Asynchronous Random Access Schemes for the ...

Asynchronous Random Access Schemes for the VDES Satellite Uplink Federico Clazzer, Andrea Munari, Federico Giorgi

Abstract—The paper focuses on the satellite uplink for the new VHF data exchange system (VDES), which aims at providing a worldwide messaging service for vessels. In this context, we investigate the use of some recently proposed asynchronous schemes for the narrowband random access mode of the standard. Remarkable improvements over the basic VDES self-organised TDMA return link access are shown in terms of both spectral efficiency and system coverage when considering realistic ships distributions. The study offers relevant insights for the ongoing discussions on the MAC layer design for upcoming versions of the VDES standard.

I. I NTRODUCTION In recent years, the automatic identification system (AIS) has experienced a critical overload of its very high frequency (VHF) channels [1], originally allocated for the sole exchange of vessels movement information to prevent collisions [2]. While the intensification of ship transport certainly played a role, the current congestion is largely due to the use of the VHF band for non-critical messaging applications, especially in regions densely populated by vessels. The situation called for the development of a new maritime communication system, dedicating specific spectrum to new services and relieving safety-critical channels from an excessive burden. To answer this demand, the International Telecommunication Union (ITU) recently released the VDES [3] standard, which extends AIS by allocating bandwidth for two application specific message (ASM) channels as well as additional 150 kHz for both up- and downlink communications. Within the latter, at least 50 kHz are granted exclusively to satellite links, while the rest is dedicated to terrestrial connections available close to the coast. To provide messaging services to vessels over such newly available yet limited resources, an efficient design of the satellite uplink channel access appears thus paramount. From this standpoint, the simplest solution would be the well-known self organizing time division multiple access (SOTDMA) policy, Federico Clazzer and Federico Giorgi are with the Institute of Communications and Navigation of the Deutsches Zentrum f¨ur Luft- und Raumfahrt (DLR), D-82234 Wessling, Germany (e-mail: {federico.clazzer, federico.giorgi}@dlr.de). Andrea Munari is with the Institute for Networked Systems of the RWTH Aachen University, D-52072 Aachen, Germany (e-mail: [email protected]). This work has been accepted for publication in IEEE/MTS Oceans’17, c Aberdeen, UK. 2017 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works.

employed in AIS and originally devised for communications among neighbouring ships. When used to send messages to a satellite, however, this approach has been shown to incur significant throughput degradation in areas with high traffic density [4]. Due to the very large footprint of a low earth orbit (LEO) satellite, in fact, the benefits of distributed medium coordination among vessels in reciprocal reception range are lost, leading to packet collisions at the receiver and to performance similar to a slotted ALOHA (SA) scheme [5]. A second drawback for the use of SOTDMA in the satellite uplink stems from its slotted time-structure, which is difficult to maintain when LEO constellations are employed to provide worldwide coverage. In such settings, in fact, vessels may be covered by more than one satellite at a time, yet be properly synchronised to only one of them due to different slant paths. In view of these remarks, while research to improve reception of SOTDMA messages at the satellite continues [6], [7], VDES explicitly foresees the possibility to employ asynchronous random access (RA) policies for the uplink channel [1]. From this standpoint, spread spectrum ALOHA and its evolutions [8], [9], [10] may first come to mind, as they are often employed for messaging applications over the satellite return link [11]. Nonetheless, the very limited amount of bandwidth granted in the maritime setting naturally tweaks the interest towards narrowband RA schemes. Recent years have, in fact, witnessed the development of a family of such solutions that go significantly beyond the performance of a plain ALOHA [12] approach. Dramatic improvements have, in particular, been grasped by sending multiple replicas in a proactive way and exploiting successive interference cancellation (SIC) and other combining techniques at the receiver [13], [14]. These protocols keep a very low complexity at the transmitter side, and their implementation would entail only minor modifications with respect to the basic AIS waveforms. Based on this ground, we explore in the present paper, the potential of a family of asynchronous narrowband RA schemes for the satellite maritime return link. We compare their performance to the one of a baseline ALOHA approach, that can be seen as representative of the present narrowband solutions of the VDES standard. By means of simulations that account for a proper link budget and distribute vessels based on real measurements, we shed light on the benefits attainable resorting to solutions that combine repetition of packets and SIC at the receiver in terms of water coverage and number of served vessels. The presented results are meant to stimulate further interest on the upcoming and still under discussion versions of the standard. We start our discussion in Section II by revising the VDES

