Faster than the Speed of Light!

A great break through in the world of science. An international team of scientists said they had recorded sub-atomic particles traveling faster than light -- a finding that could overturn one of Einstein's long-accepted fundamental laws of the universe. More details on the link below:

http://www.reuters.com/article/2011/09/22/us-science-light-idUSTRE78L4FH20110922

MATLAB Functions of the Week!

1) fminunc 

fminunc attempts to find a minimum of a scalar function of several variables, starting at an initial estimate. This is generally referred to as unconstrained nonlinear optimization.

Details: http://www.mathworks.com/help/toolbox/optim/ug/fminunc.html

2) fmincon

fmincon attempts to find a constrained minimum of a scalar function of several variables starting at an initial estimate. This is generally referred to as constrained nonlinear optimization or nonlinear programming.

Details: http://www.mathworks.com/help/toolbox/optim/ug/fmincon.html

Antennas in WiFi (WLAN)

Some Radio network interface cards (NICs) and access points have integrated antennas that you can't change. For example, laptops such as Apple iBook integrate the antenna within the cover or body of the device, which is not visible or changeable by the user. Some radio NICs and access points also use permanently mounted antennas. With these types of products, you have no choice but to use the antenna the vendor supplies.

Other wireless LAN devices have antennas that are interchangeable. In fact, it's a good idea to purchase access points with removable antennas. These allow more flexibility by enabling the selection of an antenna having characteristics better suited for your specific application. The more common antenna types for wireless LANs have omni-directional and directional radiation patterns. Omni-directional antennas propagate RF signals in all directions equally on a horizontal plane (i.e., throughout the facility), but limit range on the vertical plane. This radiation pattern resembles that of a very large doughnut with the antenna at the center of the hole.

Omni-directional antennas, having gains ranging up to 6 dB, apply to most applications inside buildings. Omnis provide the widest coverage, making it possible to form somewhat circular overlapping cells from multiple access points located throughout the building. Most access points ship standard omnis having relatively low gain.

References:

[1]-http://www.wi-fiplanet.com/tutorials/article.php/1144391/Antennas-The-Key-to-Maximizing-RF-Coverage.htm

[2]-http://www.wlanantennas.com/index.php

MIMO Technology

The radio environment on electrically small platforms is changing rapidly. Until recently one radio was used in isolation and was usually connected to only one antenna. The situation today is very different: there is usually more than one radio used at once for example a handset may have 4 cellular bands, GPS and BluetoothTM. Sometimes WLAN radios are also present. This means that more RF filtering of signals is necessary. It is also becoming common for each radio to use more than one antenna in order to create diversity or for MIMO applications.

Antenna diversity is already used with WLAN radio in order to counter multipath, reduce outages and improve the quality and reliability of the communications link. Generally three types of diversity are used, two antennas can be deployed as far apart as possible to create some spatial diversity, they can be oriented orthogonally to give polarisation diversity or they can have different beams patterns. Diversity in current WLAN systems is usually restricted to two antennas for each radio as this is enough to ensure that if one antenna is in an RF null, the other is generally not, thereby providing better performance in multipath environments. Only one radio is present and so the receiver listens to one antenna at a time and a RF switch is used to select the antenna giving the best signal.

Reference:

Antenova, "Antenna Designs for MIMO Systems", Queen Mary University of London 2004.

Atmospheric Effects in RF Propagation

The atmospheric effects of interest for RF propagation are refraction/reflection, scattering, and absorption/attenuation.With the exception of refraction, these effects are all minimal below 30MHz. Between 30MHz and 1GHz, refraction/reflection is the primary concern. Above 1GHz or so, attenuation starts to be a significant factor and refraction/reflection becomes less of an issue except for nearly horizontal paths. Atmospheric multipath also starts to be observed above 1GHz and can cause extreme fading on terrestrial microwave links. The effects of interest for propagation analysis are in the troposphere and, to a limited extent, the tropopause.The stratosphere is well approximated as free-space.

Refractive and scattering effects of the atmosphere include:

• Refraction on horizontal paths resulting in alteration of the radio horizon due to ray curvature.

• Troposcatter, from localized fluctuations in the atmospheric refractive index, which can scatter electromagnetic waves.

• Temperature inversion, abrupt changes in the refractive index with height causing reflection.

• Ducting, where the refractive index is such that electromagnetic waves tend to follow the curvature of the earth.

