5. CONCLUSION AND FUTURE WORK

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1 CONCLUSION AND FUTURE WORK 5.1 CONCLUSION The MIMO systems are capable of increasing the channel capacity and reliability of wireless channels without increasing the system bandwidth and transmitter power. The advanced wireless technologies like WLAN and WiMAX have adopted the MIMO technology in their wireless systems and already the MIMO technology is being used in 3G and 4G mobile communications. Recently in India, the 3G service provider Reliance has started using the MIMO technology, providing the data rate at a speed of 28 Mbps. The recent researchers of MIMO technology have mainly focused on signal processing, channel modeling and coding aspects rather than the antenna design issues. Hence, in the present thesis the main importance is given for this particular area of MIMO technology. The MIMO technology can be implemented to both base station antennas and small handheld devices like mobile, laptop, Personal Digital Assistants (PDAs), etc.. However, implementing the MIMO technology to small portable devices is more challenging mainly due to two reasons. The first reason is, the antennas of the small handheld devices must have wideband characteristics to support the large data

2 129 rates of the current advanced wireless systems. The second reason is, when the multiple antennas are employed in small devices, mutual coupling comes into picture due to the interaction of electromagnetic waves of neighboring antennas. The Microstrip antennas and Dielectric Resonator Antennas (DRAs) are usually preferred for many wireless applications due to their low profile. However, the main limitation of both these antennas is their low bandwidth characteristics. Though DRAs give comparatively better bandwidth, it does not meet the requirements of most of the present wireless applications. Also, when these antennas are employed in small handheld devices, mutual coupling dominates, reducing the overall system performance. The researchers are focusing on either improving the bandwidth or reducing the mutual coupling of these antennas and a little amount of work is contributed in the literature to handle both the problems simultaneously. The microstrip antennas are available in various shapes like rectangular, E shaped, H shaped, U shaped, etc., and the most popular one among all these antennas is E shaped patch antenna due to its better bandwidth and isolation characteristics. In the present work, a modified E shaped patch antenna is designed giving an improvement in the bandwidth by 9%.

3 130 The main contribution of the thesis is the development of a novel triband Swastika shaped patch antenna with improvement in both bandwidth and isolation. The Swastika shaped patch antenna with substrate permittivity value εr=1 is shown to give an improved impedance bandwidth of 37% with an isolation of 33 db in a 2 2 MIMO system, whereas the E shaped patch antenna of almost same size, substrate and with same separation between the antennas in a 2 2 MIMO system gives a bandwidth of only 14% with an isolation of 25 db. The empirical equations are developed for calculating the resonant frequencies of the proposed Swastika shaped patch antenna giving a maximum error of 8.7% compared to the simulation results. The researchers have worked on various bandwidth improvement techniques by reducing the ground plane. The main reason for the improvement in bandwidth is due to the change in resonant behavior of the antenna with the reduction of ground plane. The proper feeding mechanism and reduction of ground area make the bandwidth to increase. Based on this concept, the bandwidth of the proposed Swastika shaped patch antenna MIMO system is increased to 51%, which is a good impedance bandwidth that can be obtained with an FR4 substrate of thickness 1.6 mm only. Also, the developed MIMO system is shown to give an isolation of 50 db due to the surface current flow on the patch in opposite directions at the adjacent sides. The antenna system is

4 131 physically fabricated and a comparative study is made between the simulated and measured results. The channel capacity of the proposed Swastika shaped patch antenna for a 2 2 MIMO system is calculated and compared with E shaped and dipole MIMO systems. The mutual coupling between a pair of rectangular microstrip antennas is analyzed using Artificial Neural Networks (ANNs). A normal EM simulation tool can give the mutual coupling between the antennas for a specified spacing only. This problem is addressed using the ANN and the mutual coupling is calculated for a set of separations by properly choosing the ANN structure, algorithm and a proper training data. Dielectric Resonator Antenna (DRA) technology is another major area of study in the field of antennas. These antennas give better bandwidth, gain and efficiency compared to the conventional microstrip antennas. A Swastika shaped DRA is designed with same dimensions as that of a microstrip antenna and a comparative study is made between them. The Swastika shaped DRA MIMO system is shown to give a wide bandwidth of 51.5%, whereas the Swastika shaped microstrip antenna of same dimensions gives a bandwidth of 20% only. However, the amount of isolation and channel capacity of Swastika shaped DRA MIMO system are lesser by 10 db and 0.2 bps/hz respectively. The bandwidth and isolation improvement of DRAs are studied for cylindrical structures as these are widely used geometries due to their

5 132 ease of fabrication. The bandwidth of a Cylindrical Dielectric Resonator Antenna (CDRA) is increased by 9% by top loading the antenna with a thin circular disk. A 2 2 MIMO system is formed using the CDRA system and the mutual coupling is reduced by an amount of 7 db by introducing a narrow slot on the ground plane between the two antennas, which is due to the reduction of surface currents. The mutual coupling between a pair of CDRAs is analyzed using the Artificial Neural Networks (ANNs). In summary, the key contributions of the thesis can be listed as follows, Key Contributions: Development of a novel tri-band Swastika shaped microstrip antenna giving improved bandwidth and isolation simultaneously. Development of empirical equations for calculating the resonant frequencies of the Swastika shaped antenna. Implementing the ANN concept for analyzing the mutual coupling between the elements of a 2 2 MIMO system employing rectangular microstrip antennas. Development of a Swastika shaped Dielectric Resonator Antenna with wideband characteristics.

6 133 The methods of improving bandwidth and isolation of CDRAs using top loading and ground plane reduction techniques respectively. Implementing the ANN concept for analyzing the mutual coupling between the elements of a 2 2 MIMO system employing CDRAs. 5.2 FUTURE WORK The design of suitable antennas for MIMO systems is a vast area of research. The work presented in the present thesis covers only a part of it due to the time constraint. Based on the conclusions drawn and the limitations mentioned in the presented work, further research can be carried out in the following areas. The proposed Swastika shaped patch antenna gives improved bandwidth and isolation due to its tri-band nature. Further designs can be worked out resonating at multiple frequency bands to give improved bandwidth and isolation. The developed Swastika shaped DRA is shown to give wideband characteristics, however its isolation properties are not much better compared to a microstrip antenna. The isolation of this antenna system can be further studied using the concept of

7 134 Electronic Band Gap (EBG) structure, which involves periodic slots on the ground plane to reduce the mutual coupling. The DRAs can be integrated on silicon substrates resulting in the possibility of System on Chip (SoC). This method highly reduces the manufacturing cost and makes the possibility of bulk production of DRAs. The study on radiation properties of DRAs, when they are employed in SOCs is a major area of research. The present work has used ANN for studying the mutual coupling in rectangular microstrip antennas and CDRAs. However, ANNs can be explored further to study the other properties of both Microstrip and DRAs such as bandwidth, resonant frequency, size reduction, etc..