Projects:2020s1-1110 Cognitive Radio System

Project students

Nicholas Paczynski and Christopher Rockliff

Project supervisors

Prof. Christophe Fumeaux, Dr. Shengjian Chen, Josh Brittain (DST) and Nick Lawrence (DST)

Abstract

The objective of this project was to research and develop a Cognitive Radio System. Cognitive Radio is a wireless system that adapts to its environment to find the optimal frequency channel to provide fast and efficient communications for data transfer and to extend the battery life of the radio system due to lower power usage. Two main parts will make up this system, that being a frequency reconfigurable antenna or tunable antenna and a radio control system. Frequency reconfigurability can be achieved by changing the physical length of the antenna radiators using devices such as discrete switching mechanisms such as the digitally tunable capacitor or RF switches. The antenna will also provide a control interface for the transmitter control subsystem. An SDR will be used as the platform for the radio transmitter design. A control system will be implemented to detect (using a VSWR detector) and adjust the characteristics of the system based on the dynamic properties that are continuously being tracked by the antenna, and will also drive the control interface of the antenna subsystem.

Introduction

The efficiency of a wireless system faces problems caused by antenna detuning, such as when placed inside a pocket, and deep frequency specific fading, caused by urban environments. These problems cause the wireless system to utilise RF spectrum and battery power in a non-optimal fashion to counter these, often being forced to reduce data rates and utilise higher power transmissions, lowering battery life.

These problems can be effectively addressed through the use of a cognitive radio system and a companion reconfigurable antenna. This would allow the wireless system to dynamically track properties of both the RF spectrum and the systems antenna itself, then utilising this information to best adapt to its environment through altering the frequency response of the reconfigurable antenna and selecting the best wireless channels over which to transmit.

Of particular relevance is tracking properties such as the RF spectral availability, done by performing spectrum sensing upon prospective transmit channels, and the antennas return loss, used to detect any detuning effects that the antenna is experiencing. These properties will be tracked and utilised in a closed-loop optimisation-driven control system to best adapt the antennas frequency response and the chosen wireless transmission channels.

As such the scope of the project is to create the aforementioned cognitive radio system and a frequency reconfigurable antenna to accompany it. This antenna will operate in the 2.4GHz range with reconfigurability allowing it to alter its frequency response around this centre frequency.

The choice of an antenna’s topology and reconfiguration mechanisms will determine the performance and behaviour of the antenna. A microstrip patch antenna will be adopted as they are simple to design and their radiation pattern is directional therefore higher gain. This has has been determined to be the ideal approach for this project. Different methods can be used to reconfigure the frequency of an antenna. In this case switching devices were chosen as they are fast, cheap and can operate in a wide bandwidth. The characteristics of the antenna change based on the specific functionality that is required at that given time, such as spectrum sensing or communications.

The cognitive radio system will utilise a simple log detector based VSWR detection method to determine the detuning factor of the antenna and compensate accordingly by altering the frequency response of the companion antenna through simple hardware with voltage changing properties. Energy density spectrum sensing will be used to determine the best channel on which to transmit, performed after the adjustment of the antenna frequency response. The spectrum sensing will be achieved using an ultra-wideband antenna and the communications with a narrowband antenna. The software driving the optimisation loop will be run on a computer, which will interface with the VSWR detector, the SDR and a voltage controlling circuit for the antenna. This software will be built using a tool chain like LabVIEW or MATLAB/Simulink.

Conclusion

References

[1] a, b, c, "Simple page", In Proceedings of the Conference of Simpleness, 2010.