Dynamic Resource Allocation for Eﬃcient Wireless Packet Data Communcations

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Dynamic Resource Allocation for Efficient Wireless

Packet Data Communcations

B. Srikrishna

Introduction Problem OFDM Resource Allocation

Our Algorithm Results CDMA Resource Allocation

Results

Summary References

Dynamic Resource Allocation for Efficient Wireless Packet Data Communcations

B. Srikrishna

Assistant Professor Department of Electrical Engineering Indian Institute of Technology Madras

Joint work with:

M. Chandrashekar V. Sandeep Parimal Parag

March 17, 2006

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B. Srikrishna

Introduction

Problem OFDM Resource Allocation

Results

Summary References

Broadband Wireless

Internet

Wireless Local Area Network (WLAN)

Wireless Cellular

I Demand for mobile wireless internet access

I Need support for multimedia data transfer

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B. Srikrishna

Introduction

Results

Summary References

Broadband Wireless: Challenges

t t

I High data rates and limited/expensive spectrum

⇒ need high spectral efficiency

I Shared resources and multiple access

I Multipath fading channel

I Bursty traffic characteristics

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Introduction

Results

Summary References

Broadband Wireless: Some Proposals

I Code-Division Multiple Access (CDMA)-based

I

1xEV-DO (HDR)

I

HSDPA

I

1xEV-DV

I Orthogonal Frequency Division Multiplexing (OFDM)-based:

I

FlashOFDM

I

IEEE 802.16e

I

IEEE 802.20

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Introduction

Results

Summary References

Key Techniques

User 1 User 2 User 3

Periodic reallocation of resources

I Adaptation to channel and traffic conditions

I Dynamic resource allocation

I

Reallocation period of the order of a few milliseconds

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Results

Summary References

Downlink Resource Allocation Problem

Channel information

User 1

User 2

User K Traffic

Basestation

I Physical resources: power and bandwidth

I Maximize system throughput

I Total transmit power constraint

I Fairness or Quality of Service (QoS) constraints

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Results

Summary References

Maximizing Capacity

User 1

User 2

User K Basestation

Select the user with best channel

Channel K Channel 2 Channel 1

I All power and bandwidth resources to one user

I User with best achievable rate chosen:

i = arg max

k R k ,

where R k is the rate that can be supported by user k.

I No fairness or QoS constraint

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Results

Summary References

Maximizing Capacity: Parallel Channels

User 1

User 2

User K Basestation

For each

best channel user with Select the parallel channel

Parallel Channels to each user

I Bandwidth resources split to achieve parallel channels

I For each channel n, user with best channel conditions chosen:

i n = arg max

k R k,n .

I Water-filling power allocation

I No fairness or QoS constraint

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Results

Summary References

Fairness and Quality of Service (QoS)

I Various notions of fairness or QoS

I Round-Robin

I Proportional Fairness [Tse02]

i = arg max

k

R k R k,av ,

where R k,av is the average rate that can be supported by user k.

I Modified-Largest Weighted Delay First (M-LWDF) [Andrews00]

i = arg max

k γ k W k R k ,

where W k is the Head-Of-Line (HOL) packet delay for user k, and γ k = R C

^k

k,av

.

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Results

Summary References

Resource Allocation in OFDM

Power

User 1 User 2 User 3

Subcarriers

I Available resources:

I

Subcarriers

I

Transmit power

I Channel is frequency-selective ⇒ subcarriers not

identical.

