es that are pre-allocated by the base station with a certain periodicity. To avoid resource utilization inefficiency, multiple UEs might be allowed to share the same resources and to operate in a contention-based manner. Hence, collisions are inevitable, affecting connection reliability and latency, which is worsen as the UE traffic increases and retransmissions become necessary. In other words, grant-free trans- mission may not be scalable as the UE density increases in a network. Fig. 3.55 shows a comparison between the contention-based (grant-free) and grant-based performance, where we observe that the efficiency of contention-based transmission degrades with increasing packet size and traffic load, while efficiency of grant-based trans- mission improves by increasing packet size and is stable over different load factors. The markings in the figure indicate that for packet sizes of 20, 30, and 40 bytes, there is a load- ing factor threshold below which contention-based transmission is more efficient than grant- based transmission. When the packet size is sufficiently small, for example, 10 bytes, contention-based transmission is always optimal within certain load factor range. Based on the analysis in [43], keep-alive messages of mobile Internet traffic are better suited to use Contention-based transmission: packet size 10 byte Grant-based transmission: packet size 10 byte Contention-based transmission: packet size 20 byte Grant-based transmission: packet size 20 byte Contention-based transmission: packet size 30 byte Grant-based transmission: packet size 30 byte Contention-based transmission: packet size 40 byte Grant-based transmission: packet size 40 byte Load factor [activation probability of users (%)] Figure 3.55 Comparison of grant-based and grant-free uplink transmissions [43]. New Radio Access Physical Layer Aspects (Part 1) 405 contention-based transmission, while the rest of the mobile Internet packet types (e.g., video or voice) need to be transmitted by grant-based transmission, if not taking into