Mobile data traffic will continue its tremendous growth in some markets, and has already resulted in an apparent radio spectrum scarcity. There is a strong need for more efficient methods to use spectrum resources, leading to extensive research on increasing spectrum reusability on flexible radio platforms. This study solves this problem in two sub topics, millimeter wave communication on wireless backhaul for spectrum reusability, and flexible prototyping radio platform using software-defined radio (SDR).
Wireless backhaul has received significant attention as a key technology affecting the development of future wireless cellular networks because it helps to easily deploy many small size cells, an essential part of a high capacity system. Millimeter wave is considered a possible candidate for cost-effective wireless backhaul. In the outdoor deployment using a millimeter wave, beamforming methods are key techniques to establish wireless links in the 60 GHz to 80 GHz to overcome pathloss constraints (i.e., rainfall effect and oxygen absorption). The millimeter wave communication system cannot directly access the channel knowledge. To overcome this, a beamforming method based on codebook search is considered. The millimeter wave communication cannot access channel knowledge, therefore alternatively a beamforming method based on a codebook search is considered. In the first part, we propose an efficient beam alignment technique using adaptive subspace sampling and hierarchical beam codebooks. A wind sway analysis is presented to establish a notion of beam coherence time. This highlights a previously unexplored tradeoff between array size and wind-induced movement. Generally, it is not possible to use larger arrays without risking a performance loss from wind-induced beam misalignment. The performance of the proposed alignment technique is analyzed and compared with other search and alignment methods. Results show significant performance improvement with reduced search time.
In the second part of this study, SDR is discussed as an approach toward flexible wireless communication systems. Most layers of SDR are implemented by software. Therefore, only a software change is needed to transform the type of radio system. The translation of the signal processing into software performed by a regular computer opens up a huge number of possibilities at a reasonable price and effort. SDR systems are widely used to build prototypes, saving time and money. In this project, a robust wireless communication system in high interference environment was developed. For the physical layer (PHY) of the system, we implemented a channel sub-bandding method that utilizes frequency division multiplexing to avoid interference. Then, to overcome a further interfered channel, Direct Spread Spectrum System (DSSS) was considered and implemented. These prototyped testbeds were evaluated for system performance in the interference environment.

Millimeter wave beamforming for wireless backhaul and access in small cell networks and practical approaches in software-defined radio

This paper investigates the application of non-orthogonal multiple access in millimeter-Wave communications (mmWave-NOMA). Particularly, we consider downlink transmission with a hybrid beamforming structure. A user grouping algorithm is first proposed according to the channel correlations of the users. Whereafter, a joint hybrid beamforming and power allocation problem is formulated to maximize the achievable sum rate, subject to a minimum rate constraint for each user. To solve this non-convex problem with high-dimensional variables, we first obtain the solution of power allocation under arbitrary fixed hybrid beamforming, which is divided into intra-group power allocation and inter-group power allocation. Then, given arbitrary fixed analog beamforming, we utilize the approximate zero-forcing method to design the digital beamforming to minimize the inter-group interference. Finally, the analog beamforming problem with the constant-modulus constraint is solved with a proposed boundary-compressed particle swarm optimization algorithm. The simulation results show that the proposed joint approach, including user grouping, hybrid beamforming and power allocation, outperforms the state-of-the-art schemes and the conventional mmWave orthogonal multiple access system in terms of achievable sum rate, and energy efficiency.

Millimeter-Wave NOMA With User Grouping, Power Allocation and Hybrid Beamforming

This paper investigates the application of non-orthogonal multiple access in millimeter-Wave communications (mmWave-NOMA). Particularly, we consider downlink transmission with a hybrid beamforming structure. A user grouping algorithm is first proposed according to the channel correlations of the users. Whereafter, a joint hybrid beamforming and power allocation problem is formulated to maximize the achievable sum rate, subject to a minimum rate constraint for each user. To solve this non-convex problem with high-dimensional variables, we first obtain the solution of power allocation under arbitrary fixed hybrid beamforming, which is divided into intra-group power allocation and inter-group power allocation. Then, given arbitrary fixed analog beamforming, we utilize the approximate zero-forcing method to design the digital beamforming to minimize the inter-group interference. Finally, the analog beamforming problem with the constant-modulus constraint is solved with a proposed boundary-compressed particle swarm optimization algorithm. Simulation results show that the proposed joint approach, including user grouping, hybrid beamforming and power allocation, outperforms the state-of-the-art schemes and the conventional mmWave orthogonal multiple access system in terms of achievable sum rate and energy efficiency.

