Hybrid Smart Antenna Array for Low-Cost High-Performance Wireless Communication Applications
Development of wireless applications utilizing the 60 GHz band is a very important task for the industry since this will result in a breakthrough in data rates achieved by the wireless communication systems and close the gap with the wired and optical data links. These systems will make uncompressed high definition video transfer possible and eliminate the bottleneck problems due to low data rate wireless links. However, the excessive propagation losses, immature circuit technology and narrow antenna beam widths render the implementation and deployment of 60 GHz wireless systems a challenging task. Use of several high gain antennas and employing antenna switching has been proposed by others for overcoming the propagation losses. This approach, however, does not provide any beam alignment mechanism and significant performance loss is expected with a small mismatch in the alignment of transmitter and receiver narrow beams. The smart antenna technology which is a significant asset providing performance enhancement in wireless communication systems has also been proposed by others to help overcome the problems associated with the 60 GHz propagation environment. However, since the required overall system gain is very high, the low gain array elements often used with the smart antenna systems result in a remarkable increase in the required number of array elements consequently increasing the cost, latency, and complexity of the system and rendering this technology infeasible for commercial applications. Therefore, development of a technology that provides adaptive beam alignment capability as well as high system gains is perceived as an important milestone in the effective development of 60 GHz wireless applications considering the current needs of the industry.
HSAA Block Diagram

Fig. 1. Overview of the proposed HSAA system

Considering the signal processing and the electromagnetics aspects of the smart antenna systems, our group at University of Hawaii proposed the Hybrid Smart Antenna Algorithm (HSAA). The HSAA combines the advantages of switched antenna systems with those of smart antenna systems while considering the array element response and array geometry as other design parameters [6]. As shown in Fig. 1, the HSAA employs several highly directive elements each with a different beam direction providing additional degrees of freedom to the array processor. The beam direction of each array element is adjusted by tilting the element with respect to the array axis and tilt angles are determined as a result of an optimization procedure. The array processor closely monitors the received signal power at each array element and selects a subset of neighboring array elements for the beamforming stage with the help of a smart switch. Since only a subset of the array elements are used in the beamforming process, the HSAA can result in significant savings in hardware cost, latency, and system complexity.

Previous studies refrained from using highly directive elements in adaptive beamforming systems since this will result in reduced beam scan range, significant scalloping effects, and considerable grating lobe problems. Element tilting to overcome these problems is first proposed by our group and an optimization tool determining the optimal tilt angles is developed that considers element radiation patterns, total number of array elements, and the number of elements used in the beamforming process as input parameters [4].

Fig. 2. The effect of element beam width on HSAA performance, Ns=2     and DOA spread is equal to 360

The proposed HSAA is first evaluated through computer simulations and possibility of achieving the same performance as a fully adaptive beamforming system is observed with using as low as one third of the array elements [3-4]. As a result of the computer simulations, the beamwidth of the array elements is identified as a critical parameter determining the system performance as shown in Fig. 2 since the bit error rate (BER) strongly depends on this parameter [3]. Subsequent computer simulations confirmed this result and indicated that the wider main lobe and higher side lobes in the beam pattern due to inclusion of fewer elements in the beamforming process can be narrowed down and suppressed with the use of highly directive elements [17].

   Fig. 3. A photo of the 2.4 GHz HSAA prototype (left), and the experiment setup in the antenna range (right)

Having evaluated the performance of the HSAA through computer simulations, an 8 channel wireless receiver prototype with HSAA capability is designed and built for experimental verification in the 2.4 GHz band [7,8]. The receiver prototype is built from scratch using off-the-shelf products and it has been fitted in a computer case together with the processing subsystem as shown in Fig. 3. The experiments conducted in the antenna range with the setup shown in Fig. 3 confirmed the simulation results indicating a performance gain with the use of highly directive array elements and confirming that the HSAA can achieve the same performance as a fully adaptive system with fewer array elements as shown in Fig. 4.

Fig.4 . Normalized receive power patterns versus the number of array

elements selected for AOA=100. 8 Dipole refers to the fully adaptive

system.

With these promising results, experimental verification efforts are focused on application of the HSAA to 60 GHz systems, and the 2-channel HSAA receiver prototype shown in Fig. 5 is designed and built for this purpose. As a result of experiments, it is found out that widely adopted far-field assumptions about antenna arrays may not hold for 60 GHz indoor applications due to close communication ranges, and employment of highly directive array elements [4]. This necessitates the development of a new digital signal formulation including the array geometry as an input variable. In these experiments, horn antennas are used due compensate for the low receiver gain, and the large element spacing created grating lobe problems. Moreover, due to very low beam widths of the horn antennas, meaningful beam scanning only in a 4 range is achieved as shown in Fig. 6. It is concluded that a receiver prototype with more channels employing less directive, lower gain array elements is required for beam scanning in a larger angular range.

Fig. 5. Photo of the 60 GHz HSAA prototype and the experimental setup Fig. 6. The receive beam patterns for various AOA

In parallel with the discussion above, the future tasks for this project include development of highly directional array elements that are small enough to prevent the grating lobes, and implementation of a full HSAA prototype at 60 GHz with 8 channels to provide beam steering capability in a larger angular range and to demonstrate the performance gain with respect to a fully adaptive system. A DSP based real-time implementation of the receiver and transmitter will be completed to further benchmark the performance of the proposed algorithm in terms of BER and tracking performance.

Deliverables

  1. A novel low cost, low complexity smart antenna algorithm for 60 GHz wireless systems
  2. A novel DSP formulation of the beamforming problem considering the array geometries and element radiation patterns
  3. Expertise in beam steering with highly directive array elements
  4. Design of highly directive array elements with less than half wavelength aperture size
  5. Optimization tools to determine innovative array geometries to provide the most efficient beamforming with lowest possible number of array elements
  6. A real-time 60 GHz wireless communication system prototype including the DSP implementation of the HSAA

Publications Related to this Project

M.F. Iskander, W. Kim, J. Bell, N. Celik, and Z.Q. Yun, “Antenna Arrays Technologies for Advanced Wireless Systems”, Modern Antenna Handbook, Constantine Balanis, Editor, chapter 25, John Wiley & Sons publication, July 2008

N. Celik, M. F. Iskander, and Z. Zhang, “Implementation and Experimental Verification of a Smart-Antenna System with Modulation,” IEEE Antennas and wireless Propagation Letters, vol.9, pp. 236-239, 2009.

N. Celik, and M. F. Iskander, “Genetic-Algorithm-Based Antenna Array Design for a 60 GHz Hybrid Smart Antenna System,” IEEE Antennas and Wireless Propagation Letters, vol. 7, pp.795-798, 2008.

N. Celik, M.F. Iskander, R. Emrick, S. Franson, and J. Holmes, "Implementation and Experimental Verification of a Smart Antenna System Operating at 60 GHz Band,” IEEE Transactions Antennas and Propagation, vol. 56, No.9, pp2790-2800, 2008.

N. Celik, W. Kim, M.F. Demirkol, M.F. Iskander, and R.Emrick, “Implementation and experimental verification of Hybrid-Smart Antenna Algorithm,” IEEE Antennas and Wireless Propagation Letters, vol. 5, pp. 280-283, 2006.

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