Mehdi Abolfathi

SDR CourseSpring 2008

A Cognitive MAC Protocol for Ad Hoc Networks

Overview

• Need for Spectrum Access• Related Works• Access Scheme and Capacity• Pus Constraints• Sensing Techniques• Statistical Channel Allocation• Proposed combination

Cognitive Spectrum Access

Basic idea• Sense the spectrum you want to transmit in

• Look for “holes” or “opportunities” in time and frequency• Transmit so that you “don’t interfere” with the licensees

Cognitive Spectrum Access

Goals

• Dynamically control access (Tx/Rx, beam, power etc.) on each time/frequency such that network capacity is maximized

• Constraints are honored

• Capacity demands are met

Related Works

• Centralized and decentralized spectrum auction and brokerage

• POMDP MAC protocol framework

• A tri-band protocol, called the dynamic open spectrum sharing (DOSS) MAC

SU Access Schemes

KS Scheme(Keep-Sensing-if-Busy)

VX Scheme (Virtual-Xmit-if-Busy)

Collision Probabilities

Primary User’s Statistics

• Idle Time=Exponential • Busy Time=General• n Channels

Modeled as a M/G/n Queue

SU’s Maximum Capacity

SU Packet

Len. Dist.

Mean PU Idle

time

SU’s Maximum Capacity

PU’s Collision Probability

Multiple SUs, one Channels

Multiple SUs, one Channel

“Random Sensing” vs. “All Sensing”

Random-Sensing: SU randomly selects a channel + Vitual Transmission

All-Channel-Sensing: Each SU senses all channels + Virtual Transmission

All-Channel-Sensing strategy does not improve the total spectral

efficiency

QoS of PU

Multi-Band, Multi-User System

• Multiple SUs has no loss/gain in terms of total throughput.

• Sensing all the frequency bands does not improve the total throughput of SUs.

• Dividing SUs into groups ≈ Scaled throughput

Consraints

For the Primary User

• Idle Time=Exponential • Busy Time=General• n Channels

Modeled as a M/G/n Queue

SU Packet

Len. Dist.

Mean PU Idle

time

Constraints

G in “M/G/n” appears in:

•Need for Sensing the Channel in KS or VX scheme

•Capacity of SU decreases by

the factor of

Statistical Channel Allocation

Successful Rate vs. Operating Range

Successful Rate vs. Aggregation, m

Proposed: “SCA” + “VX Scheme”

• Identifying PU’s parameters, Historical Study

• Search for the maximum available rate

• Accessing the channel using the VX scheme

“SCA” + “VX Scheme”, Benefits

1. Distributed calculation, Ad Hoc networks

2. Not to sense the channel in unnecessary occasions, Hardware savings.

3. Smart access to the channel.

4. Finding the best combination of the channels to achieve the maximum throughput.

5. Acting as an independent layer for upper layers by transparency in the case of busy channel.

Questions

Some Results

• SU’s Capacity Bound:

• Under the VX scheme, the SU achieves the maximum throughput when it transmits fixed length packets, i.e., L2 = l2.