Millimeter-Wave NOMA with User Grouping, Power Allocation and Hybrid Beamforming

Over the last few years, interference has been a major hurdle for successfully implementing various end-user applications in the fifth-generation (5G) of wireless networks. During this era, several communication protocols and standards have been developed and used by the community. However, interference persists, keeping given quality of service (QoS) provision to end-users for different 5G applications. To mitigate the issues mentioned above, in this paper, we present an in-depth survey of state-of-the-art non-orthogonal multiple access (NOMA) variants having power and code domains as the backbone for interference mitigation, resource allocations, and QoS management in the 5G environment. These are future smart communication and supported by device-to-device (D2D), cooperative communication (CC), multiple-input and multiple-output (MIMO), and heterogeneous networks (HetNets). From the existing literature, it has been observed that NOMA can resolve most of the issues in the existing proposals to provide contention-based grant-free transmissions between different devices. The key differences between the orthogonal multiple access (OMA) and NOMA in 5G are also discussed in detail. Moreover, several open issues and research challenges of NOMA-based applications are analyzed. Finally, a comparative analysis of different existing proposals is also discussed to provide deep insights to the readers.

/pdf/a-systematic-review-on-noma-variants-for-5g-and-beyond-55xdyacniv.pdf

A Systematic Review on NOMA Variants for 5G and Beyond

The integration of non-orthogonal multiple access in millimeter-Wave communications (mm Wave-NOMA) can significantly improve the spectrum efficiency and increase the number of users in the fifth-generation (5G) mobile communication and beyond. In this paper, we consider a downlink mm Wave-NOMA cellular system, where the base station is mounted with an analog beamforming phased array, and multiple users are served in the same time-frequency resource block. To guarantee user fairness, we formulate joint beamforming and power allocation problem to maximize the minimal achievable rate among the users, i.e., we adopt the max–min fairness. As the problem is difficult to solve due to the non-convex formulation and high dimension of the optimization variables, we propose a sub-optimal solution, which makes use of the spatial sparsity in the angle domain of the mm Wave channel. In the solution, the closed-form optimal power allocation is obtained first, which reduces the joint optimization problem into an equivalent beamforming problem. Then, an appropriate beamforming vector is designed. The simulation results show that the proposed solution can achieve a near-upper-bound performance in terms of achievable rate, which is significantly better than that of the conventional mm Wave orthogonal multiple access (mm Wave-OMA) system.

User Fairness Non-Orthogonal Multiple Access (NOMA) for Millimeter-Wave Communications With Analog Beamforming

We consider a 5G millimeter-wave (mmWave) massive MIMO system with subconnected hybrid structure, where each transceiver unit (TXRU) is connected to a subarray at the transmitter (TX) or the receiver (RX). Beam training is the first step for codebook-based beamforming in the absence of ideal channel state information (CSI). The complexity of the optimal exhaustive search beam training is exponentially increased with the number of TXRUs (or subarrays) at the TX and the RX, which is infeasible in practice. The conventional beam training schemes are mainly based on the idea of mathematical search algorithms and novel beam codebook designs. In this study, we explore a novel low-complexity beam training scheme by fully exploiting the sparse nature of the mmWave channel and the strong correlations of large-scale antenna arrays. Specifically, we propose a training scheme to reduce beam search space for 5G mmWave system, which consists of the following two key stages: 1) a beam subset optimization method from the perspective of array gain for capturing the energy of dominant channel propagation paths, and 2) a class of linear iterative beam search algorithms within the optimized beam subset utilizing the idea of limiting the degrees of freedom of the subarray indices during beam training. The range of the optimized beam subset size and linear search condition are also given. The performances of the proposed two algorithms, beam subset optimization-single subarray linear search and beam subset optimization-subarray pair linear search, are evaluated in a 5G mmWave system. Analysis has shown that the complexity of the proposed scheme is linear with the number of channel propagation paths, which can considerably reduce the complexity of conventional schemes. Simulation results have verified the near-optimal performance of the proposed schemes when compared with exhaustive search as well as other existing training methods.

