Opportunistic Scheduling in Wireless Networks

Mohammed Eltayeb

Obaid Khattak

Project Outline This report gives an overview of different scheduling

algorithms, from the simple round robin algorithm, to opportunistic scheduling algorithms considering QoS, with simulation of system capacity feedback load and fairness.

We divided the algorithms into fair, semi-fair and greedy algorithms.

All simulations are done with Matlab 7.0 with an average SNR of 15dB and 1000 Ts for 30 users.

Back Ground Theory

A scheduling system is implemented both in the mobile station (MS) and in the base station (BS).

The BS uses a TDMA scheme and during one time slot, only one user can receive or transmit, and this user is selected by the scheduler.

Fair AlgorithmsRound Robin

• The RR scheduler is the simplest scheduling algorithm, and it is not opportunistic. • When a user connects to the base station (BS), it is given a position in the queue of users, and the scheduler will iterate through the queue.

Fair Algorithms - RR

Fair Algorithms - RR

Fair Algorithms

Opportunistic Round Robin (ORR)

• The ORR algorithm is a Round Robin scheduler.• Channel conditions are taken into account.• The scheduler iterates the list of users, and every time the best user is selected and removed from the list.

Fair Algorithm - ORR

Fair Algorithm - ORR

SEMI-FAIR SCHEDULING ALGORITHMS

EXAMPLES AND PERFORMANCE

Semi-Fairness

Middle ground between Fair & Greedy Provide Fairness in terms of scheduling

outage Feedback load not zero but not rate optimal

either

Example: Switched Diversity Scheduling (SDS)

SDS

Family of algorithms based on multi-antenna systems schemes

Specific Threshold γth is set Scans users to find CNR > γth If user found, selected At each time slot, sequence may be randomized or

organized in special way Examples

Selection Combining Transmission (SCT) SET with Post-Selection (SETps)

SCT

Checks ALL users, selects user with highest CNR

Fair if all users are i.i.d Advantage

Only form of SDS which is rate optimal Disadvantage

Normalized feedback load (NFL) unity

MASSE Performance of SCT

Throughput Fairness in SCT

SETps

Extension of Switch-and-Examine Transmission (SET)

First scanned user with CNR > γth selected

If no user CNR > γth User with greatest CNR selected Combats scheduling outage

At each time slot, list randomized Provides level of fairness

MASSE of SETps

Throughput Fairness of SETps

Time-slot Fairness of SETps

NFL of SETps

GREEDY SCHEDULING ALGORITHMS

EXAMPLES AND PERFORMANCE

Greedy Algorithms

More concerned with maximizing system throughput, not fairness to individual users

Do provide fairness when all users have i.i.d. channel conditions

Rate optimal, MASSE values equal Examples

Maximum CNR Scheduling (MCS) Optimal Rate, Reduced Feedback (ORRF)

MCS

All users report their CNR to BS

User with best channel selected Rate optimal

Large overhead in reporting CNR values Normalized feedback load (NFL) unity

Poor throughput and time-slot fairness Same as SCT

MASSE of optimal schedulers

Optimal Rate, Reduced Feedback (ORRF) Scheduler decides threshold CNR

Distributed to all users Users with CNR > Threshold reply Best user selected If no user replies

Scheduler requests full feedback Every user returns CSI (Channel State Information)

After full feedback or without it, best user selected

NFL of ORRF

Time-slot Fairness of ORRF

Throughput Fairness

MASSE-based Comparison

NFL-based Comparison

References

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