Testing eCall systems: how to benefit from synergies with existing GSM test platforms

In the event of an emergency or accident, the eCall e112 flag system will provide the ability to call the local emergency services via a Public Safety Answering Point (PSAP) using any available cellular network. While eCall will not prevent accidents, it will speed the arrival of emergency assistance. A key requirement is that data and voice calls must use the same voice channel because SMS and GPRS do not provide the necessary service priority or availability. Routing data over the voice connection will enable eCall to use the e112 routing protocol standards deployed in the existing cellular network. The minimum set of data (MSD) required to be transmitted by EN 15722:2011 comprises the exact vehicle location (from the vehicle's GPS device), time and date stamps, number of occupants and Vehicle Identity Number. A number of cellular standards have the potential to manage and deliver the various protocol control and data required for eCall to function correctly. But, as high data bandwidth is not the primary requirement, the initial deployment of eCall will focus primarily on the 2G network (GSM/GPRS/EDGE) widely available within Europe. However, multiple cellular standards are likely to be supported on future eCall chipsets.








Typically, eCall devices and modules will be integrated in the vehicle's telematics systems and will play an increasingly prominent role in automotive electronics design. The following system blocks are necessary for eCall to work successfully: • In Vehicle System (IVS). Alongside the automotive telematics unit, the module will include such sub systems as the GPS module, multiple vehicle sensors, microphone/speakers, IVS data in-band modem, 2G/3G communications modem and the vehicle application software. The eCall voice and data message can be originated and activated automatically or with driver intervention. • Mobile operator network. This is responsible for transmitting and routing the eCall emergency e112 flag message to the emergency call response centre (PSAP). • Public Safety Answering Points (PSAP). Call centres responsible implementing the infrastructure required to receive eCalls and for answering them. The PSAP transmission section is responsible for sending control messages to the IVS to initiate transmission of MSD information and for providing ACK/NACK feedback for the hybrid automatic repeat request (HARQ). In the event of a collision, an eCall flag is triggered and two way voice communication is established between the PSAP and the driver. In addition, eCall can transfer data from the vehicle over the same cellular network connection. IVS in-band data modem The primary blocks of the in-band transceiver are cyclic redundancy check (CRC), forward error correction (FEC) codecs, HARQ, data modem and a sync/multiplexing block. MSD information is input to the IVS modem via the CRC section, where cyclic code data bits are appended. This additional code will be used by the PSAP's data modem to determine whether the original message has been corrupted. If the verification check reveals errors, the system will send ACK/NACK feedback messages requesting repeat transmission (ARQ) of problem data blocks. MSD information bits are then subjected to channel encoding in the HARQ encoder using FEC, where redundant error detection bits are added to the already modified data. The HARQ encoder is a combination of ARQ and FEC coding and typically contains a powerful Turbo coding scheme with incremental redundancy added to each data retransmission. The FEC technique reduces the susceptibility of data to errors during transmission over noisy or inefficient cellular rf channel links. FEC enables the receiving PSAP modem to correct errors without needing to request retransmission of the original message. However, while HARQ offers better performance in poorer channel conditions, it has the disadvantage of significantly lower data throughput in improved channel conditions. The signal modulator up converts the data stream by mixing it with a carrier waveform suitable for it to be applied to a speech codec. The voice speech encoder and decoder can support adaptive multirate (AMR) and full and half rate (FR/HR) GSM audio data compression schemes. These generate a compacted data bit stream output representation of the analogue speech signal, whilst providing an adequate level of audio quality. These speech coded standards are commonly employed in GSM and UMTS systems. The receiver section demodulates and monitors the corresponding ACK/NACK message sent by the PSAP modem. Once MSD transmission is completed and a successful ACK message has been received, the IVS and PSAP modems are placed into idle state by deactivating the transmitter signal paths. The equivalent PSAP receiver and transmitter sections have similar building blocks, but function in the reverse order. The HARQ mechanism block is not used and there is a different FEC implementation. Test challenges and limitations The eCall simulation system overcomes challenges presented using a deployed live network system, including emergency services testing without the need to contact an operator. This prevents an emergency services response being triggered accidentally. If emergency services testing is required, the Anritsu eCall system can function as a development stage test and simulation solution in advance of the live network becoming available. The IVS DUT contains various functional blocks which require independent verification and testing. To verify the raw MSD data, a logic analyser confirms the transmission of the low serial data rate defined by its set of requirement definitions.













eCall test solution The MD8475A simulates the PLMN and PSAP sections of the live network whilst providing a convenient platform for verifying the voice call connection and the MDS content transmitted by the IVS device under test (DUT). When using the eCall test solution, the user is not restricted to testing on a live cellular network and PSAP provider. Using a simulated and controlled test environment means the DUT will not be subjected to cell and connection link quality issues; beneficial when testing the higher layers of the software protocol implementation. The eCall tester provides functions to test the MSD and voice call communication sequence between the IVS DUT and PSAP. Current communication sequence functions supported include voice codec (AMR, GSM FR/HR), in-band modem (push and pull mode), voice operator and loopback calls and voice quality. The tester displays the current MSD, voice and in-band modem communication status between the IVS and PSAP and, as the MD8475A supports all major technology standards, the platform is equipped to provide an upgrade path to supporting all eCall technology implementations. The Anritsu MD8475A does not verify in-band modem block functionality in isolation. By combining the IVS elements, the eCall solution can verify the system as a whole, enabling end to end system test and providing the ability to simulate a complete operational solution. As the MD8475A includes a base station cell emulator, the radio modem section can also be verified independently. The implementation of simple test routines and integration of future proof cellular standards allow the tester to be easily upgraded when enhancements and amendments to the eCall standards are introduced. Lee Roberts is business development manager for Anritsu (UK).