SIGNAL ENHANCEMENT IN MULTIUSER COMMUNICATION THROUGH ADAPTIVITY ON TRANSMIT Abdellatif Medouri 1 and Tapan S. Sarkar 2 1 Ecole National des Sciences Applique ´ es Abdelmalek Essadi University B.P. 416 Tangier, Morocco 2 Dept. of Electrical Engineering & Computer Science Syracuse University 121, Link Hall Syracuse, NY 13244-1240 Received 31 January 2003 ABSTRACT: This paper presents a technique to enhance the received signals in a multiuser communication environment through the use of adaptivity on transmit. This technique is based on the principle of reci- procity and is independent of the material medium of transmission and the near-field environments. The objective here is to select a set of weights to be applied to each transmitting antenna, which is a function of the user location. The methodology is not a function of the multipath environment. Furthermore, the transmitted signal may be directed to a particular receiver location and simultaneously be minimized at other receiver locations while operating on the same frequency. Numerical simulations have been made to illustrate the novelty of the proposed approach. © 2003 Wiley Periodicals, Inc. Microwave Opt Technol Lett 38: 265–269, 2003; Published online in Wiley InterScience (www. interscience.wiley.com). DOI 10.1002/mop.11034 Key words: adaptive antennas; wireless communications; multiuser communications 1. INTRODUCTION Many methods to enhance reception of signals in a multiuser environment [1, 2] have been developed and are continuing to evolve in recent years. This is necessary for mobile communica- tion in order to enhance the quality of the reception in the presence of multipath fading, near-field scatterers, and so on. In order to have such enhancements it is generally necessary to have spatial diversity. One way to counteract the problems of interference at the receiver due to multipath and the presence of other near-field scatterers such as buildings, trees, platforms, and so on, is to provide spatial diversity, as shown in Figure 1. However, at a mobile receiver there is generally no spatial diversity because the footprint of the receiver is quite small. On the other hand, it is possible to have multiple antennas located at a base station. By weighting the signals being fed to each of the antennas, it is possible to provide the necessary spatial diversity on transmit. Thus, in this paper we attempt to address the problem of what can be done in terms of adaptivity on transmit in order to direct the energy destined for a pre-specified receiver and simultaneously minimize the reception of that same energy at other receivers. In this way, transmitted energy will be effectively re- ceived at the designated receiver while it would cancel out at the other locations. By providing spatial diversity on transmit, it is possible to mitigate the effects of multipath fading, as the directed energy from the transmitted antennas would combine vectorially at the selected receiving element to produce either a maximum or a minimum. Many available papers and books discuss the problem of spatial diversity for wireless communication [1, 3]. In addition, methods to counteract multipath fadings in an adaptive antenna which can significantly improve the performance of a system, have also been discussed [4 –7]. In this paper, we present an alternate way of directing the energy from the transmitter to a specific receiving antenna while simultaneously minimizing the signal directed to- wards the other receiving antennas. This methodology is based on the reciprocity theorem [8 –9] and has been applied to a collection of receiving and transmitting antennas. The scenario has been simulated using the WIPL-D code [10], which can provide a complete description of the voltages and currents on the structures. Numerical examples have been presented to illustrate this novel methodology. 2. DESCRIPTION OF THE PROPOSED METHODOLOGY In this procedure we simultaneously employ the concept of reci- procity and use the principle of adaptivity on transmit. In this way, it is possible to direct the energy transmitted from a base station to a preselected mobile station without worrying about either the presence of other near field scatterers or the existence of a mul- tipath environment. For example, consider the system represented in Figure 1. Here let us assume, for example, that the transmitter is sending signals using spatial diversity, that is, using a number of antennas. We assume that the transmitting antenna #1 is transmit- ting a voltage V T 1 at a frequency f 0 . The internal impedance of this voltage source is R T 1 . At the second antenna, we have a voltage source V T 2 transmitting at the same frequency with an internal impedance of R T 2 . In addition, there are two receivers represented by R 1 and R 2 . The goal of this transmitting system is that we want to maximize the received power at receiver 1 by exciting trans- mitting systems 1 and 2 with voltages V T 1 1 and V T 2 1 . Our goal is to minimize the received power at receiver 2 while directing the energy to receiver 1. Similarly, we want to find another set of voltages V T 1 2 and V T 2 2 which will excite the two transmitting an- tennas. The characteristics of these two voltage sets are such that they will direct the signal to receiver 2 and will induce little or no energy at receiver 1. In this way, we can essentially do adaptivity on transmit. This is ideally suited, for example, to a CDMA environment where a unique code is assigned to each receiver, so that when we receive any signal we know from which receiver it is from. When we are going to transmit that particularly weighted signal simultaneously broadcast over all the transmitting antennas, we can spatially direct the energy to a prespecified receiver only and try to minimize the received signals at the other receivers. In this way, it is possible to direct the energy to a preselected receiver. Figure 1 A multiple user transmit/receive scenario MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 38, No. 4, August 20 2003 265