ENTERPRISE EQUIPMENT MICROWAVE RADIO: The Still Good Factor

TECHNOLOGY TRENDS



Free Space Loss: As signals spread out from a radiating source, the energy is
spread out over a larger area. As this occurs, the strength of that signal gets

weaker.

Free space loss (FSL), measured in dB, specifies how much the signal has

weakened over a given distance. The type of antenna used has no effect on FSL

since at any appreciable distance all antennas look like a point-source

radiator. The difference in FSL between a 2.4 GHz link and a 5.8 GHz link is

always about 8 dB, regardless of the distance. This is one of the reasons why

802.11a WLAN devices will have less than half the range of a 2.4 GHz WLAN device

(e.g., 802.11b).

l Fresnel Zone: Radio

waves travel in a straight line, unless some obstruction refracts or reflects

them. But the energy of radio waves is not "pencil thin."

They spread out and get weaker the farther they move from the radiating

source–like ripples from a rock thrown into a pond. The area that the signal

spreads out into is called the Fresnel zone. If there is an obstacle in the

Fresnel zone, part of the radio signal will be diffracted or bent away from the

straight-line path.

The practical effect is that on a point-to-point radio link, this refraction

will reduce the amount of RF energy reaching the receive antenna. The thickness

or radius of the Fresnel zone depends on the frequency of the signal–the

higher the frequency, the smaller the Fresnel zone. Therefore, the Fresnel zone

is fattest in the center. As with FSL, the antennas used have no effect on the

Fresnel zone.

l Receive Signal Level:

Receive signal level is the actual received signal level (usually measured in

negative dBm) presented to the antenna port of a radio receiver from a remote

transmitter.

l Receiver Sensitivity: Receiver

sensitivity is the weakest RF signal level (usually measured in negative dBm)

that a radio needs receive in order to demodulate and decode a packet of data

without errors.

l Antenna Gain: Antenna

gain is the ratio of how much an antenna boosts the RF signal over a specified

low-gain radiator. Antennas achieve gain simply by focusing RF energy.

If this gain is compared with an isotropic (no gain) radiator, it is measured

in dBi. If the gain is measured against a standard dipole antenna, it is

measured in dBd. The gain applies to both transmit and receive signals.

l Transmit Power: The

transmit power is the RF power coming out of the antenna port of a transmitter.

It is measured in dBm, watts or milliwatts and does not include the signal loss

of the coax cable or the gain of the antenna.

l Effective Isotropic Radiated

Power: Effective isotropic radiated power (EIRP) is the actual RF power as

measured in the main lobe (or focal point) of an antenna. It is equal to the sum

of the transmit power into the antenna (in dBm) added to the dBi gain of the

antenna. Since it is a power level, the result is measured in dBm. Using an

amplifier, +24 dBm of power (250 mW) can be "boosted" to +48 dBm or 64

Watts of radiated power.

l System Operating Margin: System

operating margin (SOM) is the difference (measured in dB) between the nominal

signal level received at one end of a radio link and the signal level required

by that radio to assure that a packet of data is decoded without error. In other

words, SOM is the difference between the signal received and the radio’s

specified receiver sensitivity. SOM is also referred to as link margin or fade

margin.

l Multipath Interference: When

signals arrive at a remote antenna after being reflected off the ground or

refracted back to earth from the sky (sometimes called ducting), they will

subtract (or add) to the main signal and cause the received signal to be weaker

(or stronger) throughout the day.

l Signal-to-noise Ratio:

Signal to noise ratio (SNR) is the ratio (usually measured in dB) between the

signal level received and the noise floor level for that particular signal. The

SNR is really the only thing receiver demodulators really care about. Unless the

noise floor is extremely high, the absolute level of the signal or noise is not

critical. The weaker signals have larger negative numbers.

BUYING TIPS

l Clearing Fresnel Zone: Irrespective

of whether the radio link planned is point-to-point or point-to-multipoint, the

first thing to do is to verify that it will have not only clear line of sight,

but at least 60 percent of the first Fresnel zone clear of obstructions as well.