uplink and presenting the recent asynchronous RA techniques. We define our scenario and system model in Section III, with particular attention to the realistic link budget and medium access (MAC) performance of the various protocols. We present the numerical results on worldwide water and vessel coverage in Section IV and we close the paper with some concluding remarks in Section V. II. VDES AND A SYNCHRONOUS R ANDOM ACCESS In this Section we discuss some of the key aspects of the VDES satellite uplink and we present some appealing solutions for the MAC layer of this communication link. A. VHF Data Exchange System Satellite Uplink The VDES standard – under discussion and development in these months – is focusing on two main data traffic scenarios for the satellite uplink. On the one hand, there is the high volume on-demand communication traffic, for which DAMAlike waveforms are defined. On the other hand, the messagingtype communication of machine-to-machine (M2M) applications, characterized by low duty cycle and small message size is also taken into consideration, as a second class of waveforms is defined. The DAMA traffic requires relatively high data rates, and most likely, is prevalently served by other communication systems on board the vessel, as very small aperture terminals. We will therefore focus on the second class of communication traffic in this work. We are considering a framed system, in which DAMA traffic and M2M traffic do not coexist. In particular, each frame is uniquely dedicated to one specific traffic type, either DAMA or M2M, and can change from frame to frame. The frame structure is inherited from the AIS subsystem within the VDES transceiver. There in fact, transmissions are organized into frames of 1 minute. Several waveforms are foreseen for the satellite uplink M2M-type of traffic [3]. They can be divided into two main classes. The first class focuses on spread spectrum techniques [15], [10], that are more resilient to interference coming from concurrent transmissions of other vessels. Typical for spread spectrum techniques, is the capability of allowing a high number of concurrent transmissions at the same time, thanks to the receiver capability to distinguish several interfering waveforms. In particular, each packet’s coded bit is multiplied by a common pseudo-random bit sequence, known both at all transmitters and at the receiver. The sequence has low crosscorrelation properties so that the receiver is able to synchronize to a detected packet, rejecting the other interferers as random noise. This is achieved at the expense of long packet durations, since the total available bandwidth is fixed, and results in lower data rates compared to narrowband transmission. Furthermore, the challenging maritime domain limits the ship-to-satellite link to only 50 kHz, and forces the use of spreading factors (or processing gain) of only 8 or 16. These values are very limited compared to typical spread spectrum systems, where it is common to enable spreading factors of 512 or more. In order to overcome these limitations, a second class of waveforms is also defined in the standard. They rely on a slot synchronous system, inherited from the MAC modes of AIS.

Nonetheless, slot synchronicity might be particularly difficult to achieve and maintain, especially when a worldwide LEO constellation providing coverage to all vessels is considered. In such a scenario, multiple satellites might be visible to a vessel simultaneously, and there is no possibility to let the vessel be slot-synchronous to all of them at the same time. This is due to the different satellites position as well as their relative orbits compared to the vessel’s position. A pragmatic and very practical approach would be to let the users skip entirely slot-synchronization. On the one hand, the transmitter would become more simple, since only frame synchronization shall be kept. On the other hand, the receiver shall be enhanced in order to achieve comparable performance to slot-synchronous systems. In order to shed the light on viable options in this regard, we review next two valuable narrowband options for the satellite uplink of the VDES standard, i.e. CRA and ECRA. B. Contention Resolution ALOHA (CRA) The CRA protocol [13] is based on two main pillars: packet repetition at the transmitter, and SIC at the receiver. In particular, each transmitter replicates its physical layer packet d times. Each replica contains, within its header, information about the location of all its twins. The d replicas are located in the frame uniformly at random, with the only constraint to fit within the frame boundaries and to avoid self-interference. At the receiver, the content of an entire frame is stored and the processing starts looking for replicas. Once the first replica is detected, decoding is attempted. If the decoded data sequence passes the cyclic redundancy check (CRC) it is declared as correctly decoded, and the header content can be reliably exploited to identify the other replica locations. In this way, the receiver knows the content of all replicas together with their location. The data content in fact, is equal for all replicas belonging to the same user. While the replica locations is known because it is stored in the header that the receiver retrieved. In this way, the replicas waveform can be reconstructed and canceled from all the locations within the frame, removing the interference contribution. This process is well known and is called Interference Cancellation (IC) or SIC. The receiver proceeds, seeking for further replicas within the frame, clearing up in an iterative sequence decoded replicas, and progressively reducing the amount of interference in the frame. The iterative cancellation stops when either all packets are correctly decoded, or a the maximum number of iterations is reached, or when no further packets can be correctly decoded. Compared to the ALOHA protocol, CRA is able to impressively improve the throughput beyond 5 times, for some channel conditions [13]. C. Enhanced Contention Resolution ALOHA (ECRA) Although very effective, SIC may be blocked if some specific interference configurations between replicas happen. If, for example, both replicas of two or more users are reciprocally causing interference, because they are received in overlapping instances of time, SIC may be unable to resolve the collision. An example is given in Fig. 1, where packets

