Synchronisation Keeping in an Aeronautical Satellite Communications Environment J. M. Cebrian, E. Monroy, J. Batlle and M. Admella Indra Espacio Avda. Diagonal 188, 818 Barcelona, Spain Email: jcebrian@indra.es, madmella@indra.es ABSTRACT The main goal of this paper is to analyse the method to keep synchronisation of an Aeronautical Satellite Communication System, based on geostationary satellites and CDMA access technique, to meet the world wide civil aviation safety communication expectations for the 215. The system must provide secure and reliable communications to the aeronautical community, and at the same time achieve a high degree of efficiency in the use of the satellite resources. For this purpose, a system based on synchronous CDMA has been analysed focussed on the definition of the synchronisation process strategy. This synchronisation strategy must provide the maximization of the number of users and must cope with the particularities of a mobile aeronautical propagation channel. The processes involved in the synchronisation strategy will be evaluated. In particular, the reference and traffic carriers synchronisation loops will be analysed by studying the behaviour of the carrier frequency/phase and code phase/frequency loops in a very low SNR ratio scenario. In addition, a proper Doppler compensation mechanism is defined to cope with the mobile channel. I. INTRODUCTION The Satellite Data Link System (SDLS) for air traffic management is an ESA/Industry initiative to promote aeronautical SATCOM systems for safety communications of civil aviation. The SDLS system was conceived thinking Aeronautical Mobile Communications Infrastructure for ATS in Europe, currently working in VHF band (118-137 MHz), will become critical around the 215 year even also after the introduction of 8.33 khz spacing and data links. In this sense, a Satellite communications network could be used in a transparent way for the end user either to carry some of traffic in VHF band, helping to alleviate congestion in European areas, or to offer communications backup capabilities in areas where it is non-existent or cannot be achieved economically with terrestrially based systems. SDLS is based on Geostationary "transparent" satellites, requiring signals at L-band for the mobile link and a transparent switching of the signals into C, or Ku bands for the feeder link. In the SDLS communications sub-network, the main constituents are the space segment, with the satellites and their respective control centres the ground segment, working at both C-Ku/L bands, with ground earth stations (GES) providing voice and data services interface with terrestrial Air Traffic Network (ATN). the user segment, working at L band, with air earth station (AES) terminals mounted on aircraft for providing A/G connectivity with ATC and AOC. In the past [2], an SDLS Demonstrator was built for operational demonstration to civil aviation community of the satellite technology readiness. Data and voice communications expected between aircraft, on one hand, and air trafficcontrol centres (ATC) as well as airline operations centres (AOC), on the other, were exchanged using ITALSAT F2- EMS spot and ARTEMIS satellites with recognized good results on feasibility, including cost factors, and achieved performances. From this point, a complementary study was carried-out to further investigate all topics necessaries to get a preliminary definition of an SDLS operational system. For this purpose, an European consortium was created in the 23 with participation of Alcatel Space, Indra, Atos Origin (old Slumberger-Sema), Skysoft and Airtel under the ESA contract (ESA 174/2/NL/US). Meanwhile, with the aim at coordinating the new interest of aviation community for Satellite communications, ESA reached an agreement of technical cooperation with the Eurocontrol initiating NexSAT activity. From that cooperation, a definition of the new generation satellite system mission requirements [1] was ready and a matching was prepared with the SDLS system services and functional requirements. In that definition, Indra was responsible of the physical level analyses. From the trade-offs performed, the synchronisation outcomes are going to be summarized.
