The first one is the type of modulation: this is the modulation scheme itself, which is used to convert the analog speech to digital signals (also called digitizing). The most widespread modulation modes are GMSK(MSK), 16QAM, π/4 DPQSK and C4FM (4-level FSK). Each of these have their benefits and disadvantages, which will be discussed later.
The second one is the type of communication: this is the method of multiplication and transmission of the modulated signals. Like I mentioned in the previous part, FDMA (Frequency Domain Multiple Access), TDMA (Time Division Multiple Access), CDMA (Code division Multiple Access) are the widespread ones. This was covered in the previous part.
The third one is the protocol: this is the procedure how the signal is rendered from the transmitter to the loudspeaker of the receiver. The protocol is probably the most important factor in the design of digital radios, even if they are using the same modulation and communication system. One of its most important elements is the method of compression, in order to reduce the amount of data, not containing useful information, and to detect and correct the faulty bits. Without the latter one, the voice in the loudspeaker of the receiver would be broken up. While for an analog radio, only the type of modulation is important, for the digital one, the type of communication and the protocol is also important for the contact.
Let's have a look at the benefits and disadvantages of modulation modes!
The GMSK modulation circuit is relatively simple. This modulation mode has been used in the mobile phones and the TETRAPOL system between 1980 and 1990. Nowadays it is considered to be obsolete. The GMSK modulation is still being used by D-STAR. The block diagram of the modulator is shown on Figure 1. You can find detailed information about the MSK and GMSK modulation on the following webpage: https://en.wikipedia.org/wiki/Minimum-shift_keying
π/4 DQPSK modulation
This is a higher level modulation mode. One of the biggest advantages it has is that the distortion of the signals is very low, although this mode requires linear RF stages (Class A stages, just like SSB), and in addition, the circuitry is rather complicated. This is also accompanied with the lower efficiency of the transmitter stage, which has a negative effect on the service life of the battery, too. Therefore this mode is not quite widespread, except for a few applications. One of them is the European TETRA system (in Hungary it is known under the name EDR), another one is the TDMA-based mobile phones (USA). The block diagram of this modulation is shown on Figure 2. If you want to take a deep dive in the world of the DQPSK modulation, a detailed description can be found at: http://wsl.stanford.edu/~ee359/doppler2.pdf
C4FM (4 level FSK) modulation
Compared to the previous two modulations, the circuitry of the C4FM is simpler, and the bit error rate (BER) is also more favourable. Because of these benefits, this one soon superseded the GMSK modulation in the development of the professional radios. One of the best known systems is the APCO P-25, which is mainly used for governmental communication (police, disaster prevention, etc.) in the USAand some other countries. The C4FM modulation system is very popular among professional radios. This is used by the DMR system, which is also known in Hungary. This modulation system is getting more and more popular among amateurs as well. The block diagram of the modulator is shown on Figure 3. The technology can be best understood by the following small illustration. While the binary signals have two stabile states, 0 and 1, the 4-level signal has four stable states, namely 00, 01, 11 and 10.
The capabilities of the different digital communication systems are different, which depend on the type of modulation, the method of multiplication and the protocol. Nowadays the C4FM modulated FDMA and TDMA (DMR) system is considered to be the most reliable, efficient in error correction, best sound fidelity, providing the best coverage and longest service life of the battery. This is why the engineers at YAESU have chosen the C4FM modulation mode and the FDMA communication technology, to develop the SYSTEM FUSION digital system for the radio amateurs.
The core of the SYSTEM FUSION is the duo-band analog/digital repeater DR-1X, which is built on the main units (transmitter and receiver) of the FTM-400 mobile radios, and has a control circuit connected to them.
The SYSTEM FUSION can be used in either C4FM digital, or analog mode, and it is not compatible with the D-STAR GMSK system. The SYTEM FUSION is equipped with an Automatic Mode Select function, which detects the mode of the received signal:
V/D Simultaneous Voice/Data Communication Mode: one of the logical channels is used for the voice signal, while the other logical channel is used for the data stream, separating the 12.5 kHz channel. The data stream also has the error correction codes.
Voice FR mode (Voice Full Rate Mode): This mode uses the entire 12.5 kHz bandwidth to transmit digital voice data. The larger voice data size allows voice communication with high sound quality.
Data FR Mode (High-speed Data Communication Mode) this mode uses the entire 12.5 kHz bandwidth for data communication. The transceivers automatically switch to this mode when sending an image taken with the camera.
FM Mode: can be used to communicate with the amateurs who do not yet have a digital radio, and it is effective for communication with a weak signal that causes voices to break up in the digital modes. The analog mode allows communication even at distances where noise and weak signals make communication almost impossible.
Automatic Mode Selection (AMS)
One of the excellent features of the DR-1X repeater, is that it can replace the existing repeater, whether it is a VHF or UHF band unit. Keeping the antennas, the duplex filters, by replacing the RF units, we have the most up-to-date digital repeater. For the smooth transition, the repeater can operate as an analog FM repeater or a C4FM digital repeater. If the incoming received signal is analog, then it is transmitted as an analog signal, and if it is a digital one, it is repeated as a digital voice signal.
New functions of the C4FM digital system
Digital Group Monitor Function
The digital GM function automatically checks whether members registered in a group are within communication range, and displays information such as distance and direction for each call sign on the screen. This function can also be used to send messages and data such as images between members of a group.
Snapshot Picture Function
When using the MH-85A11U speaker microphone camera, a snapshot (photo) can be taken, and transmitted through the C4FM system.
Smart Navigation Function
This is a real-time navigation function that records the location and direction of the stations. In digital V/D mode communicates information such as position data at the same time as the voice signal, allowing you to view the distance and direction of the other party in real time while communicating.
It is possible to navigate the transceiver to a given point. For example, in situations such as hiking where your positions are constantly changing, providing an easy way to meet up or join routes.
The detailed description and specifications of the DR-1X C4FM repeater in Hungarian, can be found at: http://www.radioamatorwebshop.eu
There is now an open Hungarian group called Yaesu C4FM System Fusion Hungary on Facebook. Those who are interested in digital communication, and are ready to share their experience, are welcome to join the group.
M.Sc. E. E.