SMART ANTENNA SYSTEM USING FIVE-PORT REFLECTOMETER A. Judson Braga, Van Yem Vu, Bernard Huyart, J. C. Cousin, and R. Freire COMELEC – ENST Paris, 46 rue Barrault 75634 Paris Cedex 13 – France Abstract The difficult design of variable phase shifters for smart antenna applications makes digital beamforming systems an interesting way for future communication systems that aim to provide better network performance. Five-port reflectometers use a third redundant mixer to decrease the system dependency from the phase and amplitude unbalance of local oscillators. We introduce a digital beamforming system for reception array using the five-port technique which performs a direct conversion of RF signals. The system performance is tested by measuring its bit error rate for different number of antenna array elements. Index Terms — array signal processing, direction of arrival estimation, five-port circuits, smart antenna system. I. INTRODUCTION New communication systems will allow users to access multimedia services with high data rate such as internet, online shopping, video telephony, etc … New challenges for the improvement of the communication link emerge as a consequence of that such as interference reduction, range and capacity increasing and energy reduction, for example. Smart antennas will be massively employed in the next years as a high performing solution for these challenges [1]. The usual communication systems use omnidirectional and sectorized antenna in base station while beamforming systems use antenna array to direct the main lobe of the diagram pattern toward the useful signal direction and nulls at jammer directions. We introduce a digital beamforming system for reception array using the five-port technique instead classical I-Q demodulators. As I-Q systems, the five-port reflectometers also perform a direct conversion but using a redundant mixer and a 120 degrees basis [4] instead the cartesian basis. This redundancy makes the five-port system more robust to phase and amplitude unbalance of the local oscillator and consequently less sensible to the error vector magnitude (EVM). Other advantage of the present system is the avoidance of phase shifters which are generally not easy to be designed. This system’s drawbacks concern the number of needed demodulators that must be equal to the number of antennas, the quantification noise which is worst for the reason that the jammer signals are also demodulated, and the DC offset intrinsic in five-port circuits because power detectors are used as mixers. The beamforming which is carried out in baseband after five-port demodulation uses a non real-time process. Originally designed to measure the reflection coefficient of microwave network ports, the six-port reflectometer has recently been applied in wireless receivers [2] and anti- collision radars [3] as phase/frequency discriminator. If the local oscillator (LO), used as reference of phase and frequency, is stable enough so that its power can be assumed to be constant, one port of the reflectometer may be neglected and the applications previously mentioned may be well accomplished in five-port system [4]. The complex envelope information when using five-port technique is obtained by making only amplitude (or power) measurements of three different linear combinations of RF and LO electromagnetic waves. This means that a five-port reflectometer is in principle simply a passive linear circuit with two input ports and three output ports (hence its name). The five-port reflectometer and the measurement system are described in section II. The estimation procedure of the direction of arrival (DoA) of the signals is supposed to be known [5] and the algorithm used to evaluate the weighting factors is presented in section III followed by experimental results and conclusions in sections IV and V, respectively. II. THE SYSTEM MODEL The five-port system, shown in Fig. 1, conceived in microstrip technology is madeup of a five access interferometric ring. Two of theses accesses are used for RF and local oscillator inputs and three for baseband outputs. It is also madeup of three diode power detectors and three low pass filters in the output of the system. This system generates a signal in the digital domain representing the complex ratio between the two input signals RF and LO as a linear combination of the three baseband outputs. This complex ratio is computed in 121 0-7803-9342-2/05/$20.00 © 2005 IEEE