The outline of the paper is as follows. In section II, the synchronisation mechanisms for the aircraft environment are presented putting special emphasis on the mobile link conditions, subject of the paper, and the different traffic services scenarios to be taken into account in the study (MAI vs efficiency, WH vs Gold, low channel speed with minimum Es/No). Synchronisation strategy followed to reach full channel synchronisation is presented in section III. In section IV, simulation schemes proposed are introduced for the different scenarios analyzed and results obtained are given. Finally, in section V, the conclusions of the study are summarized. II. SYNCHRONISATION MECHANISM DEFINITION From the point of view of the physical channel, the synchronisation mechanism must compensate the effect of mobility due to the aircrafts movement, mutual interferences among other users, and the consequence of multipath originated in the aircraft itself and in the ground or sea. The AES movement with respect the satellite considered in the analysis carried-out corresponds to an AES speed of 42 m/s and acceleration of.2 m/s2, and the signal to noise plus interference ratio was established in 2 3 db.the multipath propagation model has been approximated to a Rice channel model, being the direct/reflected link power difference of K= 9 db mainly due to the reflections in the aircraft. The traffic characteristics to be supported in the SDLS system for data applications and voice services are the following: Application Short data Service CPDLC high ACL CPDLC medium CAN, DLL, DCL, DSL CPDLC low DFIS AOC/FANS ACARS ADS-C FLIPCY,FLIPINT, DYNAV,COTRAC ADAP QoS QoS 1 QoS 2 QoS 3 QoS 4 QoS 5 QoS 6 Transit Delay 5s 5s 1s 3s 3s 5s RER 1-8 1-8 1-7 1-6 1-7 1-7 Priority 1 1 2 3 2 2 Symmetry SDLS Bearer Service Unidirectional Bidirectional Bidirectional Bidirectional Bidirectional Unidirectional Air to Ground Air to Ground Data 5 Data 1 Data 2 Data 3&6 Data 3 Data 4 Table 1: SDLS Data services[3] ATS/Safety Voice AOC/Telephony Party-Line Vocoding rate (kbps) 4.8/2.4 4.8/2.4 4.8/2.4 Communication establishment <2s <2s N/A (Permanent) time (s) Voice latency 4ms 4ms 4ms Communication configuration Point to Point Point to Point Point to multipoint Residual Error Rate (RER) 1-3 1-3 1-3 Priority 1 2 1 SDLS Bearer Service Voice 2 Voice 1 Voice 3 Table 2: SDLS Voice services[3] From the above traffic characteristics, the synchronisation mechanisms to be defined must cope with: Circuit oriented communications, mainly devoted to voice services, and, Packet oriented communications, with different constraints in the transit delay for supporting data applications. The circuit oriented connections require the establishment of dedicated links that involve the capacity to setup a synchronous CDMA channel in a short period of time. In addition, the packet oriented connections call for a mechanism not penalised by a channel establishment time, but at the same time with a high probability of detection and collision avoidance. In addition, a low transit delay requires means from the access scheme to allow an almost immediate availability of resources. This implies the maintaining of permanent synchronized CDMA channels and the definition of a TDM structure on top according to transit delay needs.
Several mechanism are considered to transmit efficiently the different traffic types: A connection by connection synchronisation scheme, for voice and for not critical data packets services transmission in strength mode. This scheme is appropriate for long connections in which the fact of requiring a connection setup with a resource assignation and a fine synchronisation process is tolerated by the service. An asynchronous burst synchronisation scheme, for data packet service transmission in a Random access channel over Spread ALOHA. It will be used for the log on of the AES to the system, because it provides a fast transmission mechanism although can be experienced collisions and a high level of mutual interference from another traffic carriers. A permanently synchronised scheme, for any type of services avoiding the use of asynchronous carriers and reducing the connection setup to a resource assignation in the worst case. In addition, a CDMA carrier can be shared among several AES following a TDM over CDMA approach. This scheme makes the system more complex because it requires a synchronisation maintenance process for all the AES in the system, but the system becomes more efficient when data traffic rate is low. III. SYNCHRONISATION STRATEGY One of the goals of the mobile link synchronisation consists in counteracting the AES movement effect, either in the carrier and in the code tracking loops. To compensate AES mobility effect a Doppler estimation process has been analysed based on the information obtained from the reception of a reference carrier. The synchronisation processes to be considered will be those used in: Log on phase, to synchronise the AES transmissions with the system reference. Synchronisation maintenance phase, carried out to keep all AES synchronised in the system. The synchronisation