The following LTE ProSe Technology White Paper describes Long-Term Evolution (LTE) Direct and Proximity Services (ProSe) and is part one of a 5-part Summary of our 3GPP Core Technologies series. You can also download and print them by clicking below.
Long-Term Evolution (LTE) Direct and Proximity Services (ProSe)
Long-Term Technology (LTE) Direct is Device-to-Device (D2D) communications which was introduced in release 12 of 3rd Generation Partnership Project (3GPP) LTE technology. 3GPP defined new communication channels for LTE Direct so that close-by devices can search and discover each other to setup one-to-one or one-to-many communications for proximity services (ProSe). Note that ProSe is not limited to only D2D communication D2D communications are meant to enable public safety as well as commercial use cases, either for discovery and/or communication purposes. D2D improves the utilization of the spectrum as well as data throughput since the data is conveyed directly among the devices with no network involvement.
Device-to-Device communication (D2D) uses sidelink[1] connectivity for discovery, time synchronization, and frequency synchronization of the devices. In order for the devices to D2D communicate, they must first time- and frequency-synchronize. For sidelink connectivity, LTE has created System Information Block (SIB) 18 for configuration parameters related to sidelink communications, and SIB 19 for configuration parameters related to sidelink discovery.
Figure 1
Figure 1 shows the LTE radio channels for sidelink connectivity. Sidelink Traffic Channel (STCH) is a logical channel and exchanging the user data for sidelink communications and is mapped to the Sidelink Shared Channel (SL-SCH), a transport channel. SL-SCH is then mapped to Physical Sidelink Shared Channel (PSSCH). For a receiving device to detect and decode the information in PSSCH, the Sidelink Control Information (SCI) is mapped to a Physical Sidelink Control Channel (PSCCH) which is transmitted in parallel to PSSCH.
Sidelink Discovery Channel (SL-DCH), a transport channel, is employed for the discovery procedure. In the physical channel, SL-DCH is mapped to Physical Sidelink Discovery Channel (PSDCH). The sidelink discovery does not have any logical channel. Therefore, there may be no Radio Link Control (RLC) to support error detection and recovery, segmentation, reassembly, duplicate detection, and flexibility of data transmission with or without acknowledgment; nor any Packet Data Convergence Protocol (PDCP) to support header compression, ciphering, integrity protection, and transfer of user data and control data.
Sidelink Synchronization Signal (SLSS) comprises Primary Sidelink Synchronization Signal (PSSS) and Secondary Sidelink Synchronization Signal (SSSS) which is associated by Sidelink Identifier (SLI) indicating whether the broadcasted SLSS is synchronized in-coverage (in the coverage area of the network) or out-of-coverage (outside the coverage area of the network) by another reference device. Sidelink synchronization may also comprise the logical channel Sidelink Broadcast Control Channel (S-BCCH) which corresponds the transports channel Sidelink Broadcast Channel (SL-BCH) and the physical channel, Physical Sidelink Broadcast Channel (PSBCH). PSBCH may be employed by devices to broadcast basic system information referred to as Sidelink Master Information Block (SL-MIB) which may convey the following information:
Both PSBCH and SLSS are transmitted in the same subframe and the same resource blocks.
Figure 2
Figure 2 shows the structure for the SLSS (PSSS and SSSS) and PSBCH. Demodulation Reference Signal (DMRS) is a reference signal which implies that the received subframe is decodable. DMRS handles the high Doppler associated with high relative speeds of up to 500 Km/h. Regularity rules may typically be concerned about what a device is transmitting to avoid the interference within a radio cell. Thus, the sidelink channels comprising PSBCH and SLSS (PSSS and SSSS), are designed to maximize the commonalities with the LTE uplink channels, even though the SLSS (PSSS and SSSS) employs the principle of the LTE downlink PSS and SSS.
Figure 3
Figure 3 illustrates an in-coverage device synchronizing to the LTE network and transmitting SLSS if this device is configured to transmit SLSS. It may also transmit SLSS when the signal power received from the LTE network decreases below a certain threshold. The devices which are in partial coverage or out of coverage may transmit SLSS when the signal power from selected devices falls under a certain threshold. The recipient devices can thereby synchronize and thereafter discover and, if needed, communicate with each other.
Proximity Services (ProSe) are services which become available when the devices are in vicinity of each other. The communication between devices may be via LTE network or may be based on D2D communication. The ProSe services employ the discovery mechanisms using Model A or Model B. Model A is based on repeated announcements by a device transmitting “I am here!” While the mechanism in Model B is based on request and response where a device repeatedly announces a request to find all group members “Who is there?” or specific group members “Are you there?” and, subsequently, the called group members respond to the announcement.
Figure 4
Figure 4 illustrates the ProSe discovery message which is employed for discovery procedures of Model A and Model B. The announced Temp ID is the service specific data which comprises the information about the group member identity and the application when employing Model A as the discovery mechanism. Depending on discovery type for Model A, the service specific data may be either “restricted” or “open”.
In the Model B discovery, the device is either a “discoverer” or a “discoveree” at the time of announcment. The discoverer device announces query codes while the discoveree device announces response codes for those query codes. The query codes and the response codes are obtained from the function and are related to the group member identities and applications. The discovery type is always set to “restricted” for Model B.
If the security measurements for a service is of great importance, the discovery mechanism of Model B is employed. There are some related defined security measurements commensurate with the threat for Model A and Model B in the specification for ProSe, 3GPP TS 23.303.
[1] Sidelink (as opposed to uplink or downlink) refers to the link used for direct communication with other devices. Sidelink uses certain uplink frequency bands that are specifically set aside for LTE Direct.
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