Research Area

      Cognitive Radio, Adaptive Wireless Ad hoc Networks, Broadband wireless communication networks, Wireless mesh networks (WiFi/WiMax), Spectrum and power efficient wireless systems, Dynamic spectrum management in wireless heterogeneous systems

Research Work

• Enabling Adaptive Wireless Networks with Cognitive Radio Technology

    - Global Control Plane Architecture for Cognitive Radio Networks

   An novel network architecture for adaptive cognitive radio networks based on the concept of a “global control plane” is proposed. The control architecture uses a predetermined common coordination channel for spectrum etiquette, network establishment and adaptation to changing interference environments. Four key components of the control protocol are designed: bootstrapping, discovery, data path establishment and naming/addressing.

• Bootstrapping: The bootstrapping protocol uses beacons to inform neighboring nodes about a node’s PHY/MAC capabilities and current status.

• Discovery: The network discovery protocol helps nodes to obtain a global view of reachability and end-to-end paths in the network by exchanging and propagating local link states.

• Data Path Establishment: Cross-layer parameters (such as frequency, power, rate, etc.) are dynamically configured and optimized along the discovered end-to-end data path. Spectrum opportunities at local transmissions are explored.

• Distributed Naming/Addressing: Nodes obtain their IP addresses and perform name to network address translations using the distributed naming/addressing scheme. Applications using permanent node names are supported.

    An ns2 simulation model of the cognitive radio network with global control has been developed and used to evaluate performance in terms of network setup time, control overhead and achievable data throughput. The bootstrapping and discovery protocols are also evaluated by experiments using ORBIT radio grid testbed.

• Reactive and Proactive Spectrum Coordination for Cognitive Radio

           - Study of Spectrum Coordination in a Co-existing WiFi/WiMax Network

   In this research, spectrum co-existence between IEEE 802.11b and 802.16a networks in the same shared frequency band is investigated using cognitive radio techniques with different levels of complexity. Simple reactive interference avoidance algorithms as well as proactive spectrum coordination policies based on etiquette protocols are proposed and compared in terms of achievable spectrum efficiency in a shared Wi-Fi/Wi-Max scenario. In reactive interference avoidance methods, radio nodes coordinate spectrum usage without exchange of explicit control information – this is done by adaptively adjusting transmit PHY parameters such as frequency, power and time occupancy based on local observations of the radio band. Because local observations provide information only about transmitters, they may not be sufficient for resolving spectrum contention in scenarios with “hidden receivers”. Proactive coordination techniques solve the hidden-receiver problem by utilizing a common spectrum coordination channel (CSCC) for exchange of transmitter and receiver parameters. Radio nodes can cooperatively select key PHY-layer variables such as frequency and power by broadcasting messages in the CSCC channel and then following specified spectrum etiquette policies. The compared schemes are:

• Reactive Methods:
  - DFS: Dynamic Frequency Selection
  - RTPC: Reactive Transmit Power Control
  - TA: Time Agility
   

• Proactive Schemes:
  - CSCC-F: Frequency Adaptation using CSCC
  - CSCC-P: Power Adaptation using CSCC

   An ns2 simulation model is developed to evaluate both reactive and proactive etiquette policies in scenarios with co-existing IEEE 802.11b and 802.16a networks. The density of radio nodes in the coverage region, and their degree of spatial clustering are key parameters in the system evaluation. Detailed simulation studies were carried out for a variety of scenarios including both single and multiple 802.11b hotspots per 802.16a cell with and without spatial clustering. Our results show that simple reactive algorithms can improve system throughput when sufficient “free space” (in frequency, power or time) is available for PHY adaptation. In more congested scenarios with spatially clustered nodes and hidden receivers, the proposed CSCC etiquette can significantly improve overall system performance over reactive schemes.

• Spectrum Etiquette Protocols in Unlicensed Bands

         - Spectrum Etiquette Protocol Evaluation in a 2.4GHz 802.11/Bluetooth Network

  A spectrum etiquette protocol is proposed for efficient coordination of radio communication devices in unlicensed (e.g. 2.4 GHz ISM and 5 GHz U-NII) frequency bands. The proposed etiquette method enables spectrum coordination between multiple wireless devices using different radio technologies such as IEEE 802.11.x, 802.15.x, Bluetooth, Hiperlan, etc. The basic idea is to standardize a simple common protocol for announcement of radio and service parameters, called the “common spectrum coordination channel (CSCC)”. The CSCC mechanism is based on the low bit-rate mode of the 802.11b physical layer, along with a periodic broadcast protocol at the MAC layer. The CSCC protocol is “policy neutral” in the sense that it provides a general mechanism which can accommodate a wide range of specific spectrum sharing rules. One possible CSCC protocol implementation is described in terms of the packet formats used and related channel access rules.

   Proof-of-concept experimental results from a CSCC prototype are presented for an example scenario in which nearby 802.11b and Bluetooth devices contend for 2.4 GHz ISM band access. Results showing file transfer delay with and without CSCC etiquette are given for comparison purposes.

 (more resources in the paper...)