vhf digital small message service rtcm special ... -...

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VHF Digital Small Message Service RTCM Special Committee 123

• RTCM has identified the need for transmitting short digital messages on maritime VHF channels.

• Current regulations permit data transmission on these channels.

• IEC 62287 (the AIS Class B”CS” standard) has established a method of “polite” use of the AIS data link, using Carrier Sense Time Division Multiple Access (CSTDMA) technology.

• “Carrier Sense” (CS) could be used to transmit data on VHF radio channels without interfering with the primary (voice) service.

• There are presently no standards for leveraging CSTDMA technology to facilitate digital messaging in the VHF maritime service.

RTCM Paper 156-2007-SC123-031

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• There is currently a very limited set of VHF marine channels in the U.S.

• FCC policy promotes the inter-service sharing of spectrum based on geographical separation and “white space”.

• Many ITU Appendix 18 (AP 18) channels have been reassigned to LMR (land-mobile radio) services, but this spectrum may possibly be sharedwith maritime Digital SMS based on an approved RTCM SC123 standard.

• SMS would share on a secondary non-interference basis with the LMR.

• Application of CS, “spectrum-efficient” modulation and authorization to use “white-space” in AP 18 would provide sufficient capacity for SMS.


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• There is currently a proposed draft new ITU recommendation for a digital VHF communications standard based on an ETSI digital land-mobile radio standard that requires reorganization of RR Appendix 18 to provide a wideband duplex 225 KHz channel for data.

• RTCM SC123 is evaluating alternative ETSI technology currently used by TETRA (Trans European Trunked Radio Association) that can provide efficient data transmission in a 25 KHz channel bandwidth and thus does not necessitate reorganization of RR Appendix 18.

• RTCM SC123 plans to develop a marine VHF standard that adapts this digital radio technology along with carrier sense (CS) technology.

• The current RTCM SC123 work is focused on transmitter emissions mask, data rate, propagation range, and channel access scheme.


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Urgent Relevant Issues at ITUProposal for Reorganization of RR Appendix 18

Not available in the U.S.

Proposed 225 KHz combined channel

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AIS 1161.975

MHz@-33dBc

AIS 2162.025

MHz

-70 dBc IEC Limit for Appendix 18

Adjacent Channel

CH 27B161.950

MHzCH 28B162.000

MHzProposed 225 KHz channel

Center Frequency of proposed new 225 KHz channel @ 161.825 MHz

-70 dBc @ ± 250 KHz

ETSI standard:

Urgent Relevant Issues at ITUTransmitter Emissions Mask

The Current ITU Proposal: Wideband 225 KHz Data Channel

Information Courtesy of ITU-R WP8B

ETS300113 v 1.5.1Emission designator: 200KF1DANRaw bit-rate:133 kbps

QUESTIONS:• Is this 225 KHz channel

really needed?

• Are all of the 9 channels available globally?

• Is there another digital radio standard that might provide an equal or higher capacity VHF digital SMS using RR Appendix 18 channels?

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ETSI: TETRA Modulation Standards

TETRA-1

TETRA-2 (TEDS)

Applicable to VHF Marine Channels

(25KHz Channels)

Compare Propagation: π/4 DQPSK @ -112dBm RSS vs. π/8 D8PSK @ -107dBm RSS

64-QAM 115

25 kHz

Л/4 DQPSK

50 kHz

36

154

54Л/8 D8PSK

230

1547716-QAM

77384-QAM

150 kHz100 kHzChannel Type

Modulation

307

461

461

691

230

Information Courtesy of ETSI and TETRA

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TETRA Transmitter Emissions vs. IEC Marine VHF Mask

NOTE:Mask overflow occurs at ±10KHz offset from the carrier.

Information Courtesy of CML Microcircuits

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TETRA Data Rates Reduced to fit the IEC Marine VHF Mask

NOTES:1. Data rates 28.8KBPS

and 43.2KBPS for the two modulation types fit the IEC mask with comfortable margin.

2. 43.2KBPS per 25 KHz channel is 2.9X more spectrum efficient than the ITU wideband 225 KHz proposal.

3. How do they compare in receiver sensitivity?

4. What is the resulting radio propagation range over sea water?


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Adjacent Channel Power Ratio (ACPR) Transmitter Emissions for SC123 Modulation


RR Appendix 18 ACPR ≥ 70 dB requirement (IEC standard)is satisfied.

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SC123 RF Coverage Study

• Environmental noise, fade margin, and receiver sensitivity wouldneed to be considered in order to predict the level at the output of the antenna that would correspond to the predicted coverage.

• Shore Station Receive Channel Assumptions• Example TSB-88-B:

– Assuming a Equivalent Noise Bandwidth of 18 kHz, To=290 K– Rated sensitivity of -107 dBm for π/8D8PSK or -112dBm for π/4DQPSK– Quiet rural environment over water IAW TSB-88-B– kTB = 290 X 1.38E-23 X 18000 = 7.20E-17 = -161 dBW or -131 dBm– SNR @ sensitivity for π/8D8PSK = -107 dBm - (-131 dBm) = 24 dB– SNR @ sensitivity for π/4DQPSK = -112 dBm - (-131 dBm) = 19 dB – Historical noise for quiet rural IAW TSB-88-B is as follows:– Noise Relative to kTB = 52-29.5logf(MHz)= 52-29.5log162 = -13.2 dB– Environmental Noise in channel = -131 dBm - (-13.2 dB) = -117.8 dBm– Signal required at the antenna for π/8D8PSK = -117.8 + 24 = -93.8 dBm– Signal required at the antenna for π/4DQPSK = -117.8 + 19 = -98.8 dBm

Information Courtesy of Harris Corporation

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SC123 RF Coverage Study (cont’d)

• Example Gulf of Mexico Shore Stations: Environments at the sites range from quiet rural to urban environment.

