Cognitive Wireless Mesh Networks: PHY Layer

1. Cognitive Wireless Mesh Networks: PHY Layer Mohamad Haidar, Ph.D. Electrical Engineering Department Ecole de Technologie Superieure

2. 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

3. 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

4. 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.

5. 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

6. 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

7. MAC • Single Channel, Single Radio

8. 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

9. 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

10. 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

11. 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;

12. Multi-Radio, Multi-Channel Analysis Analysis was conducted on an IEEE 802.11b/g mesh network

13. 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

14. 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.

15. 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

16. 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.