process during the Log-on phase consists in the next main stages. The AES synchronises the forward reference carrier from the GES. The synchronisation consists in locking the carrier frequency and code phase/frequency of the AES receiver to the GES reference carrier by performing local carrier frequency and the code phase/frequency tracking synchronisation loops. The Doppler shift and rate is estimated from the reference tracking loops and pre-compensated in the AES transmitter. The AES sends an asynchronous burst to the GES, through an specific random access channel, to allow a coarse estimation of the timing error in the transmission. The GES detects and process the asynchronous burst and sends back to the AES the coarse correction. The fine synchronisation process is then carried out, which consists in performing a long remote loop of AES transmissions once compensated the Doppler rate effect and GES accurate measurements of the synchronisation errors, being finally transmitted to the AES the corresponding corrections. When the errors are below a threshold, synchronisation is declared by the GES and the log on process is finished. At this moment, the AES is ready to transmit traffic. The synchronisation process during the synchronisation maintenance phase consists in a periodic transmission from each AES of a predefined sequence and a remote measurement in the GES of the synchronisation errors. In addition, the AES is continuously estimating the Doppler dynamics and pre compensating the transmitter frequency and timing. The main synchronisation steps simulated and results presented in the following chapters correspond to: Reference carrier frequency and code phase / frequency synchronisation local loops. Asynchronous bursts detection in presence of traffic carriers that causes a high level interference. Traffic carrier frequency and code phase synchronisation remote loops to keep the AES transmitter fine synchronisation. IV. SYNCHRONISATION SCHEMES SIMULATION Reference Carrier Synchronisation Local Loops The reference carrier process together with the Doppler pre-compensation mechanism to be simulated is depicted in Figure 5. The main objectives for the reference carrier synchronisation loops are to cope with the tracking of worse ramps conditions expected in the system: Maximum carrier frequency Doppler rate of 1.6 Hz/s, Maximum code frequency shift of 1.5 chip/s and Maximum code frequency rate of.46 Hz/s. The reference carrier synchronisation local loops include two phases: acquisition and tracking. The acquisition process compensates for the initial carrier frequency error and synchronises the receiver code phase or time to the received
reference one. Once the acquisition is completed, the tracking process is performed, comprising the carrier frequency and code phase and frequency tracking loops. The work carried out is focussed on the tracking loops which are the ones more affected by the dynamic Doppler. The carrier frequency tracking loop consists in an Automatic Control Loop that governs the receiver carrier frequency oscillator. The transient behaviour of the loop with the specified dynamic Doppler rate is shown in Figure 3. As the reference frequency is continually received and tracked, the transient duration has not stringent requirements. In consequence, the loop bandwidth has been optimised to reduced the steady state jitter (shown below), at the price of a longer transient time. When the system is fully loaded, the frequency error detector is degraded, causing spikes in the measured error, as shown in the figure below for an initial error of 9 Hz. To solve that, the coherent integration time has been increased, avoiding the problem as it can be seen in Figure 1. Figure 1: carrier frequency error (left) degraded and (right) improved with a coherent integration time The code phase and frequency tracking loop, with transient behaviour depicted in Figure 4, is based on an standard Delay Locked Loop. One of the main difficulties of the loop rises when the system is fully loaded. In that case, the phase detector curve is degraded, decreasing its slope dramatically and producing a very high jitter that makes the loop very unstable. The phase error curve (S-curve) was simulated in presence of mutual interference from other users carrier. The Figure 2 shows the S curve degradation when the number of users is increased. In order to solve this problem, the reference carrier includes known symbols to allow to increase the coherent integration time and reducing the effect. The output of the reference carrier loops feed to the Doppler estimator process, that estimates the Doppler dynamics to pre-compensate the transmitter. SFpilot = 32 SFuser = 32 Code phase error estimation NO-COHERENT 2 Adv2-Del2/Ont2 1-1 1 user 16 users 3 users -2-1 -.8 -.6 -.4 -.2.2.4.6.8 1 Chips 1 Adv2-Del2/Adv2+Ont2+Del2.5 -.5 1 user 16 users 3 users -1-1 -.8 -.6 -.4 -.2.2.4.6.8 1 Chips (a) MAI effect with different users NON-COHERENT (b) MAI at maximum load with symbols accumulation COHERENT Figure 2: Code phase error estimator curve, with mutual interference.
