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One of the essential requirements of 5G wireless systems is to minimize packet transmission latency for ultra-reliable, low latency services (URLLC). One of the most prominent examples will be Vehicle-to-Any (V2X) communications. V2X certainly includes vehicle access with broadband services, but latency is not an issue there. Low latency in cellular networks is a prerequisite for the security of autonomous vehicles.
Autonomous vehicles will need to detect other vehicles, road conditions, pedestrians and other obstacles. Often there will be environmental sensors to supplement this information; this data will frequently be available for autonomous vehicles via road units (RSUs) or other vehicles. Low latency connections between these RSUs, V2X application servers, and embedded systems will allow faster decision making, which will improve security.
As an example, consider stopping distances. When a person driving a vehicle on a highway moving at 70 mph recognizes a danger, the traditional stopping distance is 96 meters, or 315 feet (Figure 1). Twenty-one meters of the total of 96m is a reflection distance, based on a reaction time of 0.67s, the remaining 75m is the actual braking distance. But with autonomous driving engaged, vehicles would recognize the dangerous situation sooner, the thinking distance would be reduced, and there would be more room for braking. The result will be a significant decrease in collision rates.
Figure 1 Typical stopping distances. Source: UK Department for Transport
The V2X system helps vehicles recognize danger quickly by alerting nearby vehicles to the dangerous situation, and 5G’s 1 millisecond (ms) end-to-end transmission latency requirement minimizes car reaction time autonomous.
The existing LTE (Long Term Evolution – a 4G technology) system has some fundamental limitations that prevent it from supporting 1ms latency. The first obstacle is that the minimum size of a radio transport block is a subframe having a length of 1 ms. This means that a period of 1 ms is devoted only to the transmission of the transport block via the radio interface, excluding the processing time on the devices and the transmission latency in the network.
To reduce response time, 5G uses an evolving Orthogonal Frequency Division Multiplexing (OFDM) framework with different numerologies. In a duration of 1 ms, six distinct slot configurations are available, for example 1, 2, 4, 8, 16 and 32 slots. The minimum size of a transport block could be reduced to a minimum of 0.03125 ms depending on the new configuration, as shown in Figure 2.
Figure 2 Example of slot configuration of several types of numerology (TTI). Source: 3GPP
Another feature of LTE that makes it difficult to reduce latency is the delay in allocating radio resources between a vehicle and the base station. When a vehicle wishes to transmit packets, a radio resource allocation procedure precedes the transmission of packets. To transmit a resource schedule request and send packets on the scheduled resource, a vehicle needs at least 8 ms. In LTE, the semi-persistent scheduling (SPS) feature was introduced for periodic data transmissions such as voice over IP (VoIP) services.
When a base station configures SPS radio resources, a mobile handset can use the periodic resources without any additional programming request procedure. When the device receives data to send to its buffer, it can transmit the data via the next periodic resource already configured.
However, the existing LTE SPS configuration is dedicated to a single device. If the device does not need periodic resources, such as transmitting data only when specific events occur, such as a collision warning, then unused SPS resources by the device are wasted.
To reduce the waste of periodically allocated resources, 5G allows multiple devices to share periodic resources, known as configured allocation (Type 1). The configured grant is based on the LTE SPS feature. In figure 3, the base station allocates the configured grant resources to multiple vehicles, and the vehicles randomly use the resources when they have data to transmit. By allocating the configured grant resources, the 5G network eliminates the delay in transmitting packets for a schedule request procedure and increases the utilization rate of the allocated periodic radio resources.
figure 3 Grant resources configured
Ofinno has developed a number of patented technologies relating to SPS and configured grant technologies. Patent technologies improve the operating efficiency of the configured dealership by providing multiple SPS configurations, improved mobility mechanisms, carrier aggregation for V2X services, sharing of information on device capabilities and methods of power control. Multiple SPS configurations allow efficient use of resources and reduce transmission latency with optimized resource configurations (eg, recurrence and size of resources) for different types of services. The enhanced mobility mechanisms allow reliable packet transmission during a V2X device handover procedure. Carrier aggregation for V2X services increases the capacity of radio resources with the introduction of multi-cell cooperation for reliable V2X services.
V2X and enabling technologies are discussed in more detail in this Oninno white paper: Vehicle-to-all traffic control and communications (V2X).
Kyungmin Park is a Principal Investigator at Ofinno Technologies and focuses on radio access network procedures for LTE Advanced, LTE Advanced Pro, and New Radio for 5G.
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