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The past few years have witnessed a phenomenal growth in the wireless industry, both in terms of mobile technology and subscribers. The large numbers of users, increasing usage of telephony services as well as new services force operators to increase the capacity offered by the networks. As the wave length is approximately 30cm for GSM900 and 15cm for DCS1800, most objects in the path will have some effect on the signal. Such things as vehicles, buildings, office fittings even people and animals will all affect the radio wave in one way or another. The main effects include Attenuation, Reflection, Scattering, Diffraction, polarization changes and shadowing. Attenuation is caused by any object obstructing the wave path causing absorption of the signal. Buildings, trees and people will all cause the signal to be attenuated by varying degrees. Reflection is caused when the radio wave strikes a relatively smooth conducting surface. The strength of the reflected signal depends on how well the reflector conducts. Scattering occurs when a wave reflects of a rough surface. The roughness, size of the objects of the surface and the wavelength will determine the amount of scattering that occurs. Diffraction is where a radio wave is bent off its normal path. This happens when the radio wave passes over an edge, such as that of a building roof or at street level that of a corner building. Polarization changes happen anytime with any of the above effects of due to atmospheric conditions and geomagnetic effects such as the solar wind striking the earth's atmosphere. Polarization changes mean that a signal may arrive at the receiver with a different polarization than that which the antenna has been designed to accept. If this occurs the received signal will be greatly attenuated by the antenna.
Due to propagation effect receiver will pick up signal reflected from many objects. As signal travelled different distances they will arrive at receiver at different time with different signal strength. The result will be that some will combine constructively resulting in gain, some will combine destructively resulting in loss of strength. In Radio communication a Fresnel zone is one which consists of several different zones, each one forming an ellipsoid around the major axis of the direct propagation path. Each zone describes a specific area depending on the wavelength of the signal frequency. Radio waves reflected in the first Fresnel zone will arrive at the receiver out of phase with those taking the direct path and so combine destructively. This results in very low received signal strength.
Network Engineers have the possibility to use updated maps, displaying all essential information needed for transmission engineering. Radio spectrum is the basic resource to carry information for wireless networks. Its efficient usage will be crucial in CPS communications, particularly facing
exponential growth data traffic. However, the increasingly CPS wireless applications make spectrum scarce situations even more worse. Dynamic Spectrum Access (DSA) is considered to be an efficient way against spectrum scarcity caused by existing static spectrum allocation schemes. Primary users (PUs) own exclusively licenses to assigned frequency bands. Secondary users (SUs) can opportunistically use the spectrum when PUs
are absent in DSA scenarios.
Fig. 1: Fresnel Zone
RR_sada_fig1.jpgFig. 2: Adjacent Cell Interference
Mobile network operators and vendors have recognized the importance of efficient networks with equally efficient design processes. This has resulted in services related to network planning and optimization coming into sharp focus.
