Communication Systems

• Communication System Model

  1. Transmitter

  2. Receiver

  3. Medium

  4. Noise
• Communication Links

  1. Simplex

  2. Half-Duplex

  3. Full-Duplex
• Data Transmission techniques

  1. Transmission Mode

  2. Synchronization

  3. Multiplexing

  4. Multiple-Access
• Components of electronic communication systems

  • Transmitter
  • Communication Channel (Medium)
  • Receiver
• Transmitter


  • A collection of electronic components and circuits designed to convert information signals to a signal suitable for transmission over a Communication Channel
• Receiver

  • A collection of electronic components and circuits that accept transmitted signals from the Communication Channel and convert them back to a form understandable by humans
• Communication Channel

  • The medium by which an electronic signal is sent from one point to another, examples include Electrical Conductor, Optical Media, and Free Space (Air)
• Noise in communication systems
• Types of Communication Links

  • 1-way (Simplex)
  • 2-way (Duplex)
• Walkie-Talkie

  • You can talk and listen, but not together at the same time
• Analogue transmission

  Conveys voice, data, image, or video information using an information signal that is continuously varying
• Digital transmission


  Transfers data over a transmission medium discretely as a series of discrete pulses representing one bits and zero bits
• ADC and DAC Conversion

  1. Most sources of information are analogue in nature

  2. Analog-to-Digital Converter (ADC) converts analogue signals to digital form before transmission

  3. Digital-to-Analogue Converter (DAC) converts digital information back into analogue form at the receiver
• Advantages of Digital transmission

  • Maintain Data Integrity (Less Sensitive to Noise & Impairments)
  • Transmit Longer Distance (Repeater can function as Re-Generators)
  • Easier to Multiplex (Integrate Voice/Video/Data)
  • Easy to Apply Encryption (Digitally Processed)
• Disadvantages of Digital transmission

  • Costly to Convert from Analogue System
  • Higher Bandwidth & System Requirements
• Digital Transmission Modes

  Two basic methods: Parallel Transmission and Serial Transmission
• Parallel Transmission

  1. All bits in a single character are transmitted simultaneously

  2. Data bits are sent in parallel (typically 8 bits at a time) using separate wires or circuits<|'Byte-wide' transmission results in a faster transmission rate than Serial Transmission
• Skew problems limit parallel transmission to shorter distances due to unique characteristics of each wire delaying pulses by different amounts
• Serial Transmission

  Data bits are sent sequentially bit-by-bit over a single wire or communication circuit<|Slower than Parallel Transmission<|Used for longer distance transmission without skew problems<|Widely used in data communications
• Digital Transmission Synchronization

  Two types: Synchronous Transmission and Asynchronous Transmission
• Asynchronous Transmission


  Uses a simple interface<|Enables sending a series of bytes (or ASCII characters) along a single wire<|Each character is transmitted independently with a 'Start' and 'Stop' signal separating characters<|Data Rate is typically < 64 kbps<|Called 'Asynchronous Communication' because Transmitter & Receiver use independent clocks
• Types of Multiple Access schemes

  • Frequency Division Multiple Access (FDMA)
  • Time Division Multiple Access (TDMA)
  • Code Division Multiple Access (CDMA)
  • Space Division Multiple Access (SDMA)
• Multiplexing refers to the simultaneous transmission of several separate information channels over the same transmission medium without interference
• The aim of multiplexing is to share an expensive resource
• In telecommunications, several telephone calls may be carried using one wire
• A device that performs multiplexing is called a Multiplexer (MUX), and a device that performs the reverse process is called a De-Multiplexer (DEMUX or DMX)
• Multiple-Access is the accessing of a common communication channel from different locations
• An analogy to the problem of Multiple Access is a room (channel) in which people wish to talk to each other simultaneously
• To avoid confusion in Multiple Access, people could take turns speaking (Time Division), speak at different pitches (Frequency Division), or speak in different languages (Code Division)
• In FDMA, the whole frequency band is divided into small frequency bands called channels
• Each channel in FDMA can accommodate a single user at a time, allowing many users to access the whole frequency bandwidth
• If a total bandwidth of 200 kHz is available and the allocated channel bandwidth is 50 kHz, then the system can accommodate 4 users at a time by giving 50 kHz channel to each user
• TDMA allows several users to share the same frequency channel by dividing the signal into different time slots
• Time Division Multiple Access (TDMA)

  1. 'TDMA' allows several users to share the same frequency channel by dividing the signal into different time slots

  2. The users transmit in rapid succession, one after the other, each using its own time slot. This allows multiple stations to share the same transmission bandwidth
• Code Division Multiple Access (CDMA)

  1. 'CDMA' is another example of multiple access to permit sharing of communication channel without undue interference amongst users

  2. CDMA employs spread-spectrum technology and a special coding scheme where each user (transmitter) uses a different code to modulate their signal

  3. The spread spectrum technique spreads the bandwidth of the data uniformly for the same transmitted power
• Space Division Multiple Access (SDMA)

  1. 'SDMA' divides the geographical space where the users are located into smaller spaces

  2. The key element of the design is a one-to-one map between the space divisions and the bandwidth divisions of time slots, frequency divisions and etc

  3. 'SDMA' is compatible with any multiple access scheme such as TDMA, FDMA, and CDMA

  4. 'Space Division Multiple Access' or 'Spatial Division Multiple Access' is a technique which is 'MIMO' (Multiple-Input Multiple-Output) architecture and used mostly in wireless and satellite communication
• Three basic components of a communication system

  • Transmitter
  • Receiver
  • Communication Channel (Medium)