These effects vary widely with altitude, geographic location, and weather conditions. The effects can permit beyond-the-horizon communication (or interference), or produce blockage and diffraction from terrain that appears to be below the line of sight and multipath fading.

Reference:

An excerpt from John S. Seybold, "Introduction to RF Propagation", John Wiley and Sons Inc., 2005.

Authors in LaTeX!

To produce

write this code just below the title

\author{ \textbf{Syed Ahsaan Rizvi \& Muhammad Ali Khalid} \\ \small Electrical Engineering Department, Military College of Signals\\ \small National University of Sciences and Technology (NUST), Pakistan}

For authors with different affiliations, check this link
http://fourlovesfour.blogspot.com/2009/10/latex-tip-author-affiliations.html

Large Scale Data Analysis

The past decade has seen the increasing availability of very large scale data sets, arising from the rapid growth of transformative technologies such as the Internet and cellular telephones, along with the development of new and powerful computational methods to analyze such datasets. Such methods, developed in the closely related fields of machine learning, data mining, and artificial intelligence, provide a powerful set of tools for intelligent problem-solving and data-driven policy analysis. These methods have the potential to dramatically improve the public welfare by guiding policy decisions and interventions, and their incorporation into intelligent information systems will improve public services in domains ranging from medicine and public health to law enforcement and security.

In Pakistan, there are so many organizations that are always in need of better mechanism to handle such a large data and use to take decisions effectively and quickly. Take the telecom sector, for instance, almost all the decisions are made by taking out KPIs from a large set of data. A lot of research work is going on worldwide in this arena and there still exists a lot of potential!

Equations in LaTeX!

1) Latex Code for

\begin{equation}
E(\theta,\phi) = -j\frac{Z_{0}}{2\lambda}\frac{e^{-jkr}}{r}\sum\limits_{m=0}^{M} le_{m}(\theta)i_{m}a_{m}(\theta,\phi)
\end{equation}


2) Latex Code for 

\begin{equation}
\min_{x}{\frac{1}{2}x^{T}Hx+c^{T}x+b}
\end{equation}

NUST graduate’s book published in Germany

NUST graduate’s book published in Germany

Reconfigurable Antennas - Beam Steering

Recently, reconfigurable antennas have attracted significant interest to implement several operating characteristics by using a single antenna. Various methods are reported in the literature to achieve reconfigurability in frequency, polarization, and radiation pattern. Printed microstrip antennas are widely preferred in reconfigurable antenna applications because of their low profile and easy integration with active devices.

Beam steering refers to the changing of the direction of the wave lobe that contains the maximum power of a radiation blueprint. This is the lobe that displays the greatest field strength, which basically means the one with the best reach and the strongest of signals. Typically, beam steering with reflective apertures is performed using mechanical rotation of the reflecting surface. Limited electronic steering can be accomplished using a focusing reflective surface by changing the feed location or direction of incidence. To achieve beam steering more than 10 degrees, different phase shifting mechanisms using varactors can be used such as 90 hybrid loaded by the varactor diodes. Furthermore, the use of MEMS capacitors will also increase the overall efficiency of the antenna because the insertion loss of MEMS capacitances is small compared with varactor diodes.

Microstrip Line

Printed transmission lines are widely used in circuits and antennas as they are compact, light weight and broadband in frequency. The microstrip line is a transmission-line geometry with a single conductor trace on one side of a dielectric substrate and a single ground plane on the opposite side. Since it is an open structure, microstrip line has a major fabrication advantage over stripline.

Another advantage of microstrip over stripline is that all active components can be mounted on top of the board. The disadvantages are that when high isolation is required such as in a filter or switch, some external shielding may have to be considered. Given the chance, microstrip circuits can radiate, causing unintended circuit response. A minor issue with microstrip is that it is dispersive, meaning that signals of different frequencies travel at slightly different speeds. Microstrip does not support a TEM mode, because of its filling factor.

All microstrip equations are approximate. The effective dielectric constant is a seen to be a function of the ratio of the width to the height of the microstrip line (W/H), as well as the dielectric constant of the substrate material.

