4 General overview

26.2673GPPeCall data transferGeneral descriptionIn-band modem solutionRelease 17TS

4.1 eCall system overview

The eCall system overview is depicted in Figure 1.

Figure 1: eCall system overview [11]

In the event of a vehicle collision, the eCall in-band modem solution is used in an automatically or manually established emergency voice call (E112) from the vehicle (IVS) via the cellular network to the local emergency agencies, i.e. the PSAP. The eCall modem allows to transfer a data message from the IVS over the cellular network to the PSAP which is denoted as eCall MSD. The MSD can include, e.g. vehicle location information, time stamp, number of passengers, Vehicle Identification Number (VIN), and other relevant accident information.

It is expected that the eCall MSD information will be sent either immediately following the establishment of the voice call or at any point later during the voice call. The integrity of the eCall data sent from the vehicle to the PSAP is ensured by the specified modem.

eCall is a European regional requirement. It shall not have an impact on the global circulation of terminals.

4.2 eCall system requirements

The eCall service requirements have been defined in 3GPP TS 22.101 [1], and are reproduced here for information. Not all of the requirements apply to the eCall modem specification in this document.

– The data may be sent prior to, in parallel with, or at the start of the voice component of an emergency call.

– Should the PSAP request additional data then this may be possible during the established emergency call.

– The realisation of the transfer of data during an emergency call shall minimise changes to the originating and transit networks.

– Both the voice and data components of the emergency call shall be routed to the same PSAP or designated emergency call centre.

– The transmission of the data shall be acknowledged and if necessary data shall be retransmitted.

– A UE configured only to transfer data during emergency calls (e.g. eCall only UE) shall not generate signalling to the network besides what is needed to place an emergency call.

– The UE shall indicate at call setup if the emergency call will carry supplementary data.

The following specific requirements are considered necessary for the satisfactory operation of the eCall service. Additionally, all existing TS12 emergency call requirements shall apply.

– An eCall shall consist of a TS12 emergency call supplemented by a minimum set of emergency related data (MSD).

– An eCall may be initiated automatically, for example due to a vehicle collision, or manually by the vehicle occupants.

– An IVS, or other UE designed to support eCall functionality, shall include in the emergency call set-up an indication that the present call is either a Manually Initiated eCall (MIeC) or an Automatically Initiated eCall (AIeC).

– The Minimum Set of Data (MSD) sent by the In vehicle System (IVS) to the network shall not exceed 140 bytes.

– The MSD should typically be made available to the PSAP within 4 seconds, measured from the time when end to end connection with the PSAP is established.

– Should the MSD component not be included in an eCall, or is corrupted or lost for any reason, then this shall not affect the associated TS12 emergency call speech functionality.

– A call progress indication shall be provided to the user whilst the MSD transmission is in progress.

– To reduce the time taken to establish an eCall an IVS whilst in eCall only mode, may receive network availability information whilst not registered on a PLMN.

– Optionally, PLMNs may make use of eCall indicators, received in the emergency call set-up, to differentiate eCalls from other TS12 emergency calls.

– The MIeC and AIeC may be used to filter or route eCalls to a dedicated PSAP operators.

Throughout the duration of the emergency call and following receipt of the MSD by the PSAP

– It shall be possible for the PSAP to send a confirmation to the IVS that the MSD has been acted upon.

– It shall be possible for the PSAP to request the IVS to re-send its most recent MSD.

– It shall be possible for the PSAP to instruct the IVS to terminate the eCall.

For the purpose of selecting the best performing eIM solution, these service requirements have been further clarified, and performance objectives under different radio channel conditions as well as design constraints are defined in Annex A.

4.3 eCall in-band modem architecture

It is a challenging task to transmit data over the mobile voice channel as required of an in-band modem since speech codecs used in digital cellular systems are optimized explicitly for speech signal compression. Therefore, modem signals may incur heavy distortion after passing through the effective transmission channel consisting of speech codec, possible degradations on the radio channel, and speech decoder with error concealment. Furthermore, in digital cellular communications frame losses occur regularly and increase the burden of data recovery by the in-band modem.

CTM was developed in 3GPP for transmitting text data for text telephony. It was evaluated as a potential solution for eIM in the technical report (3GPP TR 26.967 [4]) and found not able to meet eCall requirements.