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Results

Summary References

OFDM Resource Allocation Algorithms

I Channel Aware Only (CAO) Scheduling

I

Proportionally Fair (PF) subcarrier allocation [Rhee00]

I

PF subcarrier allocation + power optimization [Shen05]

I

Max utility subcarrier allocation + power optimization [Song05]

I Channel Aware Queue Aware (CAQA) Scheduling

I

MLWDF for OFDM-TDMA [Andrews00]

I

MLWDF at subcarrier level [Parag05]

I Our Work

I

Joint Subcarrier and Power Allocation (JSPA) approach

I

Optimize power allocation after each subcarrier is

allocated

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Results

Summary References

MLWDF for OFDM-TDMA

I All subcarriers allocated to a single user in each slot

I Select user i as:

I

i = arg max

k

γ

_k

W

_k

R

_k

I

W

_k

: Head-Of-Line (HOL) packet delay for user k

I

R

_k

: Rate achievable for user k (water-filling)

I

γ

k

= C

k

R

_k,av

I

C

k

=

⁻^log_D ^δ^k

k

to achieve P[delay > D

k

] < δ

k

I Throughput optimal single-user scheduling rule

I

Maximum stability: achieves stable queues if any algorithm can achieve it

I Single-user scheduling in each time slot not optimal

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Results

Summary References

Subcarrier-wise Allocation

I Approach 1: MLWDF at the subcarrier level [Parag05]

I

For each subcarrier n:

I

i

n

= arg max

k

γ

k

W

k

R

k,n

I

W

k

: Head-Of-Line (HOL) packet delay for user k

I

R

k,n

: Rate achievable for user k on subcarrier n

I

Power allocation needed to allocate subcarriers

I

Uniform/fixed power allocation assumption

I Approach 2: [Song04]

I

Mean packet waiting time instead of Head-Of-Line (HOL) packet delay

I

Other utility functions based on mean packet waiting

time

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Results

Summary References

Joint Subcarrier and Power Allocation

power Split power equally

amongst subcarriers Start

Check if all subcarriers

are allocated

Allocate a subcarrier to a user Start

Split power equally amongst subcarriers

Allocate all subcarriers to users

Update each user’s queue

No Yes

End

Optimize power allocation with

Update each user’s queue

End Optimize power

allocation with

power

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Results

Summary References

Joint Subcarrier and Power Allocation (JSPA)

I Optimal JSPA too complex

I Sub-optimal JSPA

I

Power optimization after each subcarrier is allocated leads to better allocation of the remaining suncarriers

I

Power allocation to each user proportional to the number of subcarriers allocated

I

HOL delay is estimated after each subcarrier is allocated

I Some practical constraints included

I

Discrete-rate constraint: Integer bit M-QAM constellations

I

Extension to band-wise allocation: reduced

signaling/feedback

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Results

Summary References

Simulation Setup

I 128 subcarrier OFDM system

I 12 users, Bernoulli packet arrival, 100 slot buffer

I 6-tap multipath channel, average channel conditions are different for each user

I QPSK to 64-QAM

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Results

Summary References

Results: Throughput vs. Arrival Rate

1 1.5 2 2.5 3 3.5 4 4.5 5 5.5

1 2 3 4 5 6 7

Arrival rate (Mbps)

Throughput (Mbps)

CAO+FPA CAO+FPA+PAO CAO+JSPA MLWDF CAQA+FPA CAQA+JSPA P_total= 5 dBW

Homogenous rate users

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Results

Summary References

Results: Max. Arrival Rate vs. Transmit Power

4 4.5 5 5.5 6 6.5 7 7.5 8

2.5 3 3.5 4 4.5 5 5.5 6 6.5 7 7.5

P_total in dBW

Arrival rate (Mbps)

CAO+FPA CAO+FPA+PAO CAO+JSPA MLWDF CAQA+FPA CAQA+JSPA Homogenous rate users

I Max. arrival rate for less than 0.5% packets dropped

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Results

Summary References

Results: Delay Performance

5 10 15 20 25 30 35 40

10

⁻¹

10

⁰

10

¹

delay (time slots)

P(delay>x)

CAO+FPA CAO+FPA+PAO CAO+JSPA MLWDF CAQA+FPA CAQA+JSPA P

_total

= 8dBW

Arrival rate = 3 Mbps Homogenous rate users

I Best and worst delay performance among users plotted

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Results

Summary References

Results: Band-wise Allocation

1 1.5 2 2.5 3 3.5

3 3.5 4 4.5 5 5.5 6

Delay spread (rms) µsec

Arrival rate (Mbps)

L = 1 L = 2 L = 4 L = 8 L = 16 P_total = 5dBW

I L = Number of subcarriers in a sub-band

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Results

Summary References

Resource Allocation in CDMA

User 1

Power

codes Spreading

User 2

I Available resources:

I

Spreading codes

I

Transmit power

I For any given user, all spreading codes are similar (in

terms of channel conditions).

