A. J. Paulraj: When I joined Stanford University in 1992, mobile wireless technology was beginning to attract attention and it seemed to be a good area to work in. Moreover, I had worked on directions-of-arrival estimation algorithms exploiting multiple antennas during an earlier visit to Stanford. So, the combination of multiple antennas and wireless appeared to be a terrific area to explore. Very quickly, the idea of using multiple antennas to implement spatial multiplexing came to me, and I knew from back-of-the-envelope calculations that the potential for such ideas was indeed high. I was lucky to attract brilliant students, including Dr. Papadias, who joined us as a postdoc and helped build our group.

“Mobile broadband will be the big trend in the next five years and will usher in a whole new range of services dominated by entertainment and multimedia.”

C. B. Papadias: I got involved with wireless systems initially during my Ph.D. thesis at ENST and Eurecom in France, and later, in 1995, when I joined Stanford’s Smart Antennas Research Group (SARG). This group, headed by Professor Paulraj, is a world-renowned team in the area of smart antennas, which is a key ingredient of 3^rd-generation (3G) and next-generation wireless networks, as well as of defense wireless systems.

The paper presents the main attributes of smart antenna systems that were known at the time, i.e., their potential to improve the coverage, capacity, and overall performance of wireless networks. The emphasis is on the signal processing techniques that are required on the transmitter and receiver in order to realize these benefits. After a short description of the main features of wireless channels, the paper goes on to provide a categorization of smart antenna scenarios. This classification takes into account the number of antenna elements residing on each side of the wireless link, the direction of communication (i.e., forward and reverse link), the co-existence of multiple users (i.e., single and multiple user systems), etc. The appropriate signal models for these categories are presented, including the effect of interference.

We also discuss the concept of space-time structures, which can in principle aid the performance of smart antenna systems. Then we describe a number of signal processing algorithms that apply to different scenarios; these techniques are again categorized according to different criteria (linear vs. maximum likelihood, trained vs. blind, etc.) and applications (interference mitigation, direction finding, etc.). We also discuss techniques as applied to the two dominant wireless communication multiple access schemes at the time, TDMA, CDMA. Finally, we provide a discussion of the dominant commercial approaches of space-time processing at the time (e.g. switched beam and SDMA systems), as well as the emerging industry trends. We predict that 3G systems would present a great opportunity to incorporate smart antenna (or space-time processing, as we preferred to call them) systems.

The paper provided an overview of the state of the art of signal processing for smart antenna systems, and as such was a good starting point for many people who worked in this field after its publication. Many of the smart antenna techniques that were incorporated in real systems since were somehow covered by the paper. Also, the presented categorization helped identify the so-called MIMO (Multiple Input Multiple Output) systems that emerged soon after and gave a new big push to the field. Our prediction about smart antennas penetrating 3G wireless networks came true, even though the specific ways in which this happened had not been invented at the time.

One could say that, due to its timing, this paper in some sense sealed what could be called the "pre-MIMO" era of wireless networks, by presenting the essential know-how in the smart antenna field before the invention of MIMO systems. Both of us have continued to work on many aspects related to wireless networks since and have devoted a large fraction of our careers (both in the industry and in academia) to space-time processing, since it became an almost integral part of most discussions related to the design of next-generation wireless networks. In particular, Professor Paulraj was one of the very inventors of the MIMO concept himself. Several of the ideas that we worked on ended up influencing 3G cellular and WiFi standards and beyond.

A. J. Paulraj: My research group now works on multiple antenna techniques in broadband wireless networks. In particular, OFDMA-based systems incorporating opportunistic scheduling. We also have a new program to study very wideband MIMO channels. I have also been spending part of my time in industry off and on. My current company, Beceem Communications Inc., builds mobile WIMAX chips sets.

C. B. Papadias: I currently work on several topics related to next-generation wireless networks, such as: space-time coding, re-configurable wireless, joint beamforming and scheduling systems for 3G and WiFi systems, mesh/multihop wireless networks, context-aware networks, cognitive radio networks, etc.

Mobile broadband will be the big trend in the next five years and will usher in a whole new range of services dominated by entertainment and multimedia. The demand for significantly higher data rates at the same or better coverage levels available today creates a strong pull for multiple antenna technologies. In the longer term, say 10 years, seamless integration of different wireless networks, giving the user a completely seamless experience, will become a reality.