Base Transceiver Station otherwise regarded as Cell is the smallest unit of the Base Station System structure. It’s the area of radio coverage of a BTS. The elements of the BTS are: Mast/Tower, Sectorial antennas, PDH & SDH Microwave, Waveguide cables, Rectifier, Generator, Radio Base Station, Duplexers, Data Distribution Frame rack, Transceiver Unit (TRU), Trunking, TX cabinet & Shelter.
COMPONENTS OF THE BASE TRANSCEIVER STATION
Shelter: This is the housing in which all installations, hardware configuration & termination is done. It usually is 10*10 ft kiosks with provision for two air-condition unit & a feeder window.
Mast/Tower: The importance of the tower on the BTS is to have a clear Line of Sight for the PDH/SDH radio & give room for easy radiation of radio signals by the sectorial antenna. The height of the tower is dependent on the topography of the land in focus but the standard recommended height is between 35-40m approximately.
Sectorial Antenna: It’s a broadband antenna capable of multiplexing dual frequency bands for transmission. In Nigeria for example where telecommunication is fast becoming a good substitute for economic backbone, dual band frequency operation is used (i.e. GSM 900 & GSM1800) in order to curb the menace of both capacity & coverage building. This antenna radiates at an angle of 120°. For a total coverage, three sectorial antennas are used on a tower to cover 360 ° circumference. It also has a radiation distance of about 35km if concentrating on capacity building for urban areas & 121km when emphasizing on coverage. A clear edge it has over omni directional antenna is that it eliminates the issue of drop calls while roaming.
PDH/SDH Microwave: Plesio synchronous Digital Hierarchy (PDH) & Synchronous Digital Hierarchy (SDH) are microwaves commonly used on the Base Station System. Point to Point (p2p) & Point to Multipoint transmission e.g. Root/Hub station to other BTS sites is facilitated by these radios via line of sight. The PDH microwave has a capacity of 16E1 making it the mostly used for BTS transmission since it gives room for upgrade and it is very efficient in terms of radio transmission to the BSC on the Abis interface. SDH on the other hand has a capacity of 75 E1.Its used basically on the Hub stations for transmission to the BSC on the Ater interface i.e. if you have the BSC & Transcoder Controller (TRC) on a single node . It has a transmission length of about 50km.
Waveguides: As a result of skin effect, waveguides were invented to eliminate or minimize loss of electro-magnetic signals passing through cables in the course of transmission. This black armored like cables have connectors at their tips to fit into the duplexers via feeder window on the shelter. They come in various sizes and have several connecting tips e.g. BNC. Some still refers to it as a jumper cable.
Radio Base Station (RBS): Radio Base Stations (RBS) handles the modulation of speech signals. The Transceiver Unit does basically base band speech processing, abis interface signaling processing, RF signal amplification, modulation & demodulation. Frequency assignment is done on the TRU and the transceiver units are multiplexed in the combiner unit i.e. Combiner & Distribution Unit (CDU). The combiner unit does the filtering of signals from 33.8kbps to 16kbps before its being sent to the PDH from Abis interface. Since the GSM system uses the TDMA technology, several speech signals can be conveyed on frequency. Each physical channel has 8 time slot under this technology. Every logic channel can connect via the slots. Ericsson has several versions but prominent are the RBS 2200 and RBS 2100 versions. RBS 2216 version has six TRU slots for both GSM 900 & 1800 frequencies. It has dummy slots for upgrade purpose The RBS provides interface to mobile station on the air interface also it interfaces the BSC on the Abis interface on the Distribution Switch Unit (DXU).
Rectifier: The RBS works on a 48V d.c. Alternating to direct current conversion is maintained by the rectifier and its output fed into the site voltage regulator. Four 12V d.c batteries are used as backup on the rectifier. The RBS takes it power directly from the rectifier.
Transmission Rack: Otherwise known as TX cabinet, it holds the PDH/SDH radio & contains the connectors on which the alarm, TX/RX installations are done.
Truncking: The trunk consists of a ladder and a bus like rail on which all installation cables/ waveguides run.
