What "noise" means in a cellular system depends on whom you're talking to. To the subscriber, it's a simple disruption in call audio, but to the carrier, noise is more complex. Noise affects cellular communications in various ways, only some of which engineers can control.

All matter at temperatures above absolute zero (0K, about -460F) radiates electromagnetic energy. The amount of energy is related to temperature -- the hotter the matter, the more energy is radiated. This energy is described by Boltzmann's Constant, 'k' (k = -198.6dBm/degreesK-Hz). This constant, multiplied by the temperature of the matter a receiver views and the system bandwidth, yields an irreducible background noise against which a desired signal must compete. This is thermal noise.

In a cellular system, the receiving antennas are designed to view the ground around the site because that's where the subscribers are. The ground temperature varies, but at 80F, it's about 300K (T=300K). RF engineers typically use this number as a rule of thumb.

The receiver bandwidth varies depending on the technology, but the same principles hold for all technologies. EAMPS, for example, uses 30kHz-wide channels. Receiver bandwidth is a bit less than 30kHz, for rejection of adjacent channels. Assume the typical EAMPS receiver has a bandwidth of 25kHz (B=25,000Hz). By making this assumption, you can calculate the amount of noise an EAMPS receiver will have in its passband if it contributes no noise of its own.

This receiver thermal noise floor often is referred to as 'kTB.' In the example, assume consistent units:

kTB = -198.6 + 10 Log(300)

+ 10 Log(25,000) in dBm

kTB = -129.8dBm

Thus, if you build a perfect EAMPS cellular receiver, it would have -129. 8dBm of noise in its passband competing with the wanted signal.

CABLE LOSS Cable, filters and other passive elements exhibit a loss and produce thermal noise.

If a cable (or other lossy element) has 10dB of loss, it will attenuate the desired signal as well as the input noise by 10dB. But at the output of the cable, you will see noise at least equal to kTB because the cable itself contributes it.

If you put a signal into the cable at -100dBm over a thermal noise of -129. 8dBm, you have a signal-to-noise ratio of 29.8dB at the cable input. At the cable output, the signal has been attenuated by 10dB to -110dBm. The noise you put into the cable also has dropped the same amount, to -139.8dBm. But the cable contributes its noise floor of -129.8dBm, so the combined (uncorrelated) noise terms are -129.8dBm. The resulting signal-to-noise ratio is only 19.8dB at the cable output. You sacrifice 10dB of signal-to-noise ratio. This is why you spend money on 7/8", 15/8" or larger coax at cell sites to reduce this loss.

ANTENNA ADVANTAGE Antennas, on the other hand, offer an advantage. If an antenna is "looking" at the ground surrounding the site, its thermal noise output remains the same, regardless of its gain. But, as gain is increased, the desired signal increases in strength. So, the signal-to-noise ratio from a limited power source such as a portable phone grows as antenna gain grows.

A simple conservation-of-energy argument shows that the higher the antenna gain, the smaller the field of view. 120 sector antennas will have three times (+4.8dB) the gain of an omni with a similar vertical pattern. This is why many PCS carriers sectorize rural sites. They don't often use sectorization for capacity; they need the antenna gain to overcome higher propagation losses at 1.9GHz. The cost of sectorization must be balanced against improved cell performance.

So, high antenna gain coupled with low cable loss yields the best receiving system performance. What about the receiver itself? There is no such thing as a perfect receiver. With a good design, the receiving equipment at a cell site may reach a "noise figure" of 3dB to 6dB. This means it will contribute its own noise in the act of processing the signal, degrading the signal-to-noise ratio by 3dB to 6dB.

What does this mean? In a cellular system, you typically can come up with all of the forward channel power necessary to reach a mobile. The mobile however, is limited by published standards. An EAMPS portable, for instance (with a unity gain antenna), only radiates 600mW (about +28dBm). A 1-way call isn't particularly useful, so the performance of the call is limited by the weaker link -- usually the reverse link. When the received power from the mobile is overcome by thermal noise, the call becomes noisy.

The EAMPS rule of thumb is 17dB carrier-to-noise ratio, which means you can sustain a path loss that results in a 17dB signal-to-noise ratio at a typical receiver. In the example (in dB), that's +28 (the transmit power of the mobile) +13 (a typical cell-site antenna gain) -2 (typical cable loss) -5 (typical cell-site noise figure) -17 (the desired signal to noise) +129.8 (kTB) = 146.8dB. When the path loss reaches 147dB, the call becomes noisy. Path loss is determined by physics. If the path loss exceeds the limit, you get a noisy call.

INTERFERENCE The term "noise" often is used to refer to what actually is interference. Carriers reuse channels routinely to handle their subscribers, often in areas small enough that some signal from each reuse reaches other areas in which the same channel is used. The trick is planning so that the interference is much lower than the desired signal.

Co-channel interference raises the thermal + interference "noise floor" with which the system must cope. This means that the maximum design path loss, hence maximum range of a cell, is less. The degree varies with how much frequency reuse is present.

To minimize the impact, use the highest antenna gain possible consistent with proper coverage of the intended area(s);downtilt antennas to reduce reuse interference; use the shortest run of the lowest loss coaxial cable possible, consistent with cost, tower loading and practical limitations; keep connectors clean and free of corrosion; and use short, low-loss jumpers. If long, lossy cable runs are unavoidable, use a tower-mounted amplifier to lower the system noise figure and overcome cable loss and select base-station options that reduce the receiving system noise figure.

ENVIRONMENTAL CONTRIBUTIONS Sometimes the environment contributes more "noise." In populated areas, any equipment that makes RF energy from corroded (arcing) electrical connections to other radio transmitters can produce unintentional noise and interference. When this happens to an objectionable degree, an engineer must drive the affected area to find the interference source.

Sometimes, the solution is easy. For example, a power company wants to fix arcing connections. Sometimes, however, the source is difficult to identify. Multiple transmitters near a cell site may each transmit a clean signal that poses no problem. However, if energy from these transmitters combines, "intermodulates," in an electrically non-linear device, products may form that fall in the cellular band. In such cases, options include adding filters to the offending equipment, replacing a tower or shutting someone down. The engineer must not only play detective, but also be a diplomat in solving the problem with others.