In the 6th generation (6G), demand for high-capacity wireless communication systems is growing due to the emergence of a wide variety of applications. Small-cell use with millimeter waves and higher frequency bands is key to increasing system capacity. To achieve a large number of small-cell installations, a flexible backhaul is required to support the foundation of high-speed 6G accesses. The orbital angular momentum (OAM) multiplexing technique is effective for achieving high-capacity wireless backhauls. This paper presents the field experimental results of real-time wireless transmission using a combination of OAM and polarization multiplexing in the E-band, from 71 to 86 GHz, using an OAM-multiplexing prototype. Experimental results indicate successful real-time transmission of 139.2 and 104.0 Gb/s at distances of 22.5 and 45 m, respectively. We also demonstrated real-time OAM multiplexing transmission in a reflection scenario using a reflector placed in the middle of the propagation path and successfully achieved 139.2 Gb/s at a distance of 22.5 m.

Millimeter-wave (mm-wave) band use is the key to meet the demand for high-speed wireless communication in future mobile networks. The mm-wave throughput performance is often unstable due to the influence of blocking objects, the fading effect, etc. A promising way to improve the throughput performance of the mm-wave channel is for multiple user equipment (UEs) to be used by the user and to integrate their links to achieve the extremely high requirements of the future mobile network. Since the mm-wave channel throughput strongly depends on where the UE is kept, the simple cooperation of multiple-UE-link of the same user enables stable throughput performance, while there is little gain in the multiple-UE-link cooperation in the micro-wave channel. In this paper, we evaluated the potential throughput of multiple-UE-link cooperation by using measured throughputs in 28 GHz channels in a fifth generation mobile communication system (5G) network. In addition, we propose the link selection by using measured reference signal received power (RSRP) cross-correlations between UE links to reduce power consumption. Experimental results show that the potential throughput of three UE links is 22.2 % greater than when using a single UE link at the 5%-ile cumulative distribution function (CDF). Also, we show that the proposed method achieves throughput performance with two UE links that is the same as when three UE links are used at the 5%-ile CDF and is 9.9% greater than that of the conventional link selection.

In orthogonal frequency division multiplexing (OFDM) with massive multi-input multiple-output (mMIMO), the reduction of the high peak-to-average power ratio (PAPR) is a challenging problem. Recently, an adaptive peak cancellation is proposed to reduce the transmitted signal's PAPR, while keeping the out-of-band leakage power (ACLR) as well as an in-band distortion power (EVM) below the predetermined and permissible value. In this paper, we propose an analytical method to evaluate achievable BER performance of downlink OFDM with the peak cancellation in massive multi-input-multioutput (mMIMO) systems using arbitrary numbers of transmit antennas and served users. In this method, bit error rate (BER) is derived based on the assumption that in-band distortion due to peak cancellation is approximated as random variable following Gaussian distribution, provided that variance of the Gaussian distribution in two user case is known. The results clarify that theoretical BER expressions for arbitrary numbers of transmit antennas and served users show good agreements with its simulation results. In addition, we clarified the impact of the increase of the number of transmit antennas on achievable BER and PAPR reduction capability in MRC precoded mMIMO-OFDM system with the peak cancellation.

Recently, an adaptive peak cancellation was proposed to reduce the high peak-to-average power ratio (PAPR), while keeping the out-of-band (OoB) power leakage as well as an in-band distortion power (EVM) below the pre-determined (permissible) level. However, the peak cancellation in MIMO-OFDM systems was not considered. In this paper, we propose a peak cancellation method for linearly pre-coded MIMO-OFDM systems using eigen-beam space division multiplexing (E-SDM). We evaluate and discuss the performance of the system using the proposed peak cancellation in terms of bit error rate (BER), complementary cum-mulative distribution function (CCDF) of PAPR and the system's computational complexity. Our results show the improvements with respect to both the achievable BER and PAPR with the proposed peak cancellation in E-SDM systems under the restriction of OoB power radiation.