(Possible) VDES Uplink f

VDES Terrestrial Uplink

100 kHz

RA

50 kHz

User 1

A1

User 2

B1

RA

RA

DAMA

DAMA

RA

RA

DAMA

DAMA

DAMA

RA

VDES Satellite Uplink

A2 B2 Collision C1

User 3 User 4

D1

C2 D2 E1

User 5 User 6

Interference free

F1

E2 F2

Fig. 1: Sketch of the possible frequency allocation for the VDES uplink and framing between RA and DAMA. It is assumed that 100 kHz are reserved for terrestrial uplink, while the remaining 50 kHz are utilized by the satellite uplink. In a time division pattern, frames are dynamically allocated to either DAMA or RA. For the RA frames, an exemplary CRA/ECRA frame with repetition rate 2 is shown at the bottom of the figure. The iterative process of SIC starts looking for replicas that can be correctly decoded. The first one to be found is the replica C2. Once correctly decoded, information about its twin, i.e. replica C1 can be retrieved form the header. Thus, the interference contribution of both instances of user’s 3 packet can be successfully removed from the frame. In this way both replica A2 and D1 are left free from interference, leading to further successful decoding. The SIC can proceed alone, until we are left with the replicas of users 5 and 6. At this stage, CRA would declare the two users as not decoded, while in ECRA, MRC combining is applied. In this way, user 5 packet can be also decoded, releasing user 6 replicas from the interference and leading to a full success in this frame.

of users 5 and 6 may not be decoded by CRA. In such scenario, different portions of the two replicas of one user may be interfered, and adopting combining techniques can resolve the collision. This idea is brought into application by an evolution of CRA, the ECRA scheme [14], [16]. The transmitter operations remain unchanged compared to CRA while the receiver side is enhanced. In the first phase, SIC is iteratively run as per CRA. Once it stops, if some replicas are left in the received signal, replicas combining takes place. Assuming that MRC is adopted at the receiver, the weighted sum of the received symbols is computed, considering the aggregate noise plus interference power of each symbol in both replicas. The weights are computed so to maximise the output of the combiner. As a consequence, the decoder sees an increased Signal to Interference and Noise ratio (SINR) as per equation (2). When instead selection combining (SC) is adopted, for every packet symbol, the one received with the highest SINR among all replicas is selected and fed into the decoder. Regardless of the combining technique, if the decoder is successful, interference cancellation takes place, and the interference contribution is removed from all the replicas location. The process is iterated until no packets are present in the received signal, or a maximum number of iterations is reached.

III. S CENARIO AND S YSTEM M ODEL In the following we will consider a population of vessels generating data traffic that is collected by a LEO satellite. The transmission is organized into frames of 1 minute duration, as per AIS. Within the frame, packets can be transmitted without any time constraint, apart from the requirement to respect the frame boundaries. The full 50 kHz band is exploited by every transmission. In CRA or ECRA every transmitted packet is replicated 2 times within the frame and information about both locations is stored in the packet header. Our focus is on a scenario where every vessel transmits 10 data packets per frame,1 or per minute. Hence, the replicas are sent with a maximum relative delay of 6 seconds. In line with the first available revision of the VDES standard [3], we select a waveform in which packets are composed by 256 information bits, encoded with code rate r = 3/4 and quadrature phase shift keying (QPSK) modulated. In this way, each physical layer packet results in 171 symbols. The LEO satellite is flying at 524 km over the Earth surface, resulting in a reception coverage radius of 2500 km, assuming an omni-directional antenna. Vessels are realistically distributed over the waters, thanks to satellite data from 2013 1 The number of data packets per frame is arbitrarily chosen. In the real system, it will be defined given the supported service.