• Measured noise floors range from kTB = 290 X 1.38E-23 X 18000 = 7.20E-17 = -161 dBW or -131 dBm

• Noise density per 1 kHz, measured at Gulf of Mexico Shore Stations range from -129 dBm/kHz to -133 dBm/khz– Noise in and 18 kHz Bandwidth range from -116 to -120

• Environmental Noise in the channel = -131 dBm + 13.2 dB = -117.8• SNR (dB) at rated sensitivity = -107 dBm - (-131 dBm) = 24 dB• Signal required at the output of antenna

– Urban: = -116+ 24 = -92 dBm (π/8D8PSK)= -116+ 19 = -97 dBm (π/4DQPSK)

– Quiet Rural: = -120+ 24 = -96 dBm (π/8D8PSK)= -120+ 19 = -101 dBm (π/4DQPSK)

• Ships Station Receive Channel Assumptions– Although ships environmental noise will vary a quiet rural environment could

be assumed, i.e. – 96 dBm for π/8D8PSK or -101 dBm for π/4DQPSK out of the antenna to meet the sensitivity requirements.


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RF Coverage Assumptions(for Talk-in to a Shore Station)

• Used Port Bolivar location as test site.• ITM-NTIA – Longley Rice coverage model

– Ground Type – Sea water– Polarization – Vertical– Radio Climate – Maritime Temperate (over sea)

• Clutter – TSB88B• Ship assumptions:

– Antenna heights: 30 ft– Antenna gain: 6 dBi– TX power: 25 W

• Base station parameters:– Antenna height: 200 ft (ASL)– Antenna gain: 9 dBi– TX power: 50 W

• Frequency – 162 MHz


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RF Coverage – Ship to Shore(NTIA-ITM Model with Noise Clutter)

25nm 50nm

- 29.4 nm- 31.6 nm

- 33.6 nm- 35.8 nm

- 38.1 nm

- 40.2 nm- 42.7 nm

- 44.9 nm

- 47.5 nm- 49.8 nm

- 55.3 nm

π/8 D8PSK

π/4 DQPSK


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Propagation Analysis: Ship to ShoreITU-R P.525 Propagation Model (noisy environment)

52nm

55nm

π/4 DQPSK

π/8 D8PSK

Information Courtesy of US Coast Guard

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Propagation Analysis: Ship to ShoreITU-R P.525 Propagation Model (quiet environment)

70nm

77nm

π/4 DQPSK

π/8 D8PSK


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Propagation Analysis: Shore to ShipITU-R P.525 Propagation Model (quiet environment)

89nm74nm

π/8 D8PSKπ/4 DQPSK


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Propagation Analysis: Ship to ShipITU-R P.525 Propagation Model (quiet environment)

35nm

41nm

π/8 D8PSK

π/4 DQPSK


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Seawater Propagation Profile: Ship to ShipITU-R P.525 Propagation Model (signal level vs. range)

CONCLUSION: Both SC123 data rates provide excellent seawater coverage.


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• Transmitter emissions- Both ETSI TETRA modulations fit the IEC emissions mask.- The higher data rate offers acceptable coverage at 25 Watts.- A typical 25 Watt marine VHF transmitter component was tested.- Vector feedback technique was used to achieve mask compliance.- A chip is under development for automatic transmitter control.- CML Microelectronics reports on transmitter control and testing.

• Access schemes (MMSI-based) under consideration- DSC can be used to selectively call and switch AP 18 channels.- AIS Message 26 (included in ITU-R M.1371-3, latest revision)

- More efficient: 500 ms (DSC) vs. 27 ms (AIS Message 26)- Has no impact on the GMDSS service on channel 70- Provides visible identification for the user- Provides synchronization (timing) for data transmission- AIS Class B”CS” (Carrier Sense) can assess “white space”

VHF Digital Small Message Service RTCM Special Committee 123 Development Status- Current Emphasis

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• DSC provides access for marine VHF radios but not for AIS equipment• AIS-based access scheme for SC123 data transmission

- Assumes a VHF radio with integrated AIS, GPS and CS capability:- MMSI-based selective calling (address of sender & receiver)- Carrier Sense (CS) capability

- Receive Signal Strength Indication (RSSI) of “white space”- Measurement method defined in ITU-R M.1371-3, Annex 7

- Integrated GPS- Position keeping- Distance determination (radio propagation assessment)- Timing and synchronization of messages in “white space”

- Assumes integration with PC-type user interface:- Electronic Chart System (ECS), e.g. RTCM SC 109- Display and identification of AIS targets- Text messaging capability


Possible Access Schemes

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VHF Digital Small Message Service RTCM Special Committee 123Integrated Marine VHF Radio of the Future

GPS

Filter & T/R

SwitchLNA

DSP-based Multi-channel Multi-function

receiver

DSP-based Multi-function transmitter w/

vector feedback25W

RF PA

CPU

• UTC Time Keeping• Message Synch• Position Keeping

Audio Interface

Speaker

MIC

Digital User Interface

MKD

PC-Based Integrated Keyboard& Display System:• ECS• AIS• SC123

IEC 61162 (NMEA) Interface

Radio Functions:• Voice Comms• DSC• AIS B”CS”• SC123 SMS

VHF

GPS

RTCM Paper 156-2007-SC123-031

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