[1] http://mcalc.sourceforge.net/

[2] http://paginas.fe.up.pt/~hmiranda/etele/microstrip_basics.pdf

Intro to Communication Satellite (COMSAT)

An important and widespread use of microwave engineering is communication satellite. What was the motivation? Telephone service overseas was exceptionally bad, and live television could not be received or transmitted over great distances. Properly positioned satellites could provide unobstructed communications for nearly all points on Earth as long as there was a method to put them in orbit. The key to this concept was the placement of space stations in geosynchronous Earth orbit (GEO), a location 35,786 kilometers (22,300 miles) above Earth. Objects in this orbit will revolve about Earth along its equatorial plane at the same rate as the planet rotates. Thus, a satellite or space station in GEO will seem fixed in the sky and will be directly above an observer at the equator. A communications satellite in GEO can "see" about one-third of Earth's surface, so to make global communications possible, three satellites need to be placed in this unique orbit (geostationary orbit).

Lets have some technical introduction to a communication satellite. A simple definition is: A set of radio receivers and transmitters (payload) based on a space-based platform (satellite bus/spacecraft) orbiting around the earth. Thus, the components of a communication satellite are Payload, Bus and Command & Control.

The satellite can have a passive role in communications like bouncing signals from the Earth back to another location on the Earth; on the other hand, some satellites carry electronic devices called transponders for receiving, amplifying, and re-broadcasting signals to the Earth.  The area to which it can transmit is called a satellite's footprint.

Tidbit: Pakistan has a leased satellite, PAKSAT-1, in the 38 degree East longitude geostationary orbit. The government of Pakistan has granted approval for the replacement of PAKSAT-1 by a new communication satellite PAKSAT 1R by 2011. SUPARCO has also developed a prototype of a communication satellite named Prototype PAKSAT-1R and is now developing anEngineering Qualification Model (EQM) [4].

Recommended Readings:

[1] http://www.encyclopedia.com/topic/communications_satellite.aspx

[2] http://www.satellites.spacesim.org/english/function/communic/index.html

[3] http://www.gma.org/surfing/satellites/sat_com.html

[4] http://www.suparco.gov.pk/pages/comm-satellite.asp

Types of Handover in GSM

Power Budget Handover:

When signal strength difference between serving cell and neighbor cell exceeds Power Budget Handover margin which is set in Handover parameters, the call is handed over to the neighboring cell. This margin is usually set to 3 to 6 dB. In case of ping–pong handovers between the same two cells, power budget handover margin between the two could be increased to reduce number of handovers. Margin should be decreased if faster handover decision is wanted.

Level Handover:

If downlink level is worse then HO Thresholds Lev DL parameter, then an immediate level handover takes place. This parameter is set to –95dBm as default. If uplink level is worse then HO Thresholds Lev UL parameter, then an immediate level handover takes place. This parameter is set to –105dBm as default.

Quality Handover:

If downlink quality is worse then HO Thresholds Qual DL parameter, then an immediate quality handover takes place. If uplink quality is worse then HO Thresholds Qual UL parameter, then an immediate quality handover takes place.

Interference Handover:

Any quality problem when downlink signal level is higher then HO Threshold Interference DL causes a handover. This level is generally set to –80dBm. Any quality problem when uplink signal level is higher then HO Threshold Interference UL causes a handover. This level is usually set to –80dBm.

Intra–cell Handover:

The Intra–cell Handover feature aims to maintain good quality of a connection by performing a handover to a new channel within the same cell when bad quality is detected. If the signal strength is very high, the interference is probably lower on another channel within the same cell. Intra–cell Handover aims at improving the carrier–to–interference ratio, C/I, for a connection when the bit error rate estimation, RXQUAL, has indicated poor quality, and the received signal strength at the same time is high. This is done by changing the channel within the cell, a so called intra–cell handover. The intra–cell handover can be triggered from bad quality in the uplink, as well as the downlink. Intra–cell handover decision is given when serving cell is the best cell and quality is worse then 4 and signal strength is lower then –85dBm. Intra–cell handover can solve temporary co– and adjacent channel interference as well as intermodulation problems, but permanent interference and time dispersion cannot be solved.

Directed Retry Handover:

When no TCH is available in the serving cell, TCH can be allocated in an adjacent cell regardless of mobile originated or mobile terminated call. It is basically handover from SDCCH to TCH.

MS Distance Handover:

MS Distance HO Threshold parameter MS Range Max should be set to desired value. If an overshooting site is needed to hand its traffic after some distance from its origin, MS Range Max value could be adjusted to limit the serving area of the site.

Umbrella Handover:

Macro site which is defined as umbrella will shift all the TCH traffic to small sites until signal level of small site becomes worse than HO Level Umbrella RX Level parameter. This parameter could be set from –80 to –90dBm.

