xiaohua li and wenyu liu

ICC 2003 - May 11, 2003 A Joint Layer Design X. Li, W. Liu

Smart Contention Resolution Random Access Wireless Networks With Unknown Multiple

Users: A Joint Layer Design Approach

Xiaohua Li and Wenyu Liu Department of Electrical and Computer Engineering, State University of

New York at Binghamton, Binghamton, NY 31902Motorola Inc., Libertyville, IL 60048


Outline

IntroductionJoint PHY/MAC Layer design

Performance AnalysisSimulationsConclusions


Introduction

Issues:• Wireless spectrum is becoming scarce• Advanced access schemes required to improve efficiency to improve throughput to support packet priority/QoS • Random channel access is more preferred • Need resolve contentions


Introduction (cont.)

Issues: (cont.)• Contention resolution is challenging in wireless networks• Traditional ways: operate in MAC layer, collided signals discarded instead of utilized• Signal processing ways: separate collided signal in PHY layer

Questions:Benefit from both PHY/MAC layer?


Introduction (cont.)

Proposed Solution:• Joint PHY/MAC layer design

– PHY: signal separation or multiuser detection– MAC: smart packet scheduling

• Compared with traditional ways: – collided signal is used instead of discarded

• Compared with other signal processing ways– Preserves packet priority, support QoS – Works with unknown multiple users– Computationally efficient and robust, i.e., does not

suffer ill-channel or near-far conditions.


System Model of Wireless Networks• The wireless network has a central controller and multiple unknown mobile users:• Central controller: PHY collision separation, MAC packet scheduling

User n

User 1 User 2

Controller(User 0)


Packet Flow in the Wireless Networks

… Access request packets

(Controller & Users)

Data packets(Users)

Access request packets

(Controller & Users)

Data packets(Users) …

Problem: if there are too many unknown users, contentions in access request slots are high


Packet Flow in the Wireless Networks (cont.)

• Solutions: divide and conquer

• Procedure:– Each active user generate I random number– Each random number is corresponding to a unique

access request packet– Actives users are grouped according to I or access

request packets


Random Access Scheduling Protocol 1. Central controller ask all active users for access

requests2. Active users generate random numbers

• Form subgroups distributedly by the numbers• Transmit access request packets according to the random

numbers

3. Central controller detects collided packets• If there are active users from different subgroups, there

access request packets collide• Collided packets can be detected successfully• From collided signal, central controller know active

subgroups• Central controller asks each subgroup one-by-one to

request again

4. Go to 2


Random Access Scheduling Protocol (cont.)• Packet priority:

– Users generate random numbers according to priority– Central controller gets priority information after separating

collided access requests – Central controller arrange active users to transmit data packets

according to their priorities.

• Efficiency:– Contention exists in access request slots only– If active users have different random integer, no collision

happens for data packet transmission– Size of access request slots is reduced by divide-and-conquer

• Collision detection:– Orthogonal codes can be used because of the reduced access

request slot size


Access Request Collision Resolution

• Key point: need K different access request packets (codes) only

– One codes for each group instead of user– All active users in a group transmit the same code, and will

be divided in the next turn

• Each user generate random number – find from

– Select the corresponding code to transmit as access request

packet

ja

ija ,

1

0,

I

i

iijj Kaa

1,,0)],(,),0([ KkMcc kkTk c



• There are at most K different collided access codes, although there are unknown number of users

• Received collided signal

kk

Hk

kk

kk

k jjjjj

T

NMcc

McNc

Lhh

MxNxN

Ch

00

x

)()0(

)()(

]),()0(,[

)]()([)(

21



issionfar transm-near

us,asynchrono multipath, in even codes, orthogonalby

ly /efficientcompletely cancelled ceinterferen access-Multi

active is group if,

activenot is group if,)(

metric Decision

if,

if, that such detectorsK find can we

,, matrices symboldifferent most at With 10

k

kNz

kl

kl

K

Hk

kH

k

kl

K

1h

0fx

0

1fC

CC



• Active group/user detection (collision separation)– Construct received sample vector – Apply detector to calculate decision

metrics – Compare with threshold to determine

whether each group k is active (feedback to MAC)

kf

kz


Performance Analysis

Probability of Data Packet Collision:

where A is the maximum range of the random number, u is the number of active users contending for channel access at the same time

A

iAuP

u

ic

1

01)(


Performance Analysis (cont.)

System Efficiency:

ratio of successful to total transmission in one frame instantaneous throughput

Without detection error:

With detection error

acd

cdc

ad

d

ILuPuL

uPuLuEuP

ILL

L

)(1(

))(1()]([))(1(

ad

d

ad

d

ILuL

uLu

ILL

L

)(


Performance Analysis (cont.)

aad

d

LL

L

0

System Throughput:

Packet delay:

],)1

([

: withinis onelast theofdelay the

, :ispacket first theofdelay the

,contendingpacket data if

d

1

1d

d

aa

I

ii

a

IuuK

uIu

I

u


Simulations

DER versus SNR, for offered load 0.5 and 1:satisfactorily low detection errors for even low SNR


Simulations (cont.)

Throughput vs. offered traffic load :Achieve much higher throughput than ALOHA and CSMA.

Especially, if the DER is not too low, it achieves almost the offered traffic load up to unity.


Simulations (cont.)

Packet delay vs. throughput:Has better (much smaller) packet delay than the ALOHA. With DER=0, 0.01,

0.05.


Conclusions

– Use signal processing, specifically, multiuser detection principles, to resolve random access contentions.

– Central controller schedules random access request packet detection

– Collisions are resolved through signal separation in systems with unknown multiple users

– Enhance throughput, support QoS


-Questions? Questions?

-Issues?Issues?

-Comments?Comments?


Thank You !!!

Xiaohua LiDepartment of Electrical and Computer Engineering

State University of New York at Binghamton

Binghamton, NY 12309

Email: [email protected]

Wenyu LiuPersonal Communication Sector

Motorola Inc.

Libertyville, IL 60048

Email: [email protected]

xiaohua li and wenyu liu

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