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In the existing dynamic wireless scenario, high capacity and efficiency is demanded by every communication system and to fulfill the ever-increasing appetite of higher data rates, smart antennas are one way or another, are becoming feasible with adaptive beamforming. The proper usage can make available high spectrum efficiency and channel capacity aiding among overall performances of systems.
When many non-directional antennas send and receive several radio waves, there simulation to a big directional antenna is concluded by beamforming. As the antennas are not able to shift physically, so electronic procedures usually adopted to point them in a specified direction, smart antennas with the implementation of beamforming algorithms through adaptive systems are useful to track the sources because they tend to decrease noise samples and recover communication quality by steering an antenna to a chosen route. On the other hand, smart antennas lower downs the interferences and fading, complexity of the system, delay spread and Bit Error Rate (BER) by incrementing capacity of available channels, spectral efficiency, coverage or range and aids in steering multiple beams for tracking while reimbursing aperture distortion. Mobile communication system is one of the applications of smart antenna arrays fulfilling the rising requirements of outsized user capacity within limited channel bandwidth.
The chosen project has following objectives.
Design & fabricate a transceiver array capable to receive and transmit the desired signal and the beams correspondingly.
Process signals for attain digital beamforming.
Devise a Graphical User interface (GUI).
Apply adaptive algorithms for the further analysis on the received signals.
Scope of works
The project incorporates both simulation aspects and implementation of hardware. In project's first stage High Frequency Structure Simulator (HFSS) was used to design a single element and then the antenna array and then MATLAB was used to simulate adaptive algorithms and their behavior was examined in different scenarios. In the second phase of project WLAN (Wireless Local Area Networks) cards combined with antenna array were used for reception of real time signals. In the third segment, with the help of LabView a GUI was developed and eventually integration of both hardware and software modules totaled the prospects properly.
SMART ANTENNA SYSTEMS
Smart antennas are participating significant in order to facilitate wireless communication system. Space diversity helps in providing higher data rates, increased coverage and capacity, and improved quality of service at an affordable cost. Mostly, in time and space, mutually; antenna arrays combined with signal processing formulates a smart antenna. The overall performance of system is improved because spatial processing provides additional freedom levels
When talking about cell sectoring, sector coverage is compiled by multiple beams by using antenna arrays, which are actually, smart antennas.
Smart Antenna Definition
Smart antennas change their patterns to regulate to interferences and multipaths. Allocation of dynamic provisions from noise by antenna arrays via signal processing (digital) is called smart antennas.
The conceptual block diagram of a smart antenna system is shown in Figure 1.
The following three main blocks can be identified:
(i) Array antenna (ii) complex weights and (iii) adaptive signal processor.
The antenna array consists of circular or linear antenna elements where each individual antenna element is identical. The received signals from different sources are collected by the array elements separately, and then complex weights are multiplied with them to conclude as addition. The adaptive signal processor continuously adjusts the complex weights by using all available information. The information to calculate the weights can be accessed by pilot or training sequences or through the fundamental characteristics of the received signal.
A smart antenna is an array of antenna elements connected to a digital signal processor, as shown in Figure-(2.1).
The basic aim is to implement a system in the company of enhanced gain and suppression of interferences which radically improves the capacity of a wireless system.
As we know that, for a given number of users increment in capacity interprets higher data rates. This is achieved by tracking the desired user and placing nulls in the direction of interferences while offering main beam to the actual user.
Smart Antenna Operation
Since the distance connecting the user and every array element is dissimilar from other receiving elements, so each element of smart antenna array will entertain the signal at special instance of time instance. Usually, it seems to be the weak point of smart antenna system but in reality by utilizing the separation among antenna elements and delay occurs, the location of a user can be estimated. Eventually, enables the transmitter to send the signal to accurate position of that user. Additionally, this same policy is used for multiple users. A smart antenna transceiver can improve performance of a wireless system by suppressing the interference and is intelligent to process the signals received by the array or transmitted by the array using proper array algorithms. An antenna array, whose spacing within the elements can vary, is prearranged in definite geometry to collect the signals coherently pooled in a way that boost the desired signal strength and decrease the intrusion from other signals.
