Digital Communication Systems: Principles and Transformations, Essays (university) of Digital Electronics

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Digital communications

ECE

Unit 1

• (^) Main purpose of communication is to transfer information from a source to a recipient via a channel or medium.
• (^) Basic block diagram of a communication system:

Communication

Source Transmitter Receiver

Recipient

Channel

• (^) Communication system converts information into electrical electromagnetic/optical signals appropriate for the transmission medium.
• (^) Analog systems convert analog message into signals that can propagate through the channel.
• (^) Digital systems convert bits(digits, symbols) into signals
  • (^) Computers naturally generate information as characters/bits
  • (^) Most information can be converted into bits
  • (^) Analog signals converted to bits by sampling and quantizing (A/D conversion)

Information Representation

Why digital?

 Digital techniques need to distinguish between discrete symbols allowing regeneration versus amplification  (^) Good processing techniques are available for digital signals, such as medium.  Data compression (or source coding)  Error Correction (or channel coding)(A/D conversion)  Equalization  Security  Easy to mix signals and data using digital techniques

• (^) Basic Digital Communication Transformations
  • (^) Formatting/Source Coding
  • (^) Transforms source info into digital symbols (digitization)
  • (^) Selects compatible waveforms (matching function)
  • (^) Introduces redundancy which facilitates accurate decoding despite errors
• (^) It is essential for reliable communication
  • (^) Modulation/Demodulation
  • (^) Modulation is the process of modifying the info signal to facilitate transmission
  • (^) Demodulation reverses the process of modulation. It involves the detection and retrieval of the info signal - (^) Types - (^) Coherent: Requires a reference info for detection - (^) Noncoherent: Does not require reference phase information

• (^) Transmitters modulate analog messages or bits in case of a DCS for transmission over a channel.
• (^) Receivers recreate signals or bits from received signal (mitigate channel effects)
• (^) Performance metric for analog systems is fidelity, for digital it is the bit rate and error probability.

Main Points

• (^) Hardware is more flexible
• (^) Digital hardware implementation is flexible and

permits the use of microprocessors, mini-processors,

digital switching and VLSI

• (^) Shorter design and production cycle
  • (^) Low cost
• (^) The use of LSI and VLSI in the design of components

and systems have resulted in lower cost

• (^) Easier and more efficient to multiplex several digital

signals

• (^) Digital multiplexing techniques – Time & Code Division

Multiple Access - are easier to implement than analog

techniques such as Frequency Division Multiple Access

Why Digital Communications?

• (^) Can combine different signal types – data, voice, text, etc.
• (^) Data communication in computers is digital in nature whereas voice communication between people is analog in nature
• (^) The two types of communication are difficult to combine over the same medium in the analog domain.
• (^) Using digital techniques, it is possible to combine both format for transmission through a common medium
• (^) Encryption and privacy techniques are easier to implement
• (^) Better overall performance
• (^) Digital communication is inherently more efficient than analog in realizing the exchange of SNR for bandwidth
• (^) Digital signals can be coded to yield extremely low rates and high fidelity as well as privacy Why Digital Communications?

• (^) To maximize transmission rate, R
• (^) To maximize system utilization, U
• (^) To minimize bit error rate, Pe
• (^) To minimize required systems bandwidth, W
• (^) To minimize system complexity, Cx
• (^) To minimize required power, Eb/No Goals in Communication System Design