Indoor Millimeter Wave MIMO: Feasibility and Performance

Abstract

Abstract-In this paper, we investigate spatial multiplexing at millimeter (mm) wave carrier frequencies for short-range indoor applications by quantifying fundamental limits in line-of-sight (LOS) environments and then investigating performance in the presence of multipath and LOS blockage. Our contributions are summarized as follows. For linear arrays with constrained form factor, an asymptotic analysis based on the properties of prolate spheroidal wave functions shows that a sparse array producing a spatially uncorrelated channel matrix effectively provides the maximum number of spatial degrees of freedom in a LOS environment, although substantial beamforming gains can be obtained by using denser arrays. This motivates our proposed mm-wave MIMO architecture, which utilizes arrays of subarrays to provide both directivity and spatial multiplexing gains. System performance is evaluated in a simulated indoor environment using a ray-tracing model that incorporates multipath effects and potential LOS blockage. Eigenmode transmission with waterfilling power allocation serves as a performance benchmark, and is compared to the simpler scheme of beamsteering transmission with MMSE reception and a fixed signal constellation. Our numerical results provide insight into the spatial variations of attainable capacity within a room, and the combinations of beamsteering and spatial multiplexing used in different scenarios.Index Terms-Millimeter wave communication, MIMO, channel capacity, antenna arrays.

Channel Models and Beamforming at Millimeter-Wave Frequency Bands

“…What makes the beamforming more relevant for future cellular applications is its potential for dense spatial frequency reuse; pointing a narrow, so-called pencil beams towards the intended user keeps interference to other users minimal, making multiple concurrent wireless data transfers feasible in a confined area [164], [165]. Furthermore, having electrically large antenna arrays at the Tx and Rx allows spatial multiplexing; not only in multipath channels, but also in LOS-only channels where the antenna arrays create multiple pencil beams that illuminate different parts of the antenna aperture on the other side [166]- [168]. When analyzing the multipath effects on beamforming performance, it is necessary to be aware of the interplay between the antennas and radio propagation since it determines the degree of channel capacity improvement through gain focusing and spatial eigenmode transmission [169], [170].…”

An efficient beam-training scheme for the optimally designed subarray structure in mmWave LoS MIMO systems

“…Although the second structure has a larger size than the optimally designed one, it still has a small performance loss due to the disadvantages of wider lobes. Figures 10 and 11 show the capacity as a function of θ and z 0 , where L t = L r = L = 64 is the same for all Tx and Rx subarrays and R = 100 m. The capacity obtained by the decomposition of the channel H into N non-interfering parallel eigenmodes in [9] is also plotted as the benchmark. In other words, the benchmark is calculated using a digital scheme.…”

“…While mmWave communications usually take place in the strong line of sight (LoS) circumstances where the channel responses are rank deficient, the spatial multiplexing gain can still be obtained by employing carefully designed antenna arrays thanks to the short wavelength [7]. Some efforts have been made on this topic, both for indoor scenarios [8,9] and outdoor scenarios [10].…”

Millimeter-Wave Communications: Physical Channel Models, Design Considerations, Antenna Constructions, and Link-Budget

“…For instance, BF is considered to be a key enabling technique for communications in the mmWave band [6], [15], [83], for the sake of combating the associated high path-loss [43]. BF is generally achieved by directing the transmitted signal towards the receiver, while suppressing it in the direction of the unintended receivers, which can be applied using digital, analogue, or combined digital and analogue beamforming techniques [84].…”

“…However, the advantage of ABF comes at the cost of handling only a single input data stream, which limits the signal processing and multiplexing capability of the system [43], [83], thus mmWave wireless systems can be further enhanced by relying on hybrid analogue-digital techniques, as proposed in [8], [13], [48], [90], [91]. Hybrid techniques empower digitally processing multiple data streams before being transmitted by the antenna arrays employed [49], [92].…”