Improving MIMO capacity with directive antennas for outdoor-indoor scenarios

• Published in: IEEE transactions on wireless communications Vol. 8; no. 5; pp. 2177 - 2181
• Main Authors: Hermosilla, C., Feick, R., Valenzuela, R., Ahumada, L.
• Format: Journal Article
• Language: English
• Published: New York, NY IEEE 01.05.2009; Institute of Electrical and Electronics Engineers; The Institute of Electrical and Electronics Engineers, Inc. (IEEE)
• Subjects: Antenna arrays; Antenna measurements; Antennas; Antennas and propagation; Applied sciences; Arrays; channel modeling; Channels; Detection, estimation, filtering, equalization, prediction; Dipole antennas; Dipoles; Directive antennas; Directivity; Exact sciences and technology; fading channels; Gain; Information, signal and communications theory; MIMO; MIMO capacity; Noise levels; propagation measurements; Radiocommunications; Rayleigh scattering; Receiving antennas; Searching; Signal and communications theory; Signal to noise ratio; Signal, noise; Systems, networks and services of telecommunications; Telecommunications; Telecommunications and information theory; Transmission and modulation (techniques and equipments); Fading channels; MIMO system; Parameter estimation; channel modeling; Wireless telecommunication; Outdoor installation; Transmission channel; Dipole antenna; Beam forming; Directivity; Directive antenna; Antenna gain; propagation measurements; Indoor installation; Free space propagation; Channel estimation; MIMO capacity; Radio communication; Signal to noise ratio
• ISSN: 1536-1276; 1558-2248
• DOI: 10.1109/TWC.2009.070662

MIMO systems are usually associated with high scattering isotropic propagation while the use of directive antennas is associated with free space conditions. We found outdoor-indoor channels to be in between these two extremes, in the sense that we observed directivity - and - MIMO gain, for the same ensemble of channels. Our observation is based on measurements with directive (8 dB) and dipole antennas. Median MIMO capacities were found to be about 80% of the ideal (Rayleigh i.i.d.), at 5 dB signal to noise ratio (SNR), for both types of antennas. Using properly aimed directive antennas, the SNR was found on average to be 5.4 dB above that obtainable with dipoles, somewhat less than the 7 dB antenna gain difference. Thus, isotropic propagation, which would have negated directivity gains, cannot be justified in general. We empirically established that aiming for largest received power is the best array pointing strategy with directive antennas. Combining MIMO processing and angular search resulted on average in gains of 70% over the median capacities obtained with dipoles. Therefore it may in some cases be convenient to arrange subgroups of antennas for beamforming, and then process the thus reduced number of radio channels for MIMO gain.