Figure-(2.2) shows a basic execution of smart antenna system.
The antenna arrays takes input and gives output in the analog domain as Radio Frequency (RF) signals. These signals are then passed from/to low noise amplifiers, mixers and analog filters. In the in transmit mode, digital to analog converters (DACs) are used for the conversion of the baseband digital signals to RF signals and in receive mode, the RF signals are converted to digital domain by analog to digital converters (ADCs). The down or up conversion from RF to baseband or baseband to RF is through IF signals respectively. In receive mode, signals from each antenna is then summed up using the algorithms (adaptive) in a digital processing unit. The digital processing unit can be implemented on a microprocessor or a Digital Signal Processors (DSP) or FPGA.
Classification of Smart Antenna
Previously, smart antennas were employed in electronic warfare as a counter measure to jamming but being a solution to capacity and interference problems, improving the received signal strength (RSS) and/or signal-to-noise ratio (SNR) and nowadays due to low cost of DSPs, Application Specific Integrated Circuits (ASICs) and innovative signal processing algorithms; they are now in great demand for commercial purposes. The smart antenna systems for can be separated into two main types, these are (i) switched beam system and (ii) adaptive arrays systems.
Switched Beam Systems
It is the simplest system and effortless to fuse with the existing wireless technologies. These systems consists of many fixed, directive, pre-defined beams that can sense a set of several beams and then selects one beam that gives the maximum received power. A switched beam antenna can be considered as an expansion of the conventional sectored antenna which splits a sector into further sectors.
On the other hand, poor quality of service can also be provided to the desired receiver if at the center of selected beam, receiver is away and the strength of the interfering signal is enough high. As the switched beam antenna systems has lack of capability to discriminate a desired user and interferer, so they are only valuable in low to moderate co-channel interfering environments.
The Butler Matrix is one of the most common examples of a switched beam antenna system.
Adaptive Array System
Advanced signal processing algorithms animatedly lower down the effect of interferences and after that locate and track the desired signals. In adaptive array, to facilitate most of the Signal to Interference Ratio (SIR) or SNR, received signals by each antenna element are weighted correspondingly and then combined collectively using complex weights in terms of magnitude and phase.
To include low co-channel interference and high antenna gain, Phased array steers the beams while an adaptive array system uses beam steering and nulling and may need smaller amount of antennas to attain a given range as compared to the phased array but it adds receiver complexity and implementation cost. As digital processing can shape the radiation pattern for reception and transmission, so beamforming aids smart antennas in receiving the signals from the exact direction of desired source and repress noise coming from the direction of interfering sources or simply we can say that adaptively steering the beams in the desired directions and placing nulls for interfering signals.
Figure (2.4) showing a beamformer (in receiving mode) using adaptive beamforming algorithm to allocate proper weights to parallel beams which puts a null for the interference and transmitting beams to desired source.
Fixed beamforming systems and fully adaptive systems are in two ways to execute beamforming systems; first one has Beamforming Network which demands low processing (BFN) as BFN followed by RF switches which choose a specific beam are controlled by a control logic and are operating in analog and in RF domain but in adaptive beamforming, weights are selected adaptively by algorithms in the digital domain.
Advantages of Smart Antennas
Mostly smart antennas are used in networks to get large user capacity which means providing simultaneous services to numerous users at a time. Co-channel interference is originated by omnidirectional antennas as if two users using frequency band confines the user capacity in that system. In order to crack this major issue, Spatial Division Multiple Access (SDMA) is exploited which enhances the user capacity as smart antennas can focus their beams towards desired user.
Figure 2.5 shows plus point of SDMA in contrast to the omnidirectional case, which can reduce co-channel interference using beamforming.
Further studies related to smart antennas reveals that they require low power, improves range and capacity of the link, and are least probable to get detected and intercepted and robust next to multipath fading ultimately recovering reliability. Multiplicity of paths occurs in wireless propagation channels as there are numbers of obstructions between transmitting and receiving end and is called multipath which creates attenuation and delay. as shown in figure-(2.6).
