A Radiation Pattern is a 3 dimensional description of how an antenna radiates power in the surrounding space. This pattern is usually measured at a sufficient distance from the antenna known as the far-field. In simple words it is the power radiated in a certain direction with reference to an omni-directional antenna (a theoretical antenna that radiates equally in all the directions). Given below are the radiation patterns for some common antenna types.
Antenna Gain and Directivity are two terms that are sometimes not that well understood. The Antenna Gain and Directivity are related through the following equation.
That is, the Antenna Gain in a particular direction is equal to the Directivity in that direction multiplied by the Antenna Efficiency. Antenna Directivity is the ratio of energy transmitted (or received) by the antenna in a particular direction to the energy transmitted (or received) in that direction by an isotropic source. This is also known as the Directive Gain.
The Antenna Gain (also known as the Power Gain) seems to be a better metric to quantify the performance of an antenna as it takes into account the efficiency in converting electrical energy supplied to the antenna into radiated energy.
The 3-dimensional plot of the Gain of an antenna is known as the radiation pattern. The Antenna Gain with reference to an isotropic source is given in dBi (decibel above isotropic source). Sometimes the Antenna Gain is given with reference to a Dipole Antenna and is labelled as dBd. The figure below shows the Directivity of a Patch Antenna embedded inside a human body .
1. An isotropic source (a source that radiates uniformly in all directions) is only a theoretical concept and does not exist in reality.
2. The sun can be considered an isotropic radiator since it radiates uniformly in all directions (almost).
3. When no direction is given the Gain refers to the maximum Gain.
In the previous post we plotted the E-field of a half wave dipole. We now turn our attention to higher antenna lengths such 1,1.5 and 2.0 times the wavelength. The E-field pattern is a three dimensional pattern, however, we only plot the E-field in a 2D plane along the axis of the dipole.
It is observed that as the antenna length is increased from 0.5*wavelength to 1.0*wavelength the antenna becomes more directional. However, as the length is further increased from 1.0*wavelength to 1.5*wavelength and 2.0*wavelength sidelobes begun to appear. These sidelobes are an unwanted phenomenon in a typical telecommunications application. When the antenna is placed vertically (shown horizontal in the above figure) it radiates uniformly along a horizontal plane and would provide coverage within a circular cell (not for 2.0*wavelength where there is no radiation at 90 degrees).
In simple terms the path loss is the difference between the transmitted power and the received power of a wireless communication system. This may range from tens of dB to more than a 100 dB e.g. if the transmitted power of a wireless communication system is 30 dBm and the received power is -90 dBm then the path loss is calculated as 30-(-90)=120 dB. Path loss is sometimes categorized as a large scale effect (in contrast to fading which is a small scale effect).
According to the WINNER-II model the path loss can be calculated as:
Here d is the separation between the transmitter and receiver in meters, fc is the frequency in GHz, A is the path loss exponent, B is the intercept and C is the frequency dependent parameter. X is the environment specific parameter such as path loss due to a wall. PLfree is the path loss in a free space line of sight environment (here A=20, B=46.4 and C=20).
The table below describes the different environments defined in the WINNER-II model. Once an environment is selected the path loss parameters A, B and C can be selected from the table further down e.g. A1 is the in-building scenario with A=18.7, B=46.8 and C=20 for the LOS case. The PL for a T-R separation of 100 m and frequency of 2 GHz is calculated as:
A separate equation for the path loss is given where the parameters A, B and C are not sufficient to describe the scenario.
1. Here CG is the concept group that developed the particular scenario. This is either Local Area (LA), Metropolitan Area (MA) or Wide Area (WA).
2. For more details visit:
L. Hentilä, P. Kyösti, M. Käske, M. Narandzic , and M. Alatossava. (2007, December.) MATLAB implementation of the WINNER Phase II Channel Model ver1.1 [Online]. Available: https://www.ist-winner.org/phase_2_model.html
Frequency Reuse is a well known concept that has been applied to wireless systems over the past two decades e.g. in GSM systems. As the name suggests Frequency Reuse implies using the same frequencies over different geographical areas. If we have a 25MHz band then we can have 125 GSM channels and 125*8=1000 time multiplexed users in a given geographical area. Now if we want to increase the number of users we would have to reuse the same frequency band in a geographically separated area. The technique usually adopted is to use a fraction of the total frequency band in each cell such that no two neighbor cells use the same frequency. Typically the frequency band is divided into 3 or 7 cells.
