11.4 Optical Fiber, Microwave, Satellite & Philippine NTC Regulations

11.4 Optical Fiber, Microwave, Satellite & Philippine NTC Regulations

Quick Answer: Optical fiber, microwave links, satellite systems, and NTC regulations form the core of modern telecommunications infrastructure. Key PRC licensure formulas include optical Numerical Aperture ($NA = \sqrt{n_1^2 - n_2^2}$), GEO satellite altitude ($35,786 \text{ km}$), link budget Carrier-to-Noise ratio ($C/N = EIRP - FSPL + G/T - k - B$), and compliance with Philippine Republic Act No. 7925.

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Principles of Fiber Optic Communications

Fiber optic communication transmits information as light pulses along ultra-pure silica glass or plastic fibers. Light guidance is governed by Total Internal Reflection (TIR) at the interface between two optical media:

1. Core: Inner glass region possessing a higher refractive index $n_1$.
2. Cladding: Outer glass layer possessing a slightly lower refractive index $n_2$ ($n_1 > n_2$).

                    Light Ray (Total Internal Reflection)
                     ┌────────────────────────────────────────┐
  Cladding (n2)      │                                        │
  ───────────────────┴───╲────────────────╲───────────────────┴───
                          ╲  θi            ╲
  Core (n1 > n2)    ------ ╲----------------╲ --------------------
                            ╲                ╲
  ───────────────────┬───────╲────────────────╲───────────────┬───
  Cladding (n2)      │                        ╲               │
                     └────────────────────────────────────────┘

Snell's Law & Critical Angle ($\theta_c$)

According to Snell's Law of Refraction ($n_1 \sin \theta_1 = n_2 \sin \theta_2$), light striking the core-cladding boundary at an angle of incidence greater than the critical angle $\theta_c$ undergoes total internal reflection:

$\sin \theta_c = \frac{n_2}{n_1} \implies \theta_c = \arcsin\left(\frac{n_2}{n_1}\right)$

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Numerical Aperture & Light Acceptance Angle

The Acceptance Angle ($\theta_a$) is the maximum semi-angle of the light cone entering the fiber core from air ($n_0 \approx 1.0$) that will undergo total internal reflection inside the fiber.

Numerical Aperture (NA) Formula

Numerical Aperture ($NA$) measures the light-gathering capability of an optical fiber:

$NA = \sin \theta_a = \sqrt{n_1^2 - n_2^2} = n_1 \sqrt{2 \Delta}$

where $\Delta = \frac{n_1 - n_2}{n_1}$ is the fractional refractive index difference.

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Optical Fiber Types & Transmission Characteristics

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Attenuation & Dispersion in Optical Fibers

Optical Attenuation Windows

Fiber loss is expressed in decibels per kilometer ($\text{dB/km}$):

• 850 nm Window (1st Window): High attenuation ($\approx 2.5 - 3.5 \text{ dB/km}$); used for low-cost LED/multimode short-reach systems.
• 1310 nm Window (2nd Window): Attenuation drops to $\approx 0.35 \text{ dB/km}$; point of zero material dispersion for standard SMF.
• 1550 nm Window (3rd Window): Minimum attenuation window ($\approx 0.20 \text{ dB/km}$); standard for long-haul Dense Wavelength Division Multiplexing (DWDM) networks using Erbium-Doped Fiber Amplifiers (EDFA).

Optical Dispersion

• Modal Dispersion: Occurs only in multimode fibers due to differing path lengths of light modes.
• Chromatic Dispersion: Pulse broadening in single-mode fibers caused by wavelength-dependent propagation velocities. Sum of Material Dispersion and Waveguide Dispersion.

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Terrestrial Microwave Communications

Terrestrial microwave links operate in the $2 \text{ GHz} - 40 \text{ GHz}$ frequency range over line-of-sight paths ($30 - 50 \text{ km}$ per hop).

