I found the following : " The purpose of a tower-top amplifier, or TTA, is to improve receiver sensitivity at the repeater site. Good sensitivity at the repeater is especially important in land mobile radio systems because it overcomes some of the link imbalance created by the high-power repeater transmitter. "
But, How is the above possible ? Any links / documents pls !
Land mobile (VHF/UHF) is somewhat different than 2.4GHz. The big difference is coax cable losses. At 2.4GHz, they're MUCH higher. If you have a tower, and about 100ft of reasonable coax cable (LMR-400), you'll see about 8dB of coax loss at 2.4Ghz. Put an antenna on top of the tower, add this 100ft of coax, and plant the access point in the shelter building. You'll get terrible reception because the 7dB of loss will reduce the range to about 40% of what it might be if the access point were located at the antenna (on top of the tower). In some cases, it's impractical to put the access point on top of the tower. For example, visualize climbing the tower in a storm. So, instead of the whole access point, a bi-directional (switched) amplifier is installed on the top of the tower. This eliminates the coax cable loss in both directions, and dramatically improves the range.
Unfortunately, the link is a bit muddled. While a tower top amplifier can usually improve range in both directions (xmit and receive), it more often creates the imbalance that the article suggests. Some tower top 2.4GHz amplifiers transmit at 1 watt, while the client radio might be lucky and transmit at perhaps 0.035 watts. That's a major imbalance caused by the TTA.
The article you found (and didn't bother citing the source) is about land mobile radio. In land mobile, repeaters typically have power outputs of perhaps 40 to 100 watts depending on system requirements. The typical handheld will deliver between 1 and 5 watts. That's a rather large imbalance and problems with handhelds hearing the repeater, but not being able to respond.
The TTA that I'll guess your article is discussing is not a bi-directional amplifier, but rather a receiver multicoupler and amplifier system, where a single antenna and tower top receive amplifier is used to feed a building full of VHF or UHF receivers. Sometimes, the amplifier is cyrogenically cooled to obtain the best possible sensitivity. By dramatically improving the receiver sensitivity, some of the imbalance in transmit powers can be compensated.
 6dB loss is half the range 12dB loss is 1/4 the range
When working in the land mobile field, and cellular field. We would use TTA at the top of the antenna for the base station recieve. This allowed us to recieve the weaker power mobile units. Using the TTA on our receive side only, allowed us to balance our high base station transmit with the low power mobiles. We would also use Bi directional amps to bring the signal into tunnels or buildings. We would either use leaky cable or a system of antennas inside the building or tunnel.
Generally speaking, you'll find several references from system manufactures explaining that receiver amplifiers introduce additional noise in the system and the overall improvement is negligible for WiFi.
Which is why you mount them at the top of the tower near the antenna. If you mount them at the bottom of the tower, than any gain is negated by the noise from the transmission line being amplified by the TTA.
The coax doesn't produce any noise at all...period. Its the noise introduced by the receiver amplifier. I seem to recall TerraWave said the noise introduced by a receiver amplifier negates any system gain and they do not even add the additional gain into the overall gain calculations.
Hi, There is no signal in this world without noise component. If signal and noise are amplified together and still sginal is above noise, that is a over all gain. Ever heard of knee of quieting or S/N ratio? Some devices are less noisier than others in amplifier application. Some LNAs can be even kept cool to lower the noise figure(or noise floor).
You haven't seen the crappy coax cable to connector interfaces that I've had to deal with. They not only produce noise, but they also act as diode mixers to produce intermod. While the coax itself doesn't produce any added noise, any dissimilar metals in the system (aluminum coax outer conductor to nickel plated connector) will produce noise and intermod.
Anyway, mounting the receive amplifier at the antenna eliminates the reduction in sensitivity cause by the coax loss. If I assume that the noise figure of the receiver input amplifier and the tower top amplifier are reasonably low (they usually are), and the gain of the tower top amplifier is a few dB more than the coax loss, what little added noise the tower top amplifier contributes will not have a significant effect on overall sensitivity and system noise figure (either better or worse). Calculations if you really want them (say no, I'm busy).
True. That's because most of the technology in use is commercialized military technology and is also done on a bulk scale. My crystal ball sees that changing with the introduction of SDF radios, where most of the receiver is cryo cooled using SME cooling. It's kinda a marginal proposition to cool just the LNA. However, cooling the LNA, filters, and A/D converter in the front end of an SDF radio makes lots of sense and may be the only way to get the noise induced digitization errors down to a reasonable level for SDF radios past about 18bits. Real-soon-now.
There are also cryogenic ceramic cavity filters and duplexers that offer very low loss and high Q filters. I've played with a rx amplifier multi-coupler for a shared receive site, where it makes sense because the cost can be distributed among perhaps 8-16 radios.
What's really expensive for cell sites is the cost of the electricity to run the current coolers. To get an LNA or ceramic cavity filter down to about 50-150K necessary to use room temperature ceramics, it takes about 60-80 watts of power. Multiply that by a large number of cell sites, and the cost of electricity alone will kill the idea.
I couldn't find a good overview of the current trends in SME. This covers most of the stuff:
 Buzzwords: SDF = Software Defined Radio SME = Superconducting MicroElectronics LNA = Low noise amplifier A/D = Analog to Digital
 Bulk max rate commercial electricity is about $0.35/kw-hr. At 80 watts, that's about $250/year for just the cooler power.
Wrong. Coax cable does not normally generate any noise by itself. There are some conditions where the coax cable can be convinced to create some noise, such a electrolytic action or braid rubbing against itself. Some microwave test set manufacturers sell low noise coax test cable to minimize these effects. However, a properly installed conventional coax run, no matter how long, does not generate any noise.
At 2.4GHz 100ft of LMR-600 is 4.4dB/100ft. You'll loose about 60% of your power in the coax. 100ft is actually not too horrible unless you have to squeeze every bit of performance out of the system. However, at much more than that, you'll need to do something else.
Please note that bi-directional 2.4Ghz amplifiers without AGC require a very specific length of coax cable and AP output power to insure that the tx power amplifier is properly driven and remains linear. Those with AGC are bit more forgiving. I've had miserable experiences with
2.4GHz tower top amplifiers. Details on request. At 600ft, I would look into a PoE system and forget both the coax cable and TTA.
Here is an article that describes noise in RF systems
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.
Yech. They sell some of the worst coax cable I've ever seen. I bought some RG-58a/u clone once that I swear had less than 75% coverage on the braid. (Don't mention PL-259 connectors). The unplated copper in the coax also seemed to corrode more easily than better cable. The air gap in the loose braid was perfect for capillary action sucking water into the coax. Strangely, their satellite grade RG-6/u cables seem just fine (except for the cheezy F connector).
Nickel plated connectors are know to cause intermod. There was an article in MRT magazine on the topic: "Intermod and Connectors: Silver Plate Beats Nickel" by Manny Gutsche, Mobile Radio Technology, Mar. 1992. It too old to be on their web pile.
As a result of the above article, several site owners went on a rampage to remove nickel plated connectors. One site, that I was involved with at the time, showed a rather dramatic improvement in intermod reduction.
Anyway, most Radio Shack connectors and adapters are nickel plated except the gold plated variety.
For all intent and purposes...the thermal noise generated by a passive coax line in a typical LMR and WiFi system is NOT EVEN a considered issue.
The original OP was referring to LMR and thinking of WiFi...not a million dollar research facility working with signal levels so low that can't be observed with a diving bell.
Ergo, it still stands - coax does not normally generate any noise by itself (miscellaneous metal junction intermod excluded) that needs to be entered in any LMR, WiFi, or WiMax deployment calculations.