collected by exactEarthr [17], [6].2 The exact vessel-satellite distance is taken into account for the path loss evaluation. Additionally, we consider a block Rayleigh fading model, since the data transmission is likely to be affected by vast amount of reflections, caused by the presence of metal objects on board of a freight or container vessels. In this way, the power at the input of the receiver is P = Pt δv ρ, being δv path-loss of vessel v, Pt the transmission power (equal for all vessels) and ρ an exponential r.v. with unit mean and Probability Density Function (PDF) fρ (a) = e−a , a ≥ 0. Specifically:   c δv = gt gr 4πf dv with gt and gr the transmitter and receiver antenna gain respectively, f ∼ = 162 MHz the link frequency, dv the vesselsatellite distance and c the speed of light constant. The selected values for the numerical evaluation are summarized in Table I and are in accordance with the VDES standard [3]. At the receiver’s input, the s-th symbol SINR γs can be written as P . γs = N + Is Where N is the noise power and Is is the aggregate average interference over the s-th symbol. The receiver decoding probability is modeled with the capacity threshold. Hence, given a selected rate R including both code rate and modulation, a replica is declared as correctly decoded when the rate R is below the average channel capacity, Ds 1 X R< log (1 + γs ). Ds s=1 2

(1)

The parameter Ds identifies the number of symbols in a packet. In case of ECRA with SC, since for each symbol the one with the highest SINR is selected among all replicas, the right hand of the inequality (1) is subject to increase. The s-th symbol SINR at the input of the receiver after SC is Pr . r=1,2 N + Is,r

γs = max

When considering ECRA with MRC, the two replicas are combined prior to decoding. An increased SINR symbol by symbol is observed at the input of the receiver, i.e. γs =

2 X r=1

Pr . N + Is,r

(2)

For reference purposes, we will consider also an ALOHA system where the receiver applies SIC. In this scenario, transmitters are still allowed to transmit without time synchronization constraint and additionally do not need to replicate the packets. At the receiver side, whenever a packet is correctly decoded, its interference contribution is removed as per SIC. We performed Monte Carlo simulations in order to evaluate the spectral efficiency and packet error rate (PER) performance 2 The vessels are uniformly distributed over the coverage area, per satellite position. Nonetheless, the total amount of vessels follows the real data collected by the exactEarthr satellite system. It has been shown that this approximation holds well for many practical situations [6].

TABLE I: Parameters of the VDES Satellite Uplink Parameter

Value

Description

Pt gt , gr f Ta B k N dv

6W 0 dB 162 MHz 200 K 50 kHz 1.379 · 10−23 J/K Ta kB computed

Transmission power Transmitter, receiver antenna gains Carrier frequency Antenna noise temperature Carrier bandwidth Boltzmann constant Channel noise Vessel-satellite distance

pf

10

Packets per frame Replicas per packet in CRA/ECRA (ALOHA-SIC) Information bits Code rate CRA/ECRA (ALOHA-SIC) Modulation Rate Symbols per packet CRA/ERCA (ALOHA-SIC)

d

2 (1)

nb

256 b

r

3/4 (1/3)

R

QPSK 1.5 (0.67) b/sym

Ds

171 (383) sym

TABLE II: Spectral efficiency results [b/s/Hz] for a traget PER of 10−2 for ALOHA-SIC, CRA, ECRA with SC and MRC Configuration

