Terrestrial Microwave Communication Systems: Concept and Design

Introduction to Terrestrial Microwave Communication

Terrestrial Microwave Communication, often referred to as Line-of-Sight (LOS) communications, utilizes radio waves within a specific spectrum to establish wireless connections between two fixed points on Earth. This technology is traditionally a common carrier service provider offering a quick and reliable method for transferring voice, data, and video across various facilities.

Spectrum Range: The microwave spectrum is generally considered to start at approximately $1\,GHz$ and extends up to $30\,GHz$ . For specific point-to-point terrestrial communications, frequencies typically range from $6\,GHz$ to $60\,GHz$ .
Advantages over Landline:
- Increased speed of deployment.
- Ease of installation in difficult terrain.
- Reduced cost compared to physical cabling.
Link Distance: High-capacity communications are reliable at ranges of a mile or two for high frequencies, while lower frequencies can reach distances of $20$ to $30$ miles ( $-2$ to $40\,km$ ).

Terrestrial Microwave System Setup and Components

A microwave system consists of various specialized hardware components designed to transmit and receive high-frequency signals.

Typical System Components

Microwave Antenna: Often parabolic reflectors used for focusing signals.
Radome: A protective cover for the antenna to shield it from environmental factors.
RF Cables and Connectors: Used to link the radio equipment to the antenna (e.g., N-Type, SMA connectors).
Tower: Provides the necessary height for line-of-sight paths.
Equipment Shelter (BTS): Housing for indoor units and power supplies.
Multiplexers/Demultiplexers (MUX/DEMUX): Combines/separates multiple signals for transmission.
Power Supply: Often includes backup systems like Diesel Generator Sets and Battery Racks.

Analog Microwave Communication System Setup

Connects to the Public Switched Telephone Network (PSTN) or Cellular Networks.
Uses items such as Flanges (PDR84) and Elliptical Waveguides (EW77 - WR112).
Signals travel from the PSTN/MUX through the analog microwave radio, through the waveguide to the parabolic antenna.

Digital Microwave Communication System Setup

Digital systems often employ a split-mount or direct-mount architecture.

ODU (Outdoor Unit Radio): Mounted near or directly on the antenna to minimize signal loss.
IDU (Indoor Unit Radio): An Intelligent Node Unit containing:
- Node Controller Card
- Node Protection Card
- Radio Access Card
- Data Access Card
Cabling: Employs Coaxial IF Cables to carry the Intermediate Frequency (IF) signal between the IDU and ODU.

Microwave Radio Frequency Spectrum and Band Designations

Band	Frequency Range	Typical Applications
L	$1$ to $2\,GHz$	Satellite, Navigation (GPS), Cellular phones
S	$2$ to $4\,GHz$	Satellite, SiriusXM, unlicensed (Wi-Fi, Bluetooth), Cellular
C	$4$ to $8\,GHz$	Satellite, Microwave relay
X	$8$ to $12\,GHz$	Radar
$K_u$	$12$ to $18\,GHz$	Satellite TV, Police radar
K	$18$ to $26.5\,GHz$	Microwave backhaul
$K_a$	$26.5$ to $40\,GHz$	Microwave backhaul
Q	$30$ to $50\,GHz$	Microwave backhaul
U	$40$ to $60\,GHz$	Experimental, Radar
V	$50$ to $75\,GHz$	New WLAN (802.11ad/WiGig)
E	$60$ to $90\,GHz$	Microwave backhaul
W	$75$ to $110\,GHz$	Automotive radar
F	$90$ to $140\,GHz$	Experimental, Radar
D	$110$ to $170\,GHz$	Experimental, Radar

Terrestrial Microwave Applications

Long-haul Backbone: Microwave links are used as backbone connections to smaller cities and towns, especially where terrain makes fiber burial difficult.
Mobile Backhaul: Approximately $80\%$ of cellular towers worldwide use microwave links to connect Base Transceiver Stations (BTS) to the Transmission Network (BSC/MSC).
Enterprise: Hospitals and universities use microwave to connect various campus locations, bypassing local telephone companies.
Metro Access: High-speed access between office buildings for incumbent and competitive carriers.
Control and Monitoring: Used by utility companies, railways, and pipeline operators to monitor infrastructure.
Military: Includes specialized Troposcatter microwave radio terminals for long-distance communication.

Engineering a Terrestrial Microwave System

Link Budget Analysis

Engineers begin designs with a Link Budget Analysis to calculate the feasibility of the signal path.