2. LINK BUDGET
In any two-way radio system, the radio path losses and equipment output powers must be taken into account for both directions. If these differences are not considered, it is possible that the BTS will have a service area far greater than that which the mobile will not be able to use due to its limited output power. Therefore the path losses and output powers in the uplink and downlink must be carefully calculated to achieve a system balance. Once the area of coverage for a site has been decided, the calculations for the power budget can be made. A Link Budget is an accounting of all the gains and losses in a transmission system. The link budget looks at the elements that will determine the signal strength arriving at the receiver. The link budget may include Transmitting power, Antenna gains, Antenna feeder losses, Path losses and Receiver sensitivity. In Radio transmission, Transmitted Power Output (TPO) is the actual amount of power (in watts) of Radio frequency (RF) energy that a transmitter produces at its output. The ratio of the power required at the input of a loss-free reference antenna to the power supplied to the input of the given antenna to produce, in a given direction, the same field strength at the same distance. The sensitivity of an electronic device, such as a communications system receiver, is the minimum magnitude of input signal required to produce a specified output. The communication going from an antenna to mobile is called downlink and when it is going from mobile to satellite is called as uplink. The standard values of losses for uplink and downlink are as shown below:
MS Transmit Power
MS Antenna Gain
Maximum Path Loss
BTS Antenna Gain
Feeder/ Cable Connector Loss
Duplexer/ Combiner Loss
Received Power at Input of Receiver
BTS Receiver Sensitivity
BTS Transmit Power
Duplexer/ Combiner Loss
Feeder/ Cable connector Loss
BTS Antenna Gain
Maximum Power Loss
MS Antenna Gain
Received Power at Input of Receiver
MS Receiver Sensitivity
Fig. 3: Uplink and Downlink Budget
3. CELL SHAPE AND FREQUENCY RE-USE
In mobile networks we talk in terms of 'cells'. One base station can have many cells. In general, a cell can be defined as the area one sector i.e. one antenna system. The hexagonal nature of the cell is an artificial shape. This is the shape that is closest to being circular, which represents the ideal coverage of the power transmitted by the base station antenna. The circular shapes are themselves inconvenient as they have overlapping areas of coverage. A practical network will have cells of non geometric shapes, with some areas not having the required signal strength for various reasons. Omni directional cells are usually not implemented because their coverage and capacity properties are the worst. The 2-sector base station is typically used for road coverage. The 3-sector configuration is used for both urban and rural areas as it provides the largest coverage areas and also the highest capacity.
Fig. 4: Cell Shape
The network planner designs the cellular network around the available carriers or frequency channels. The frequency channels are allocated to the network bands as shown below:
Number of RF Carriers
Within this range of frequencies only a finite number of channels are allocated to the each operation in given service area. There has to be great care taken when selecting/ allocating the channels. Installing a greater number of cells will provide greater spectral efficiency with more frequency re-use of available frequencies. However, a balance must be stuck between spectral efficiency and all the costs of the cell. The size of cells will also indicate how the frequency spectrum is used. Maximum cell radius is determined in part by output power of the mobile subscriber (MS) and interference caused by adjacent cells. Also it has to be kept in mind that the output power of the mobile subscriber is limited in all frequency bands. Therefore to plan a balanced transmit and receive radio path, the planner must make use of the path loss and thus the link budget. The effective range of a cell will vary according to location, and can be as much as 35km in rural areas and as little as 200m in a dense urban environment. The total number of radio frequencies allocated is split into a number of channel groups or sets. Channel groups are assigned on a per cell basis in a regular pattern which repeats across all of the cells. Thus, each channel set may be re-used many times throughout the coverage area giving rise to a particular re-use pattern. For example a 7 cell re-use pattern, 4site - 3 cell re-use pattern.
Fig. 5: 7 Cell Re-use Pattern
A key parameter determining the efficient reuse of spectrum is governed by multiple-access technique used. The access technique defines how the frequency spectrum is divided into channels and affects reuse of the channels.
The oldest technique used in wireless access, especially in mobile communications, is Frequency Division Multiple Access (FDMA). Here the available frequency spectrum is divided in a number of orthogonal frequency channels and these channels are assigned to the user on demand. FDMA can be used both for analog as well as a digital communications. This simple technique used extensively in first generation analog mobile system, however, had poor reuse and the same channels can be reused only once in 14 or 21 cells. One way to increase re-use efficiency is by employing sectored or directional antennas at the cell site. Even with sectorisation, say 3 sectors per cell the best planning gives a typical reuse of once in 7 cells implying reuse factor of 1/7 = 0.143 per cell.
