1. 1.    What is Distributed Antenna System?
2.  

A Distributed Antenna System, or DAS, is a network of spatially separated antenna nodes connected to a common source via a transport medium that provides wireless service within a geographic area or structure. DAS antenna elevations are generally at or below the clutter level and node installations are compact.

2.What is PIM and why is it so important?

PIM (passive inter-modulation) occurs when 2 or more signals are present in a passive device that exhibits a non-linear response. The nonlinearity is caused by dissimilar metals, anodic effects or loose connections. This nonlinearity does not manifest itself at low input signal levels, and is therefore also a function of the high power level transmit signals in the system. The resultant products are referred to as 3rd, 5th, order inter-modulation products, etc. The 3rd order products are typically the focus in most system design. The real problem occurs if the 3rd order PIM products generated by the transmitter channels fall within adjacent receiver channels. This upsets the adaptive settings of the receiver and desensitization will occur. The result is typically a dropped call

3. What is the Point of interface / Multi-Operator Combiner?

In Telecommunications a Point of Interface (POI) is used to show the physical interface between two different carriers, such as a local exchange carrier (LEC) and a wireless carrier or an LEC and a IntereXchange Carrier (IXC). This demarcation point often defines responsibility as well serving as a point for testing. In many cases, a POI exists as a point of demarcation ("DEMARC") within an LEC building, and is established under "Co-Location" agreements. A long distance, wireless, or competitive local carrier 'rents' space at the local telephone (usually tandem switch) location. This space is physically a 'cage' in which a device for interconnecting telecom services is installed. This 'device' was originally a wire frame with one side being accessed by the LEC, and the other side accessed by the "other" carrier. In recent years, 'electronic frames' such as a digital cross-connect systems have been used as POI devices. Local exchange services are ordered from the local telephone carrier who delivers the service to 'their side' of the POI. The 'other' carrier then arranges to bring its own facilities (fiber, or other type of transport) into the POI and transport the service to its own network facilities.

4. Power dividers and directional couplers

Power dividers (also power splitters and, when used in reverse, power combiners) and directional couplers are passive devices used in the field of radio technology. They couple a defined amount of the electromagnetic power in a transmission line to another port where it can be used in another circuit. An essential feature of directional couplers is that they only couple power flowing in one direction. Power entering the output port is not coupled.

Directional couplers are most frequently constructed from two coupled transmission lines set close enough together such that energy passing through one is coupled to the other. This technique is favoured due to the microwave frequencies the devices are commonly employed with. However, lumped component devices are also possible at lower frequencies. Also at microwave frequencies, particularly the higher bands, waveguide designs can be used. Many of these waveguide couplers correspond to one of the conducting transmission line designs, but there are also types that are unique to waveguide.

Directional couplers and power dividers have many applications, these include; providing a signal sample for measurement or monitoring, feedback, combining feeds to and from antennae, antenna beam forming, providing taps for cable distributed systems such as cable TV, and separating transmitted and received signals on telephone lines.

5. Low-noise amplifier

Low-noise amplifier (LNA) is an electronic amplifier used to amplify possibly very weak signals (for example, captured by an antenna). It is usually located very close to the detection device to reduce losses in the feedline. This active antenna arrangement is frequently used in microwave systems like GPS, because coaxial cable feedline is very lossy at microwave frequencies, e.g. a loss of 10% coming from few meters of cable would cause a 10% degradation of the signal-to-noise ratio (SNR).

An LNA is a key component which is placed at the front-end of a radio receiver circuit. Per Friis' formula, the overall noise figure (NF) of the receiver's front-end is dominated by the first few stages (or even the first stage only).

Using an LNA, the effect of noise from subsequent stages of the receive chain is reduced by the gain of the LNA, while the noise of the LNA itself is injected directly into the received signal. Thus, it is necessary for an LNA to boost the desired signal power while adding as little noise and distortion as possible, so that the retrieval of this signal is possible in the later stages in the system. A good LNA has a low NF (like 1dB), a large enough gain (like 20dB) and should have large enough intermodulation and compression point (IP3 and P1dB). Further criteria are operating bandwidth, gain flatness, stability and input and output voltage standing wave ratio (VSWR).

For low noise, the amplifier needs to have a high amplification in its first stage. Therefore JFETs and HEMTs are often used, and distributed amplifiers could be used. They are driven in a high-current regime, which is not energy-efficient, but reduces the relative amount of shot noise. Input and output matching circuits for narrow-band circuits enhance the gain (see Gain-bandwidth product) and do not use resistors, as these would add noise. Biasing is done by large resistors, because energy efficiency is not needed, and a large resistor prevents leakage of the weak signal out of the signal path or noise into the signal path.

6. Continuous-wave radar

Continuous-wave radar is a type of radar system where a known stable frequency continuous wave radio energy is transmitted and then received from any reflecting objects. Continuous wave (CW) radar uses Doppler, which renders the radar immune to interference from large stationary objects and slow moving clutter.

CW radar systems are used at both ends of the range spectrum.

•     Inexpensive radio-altimeters, proximity sensors and sport accessories that operate from a few dozen feet to several kilometers

•     Costly early warning CW angle track (CWAT) radar operating beyond 100km for use with surface to air missile systems.