Range of Radio Communication
The range of any communication circuit is based on the signal level at the receiver and the receiver sensitivity. The signal level is based on:
- transmitter power,
- transmit antenna gain,
- path loss,
- receiver antenna gain, and
- transmission line loses in any part of the system.
Receiver sensitivity is based on the design of the receiver, and typically is specified by the manufacturer. Sensitivity is specified by two terms: a signal level, and the success at which the original modulation of the signal is recovered at that signal level.
To compare the range of a VHF Marine Band transmission by voice to an AIS data transmission, we look at the the path loss equation and the receiver sensitivities
The path loss will be the same for both systems. If the transmit and receive antennas and their feed lines have the same gains and losses, these variables will be the same for both VOICE and AIS transmission. We assume all elements of the signal level equation will be the same, except transmitter power. Typically the VOICE transmission will be at nominally 25-Watts while an AIS Class B will be at nominally 2.5-Watts. This gives a 10-dB advantage to VOICE compared to AIS.
We next look at receiver sensitivity. For an AIS receiver, the sensitivity is typically specified as better than -107dBm for 20-percent packet error rate (PER) or bit error rate (BER).
A typical FM VHF voice receiver is rated for a sensitivity of 0.25-microVolt for 12dB-SINAD. Converting this voltage to dBm for a 50-Ohm antenna system results in a sensitivity -119dBm
We see that the sensitivity of a VHF Marine Band voice receiver is 12-dB better than the AIS receiver
Another factor to consider is the demodulation and readability of the signals. The demodulated VOICE signal is interpreted by human hearing. The AIS signal is demodulated and interpreted by a combination of hardware and software. In both cases there is some variability. Not all humans can copy a weak signal on a radio receiver at the same readability, and I am also certain that some variation exists among AIS receivers.
Summary of Range Comparison
I don't see a particular advantage to the AIS transmission compared to VOICE, as the VOICE transmission has a 10-dB greater signal and a 12-dB advantage on receiver sensitivity, or 22-dB. An improvement in signal of 22-dB often represents a very marked improvement in readability, either with demodulation by human ear or by machine.
On this basis and particularly in the comparison of a 25-Watt VOICE signal to a 2.5-Watt AIS signal, I am not inclined to accept the notion that AIS can go farther than VOICE. If comparing a 2.5-Watt VOICE signal to a 2.5-Watt AIS signal, the received signals are now equal. In this case there may be some advantage to the AIS signal because it will be demodulated by hardware and software, rather than by ear.
Comparing Demodulation and Interpretation in AIS and Voice
The specification of the demodulator in a typical AIS receiver is for a packet error rate of 20-percent at the specified sensitivity. To compare this to the voice receiver rated for 12-db SINAD, we have to make a judgement about the ability of the operator to copy a voice signal with a SINAD of only 12-dB. Generally having a voice signal above the noise by 12-dB will give good communication quality copy for an experienced operator. The readability obtained by a skilled operator may be at least 80-percent, that is, no more errors than the 20-percent error rate occurring in the AIS demodulator at that same signal strength.
An advantage for AIS may be in the duration of the transmission and the density of the data. A typical AIS transmission lasts for only a fraction of a second, and it very efficiently encodes and sends a great deal of data. In comparison, a VOICE transmission will be much longer, several orders of magnitude longer, and be very inefficient in encoding data.
To send a position by AIS takes only a few bytes of the message, which will occur in a few milliseconds. In contrast, to send a position by voice requires a long duration. The operator must say, "Position is forty-two degrees twenty-one decimal one two three North and eighty-three degrees decimal nine eight seven West." To obtain clear and error free copy of the VOICE transmission will require several seconds of very stable signal, no fading, and no interference.
AIS transmission repeat themselves, but at varying intervals. When a Class-B AIS transponder is sending at its shortest interval, it transmits every five seconds. This suggests than an AIS signal might be sent several times during the interval that it would take for a VOICE transmission to send the same amount of data. On the other hand, at its longest interval, a Class-D AIS waits three minutes between transmission. If its brief burst of information occurs during a period of path fading, there may be no communication possible, and no data is received.
