1. The System Issues of Rate Adaptation
May 19
The System Issues of Rate Adaptation
Hujun Yin
Hujun Yin,Vivato

2. Motivation Traditionally, rate adaptation is done at link level
May 19
Motivation
Traditionally, rate adaptation is done at link level
Ignoring the impact of other links at MAC
Ignoring the impact of link adaptation on other links
Understand the impact of rate adaptation on system performance
Considering the synergy of multiple links
Identify the issues of link based rate adaptation
Provide insight into system based rate adaptation
Hujun Yin,Vivato

3. Assumptions Network is operating in infrastructure mode
May 19
Assumptions
Network is operating in infrastructure mode
One AP and multiple clients
Communication only between AP and clients
No multiuser reception capability
If collision happens, at most one packet can be successfully received
No non interference
Packet error is only introduced by low SNR or packet collision
Hujun Yin,Vivato

4. Link Based Rate Adaptation
May 19
Link Based Rate Adaptation
SNR based rate adaptation
Chooses data rate from a rate-SNR lookup table
PER based rate adaptation
Monotonic relation between data rate and PER
Adjust data rate so that PER keeps constant
Decreases data rate when PER raises
Increases data rate when PER drops
Rate adaptation is done from link perspective without considering the impact on system performance
Hujun Yin,Vivato

5. System Performance Criteria
May 19
System Performance Criteria
Maximize total throughput under certain constraints
Only consider PER constraint here
S : total throughput; N: number of nodes;
Ri: data rate of node i; pi: PER of node i;
: utilization time of node i; : channel idle time;
: channel busy time; p0: PER constraint
Hujun Yin,Vivato

6. System PER Model The packet error is contributed by two sources:
May 19
System PER Model
The packet error is contributed by two sources:
pe: wireless channel & noise (SNR related)
pc: MAC collisions
pe is a function of data rate and SNR
pc is a function of packet arrival rate and channel access mechanism
Hujun Yin,Vivato

7. Rate Adaptation Criteria Revised
May 19
Rate Adaptation Criteria Revised
Optimal data rates are not only a function of SNR, but also a function of network traffic
Data rate adaptation is link dependent
Traditional link adaptation only considers SNR is suboptimal
Data rate shall be optimized with regard to interaction between links (collision)
Hujun Yin,Vivato

8. Examples: TDMA MAC Each node is assigned to unique time slots
May 19
Examples: TDMA MAC
Each node is assigned to unique time slots
No MAC collisions: pc=0
Links are isolated
Maximize overall throughput equivalents to maximize individual link throughput
Link based rate adaptation is optimal
Hujun Yin,Vivato

9. Examples: ALOHA Random Access
May 19
Examples: ALOHA Random Access
Total throughput
Probability of collision
If N>>1 nodes are identical with aggregated traffic rate l and data rate R
Pc decreases with data rate R
pe increases with data rate R
PER=pe+pc is no longer a monotonic function of rate R
Hujun Yin,Vivato

10. Instability of Link PER Based Rate Adaptation
May 19
Instability of Link PER Based Rate Adaptation
Consider two rates:
R1=R, pe1=10%
R2=R/10, pe2=0
Network operates at 20% PER
Pc=10%, l=R/10, S=0.081R
pe1->15%
Switch to R2: pe2=0, pc=63%, PER=63%, S=0.037R
Stay at R1:pe1=15%, pc=10%, PER=24%, S=0.081R
Rate adaptation to improve PHY PER may result in much worse system performance
Hujun Yin,Vivato

11. Examples: 802.11 DCF MAC CSMA/CA Exponential Backoff Packet 2 DIFS
May 19
Examples: DCF MAC
CSMA/CA
Exponential Backoff
Packet 2
DIFS
Packet 1
Ack
Tx Starts Sensing
SIFS
DIFS: Distributed Interframe Space
SIFS: Short Interframe Space
DIFS + Backoff
Hujun Yin,Vivato

12. 802.11 Packet Collision Probability
May 19
Packet Collision Probability
802.11a initial backoff window only has 16 slots
The probability of collision is close to 10% with two saturated nodes
The probability of collision is close to 30% with 5 saturated nodes*
The PER caused by packet collision can not be ignored in CSMA based MAC like a
Rate adaptation in is an involved system problem**
High packet collision probability
Variable packet size
*G. Bianchi, ”Performance analysis of the IEEE distributed coordination function”,
IEEE JSAC, March 2000.
** H. Yin, “Multirate a Networks: System Performance Evaluation”,
CISS, March 2002.
Hujun Yin,Vivato

13. Conclusion Link adaptation depends on: Channel condition (SNR)
May 19
Conclusion
Link adaptation depends on:
Channel condition (SNR)
Activity of other links
Rate adaptation considering PHY only is sub-optimal in random access MAC
Rate adaptation in PHY simulation may provide biased results to random access MAC
Limiting PHY PER at low level may be artificial if the nature of MAC contention is not considered
Data rate adaptation has to be considered at system level
The black-box approach can not provide accurate PHY-MAC interface abstraction for all possible MAC proposals
Hujun Yin,Vivato

14. Ways to Improve Rate Adaptation Abstraction
May 19
Ways to Improve Rate Adaptation Abstraction
Partition rate adaptation
Coarse rate adaptation at MAC
Deals the network aspect of rate adaptation
Fine rate adaptation at PHY
Deals the channel aspect of rate adaptation
Link adaptation localization
Other links only impacted if the rate of one link varies significantly
Coarse adaptation at MAC sets constraints on the fine rate adaptation at PHY
Hujun Yin,Vivato