Optimal Selection of Power Saving Classes in IEEE 802.16e

Lei Kong, Danny H.K. Tsang

Department of Electronic and Computer Engineering Hong Kong University of Science and Technology

IEEE Wireless Communication & Network Conference ( WCNC 2007 )

Outline

Introduction Proposed Algorithm

– System model– Cost metric and Delay metric

– Policy Optimization

Simulation Conclusion

Introduction

According to the mobility extension,– IEEE 802.16e defines the sleep mode operation for the power

saving which is one of the most important features for MSSs to extend their

lifetime.

In order to support different service connections– IEEE 802.16e offers several sleep mode types, called Power

Saving Classes (PSCs)

Introduction – 802.16e sleep mode operations

Power Saving Class of Type I

Power Saving Class of Type II

Power Saving Class of Type III

…

TS_init (Initial sleep window)

2 x TS_init TL

4 x TS_init TS_max

Incoming packet

TL TS

Incoming packets

…

Incoming packets Incoming packetsIncoming packets

TS

normal operation

sleep windows

listening windows

MOB_TRF-IND

Relative Work

Sleep mode in IEEE 802.16e has been generally recognized as effective in discontinuous reception.

However, their model only applies for PSCs of type I and does not capture the new characteristics of PSCs of type II.

Relative Work

Lei Kong and Danny H. K. Tsang, “Performance Study of Power Saving Classes of Type I and II in IEEE 802.16e”, To appear in the Proc. of of the 31st IEEE Conference on Local Computer Networks, Tampa, Florida, US, November. 2006.

– The original models have captured the inherent properties of two PSCs accurately and also show the energy delay trade-offs on different PSCs

Motivation

The static timeout policy may solve the problem of when to perform the sleep mode switching, but it cannot resolve the issue on PSCs selection.

Goal

Our main goal is to find the optimal selection for PSC of types that achieves the minimum energy cost or traffic delay under different traffic requirements.

Assumptions

We assume that packet arrival rate λ ( packets/frame ).

We also denote μ ( packets/frame ) as the service rate between BS and MSS .

In PSC of type I, we assume the energy for device to switch on and off in TL is negligible

Assumptions

In PSC of type II, we assume that MSS would remain in sleep mode if the arriving packets from the pervious sleep interval is less than or equal to the maximum number of packets that it could transmit during TL.

TL TS

Incoming packets

System model

Definitions– I = { SN, SI, SII } representing normal mode, sleep mode of type I an

d type II– A = { s_N, s_I, s_II } with the intuitive meaning of ”switching to no

rmal mode”, ”switching to type I” and ”switching to type II”, respectively

– SI (k) represent the multi-sleep state of type I, where 0 ≤ k ≤ w and w is the final sleep stage

– Pk is the probability that there is packet arrival at BS in sleep stage SI (k)

– Q is the transition probability that PSC of type II to normal mode– Ri is a decision epoch in state i

System model

Examples s_N

Stay On Normal mode

The next decision takes place when the system endures a busy period and becomes idle again.

Examples s_I

Switch to PSC I

The next decision takes place when the system endures normal mode and becomes sleep mode again.

Examples s_II

Switch to PSC II

The next decision takes place when the system endures normal mode and becomes sleep mode again.

Cost metric and Delay metric

Definition – τi (a) = the expected time duration until the next decision epoch if

action a is chosen in state i.– c i (a) = the expected power consumption incurred until the next d

ecision epoch if action a is chosen in state i.– d i(a) = the expected packet delay if action a is chosen in the prese

nt state i

Goal

Cost metric and Delay metric

Definitions– Denote PB, PI , PS, PL as the power consumption level

of busy period B, idle period I, sleep interval and listen interval TL, respectively.

– E [B], E [I] is means busy period duration and idle period duration

– S is the random variable of service time for each packet and E [S] = 1 /μ

Cost and Delayin Normal mode

Expected time duration for normal mode

Expected power consumption for normal mode

The mean waiting time for the packet – E[W] is the mean waiting time for the packet arrival du

ring the busy period. – ρ=λ/μ is traffic intensity– E[R] = E[S2]/(2E[S]) is the residual processing time– According to Pollaczek-Khintchine(PK-) mean value f

ormula [5], E[W] is

Cost and Delayin Normal mode

Cost and Delayin Normal mode

Expected packet delay for Normal mode

Cost and Delayin PSC I

Packet arrival Probability for PSC I– Pk is the probability that there is packet arrival at BS in sleep stag

e SI (k)

Cost and Delayin PSC I

The Vacation time and busy period for PSC I– Vk is the vacation time (i.e. the sleep window size plus the listen i

nterval) at stage k

– is the mean time of exceptional busy period to transmit previously buffered traffic accumulated in sleep stage k and can b

e derived from the following equation:

Cost and Delayin PSC I

Expected time duration until vacations k – is the expected duration until the next

decision epoch after sleeping for k vacations

Cost and Delayin PSC I

Expected time duration for PSC I

Cost and Delayin PSC I

Expected Power consumption until vacations k – is the total expected energy consumption when

system wakes up after k vacation cycles and becomes idle.– ε i = PS2iT0 + PLTL is the power consumption at the sleep stage i

Cost and Delayin PSC I

Expected Power consumption for PSC I– Esw denotes the power consumption in switch-on and s

witch-off the transceiver at physical layer.

Cost and Delayin PSC I

Expected packet delay for PSC I– is the total expected packet delay when

system wakes up after k vacation cycles.

ρ=λ/μ is traffic intensity

Cost and Delayin PSC II

Packet arrival Probability for PSC II– VII is the vacation time for PSC of type II (i.e. the sleep window

size plus the listen interval)

– d is maximum number of packets that it could transmit during TL

– (1 − Q) is the transition probability that MSS keeps in state SII

Cost and Delayin PSC II

Expected time duration for PSC II

Cost and Delayin PSC II

Expected power consumption for PSC II

Cost and Delayin PSC II

Expected packet delay for Type II

Policy Optimization

Let xi(a) are the expected number of times that the system is in state i and command a is issued.

We define in our model that every decision epoch only tak

es place as soon as MSS becomes idle in SN.

In other words, xi(a) = 0, a ∀ and i {S∈ I, SII}.

Policy Optimization

We formulate two probabilistic constrained optimization problems: – power optimization under delay constraint

maximal expected delay with a upper bound δ.

Policy Optimization

– delay optimization under power constraint

meaning that the battery life time would be extendedσ times in the long run

Simulation

PB = 750mW, PI = PL = 170mW and Ps = 50mW, and for switching cost Esw = 1.5J

Simulation

A. Cost Metric Comparison

Simulation

B. The Space of Optimal Policies (delay 20 frame)

Simulation

B. The Space of Optimal Policies (delay one frame)

Conclusion

In this paper, we propose a semi-Markov Decision Processes (Semi-MDP) method into the sleep mode operation in IEEE 802.16e.

We find out the optimal selection for PSC and efficiently under different traffic conditions that minimize energy consumption for MSS