Speed, Wavelength, Frequency

c = λ * f

• The lightspeed is constant. (3 * 10^8 m/s)

Types of Waves

• Space wave (upper Ionosphere)
• Sky wave
• Ground wave

Ground wave

• follows the contour of the earth
• can propagate over the visual horizon
• <= 2MHz
• AM radio

Skywave

• bounces between Ionosphere and the earth's surface
• 2MHz ~ 30MHz

Line-of-sight propagation

• no bounce, straight propagation
• for satellite communications and ground communications
• 30MHz ~

Signals

• analog / digital
  • analog = continuous time, continuous values
  • digital = discrete time, discrete values
• periodic signals
  • is a sine wave, a model for signal

Sine wave

• is the fundamental periodic signal
• s(t) = A[i] sin(2πf[t]t + Φ[t])
  • peak amplitude (A)
  • frequency (f), period (T=1/f)
  • phase (Φ)

Composite signal

• addition of sine waves

Square wave

infinite sum : { sin(f=1, A=1), sin(f=3, A=1/3), sin(f=5, A=1/5), ... }

Antennas

• radiation pattern: measurement of radiation

Isotropic radiator

• only in a theoretical model
• equal radiation in all directions

Simple dipoles

Antenna gain

• maximum power in the direction of the main lobe compared to the isotropic radiator
• G = 10 lg(P2/ P1) dB
  • isotropic power (P1)
  • diopole power (P2)

Directed and sectorized antennas

Signal Propagation

Signal Propagation Ranges

• Transmission range; communication possible, relatively low error rate
• Detection range; detection possible, no communication possible
• Interference range; may not be detected, may interfere other transmissions

Signal propagations are affected by

• fading
• shadowing
• reflection; at large obstacles
• refraction; density of medium
• scattering; at small obstacles
• diffraction; at edges

Radio propagation effects

• direct signal (Line-of-Sight)

Propagation model

Free-space

• received signal power at distance d

  P = A G Pt / (4 π d^2)

  • Pt : transmitting power
  • A : effective area of antenna; λ^2/π
  • G : antenna gain

Land

P = Gt Gr Pt / L

• Gr : receiver antenna gain
• Gt : transmitte antenna gain
• L =

Path Loss

Free-space

• proportional between d^2 ~ d^4

  Lp = Pt / Pr = (4πd / λ)^2

Land

• Lp = A d^β
  • A : constant
  • d : distance between t ~ r
  • β : constant in typical urban area

Fading

• attenuation of the transmitted signal power
• caused by interference from multiple copies of TX -> RX arriving at different times
• consequence : signal error

Small-scale fading; Fast fading

• due to movement, doppler effect, multipath propagation
• short-term fluctuation in amplitude caused by the local multipath

Large-scale fading; Slow fading

• due to shadowing

Shadowing

• raining, leaves, etc

Doppler Shift

Moving speed effect

Delay spread

• measure of the multipath profile
• difference between the time of arrival of the earliest and the latest multipath component

Inter-symbol interference (ISI)