- What is the difference between a Hybrid Combiner and a Same-Band Combiner?
Although both types of products can essentially be used for the same or very similar application, we chose to classify these products either as Hybrid or Same-Band Combiners in order to allow the customer to differentiate the basic design advantage or disadvantage of each type.
The main building blocks of Hybrid Combiners are 3 dB couplers. Kathrein generally deploys wide-band 3 dB couplers, thereby enabling the decoupled combining of 2 transmitter or receiver signals onto one output. No frequency spacing is required. However, in the case of 2:1 Hybrid Combiners half the power is dissipated over a 50 ohm load. For such applications, Kathrein has designed 2:1 Hybrid Combiners with integrated 50 ohm loads that have a very good IM-Performance (< -160 dBc).
The term Same-Band Combiner is used to define combiners based on cavity or resonator band-pass filters which combine two neighbouring frequency bandwidths within one mobile communications spectrum onto a common output. A Guard-Band is required to separate the neighbouring frequency bands; i.e. a minimum frequency spacing needs to be respected when combining adjacent frequency bands. Kathrein’s portfolio of Same-Band Combiners is tuned to order and has a significantly lower insertion loss compared to Hybrid Combiners.
- What is the difference between a 2:1 and a 4:4 or even higher-order Hybrid Combiners?
A 2:1 Hybrid Combiner is made up of a single 3-dB Coupler, 2 inputs, one common output and has an integrated absorber/load. This product is used for decoupled combining of 2 transmitter or receiver signals onto a common single output, whereby half the power is dissipated over the absorber. For such applications, Kathrein has designed 2:1 Hybrid Combiners with integrated 50 ohm absorbers which have a very good IM-Performance (< -160 dBc).
A 4:4 Hybrid Combiner is made up of a cascaded system of 3 dB Couplers and distributes the signal power available on 4 inputs equally onto 4 outputs. One quarter of the power of each input signal is then available on each output (and vice-versa). The 3 dB losses normally associated with Hybrid Combiners are not experienced as no power is dissipated over an absorber. The insertion losses are stated in our data sheets.
8:4, 12:4 and 16:4 Hybrid Combiners are also available. These types of combiners are often used for the decoupled combining of various mobile communication network services onto a passive indoor antenna distribution system. These types of solutions are achieved by cascading Hybrid Combiners together with Dual-Band or Triple-Band Combiners such that all available services are inserted into the system.
- What is a Duplex Hybrid Combiner and when can this be deployed?
The name Duplex Hybrid Combiner gives a first insight into the functionality of this product. This product consists of both 3 dB Couplers (Hybrid) and Tx/Rx Band-pass Filters (Duplex).
A Duplex Hybrid Combiner can be deployed for same band combining of two base stations onto common feeder cables. The advantage over standard Hybrid Combiners is that the insertion loss on the Tx path is extremely low; i.e. insertion loss of less than 0.4 dB compared to 3 dB in the case of standard 2:1 Hybrid Combiners. There is a 3 dB loss on the Rx-Path, but this can be compensated by using a DTMA if not already in use.
However, successful operation of Duplex Hybrid Combiners requires that the corresponding Base Stations have Tx/Rx and Rx diversity on their respective outputs only. Base Stations supplying Tx1Rx1 and Tx2Rx2 cannot be combined using a Duplex Hybrid Combiner. In such case, customers may have to deploy a standard 2:1 Hybrid Combiner or Same Band Combiner while allowing for a Guard Band.
- How high is the insertion loss of a Combiner?
The insertion loss of a combiner is a determining factor in the selection of the technology used during development of each product. Further criteria in deciding which technology should be deployed in the design of the combiner are the bandwidth of the frequency under consideration, the required isolation between the bands and the power rating.
The insertion loss experienced with Multi-Band Combiners tends to be significantly reduced the further apart the frequencies are. For example, values of 0,1 dB are typical for many Kathrein multi-band combiners where 900 and 1800 or 2100 MHz are being combined. The insertion loss for Multi-Band Combiners for the combining of neighbouring frequencies such as 1800 and 2100, on the other hand, may increase to < 0,3 dB.
Occasionally we are confronted with misconceptions from the field that all combiners have 3 dB insertion losses. In such cases it is generally quite clear that multi-band combining applications have been confused with same-band combining using Hybrid Combiners.
- Why and where are TMA’s deployed in a network?
A Tower Mounted Amplifier (TMA) is often used in order to increase the sensitivity of the receiver in the system. Generally the radio link from the Base Station to the end-user in the cell is satisfactory (downlink). The radio link from the end-user to the Base Station (uplink), on the other hand, is often lower in power and is prone to fading and other site specific effects. This can cause dropped calls. In particular, the Base Station may lose end-users located at the edge of the cell due to poor uplink performance.
TMA’s should be deployed in order to amplify the uplink signal (Rx) directly below and as close as possible to the Rx Base Station Antenna output. Unnecessary lengths of jumper cable between the TMA and the antenna only lead to a reduction in the Signal-to-Noise ratio which is an important criterion in the management of the receiving system sensitivity.
Kathrein offer a variety of legacy TMA products. However, in recent months and years we have seen a revival in TMA business with focus now being set on new TMA’s for LTE applications (800, 1800 and 2600) and also Dual-Band versions, which can be used in conjunction with either existing broad-band antenna or new antenna site configurations.
Please contact your Kathrein sales partner for details.
- Does Kathrein also supply Boosters or GMAs (Ground Mounted Amplifiers)?