2.5 Carrier frequency estimator output. WH+EG codes. EsNo:3dB. Freq.ramp:1.62Hz/s.8 Code phase error. WH+EG codes. EsNo:3dB. Freq.ramp:.463Hz/s.7 2.6 Carrier frequency error detected (Hz) 1.5 1.5 Code phase error (chips).5.4.3.2.1 -.5 -.1-1.5 1 1.5 2 2.5 3 3.5 4 Symbols x 1 4 -.2.5 1 1.5 2 2.5 3 3.5 4 Symbols x 1 4 Figure 3: Transient of reference carrier frequency estimation loop Figure 4: Transient of the reference code phase estimation loop The Table 3 presents the results for the reference carrier frequency loop and the Doppler estimator output for different estimator bandwidths. The loop bandwidth was selected to minimise the thermal and mutual interference noise at the loop input. The input signal was affected by a Doppler dynamic effect of 1.62 Hz/s, which is followed by the loop very accurately. The residual effect of the Doppler estimator is the slightly increment of the output jitter. E s /N = 3 db E s /N = 2 db mean (Hz) jitter (Hz) mean (Hz) jitter (Hz) frequency error.32.135 -.143.1477 Doppler estimator BW (Hz) mean (Hz) jitter (Hz) mean (Hz) jitter (Hz) 2 1.628.4415 1.6181.58 1 1.625.3735 1.6178.4216 6 1.623.3117 1.6177.3494 2 1.62.1774 1.6181.1899 Table 3: Reference carrier frequency loop results, for different Doppler estimator bandwidths The Table 4 presents the results for the code phase loop and the code frequency Doppler estimator. The input signal was perturbed by a code frequency Doppler of.46 Hz/s. It can be seen that a wide Doppler estimator BW produce a high code frequency jitter. phase error (chips) Doppler estimator BW (Hz) E s /N = 3 db E s /N = 2 db mean Jitter mean jitter -.263.323 -.311.344 mean (Hz) jitter (Hz) mean (Hz) jitter (Hz) 5.7 5.4676.374 5.864 2.4815 2.6941.4411 2.8777.5.482.7645.567.8137.25.4281.3886.4896.4136 1 Table 4: Reference code phase loop results, for different Doppler estimator bandwidths 1 By integrating N remote error corrections of code phase, this jitter can be further reduced.
Asynchronous Burst Synchronisation Loops The main goal of the asynchronous burst synchronisation loop consists in minimising the length of the preamble required to detect it and analyse the resulted jitters in the carrier frequency and code phase loops, as its output is sent back to the AES as coarse transmitter correction. This process must work in a worst case scenario due to all other traffic carriers are asynchronous to the one to be received. In order to reduce the required preamble length and minimise the final jitter, the loop starts with a high loop bandwidth to cope with the initial carrier frequency and code phase errors (178Hz and.5chips/.4 2 Hz in the worse case), and then the loop bandwidth is narrowed once the initial frequency errors are reduced sufficiently (up to 1-15 Hz of carrier frequency correction and.1-.2chips of code phase correction). The goal of the transient of the carrier frequency loop is to achieve a maximum error of 1 to 15 Hz. The behaviour of the loop is summarized in the table below, with the peak error at the end of the transient and the steady state jitter. Es/(No+Io) (db) Transient duration (symbols) Carrier frequency correction jitter after transient (peak: 3σ) Carrier frequency error jitter in Steady State(Std:σ) Carrier frequency correction jitter in Steady State(Std:σ) 3dB 3 symbols 11 Hz 3.3 Hz.4 Hz 2dB 325 symbols 12 Hz 3.7 Hz.5 Hz Table 5: Asynchronous carrier frequency loop errors The code phase loop is critical due to the asynchronous nature of the carrier. The loop is also implemented with different loop bandwidths for the transient and the steady state. In the table below, it can be observed that the maximum code phase error at the final stage of the transient reaches a high value, due to the initial code frequency error. But even in this worse case scenario, the loop accomplish to reduce the final jitter in the steady state. Es/(No+Io) (db) Transient duration (symbols) Code phase correction jitter after transient (peak: 3σ) Code phase error jitter (Std:σ) in steady state Code frequency correction jitter (Std:σ) in Steady state 3dB 4 symbols.11 Hz.11 chips 2 Hz 2dB 4 symbols.13 Hz.12 chips 2.3 Hz Table 6:Asynchronous code phase loop errors Traffic Carrier Synchronisation Remote Loops The traffic carrier long loop synchronisation process is intended to remotely synchronize and keep synchronised the AES transmitter from the GES. Basically, the GES measures the synchronisation error of the AES transmission with respect to the reference carrier, compute the corrections and sends them to the AES. The loop is designed to minimise the time required to reach the synchronisation state during the log-on phase, and at the same time reduce the jitter to maximize the period between transmission required to keep the synchronisation. The traffic carrier loop block diagram is depicted in Figure 6. The requirements of the carrier frequency and code phase remote loops are: a maximum initial carrier frequency error of 2 Hz, a maximum initial code frequency error of.4 2 Hz, and a maximum initial code phase error of.5 chips. The results of the Figure 7 and Figure 8 show that after 3 to 4 corrections the AES remote transmitter has reached the steady state, considering as the maximum error at the end of transient the carrier frequency value of 5Hz (3σ) and,125(3σ) of code phase. In Table 7, it can be observed that incrementing the error measurement time, the steady state jitter is reduced at the cost of a small reduction in efficiency, but improving the capacity to track the system instabilities. 2 Value due to GES/AES clocks and Satellite Doppler residual errors.
