# On the Needs and Requirements Arising from Connected and Automated Driving
## 4 Technical Requirements: from Automotive to Network Perspective
### Definition of KPIs  
From an automotive perspective, it is possible to identify the following types of requirements and key performance indicators (KPIs):  
**(1) Completion time**: denotes the time that is needed to complete a certain vehicle activity. For a specific maneuver, the completion time specifies the total time it takes from when the maneuver is initiated until it has been completed. In terms of the see-through use case, the completion time takes the form of **overtake time**, the amount of time taken to perform the overtaking maneuver, whereas **intersection crossing time** represents the completion time for crossing the intersection safety [13] or lane merging procedure into the highway. Figure 5: Remote driving for automated parking use case. * **Localization accuracy** is used to specify the needed geographical position accuracy. The localization error is defined as the distance of a measured position to the ground truth position. Additionally, the confidence limit is considered, often being selected as 95% [14]. * **Inter-vehicular time** is the recommended/required distance to be kept between two vehicles. It depends on regulatory requirements (traffic safety) and business requirements (traffic efficiency) [15]. * **Mobility** is defined as the velocity (speed and heading) at which an object is moving with respect to a reference object or a geographical point. The maximum relative velocity between the objects is considered if the reference object is also moving [16]. * **Relevance area** is defined as the area where the messages have to be distributed to ensure the automotive service. The relevance area is application-dependent. The automotive KPIs set boundary conditions for the top layer network KPIs, which we refer to as the application layer. The application layer KPIs are then broken down into the requirements on lower layers. However, this process is implementation-specific; for simplicity, we limit the discussion to application layer KPIs. From the communication network perspective, it is possible to set the focus on the following requirements:  
* **Maximum service data unit (SDU) size** is the maximum size of payload that is required by a specific service and generated by the application. * **Latency** is the time from when an SDU is requested to be transmitted by an end-node application until it is made available to the other end-node application. In case an SDU is not delivered, due to transmission errors or other causes, then, by convention, the SDU has infinite latency. This is also referred to as \"end-to-end latency\". The latency requirement (or deadline) is the longest latency that the application can tolerate in the sense that the probability that the latency is less than the deadline is at least equal to the reliability requirement (see next bullet). * **Reliability** is defined as the probability that the SDU is correctly received within a specified maximum latency (deadline), subject to the other relevant requirements and conditions (such as SDU size, communication range, transmission power, propagation conditions, and mobility). Figure 6 depicts the Cumulative Distribution Function (CDF) of the latency (\(\tau\)), and its relationship with the reception deadline (\(\tau_{\text{dl}}\)), the reliability, and the packet drop probability. The packet drop probability is represented by \(\Pr\{\tau=\infty\}\), since a dropped packet is, by convention, thought to be equivalent to a packet being delivered with infinite delay. Hence, the latency CDF \(\text{F}_{\tau}(\text{x})=\Pr\{\tau\leq x\}\leq 1-P_{e}\) for all finite x (nevertheless, with some abuse of notation, \(F_{\tau}(\infty)=1-P_{e}+P_{e}=1\) as would be expected). It is worth noting that an arbitrarily low packet drop probability does not necessarily translate to attaining a high reliability. In fact, allowing for an unbounded number of retransmissions can achieve virtually error-free transmission from the application perspective, which, however, comes at the cost of potentially very long latencies.