GMAs have essentially the same functionality as TMAs. However, GMAs have the distinct disadvantage of being mounted at the foot of the mast which is often a considerable distance from the receiving antenna system. The Rx signal will be subject to additional losses due to the length of RF feeder cable between the receiving antenna and the GMA. Hence any noise created on the RF path from the antenna to the GMA will be amplified together with the reduced Rx signal. The overall Signal-to-Noise ratio of the system is thereby reduced. For this reason Kathrein does not recommend deployment of GMAs, however, TMAs can theoretically also be used for this type of application.
Boosters increase the output power on the transmitting radio link from the Base Station (downlink). The uplink/downlink balance is of paramount importance when designing a mobile communications site. Care should be taken to ensure that the cell coverage is not biased in favour of either the downlink or the uplink. Generally the downlink from the Base Station to the end-user is not the cause of concern. On the contrary, the uplink from the end-user to the Base Station is more often the reason for dropped calls. In particular, the Base Station may lose end-users located at the edge of the cell due to poor uplink performance. Hence, the deployment of TMAs is much more predominant in the rollout of mobile communication networks. Kathrein does not supply Boosters.
- How can I select the correct TMA for my network requirements?
Kathrein offers a wide range of DTMAs for practically all frequency spectrums. In addition, Kathrein have a number of additional features on selected DTMAs available which make deployment in multi-band or broad-band antenna configurations possible.
For instance, Kathrein offer a variety of DTMAs with an RF-Bypass feature so that amplification in one frequency spectrum is possible and the other service is by-passed onto either a separate or common output. This enables feeder sharing without the use of additional combiners.
Alternatively, the deployment of dual-band DTMAs is now also possible, thereby making the retrofitting of existing sites with an additional service a relatively simple exercise.
Please see our selection guide “DTMA Selection Guide” in our customer portal for a full overview.
- How can feeder sharing be performed in spite of a mixed protocol environment at the mast head (CWA, AISG or proprietary communication protocol)?
Legacy site configurations often make use of CWA controlled DTMA’s. In addition certain vendors have, or had, proprietary communication protocols for the control of mast-head devices. Quite often these communication protocols were injected onto the RF feeder cable, decoupled at the mast-head and directly fed into the individual devices.
If additional services are being installed on such existing sites, then the provider is faced with a potential design challenge if existing RF feeders need to be used and multiple communication protocols (AISG 1.1, AISG 2.0, CWA and or vendor proprietary protocol) must operate in parallel via the RF feeders.
Kathrein have developed special Dual-Band Combiners which solve this issue with a minimum of additional investment. Kathrein’s SmartPlex [78210900/-901] allows the continued operation of legacy masthead devices without any restrictions. In addition, further protocols can be used for additional mast-head devices without separate cable runs. Additional Bias Tees and DC Stops are not required. Kathrein’s SmartPlex product family offers a cost effective solution for enabling simultaneous multi-protocol communication transparency while allowing RF feeder sharing. The SmartPlex also intuitively provides the correct DC supply voltages where needed.
For further applications please see our „Co siting application notes download“ via the customer portal.
- When are DC-Stops deployed in an RF-Configuration?
Base Station Antennas must be DC-grounded. Kathrein Base Station Antennas are DC grounded in accordance with EN 50083-1.
Masthead devices such as DTMA’s are active devices and require DC power for operation. The DC voltage is generally provided by the BTS via the RF feeder cable to the mast top. Alternatively the DC supply voltage is inserted onto the RF feeder from an external Power Distribution Unit (PDU) via a Bias Tee.
In order to insure that a short circuit at the Base Station Antenna does not occur, it is imperative to design the RF-Configuration such that DC-Stops prevent the DC supply voltage reaching a Base Station Antenna.
Kathrein provide either external DC-Stops as an accessory or as an integrated part of the combiner. Please see the individual data sheets of the respective multi-band combiner for details. Kathrein generally offer a variety of versions with the DC-Stop functionality on different ports.
- What IP rating do Kathrein Filters, Combiners and DTMA’s fulfill?
Some specific products are designed solely for indoor use, however the majority of Kathrein Filter, Combiners and DTMA product portfolio is designed for outdoor applications and are rated either IP 66 or IP 67. Certain accessories such as attenuators and loads are rated IP65.
In some cases, we make recommendations on the orientation of the product when mounting. Kathrein products are not designed for permanent operation under water. If a product is rated as IP 67, then this was subjected to submersion under 1 m of water for 1 hour during type approval testing.
Please see the individual data sheets for details and confirmation.
- What is the difference between a Bias Tee and a Smart Bias Tee?
A Bias Tee is used to inject or decouple a DC voltage and, where applicable, also AISG control signals into or from the feeder cable in order to provide operating voltage and, where applicable, controls signals via the RF feeder cable to a TMA or antenna tilting actuator (Remote Control Unit – RCU).
Generally speaking, Kathrein offer three different types of Bias Tees with variants depending on RF operating frequency range:
- Bias Tees where a DC voltage can be injected into, or decoupled from the RF feeder via an SMB male or SMA female connector respectively.
- Bias Tees where a DC voltage together with an AISG control signal, which has already undergone Layer-One-Conversion (LOC), can be injected into the unit via an SMB male connector. In such cases, the AISG signal has already been modulated on a carrier at 2.176 MHz prior to being made available to the Bias Tee input.
- Smart Bias Tees allow injection or decoupling of the DC Voltage and also performs the LOC of the AISG control signal via an internal modem. These units have an 8-pin connector for an RS485 interface and allow direct connection with standard primary and secondary AISG devices via standard RET control cables.