# Wireless Multi-Node uRLLc B5G/6G Networks for Critical Services in Electrical Power Systems
###### Abstract  
The successful operations of power system require the support of communication networks with massive nodes access and two-way reliable and latency-critical transmission, which is the most-important use case of ultra-reliable low-latency communications (uRLLc) in the Fifth-Generation and Beyond and Sixth-Generation (B5G/6G) networks. In this paper, based on the orthogonal frequency division multiplexing (OFDM) signal in 5G, a new OFDM employing the hopped subcarrier technique (OFDM/FH) is proposed to attain the multiple nodes connectivity and the enhancement of transmission reliability. Considering the short and bursty packet types in the control information of power systems, the OFDM/FH signal can be naturally integrated with the mini-slot frame structure and automatic repeat request (ARQ) scheduling scheme to reduce the latency. The trade-off between the reliability in terms of the bit-error rate (BER) and the retransmission latency is investigated by semi-analytic simulations. Through the new joint design of the signal waveform and frame structure for uRLLc, we find that the proposed system can attain up to 99.999% reliability and ms-level latency for electric power services, even in the harsh wireless environment. uRLLc networks; system design; mini-slot; services in power system; performance analysis +
Footnote †: journal:  
0000-0001-999-4300-3300-3300-3300-3300-3300-3000-33000-3300-3300-3300-33000-  
With respect to the physical layer (PHY), the OFDM signal has been widely adopted as the basic transmission waveform since the 4G cellular networks [8]. OFDM can efficiently eliminate the influence of frequency-selective fading on performance in the wideband communication scenario to aid the serial-to-parallel conversion and the cyclic prefix. However, the original OFDM normally occupies wide and contiguous frequency resources, thus being vulnerable to the interferer and jammer, which emit malicious signals on the specific frequency bands. It is necessary to improve the reliability of signal transmission via the new design of a reliable OFDM waveform, particularly in the critical communications in power systems. Frequency hopping (FH) is a superior signal processing method to implement reliable and secure communications [10]. The FH system can flexibly select the frequency bands/slots with good quality and enable the instantaneous frequency hopping randomly over these bands/slots according to the specific hopping strategy (i.e., hopping sequence), so as to avoid channel fading and various interferences/jamming. Due to these merits, FH has been suggested to be deployed in some critical scenarios such as smart grid communications [11] and 5G PDCCH [12]. Therefore, the integration of OFDM and FH (OFDM/FH) could become a potential waveform for reliability-critical applications in the future B5G/6G, which is one of the contributions of our paper. Besides, some other solutions to reliable transmission such as multi-connectivity, hybrid ARQ based on a short frame structure, the design of the control channel, a short coding scheme, etc., have been reported in [13; 14]. In order to guarantee the stringent latency requirement in the power system scenario, a flexible frame structure, the so-called mini-slot, which enables lower latency by using a shorter transmission time interval (TTI), has been proposed in the 3 GPP Release-15 standard from the medium access layer (MAC) [8]. In the contrast with the previous standard versions of 4G/4G-LTE, the subframe with 1 ms TTI is a basic scheduling unit, which needs a longer waiting time for (re)transmission. Moreover, the mini-slot structure implies that time-frequency resource scheduling (i.e., uplink-to-downlink conversion) can  
Figure 1: The communication networks for electric power systems accommodating multiple nodes and services. initiate at any mini-slot other than the subframe, like in 4G/4G-LTE, thus leading to a scheduling interval less than 1 ms (the exact scheduling interval is determined by the selection of OFDM numerology).