--- title: "Duplex digital communication system vs analog" subject: "Communication Systems" chapter: 1 key: "semester_6/communication_systems:digital-comm-duplex-vs-analog" source: https://www.bctengineer.com/q/semester_6/communication_systems/digital-comm-duplex-vs-analog site: BCT Engineering --- # Duplex digital communication system vs analog Communication Systems · Chapter 1: Introduction - Asked in: 2081 Chaitra, 5+3 marks - Video: [Lecture 1: Differences Between Analog and Digital Communication](https://www.youtube.com/watch?v=KJY04jQ-Ly0) — Electronic Waves - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/communication_systems/digital-comm-duplex-vs-analog - Whole chapter: https://www.bctengineer.com/md/semester_6/communication_systems/chapter-1 ## Question Sketch and describe the functional digital communication system working in a duplex mode and contrast it with an analog communication system. Mention its impairments and performance parameters. ## Short answer A duplex digital system uses source and channel coding, modulation, and a duplexer for simultaneous two-way communication. Unlike analog, it fights noise through regeneration and error control, but needs more bandwidth. Impairments include noise, ISI, fading; performance is measured by BER, data rate, bandwidth efficiency, and Eb/N0. ## Answer In **duplex (full-duplex)** mode both terminals send and receive at the same time, e.g. a mobile phone call. Each terminal therefore has a complete digital transmitter and receiver, and the two directions share the channel by separate frequencies (FDD) or time slots (TDD) through a **duplexer**. #### Functional digital communication system (duplex) ```text TERMINAL A TERMINAL B +-----------+ +-----------+ | Source | | Sink | | Formatting| | Formatting| | Src encode| | Src decode| | Encrypt | | Decrypt | | Chan code | | Chan dec. | | Modulator | | Demod. | +-----+-----+ +-----^-----+ | +-------+ CHANNEL +-------+ | +-->| |==========>| |--+ |Duplex-| (noise, |Duplex-| +<--| er |<==========| er |<-+ | +-------+ fading) +-------+ | +-----v-----+ +-----+-----+ | Demod. | | Modulator | | Chan dec. | | Chan code | | Decrypt | | Encrypt | | Src decode| | Src encode| | Sink | | Source | +-----------+ +-----------+ ``` Main blocks of each direction: 1. **Formatting:** converts the source output to digital form (sampling, quantization, PCM). 2. **Source encoding:** removes redundancy to reduce bit rate (e.g. Huffman, speech codecs). 3. **Encryption:** secures the data. 4. **Channel encoding:** adds controlled redundancy for error detection/correction (parity, Hamming, CRC, convolutional codes). 5. **Multiplexing / multiple access:** combines several streams or users (TDM, FDMA, CDMA). 6. **Modulation:** maps bits to waveforms (ASK, FSK, PSK, QAM); pulse shaping limits bandwidth. 7. **Duplexer and channel:** separates the transmit and receive paths over the shared medium. 8. At the receiver, the blocks work in reverse: demodulation and detection, channel decoding, decryption, source decoding, and formatting back to the user's form. #### Contrast with analog communication | Point | Digital system | Analog system | |---|---|---| | Message | Discrete symbols / bits | Continuous waveform | | Noise effect | Regenerative repeaters remove noise | Noise accumulates along the link | | Error control | Coding can detect and correct errors | Not possible | | Security | Easy encryption | Difficult | | Multiplexing | TDM, easy mixing of voice/data/video | Mostly FDM | | Bandwidth | Usually more | Usually less | | Hardware | Complex, needs synchronization | Simpler | | Quality measure | Bit error rate (BER) | Output SNR, fidelity | #### Impairments - **Additive noise** (thermal, shot, atmospheric) causes bit errors. - **Attenuation** reduces signal strength with distance. - **Bandwidth limitation and dispersion** cause **inter-symbol interference (ISI)**. - **Interference:** co-channel, adjacent channel, crosstalk, echo in duplex links. - **Fading and multipath** in wireless channels. - **Timing jitter** and carrier/clock synchronization errors. #### Performance parameters - **Probability of bit error (BER)** at a given $E_b/N_0$. - **Data rate** (bit/s) and **bandwidth efficiency** (bit/s/Hz). - **Power efficiency:** $E_b/N_0$ needed for a target BER. - **Channel capacity** limit $C = B\log_2(1 + S/N)$. - **Latency/delay** and system complexity and cost. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What defines full-duplex operation, and how do the two directions share the channel? A. Only one terminal sends at a time; they take turns manually B. Both terminals send and receive at the same time, separated by frequency (FDD) or time slots (TDD) via a duplexer C. One terminal transmits and the other only listens D. Both directions use the same frequency and same time with no separation Answer: B. Full-duplex has a complete transmitter and receiver at each end, with a duplexer separating the paths. (Key point: Duplex mode) 2. What is the purpose of channel encoding in the digital chain? A. To reduce the bit rate by removing redundancy B. To keep the data secret from eavesdroppers C. To convert the analog message to bits D. To add controlled redundancy for error detection and correction Answer: D. Source coding removes redundancy; channel coding adds controlled redundancy (parity, Hamming, CRC, convolutional) to catch and fix errors. (Key point: Channel encoding) 3. Why does noise not accumulate along a digital link as it does in an analog link? A. Digital signals have no noise B. Digital signals use wider carriers C. Regenerative repeaters detect the bits and retransmit clean pulses D. Digital systems use analog amplifiers only Answer: C. Repeaters regenerate the symbols, removing noise at each stage, whereas analog amplifiers boost noise with the signal. (Key point: Digital vs analog noise handling) 4. Which set lists performance parameters of a digital communication system? A. Output SNR and fidelity only B. Antenna height and carrier amplitude C. Modulation index and deviation ratio D. BER at a given Eb/N0, data rate, bandwidth efficiency and channel capacity Answer: D. Digital quality is judged by BER versus Eb/N0, rate, bit/s/Hz efficiency, and the limit C = B log2(1 + S/N). 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