The AIS signal is a broadcast signal, it is just sent out, and the sender never knows if the receiver of the signal got it without error, and, on that basis, there is no error correction. While it is possible to send the signal with an encoding method that provides for some redundancy for error correction, a method called forward error correction, this is not done in AIS signals--at least as far as I know. I base my comment on the findings summarized in an interesting white paper on AIS signals. See
AIS: Guide to System Development
http://www.fidus.com/downloads/hardware ... h_2009.pdf
In particular, see Section 3.1 in which describes the data link layer as follows:
Scattered throughout the AIS specifications are hints about processing in the data link layer. There is some mention of what isn't done (forward error correction, interleaving, bit scrambling, etc.) and some about what is (NRZI and HDLC encoding, byte reversing, etc.),....
This suggests to me there is no forward error correction provided in AIS signals. The signals contain a cyclical redundancy check or CRC code, but that only allows errors to be detected. The usual application of CRC for error detection is to have the receiver initiate some action such as asking the sender to retransmit the message. In AIS, there is no communication back to the sender from the receiver, as the AIS transponder just sends data. This is inherent in a broadcast type system. There is one sender and may listeners. It would not be practical for the sender to transmit and then listen to see if all listeners acknowledged error-free reception.
I do acknowledge that the method and technique of demodulating a digitally encoded signal might include some sophistication that enhances the performance of the demodulator, but this advantage, if there is one, will be represented in the receiver sensitivity specification. If a typical AIS receiver were able to copy, using its sophisticated methods, an AIS signal with very weak levels and buried in the noise, then the rated receiver sensitivity would reflect this advantage. The typical AIS receiver sensitivity (-107dBm) is rated as actually worse than a typical voice receiver (-119dBm). To better understand receiver sensitivity ratings, see
Understanding Receiver Sensitivity Specifications
Inferences about intended uses and ranges
Regarding the relationship between the range of VOICE and AIS transmissions, one perhaps can make an inference from the intended use of AIS about the two signals and their ranges. AIS is intended to be used as a collision avoidance aid. In use, one would suppose that two vessels could each receive an AIS transmission from the other vessel at some distance apart, thus permitting the identity, course, speed, range, and bearing of the other ships to be known to each other. If the ships are on courses that may produce a collision, the most likely action is for the ships to communicate with each other by radiotelephone to arrange some mutually agreed alteration of their courses.
If the AIS signal could be received at a significantly longer range than voice communication would be possible, the ships would be in a situation of being able to receive data about the impending collision, but be too far apart to communicate their intentions to each other via radiotelephone.
The correlation between AIS reception and follow-on radiotelephone reception is further supported by the feature set of many AIS devices: the AIS device generally provides for a very simple method to initiate a radiotelephone call by digital selective calling to the AIS target vessel using its MMSI. This feature, and its incorporation into so many AIS receivers and integration with voice transceivers, suggests that it is reasonable to anticipate that an AIS target vessel in range for AIS will also be in range for radiotelephone communication.
Forward Error Correction in AIS
If there were forward error correction in AIS signals--there is none--then the effect of the forward error correction would be expressed in the receiver sensitivity specification. A receiver would include the effect of the forward error correction when it described the signal level and packet error rate. For example, the typical AIS receiver says it can copy a signal at -109dBM with a packet error rate of 20-percent. If there were some forward error correction sent in the AIS signal, one would have to assume that the effect of that forward error correction would be to reduce the packet error rate to be lower than it would be without forward error correction. We do not see any specification in the receiver that suggests there is an improvement obtained when forward error correction is used or a degrading of packet error rate when it is not used, so we are left with the assumption that the stated sensitivity and packet error rate must be the best outcome. It is not reasonable to assume that an AIS receiver would list its receiver sensitivity to be worse that it could actually deliver if the receiver included some method for taking advantage of forward error correction and such error correction were employed in the signal it was receiving. It seems only reasonable to assume that the stated sensitivity of the receiver represents the best it can do. One should not assume it can do better because of some unstated ability to employ error correction (which is not incorporated in the transmitted signals).