DIGITAL CELLULAR CELL AND SPECTRUM PLANNING
Md. Mahbubur Rahman
The pioneering work of Hertz the experiments of Marconi at the end of 19th century demonstrated the feasibility that radio communications could take place between transceivers that were mobile and far apart. Henceforth telegraphic and voice communications were not inherently limited to users’ equipment tethered by wires. Instead freedom to roam and yet still communication was possible.
Morse-coded on-off keying was mainly used for mobile radio communications until the 1920s. In 1933 a two-way mobile radio voice system was introduced for police of New Jersey, USA. The early mobile radio franks receivers were noisy, bulky and heavy. They used power hungry valves, operated in the lower frequency part of the VHF hand and had a range of some 10 miles.
While fixed commercial analog telephone networks were evolving into digital networks the mobile radio scene was also slowly altering. Private land mobile (PLM) radio system came into use. Although Bell Laboratories conceived cellular radio in 1947, the other company did not start to deploy a cellular network until 1979 due to lack of necessary development in technology. The 1980s witnessed the introduction of a number of analog cellular radio systems. They are operating in the UHF band. They enabled mobile users to have telephone conversations while on the move with any other users who were connected to the public switched telephone networks (PSTNs) or integrated service digital network (ISDNs). In the 1990s, another leap forward takes place in mobile communications with the development of digital cellular networks and digital cordless telecommunication system. This second generation mobile radio system are providing a range of services in addition to telephony.
To produce a preliminary design assumption, a s called nominal cell plan is drawn up. As shown in the figure, it normally takes the form of the familiar honeycomb pattern. The figure shows a typical traffic situation for large city area with high traffic density at the center, which require a finer mesh cell structure i.e. small cell. The references show the suggested location of base stations and the distribution of frequency groups to obtain optimal reuse of channels. A typical nominal cell plan has been super imposed over the topographical map. At this stage the plan can consider not only the present need for traffic capacity but also how the system can upgrade later by adding new cell sites in between the existing ones. This is called cell split.
A program known as TOP used to produce digitized maps and data base with the information on elevation contours, land usage characteristics such as open areas, roads, forests, cities, lakes etc. and other map background information.
Another package of program known as PRED that is used to predict the radio wave propagation, which is a crucial parameter when defining the cell sites. This prediction is based upon the digitized terrain data from TOP, or from external terrain databases. Additional input data are information on frequency hands, output power, antennas and other equipment.
With the help of these TOP and FRED a series of radio coverage prediction can he produced in the form of graphic diagrams, where radio signal level and other relevant information are indicate.
Prediction of co-channel interference is very important in cellular telephony, as the radio frequencies are constantly reused at certain intervals in the system.
The prediction models are verified beforehand by radio surveys in the specified area. The specially developed test transmitter with its antenna is mounted in the proposed location. Radio surveys are performed with the help of the special mobile equipment. All collected data are then processed and analyzed.
The trend of modern network planning is going in Die direction of computer aided design (CAD). This is because high accuracy of planning is required and a high amount of information handling capacity is required.
BASE STATION ARCHITECTURE:
A radio station comprises of a number of base station controller (BSC) and base station transceivers (BSC) As radio network evolve and smaller cells are required to provide for capacity, more sophisticated management of the radio access layer becomes essential. In this respect there are three types of architecture in use:
I) Homogeneous cell layer
A Homogeneous cellular architecture is the conventional method of providing radio coverage and capacity. All cell can be accessed to the control channel and there no hierarchy of importance. Sonic cell may carry more traffic than others. In the coverage term such architecture provide ‘blanket’ coverage across the area of interest. It gives a clear objective to the target.
ii) Mixed cell
In the case with traffic load, the radio coverage environment is highly variable (buildings etc.) and occurrence such as coverage holes and excessive cell overlap are inevitable. Then it leads the evolution of cell architecture from homogeneous to mixed cell. Mixed cell means there is a wide range of cell sizes in the network, with large cell alongside small cells. The control of the radio network in a mixed cell environment is more depending. It is very demanding because of problems which rise due to the use of very small cell.
A multi-layer cellular architecture is one whereby all system access is managed hierarchically through a ‘top tier’ or access layer, Traffic channels are then assigned either on the top tier or on a low ‘lower tier’, depending on the criteria such as mobile speed and channel occupancy.
Micro cells can be introduced either in a mixed cell or a multi-layered architecture depending on circumstances. A misture of mixed cell and multi-layered architecture known as hybrid architecture may be introduced depending on the fundamental situation of the radio environment lbr small cell.
Frequency planning means the allocation of the radio frequencies to cell in the way that harmful interference is made minimal. The re-use of frequencies usually allows the planner to concentrate on minimizing the co-channel interference.
Frequency planning efficiency is defined by two terms
Spectrum efficiency is the Earlangs/(km2xMflz)
This is the Earlangs/kni2 or Subscriber/km2 with certain grade of service, This indicates the requirements of traffic channels/km2 i.e.
S = total available spectrum (MHz)
n = frequency re-use factor
A = cell area (kin2)
B = bandwidth per traffic channel (MHz)
The total available spectrum and bandwidth per traffic channel in most cases is fixed. For example the Global System for Mobile communication (GSM) operates in two pair of hands 890-915 MHz for uplink transmission, where the mobile transmits and the base station receives and 93 5-960 MHz for down link transmission, where the base station transmits and the mobile receives. A guard band of 200 KHz is provided at the lower end of each duplex band and the remaining spectrum is divided into 124 paired duplex channels with 200 1KHz channel spacing in each band. The spacing between the duplex bands is 45 MHz. Therefore, increasing of capacity is possible by reducing the cell size or reducing the frequency re-use factor. It is possible to reduce the cell size by introducing microcells or piccolos. Reducing the frequency re-use factor is more complicated and depends on the allowed level of interference and system tolerance of co-channel interference.
The third generation mobile services, such as Future Public land Mobile Telecommunications System (FPLMTS) put forward by the ITU and the Universal Mobile Telecommunication Systems (UMTS) can be seen as a further development of GSM. The cellular is clearly preferring a more developing approach in introducing this third generation. Current GSM based personal communication networks are growing rapidly.
Current 900 MHz GSM hand allocations are already constraining the potential of cellular networks in some Asian countries. The next logical step is therefore to organize Digital Cordless System (DCS) Technology which is operating in 1.8 MHz band, which represents the most significant growth potential with its 75 MHz bandwidth the anticipated high growth rates of service penetration and new non-voice services, however, the DCS 1.8 GHz band could he easily crowed as well. The wide spreads of cellular mobile telecommunication system will soon force the regulators to consider how and when to make FPLMTS bands available.
Engr Md. Mahhubur Rahnian was torn in 1964 in Feni. He joined 8778 in 1991. Afler completion of his 2 (two) years probationary period he was posted in Greater Dhaka Telecommunication Network Improvement Project (Phase-If,) and is working there till date. Additionally he is working at 551 Cell since September 1995.