Figure-(2.6) Multipath propagation
Smart antenna systems struggles against multipath propagation. Last but not least, some other advantages of smart antennas are:
Some of the advantages of the smart antenna are as follows:
(i) Increased range/coverage
As combined signals are received or transmitted by the all array elements so they increase the signal power. It is directly proportional to the number of antennas.
(ii) Lower power requirements and/or cost reduction
As antenna knows about the exact position of its respective user so low power consumption and amplification cost fulfills the need.
(iii) Improved link quality/reliability
Independent copies of same signal are received through diversity gain as it is expected that not all of these will be corrupted.
(iv) Increased spectral efficiency
The pilot or training sequence permits less interference.
Disadvantages of Smart Antennas
The significant drawback of smart antennas is its design and implementation in hardware, as amalgamation of several RF devices not only adds up into the expenditures but it will also make the transceiver weighty. Periodic verifications are mandatory as many nonlinear types of equipment are used such as mixers, amplifiers and ADCs devices. Additionally antenna arrays limit data rates.
The placement of the antenna array is still a challenge itself; but with the evolution of microstrip patch antennas many elements can be combined within a same material.
The basic idea to write his chapter is to get familiar with the characteristics of antennas and arrays with reference to beamforming; also some elucidation on the assembling of beamformer unit's hardware is also discussed.
Actually the regulation of side lobe levels and steering nulls is employed phase during the combined effect of amplitude as phase and amplitude of every individual element antenna element is controlled by the process of beamforming. Complex weights are represented by constants wk, where the ak and qk are amplitude and phase shift respectively.
Since its name implies digital, so it is obvious that in every element of antenna phase shifting and scaling of amplitude, and addition is completed digitally.
Due to the nature of radio waves, their direct digitization is not feasible where requirements can be fulfilled by ADC but habitually down conversion of frequency with the help analog RF translators is done prior to the arrival of signals to ADC. The following figure shows a translator that shifts the entire cellular telephone uplink band at 824-849 MHz down to the 1-26 MHz range.
After being digitization of radio waves, a quadrature baseband output waveform is produced by digital down converters which furthermore transfers the central frequency virtually 0 Hz.
The following figure shows a complete digital beamforming receiver. Many beamformers can share couple of ADCs, RF translators, and an array element. It can be seen that to construct a well-matched phase shift, a common oscillator and clock can be distributed with various RF translators and ADCs. Moreover, each digital down converter uses identical clock in order to adjust identical central frequency. Finally, baseband outputs after getting multiplied with complex weight are added to generate one baseband signal with directional properties.
In direction-finding the complex weights are precisely adjusted to offer a fundamental beam which can be further guided in anticipation of a signal eventually making the communication channel superlative. Frequent scheme which are employed for Adaptive Beamforming are:
Minimum Error (Mean-Square) Regulation of complex weights is made to decrease error between the probable waveform and output from the beamformer.
Maximum SIR Strengths of preferred and expected signals is estimated and are customized to maximize the Signal to Interference Ratio.
Minimum Variance If a receiver recognizes both, shape and direction of signal and source correspondingly, then it can dynamically decide the complex weights to lessen the interference.
Sometimes, there are also some restrictions to no to amend the complex weights in a specific adaptive beamformer so incorporation of pilot signals is another solution which rapidly synchronize the both ends.
Spatial Domain Multiple Access (SDMA) is a practice which is generally applied to twice the capacity of a wireless system, when a singular smart antenna is deployed to accommodate more than one user operating at the same frequency but each user in different direction, by navigating a separate beam to every user.
FFT's in Beamforming
A FFT is capable to form a lot of beamformers with their elementary beam in individual paths in digital beamforming, while allowing them to consume a single array, RF translator and ADC. The beamformers may have their central beams pointed in different directions.
In Beam Space Beamforming, arbitrary radiation patterns are constructed by adding up; all the complex weights came up from FFT outputs, on the other hand, summation of baseband waves from various sources is called Element Space Beamforming.
Super-Resolution Direction Finding
If beam width of elements of array is hardly any, then the capacity of a receiver to estimate the arriving angle of a radio wave is called Super-Resolution Direction Finding but the limitation of this technique is that two or more than two signals should not function at the same frequency. The phase of the signals must be known very precisely to compute the arriving angle.
