Xiaohang Song

On Spatial Multiplexing of Strong Line-of-Sight MIMO With 3D Antenna Arrangements

In recent works, it has been shown that specific 2D antenna arrangements for multiple-input multiple-output (MIMO) systems can achieve similarly high spatial multiplexing gains under deterministic line-of-sight (LOS) conditions as non-line-of-sight channels with strong scattering considered in classical papers. However, the question whether 3D antenna arrays could provide an additional advantage was not addressed. In this work we show that the capacity of dominant LOS MIMO channels is invariant w.r.t. small offsets of the antenna elements along the transmit direction. This proves that the optimal 2D arrangements for point-to-point communication of LOS MIMO arrays are equivalent to 3D arrangements, whose projections of the antenna positions into a plane perpendicular to the transmit direction reproduce the optimal 2D arrangements. This insight also leads directly to the optimal designs for antenna arrays that communicate with each other along a transmit direction that is oblique w.r.t. the array plane(s).

A 60GHz LOS MIMO Backhaul Design Combining Spatial Multiplexing and Beamforming for a 100Gbps Throughput

In this work, a two-level hierarchical MIMO system is proposed to combine the spatial multiplexing gain and beamforming gain in a strong LOS channel. The superior is a MIMO system that consists of specially arranged sub-arrays to fully exploit the spatial multiplexing gain in deterministic channels. Additionally, a deterministic spherical-wave channel model is introduced. This channel model includes the radiation patterns of the sub-arrays, orthogonal phase relations introduced by the specific sub-array arrangement and the path loss considering deployment in practical scenarios. The attenuation includes the free space path loss, the oxygen absorption, the rain attenuation in bad weather and the front-end loss. The regulations for the maximum radiated power and the available bandwidth at 60 GHz were also investigated. Furthermore, the maximum transmission rate and upper bound of the energy efficiency are modeled and calculated for the proposed system operating at 60 GHz compliant to those regulations, as well for a constraint of the maximum available transmit power on-board. The result shows that the proposed system architecture is promising to achieve over 100 Gbps for macro-cell backhaul links with reasonable antenna sizes and high energy efficiency.

Network coding for the broadcast Rayleigh fading channel with feedback

A broadcast system with ACK/NAK feedback is considered where each link is modeled as a Rayleigh fading channel. The performance improvement is investigated if the erroneously received packets are not discarded and the soft information within them is exploited to reduce the number of transmission attempts. An outer bound on the packet reception rate region on the physical layer is provided by extending that region on the packet layer. A new transmission algorithm is proposed that combines log-likelihood ratios on the physical layer with network coding. This transmission algorithm does not require the transmitter to know any prior or instantaneous information about the channel and considers network coding on both the packet layer and the physical layer. Simulations show that the transmission algorithm with soft decoding achieves rates close to an outer bound for the packet reception rate region.

Analog and successive channel equalization in strong line-of-sight MIMO communication

In this work, we show a new design of analog equalizing network for N-stream strong Line-of-Sight MIMO communication, aiming at improved robustness. The design includes a core fixed equalizing network that equalizes ideal spatially orthogonal channels. Existing works show that the fixed equalizing network can equalize a spatially orthogonal MIMO system with parallel arrays. However, it is observed that such a fixed equalizing network is very sensitive to displacement errors. To make the system robust, state-of-the-art approaches use N2 fully controlled analog elements to perfectly equalize the channel via aligning the structured interferences. In this work, the terms causing the sensitiveness of fixed analog equalizing networks are identified. By compensating the tackled sensitive terms, the proposed design uses only 2N fully controlled analog elements and the robustness of the system is improved significantly. Meanwhile, by exploring the channel property, this work shows that if the spatially orthogonality is achieved by uniform rectangular arrays, the equalization can be applied with a new two-stage scheme. The scheme can be applied to spatially orthogonal MIMO systems with digital and/or analog equalization. Meanwhile, the computational complexity, required components number, as well as the complexity of the hardware design are significantly reduced.

Analog equalization and low resolution quantization in strong line-of-sight MIMO communication

In this work we show that the analog equalizing networks are suitable for low resolution quantization with limited mutual information loss in strong line-of-sight MIMO communication. More specifically, a simplified analog equalizing network design in comparison with state-of-the-art works is proposed in this work. Additionally, the new network design works equally good for larger displacement ranges. Furthermore, by using analog equalizing networks in line-of-sight MIMO systems, it is shown that the drawbacks of low resolution quantization can be minimized. This increases the energy efficiency of the analog-to-digital converters via minimizing the required magnitude resolution. Generally, low resolution quantization causes low entropy on the receive vectors which will effectively reduce the mutual information of the desired transmission. The analog equalizing network reshapes the distribution and reduces the dynamics of the received signals in the complex constellation plane before quantization. Therefore, the entropy loss after low resolution quantization is reduced and the system performs essentially as good as a system with higher resolutions. Finally, an algorithm is proposed to remove ambiguities which arise after analog-to-digital conversion with a given low resolution.

Strong LOS MIMO for Short Range mmWave Communication - Towards 1 Tbps Wireless Data Bus

In this paper, we propose a wireless data bus system design which relies on a strong Line-of- Sight MIMO approach. An analog MIMO equalizer, which equalizes the deterministic MIMO channel is involved. Instead of interference suppression, the analog MIMO equalizer aligns the phases of the received signals and enhances the desired signal while it suppresses the undesired ones simultaneously. Although the magnitudes of different signal components of the received signals are preferred to be unique, further studies are applied in our work to investigate the validation of our system design with non-unique magnitudes by using practical on-board antennas. It is shown that the proposed system works well with non-unique magnitudes where undesired remaining interference occurs with limited power in comparison with the desired ones. Furthermore, the proposed design shows a great potential for putting a 1 Tbps wireless data bus into practical systems with moderate transmit power and fairly simple modulation schemes which provide high energy efficiency.

High Throughput Line-of-Sight MIMO Systems for Next Generation Backhaul Applications

Abstract The evolution to ultra-dense next generation networks requires a massive increase in throughput and deployment flexibility. Therefore, novel wireless backhaul solutions that can support these demands are needed. In this work we present an approach for a millimeter wave line-of-sight MIMO backhaul design, targeting transmission rates in the order of 100 Gbit/s. We provide theoretical foundations for the concept showcasing its potential, which are confirmed through channel measurements. Furthermore, we provide insights into the system design with respect to antenna array setup, baseband processing, synchronization, and channel equalization. Implementation in a 60 GHz demonstrator setup proves the feasibility of the system concept for high throughput backhauling in next generation networks.

Interference-aware multi-iterative equalization and detection for frequency-selective MIMO channels

Turbo equalization has demonstrated to be a powerful approach for wireless transmission over frequency-selective channels introducing intersymbol interferences (ISI). Regarding multiple-input multiple-output (MIMO) systems, tree-search based MIMO detection techniques (e.g., sphere detection) are well suited for significantly reducing multi-antenna interferences. However, direct application of these detection techniques under the presence of ISI leads to vast processing complexity, increasing exponentially with the channel length and the number of transmitting antennas. In this paper, a low-complex two-stage approach is described splitting the interference reduction into a frequency-domain equalization stage and a time-domain sphere detection stage. Both are able to take a-priori information into account. Based on this, we derive a novel multi-iterative receiver, enabling powerful and adaptable processing with respect to the dominating interferences.