# Cluster-Based Transmission Diversity Optimization in Ultra Reliable Low Latency Communication
## 2 Related Work
In the sequence of development, 5G URLLC standards might be ensured collision-free and easy-to-maintenance intra-vehicular communication. The 5G URLLC Quality of Service (QoS) for guided vehicle and automated industries allow packet error rate up to \(10^{-5}\) and latency 1 ms is found in [28]. The 3 GPP also sets the same standards (packet error rate and latency) for 32 bytes packet size [29]. The end-to-end latency estimation for spacecraft automation and sensor based communication was found 5 ms in a TDMA protocol [30]. 5G URLLC has been investigated in [31] for real-time application, which is a newer frame structure, well beyond the LTE-advanced frame that can use URLLC latency. This seems to have a reliability requirement for different packet sizes evaluated in a different modulation technique. Using polar OFDM waveform technology, an automated robot has been built [32] ensuring higher reliability. A single OFDM symbol is capable of detecting a packet, minimizing the latency of packet detection to achieve 99.999% reliability [33]. However, in the OFDM system, the packet corresponding bits are divided into subcarriers, and cyclic prefix bits are associated bits to extend the packet length. As a consequence, the OFDM system takes more time. In the intra-vehicular condition, 5G URLLC is extremely challenging due to network system schemes and transmission techniques for wireless sensors as it moves at around 120 Km/h to 250 Km/h. At such speed, a 5.9 GHz band has been used for V2X safety concerning application in the transport industries [34]. 5G URLLC is optimized adequately for mission-critical systems [35, 36] where the transmission latency is stringently maintained with fixed reliability. To maintain the 5G URLLC standard, wireless sensor-based Orthogonal Frequency Subcarrier-based Multiple Access (OFSMA) MAC protocol has been accepted in [37]. The OFSMA ensures reliability of 99.999% within 0.1 ms transmission latency for robots internal wired to wireless sensor conversion [11]. A Hybrid Access Scheme (HAS) considers audio, video, and general sensors and utilize OFSMA MAC protocol to ensure transmission reliability of 99.999% having a latency of 0.1 ms [12]. URLLC packet designing is another vital issue. With an accurate packet structure, the packet processing and transmission time latency can be decreased [38]. For this, uplink contention-based access [39] is considered for sporadic arrivals for the URLLC packets. A non-square-shaped packet for the frequency domain is employed in the 5G NR system. The Low-Density Parity-Check (LDPC) helps to reduce transmission latency of the data channel [40]. The packet diversity principle is another influencing concept to increase the transmission reliability of a communication system. A contention-based multi-channel slotted-ALOHA protocol that has been proposed in [39] that used packet diversity principle to improve the transmission reliability. As multiple duplicate packets are sent into different slots, inner packet collisions are reduced. A quick retrial technique to forward a packet into a frequency diversity structure just after collision in a slotted-ALOHA system is proposed in [41] for multi-channel random access in an OFDMA wireless network. A random frequency band is generated in each TTI to improve the throughput, which is analyzed in [42]. The receiver should work more efficiently to receive these data packets to transmit duplicate data packets from different clusters. For multi-connectivity and interface diversity, a Massive Multiple-input Multiple-output (MIMO) is adopted to increase transmission accuracy [9, 43] as it utilizes a vast number of antennas. Some of the discussed network schemes are incorporated into MANET and VANET too. However, the proposed work considered only intra-vehicular networks to serve only 1-6 m distance inside a vehicle. Low latency and higher reliability are extremely difficult to achieve for this short-range communication.