Tutorial 1: Non-orthogonal Multiple Access: Current State of the Art and Future Directions
- Monday, 12 June 2017, 9:00-12:30, Room IT105
- Zhihuo Ding (Lancaster University, UK)
- Robert Schober (Friedrich-Alexander University Erlangen-Nürnberg, Germany)
Motivation and Context
Multiple access in 5G mobile networks is an emerging research topic, since it is key for the next generation network to keep pace with the exponential growth of mobile data and multimedia traffic. Non-orthogonal multiple access (NOMA) has recently received considerable attention as a promising candidate for 5G multiple access. The key idea of NOMA is to exploit the power domain for multiple access, which means multiple users can be served concurrently at the same time, frequency, and spreading code. Instead of using water-filling power allocation strategies, NOMA allocates more power to the users with poorer channel conditions, with the aim to facilitate a balanced tradeoff between system throughput and user fairness. Recent demonstrations by industry show that the use of NOMA can significantly improve the spectral efficiency of mobile networks. Because of its superior performance, NOMA has been also recently proposed for downlink scenarios in 3rd generation partnership project long-term evolution (3GPP-LTE) systems, where the considered technique was termed multiuser superposition transmission (MUST). In addition, NOMA has been included into the next generation digital TV standard, e.g,, ATSC (Advanced Television Systems Committee) 3.0, where it was termed Layered Division Multiplexing (LDM). In this tutorial, we will provide a progress review for NOMA, including an information theoretic perspective of NOMA, the design of multi-input multi-output (MIMO) and cooperative NOMA, the application of NOMA in millimeter-wave (mmWave) networks, the interaction between NOMA and other types of multiple access techniques, resource allocation for NOMA, and the impact of practical constraints, such as imperfect channel state information (CSI) and limited feedback, on the performance of NOMA.
Structure and Content
- Review of the overall requirements to realize spectrally efficient 5G communications.
- The tutorial will start by introducing the basic concepts of NOMA in a simple scenario with one base station and multiple users, where each node is equipped with a single antenna. The performance gains of NOMA will be illustrated from an information theoretic perspective.
- The combination of MIMO technologies and NOMA will be described. Unlike conventional multiple access techniques, the design of MIMO-NOMA is challenging. For example, power allocation of NOMA requires a step to order users based on their channel conditions. This user ordering is straightforward for the single-input single-output (SISO) case since it is easy to compare scalar channel coefficients, but it is difficult in MIMO scenarios in the presence of channel matrices/vectors. A few designs of MIMO-NOMA with different trade-offs between system performance and complexity will be illustrated.
- The design of cooperative NOMA will be discussed. In a NOMA system, successive interference cancellation is used, which means that some users know the other users’ information perfectly. Such a priori information should be exploited, e.g., some users can act as relays to help other users experiencing poorer channel conditions. A few examples of cooperative NOMA protocols will be introduced and their advantages/disadvantages will be illustrated.
- The application of NOMA in mmWave networks will be investigated. Similar to NOMA, the motivation for using mmWave communications is motivated by the spectrum crunch, but the solution provided by mmWave communications is to use mmWave bands which are less occupied compared to those used by current cellular networks. We will show that the use of NOMA is still important to mmWave networks, in order to fully explore those bandwidth resources available in very high frequencies.
- The interaction between NOMA and other types of multiple access techniques will be described. Particularly we will focus on how to design hybrid multiple access schemes by combining NOMA with orthogonal frequency division multiple access (OFDMA), where users are grouped in small-size clusters and different clusters are served by different OFDM subcarriers. Designing such a hybrid multiple access scheme can be formulated as a joint subcarrier and power allocation problem for which both optimal and suboptimal solutions will be presented.
- The impact of CSI on the performance of NOMA will be investigated. Three types of CSI assumptions will be considered. The first is the case of imperfect CSI, which introduces an error floor to the probability of detection. The second is that the base station has statistical information about the CSI, and particularly we will focus on the case when the path loss is known perfectly at the transmitter. The third is the case with limited feedback, such as one or a finite number of feedback bits.