Duplexers: The duplexer does de-multiplexing function between the sectorial antennas and the RBS. The sectorial antenna has a dual frequency input. Waveguides connects the input of these frequencies then de-multiplexes. The duplexers have two input and four outputs. Each output connects to each 1800 TRU card on the RBS. A pair of the output on the duplexer for 900 connects to a TRU card.
Data Distribution Frame: The Data Distribution Frame (DDF) or krone box acts as the interface between the Distribution Switch Unit (DXU) and the radio. Its just a connector where all E1 connections & DXU connection is terminated. The PDH radio is hold or supported on the rack.
Having looked at some of the major constituents of the BTS, we can go ahead to examine the step by step way of installing the RBS. We are going to use the Ericsson RBS as a model for our study.
-Firstly, the Ericsson RBS comes with a base which holds it firm & prevents it from any form of corrosion from the surface of the shelter. In placing the base of the RBS care should be taken because there has to be an edge alignment between the RBS and the base. This can be achieved using a plum. The screw at the top of the RBS can then be turned to hold the RBS firmly to its base.
-The next thing is to determine the length of the cables from the power point of the RBS to the rectifier. Note that the power point for the RBS has three points each having provision for live and neutral. It is advisable that the power connection is done last but it is very good to do all measurement before doing the actual cable laying.
-The alarm cable should be plugged into the alarm port on the RBS meant for the 900 TRUs. The other end of the cable should be taken to the DDF rack.
-On the DXU of the 900 RBS, plug the DXU cable into port A & port B of the card. Ports C & D should be left open. This action should be done on both the 900 & 1800 RBS. The other end of the cable is also taken to the DDF rack.
-Fix TRU cards into the slots noting that the number of cards fixed is a factor of the cell configuration. For example, a typical BTS sited in an urban area will use a configuration of 2/2/2 to 6/6/6 signifying that much emphasis is made on capacity building at the expense of coverage. The 2/2/2 to 6/6/6 configuration tells that we will use a total of 3 TRU cards for 900 RBS and 9 TRU cards for 1800 RBS. An understanding of the configuration helps to determine how many TRU cards we are to use on a cell and even provision dummy slots for upgrade purpose.
-Connect your waveguide or jumpers to the input of the duplexer from the feeder window and tighten very well.
-In connecting to the input of the duplexer we should note that the dual input for a duplexer is coming from a single sectorial antenna so that tells us that since we have three sectorial antennas, we will be having three duplexers each for a sectorial antenna. The input of the duplexer has a provision for both GSM 900 & 1800 frequencies separately and this is indicated on the duplexers. For the output port, we’ll see that two ports are provisioned for GSM 900 & two for GSM 1800. Since we are using the 2/2/2 to 6/6/6 configuration as a model for our installation.
-Connect the dual output of the GSM 900 band from the sectorial with your waveguide to a single TRU card on the RBS and repeat the same for the other two duplexers meant for the other sectorial antennas.
-Also, connect the outputs of the GSM 1800 band from the duplexers to the 1800 TRU cards but in doing this we should note that each output from the duplexers goes to a duplexer. We will note that in doing this installation this way, achieving our configuration is not feasible since Ericsson’s RBS 2216 has six slots for TRU cards and we have to use 9 cards for our 2/2/2 to 6/6/6 configuration. Another RBS needs to be deployed known as an extension to augment the other 3 TRU cards needed and give room for upgrade during capacity building but we need to cascade the bus of the extra RBS in other to achieve our configuration.
-Run the cables from the DXU to the krone connector on the rack and the connection should be done based on the number of E1 we are actually using on the site. A CAT 5 cable contains 4 twisted pair cables. Let’s assume we are using two E1 on this site; that means at the other terminal of the DXU cable going to the rack, we are going to cut off two pairs of the twisted cable and use just two pairs. Note that two pairs of the twisted cable make an E1. The crimping of an E1 on the RJ 45 connector is pin 1,2,4,5 with the twisted cables.
-Finally we are going to connect the ESB port on the DXU. The connection goes thus:
900RBS ESB1 ESB2
The hardware configuration involves the determination of the number of TRU units that will be used on the RBS but the software configuration will be done on the Base Station Controller (BSC). Amongst the configuration that will be done is
-Frequency channel assignment
-Base Station Identity Code (BSIC) configuration.
We have little or nothing to do after our configuration but to commission the site launching the site on our own local network as the case may be.