1. A Relay-based MAC Protocol for Multi-Rate and Multi-Range Infrastructure Wireless LANs
Dept. of computer Science and Information Management
Providence University
Taichung, Taiwan 433, R.O.C.
劉建興 靜宜資管系

2. Outline
Introduction
Relay-based Adaptive Auto Rate Control protocol (RAAR)
Throughput of IEEE MAC and RAAR in Single Node Environment
Throughput of IEEE MAC, RAAR and D-RAAR in Multiple Node Environment
Throughput in Multiple Nodes Environment with Imperfect Channel
Conclusion

3. Introduction Adaptive transmission techniques
varying the transmission power, packet length, coding rate/scheme, and modulation technology over the time-varying channel
With the new high-speed IEEE a MAC/PHY,
we can transmit packets with different data rates ranging from 6 to 54 Mbps  a Multi-rate, Multi-range IWLAN (MMI-WLAN)
However, all mobile hosts (MHs) can not insist on using the highest-level modulation scheme
the data rate is inversely proportional to the transmission distance between a pair of MHs

4. Introduction Related Works Auto Rate Fallback (ARF)
Bell Labs Technical Journal, 1997
Receiver-Based Auto Rate (RBAR)
ACM MOBICOM, 2001
Opportunistic Auto Rate (OAR)
ACM MOBICOM, 2002
Network layer routing/forwarding: find intermediate nodes to forward data packets
many related works

5. Related Work: Auto Rate Fallback (ARF)
The sender selects the best rate based on information from previous data packet transmission, incrementally increasing or decreasing the rate after a number of consecutive successes or losses
The rate selection is performed by the sender, the channel quality estimation is also performed by the sender
ARF provides a performance gain over pure single-rate IEEE

6. Related Work: Receiver-Based Auto Rate (RBAR)
The sender Src chooses a data rate based on some heuristic, and then stores the rate and the size of the data packet into the RTS
Node A, overhearing the RTS, calculates the duration of the reservation Drts.
The receiver Dst uses information about the channel conditions to generate an estimation
Dst then selects the rate based on that estimate, and transmit it and the packet size in the CTS back to the sender
The Reservation SubHeader (RSH) in MAC header of the data packet is used to update the NAV to finally confirm the reservation
In RBAR, the rate selection and channel quality estimation are located on the receiver
RBAR yields significant throughput gains as compared to ARF

7. Related Work: Opportunistic Auto Rate (OAR)
The key OAR mechanism is for a flow to keep the channel for an extend number of packets (instead of for a single packet) once the channel is able to transmit at rates higher than the base rate
OAR uses the fragmentation mechanism in IEEE as the basic transmission scheme
OAR receivers continually monitor the channel quality, and if a significant change is detected, additional RSH messages are used to adapt the rate

8. Introduction: the problem not considered in above approaches
The relay concept in network layer is not easily adopted in MAC layer
In particular, IEEE Distributed Coordination Function (DCF) requires the 4-way handshake of RTS/CTS/DATA/ACK for its data transmission
Possible solutions in MAC layer: MPLS-based cut-through mechanisms

9. The solution given by A. Acharya et. al
The solution given by A. Acharya et. al., IEEE Wireless Communication Magazine

10. A. Acharya et. al.’s solution

11. A. Acharya et. al.’s solution

12. The solution given by Guido R. Hiertz et
The solution given by Guido R. Hiertz et. al, the IEEE Circuits and Systems Symposium on Emerging Technologies

13. Relay-based Adaptive Auto Rate Control protocol (RAAR)
System model and environment under consideration
Design Objects
Label Switching Mechanism in RAAR
Control Message Flow
Transmission with Best Modulation Scheme

14. System model and environment: the multiple-data rates achievable in recent IEEE 802.11 PHY

15. System model and environment: the Multi-rate Multi-range Infrastructure WLAN (MMI-WLAN)

16. Design Objects
The first aim is to keep the channel for an extended number of packets once the channel is measured to be of sufficient quality to allow transmission at rates higher than the base rate.
The second aim is for nodes near the fringe of AP's transmission range to transmit packets at a higher rate.

17. Label Switching Mechanism in RAAR: basic model

18. Label Switching Mechanism in RAAR: the extension for RAAR

19. Control Message Flow

20. Transmission with Best Modulation Scheme
AP can uses, for example, broadcasting approach to let all MHs know their locations and use the best modulation scheme
In addition, each MH can overhear packets transmitted in air, measure their signal to noise ratios (SNRs), estimate the distances and the modulation schemes to be adopted between itself and the transmitting nodes, and finally record these information in a so-called neighbor-list.
AP can then obtain the information about the MHs in its transmission range, and collect these MHs' neighborhood information by means of the neighbor-list delivered with some kinds of periodical reports or routing information exchanges.
With these, AP can decide the relay node for each MH without the requirement of exact knowledge about the directions of these MHs.

21. Throughput of IEEE 802.11 MAC and RAAR in Single Node Environment
IEEE a PHY Models

22. Variable times adopted in our throughput calculation for IEEE 802.11a

23. IEEE Throughput
Throughput calculation

24. RAAR Throughput

25. Uplink throughput comparison of IEEE 802.11 MAC and RAAR

26. Throughput in Multiple Nodes Environment with Imperfect Channel
IEEE Anomaly
Transmit probability and frame error probability in IEEE

27. IEEE 802.11 Anomaly The overall transmission time
The time for the jth transmission attempt in IEEE

28. IEEE Anomaly
Assume Ptx and n are fixed, and s and m are given according to the locations, the overall transmission time in region j can be defined as
Because the long term fairness of CSMA/CA, the utilization factor in region j can be obtained by

29. IEEE 802.11 Anomaly Throughput Anomaly
In above no particular terms with regard to Rj are involved in the final formula, which implies that a node located in an inner region and using a higher rate to transmit l-bytes data obtains the same throughput as a node located in an outer region and using a lower rate for the same data.
Contrary to all expectations of the multiple-rate PHY, this result exhibits the fact that a higher data rate does not bring the throughput higher than the others if it competes with a lower rate.

30. Solving the Performance Anomaly Problem of IEEE 802
Solving the Performance Anomaly Problem of IEEE MAC with Degraded RAAR
An example of the control flow of D-RAAR

31. Throughput of D-RAAR
If the value of the denominator is similar to that of IEEE MAC, in the numerator in fact represents the gain factor that D-RAAR can increase on the throughput

32. Throughput of RAAR

33. Throughput Comparison of IEEE 802.11 MAC, RAAR, and D-RAAR
Experiment Environments

34. Experiment Result 1

35. Experiment Result 2
Gain ratio Throughput ratio

36. Experiment Result 3

37. Experiment Result 4

38. Conclusion
In Relay-Based Adaptive Auto Rate protocol, a relay node is added to increase the system throughput
RAAR will revert to a degraded version, namely D-RAAR, and perform the normal IEEE fragmentation with variable data rates suggested by AP, when no relay node can be found or the relay node is missed due to mobility
Analysis and simulation both indicate that with this scheme, significant throughput improvement can be achieved for nodes located at the fringe of the AP's transmission range, thus significantly improving the overall system performance
Experimental results show that, based on the measured data in indoor environment, adopting RAAR in the outermost four regions while using D-RAAR in the innermost four regions can effectively combine both methods' benefits and remarkably improve the system throughput