Impedance Taper Selection Guide

As mentioned earlier, Tapered microstrip baluns can be classified according to the type of taper used. Four widely used tapers are:

• Linear
• Exponential (E)
• Triangular (T)
• Klopfenstein (K)

The selection of taper can have multiple criteria but the general important considerations are the simplicity of the design and the production of a smooth transition of impedance from one end to other. The size of the impedance section is also a critical factor in certain design situations where length of the section can not exceed a given limit. Linear taper is the simplest of all the tapers and can be designed quickly, however, the trade-off is in the length of the taper. Similar is the case with the triangular and exponential taper, however, but as there designs are a little complex from the the linear taper, they provide a better response in impedance transformations (the reflection coefficient are lower than that of linear taper along the lenght).

The klopfenstein taper is defined by a very complicated equation, however, the advantage of this taper is that the reflection coefficient is minimum over the passband. Alternatively, the Klopfenstein taper provides the minimum taper length for a maximum reflection coefficient specification.

Recommended Readings:

[1] http://www.microwaves101.com/encyclopedia/Klopfenstein.cfm

[2] David Pozar, "Microwave Engineering"

Drop Calls in GSM

If the radio link fails after the mobile sends the Service Connect Complete Message then it is considered a dropped call. Dropped call analysis can consume a considerable amount of time. Using good post–processing analysis tools, the root cause of some of the drops can be determined from mobile data alone. However, there will be cases where the cause cannot be reliably confirmed unless system data is also used.

Calls often drop when strong neighbors suddenly appear. When the MS is suddenly confronted with a strong new signal, or when the signal it is using takes a sudden deep fade, it will have poor C /I and high forward FER. The call will drop unless it gets help quickly.

Using a post– processing tool, display a map of the locations of dropped calls that exhibit symptoms of poor coverage. Verify this type of drop is not occurring in good– coverage areas. If so, suspect and investigate hardware at the serving site.

Another technique is to examine the dropped call message files and identify the BTS from which the sync channel message is received immediately after each drop. This could be achieved by analyzing Layer 3 messages in log files or running traces from NMS/OSS.

Drop calls can be classified by looking to their orientations:

• TCH radio drops are the drops that occurred due to summation of radio and ABIS reasons.
• TCH non–radio drops are the drops that occurred due to summation of network management, BSCU reset, BTS fail, LAPD failure, user and A interface.
• TCH handover drops are the drops that occurred in handover phase while the call tries back to old serving channel but fails and drops. These drops may occur due to RF, ABIS and A interface reasons.

General Reasons for Drop Calls are as follows:

• Drop Call due to Low Signal Strength.
• Drop Call due to Missing Neighbor.
• Drop Call due to Bad RX Quality.
• Drop Call due to Interference.
• Drop Call due to Handover Failures.

General Recommendations:

• Defining missing neighbor relations.
• Proposing new sites or sector additions.
• Proposing antenna azimuth changes.
• Proposing antenna tilt changes.
• Proposing antenna type changes.
• Re–tuning of interfered frequencies.
• BSIC changes.
• MHA/TMA adds.
• Changing power parameters.
• Adjusting Handover margins.

Tapered Microstrip Balun

Tapered microstrip balun is used as an impedance transformer network in feeding sections of spiral antennas as it provides impedance transformation over a large range of frequency and also serves the purpose of conversion of single ended port to a symmetric port. The conversion from unbalanced to balanced line relies on a gradual change of cross section of the line.

A tapered microstrip balun consists of two tapered lines etched on either side of a substrate. One line is atleast three times wider than the other at the unbalanced end, and together the two form a microstrip line. At the balanced end, the lines are of equal width and appear as parallel strips. At the input, the cross section of the line resembles microstrip, while at the output the strips are of equal width, constituting a balanced line. Ideally the

balanced line should support only odd modes. In order to design a tapered microstrip balun we need the characteristic impedances of this line at the input and the output ports. The impedance at the balanced end is known beforehand (which is according to the requirement). The widths of the parallel strips at this end can be found iteratively using the equations of the selected taper. The length of the taper is also given by a criteria which is specific to each taper. The structure is then simulated in any EM modeling software like HFSS and the impedances at the two ports are observed. If they are offset from the designed values, the widths are adjusted and procedure is repeated till the values agree sufficiently with the desired ones.