The division of the frequency band in to smaller chunks reduces the system capacity e.g. one cell with 25 MHz bandwidth would have much higher capacity then 7 cells having 3.5 MHz each. To overcome this problem a frequency reuse of 1 has been proposed i.e. each cell has the full system bandwidth (nearly). The problem of co-channel interference at the cell boundaries is resolved by dedicating a small chunk of the available spectrum for the cell edges.
In Soft Frequency Reuse (SFR) the cell area is divided into two regions; a central region where all of the frequency band is available and a cell edge area where only a small fraction of the spectrum is available. The spectrum dedicated for the cell edge may also be used in the central region if it is not being used at the cell edge. The lack of spectrum at the cell edge may result in much reduced Shannon Capacity for that region. This is overcome by allocating high power carriers to the users in this region thus improving the SINR and the Shannon Capacity.
1. The Signal to Interference and Noise Ratio is given as:
SINR=Signal Power/(Intercell Interference+Intracell Interference+AWGN Noise)
2. Typically the term capacity was used to describe the number of voice channels (or users) that a system can support. But with modern digital communication systems it usually refers to the Shannon Capacity that can be achieved (in bits/sec/Hz).
 Yiwei Yu, Eryk Dutkiewicz, Xiaojing Huang, Markus Mueck and Gengfa Fang, “Performance Analysis of Soft Frequency Reuse for Inter-cell Interference Coordination in LTE Networks”, ISCIT 2010.
Since our last post on Verizon LTE coverage within California, Verizon has removed the LTE Coverage Map from its site. Now it only gives a list of cities that have 4G LTE service (just like T-Mobile). So we now move from the West Coast to the East Coast i.e. Virginia. The state that is home to Virginia Tech, one of the finest schools in the country and a breeding ground for Wireless Engineers. It is thus somewhat of a shock to see that Verizon Wireless has no 4G LTE footprint in the state of Virginia. The only place that it intends to deploy 4G in near future is Bristol Virginia. It claims that by the end of 2013 it would have 4G coverage throughout the US where 3G service is currently available.
As in California T-Mobile has a much wider coverage with many smaller cities getting 4G service. The list includes: Alexandria, Mclean, Newport News, Norfolk, Petersburg, Portsmouth, Reston, Richmond, Roanoke and Lychburg. So although Verizon might be winning the speed race it is definitely not winning the coverage race (at least in CA and VA). And with AT&T T-Mobile merger also a possibility early next year Verizon is set to face some stiff challenge.
Given below are the results of a 4G speed test conducted by PC Magazine in the Northeast.
The above results show that in areas where 4G coverage is available Verizon allows for average download speeds that are twice that of T-Mobile. The upload speeds are somewhat similar. Overall Verizon is by far the best in terms of the Mobile Speed Index, with T-Mobile in second spot and AT&T at third.
We have previously looked at the birds eye view of 4G LTE coverage within the US. We know that Verizon 4G services are now available to more than 50% of the US population. However, geographically, the service is only available in very small islands of population. Now, we take a closer look at 4G LTE coverage within California.
We see that the coverage is available in most of the population centers such as Sacramento, San Francisco, Oakland, San Jose, Fresno and Bakersfield. Further south the coverage is also available in areas around Los Angeles and San Diego. But what is not shown on this map is that there is no coverage in many smaller cities such as Stockton, Modesto, Santa Rosa and Visalia. Also there is no coverage on the highways connecting these cities e.g. there is no coverage on I-5 which runs along the length of the state.
Bottomline: You may get very good LTE coverage when you are at home but don’t expect the same when you are on the highway. You will most probably have to fall back to 3G.
Verizon Wireless 4G LTE is now available to 160 million people with coverage in 117 cities within the US. This has been achieved within eight months of the initial deployment. Verizon hopes to increase the coverage to 185 million people by the end of 2011. The company claims that with its current deployment strategy users can experience data rates of 5-12Mbps on the downlink and 2-5Mbps on the uplink. When users do not have access to the 4G LTE network the phones will automatically switch to 3G which is available around most of the US.
This push to 4G creates a big gap between the developed and underdeveloped parts of the world where many nations have still not migrated from 2G to 3G.