The most widely used multi-access technique today, both for mobile as well as in wireless local loop, is Time-Division Multiple Access (TDMA). Here the frequency spectrum available is again divided, but into a few (wide) bandwidth channels or carriers. Each carrier is used for transmission of multiple time-multiplexed channels. Each such orthogonal channel (or time-slot as is commonly referred to) could be assigned to a user on demand. The technique can be used only for digital communication, and the ability to work with smaller signal to interference ratio in digital domain, gives this technique better reuse factor as compared to the analog FDMA. For example, with three sectors, a cell reuse factor of 1/4 or even 1/3 is achievable
Late in the eighties emerged a multiple-access technique referred to as Direct Sequence, Code Division Multiple Access (DS-CDMA). Based on spread spectrum techniques used extensively in defense applications for over twenty years, this technique enables definition of near-orthogonal channels in code-space. CDMA enables multiple channels to use the same frequency and time slots. Each bit to be transmitted by or for a user is uniquely coded by spreading the bit into 64 or 256 or even 1024 chips. The receiver separates the data of a user by a decoder which correlates the receive signal with the code vector associated with that user. On correlation, the interference from other users would become nearly zero and add only a small amount of noise, where as the desired signal will be enhanced considerably. The technique is useful in exploiting the inherent time-diversity from multipath delay-spread, especially if the spreading is significant (Chip time of 0.1 ïsec to 1 ï sec. The only problem with the technique is that as completely orthogonal codes are not possible, especially on the uplink, the total bit-rate supportable from all users using this technique is significantly less than the total bit-rate supportable with TDMA and FDMA technique using the same frequency spectrum.
This disadvantage in the CDMA system is made up by better reuse efficiency, as the same spectrum with different set of codes can almost totally be reused in every cell. The theoretical reuse efficiency could be as high as 1.0, but in practice less. With sectored antennas, it is possible to reuse the spectrum in each sector, with a 3-sector cell site resulting in a reuse efficiency of nearly 0.5 per sector.
An issue that is as important as reuse of frequency spectrum is fine power control so that more or less equal power from each subscriber set reaches a base station. Such a control mechanism was difficult to implement and delayed widespread use or CDMA for some time. Fortunately, the problem has been largely overcome today.
Fig. 6: FDMA, CDMA and TDMA
4. SECTORIZATION AND HOPPING TECHNIQUES
As cell sizes are reduced, the propagation laws indicate that the levels of carrier interference tend to increase. In an Omni cell, co-channel interference will be received from surrounding cells, all using the same channel sets. Therefore, one way of significantly cutting the level of interference is to use several directional antennas at the base stations, with each antenna radiating a sector of the cell, with a separate channel set. The more number of traffic channels available at a cell site which means more traffic channels available for subscribers to use. By installing more capacity at the same site, there is significant reduction in overall implementation and operation costs experienced by the network operator. Sectorization allows the use of geographically smaller cells and a tighter more economic re-use of the available frequency spectrum. Better network performance to the subscriber and greater spectrum efficiency. These are the few advantages of Sectorization.
Baseband hopping is used when a base station has several transceivers available. The data flow is simply routed in the baseband to various transceivers, each of which operates on a fixed frequency, in accordance with the assigned hopping sequence. It requires as many transceivers as the number of allocated frequencies. Synthesizer hopping on the other hand instead of providing as many transceivers as the number of allocated frequencies, there is only a need to provide as many transceivers as determined by traffic.
5. PLANNING PROCESS
The objective of radio network planning is a technical realization of the marketing requirement, taking into account of the following constraints: License requirements, GSM specific parameters.
5.1 License requirements:
According to TRAI guidelines it is mandatory to satisfy the coverage requirements which include 60% coverage within 12 months of launch and 90% coverage by the end of 2nd year. The quality of service should be such that it should be available in 90% of the declared area and for 90% of the time. The Grad of Service (GOS) should be good i.e. it should endeavor to achieve 3% or better. Moreover the frequency allocation should be done properly as the major limitation is that most of the operators are limited to 30-60 frequencies for handling all traffic.
5.2 GSM specific parameter:
While assigning parameters we must keep in mind few factors like the Frequency band we're selecting, the MS transmit power, the BTS transmit power, the receiver sensitivity of MS and BTS and most importantly the C/I ratio. When a channel is reused there is a risk of co-channel interference. As the number of channels sets increases, the number of available channels per cell reduces and therefore capacity reduces. But the interference level will also reduce, increasing the quality of service. The capacity of any cell is limited by the interference that can be tolerated for a given grade of service. A number of other factors, apart from the capacity, affect the interference level are power control (both BTS and MS), hardware techniques, frequency hopping (if applied), sectorization, discontinuous transmission.