sp. eff. @ PER=10−2

Scheme

degree

1

2.35

ALOHA-SIC

1

2

4

CRA

2

3

3.15

CRA

3

4

4.17

ECRA-SC

2

5

3.23

ECRA-SC

3

6

4.57

ECRA-MRC

2

7

3.83

ECRA-MRC

3

of all competing schemes, i.e. ALOHA with SIC (in the following called ALOHA-SIC), CRA and ECRA. We considered as target PER the value 10−2 , and we collected the maximum spectral efficiency for which the target PER is not exceed. Compared to CRA and ECRA, we experienced a much higher PER for ALOHA-SIC, that forced us to lower the coding rate to 1/3. Although not present in the standard we stress the fact that higher code rates lead to unacceptable losses when ALOHA-SIC is adopted. The results are collected in Table II. The use of replicas does provide an outstanding benefit. Compared to ALOHA-SIC, CRA with 2 replicas is able to increase the spectral efficiency by 70%, from 2.35 [b/s/Hz] to 4 [b/s/Hz]. A further boost is given exploiting combining techniques – especially MRC – reaching 4.57 [b/s/Hz] or 95% of increase in spectral efficiency compared to ALOHA-SIC. The use of selection combining leads to minor improvements compared to CRA. Moreover, increasing the number of replicas from 2 to 3 does not give any advantage in the considered scenario, confirming that the system adopting 2 replicas is the optimal choice.

TABLE III: Percentage of world, waters and ship coverage for various MAC schemes.

TABLE IV: Percentage of world, waters and ship coverage for various MAC schemes when vessels in the coast proximity use the terrestrial VDES link.

MAC

World Coverage

Waters Coverage

Ship Coverage

ALOHA-SIC

78.1%

81.0%

29.2%

MAC

World Coverage

Waters Coverage

Ship Coverage

CRA-2

88.6%

89.8%

47.2%

ALOHA-SIC

84.9%

84.8%

44.4%

ECRA-2 MRC

91.2%

92.4%

53.2%

CRA-2

95.6%

95.5%

74.5%

ECRA-2 MRC

98.7%

98.9%

89.8%

IV. N UMERICAL R ESULTS We are interested in comparing the different access schemes as a function of the satellite position projection over the Earth surface. The first step is to compute the data traffic generated by all vessels in the satellite’s coverage range. We distribute the vessels according to measurement data provided by the exactEarthr satellite system, which is based on the collection of AIS packets in January 2013. The aggregated data traffic measured in [b/s/Hz] is presented in Fig. 2a. We make the assumption that each vessel transmits 10 packets per minute or per MAC frame, which corresponds to an average data rate of 0.71 b/s per vessel. Nonetheless, due to the vast satellite footprint and the corresponding huge vessel population, the aggregate data rate peaks at more than 10 b/s/Hz. In Fig. 2b, the world area that can be covered assuming that the receiver applies SIC only to an ALOHA-based access scheme is presented. In green are the areas where all the vessels can be guaranteed to perceive a PER below 10−2 , while vessels in the orange/red areas perceive a PER that exceeds this value. Heavily populated vessel regions, as the Mediterranean and Red Sees or the Indian and Chinese coasts as well as the Caribbean Sea, cannot be covered by an ALOHA with SIC system without exceeding the target PER. In particular, only 29.2% of the total vessels are provided with a PER lower than 10−2 and 81% of the world waters can be covered. The wide range between the covered waters and the percentage of vessels relies on the fact that the vast majority of the vessels is concentrated in coastal regions and very few of them are undergoing oceanic travels. In Fig. 2c, we present the results for CRA with 2 replicas under the same scenario. The increase in supported spectral efficiency results in a higher coverage for the same target PER. The Red Sea and vast part of the Indian Ocean is now covered and the Caribbean as well as the Atlantic coast of Europe are better served by the system. Impressively, the percentage of supported ship is almost doubled compared to the ALOHA with SIC case, reaching 47.2%, while also the waters coverage slightly increased at 89.8%. Finally, observing the results for ECRA with MRC and 2 replicas in Fig. 2d, we observe a further improvement although less pronounced. Now the Caribbeans are fully covered by the system, as well as the entire west coast of India. The ship coverage exceeds 50% in this case, at 53.2%, while the waters coverage reaches 92.4%. The improvement compared to CRA is quite limited, since the link budget is particularly favourable. In most of the cases, the SINR on single replicas is sufficient for the correct decoding and there is no need to MRC operating on both of them. In