The present eIM solution consists of an IVS data modem and a PSAP data modem, employing signals that have been designed to pass through modern speech codecs with only moderate distortion, yet providing sufficiently high data rates for quick MSD transmission.

The overall cellular system architecture, including the IVS and PSAP data modems, is given for information in a simplified diagram in Figure 2.

After an emergency voice call has been (automatically or manually) established, the IVS modem receiver constantly monitors the incoming signal from the speech decoder output. When prompted by a request from the PSAP operator for MSD, the IVS connects the IVS data modem transmitter to the input of the speech coder and mutes any speech from the motorist for the duration of MSD transmission to prevent it from interfering with the eCall data transmission. Alternatively, it can be the IVS that may trigger the MSD transmission. In this case, the IVS asks the PSAP to request an MSD transmission.

The first operation mode shall be referred to as the pull mode whereas the latter one is the push mode. Essentially, push mode is realized by a request from the IVS to the PSAP to pull the MSD.

The requirement about the modem to be configured in either push or pull mode is beyond the scope of this specification. Refer to clause 4.2 for a reproduction of eCall service requirements.

In general, the mircophone has to be detached from the signal path whenever the eCall modem is actively transmitting.

The operational principles of the IVS and PSAP modems within the environment illustrated in Figure 1 are further explained in the following. Details of the employed algorithms and functions are given in clauses 5 and 6.

Figure 2: eCall system within the cellular system architecture

4.3.1 Principle operation of the IVS data modem

The main components of the IVS data modem are illustrated in Figure 3. The MSD information input into the IVS transmitter is first appended with CRC information. These bits are then encoded in the hybrid ARQ (HARQ) encoder using FEC coding to reduce the susceptibility to transmission errors. The HARQ encoder employs a powerful state-of-the-art turbo encoding scheme with incremental redundancy added for each retransmission. The signal modulator converts the encoded data into waveform symbols which are especially suitable for transmission through speech codecs employed in present mobile systems, including the GSM Full-Rate (3GPP TS 46.001 [5]) and the various modes of AMR codecs (3GPP TS 26.071 [7]).

The IVS receiver continues to monitor the feedback messages from the PSAP data modem. As long as the received feedback messages are NACK messages, retransmissions of the MSD with incremental redundancy are automatically continued until a sufficient number of link-layer ACK or higher-layer ACK messages has been received by the IVS, or operation is terminated by the PSAP. After the transmission of the MSD information and the ACK messages is completed, the eCall modem transmitters in both the IVS and PSAP return to idle state and the signal paths from the transmitters are switched off to avoid interference with the normal voice call.

In push mode, the IVS reuses the downlink message format for requesting the PSAP to pull the MSD. Request messages are transmitted until the IVS receiver detects START messages from the PSAP or a timeout occurs. Upon detection of the START messages the IVS continues as if it was in pull mode.

This document only specifies the eCall modem for the transmission of one MSD of length 140 bytes. Messages shorter than 140 bytes are assumed to have been padded, e.g., with zeros before being fed to the IVS transmitter. Longer message lengths would require a packet segmentation mechanism as well as adaptations to the transmission protocol, which are out of scope for this document.

Figure 3: eCall IVS data modem overview

4.3.2 Principle operation of the PSAP data modem

The main components of the PSAP data modem are illustrated in Figure 4. After having triggered the IVS data modem for transmission of MSD, the eCall PSAP receiver continuously monitors the incoming signal from the PSTN. When the eCall data signal is detected and synchronized, the signal demodulator demodulates the incoming data symbols. The HARQ decoder soft-combines the first MSD transmission with any retransmissions of the information and decodes the FEC to determine the information bits, i.e. its estimate of the CRC protected MSD information. If a CRC error is detected in the decoded MSD, the PSAP receiver returns NACK and thereby prompts the IVS transmitter to provide retransmissions with incremental redundancy. Otherwise, the MSD information is provided to the PSAP operator and the IVS transmitter is notified with link-layer or higher-layer ACK messages that retransmissions are no longer required.

In push mode, the PSAP monitors the received signal for a trigger from the IVS. Upon detection of a trigger it transmits a request for MSD transmission as it would do in pull mode and continues as described above.

The outgoing speech path is switched off when the PSAP transmitter needs to use the voice channel for feedback messages. Once the MSD is correctly received and the ACK messages are transmitted, the speech path is unmuted to avoid interference with the normal voice call.

Figure 4: eCall PSAP data modem overview