The site visits and the U.S. 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. 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, e.g., those having capacity problems or creating the most interference, can achieve most of the gain of complete deployment. In particular, Ericcson's results show that deploying smart antennas at only 10% of the base stations resulted in a 40% increase in capacity.
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. For diversity, multiple antennas are needed on the base stations and/or terminals. As mentioned above there are three types of diversity: spatial, polarization, and angle (pattern) diversity. Spatial diversity3/4spatial separation of the antennas3/4is difficult on a small handset. Even though only a quarter wavelength separation is required for low correlation of the multipath fading between antennas on a handset, it also is difficult for base stations where the angular spread is small and large separation is required for low correlation. Spatial diversity is even more difficult to achieve in point-to-point systems where a near line-of-sight exists between the transmitter and receiver, and, further, at higher frequencies, sufficient spatial separation does not appear feasible. This problem can be partially avoided by the use of polarization diversity, where both vertical and horizontal polarizations are used to obtain dual diversity without spatial separation. For example, at Philips and other companies, researchers are using dual polarization diversity on handsets. Others are studying and implementing dual polarization on base station antennas. Polarization diversity provides only dual diversity, though polarization diversity can be used in combination with other forms of diversity to obtain higher orders of diversity.
Finally, companies are using angle diversity. That is, the signals from two or more beams (generally the beams with the highest signal powers) are used to obtain diversity. But performance depends on the angular spread. If the angular spread is small, then the received signal is mainly arriving on one beam and angle diversity will not provide a significant diversity gain. Also, some companies are studying pattern diversity, where antennas have different antenna patterns. In particular, researchers at Nokia are studying the use of multiple antennas in the handset, where some of the antennas may be covered by the hand, and moving the hand around changes the antenna pattern. These researchers believe that by adaptively combining the signals from such antennas, perhaps only using those antennas not blocked by the hand or adjusting the antenna impedance to compensate for hand placement, it may be possible to obtain much better performance (including diversity) with multiple internal antennas as compared to an external antenna.
A fourth issue is signal tracking, i.e., determining the angle-of-arrival of the desired signal with phased arrays to determine which beam to use and adjusting the weights with adaptive arrays to maximize the desired signal-to-noise-plus-interference ratio in the output signal. At none of the sites the panel visited did the researchers feel that signal processing power was a significant issue for tracking in future systems. Instead, they felt that increases in signal processing power would permit new tracking algorithms to be implemented without substantial consideration of the processing requirements. Researchers at Ericsson, for example, noted that, although angle-of-arrival techniques for phased arrays use MUSIC or ESPRIT algorithms in 2000, improvements are needed to make these algorithms more robust with angular spread and to obtain higher resolution. For adaptive arrays, better subspace tracking methods are needed since higher data rates will require longer temporal equalizers, which require longer training sequences and greater overhead.
A fifth issue is spatial-temporal processing, i.e., equalization of intersymbol interference due to delay spread at high data rates, with cochannel interference suppression. During the WTEC panel's European site visits it was noted that better architectures are needed for spatial-temporal processing, as current architectures have room for significant improvement. However, the use of OFDM is being considered for fourth generation systems (as brought up during Japanese site visits), which may simplify spatial-temporal processing at high data rates, but further research is needed. Also, space-time coding is an area of significant research, primarily in the United States, but research on improved interference suppression and tracking with these codes is needed. Finally, multiple transmit/receive antenna systems (referred to as multiple input multiple output (MIMO) or BLAST for the Lucent Bell Labs version) are being touted mainly in the United States as a means for achieving very high capacities in wireless systems. With MIMO, different signals are transmitted from each antenna simultaneously in the same bandwidth and then are separated at the receiver, thus increasing the potential to provide an M-fold increase in capacity without an increase in transmit power or bandwidth. For example, Lucent has demonstrated 1.2 Mbps in a 30 kHz channel in an indoor environment using 8 transmit and 12 receive antennas. To be useful in a wider variety of wireless systems, however, research is needed to extend the technique to the outdoor environment, including determining the multipath richness of this environment, which is required for the technique to work properly, and to the cochannel interference environment of cellular systems.
A sixth issue involves putting the necessary hooks in the standards such that smart antenna technology can be used effectively. In second generation cellular systems, ANSI-136 and IS-95, implementing smart antennas had problems because the standards did not consider their use. In particular, ANSI-136 required a continuous downlink signal to all three users in a frequency channel, which precludes the use of different beams for each of these three users. In IS-95, there is a common downlink pilot, which also precludes the use of different beams for each user, as all users need to see the pilot. For third generation systems, smart antennas were taken into account in WCDMA, where downlink pilots are dedicated to each user, and therefore smart antennas can be effectively used on the downlink. In the EDGE system, the continuous downlink requirement is no longer present, but some signals from the base station still need to be broadcast to all users. Thus, further research is needed to ensure that smart antennas can be effectively used in this system. For fourth generation systems, therefore, smart antennas must be taken into account in standard development. Specifically, any packet or multimedia access to all users, as well as pilots, must be transmitted or done in such a way as to not preclude the use of smart antennas, if this technology is to be used to its full benefit. Since these standards are international, research in this area needs to be done globally.
The previous issue leads to the seventh and final issue: vertical integration or an interdisciplinary approach. Research on smart antennas will require multiple factors/expertise to be considered-smart antennas cannot be studied in isolation. This issue was brought up repeatedly during the WTEC site visits. As discussed above, smart antennas must be considered in protocol development, i.e., expertise in both physical and media access layers is required. Also, smart antennas need to be considered in combination with other techniques, such as frequency hopping, power control, and adaptive channel assignment. Researchers at Nokia and Philips noted that smart antennas need to be considered in combination with RF matching, particularly with multiband antennas. At Nokia, the issue of adapting the antennas to hand position was noted. Ericsson has studied the limited introduction of smart antennas with nonuniform traffic. Another issue was the interaction when ad hoc networks are used. Furthermore, propagation measurements and channel modeling are needed to determine the performance of smart antennas in specific environments. Issues of base station versus terminal antenna (complexity) tradeoffs were also noted, as well as transmit diversity with space-time coding. From the above issues, it seems important that smart antenna research be multidisciplinary. However, few people have such a wide range of expertise, and it is often difficult for researchers with such different expertise to work together effectively. Thus, even though the critical need for such research was noted over and over again worldwide, there were few instances in any region where this was being done, or even planned in the future, as this type of research is different from the general method used in the past. Thus, this type of research appears to require a change of approach, but there was general agreement that the companies that can do this will make the greatest progress in smart antennas.