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Results

Summary References

Multiuser Scheduling

I Most algorithms are single-user scheduling algorithms

I

Proportionally Fair [Tse02]

I

MLWDF [Andrews00]

I Recent results on multi-user scheduling algorithms

I

Greedy and pairwise greedy allocation [Kumaran05]

I

Gradient-based scheduling [Agrawal04]

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Results

Summary References

Results: Maximum Supportable Traffic

0 1 2 3 4 5 6 7 8 9

0 0.1 0.2 0.3 0.4

Average Arrival Rate (Mbps)

Fraction of packets dropped

MLWDF TWO USER

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Results

Summary References

Results: Delay Performance

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4

10⁻² 10⁻¹ 10⁰

Delay d (in sec)

Prob[delay > d]

MLWDF TWO−USER

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Results Summary References

Summary

I Dynamic resource allocation is essential to achieve high spectral efficiency

I Adaptation based on both channel and traffic information

I Some new results for OFDM and CDMA systems

I

Joint subcarrier and power allocation in OFDM

I

Multiuser scheduling in CDMA

I Several open problems:

I

Optimality

I

Quantifying the signaling/feedback overhead

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Results

Summary References

References

E. F. Chaponniere, P. Black, J. M. Holtzman, and D. Tse, “Transmitter directed multiple receiver system using path diversity to equitably maximize throughput,”

U. S. Patent No. 6449490, September 2002.

M. Andrews, K. Kumaran, K. Ramanan, A. Stolyar, R. Vijayakumar, and P. Whiting, “CDMA data QoS scheduling on the forward link with variable channel conditions,” Bell Labs Technical Memorandum, 2000.

W. Rhee and J. M. Cioffi, “Increase in capacity of multiuser OFDM system using dynamic subchannel allocation,” in Proceedings of the 51st IEEE Vehicular Technology Conference, vol. 2, Spring 2000, pp.

1085–1089.

Z. Shen, J. G. Andrews, and B. L. Evans, “Adaptive

resource allocation in multiuser OFDM systems with

proportional rate constraints,” IEEE Transactions on

Wireless Communications, vol. 4, no. 6, pp. 2726–2737,

November 2005.

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Results

Summary References

References

G. Song and Y. Li, “Cross-layer optimization for OFDM wireless networks-Part II: Algorithm development,” IEEE Transactions on Wireless Communications, vol. 4, no. 2, pp. 625–634, March 2005.

P. Parag, S. Bhashyam, and R. Aravind, “A subcarrier allocation algorithm for OFDMA using buffer and channel state information,” in Proceedings of the 62 nd IEEE Vehicular Technology Conference, vol. 1,

September 2005, pp. 622–625.

G. Song, Y. G. Li, J. L. J. Cimini, and H. Zheng, “Joint channel-aware and queue-aware data scheduling in multiple shared wireless channels,” in Proceedings of the IEEE Wireless Communications and Networking

Conference, vol. 3, March 2004, pp. 1939–1944.

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Results

Summary References

References

R. Agrawal, V. Subramanian, and R. Berry, “Joint scheduling and resource allocation in CDMA systems,”

in 2nd Workshop on Modeling and Optimization in Mobile, Ad Hoc, and Wireless Networks (WiOpt ’04), Cambridge, UK, March 2004.

K. Kumaran and H. Viswanathan, “Joint power and

bandwidth allocation in downlink transmission,” IEEE

Transactions on Wireless Communications, vol. 4, no. 3,

pp. 1008–1016, May 2005.