Reference:

Inanc Yildiz, 'Design and Construction of reduced size planar spiral antenna in the 0.5-18 GHz frequency range', METU 2004

Real-Time Hardware Control and Signal Processing using Simulink

If you don't want to use a microcontroller in your hardware project, dont worry! You can use the far more easy and advanced Simulink blocks to realize a real-time control system or signal processor. You can  use a Data Acquisition Device or Serial/Parallel Port of your PC for this purpose. Just follow these steps.

1) After installing the driver for the Data Acquisition Device, open MATLAB and write rtwintgt -setup in the command window. This will install the Real-Time Windows Target kernel.

2) Start Simulink and create a new model. Press Ctrl+E, Configuration Parameters will appear. In Solver options, change 'Variable-step' to 'Fixed-step'. Now select 'Real-Time Workshop' form the left column and change system target file to 'rtwin.tlc' and click ok.

3) Open the Simulink Library Browser, select 'Real-Time Windows Target' from the left column and drag the 'Analog Input' block to your model.

4) Double-click on the 'Analog Input' block inside your model, click on 'Install new board', select your device's manufacturer and model. Serial port, parallel port, mouse, etc. appear in 'Standard Devices'. Set your desired parameters, and click on 'Test' to see if the device is properly plugged in.

5) In the 'Block Parameters' window, you can select the sampling time period, input channel no., input range, etc.

6) Now you can do anything with the input signal by inserting Simulink blocks, and if you want to take an output depending upon the input, insert an 'Analog Output' block at the end.

7) Your system is just ready to run. In the toolbar at the top, set the Simulation Mode to 'External' and Simulation Stop Time as desired (use 'inf' here if you want to stop simulation on will). Click on 'Incremental Build' icon, then 'Connect to Target' and start the simulation.

Speech Quality Index in GSM

SQI, Speech Quality Index is another expression when Quality is concerned:

The need for speech quality estimates in cellular networks have been recognized already in the GSM standard, and the RxQual measure was designed to give an indication of the quality.

However, the RxQual measure is based on a simple transformation of the estimated average bit error rate, and two calls having the same RxQual ratings can be perceived as having quite different speech quality. One of the reasons for this is that there are other parameters than the bit error rate that affects the perceived speech quality. Another reason is that knowing the average bit error rate is not enough to make it possible to accurately estimate the speech quality. A short, very deep fading dip has a different effect on the speech than a constant low bit error level, even if the average rate is the same.

The TEMS Speech Quality Index, which is an estimate of the perceived speech quality as experienced by the mobile user, is based on handover events and on the bit error and frame erasure distributions. The quality of speech on the network is affected by several factors including what type of mobile the subscriber is using, background noise, echo problems, and radio channel disturbances. Extensive listening tests on real GSM networks have been made to identify what type of error situations cause poor speech quality. By using the results from the listening tests and the full information about the errors and their distributions, it is possible to produce the TEMS Speech Quality Index. The Speech Quality Index is available every 0.5 second in TEMS and predicts the instant speech quality in a phone call/radio–link in real–time.

Balun Transformer - Impedance Matching Device

The word Balun is derived from two words Balanced and Unbalanced. Thus, Balun is a device that transforms balanced signal to unbalanced signal or vice versa. Balanced signals are double ended, ie, both the terminals of a balanced port are hot with voltage whereas in an unbalanced port, only one terminal carries the signal and other is ground. A balun is a device that connects a balanced impedance to an unbalanced impedance. It usually also provides impedance transformation, and is hence named as balun transformer.

Microwave baluns can be coaxial or planar. Planar baluns are commonly implemented as microstrip structures. The two most popular microstrip baluns are the Marchand balun and the tapered line balun, both of which are capable of decade bandwidths if designed properly.

Tapered microstrip baluns typically consist of two broadside-coupled tapered lines. The two lines are of equal width at the balanced end, while at the other end, one line is at least three times wider than the other so as to form a conventional microstrip line.

Tapered microstrip baluns can be classified according to the type of taper used. Four widely used tapers are:

• Linear
• Exponential
• Triangular
• Klopfenstein

There are mathematical functions that define the tapering of the above tapers. The equations are complex but can be implemented in MATLAB and thus the dimensions of the device can be found according to the impedances required at each end. These equations can be found in [1].

Recommended Readings:

[1] David M. Pozar, “Microwave Engineering”, Addison-Wesley, 1993.

