Written by Simon Chege
With the current trends that shape various types of wireless systems into a smart city infrastructure, the question of what would constitute efficient multi-radio access protocols for the networking architectures deployed in smart cities becomes an important consideration when integrating into the core city network. Smart cities are required to provide services such as intelligent transportation services that can be used to enhance route planning and congestion avoidance in city streets, intelligent traffic light controls and parking services, enhance vehicular safety, and enable self-driving cars. Such services present diverse quality of service (QoS) application requirements of ultra-low latency, ultra-reliability and enhanced data rates for the surging number of massively connected devices.
IntroductionThese ubiquitous service requirements necessitate a paradigm infrastructural and architectural shift not only in the network heterogeneity, traffic intelligence and management, energy and cost efficiency and latency management, but also in the multi-access of the scarce wireless radio spectrum. Several smart city communication models including intra-vehicle, vehicle to vehicle (V2V), vehicle to infrastructure (V2I), vehicle to broadband cloud (V2B) through to the broader vehicle to everything (V2X) systems have gained interest across different smart city application areas, as illustrated in the Figure 1 .
Figure 1. V2X Communication scenarios
Taking advantage of the widely deployed infrastructure, the LTE-A network (4G) was considered promising candidate to support the V2X services. Even so, due to the massive growth of devices requiring to access the V2X network, conventional OFDMA-based LTE-A network experiences congestion issues occasioned by the low efficiency of orthogonal access, resulting in significant access delay and posing a great challenge especially to safety-critical applications. Proposed for the fifth generation (5G) networks is a shift from orthogonal multiple access (OMA) to non-orthogonal multiple access (NOMA) technologies for supporting dense internet of things (IoT) and V2X in smart cities. NOMA techniques have been well recognized as an effective solution for the future 5G cellular networks to provide low latency and high reliability (LLHR), broadband communications and massive connectivity.
Overview of NOMA
OMA techniques allow only one user at each transmission period to use a resource unit i.e., orthogonal frequency division multiple access (OFDMA) subcarrier, time division multiple access (TDMA) timeslot or code division multiple access (CDMA)’s code. Differently, NOMA allows multiplexing of several users on the same time-frequency resource, permitting controlled interference, through non-orthogonal resource allocation resulting in high spectral efficiency, user fairness and massive connectivity. NOMA achieves these by employing three broad approaches; Interleave division multiple access (IDMA), power-domain multiplexing (PD-NOMA) and code-domain multiplexing (CD-NOMA). IDMA utilizes user specific interleaves for multiplexing, PD-NOMA superimposes multiple users by allocating distinct power levels to different users at the transmitter and employing successive interference cancellation (SIC) at the receiver and lastly, in CD-NOMA, multiple users share the same time-frequency resources by mapping incoming bits to unique user-specific sparse, non-orthogonal low cross-correlation spreading sequences. At the receiver, CD-NOMA employs an iterative multi-user detector based on message passing algorithm. Based on its merits to achieve high overloading transmission compared to the available resources making it particularly suited for supporting vehicular communications, NOMA provides a new dimension for V2X services to alleviate resource collisions, thereby improving the spectrum efficiency and reducing the latency.
Hybrid NOMA techniques are emerging to enhance the NOMA experience in advancing efficient spectrum access and latency reduction of multiple V2X devices, hybrid NOMA techniques are emerging. In particular, a novel hybrid radio access schemes based on the integration of both power- and code-domain known as power domain sparse code multiple access (PD-SCMA) has been proposed in , . The hybrid technology thrives in its ability to connect multiple users in a limited resource scenario. This allows for effective spectrum usage but comes at a cost of increased detection complexity resulting in loss of performance in terms of outages. To overcome the challenges that come with such a technology, hybrid resource allocation schemes of codebook assignment, user clustering and power allocation are proposed for the transmitter. Besides, to achieve acceptable latency and error rate, a joint multi-user detector (MUD) with low complexity order and low computational time is proposed. The joint MUD is based on successive interference cancellation (SIC) that cancels interference in the power domain. In the code-domain, the received signal is estimated by using either the message passing algorithm (MPA) or the expectation propagation algorithm (EPA). The performance analysis validates the multiplexing performance of the hybrid NOMA and hence make it a feasible scheme in enabling 5G NOMA-based V2X communications for smart cities applications.
NOMA – V2X Implementation
The non-orthogonal nature of NOMA as well as the mobility and dense topology of vehicular network necessitate a paradigm shift in the design aspects of scheduling and resource allocation schemes for NOMA-based cellular V2X services. In particular, NOMA-based V2X need to adapt a scheduling scheme that combines dynamic power control with semi-persistent scheduling (SPS) unlike in OMA-based V2X employing only SPS in which the resources are booked by the vehicles every few transmission periods, hence latent. Concerning spectrum management aspect, NOMA-based V2X network introduces co-channel interference by allowing multiple vehicles to share the same sub-band, which provides an extra dimension influencing the spectrum efficiency as well as user fairness compared to OMA-based V2X. Another important design aspect is the power control. The dense topology of vehicular network exhibits elevated cross-interference amongst users overlapping in the same resource element. This warrants a sophisticated power control strategy to optimise SIC and guarantee an acceptable receiver experience. Lastly, the NOMA-based V2X requires dynamic signalling control techniques. In the conventional OMA-based case, the prior CSI knowledge required for joint decoding is usually provided by the BS and may introduce great latency to V2X applications. A signalling control scheme that can automatically adapt to unexpected changes in traffic conditions, improve travel-time reliability, reduce congestion and make traffic signal operations proactive by monitoring and responding to gaps in performance can effectively improve NOMA-based V2X applications .
With the recent development of smart cities and increasing number of vehicles that make traffic control particularly challenging, there is an urgent need to investment deeply in modern technologies that address the growing issues of such developments. V2X implementation that blends into roads with both human-driven and autonomous vehicles would require careful network planning as well as resource allocation. In this article, we have presented how NOMA can revolutionize the multi-access of the radio spectrum resources for V2X in smart cities applications. Our proposed hybrid NOMA technology is tractable and feasible in V2X applications. Moreover, we have highlighted the implementational aspect of NOMA-based V2X with the aim of achieving acceptable QoS requirements particularly, ultra-low latency requirement that is indispensable for efficient smart cities operation.
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This article was edited by Bernard Fong