Interference Avoidance/Reduction

• Power Control

A critical problem with DS-CDMA is the near-far problem. This problem occurs in the absence of power control—if all mobiles were to transmit at the same power level, the signal from the mobile closest to the base station will cause very large interference to the signals from all other users (since the signal power drops exponentially with the distance). Yet another reason for power control is battery life—if the mobile station were to continuously transmit at a power higher than that needed to maintain an acceptable (signal to interference-ratio) SIR, the battery lifetime is reduced. Using power control, each mobile station may transmit using the minimum power needed for maintaining the required SIR ratio, thus conserving its battery life.

To minimize co-channel interference and to conserve power, both the mobiles and the base station (BS) operate at the lowest power level at which an acceptable signal quality can be maintained.

• Power Control and Rate Adaptation Based on Game Theory

In recent years, game theory has emerged as a promising approach to solving the power control problem in wireless networks. This thesis extends the reach of game-theoretic analysis to embrace link adaptation, thereby constituting a generalization of the power control problem. A realistic and natural problem formulation is attempted, wherein transmitter power and a discrete-valued Adaptable Link Parameter (ALP), e.g. code rate, constitute the action set of a player in this game. The dual goals of maximizing throughput and minimizing power consumption are reflected in the utility function selection, which uses the accurate sigmoid model for approximating throughput. The discrete action space makes it difficult to verify the existence of a Nash Equilibrium (NE) in this game using standard techniques. To circumvent this limitation, a heuristic algorithm is proposed. This algorithm is analytically shown to always converge to a NE. The subsequent results probe its validity and sensitivity. Favorable comparisons are drawn between these game-theoretic results and those arising from parallel systems techniques. A linear programming system optimization that exploits properties of the dominant eigenvalue of the system gain matrix is also presented in a comparative context.

Work on game theoretic power control was carried out by Samir Ginde, as part of his Masters thesis.

• Multi-user Scheduling

• Network-Based Characterization & Management of Interference in IEEE 802.11 WLANs  

IEEE 802.11 WLAN has been the fastest growing segment of telecom industry in recent years. Meta Group predicts that 95% of corporate laptops will ship with Wi-Fi embedded directly into the platform by 2005. As a result, wireless connectivity within the enterprise will become a reality, whether or not the business is ready.

ISM band is unlicensed (virtually unmanaged) wireless medium. Therefore, 802.11 WLANs must contend with disparate numbers and varieties of interferers, changing dynamically over very short time frames. The 2.4 GHz ISM band and 5 GHz UNII band over which 802.11 WLANs operate are shared by an increasing number of devices and networks, including but not limited to, microwave oven, cordless phones, VoWiFi phones, Blue-tooth devices, and adjacent 802.11 networks. The signals emitted by these devices and networks employ many different modulation formats, ranging from narrow-band CW, agile PSK and FM modulation, FHSS to wideband direct-sequence spread spectrum (DSSS) and orthogonal frequency division multiplexed (OFDM) formats. Moreover, these signals will be highly dynamic in nature, due to mobility of users and multipath fading in the environment, and of the traffic being carried over these networks.

The costs of maintenance of WLAN networks are growing up rapidly; however, current WLANs typically have very limited interference characterization/management capability.

The accelerating adoption of WLANs based on the IEEE 802.11 standards creates a strong need for developing sophisticated network-based tools to characterize and manage WLAN interference, therefore optimize the network operation in terms of throughput, coverage, QoS, etc.

Cognitive radio is drawing more and more attention, which has been regarded as an enable technology that makes wireless communication even more smart and personal. Interference-awareness (including interference detection, classification, geolocation and management) is one important step towards realizing cognitive radio from concept.

1. Develop novel/effective network-based means to detect/classify radio network interference, by employing offline processing of single-site or multi-site snapshot data.
    

2. Develop advanced interferer geo-location techniques, by employing RF fingerprinting technique and exploiting spectral coherence of the signal.
    

3. Develop network evaluation and refinement techniques to optimize overall performance of radio network, by employing interference avoidance and management techniques based on the knowledge of interference obtained through interference detection, classification and geo-location.
    

4. Develop device and radio network architectures that can most effectively implement and take advantage of above mentioned interference detection, classification, geo-location and management techniques, paving the way toward realizing cognitive radio network.

For more information, contact Youping Zhao