The basic features of an antenna which are assumed to be inherited are that is must be competent enough to transmit and receive magnetic and electric fields of all kinds of polarization, and it has to be light weight, portable, less in volume and omnidirectional. Also, it should have as a feature to cater high power as it is supposed to generate high magnetic and electric fields. Where high gain is concerned, in addition to maintain fine range and is equipped with narrow beamwidth.
Types of Antennas
Antennas are categorized through the modes in which they emit or collect electromagnetic radiation. It can be directional, isotropic or omnidirectional.
An isotropic is an ideal antenna which homogeneously radiate in every direction with equal magnitude of electric field at each location of a sphere and a guess to be hertzian-dipole, where the length of hertzian-dipole is 100's of operating wavelength but still not depicting true isotropic behavior.
Figure 3.1 - radiation from an isotropic antenna
Monopole, dipole and biconical are the common examples of an omnidirectional antenna as they radiate homogeneously in 1 direction. Vertical radiation is just like the decimal digit of 8 while horizontally it is homogeneous.
Figure 3.2 - radiation from a vertical antenna
The primary intention of a directional antenna is to radiate the most part of its power in a specific direction. Horns, Reflector, Log-periodic, and Yagi-Uda arrays are the common examples of directional antennas.
Figure 3.3 - radiation from a directional antenna
Main Characteristics of an Antenna
The selection criteria of an antenna can be carried according to scenario and performance requirements. Given features can be considered to choose, but they are not same for all antenna family:
The Gain from the main lobe
Physical and effective aperture
Radiation pattern of an antenna
Offered resistance in terms of radiation
The arrangement and degree of its side and back lobes
Azimuth and elevation are two major planes, in which antenna's uniqueness is measured. Other names for them are horizontal and vertical planes and are often denoted by phi and theta.
Ideally, an antenna has resistive impedance (Ra) but in real time is includes self impedance (Za, real part) calculated by the end points at free space whereas, the self reactance (Xa) is an imaginary part.
Za = Ra + j Xa
No doubt, the resistive impedance is the absorbed power with the inclusion of ohmic losses (Ro) and radiation resistance (Rr).
Matching of an antenna is done when both the transmission line and the corresponding antenna has the same impedance, and to assure the transference of all the power, of tuning circuits are used generally LCR circuits.
The radiation patterns are identical in both modes whether transmitting or in receiving unless or until any variation amid feeding networks. Mostly, a radiation pattern gives a transparent learning of the major properties of an antenna which are not clear from the textual, verbal or written details. Always far field is considered to estimate the radiation pattern at a fixed distance and is plotted in dB as the actual scale could be outsized and in order to compensate the entire range dB is preferred.
We can consider is as a plot between radiated power / unit angle from a source. Azimuth and the elevation planes are adapted to measure the radiation patterns. Rectangular or polar coordinates are normally used to plot the radiation patterns.
A Rectangular plot for radiation pattern of an antenna is shown in Figure 3.4
Figure 3.4 - rectangular plot of an antenna radiation pattern
Figure shows a polar plot for radiation pattern of an antenna, which easy imagine than that of rectangular plot.
Figure 3.5 - polar plot of an antenna radiation pattern
If a pattern is surrounded by comparatively weak radiation concentration, then it is a lobe
Lobes are divided in two categories:
It includes main lobe. A direction in which an antenna is radiating with maximum power is the main lobe. Gain and beamwidth are found by the information provided by the main lobe. Figure 3.6 depicts main lobe in a rectangular radiation pattern
It includes back lobes and side lobes. Basically, minor lobes correspond to unwanted radiation pattern. To ensure no ambiguity, sometimes they are distinguished as the crest, where the difference between the crest and its next trough is no less than 3 dB.
Any lobe pointing in any other direction expect the main lobe is considered as side lobe while back lobe is the one which is 180 o in angle when main lobe is taken as reference.
Figure-2.: An illustration of major and minor lobes of radiation pattern.
There are two types of region which are present
Fresnel Field Region (Near)
The area closely surrounding antenna is near field.
