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lecture06-physical4.md

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Lecture 6 Physical Layer IV

Wired

Twisted Pair

  • Two insulated copper wires arranged in a regular spiral pattern to minimize interference
  • Low cost

Optical Fiber

  • Light sources generate pulses of light that are transmitted on optical fiber
  • Profound influence on network architecture
    • Dominates long distance transmission
    • Plentiful bandwidth for new services
  • Core/Cladding/Jacket
  • Multimode fiber: multiple rays follow different paths
  • Single-mode fiber: only direct path propagates in fiber
  • Optical range from $\lambda_1$ to $\lambda_1+\Delta\lambda$ contains bandwidth $B=f_1-f_2=\frac{v}{\lambda_1} - \frac{v}{\lambda_1+\Delta\lambda}$
  • Different wavelengths carry separate signals
    • Multiplex into shared optical fiber
  • To extend range, use combinations of optical amplifiers and regenerators

wavelength_division_multiplexing

Wireless

  • Wireless is flaky
  • Wireless is a broadcast medium
  • Wireless is half-duplex

Wifi

  • Basic Equation: $y(t)=hx(t)+n(t)$
  • More generally: $y(t)=\Sigma_{i=0}^{i=k}h(i)s(t-i\tau)$
  • But time is continuous: $y(t)=\int h(\tau)s(t-\tau)+n(t)=h(t)*s(t)+n(t)$
  • How to estimate $h$?
    • Send known $x(t)$ as "preamble"
    • $h=y(t)/x(t)$
  • In channel, there is attenuation and phase shift
    • $h=1/d \times e^{j2\pi d/\lambda}$
    • Consistent with $1/d^2$ power fading
    • $d/\lambda = df/c = ft$

wifi_processes

Frequency Division Multiplexing (FDM)

  • Divide bandwidth into small chunks: subcarriers
  • Orthogonal Frequency Division Multiplexing (OFDM)
    • WiFi, LTE uses OFDM

orthogonal_frequency_division_multiplexing

Mutiple Input Multiple Output (MIMO)

  • SISO: Assuming narrowband, $y=hx+n$
  • Estimating channels

estimating_channels

  • n antennas -> n more data

mimo_matrix

Cellular Whirlwind

  • To increase network capacity:
    • Multiple low-power transmitters (100W or less)
    • Small transmission radius -> area aplit in cells
    • Each cell with its own frequencies and base station
    • Adjacent cells use different frequencies
  • The Hexagonal Pattern
    • A hexagon pattern can provide equidistant access to neighboring cell towers
    • $d=\sqrt3R$
  • Cell Sectoring
    • Cell divided into wedge shaped sectors
    • 3-6 sectors per cell, each with own channels
    • Use of directional antennas

Cellular Standards

  • 1G: analog voice
  • 2G: digital voice
  • 2.5G: voice and data channels
  • 3G: voice (circuit-switched) and data (packet-switched)
    • Uses Code Division Multiple Access (CDMA)
  • 4G: 10Mbps and up, seamless mobility between different cellular technologies
    • LTE the dominating technology (OFDM-A)
      • Assign each user a chunk of resource blocks coordinated by the cell tower
    • Packet switched
  • 5G: mm-wave, more bandwidth, massive MIMO
    • 10-20MHz
    • 5G plays three games to increase based on $C=nB\log(1+S(I)NR)$
      • Increase n: Massive MIMO
      • Increase B: mm-wave frequencies
      • Increase B: buy more spectrum
      • Reduce I: smaller cells