[2] Inanc Yildiz, “Design and Construction of Reduced Size Planar Spiral Antenna in the 0.5-18 GHz Frequency Range”, MS Thesis, Middle East Technical University.

[3] Franco di Paolo, “Networks and Devices using Planar Transmission Lines”, CRC Press, 2000

Code Optimization in MATLAB

During the undergraduate studies, whenever a student is given a programming problem to write code for it, the most important thing for him is to write a piece of code that gives the desired output. Most of the time, he never bothers to think how much time the code takes to give the desired output. This approach can work well as long as you are a student, but professionals think differently. They write a code that produces the desired result in the least possible time and using minimum possible resources. And for that matter, they have to optimize their code.

Usually, for a given programming problem, there may exist a number of solutions that output the required result. But the best of them is the one that is the shortest and the fastest. So, have you ever thought how much time your piece of code takes for execution?

In MATLAB, there is a simple command to find out the time taken by a code for execution: tic, toc. Write the code between tic and toc and run it. After the execution completes, MATLAB will tell the amount of time the code took to run.

tic;

% here goes the code, for example:

membrane;

toc;

% Output: Elapsed time is 0.150652 seconds.

Whenever you write a code, try to minimize this elapsed time and eventually you will get to the optimum code.

Solutions to Low Signal Level Problems in Cellular Networks

Possible solution ways can be listed as below:

• New Site Proposal
• Sector Addition
• Repeater
• Site Configuration Change (Antenna Type, height, azimuth, tilt changes)
• Loss or Attenuation Check ( Feeders, Connectors, Jumpers).

The best thing to do in case of low signal strength could be recommending new site additions. A prediction tool with correct and detailed height and clutter data supported with a reasonable propagation model could be used to identify the best locations to put new sites. If client is not eager to put new sites because of high costs to the budget or finds it unnecessary because of low demand on traffic, then appropriate repeaters could be used to repeat signals and improve the coverage. Adding repeaters always needs extra attention because they can bring extra interference load to the network. The received level in the repeater should be above -80dBm (or desired limits) so that it can be amplified and transmitted again. The mobile should not receive both the original and the repeated signals at the same area, cause signal from the repeater is always delayed and it will interfere with the original signal. A repeater should not amplify frequencies outside the wanted band.

If none of the above recommendations are accepted by the client, then cheaper and easier ways should be followed. First things to be checked would be possible attenuation on the cells. Faulty feeders–jumpers–connectors or other faulty equipment, high combiner loss, reduced EIRP, decreased output power, the orientations and types of antennas, unnecessary downtilts, existence of diversity and height of the site should be deeply investigated. Putting higher gain antennas, increasing output power, removing attenuations, changing antenna orientations towards desired area, reducing downtilts, replacing faulty equipment or usage of diversity gain could improve the coverage.

Please note, amplifiers (TMA or MHA) could be used to improve uplink or compensate the loss caused by long feeder. Be careful, because they will also amplify interfering signals and they will be received at higher level.

RF and Microwave Diodes

 

We will describe here most important of these:

Varactor Diodes: A varicap diode or varactor diode is a type of diode which has a variable capacitance that is a function of the reverse voltage impressed on its terminals. This property of varying the diode capacitance is useful in tuning circuits, Automatic Frequency Control (AFC) circuits, Antenna tuning, etc.

PIN Diodes (Positive Intrinsic Negative): PIN diode is a semiconductor device that operates as a variable resistor at RF and Microwave frequencies. It can also be used as a switch and Limiter. The variable resistor property makes it usable as an Attenuator.

Step Recovery Diode (SRD): The term step recovery relates to the form of the reverse recovery characteristic of these devices. After a forward current has been passing in an SRD and the current is interrupted or reversed, the reverse conduction will cease very abruptly (as in a step waveform). SRDs can therefore provide very fast voltage transitions by the very sudden disappearance of the charge carriers.

Gunn Diodes: These are similar to tunnel diodes in that they are made of materials such as GaAs or InP that exhibit a region of negative differential resistance. With appropriate biasing, dipole domains form and travel across the diode, allowing high frequency microwave oscillators to be built. Thus GUNN diodes can be used as oscillators.

Parametric Amplifier Diodes: Parametric amplifier diodes are similar to varactor diodes. Here, the parametric variation (reactance variation with the voltage) is made use of to amplify RF signals. A rectangular wave called ‘pumping signal’ is applied to the diode to vary the reactance. This signal frequency is selected such that it is twice the frequency of the signal to be amplified.

