Fixed Terrestial Wireless

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This article covers the practical aspects of wireless access to the Internet via Fixed Terrestial Wireless (radio).


Contents

Introduction

While not widely deployed, fixed terrestrial wireless is nonetheless alive and well in some locations.

Network Type and Infrastructure

Fixed Terrestrial Wireless is a point-to-multipoint or 'star' topology, much like Wi-Fi, consisting of a single central hub radio (access point) with internet connectivity, and a number of fixed client radios (subscribers).

  • Clients can connect to one and only one access point, and not to other clients.
  • An access point can only connect to clients, not to other access points.
  • Some service providers may daisy chain radio access points, if not by routing traffic from one to the next, then by daisy chaining the backhaul to the Internet. This can introduce substantial latency for those serviced by the last radio hub on the daisy chain.

Hardware

Access Point

A wireless access point (AP) is the central bridge device used in an Infrastructure wireless network. Traffic from the wireless side of the bridge is sent to the Internet side of the bridge, and vice versa. The wireless access point controls all traffic with wireless client radios. The access point is normally pole or tower mounted at a location that provides good line of sight coverage to as much of the desired service area as possible. The pole or tower will often contain two radios. One is the access point radio that the client radios communicate with. The other is a point-to-point backhaul radio that communicates back to a central office, carrying traffic between the internet pop and the wireless access point.

Client Radio

Typically there is a radio/antenna unit mounted outside the house, in a location that has a clear view, or line of sight, to the central hub radio. The size will normally be smaller or about the same as a satellite dish. Depending on distance from the hub, the radio/antenna unit may be just a single weatherproof box, or for more distant locations, the radio/antenna may be mounted mounted on and pointed into a larger reflector that gives more gain. Unlike a satellite receiver system, the radio and the antenna are typically both located in the box mounted outside the home. The subscriber side of the radio/antenna is Ethernet plus power for the radio. Putting the radio at the antenna eliminates radio to antenna coax losses, and therefore the length of the cable run from the outside equipment indoors is not a limiting factor for signal strength. An adapter box is used inside the house to put power in the Ethernet cable to the radio, but does not feed power back towards the client side equipment.

Coverage Area

Overview

Fixed Terrestrial Wireless is basically a cellular network, as are Wi-Fi, WiMAX and cellular telephone. The hub defines the 'central' point of the cell, and the distance covered will depend on antennas used, transmitter powers, receiver sensitivity, and the terrain and any interfering obstacles. This type of system can easily cover an area where subscribers are 5 or more miles from the hub. The coverage is thus similar to WiMAX, much larger than Wi-Fi, but limited to fixed subscriber side installations. The deployment of this system would usually be designed to cover a predetermined area of land, which may or may not be circular. Therefore, the hub may not be at the actual center of the cell, and the cell may not be circular.

Link Calculations

For an idea of how to calculate the area covered, you need to know the specifications of the hardware at each end, and the specifics of the radio path. You can check example calculations in Wi-Fi#Link_Calculations. (Coax loss is normally zero for Fixed Terrestial Wireless since the antenna is normally attached to the radio.) However, in nearly all cases, you will be using equipment provided by the wireless service provider, and you probably won't have access to the radio specifications. The service provider will probably not make any path calculations, but will ask your location, and if it sounds possible, come out and 'take a look'. If line of sight seems OK, or even close, and the distance is OK, then they will likely just give it a try and see what they get. You should not expect your salesman or installer to be a radio engineer. You should expect the installer to be familiar with the area and know what normally will or will not work.

Path Variables

Line of Sight

If you have line of sight, then tree growth or someone building and consequently obstructing the line of sight might well create a problem. If you don't have line of sight, then the path might still work, but might change with the seasons if trees are the reason for it not being line of sight.

Fresnel Zone

The remaining issue is one of ground clearance, commonly known as fresnel zone clearance, which gives you an indication of what effect ground along the path will have. If the path is very close to ground, then you probably won't have enough clearance, and will lose a few dB, but the signal should be stable.

Ground Reflection

If the path is high above ground, and crosses over a flat area along the path that can be seen from the antennas at both ends, then you have the possibility of signal reflections off the ground arriving at the receive antenna delayed and out of phase with the direct path, and the two signals can cancel. It is an unfortunate fact of life that the reflectivity of the ground can change drastically. If you have a rough surface, or a nice crop of corn, it will attenuate any signal that would otherwise be reflected. But at another time of year, the ground can be bare, maybe have a nice coating of dew or water, and be very reflective. The solution to this type of problem normally is to alter the height of one antenna and change the reflection from a cancelling mode to an additive mode.

Rain Attenuation

With a radio path of less than 10 miles, the path should not normally be subject to too many variables. Rain attenuation would not normally be a problem.

Service Speeds

High speeds of 4.8 Mbps downlink and 1.8 Mbps uplink are quite possible. Actual speeds can vary greatly depending on provider, geography, and class (cost) of service.

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