Abstract
Equivalent discrete-time models for a variety of spatial combining techniques operating in a frequency-selective multipath fading channel are derived. The equivalent discrete-time models are used to perform computer simulations of the post-equalizer bit error rate over a frequency-selective multipath channel whose derivation preserved polarization state information. Two sets of computer simulations were performed. In the first set, the performance of co-located cross-polarized antenna elements was investigated. The results showed that maximum likelihood combining maximizes polarization diversity, but that maximum ratio combining and selection combining were very competitive in the case where the cross-polarized antennas produce one strong channel and a relatively weak channel. Elliptical combining, using a 90° hybrid coupler, produced the worst results. The second set of simulations used a combination of spatial and cross-polarized antenna elements, for a total of eight antenna elements. The simulation results showed that maximum likelihood combining was best, followed by maximum ratio combining, equal gain combining, and selection combining. Again, elliptical combining was the worst, leading to the conclusion that other combining techniques are preferred in frequency-selective fading environments.
Highlights
To support high data-rate downlinks in 5G systems [1], wideband channels are required
The performance of spatial diversity over a frequencyselective multipath fading channel has been investigated where an emphasis has been placed on 5G cellular systems operating in the mmWave band in a small cell urban environment
The propagation scenario used as an example is a downlink to a mobile equipped with four pairs of co-located cross-polarized antennas
Summary
To support high data-rate downlinks in 5G systems [1], wideband channels are required. The multipath modeling and analysis performed in [12]–[18] confirm the frequencyselective nature of the fading and the need for equalization [19] For this reason, the examples used in the paper focus on operation in the mmWave band at 28 GHz. One of the greatest challenges of using the mmWave band for this application is the requirement of line-of-sight (LOS) propagation between the basestation and the mobile. Single-carrier modulations have a performance advantage when operating over frequency-selective channels [28] in the sense that single-carrier modulations require no coding or high-rate codes in contrast to multicarrier modulations that require relatively power codes to achieve acceptable bit error rate performance These three observations recommend singlecarrier modulations as a possibility in mmWave systems.
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