Physical: Modulation & multiplexing

modulation schemes — send bits as signals multiplexing — share channel among users

Baseband transmission (directly convert bits to signal):

• send symbols representing bits
• symbol rate — rate at which signal changes (also “baud rate”)
• bit rate — symbol rate times bits per symbol
• NRZ (non-return-to-zero): +1V=“1”, -1V=“0”
• others:

• to decode symbols, signals need enough transitions
• problem is, after enough zeros you can’t tell when the next symbol starts
• Manchester encoding — add in clock signal by XORing with data signal
  • high-to-low transition is 1
  • low-to-high is a 0 (just the clock)
• NRZI — transition means 1, no transition means 0
  • long runs of 1 is no problem, 0 still is
  • 4B/5B — every 4 bits mapped to 5-bit pattern using fixed table
  • scrambling — XOR with pseudorandom data, XOR again to decrypt

Passband transmission (regulate amplitude/frequency/phase of carrier signal to mean bits)

• Amplitude Shift Keying — two different amplitudes mean 0 or 1
• Frequency SK — two different tones mean 0 and 1
• Phase SK — wave is shifted 0 or 180 degrees to mean 0 or 1
• Quadrature Phase SK — shifts of 45/135/225/315 degrees to transmit two bits at once (four different levels)
• combine to represent more bits:
  • Quadrature Amplitude Modulation (QAM-16): phase (angle with X axis on constellation diagram) and amplitude (distance from origin on constellation diagram) is modulated, can transmit 4 bits
  • QAM-64 — more combinations
• constellation diagrams:

Multiplexing

• FDM (Frequency Division Multiplexing) — place channels on specific frequency bands
  • WiFi and Bluetooth change frequencies — “frequency hopping”
• TDM (Time Division Multiplexing) — shares channel over time, users take turns on a fixed schedule (round-robin style)
• CDMA (Code Division Multiple Access) — narrow band signal is spread out over wider frequency band
  • each bit time divided into m short intervals (‘chips’), typically 64 or 128
  • each station is assigned m-bit code (‘chip sequence’)
  • to transmit 1 bit, a station sends its chip sequence
  • to transmit 0 bit, a station transmits negation of chip sequence.
  • if more stations transmit at the same time, bits add linearly
  • to recover, compute normalised inner product (station chip sequences must be known in advance)