The wireless technology has changed the world with technological expansion in wireless communication and brought revolution in the wireless networks. Data transmission has seen a rapid increase and this rapid increase have resulted in what is often called “data explosion”. Cellular technology has been developed to cater for this and the increased demands for increased data transmission speeds and lower latency. To handle the growth in mobile data traffic a feasibility study on the UTRA and UTRAN long term evolution was started in December 2004. When it comes to the enhancement of the mobile communication and to improve the future requirements, LTE is a better choice in wireless communication networks.
LTE-Advanced is the upcoming global cellular technology that offers very high throughput on air its interface. LTE is the forward step in moving to 4th Generation services from 3rd Generation but it is not completely companionable to 4G standards. Long Term Evolution (LTE) is a wireless technology, which describes interoperable implementations and standards of UMTS/3GPP. Its further improved version is called LTE advance which is 4G compatible technology. Both LTE & LTE advance uses the same frequency band. Due to historical and political reasons, there are different frequency bands allocated for cellular communication systems in US, Europe and Asia.
The LTE Frequency bands are planned as follow:
LTE is depended upon the 3rd Generation Partnership Project (3GPP). LTE is also being referred more formally as Evolved UMTS Terrestrial Radio Access (E-UTRA) and Evolved UMTS Terrestrial Radio Access Network (E-UTRAN). The advantages of LTE are; cost effectiveness, best use of new spectrum, better compatibility with other standards and improving of spectral efficiency. Based on the features of LTE newer an richer services can be provided like
The following are the main objectives for LTE:
Downlink speed and an uplink of LTE are almost of 100 Mbps and 50 Mbps. It supports both Frequency Division Duplex (FDD) and Time Division Duplex (TDD). Time Division Duplexing (TDD) – The communication is made using one frequency, but the time for transmitting and receiving is different. This method emulates full duplex communication using a half duplex link. Frequency Division Duplexing (FDD) – The communication is made using two frequencies and the transmitting and receiving of data is simultaneous. Above data rates (uplink and downlink) can be further increased by employing multiple antennas both at the transmitter and receiver. LTE-Advanced, on the other hand, is set to provide data rate up to 1 Gbps and 500 Mbps in down link and uplink, respectively. Different modulation schemes are used for diverse types of communication in wireless technology. It is imperative to select the most competent modulation scheme that fulfills all the requirements. It uses Orthogonal Frequency Division Multiple Access (OFDMA) in the downlink and Single-Carrier Frequency Division Multiple Access (SC-FDMA) for uplink.
This latest technology has resulted in increased data transmission speed, improved spectral efficiency. But path loss do affect its performance so it is imperative to select a most appropriate propagation model for LTE as it describes the behavior of the signal when it is transmitted from the transmitter towards the receiver and gives a relation between the distance of transmitter & receiver and the path loss and from this one can get an idea about the allowed path loss and the maximum cell range.
The Channel is the basic part of the communication refers to the medium through which information travels between the transmitting and the receiving antenna system as shown As the signal travels through medium various changes happens in its characteristics and its level reduces and this degradation estimation is very important for any network designer for designing. This degradation is termed as path loss. Thus Path loss is the decrease in signal strength of an electromagnetic wave as it propagates through space.
It is due to numerous effects, such as refraction, diffraction, free space loss, reflection, aperture medium coupling, and absorption. Apart from this, topography, environment (urban or rural), medium (dry or moist air), the distance between the transmitter and the receiver, and the height and location of antennas also plays an important role in influencing the path loss. When radio signal travels it suffers with fading, distortion, attenuation.
The mobile communication is exposed to three basic mechanisms namely
1) Diffraction-occurs when the path between the transmitter and the receiver is obstructed by an object that has sharp irregular edges and thus waves bend around and propagate behind the obstruction reaching the receiver even in the absence of LOS path.
2) Reflection which happens when light waves get impinge on another object which is large as compared to the wavelength examples are earth surface, walls etc.
3) Scattering which involves radio signal reaching object of smaller size as compared to wavelength leading to scattering in many different directions. Examples of object responsible for scattering are sign, foliage and lamp posts.
