20.1 Transmission Types, Coding & Multiplexing

20.1 Transmission Types, Coding & Multiplexing

Quick Answer: Simplex = one way at a time; half-duplex = alternate TX/RX; full duplex = simultaneous TX/RX. Spread spectrum = wide bandwidth + carrier sequence (direct sequence or frequency hopping). TDMA = separate time slots; FDM = separate frequencies; CDMA = separate codes. PCS: CDMA and GSM. SSB voice → J3E. CODEC = coder/decoder for digital voice formats.

Topic 3-M (Communications Technology) opens with how information occupies a radio channel—who talks when, how many users share spectrum, and how analog voice becomes digital symbols. Key topics 082 (Types of Transmissions) and 083 (Coding and Multiplexing) are the bridge between classical AM/FM/SSB (Ch14) and the marine/aircraft digital systems later in Element 3.

Simplex, half-duplex, and full duplex

Before multi-access schemes, nail the three classical directionality modes. Element 1 marine VHF lives on these words; Element 3 expects the same vocabulary when you service channelized gear and phone-patch style links.

Why it matters on the bench

• A simplex failure is often antenna, power, or one transceiver path—not “the other half of a duplex pair.”
• Duplex systems need a duplexer or separate antennas/filters so the local transmitter does not desense the local receiver. (Element 3 may ask what a duplexer does in a repeater: isolate TX from RX on nearby frequencies.)
• Marine USA–INT switching changes some duplex channel pairs into simplex ship–ship use—an operational control you will meet again in §20.3.

Memory hook: Simplex = single path; half-duplex = take turns; full duplex = talk and listen at once.

Spread spectrum — wide bandwidth by design

Element 3 defines spread spectrum communication as a wide-bandwidth system in which the RF carrier frequency varies according to some predetermined sequence. Energy is deliberately spread so that a narrowband interferer (or eavesdropper) no longer owns the whole information bandwidth.

Two types used in most RF applications

Pool distractors to reject: “AM and FM,” “QPSK or QAM alone,” “time-domain frequency modulation,” or “amplitude compandored SSB” as the definition of spread spectrum.

Service view for GROL techs

1. Spread-spectrum gear often looks “weak” on a narrow spectrum analyzer peak because power is spread—use the correct resolution bandwidth and code-aware test sets.
2. Frequency-hopping systems that “won’t talk” may share a synchronization / hop-table problem, not a dead PA.
3. Do not confuse intentional spread spectrum with accidental broadband spurs from a defective transmitter—spurs are illegal pollution; spread spectrum is a designed emission family.

Multiplexing and multiple access

Multiplexing combines several information streams onto one medium. Multiple access is the multi-user version of the same idea on a shared radio channel.

FDM / FDMA — separate frequencies

Frequency-division multiplexing (FDM) and frequency-division multiple access (FDMA) give each user (or subchannel) a different frequency slice. Classic analog telephone FDM and many narrowband radio channel plans are FDM ideas: separate carriers, separate filters.

TDMA — separate time slots

A TDMA radio carries multiple conversations sequentially using separate time slots on the same RF channel. Each user transmits in short bursts; the receiver demultiplexes by slot timing. Element 3’s one-line TDMA fact: multiple conversations → separate time slots (not separate pilot tones, power levels, or permanent exclusive frequencies).

GSM (a major PCS air interface) allows multiple conversations to share one radio channel by the principle of time-division multiplex. Remember: when the stem says “GSM channel sharing,” answer TDM/TDMA, not double-sideband or pure FDM alone.

CDMA — separate codes

Code-division multiple access (CDMA) lets users share the same band at the same time by assigning each a spreading code. The two most-used PCS coding techniques Element 3 names for separating different calls are CDMA and GSM. (GSM itself uses a TDMA/FDMA hybrid; the pool still pairs “CDMA and GSM” as the PCS answer set.)

Coding, CODECs, and digital voice formats

Coding here means turning human speech or computer data into symbols that survive RF links—and packing many users efficiently.

What is a CODEC?

A CODEC is a coder/decoder IC or circuit that converts a voice signal into a predetermined digital format for (often encrypted) transmission, and reconstructs audio on the far end. It is not “a Morse reader,” a generic “compandor chip,” or a random label for any DSP block.

ASCII for computer data exchange

Which code has gained the widest acceptance for exchange of data from one computer to another? ASCII (not Gray, Baudot, or Morse—though those remain historically important in radio telegraphy).

Error-control overview (bridge to §20.2–20.3)

Error control appears in two families you will detail later:

1. Forward error correction (FEC) — add redundancy so the receiver can fix some errors without a reverse channel.
2. Automatic repeat request (ARQ) — detect errors and ask for retransmission on a two-way link.

SITOR/NBDP maritime telex (Topic 3-N) implements both: ARQ for two-way ship–coast traffic and FEC for one-way broadcasts. DSC selective calling uses a 10-bit error-correcting character structure. File those as “coding for reliability,” then master the bit patterns in §20.2.

Packet radio and the ISO seven-layer model

The International Organization for Standardization (ISO) seven-level reference model (OSI stack) still appears on Element 3 for packet-radio structure:

When a stem asks which level moves bits onto the medium and handles low-level handshaking → physical. When it asks which level frames bits and manages flow control → link.

SSB voice emission designator (J3E)

Which statement about SSB voice is correct on this topic? They normally use J3E emissions: one sideband and a suppressed carrier. Reject A3E (full-carrier AM telephony), F3E (FM telephony), and A1A (CW) as the “normal SSB voice” designator.

Putting transmission types together for the maintainer

Exam-day checklist (3-M 082–083)

1. Define simplex / half-duplex / full duplex by simultaneity, not by “important vs unimportant traffic.”
2. Spread spectrum ≠ random illegal broadband; it is designed wide bandwidth with a sequence.
3. TDMA → time slots; FDM → frequencies; CDMA → codes.
4. PCS pair: CDMA + GSM; GSM channel sharing → TDM.
5. CODECs: definition + RPE (GSM) + AMBE (P25 Phase 2).
6. Packet stack: physical moves bits; link frames them.
7. SSB voice → J3E.

Master directionality, multi-access resource type, and the named CODECs—and 082–083 become vocabulary drills instead of guesswork. Next section adds DSP, software-defined radio, and the bit-level codes used in DSC and SITOR.