With the evolution of the mobile networks towards more dense and flexible configurations, novel wireless backhaul solutions that can match the high capacity and flexibility demands are required in addition to the fixed fiber optics solutions. In this work we analyze the core backhaul requirements and the related system design challenges for utilizing the mm-wave frequency band due to the large available bandwidth. We determine the key performance metrics in terms of achievable throughput and energy efficiency of several transmission schemes seeking for a viable solution for the small cell backhaul scenario. Finally, an insight to the structure of the baseband domain required for processing multiple transmitted streams at the full system bandwidth is presented for a proposed LOS SM-MIMO system concept.

Next generation mm-Wave wireless backhaul based on LOS MIMO links

The dawn of 5G requires significantly higher data rates, not only for the Radio Access Technology (RAT), but also for the transport network. Line-of-Sight MIMO links based on mmWave radios promise reaching ultra-high throughput with reasonably small antennas. mmWave links with optimally arranged antennas are one viable concept for spatial multiplexing, capable of potentially achieving Gb/s data rates. In the context of mmWave MIMO system design, providing hardware implementation solutions is essential for bridging the gap from theoretical concepts to real-time operational prototypes. In this paper, we focus on the investigation of the BER performance of an orthogonal spatial-multiplexing LoS MIMO link and its robustness to x- and z-axis antenna displacements. Hardware-in-the-loop measurements are performed with a custom demonstrator built for 60GHz LoS MIMO system verification achieving system data rates of 2.5Gb/s at uncoded BER of 1e−4. The results largely confirm the theoretical assumptions and the feasibility of LoS MIMO for short to medium range wireless backhaul applications.

Hardware-in-the-loop demonstration of a 60GHz line-of-sight 2×2 MIMO link

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.

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

This work focuses on the initial beam acquisition/alignment of millimeter wave (mmWave) communication systems. To detect the angle of arrival (AoA) and/or angle of departure (AoD), we propose a training protocol which probes all beamformers from a given codebook simultaneously by exploiting the sparse nature of mmWave channels. By applying a frequency-selective beam probing network, we can map each beamformer from the codebook to different frequencies and a spectral analysis at the receiver allows us to deduce favorable beamformers or AoDs. For practical reasons, we elaborate this idea of steering direction to frequency mapping for an orthogonal frequency division multiplexing (OFDM) communication system, i.e., we map each beamformer to specific pilot subcarriers. Under two different hardware designs, we investigate the feasibility of building such beamformer to frequency mappings for one additional radio frequency (RF) chain next to an existing OFDM communication system. We show that parallel beam training is able to achieve better effective transmission rates than exhaustive search in fast-time varying environments due to high temporal efficiency. This is crucial for mmWave communication systems which have access to large beamforming codebooks but suffer from short coherence times due to mobility and high spatial resolution.

Frequency-Selective Analog Beam Probing for Millimeter Wave Communication Systems

This paper presents a link adaptation algorithm dedicated for 100 Gbps wireless transmission. Interleaved Reed-Solomon codes are selected as forward error correction (FEC) algorithms. The redundancy of the codes is selected according to the channel bit error rate (BER). The uncomplicated FEC scheme allows implementing a complete data link layer processor in an FPGA (field programmable gate array). In our case, we use the Virtex7 FPGA to validate the functionality of our implementation. The proposed FPGA-processor achieves 169 Gbps throughput. Moreover, the implementation is synthesized into 40 nm CMOS technology and the described link adaptation algorithm allows reducing consumed energy per bit to values below 1 pJ/bit at BER

Improving energy efficiency using a link adaptation algorithm dedicated for 100 Gbps wireless communication

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.

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

On the Mapping between Steering Direction and Frequency of a Uniform Linear Array with Fixed True Time Delays.

Fast Beam Alignment through Simultaneous Beam Steering and Power Spectrum Estimation Using a Frequency Scanning Array.

In this work, we investigate the energy efficiency (EE) of future wireless communications systems. We argue that operating at high bandwidth and a comparatively low signal-to-noise ratio (SNR) is beneficial in terms of EE. Furthermore, we discuss how employing single-carrier modulations combined with 1-bit quantization and temporal oversampling at the receiver enables energy-efficient wideband wireless communications systems. Our main contribution is to compare the EE of such a system to an idealized conventional system under various assumptions on the ADC power consumption. Our numerical results suggest that the EE can be improved significantly by employing 1-bit quantization and oversampling at the receiver at the cost of increased bandwidth.

Christoph Jans

Papers

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

Frequency-Selective Analog Beam Probing for Millimeter Wave Communication Systems

On the Mapping between Steering Direction and Frequency of a Uniform Linear Array with Fixed True Time Delays.

Fast Beam Alignment through Simultaneous Beam Steering and Power Spectrum Estimation Using a Frequency Scanning Array.

Enabling Energy-Efficient Tbit/s Communications by 1-Bit Quantization and Oversampling