13.1.1 Simplex Systems
As shown in Figure 13.1, communication system carries information in one direction only—from an information source to an information sink. The receiver accepts information but does not transmit information back to the source. Simplex transmission is therefore the simplest form of communication and requires only a single transmission path.
Because communication occurs in only one direction, simplex systems do not require channel turn-taking or contention protocols. They do, however, still require many of the same communication mechanisms used by more complex systems, including framing, synchronization, addressing (where appropriate), and error-control coding to ensure that information is received correctly.
Simplex transmission is particularly attractive whenever information naturally flows in only one direction or where providing a return channel would unnecessarily increase cost, complexity, power consumption, or the risk of interception. Typical applications include broadcasting, telemetry, remote monitoring, navigation systems, and data collection.
The information source may range from a single sensor transmitting periodic measurements to a sophisticated multiplexer combining data from hundreds of sensors before forwarding the resulting information stream to a monitoring or control center. Regardless of the complexity of the source, information always flows in one direction.
It is important to distinguish between simplex and broadcast communication. Simplex describes the direction of information flow, whereas broadcast describes the number of receivers. A simplex system may therefore be either point-to-point, where one transmitter communicates with one receiver, or point-to-multipoint, where one transmitter simultaneously serves many receivers. Broadcasting is simply a specialized form of simplex communication in which a single transmitter serves multiple receivers.
13.1.1.1 International Radio Broadcasting
International radio broadcasting provides one of the best-known examples of simplex communication. A single high-power transmitter can simultaneously serve thousands or even millions of inexpensive receivers distributed over an enormous geographical area.
Unlike local AM or FM broadcasting, international HF broadcasting often employs multiple simultaneous frequencies. National broadcasters must serve listeners located at widely differing distances and in every direction around the transmitter. Because HF propagation depends strongly on ionospheric conditions, no single frequency can provide reliable communication to all listeners throughout the day.
In a conventional point-to-point communication system, an optimum transmission frequency could be selected individually for each receiver. Such an approach is clearly impractical for a broadcast service containing thousands of unknown listeners. The broadcaster generally has no knowledge of the location of individual receivers and cannot optimize transmission for each one.
Instead, the same program is transmitted simultaneously on several carefully selected frequencies distributed across the HF spectrum. This significantly increases the probability that at least one transmission frequency will provide satisfactory reception under the prevailing ionospheric conditions.
The receivers do not require any knowledge of propagation conditions beyond the published transmission schedule. Listeners simply select the frequency providing the best reception at their location. If changing ionospheric conditions render one frequency unusable, the listener simply retunes to another frequency carrying the same program.
Many international broadcasters now supplement or replace conventional analogue AM transmissions with Digital Radio Mondiale (DRM), which provides improved audio quality and greater spectral efficiency while retaining the same fundamental simplex broadcasting concept.
13.1.1.2 Navy Multi-Channel Broadcast
Navies employ a similar broadcasting concept to distribute operational information from shore headquarters to ships and submarines deployed over very large geographical areas. Because the fleet is continually moving, the transmitter generally does not know the precise location of every vessel, nor would it be practical to engineer a separate communication link for each one.
Fleet Broadcast, often referred to as multi-channel broadcast (MCB), overcomes this problem by transmitting identical information simultaneously on numerous frequencies. Ships know both the location of the transmitting station and the frequencies being transmitted, allowing each vessel to determine which frequency is likely to provide the best reception under current propagation conditions. Should propagation change, the ship simply retunes to another broadcast frequency without requiring any action by the transmitting station.
HF transmissions typically occupy numerous frequencies distributed across the HF band, providing flexibility as ionospheric conditions change throughout the day. Additional LF and VLF transmissions are often provided for specialized applications requiring greater propagation reliability or communication with submerged submarines.
Although omnidirectional transmission is possible, naval broadcasts are more commonly directed towards those regions in which the fleet is operating. Directional antennas concentrate transmitter power into the required coverage area, improving communication efficiency while reducing unnecessary radiation in other directions.
Most modern fleet broadcast systems employ digital communication techniques while retaining the same underlying multi-frequency broadcasting philosophy. The transmitted information may include radioteletype (RATT), digital messaging, encrypted operational traffic, or other data services depending upon operational requirements.
Communication with submerged submarines presents additional challenges. After travelling through the ionosphere, HF signals penetrate only a very short distance into seawater because of its relatively high electrical conductivity. Signal penetration improves as frequency decreases, so submarine broadcast systems primarily employ very low frequency (VLF) transmissions and, historically, extremely low frequency (ELF) systems for specialized applications. Because the available bandwidth at these frequencies is extremely limited, such systems typically support only low data rates and short command messages rather than continuous two-way communication.
13.1.1.3 Special Forces Broadcasts
Simplex broadcasting also provides an effective means of communicating with small, covert military units operating behind enemy lines. In these situations, establishing a two-way communication link may reveal the location of the receiving unit through radio direction finding or other electronic surveillance techniques.
Instead, the transmitting station repeatedly broadcasts the same encrypted message on several frequencies according to either continuous or prearranged transmission schedules. The receiving unit monitors the published frequencies and receives the message without transmitting any radio signal of its own.
By eliminating the need for routine acknowledgements or replies, this technique significantly reduces the risk of detection and interception. The covert unit transmits only when operationally essential and then only using carefully planned transmission procedures designed to minimize the probability of interception or location.
This example illustrates one of the principal advantages of simplex communication. Although it does not support interactive conversation, it can provide highly reliable, economical, and secure one-way dissemination of information to many geographically dispersed receivers while keeping the receiving equipment simple and, where necessary, electronically silent.
Simplex systems therefore remain widely used wherever information naturally flows in only one direction or where receiver simplicity, low cost, wide-area coverage, or transmission security are primary design objectives. In the next section we examine half-duplex systems, which extend the capabilities of simplex communication by allowing information to flow in both directions, although not simultaneously.
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