Fraunhofer Field Region (Far)
The area away from antenna is far field. In reality, we consider far fields as distances matters a lot and is given by
Actually dimensions of an antenna are inversely proportional the beamwidth whose information is acquired only by the main lobe; and the beamwidth is inversely proportional to side lobes, means decreasing the beamwidth gives increment in side lobes The half power or 3 dB beamwidth (HPBW) is basically a width on any side of main beam, where the power (radiated) is half of maximum value and the angular width in between first nulls is First Null Beamwidth (FNBW).
There is a lot of confusion in gain and directivity as masses consider both of them a same quantity. Actually antenna is not an active appliance like amplifier; so its gain is quite different from those and is the efficiency is measured in terms of gain as it is product of the directivity and the efficiency. According to IEEE, gain is comparison to the power (radiated) by a particular antenna to an isotropic antenna.
The gain G as a linear ratio is defined as
G = (power (radiated) by a particular antenna) / (power (radiated) by an isotropic antenna)
The gain is represented in dBi by the following formula
GdBi = 10 log10(G)
To reconvert the dBi in a linear term, the given equation is used
G = 10^(GdBi/10)
When a gain is measured with comparison to a standard gain reference antenna, the gain came is the absolute gain.
Standing Wave Ratio ((Voltage) SWR)
If there is no proper matching between transmission line and end terminals, there is chances of producing standing waves are set up by the reflections which eventually results in heat and other losses and is abbreviated as VSWR, which is a technique to compute the extent of mismatch. For example, an impedance of an antenna is 50 W and of the transmission line is 70 W. The aim of an antenna designer is to get 1:1 VSWR notifying all energy is transporting from transmitter to receiver without any reflection but sensibly it is not feasible.
Ratio between the power inserted in between the terminals and radiated power is efficiency and is dependent on major lobe. Equation for beam efficiency is given by:
The front-to-back ratio (F/B) is uttered is dB and is the capability of a directional antenna to focus the beam in the essential forward path.
The magnitude of beamwidth is primarily related to the aperture of an antenna. Commonly, aperture is inversely proportional to the beamwidth.
Polarization is variation of nature and direction of electric field vectors with respect to time. The ratio of the major to minor axis is considered as axial ratio and it defines the type of polarization.
Variation of electric field vectors in one plane with sinusoidal pattern with respect to time is linear polarization, which can be further horizontal or vertical as shown in figure.
Elliptical and Circular Polarization
If axial ratio is one it's circularly polarized but if it is infinite in the case of linear polarization and for elliptical polarization magnitudes differ for both axis.
Figure 3.14 shows two examples of elliptical polarization.
Return Loss is almost similar to VSWR; return loss is uttered in dB and is sum of reflected power from an inaccurately ended line to the source. It must be as large as possible but in negative which means too small to ensure highest transfer of power.
How much numerous frequencies, an antenna can radiate or receive is hypothetically the bandwidth or we can say that it is the frequency spectrum with which an antenna can transmit or receive 50% power efficiency or 70.7% of voltage efficiency and is very significant input factor The ratio between lower and upper frequency is bandwidth and is reliant on:
Direction of beam
From a transmitting end, it is not sure that all the broadcasted signals will move in line of sight (LOS) not definite in case of the directional antennas because there are many obstructions and hurdles in open environment by which radio waves meet intervention and spread into many directions with various angles which eventually outcomes as existence of manifolds on the reception part, and this whole experience is known as fading (multipath). So, at the receiving end; different copies of transmitted signals appear generating a summed signal called non line of sight (NLOS).
Figure 2 Diagram showing multipath fading.
There are diverse method at the back hand of propagations in case of electromagnetic waveslike reflection, diffraction and scattering.
An entity has enormous size in contrast to the wavelength of corresponding wave can cause reflection and is because of houses, advertisement sign boards etc.
An entity when bumps onto the boundaries of dense things in contrast to the wavelength of corresponding wave can cause diffraction.
If a transmitted wave knocks an object whose dimensions are small when compared to the wavelength of the arriving signal.
There are three categories of mechanism (radiation):