Fading is defined as the attenuation or deviation that a modulated radio wave signal experience over a communication channel. Fading changes with respect to time, geographical position and/or radio frequency and is often modeled as a radio process
Fading can be of two types:
a) Large scale fading
b) Small scale fading
Large scale fading – The phenomenon of large-scale fading is affected primarily by the presence of hills, forests, and buildings between the transmitter and the receiver and is also known as shadow fading. It occurs when an obstacle gets positioned between transmitter and receiver and due to this some part of the transmitted signal is lost through absorption, reflection, scattering, and diffraction. This effect is called shadowing.Shadowing is very significant for link budget scrutiny; handoff analysis, co-channel interference and frequency reuse investigation, and diversity design.
In the above figure, the middle building cast shadow on the antennae and due to this some part of the transmitted signal is lost through absorption, reflection, scattering, and diffraction. This effect is called shadowing. Small scale fading: is the rapid variation of the amplitude of radio signal over a short period of time ordistance. It occurs due to multipath propagation. It is caused by the interference between two or more version of the transmitted signal that arrive at receiver at different instant of time and these multipath waves combine at receiver to give a signal which may either vary in amplitude or in phase. Multipath propagation causes severe dispersion of the transmitted signal.
Factors that influence small scale fading are
a) speed of mobile- this is due to the motion between the mobile station and base station and eventually results in frequency modulation.
b) speed of observer-this occurs when objects in the environment are in motion and this results in time varying Doppler shift.
c) multipath propagation- this occurs due to the presence of reflecting and scattering objects
Multipath propagation can cause multiple copies of signal to arrive at different time with different phases and it can also result in inter symbol interference. The large scale fading is required to deduce the power required to send the signals to the receiver while small scale fading gives us the measure of how the receivers and transmitter modulation and coding should be designed.
A channel is said to be flat fading, if the signal bandwidth is smaller than the coherence bandwidth of the bandwidth. It is also known as narrow band. In this case different frequency component of the received signal goes fading to the same extent. Fig below shows flat fading Flat fading is caused by absorbers between the two antennae and is countered by antennae placement and transmit power level
Frequency selective fading occurs when the signal bandwidth is greater than the coherence bandwidth of the channel. In this case, different frequency components of the signal undergo fading to a different extent. This type of fading is much more difficult to models than fading channels. In Frequency selective fading the bandwidth of the spectrum of the transmitted signal is greater than the coherence bandwidth of the channel. Frequency selective fading is caused by multipath delays which come close to or surpass the symbol period of the transmitted symbol. Frequency selective fading channels are also known as wideband channels.When the environment is random or when multiple indirect paths are presents between the transmitter and the receiver and no LOS path then Rayleigh fading dominates. And when the propagation path is on a line of sight, as in tunnel and underground passages, then Rician fading dominates.
In order to study the nature of radiation pattern and quality of signal persisting in a particular region we definitely require certain kind of modeling to bring out the natural characteristics of the environment in to machine coordinated implementation and for the same we deploy radio propagation models. Propagation models are developed to forecast the loss of signal power or coverage in a particular location. The path loss models are derived on a basis of an extensive measurement campaign conducted in a typical suburban environment. Prediction is a must for optimum signal level estimation and to provide efficient and reliable coverage locale measurement of signal strength must be taken into consideration. Finding the service area of each base station through measurement is impractical since it can be very expensive and time consuming process. Instead, engineers depend upon propagation modeling that estimates the average signal strength and consequently the path loss at any particular distance from the base station.
Attenuation prediction is very important and if we are able to predict it then we can also estimate received strength. The prediction capability of a propagation model also depends on its suitability for that environment. Improvement of accuracy of propagation models will continue to be an integral part of wireless communication systems. This will be even more apparent with the recent convergence of mobile cellular networks and the Internet. Numerous propagation models have been derived and studied, however; there is no single model that can be applied for all the environments. As a result, the Quality of Service (QoS) of the whole cellular network depends on the selection of most suitable of the radio propagation model which depends on descriptions of terrain and effect of vegetation which vary widely from one place to another.
Propagation models are of three types.
a. Empirical Models
b. Deterministic Models
c. Stochastic Models
Empirical model are the most widely used due to their simplicity and ease of calculation and are based on observations and measurements alone. Among these empirical models are most sought and are mainly used to predict the path loss. Okumura, hata, cost 231 hata, erricson are the most common types of models used today. The deterministic models make use of the laws governing electromagnetic wave propagation to determine the received signal power at a particular location. . Stochastic models, on the other hand, model the environment as a series of random variables. Propagation model that are considered in this thesis are free space model, cost 231 hata, Ericsson model, ECC model and SUI model.
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