Digital Signal Transmission Basics

Sources

1. (Lecture Slides) Digital Signal Transmission Basics
2. Class lecture

Data Transmission Processes

• Although data transmission and digital transmission are practically synonymous, they differ in that digital focuses more on form, whereas data focuses more on the communication method (i.e., transmitting information in the digital format).

• Top part is the transmitter side

  • Format takes in the input
  • Processes the input signal to transmit it to the medium
  • Transmitter circuit passes the information to the receiver side through the channel

• Center with two arrows is the channel or medium section¹

  • The medium acts like a filter

• Bottom is the receiver side

  • Produces the output signal
  • After the transmitter transmits the information using the channel, the receiver needs to amplify the information for the other circuits (like the detect circuit) to sense it.
  • Format converts and outputs it in the desired format

Essential Transmission Approaches

• Baseband transmission: Converts binary data into electrical pulses for data transmission
  1. Convert input information to binary equivalent
     • If it is in text format, it is already stored in a file that is already in bytes (characters have binary equivalents). In other words, you can skip the format block when given a textual input.
       • Requires the use of an encoding scheme—such as ASCII and Unicode—to convert it to bits
     • In the contrary, analog information is more challenging to convert into digital information
       • Involves sampling, quantization, and encoding
  2. Modulate it into current or voltage pulses
  3. Transmit it to the channel
     • transmitter circuit is often an amplifier
  4. Receive the signal and demodulate/detect it back into digital information (binary values)
     • Receiving is usually done through an amplifier
  5. Decode it back to its original format
     • Received bits are converted back to the required format by the information sink
• Bandpass transmission: Uses carrier signal for data transmission, like frequency modulation and amplitude modulation
  • composed of high and low amplitude carrier signals (if binary level)

Pulse Modulation

• Process of converting digital information, bit, or bit groups into current or voltage signals/pulses.
• Digital signals are discrete in nature

Modulation Techniques

1. NRZ-L is the most basic way to represent bit values into waveforms wherein 0 means high and 1 means low (or vice versa)
2. NRZI is not as straight forward as NRZ-L and also includes the encoding scheme
3. Manchester represents bit values with two levels

Messages and Characters

• Analog messages are transformed into a sequence of bits using a analog-to-digital conversion process
  • An m-ary alphabet specifies a set of symbols, wherein each symbol represents digital information in terms of a combination of k bits
    • high and low pulses essentially represents symbols
    • m represents the number of symbols
    • k refers to the number of bits representing each symbol
    • The more bits a symbol can carry, the better. To put it simply, the greater the k the better.
    • Physical signals are used to represent symbols for long distance transmission

Transmission Characteristics

1. Signaling rate/baud rate/ symbol rate
   • The number of times a symbol changes per second
   • Expresses in baud (symbols per second)
2. Data rate/bit rate
   • The number of bits transmitted per second
3. Bandwidth
   • Any signals that can be decomposed into multiple sinusoidal frequencies of different frequencies (Fourier analysis). It is the span from the lowest frequency to the highest frequency.
   • Bandwidth limits the signals transmitted in a medium, and, as such, also limiting Baud Rate
   • Bandwidth is associated more with Baud Rate than bit rate because it is more concerned with the symbols involved rather than the bits.
   • Signal Property: Range of frequencies spanned by significant components of a signal
   • Medium Property: Range of frequencies that a medium can pass with no or little attenuation
   • Bandwidth can also refer to the speed of transmission
   • Bandwidth issues
     • A perfect sine wave only has 1 component
     • Theoretically, a square wave has infinite components if we consider an infinite amount of possible values, however the there is a limit when it is examined pragmatically
     • Band limit
       • Can act like a low pass filter, limiting the bandwidth into a specified frequency
       • It can alter the form of the signal (e.g., changes square wave into more sine-wave like), and therefore introduces distortion²
     • Pulse shape affect bandwidth requirements
       • The modulation technique can reduce the duty cycle—the ratio of time when it is high compared to when it is low—to 20%.³
4. Signal levels/Signal strength
   • Expressed in voltage or power units
     • It is commonly expressed in decibel units
       • dB - power relative to 1W
       • dBm - power relative to 1 mW
       • dBV - voltage level relative to 1Vrms
       • dBuV - voltage level relative to 1uVrms
     • Can also be expressed in decibel units per unit distance
       • dB/meter, dBuV/meter, etc.
5. Signal-to-noise ratio
   1. ratio of desired level to the noise signal level.
6. Data throughput
   • Effective transmission rate of useful data
7. Error rate
   • the probability that an error in transmission occurs
   • Bit-error rate (BER) - number of error bits per unit time
   • Bit error ratio (BER) - ratio of bad bits / transmitted bits measured in a time interval (to divide bits, use XOR)
   • Different signaling methods have different error rate traits
   • Negatively affects throughput and data rate

Computing for Bit Rate given Baud Rate

1. Find the m of the m-ary
2. Get
3. Multiply with the Baud Rate

Footnotes

1. Channel and medium are not exactly the same thing: medium is often associated with physical features, while a channel can be non-physical (e.g., wireless) ↩

2. To retain the actual shape of the signal, you would need to pass all significant components of the signal ↩

3. This is because it can make the square wave narrower ↩