Low-Complexity Beam Training for 5G Millimeter-Wave Massive MIMO Systems

In this paper, we propose a non-orthogonal multiple access(NOMA) based hybrid beamforming design in 5G mmWave systems. The proposed design depends on the known array geometry and incurs a low training and feedback overhead. Our model assumes that each user employs analog-only beamforming while the BS performs hybrid analog and digital beamforming. To achieve a higher sum capacity, multiple users are allowed to share the same beam based on NOMA mechanism. However, the use of analog beamformer from different localized users means that the BS's digital beamformers are not perfectly aligned with the users' baseband effective channels and multiple users may be assigned similar or even identical beamformers. In the meantime, a user pairing and power allocation algorithm is proposed to mitigate interference from other beams so as to maximize the sum capacity. Simulation results verify the significant advantage of the proposed scheme in sum capacity.

Non-orthogonal multiple access based hybrid beamforming in 5G mmWave systems

Hybrid beamforming architecture that combines analog beamforming and digital beamforming has been widely accepted in the emerging millimeter-wave (mmWave) systems with large antenna arrays. However, the design of codebooks and beam training procedures at the analog stage is problematic in mmWave system. In this study, we propose a feasible two-stage 3D codebook design consisting of a primary codebook and an auxiliary codebook. The primary codebook generates a basic directional beam, and the number of phase shifts is quite limited in order to keep low hardware complexity. The small-size auxiliary codebook creates the finer beams that are centered on each primary beam. Furthermore, a beam search scheme designed for the two- stage codebook is proposed. We use a binary tree search method with antenna selection for the primary beam search, and a directed beam scan for the auxiliary beam search. Simulation results verify the performance advantages of our proposed codebook in beamforming gain and spectral efficiency for multi-user multiple-input multiple-output (MU-MIMO) systems. Moreover, training complexity comparisons indicate that our specially designed two-stage beam search scheme significantly outperforms other existing solutions.

Two-Stage 3D Codebook Design and Beam Training for Millimeter-Wave Massive MIMO Systems

In an overloaded MU-MIMO system with hybrid beamforming that supports a very large number of users, user scheduling is necessary as the base station (BS) cannot simultaneously serve such a large number of users. Since hybrid beamforming provides a new resource to schedule, i.e. beams, user pairing becomes a joint optimization on user selection and beam selection, which is computationally demanding. In this paper, a low-complexity two-stage user scheduling scheme considering both the inter-user channel correlation and the channel energy is proposed. The idea is motivated by the fact that both the orthogonality between the user channels and the channel energy can affect the system achievable sum rate in a MU-MIMO system. Therefore, we propose to firstly sort the users from the perspective of channel orthogonality, and then carry out user and beam selection within a narrowed range for channel energy maximization. Simulation results validate the proposed scheduling strategy not only has a significant advantage in reducing computational complexity, but also achieves higher sum rate than other non-exhaustive schemes.

A low complexity two-stage user scheduling scheme for MmWave massive MIMO hybrid beamforming systems

The embodiment of the invention provides a codebook forming method and device, a base station and a mobile station. The codebook forming method provided by the embodiment of the invention comprises the steps that a first main codebook is formed according to a first phase shift parameter, wherein the first main codebook is used to generate a first basic beam sequence in a first spatial dimension; afirst rotation parameter is determined, wherein the first rotation parameter is used to carry out phase rotation on the first basic beam sequence to generate a first auxiliary beam sequence in a first spatial dimension; and a first spatial dimension codebook is formed according to the first main codebook and the first rotation parameter.

Wei Wu

Papers

Low-Complexity Beam Training for 5G Millimeter-Wave Massive MIMO Systems

Non-orthogonal multiple access based hybrid beamforming in 5G mmWave systems

Two-Stage 3D Codebook Design and Beam Training for Millimeter-Wave Massive MIMO Systems

A low complexity two-stage user scheduling scheme for MmWave massive MIMO hybrid beamforming systems

Codebook forming method and device, base station and mobile station