Key Technologies and Challenges

To achieve the requirements as proposed in Section 2.4.1, the combined gains from three categories below need to be developed and harmonised such that the promises of 5G are achieved [18, 45, 46].

• Spectral Efficiency: Increased spectral efficiency, primarily through advances in multiple access and massive MIMO techniques to support more bits/s/Hz per node. Extreme densification, cell cooperation and offloading to increase the overall net- work capacity and area spectral efficiency.

• Bandwidth: Increased bandwidth by utilizing carrier aggregation, WiFi’s unli- censed spectrum in the 5-GHz band, and moving toward the mmWave spectrum. The dominant multiple access technique for high speed wireless communication has

been OFDM and OFDMA. They form the basis of most current standards such as WiFi, 4G, digital TV e.t.c. and is expected to be incorporated into 5G. This is due to the impress- ive characteristics they provide such as its natural ability to combat frequency selective fading, computationally efficient implementation via FFT/IFFT blocks, simple frequency- domain equalization, excellent pairing with MIMO without the added complication of Inter-Symbol Interference (ISI).

Despite the strengths of OFDM/OFDMA, there are several weaknesses inherent which could possibly become more pronounced in 5G networks. First, the PAPR is higher in OFDM than in other formats since the envelope samples are nearly Gaussian due to the summation of uncorrelated inputs in the IFFT. Although a Gaussian signal distribution is capacity achieving under an average power constraint, a high PAPR presents a poor trade-off between the linearity of the transmitted signal and the cost of the power ampli- fier. This problem largely overcome in LTE uplink DFT-OFDMA by precoding the OFDM signals at the cost of a slight power penalty and higher computational equalization pro- cess at the receiver. Secondly, although the spectral efficiency in OFDM is satisfactory, further relaxations of strict orthogonality and smaller CP could further improve per- formance.

To address OFDM’s weaknesses, alternative approaches are being actively investig- ated in the research community. Most of these alternative are incremental changes to OFDM rather than a complete change to the signalling format. A key technology cur- rently being researched is the harmonization of OFDM with multi carrier non-orthogonal multiple access, which is based on the principles of superposition coding where multiple access in provided in the power domain. The scheme provides superior performance compared to OFDMA by actively utilizing the near-far effect of geographically separ-

ated users with different power requirements. A key drawback to NOMA however, is the required large power separation between users which comes at a cost to the weaker users.

Other OFDM alternative technologies, although not the scope of this thesis are time- frequency packing and faster-than-Nyquist signalling to circumvent the limitations of strict orthogonality and CP, filterbank multi-carrier OFDM that does not require prior synchronization of distributed transmitters, universal filtered multi-carrier where fil- tering is performed on groups of adjacent subcarriers such that inter-carrier interfer- ence resulting from poor time/frequency synchronization is minimized, generalized fre- quency division multiplexing that adopts a shortened CP through the tail biting tech- nique which provides attractive spectrum sharing characteristics where the frequency- domain holes are adaptively filled, single-carrier transmission due to the development of low-complexity non-linear equalizers implemented in the frequency domain.

Chapter 3

Uplink NOMA with Constellation

Precoding (UL-NCPr)

3.1

Introduction

This chapter focuses on the multiple access interference and the problem of large power separation requirements so as to improve the error performance and sum rate capacity. This is achieved by introducing a novel NOMA scheme employing a new signal design where precoding is designed at the eNB, such that component users form a single and uniquely decodable composite constellation. As the composite constellation is as that of a single user transmitting that same constellation, MAI can be viewed as absent from the system which allows multiple users to transmit at their full rates. The precoding is designed by the eNodeB which searches and allocates power and phase rotations for each user that maximizes the minimum distance of the joint received signal constellation points. The resulting joint composite constellation belongs to higher constellation with rate equal to the number of multiplexed users. Users utilise a simple common periodic

pilot broadcasted by the eNB so as to adjust their transmissions adaptively to maintain the desired received composite signal. UL-NCPr provides a practical solution for im- plementing the Multiple Access Channel (MAC) in fading environments. The proposed scheme is evaluated with uplink SC-FDMA to show its compatibility and superior per- formance compared with conventional OMA and power domain NOMA schemes. It can also be easily integrated with conventional multiple access techniques to extend user capacity and improve link utilization.

The contributions of this chapter are as follows

• The requirement for large power separation has limited the flexibility of conven- tional PD-NOMA in terms of the DoF at the component user constellations and thus, by precoding in both power and phase domains, we allow multiple users transmit at their full rates leading to increased sum rates, subject to individual power constraints, compared to conventional NOMA schemes employing SIC • As the individual users form a unique composite constellation, the receiver treats

the received signal as that of a single users’, thereby eliminating MAI perceived by conventional OMA and PD-NOMA with SIC. This enables our scheme achieve the optimum rate regions, rectangular in the case of 2 users transmitting with equal power.

• As the works in literature focus on rate maximization, by optimizing the power and phase of the component constellations, we provide superior bit error rates in terms of the minimum distance of the received composite constellation.

• Our scheme utilizes a common periodic pilot on which the users perform channel estimation. This reduces piloting overhead compared to current SC-FDMA/OMA

systems.