 08 July 2004 COST th MCM - Budapest, Hungary 19 Comparison with IEEE % >50%

Università degli Studi di Firenze 08 July 2004 COST th MCM - Budapest, Hungary 20 Conclusion & Future Developments 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 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