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Cognitive Wireless Mesh Networks: PHY Layer. Mohamad Haidar, Ph.D. Electrical Engineering Department Ecole de Technologie Superieure. Table of Contents. Overview Wireless Mesh Networks (WMNs) Characteristics Related research in PHY layer Related research in MAC layer MAC

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Cognitive wireless mesh networks phy layer l.jpg

Cognitive Wireless Mesh Networks: PHY Layer

Mohamad Haidar, Ph.D.

Electrical Engineering Department

Ecole de Technologie Superieure


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Table of Contents

  • Overview

  • Wireless Mesh Networks (WMNs) Characteristics

  • Related research in PHY layer

  • Related research in MAC layer

  • MAC

    • Different access schemes

    • Number of Channels and number of radios

  • Cognitive Radio

  • Proposed PHY Architecture for WMNs

  • Conclusion


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Overview

Node Types

  • Wireless routers

  • Gateways

  • Printers, servers

  • Mobile clients

  • Stationary clients

Link Types

  • Intra-mesh wireless links

  • Stationary client access

  • Mobile client access

  • Internet access links


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WMNs Characteristics

  • WMANs have the following characteristics:

    • Multi-hop wireless network: objective to extend the coverage range of current wireless networks.

    • Support for ad-hoc networking and capability of self-forming, selfhealing, and self-organization:

      • Easy deployment and configuration

      • Multi-point to multi-point communications

      • Fault tolerance

      • Flexible network architecture

    • Mobility dependence

    • Compatibility and interoperability with existing wireless networks: integration with IEEE 802.11, cellular networks, IEEE 802.16, etc…

    • Power-consumption: Mesh clients may require power efficient protocols.


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Related Research in PHY

  • Advanced Radio Technologies

    • Directional and smart Antennas (reduce exposed nodes, increase hidden nodes)

    • MIMO (IEEE 802.11n) (current MAC protocols perform poorly)

    • Cognitive Radio (is still in the early phase of research)

    • Software Radios (programmable MAC protocols need to be developed)

  • Scalability: Performance degrades with number of hops

    • Routing protocols may not be able to find a reliable routing path

    • MAC protocols experience significant throughput reduction (example: IEEE 802.11 MAC protocol)

All these techniques require revolutionary design in the MAC and routing protocols


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Related Research in MAC

  • MAC

    • Scheduled

      • Fix scheduled TDMA (or CDMA)

      • Polling

      • Impractical due to lack of:

        • Central coordination point

        • Reasonable time synchronization

    • Random Access

      • CSMA – simple and popular

      • RTS/CTS – protects the receiver

        • Contention and overhead

    • OFDM

      • Channel bandwidth is divided into multiple orthogonal sub-channels to reduce ISI and frequency-selective fading

      • Increased data rate throughput

      • Robustness against interference

      • Multipath signal effects are greatly reduced


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MAC

  • Single Channel, Single Radio


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2

2

1

1

Ch-1

Ch-1

3

4

3

4

Ch-1

Ch-2

User bandwidth = B/2

User bandwidth = B

MAC (cont’d)

  • Multi-Channel

    • Increases network capacity

B = bandwidth of a channel


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IP

MCCL

802.11

PHY

MAC – Multi-channel, Single Radio

  • A new Multi-Channel Coordination Layer (MCCL) should be introduced between MAC and Network layers to:

    • switch channels

    • Synchronize traffic on various channels

  • Increase in the capacity of the network

  • Other Challenges: Require to find an optimal channel assignment


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MAC – Multi-channel, Multiple Radios

  • Standard MAC

    • A node now can receive while transmitting

    • Practical problems with antennas separation (carrier sense from nearby channel)

    • Optimal assignment

    • Solutions

      • Centralized

      • Distributed


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MAC – Multi-channel– Multiple Radios

  • Custom MAC

    • Nodes can use a control channel to coordinate and the rest to exchange data.

    • In some conditions can be very efficient.

    • However the control channel can be:

      • an unacceptable overhead;

      • a bottleneck;


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Multi-Radio, Multi-Channel Analysis

Analysis was conducted on an IEEE 802.11b/g mesh network


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Cognitive Radio

  • Main Objectives:

    • Spectrum Sensing:

      • detecting the unused spectrum and sharing it without harmful interference with other users.

      • detecting primary users is the most efficient way to detect spectrum holes

        • Spectrum sensing techniques can be classified into three categories:

          • Transmitter detection

          • Cooperative detection

          • Interference based detection.

    • Spectrum Management:Capturing the best available spectrum to meet user communication requirements.

    • Spectrum Mobility:is defined as the process when a cognitive radio user exchanges its frequency of operation

    • Spectrum Sharing:providing the fair spectrum scheduling method


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Proposed PHY Architecture

  • Spectrum: Unlicensed spectrum (Free but deals with interference and limits on Tx power).

    • 2.4 GHz band offers around 80 MHz of Bandwidth.

    • 5 GHz band:

      • 5.150 GHz – 5.350 GHz (BW = 200 MHz)

      • 5.470 GHz – 5.725 GHz (BW = 255 MHz)

      • 5.725 GHz – 5.850 GHz (BW= 125 MHz)

      • 700 MHz (TV BAND)

  • Multiple Radios (Multi-Transceiver NICs)

    • Radio 1 (Multi-Radio): routing and configuration between mesh routers.

    • Radio 2: access to the network by mesh clients through mesh routers.


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Proposed PHY Architecture (cont’d)

  • Medium Access Control:

    • OFDM:

      • allows only one user on the channel at any given time

      • To accommodate multiple users within a sub-channel, OFDM must employs TDMA (separate time frames) or FDMA (separate channels).

    • OFDMA:

      • is a multi-user OFDM that allows multiple access on the same channel

        • several stations can transmit at the same time slot over several sub-channels.

FDM

OFDM

OFDMA


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Conclusions

  • A possible Cognitive Wireless Mesh Network PHY layer implementation:

    • Multi-Radio Transceivers and multi-channels at the Wireless Mesh Routers.

      • Cognitive spectrum utilization between the 3 Unlicensed spectrums (the Licensed TV band will be considered as well)

      • MIMO technology is definitely a good choice due to the low cost and high efficiency.

      • OFDMA could be the MAC access scheme.

        • Power consumption at the intended user.

      • Development of efficient MAC protocols to take advantage of the advanced radio technologies involved is necessary.


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