more challenging conditions, the benefits of MRC are be more evident. Since the uplink of VDES comprises both the satellite and the terrestrial component, it is beneficial from a system level perspective to let vessels in proximity of the coast select the best communication link. Terrestrial coverage, when available, has undoubtable advantages, such as better link budget, lower round trip delay, lower channel load conditions, just to mention a few. In this regard, it is reasonable to assume that whenever a vessel detects the availability of the terrestrial link, it switches to the reserved terrestrial channels and starts transmitting there, without loading the satellite link. In order to take into account this behavior, we evaluated the satellite link channel load, assuming that all vessels closer than 40 nm to the coast only transmit on the terrestrial VDES link. In Fig. 3a, we can observe the new channel load. Now, only up to 5 b/s/Hz can be observed in the densely populated regions. For the same conditions we present the results for ECRA with 2 replicas and MRC. As we can observe in Fig. 3b, the vast majority of the world can be now supported with ECRA and only very minor spots around the European and Chinese coasts are left uncovered. In Table IV, the results show that for ECRA 98.9% of the waters are provided with PER below 10−2 and impressively 89.8% of the total vessels can be supported. The presented results clearly show the potential benefits of adopting novel asynchronous RA protocols in the context of VDES. With the aim of stimulating further discussions in the standardisation process and among the standardisation bodies, we have brought to attention what are possible system gains when multiple replicas are sent by the VDES transmitters. In light of this, enabling transmitter modes where replicas are contemplated can bring major advantages without the need of deeply modifying the standard. V. C ONCLUSIONS In this paper we analyzed the performance of recent asynchronous random access schemes, for the novel maritime standard VDES. Our focus has been on the satellite component, and specifically on the vessel-to-satellite link. After a brief introduction on the content of the VDES standard, we presented a realistic scenario, with a LEO satellite covering the Earth surface and providing connectivity to the vessels. We first investigated the aggregate data traffic generated by all vessels in the satellite’s coverage and show for the recent random access protocols their performance. For a target PER of 10−2 the percentage of vessels that can be covered almost doubles compared to more conventional schemes as ALOHA

b/s/Hz

75° N

75° N

10

60° N

60° N 8

°

45 N °

30 N 15° N 0° 15° S 30° S

6

4

45° S

45° N 30° N 15° N 0° 15° S 30° S 45° S

2

60° S

60° S

0

150° W

90° W

30° W

30° E

90° E

150° E

(a) Channel load in packets per packet duration. Derived using the realistic vessel distribution from data of January 2013 of exactEarthr [17].

150° W

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60° N

60° N

°

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30° N 15° N 0° 15° S 30° S

30° N 15° N 0° 15° S 30° S

45° S

45° S

°

60° S

150° W

90° W

30° W

30° E

90° E

150° E

(c) Communication coverage (in green) guaranteing a PLR below 10−2 for CRA under the channel load conditions of Fig. 2a.

30° W

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150° E

(b) Communication coverage (in green) guaranteing a PLR below 10−2 for ALOHA with SIC under the channel load conditions of Fig. 2a.

75° N

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150° W

90° W

30° W

30° E

90° E

150° E

(d) Communication coverage (in green) guaranteing a PLR below 10−2 for ECRA under the channel load conditions of Fig. 2a.

Fig. 2: Channel load and communication coverage for the basic VDES narrowband RA mode and the advanced solution with ECRA.

with SIC. In order to take into account also the presence of the VDES terrestrial component, we modified our analysis so to exclude from the satellite traffic vessels in coastal proximity. In this second case, ECRA with 2 replicas and MRC is able to provide water coverage for 98.9% and providing connectivity to 89.8% of the vessels, guaranteeing the target PER of 10−2 . R EFERENCES [1] Electronic Communications Committee, “Information Paper on VHF Data Exchange System (VDES), CPG PTC(13) INFO 16,” CEPT, Tech. Rep., 2013. [2] Recommendation ITU-R M.1371-4, “Technical characteristics for an automatic identification ssystem using time-division multiple access in the VHF maritime mobile band,” ITU, Tech. Rep., 2010. [3] Technical characteristics for a VHF data exchange system in the VHF maritime mobile band, International Telecommunications Union Std. M.2092-0, October 2015.

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b/s/Hz

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°

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(a) Channel load in packets per packet duration. Derived using the realistic vessel distribution from data of January 2013 of exactEarthr [17]. Vessels in proximity of the coast, i.e. closer than 74 km or 40 nm are assumed to be using the terrestrial VDES link and therefore are not contributing to the data channel load.

150° W

90° W

30° W

30° E

90° E

150° E

(b) Communication coverage (in green) guaranteing a PLR below 10−2 for ECRA under the channel load conditions of Fig. 3a.

Fig. 3: Channel load and communication coverage for ECRA with MRC when smart link selection is activated and vessels in the coast proximity use the terrestrial VDES link.

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