Es/(No+Io) Nº of known symbols to compute the error Code phase error jitter (at σ) Carrier frequency error jitter (at σ) 3 db 2 symbols.5 chips 1.1 Hz 3 db 1 symbols.6 chips 2. Hz 2 db 2 symbols.5 chips 1.3 Hz 2 db 1 symbols.7 chips 2.3 Hz Table 7: Traffic carrier long loop behavior with time to compute error. RX Carrier Carrier Pilot Acquisition Pilot Carrier Tracking (AFC) AES Remote Transmitter GES receiver A/D X T De-spreaded samples T X D/A Channel Rx Code Code Phase Correction Code Pilot Code Phase Tracking (DLL) Code Osicllator Carrier Remote Carrier Tracking Remote Code Tracking Carrier Carrier TX Doppler Precompensation process Round Trip Delay + Mobility Carrier Code Phase/frequency D/A X T Code Code Figure 5: Local carrier frequency/code phase block diagram with Doppler pre-compensation Figure 6: Remote carrier frequency/code phase block diagram 26 Carrier frequency correction. Gold codes. EsNo:3dB. Freq.step:2Hz. 2symbols.1 Code phase error. WH+EG codes. EsNo:3dB. Freq.step:.4Hz. Code phase step:.5chips 25.5 24 Carrier frequency correction (Hz) 23 22 21 2 Code phase error (chips) -.5 -.1 -.15 -.2 -.25 19 -.3 18 -.35 17 5 1 15 2 25 -.4 5 1 15 2 25 Figure 7: Remote transient of carrier frequency corrections Figure 8: Remote transient of code phase errors
V. CONCLUSIONS AND FUTURE LINES OF WORK As summary of the synchronisation work performed the following conclusions are taken. Three different type of CDMA carriers will be needed. Firstly, the return and forward reference carriers will be used to allow locking in the system respectively the GES and AES receivers. Secondly, a limited set of Asynchronous carriers will be needed to carry the initial AES signalling requested to perform log-on in the SDLS system in spread-aloha mode. Finally, the User synchronous carriers will carry voice and data traffic in both continuous mode for AES single use and TDMA mode for several AES sharing one CDMA carrier. With the reception of Reference carriers, the AES computes the reference carrier synchronisation local loops with the purpose of tracking worse case dynamic Doppler effect provoked by AES mobility. This objective is accomplished with the continuous use of a Doppler pre-compensation mechanism and increasing the coherent integration symbol time. By receiving Asynchronous carriers, the GES is able to detect the Asynchronous burst sent by AES trying to log on the system performing the asynchronous burst synchronisation process in a reduced preamble length by implementing different loop bandwidth from the transient and steady stages. This allows maximizing the number of AES log-on per unit of time because the spread-aloha time window is reduced. Finally, the GES is able to keep the synchronisation of all AES transmitters logged in the system by implementing the traffic carrier synchronisation remote loops when receiving periodic known symbols from each AES through the User synchronous carriers. Finally, and as a matter of improvement of results obtained in the code frequency in the Doppler estimator process, the AES could integrate N remote code phase corrections received from the GES when performing the traffic carrier synchronisation remote loops and provide this value directly to the TX code frequency oscillator instead of using the corresponding output from the Doppler pre-compensation module, see Figure 5 and Figure 6. Moreover, to reduce the residual 2 code frequency error of the traffic carrier synchronisation remote loops a possible broadcast of the reference carrier synchronization local loop corrections could be done by the GES reference station. REFERENCES [1] EUROCONTROL, New Generation Satellite Communication System(s) Mission Requirements, Edition E, October 23. [2] Mathieu Dabin, Erick Flores, Jean-Marc Gaubert, Satellite Data Link System (Sdls) Demonstrator. Towards a Future Generation Aeronautical SATCOM for ATM, IAF conference. [3] Alcatel Space, SDLS Slide 3 - Executive Summary, ESA contract 174/2/NL/US, 23. [4] Indra Espacio, SDLS Slide 3 - Access Mode Analysis Report, ESA contract 174/2/NL/US, 23. [5] Indra Espacio, SDLS Slide 3 - Waveform Analysis Report, ESA contract 174/2/NL/US, 23. [6] ICAO, Annex 1 - AMSS SARPs, Volume III + Amendment 75, July 1995 and November 2. [7] Claude Loisy, Satellite Data Link System (SDLS). A dedicated mobile satellite communication system responding to the highly demanding requirements of civil aviation, proceeding of the AIAA Conference 21.