Cell-Free Cooperative Non-Orthogonal Multiple Access for 6G Wireless Systems

Introduction

The revolution of wireless industry paved the way for successful deployment of 5G networks in various sectors such as defense, healthcare, agriculture, education, commerce, entertainment etc. The diversified opportunities and challenges in these sectors necessitate development of sixth-generation (6G) wireless systems to meet the requirements such as extremely high data rate, massive connectivity, minimal latency, good coverage, high reliability, optimal resource allocation, good spectral and energy efficiency.

Multiple access is one of the key techniques in the wireless technological evolution which provides access to multiple users with the available resources. Older generations involved frequency division multiple access (FDMA), time division multiple access (TDMA), code division multiple access (CDMA), space division multiple access (SDMA) and Orthogonal Frequency division multiple access (OFDMA). FDMA, TDMA, CDMA and SDMA wasted the resources significantly and increased the design complexity. Even though OFDMA offers more flexible multicarrier communication designs in eliminating inter symbol interference, it needs complex equalizer at the receiver which increases cost of the system and fails to achieve good coverage due to non-linearity in RF power amplifier. The requirements put forth by 3GPP is kept alive by various technologies like Massive MIMO, Internet of Things (IoT) and Device-to-Device (D2D) communication which makes use of highly directional antenna. Data rate and user capacity has been significantly improved by the use of beamforming techniques for short distance communication. Even though 5G system promises data rate in Gbps and latency below 1ms with a wide bandwidth ranges from 30 GHz to 300 GHZ, its range of coverage was limited. The performance of the system degrades in long distance propagation when its operating frequency is from 30 GHz to 300 GHZ. Hence the infrastructure needs to be rebuilt to overcome the limitation.

Non-orthogonal multiple access (NOMA) is the key enabler for bringing down the interference effects and sharing the spectrum among multiple users in an efficient way. Cooperative communication is another promising technology which can improve coverage and capacity of the wireless system.