GSM BTS Tutorial and learn

(1.) GSM Frequency Bands

International Telecommunication Union (ITU), which regulates international radio spectrum allocation, has been divided into three GSM frequency bands namely GSM 900MHz, GSM 1800 MHz, GSM 1900MHz. Every foreigner that frequency is divided into two sub bands:

(A). Uplink (UL): the radio transmission from MS to BTSE

(B). Downlink (DL): a radio transmission from the MS BTSE

Differences in the frequency band used in the Uplink and Downlink transmission is called Frequency Division Duplex (FDD).


1. 900 GSM
- Uplink: 890-915 MHz (MS)
- Downlink (DL): 935-960 MHz (BSS to MS)
- Duplex Distance: 45 MHz

2. GSM 1800
- Uplink: 1710 - 785 MHz (MS to the BSS)
- Downlink: 1805 - 1880 MHz (BSS to MS)
- Duplex Distance: 95 MHz

3. GSM 1900
- Uplink: 1850 - 1910 MHz (MS to the BSS)
- Downlink: 1930 - 1990 NHz (BSS to MS)
- Duplex Distance: 80 MHz

GSM dual band antenna can be also a single band. If the BTS is supporting dual-band technology, the GSM900 and GSM1800 base stations can be connected to the same antenna. If using single band antenna, the antenna is usually larger than the antenna GSM900 GSM1800.Based On GSM system, each BTS has had its own frequency allocation, which is usually called the FA (frequency assignment).

GSM uses a combination of Time-Frequency Division Multiple Access (TDMA, FDMA). FDMA technique is used to divide the 25 MHz of available bandwidth into 124 carrier frequencies, each of which has a wide bandwidth of 200 KHz. One or more carrier frequencies allocated to a BTS. Because every GSM also adopts a TDMA (time division multiple access), each frequency can be divided into 8-time slot. Each time slot, can only be filled by one customer, at a time. In each BTS, can be allocated multiple frequency (FA), depending on the design network. If the designer wants the number of subscribers that can be served in one area a lot, then he should put a lot of base stations in one area. Vice versa, if the designer wants broad coverage for each BTS, then he must sacrifice the number of subscribers in these areas.

(2.) Frequency Reuse

The cells in the cellular system can use the same frequency, which is used by other cells in different regions with a certain minimum separation protects it from interference. Tues shows the extent of signal coverage. Theoretically, hexagonal or hexagonal pattern is the best choice because it has the largest area and did not result in overlapping patterns. Because the pattern is irregular then the distance between the center point of the polygon and the farthest point on the most distant cells compared with other patterns. In field conditions, the cells do not have a regular geometric shape that fit the contours of the areas concerned.

BTS configuration is most commonly used configuration of base stations using antenna sector with BTS location is at the corners of the cell. The use of a single transmitter powered by large towering antenna towers for mobile communications systems in order to reach a wider area, impact on the impossibility to use the same frequencies are repeated (frequency reuse).This was due to any use of the same frequency, will cause interference. To make tactics then at each BTS will be used different frequencies and repeat the same frequency to another BTS, provided the same frequency between adjacent base stations may not.
At the frequency reuse we will also be introduced to the term cluster (denoted by K). Cluster is a group of cells; each cell has one set of frequencies that are different from other cells. Cluster size depends on the total cells contained in one cluster.

Suppose in the figure below has a value of K = 3.








(3.) Frequency splitting

Added this customer will cause problems later on, because each cell has a limited ability on the BTS in providing services to its customers. Solutions to overcome this, the application of cell splitting will increase the capacity of cell lines or in areas that have high demand. As for adding the cell can be done by dividing the unit cell becomes smaller. Cell splitting is a process of dividing a cell into the cells - cells that are smaller, which each have their own base station. This will allow a reduction in antenna height and transmitter power, because the cell splitting to increase the capacity of cellular systems by increasing the number of times, the channel used. It also defines new cells that have a radius smaller than the first cell and by applying the smaller cells between existing cells, the capacity can be increased.

Handover

In addition, several techniques above, the BTS also has the function to always make contact with the mobile station (MS) or handset, if at any time MS will do or is receiving a call. Therefore, to select the best cells MS and BTS is always measuring the strength and quality signals in the cell and neighboring cell signal strength. Measurement of the BTS and MS is sent to the SC in the form of measurement reports. Based on this report, the BSC determines handover is needed or not, and determine the appropriate cell. Handover is the process when the cell changes are made the call (call), or when idle. During the neighboring cells have considered better quality than the currently serving cell, a handover is required. Another reason besides the need for a handover due to the strength and quality, handover is also determined by distance and balance the network. This usually occurs when an MS moves from cell to another quickly. Therefore, the handover process is thus an important task in any cellular radio system to ensure the continuity of conversation.

Each handover have been successfully conducted, and the event sought as seldom as possible in the system, and is not disturbing for its users. To be able to accomplish that, the designers have to determine an optimum signal level to initiate a handover. Once the signal level is determined as the minimum for the use of signal reception sound quality at the receiver in the BTS system, the signal level is slightly stronger than the level threshold is used as a handover processing. This limit is given in the form:

D = Pr handoff - (Pr) minimum power

This limit must not be too big or too small. If the value of D is too small, its impact is not enough time to do the entire procedure for handoff, the conversation already been lost due to the weak signal conditions. Conversely, if D is too large, then the handoff will occur too frequently, the work burden of MSC. Therefore, D is set carefully, so that both the above effects can be avoided.