5.3 Manufacturer specifications:
The main manufacturer specific parameters include BTS transmit power, receiver sensitivity, combiner performance, cable losses, antenna performance, frequency hopping, power control and handover algorithms.
5.4 Radio communication fundamentals:
The radio communication fundamentals include propagation losses, shadowing, multipath fading, time dispersion, power link budget, interference effect and also the budgetary factors like the business plan and funds provided for investment.
6. PLANNING METHODOLOGY
A GSM digital cellular system is usually made up of several BSS's. The planning cycle begins with defining the BSS cell, followed by the BTS(s), then the BSC(s), and finally the RXCDR(s).
6.1 Steps in planning a BSS:
The first step is choosing a cell configuration, Omni or sectored and the frequency re-use scheme that satisfies traffic, interference and growth requirements. Secondly we plan all BTS sites using an appropriate RF planning tool to determine the geographical location of sites on and the RF parameters on the chosen terrain. Lastly we determine which equipment features are required at each site like diversity, frequency hopping.
6.2 Planning sequence:
Initially we need to survey the sites with reference to cluster heights, vegetation level, obstruction, sector orientation, building strength and other civil requirements. Next, we are required to prepare power budgets and conduct propagation tests. Calculations based on coverage probabilities and verification against predictions and modifying tool parameters are to be done. Finally we are expected to prepare a Final Coverage Map.
6.3 Planning tools:
The planning tools are used to predict the signal strength in a cell area. It would be necessary to make any calculations, at regular intervals, from the BTS. Smaller the interval, the more accurate the propagation model. The calculation would need to be performed at regular distances along each radial arm from the BTS, to map the signal strength as a function of distance from the BTS. The result, is necessity to perform hundreds of calculations for each cell. This would be time consuming in practice, so the software planning tool is used. Software tool is fed with all the details of the cell, such as the type of terrain, environment and height of antennas. It can perform the necessary number of calculations needed to give an accurate picture of the propagation paths of the cell. After calculation and implementation of the cell, the figures should then be checked by practical measurements.
7. SITE PLANNING, CELL PLANNING AND OPTIMISATION
The information required to plan each site includes the place where BTSs will be located, local restrictions affecting antenna heights, equipment shelters, number of sites required, re-use plan, spectrum availability, number of RF carrier frequencies available, antenna types and gain specifications, diversity requirement, redundancy level requirements and supply voltage. Several requirements must be taken into consideration in the stages of planning process like costs of building the network, capacity of the network, coverage and location of the network elements, maximum congestion allowed, quality of calls and future development of the network. Optimization and Monitoring is the last part in radio network planning. Monitoring is linked to optimization because they both partly include the same measurements. The primary purpose of optimization is to verify and improve the actual radio plan by taking into account the radio network evolution as subscriber growth, radio interface traffic, coverage demands, radio quality and overall radio network functionality.
8. FUTURE PLANNING
Normal practice in network planning is to choose one point of a well known re-use model as a starting point. Even at this early stage, the model must be improved because any true traffic density does not follow the homogeneous pattern assumed in any theoretical models. Nowadays, cyber-physical system (CPS) relies on wireless networks for devices control and information backhaul. But the mass deployment CPS devices make operator's spectrum scarce situations even more worse. Hence, cellular network operators anticipate the Dynamic Spectrum Access (DSA) technology to solve the spectrum shortage problem in the context of cognitive radio (CR) with the help of femtocells. Femtocells are low-power, short-range, low-cost indoor cellular base stations for better coverage and capacity. Femtocells, acting as gateways in CPS, integrate CPS devices into cellular networks in a seamless manner. The concept of cognitive femtocell can solve the spectrum congestion problem even within a massive network on the CPS scale.
The factors relating and the parameters affecting the GSM radio network must all be considered while planning and designing a GSM radio network. The network designers should find a plan that both fulfills the given requirements and keeps within practical limitations. When making network plans, the designers should always remember that every location in a network has its own conditions, and all local problems must be tackled and solved on an individual basis.