Title: The gain of cross-layer scheduling and advanced antenna techniques
Description: While the simultaneous transmission using multiple antennas in multiple-input/multiple-output (MIMO) systems has been known to increase the spectral efficiency significantly, this method inherently needs elaborated signal processing units as well as multiple RF units. This cost sometimes prohibits MIMO systems from deploying a large number of antennas. Therefore, among wireless transmission scenarios with multiple antennas, we investigate selection strategies since they are more scalable to the use of a large number of antennas. We first find the asymptotic performance of transmit antenna selection in Rayleigh fading channels. Then, we consider beam selection and show that beam selection outperforms antenna selection under a more general Rician channel assumption. To investigate transmit antenna selection systems, we compute the asymptotic distribution of antenna selection gain when the transmitter selects the transmit antenna with the strongest channel. We use this to asymptotically estimate the underlying channel capacity distributions. This estimate is compared with upper and lower bounds. This analysis demonstrates that unlike MIMO systems, the channel for antenna selection systems hardens at a slower rate, and thus a significant multiuser scheduling gain can exist - Θ(1/ log m ) for channel selection as opposed to Θ(1/ [Special characters omitted.] ) for MIMO, where m is the number of transmit antennas. Additionally, even without this scheduling gain, it is demonstrated that transmit antenna selection systems outperform open loop MIMO systems in low signal-to-noise ratio (SNR) regimes, particularly for a small number of receive antennas. This may have some implications on wireless system design, because most of the users in modern wireless systems have low SNRs.

In contrast to the first work considering only non-line-of-sight (NLOS) components, we next answer the question of how much we can improve our systems under the presence of line-of-sight (LOS) components. We consider beam selection using a fixed beamforming network (FBN) at a base station with m array antennas. In our setting, a Butler matrix is deployed at the RF stage to form m beams pointing at predetermined azimuthal angles, arid then the best beam is selected for transmission. We derive the distribution of the beam selection gain for this scenario under a Rician channel assumption as a function of both the azimuthal location of the remote unit and the Rician K -factor. We provide the key properties of the noncentral chi-square distribution and the resulting properties of the beam selection gain showing that beam selection is superior to antenna selection in Rician channels with any K -factors. Furthermore, we find asymptotically tight stochastic bounds of the beam selection gain, which yield approximate closed form expressions of the expected selection gain and the ergodic capacity. Beam selection has the order of growth of the ergodic capacity Θ(log m ) regardless of user location in contrast to Θ(log(log m )) for antenna selection.