Fresnel Zone Clearance

To avoid diffraction loss from terrain obstacles, the First Fresnel Zone ($R_1$) must remain at least $60%$ unobstructed:

$R_1 = 17.3 \sqrt{\frac{d_1 \cdot d_2}{f_{\text{GHz}} \cdot D}} \quad \text{(meters)}$

where $d_1, d_2$ are distances from path ends to obstacle (km), $D$ is total path length (km), and $f$ is frequency (GHz).

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Satellite Communications & Orbital Physics

Communication satellites act as high-altitude microwave relay stations receiving uplink signals, shifting frequencies via transponders, and retransmitting downlink signals.

                       ┌─────────────────────────┐
                       │   GEO Satellite         │
                       │   Altitude: 35,786 km   │
                       └───────────┬─────────────┘
                        ▲          │
               Uplink   │          │ Downlink
             (e.g., 6 GHz)        │ (e.g., 4 GHz)
                        │          ▼
                  ┌─────┴──┐   ┌───┴────┐
                  │ Earth  │   │ Earth  │
                  │ Station│   │ Station│
                  └────────┘   └────────┘

Satellite Orbits

1. Low Earth Orbit (LEO): Altitude $160 - 2000 \text{ km}$. Fast orbital period ($90 - 120 \text{ mins}$). Requires satellite constellations (e.g., Starlink).
2. Medium Earth Orbit (MEO): Altitude $2000 - 35,786 \text{ km}$. Used for GPS navigation constellations ($20,200 \text{ km}$).
3. Geostationary Earth Orbit (GEO): Positioned directly over the equator at an altitude of exactly $35,786 \text{ km}$ (orbital radius $R = 42,164 \text{ km}$). Orbital period equals Earth's rotational period ($23 \text{ hours } 56 \text{ mins } 4 \text{ secs}$), making the satellite appear stationary from ground stations. 3 GEO satellites spaced $120^\circ$ apart provide complete global coverage (excluding polar regions).

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Satellite Link Budget Analysis & Calculation

A Link Budget tabulates all power gains and losses from transmitter to receiver.

1. Effective Isotropically Radiated Power (EIRP): $EIRP_{\text{dBW}} = P_{t,\text{dBW}} + G_{t,\text{dBi}} - L_{t,\text{dB}}$
2. Receiver Figure of Merit ($G/T$): Ratio of receive antenna gain to system noise temperature in Kelvin ($K$): $(G/T)_{\text{dB/K}} = G_{r,\text{dBi}} - 10 \log_{10}(T_{\text{sys}})$
3. Carrier-to-Noise Ratio ($C/N$): $C/N_{\text{dB}} = EIRP_{\text{dBW}} - FSPL_{\text{dB}} + (G/T)_{\text{dB/K}} - k_{\text{dBW/(Hz K)}} - B_{\text{dB-Hz}}$ where $k = 10 \log_{10}(1.38 \times 10^{-23}) = -228.6 \text{ dBW/(Hz\cdot K)}$ is Boltzmann's constant, and $B$ is channel bandwidth.

Step-by-Step Link Budget Example

Problem: A GEO satellite downlink operates at $4 \text{ GHz}$ with an $EIRP = 40 \text{ dBW}$. The free-space path loss is $196 \text{ dB}$, receiver figure of merit $G/T = 20 \text{ dB/K}$, and channel bandwidth is $36 \text{ MHz}$ ($75.56 \text{ dB-Hz}$). Calculate the received Carrier-to-Noise ratio ($C/N$).

Solution: $C/N = EIRP - FSPL + (G/T) - k - B$ $C/N = 40 - 196 + 20 - (-228.6) - 75.56$ $C/N = -136 + 228.6 - 75.56 = 17.04 \text{ dB}$

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Philippine NTC Regulations & Telecommunications Laws

Telecommunications in the Philippines is regulated by the National Telecommunications Commission (NTC), an agency attached to the Department of Information and Communications Technology (DICT).

Summary of Core Philippine Telecom Regulations