词条 | Tower Mounted Amplifier |
释义 |
A Tower Mounted Amplifier (TMA), or Mast Head Amplifier (MHA), is a low-noise amplifier (LNA) mounted as close as practical to the antenna in mobile masts or base transceiver stations. A TMA reduces the base transceiver station noise figure (NF) and therefore improves its overall sensitivity; in other words the mobile mast is able to receive weaker signals. Benefits in mobile communicationsIn two way communications systems, there are occasions when one way, one link, is weaker than the other, normally referenced as unbalanced links. This can be fixed by making the transmitter on that link stronger or the receiver more sensitive to weaker signals. TMAs are used in mobile networks to improve the sensitivity of the uplink in mobile phone masts. Since the transmitter is a mobile phone it cannot be easily modified to transmit stronger signals. Improving the uplink translates into a combination of better coverage and mobile transmitting at less power, which in turn implies a lower drain from its batteries, thus a longer battery charge.[1] There are occasions when the cable between the antenna and the receiver is so lossy (too thin or too long) that the signal weakens from the antenna before reaching the receiver; therefore it may be decided to install TMAs from the start to make the system viable. In other words, the TMA can only partially correct, or palliate, the link imbalance. Drawbacks/pitfalls
Mathematical principlesIn a receiver, the receiving path starts with the signal originating at the antenna. Then the signal is amplified in further stages within the receiver. It is actually not amplified all at once but in stages, with some stages producing other changes (like changing the signal's frequency). The principle can be demonstrated mathematically; the receiver's noise figure is calculated by modularly assessing each amplifier stage. Each stage consists of a noise figure (F) and an amount of amplification, or gain (G). So amplifier number 1 will be right after the antenna and described by and . The relationship of the stages is known as the Friis formula. Note that:
Applying the Friis formula to TMAsTypical receiver without TMAStart with a typical receiver: Antenna - Connecting Cable (stage 1) - Receiver (stage 2). The first stage after the antenna is actually the connecting cable. Therefore:
What can be done to improve the receiver to pick up very weak signals? It must have a lower noise figure; that is when the TMA comes into use. Typical receiver with TMAIt is a chain of 4 modules: antenna - short connecting cable (stage 1) - TMA (stage 2) - longer connecting cable (stage 3) - receiver (stage 4)
Updating the Friis formula with this case, the noise figure is now: In this way, the cable losses are now negligible and do not significantly affect the system noise figure.[6] This number is normally expressed in decibels (dB) thus: See also
References1. ^1 {{Cite book | last1 = Laiho | last2 = Wacker | last3 = Novosad | title = Radio Network Planning and Optimisation for UMTS | place = London | publisher = John Wiley & Sons, Ltd. | isbn = 0-471-48653-1 | postscript = }} 2. ^{{cite web| url = http://abc.net.au/reception/tv/mast_head.htm| title = TV distortion cause by too strong received signals }} 3. ^{{cite web| url = http://www.powerwave.com/tma.asp| title = Product description of a TMA manufacturer| publisher = Powerwave}} 4. ^{{cite web| url = http://www.adcbroadband.net/Library/Techpub/75177.pdf?refer=Library&C=Tower-Mounted+Amplifier+Systems&L=ClearGain+Amplifier+Systems| title = TMA description, installation and trouble shooter of a TMA manufacturer|publisher = ADC }} 5. ^{{Cite book | first = F.R. | last = Conner | title = Noise | edition=2nd | place = London | publisher = Edward Arnold | year = 1982 | isbn = 0-7131-3459-3 | postscript = }} 6. ^1 {{Cite book | first = Simon R. | last = Saunders | title = Antennas and Propagation for wireless communication systems | place = London | publisher = John Wiley & Sons, Ltd. | isbn = 0-471-98609-7 | postscript = }} 7. ^Noise Figure of lossy stage External links
2 : Electronic amplifiers|Mobile technology |
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