The longer the distance, the more important it is for this verification. If the

Fresnel zone is blocked, then you will get a lower signal level on the distant

end than expected–even if you can literally ‘see’ the other antenna in the

distance.

l Perform RF-path Analysis:

Even if the Fresnel zone is partially blocked, it is still possible to get a

link, provided that the radio system is designed to have a strong signal at the

other end of the link. In planning long-range microwave links where unobstructed

line-of-sight and clear Fresnel zone are not certain, an RF path analysis should

be done. There are many software packages available that have terrain data and

can create a path profile from a set of latitude/longitude coordinates.

l Have

Unobstructed line of Sight: The software programs for RF path analysis can

only indicate for certain if a link will not work due to terrain obstruction. A

clear path on paper is not a guarantee that your link will work, since it does

not show trees or buildings. So, even if there is a ‘clear’ link on paper,

RF analysis may show if there are 80-feet trees that can block the signal.

l Perform SOM

Calculation: In case of both clear line of sight and 60 percent of the first

Fresnel zone clear (or nearly clear), how can you know if you will have a good

link or not? How much gain do your antennas need to have? How much coax cable

loss is too much? If your link is at 2.4 GHz, should external amplifiers be

used?

Or, given your fixed base station antenna with a pre-set

gain, how far can you reach with the different types of client antennas? And,

which clients will need amplification? By doing an SOM calculation, you can test

various system designs and scenarios to see how much fade margin (or

"safety cushion") your link will theoretically have.

The SOM is determined by computing the difference between the

received signal and the radio’s receiver sensitivity. The typical formula used

is

RX Signal = EIRP — FSL + RX

Antenna Gain — Coax Cable Loss

Regarding the minimum SOM needed, there is no absolute answer

to this question, but the higher it is, the better. Most engineers agree that 20

dB or more is quite adequate. Some think as low as 14 dB is still good. Others

operate systems down to 10 dB or less.

The problem with accepting a lower SOM is that you have a

smaller safety margin. You run the risk of your link going down in case of

interference, an antenna off its aim, atmospheric conditions, moisture in your

coax, ice/rainwater on the radome, or a host of other factors.

l Determining

Interference: The SOM is not the only determining factor. It is the actual

SNR at the receiver that makes a link reliable. If there is noise or

interference on the channel, the SNR will deteriorate. This could be an issue if

you are co-locating at a site with other radios operating in the same band.

You need to find out what frequency spectrum these radios are

occupying. If these transmitter have energy or sideband noise on your receive

channel and their antennas are close to yours, you will likely get interference

from them, perhaps to the point where your link will not work.

l Factoring

Atmospheric Absorption: The SOM calculation holds perfectly for a vacuum. In

reality, there is some atmospheric absorption of the RF energy that scatters and

attenuates the signal. For example, tests on a 23-mile 5.8 GHz link vary as much

as +/-6 dB over course of a day. This variation is mostly caused by multipath

interference and other atmospheric variations.

l Using an

Amplifier: With coax cable at the receiver and no amplifier at the receiver

antenna, the SNR at the antenna does not survive when it actually reaches the

radio itself. In this case, the noise generated in the RF front-end of the radio

is a factor.

If an amplifier is used on the receive end, the SNR as it

appears at the antenna is preserved all the way down the co-ax to the radio.

This phenomenon largely occurs because the low-noise amplifier mounted on the

pole sets the noise floor for the system.

MARKET INFORMATION

While the market for ISPs has stagnated, enterprises, especially from the

banking and media sectors are some of the biggest clientele of microwave radio

systems. These include names like Punjab National Bank, Bank of Punjab, NDTV,

Radio Mirchi, Mid Day, Star News and Hindustan Times. Though last year, the

market was nearly flat, it is slated to grow by more than 50 percent this year

to end at nearly Rs 150 crore.

Radios typically excel in remote locations and in smaller

towns where it is difficult and more expensive to deploy optic fiber. Therefore,

as BSNL plans large-scale expansion in the North East, radio systems could

receive a big thrust this year. Cisco and Proxim are the main players, though

HNS, Alcatel, and Fresnel could come into the reckoning in the near

future.