CHAPTER 6

1 53. CHAPTER 6. SMART ANTENNAS. Jack H. Winters INTRODUCTION. Throughout the world, including the United States, there is significant research and development on smart antennas for wireless systems. This is because smart antennas have tremendous potential to enhance the performance of future generation wireless systems as evidenced by the antennas' recent deployment in many systems. This CHAPTER covers smart antenna technology, including software and system aspects. First the two basic types of smart antennas, adaptive and phased arrays, are described and then their current use and proposed use in future wireless systems is discussed. Then the key research issues that came up at the workshop and at the various sites the WTEC panel visited are presented.

2 Finally, conclusions are presented along with the technology assessment of the , European, and Japanese companies. SMART antenna DESCRIPTION. There are two basic types of smart antennas. As shown in Fig. , the first type is the phased array or multibeam antenna , which consists of either a number of fixed beams with one beam turned on towards the desired signal or a single beam (formed by phase adjustment only) that is steered toward the desired signal. The other type is the adaptive antenna array as shown in Fig. , which is an array of multiple antenna elements, with the received signals weighted and combined to maximize the desired signal to interference plus noise power ratio. This essentially puts a main beam in the direction of the desired signal and nulls in the direction of the interference.

3 A smart antenna is therefore a phased or adaptive array that adjusts to the environment. That is, for the adaptive array, the beam pattern changes as the desired user and the interference move; and for the phased array the beam is steered or different beams are selected as the desired user moves. Nearly every company the WTEC panel visited is doing significant work in smart antennas. Indeed, some companies placed strong emphasis on this research. In particular, researchers at NEC and NTT stated that they felt that smart antenna technology was the most important technology for fourth generation cellular systems. Researchers at Filtronics and other companies agreed that smart antenna technology was one of the key technologies for fourth generation systems.

4 The reasons appear below. 54 6. Smart Antennas DESIRED SIGNAL. SIGNAL OUTPUT. Fig. Phased array. DESIRED SIGNAL. SIGNAL OUTPUT. INTERFERENCE. BEAMFORMER WEIGHTS. Fig. Adaptive array. FUTURE WIRELESS SYSTEM REQUIREMENTS. Future wireless systems generally may require higher data rates with better coverage for a wide variety of users operating with a large variety of different systems. To achieve these goals, greater power, interference suppression, and multipath mitigation are needed. As users operate at higher data rates, they need higher power for adequate reliability. For higher bandwidths, higher carrier frequencies that have higher propagation and circuit losses are needed. So some way to recover this power must be developed.

5 In addition, interference suppression is needed for higher capacities. Particularly as higher frequency reuse is used to increase capacity, there will be more cochannel interference, which requires greater interference suppression. Finally, multipath mitigation to have more reliable and robust communications is necessary. Jack H. Winters 55. SMART antenna ADVANTAGES. Smart antennas can help systems meet these requirements in the following manner: First, both phased and adaptive arrays provide increased power by providing higher gain for the desired signal. Phased arrays use narrow pencil beams, particularly with a large number of antenna elements at higher frequencies, to provide higher gain (power) in the direction of the desired signal.

6 Adaptive arrays place a main beam in the direction of the desired signal for an M-fold power gain with M antenna elements. In terms of interference suppression, phased arrays reduce the probability of interference with the narrower beam, and adaptive arrays adjust the beam pattern to suppress interference. For multipath mitigation, smart antennas can provide diversity, of which there are three basic types: spatial, polarization, and angle (or pattern) diversity. These appear in more detail below. USE OF SMART ANTENNAS. From the site visits and company workshop, it appears that phased arrays and adaptive arrays are considered and researched about equally. Although some companies studied only one type exclusively, others work both on phased and adaptive arrays.

7 Phased arrays are mainly being studied for point-to-point wireless systems, , for wireless local loops. They are also being considered for macrocellular base stations. For example, in Europe there is work on using 8-element phased arrays on GSM base stations. In Japan, there is work on using very large phased arrays on satellites, as well as on satellite terminals such as on car tops. Adaptive arrays are being studied for indoor systems, , systems with wide angular spread where the received signals arrive via widely separated paths where a phased array may not be useful in achieving gain. Also they are being studied in microcells and in some cellular base stations. For example, currently in the TDMA system ANSI-136 adaptive antenna algorithms have been widely deployed commercially in the United States.

8 Also adaptive arrays are being considered on cellular terminals where local scattering causes wide angular spread. KEY RESEARCH ISSUES. The site visits and the workshop raised several key research issues. The first research issue is cost, including the cost of power. For example, at Philips, researchers noted that 50% of the power in the handset is in the RF electronics. Therefore, multiple antennas in the handset not only increase the dollar cost of the handset, but also increase the power and thus decrease battery life. Research to reduce the power that each of these antennas requires needs to be undertaken. Similarly, the number of required receiver chains must be reduced because the RF electronics and the A/D converter required with each antenna are expensive.

9 One method being considered is a low-cost phased array. At higher frequencies, some companies are considering using large phased arrays to create very narrow beams to provide higher gain. But the issue is how to have, for example, hundreds of antenna elements and mass produce them at a reasonable cost. Thus, cost is limiting the number of antenna elements that can be used. Various solutions are being considered. For example, ATR is considering using optical beamforming for large phased arrays. Another solution being considered is integrating the antennas onto the RF electronics IC itself. Also, researchers at Ericsson are considering a limited introduction of smart antennas, because their research has shown that using smart antennas at just a small portion of the base stations, , those having capacity problems or creating the most interference, can achieve most of the gain of complete deployment.

10 In particular, Ericcson's results show that deploying smart antennas at only 10% of the base stations resulted in a 40% increase in capacity. 56 6. Smart Antennas The second key research issue is size. Large base station arrays are difficult to deploy for aesthetic reasons, and multiple external antennas on terminals are generally not practical. For base stations, companies are using dual polarization, but at the terminal some companies are researching putting antennas on the RF. electronics IC in an antenna -less terminal (since an external antenna is not present). However, issues of gain and efficiency and the effect of hand placement on the terminal need further research. The third issue is diversity, which, as discussed above, is needed for multipath mitigation.