System Gain: Depends on radio design and modulation.
Antenna Gain: Added to the system gain (larger antennas provide higher gain).
Free-Space Loss (FSL): Subtracted from the total gain; increases with distance.
Fade Margin: The resulting buffer that protects against signal fluctuations. If signal loss exceeds this margin, the link fails.

Factors Affecting Signal Propagation

Rain Attenuation: Rain absorbs microwave energy. Engineering is based on ITU or Crane global models which map the world into "Rain Climate Regions."
Multipath: Primarily affects lower frequencies. Occurs when a receiver gets multiple signals (direct and reflected). It is combated by:
- Adjusting antenna height.
- Diversity Reception: Deploying two parallel radio links at varying heights.
Interference: Caused by adjacent channels or other radio systems. Engineers must maintain a specific Carrier-to-Interference ( $C/I$ ) ratio.

Automatic Transmit Power Control (ATPC)

ATPC is a closed-loop algorithm that manages interference and power consumption:

Maintains low transmit power during clear conditions to minimize interference with nearby links.
Automatically increases power if rain or interference is detected to maintain the link.

Terrestrial Microwave Path Design Process

Site Selection: Choosing physical locations and repeaters.
Determination of Line-of-Sight: Calculating earth bulge and checking for obstructions.
Refractive Effects: Evaluating path clearances with regard to the effective earth's radius factor ( $K$ ).
Fresnel Zones: Evaluating clearances relative to the first Fresnel zone ( $F_1$ ).
Path Reflections: Considering reflection points and using passive billboard repeaters to bypass mountains.
Power Budget Calculations: Computing Receiver Signal Level (RSL) based on gains and losses.
Fade Margin Determination: Calculating the difference between RSL and the receiver threshold.
Path Reliability: Determining the system unavailability and reliability (e.g., $99.99999 \%$ targets).
Improving Reliability: Implementing reception diversity, protection switching, and redundancy (e.g., $1+1$ Hot Stand-by).

Technical Case Study: Richmond to Yellow Point Road

Locations: Richmond, British Columbia (Site A: $49^\circ 7' 40.42'' N$ , $123^\circ 9' 12.94'' W$ ; Elevation: $2\,m$ ) to Yellow Point Rd, Ladysmith, BC (Site B: $49^\circ 3' 1.69'' N$ , $123^\circ 49' 44.78'' W$ ; Elevation: $26\,m$ ).
Frequency: $7.279\,GHz$ and $7.895\,GHz$ .
Obstructions: Managed using a passive billboard repeater located $29.55\,km$ from Richmond.
Reliability: Expected at $99.999999769\%$ .
Fresnel Clearance: $60\%$ .
Fade Margin: At least $73.915666\,dB$ .
Earth Radius Factor ( $k$ ): Range from $2/3$ to infinity.

Working Formulas for Path Design

Signal and Loss Formulas

Received Signal Level (RSL): $RSL = P_t - TLL_{TX} - CL_{TX} + G_{TX} - FSL_{TX-BB1} + P-G_B - FSL_{BB2-RX} + G_{RX} - CL_{RX} - TLL_{RX}$
Free Space Loss (FSL): $FSL = 92.4 + 20 \log(D_{km}) + 20 \log(f_{GHz})$
Antenna Gain: $G = 7.5 + 20 \log(D) + 20 \log(f_{GHz})$
Fade Margin (FM): $FM = RSL - IT$ (where $IT$ is the improvement threshold).

Path Obstruction and Clearances

Earth Bulge ( $h$ ): $h = \frac{d_1 d_2}{12.75 K}$ ( $h$ in meters, $d$ in km).
First Fresnel Zone ( $F_1$ ): $F_1 = 17.3 \sqrt{\frac{d_1 d_2}{f_{GHz} D_{total}}}$
Total Obstruction Height: $h_{obs} = h_{elev} + T.G. + h$

Reliability and Unavailability

Path Reliability: $R = (1 - UNDP) \times 100\%$
Non-Diversified Outage Probability (UNDP): $UNDP = (a \times b) \times 6 \times 10^{-7} \times f \times D^3 \times 10^{-FM/10}$ (where $a$ is the terrain factor and $b$ is the climate factor).

Passive Repeater Gain ( $G_{passive}$ )

$G_{passive} = 22.2 + 40 \log(f_{GHz}) + 20 \log(A_{ft^2}) + 20 \log(\cos(\theta))$