Tunnel Diodes: These have a region of operation showing negative resistance caused by quantum tunneling, allowing amplification of signals and very simple bistable circuits. Due to the high carrier concentration, tunnel diodes are very fast, may be used at low (mK) temperatures, high magnetic fields, and in high radiation environments. Because of these properties, they are often used in spacecraft.

For Details:
http://en.wikipedia.org
RF and Microwave Diodes - An Introduction by Anoop N. K. (http://www.avtechpulse.com/faq.html/IV.28/uwdiode.pdf)

Impedance Matching

One of the problems that a microwave design engineer frequently faces is that of impedance matching of source and load is important to get maximum power transfer. If you have a 75 ohm load, you don't want to drive it with a 50 ohm source, because it is inefficient. Due to these unequal impedances, there will be a reflection of some power of the signal as it traverses from source to load. The greatest amount of engineering time is spent in searching for ways to provide efficient impedance matching, especially to active circuit elements, so it pays to know some of the many useful impedance-matching methods and their limitations. Maximum power transfer theorem should be remembered as it serves as a great reference in resolution of a impedance matching problem. The theorem simply states that the maximum amount of power will be dissipated by a load resistance when that load resistance is equal to the Thevenin/Norton resistance of the network supplying the power. If the load resistance is lower or higher than the Thevenin/Norton resistance of the source network, its dissipated power will be less than maximum.

Microwave instruments for measurement of impedance by way of direct measurement or S-parameters are among the most widely used tools of the microwave engineer. There are many ways to match impedances, some common methods are:

i) Impedance Sections

ii) Transformers

iii) Matching networks

iv) Tapered baluns

etc.

One of the great advantages of tapered baluns is that it provides broadband impedance matching and also performs an additional functionality of converting a single ended port to a double ended port. More detail on this later!

[1] http://www.microwaves101.com/encyclopedia/maxpower.cfm

[2] EE246 — Microwave Engineering, Lesson Autumn 1999

Battery Life of a Cell Phone

One of the main factors which restrict reducing the size of a MS is the battery. A battery must be large enough to maintain a telephone call for an acceptable amount of time without needing to be recharged. Since there is demand for MSs to become smaller and lighter the battery must also become smaller and lighter.

Four features which can enable the life of a GSM MS battery to be extended are:

• Power Control
• Discontinuous Transmission (DTX)
• Discontinuous Reception (DRX)
• Use of Adaptive Antennas

Power Control: This is a feature of the GSM air interface which allows the network provider to not only compensate for the distance from MS to BTS as regards timing, but can also cause the BTS and MS to adjust their power output to take account of that distance also. The closer the MS is to the BTS, the less the power it and the BTS will be required to transmit. This feature saves radio battery power at the MS, and helps to reduce co-channel and adjacent channel interference.

Discontinuous Transmission (DTX): DTX increases the efficiency of the system through a decrease in the possible radio transmission interference level. It does this by ensuring that the MS does not transmit unnecessary message data. DTX can be implemented, as necessary, on a call by call basis. The effects will be most noticeable in communications between two MS. DTX in its most extreme form, when implemented at the MS can also result in considerable power saving. If the MS does not transmit during ‘silences’ there is a reduction in the overall power output requirement.

Discontinuous Reception (DRX): DRX allows the MS to effectively “switch off” during times when reception is deemed unnecessary. This allows the MS to ‘go to sleep’ and listen-in only when necessary, with the effective saving in power usage. DRX may only be used when a MS is not in a call.

Use of Adaptive Antennas: Since the antenna in cellular phones is an omni-directional one (this is done to enable signal reception from all sides), its gain is low. For a constant required EIRP (Pt*Gt), this increases the required transmit power, thus reducing battery life. If the antenna is made directional and if somehow it's main-lobe can be steered whenever the MS changes it's direction w.r.t. to the BS, the EIRP could be served with a lower transmit power.

Microcontroller - its easy and cool!

Microcontrollers are not at all difficult as they sound. Mostly students at MCS are hesitant to include microcontrollers in their projects as they are neither taught in any course nor there have been any workshop or seminar on this topic as far as I know. But really they are as simple as writing a basic program in C++. A microcontroller operates by changing the voltages of its output pins. Output pins typically take on two discrete voltages (usually 0 and 5 volts). Each output pin reflects the value of some bit in the microcontroller's memory. When the bit is set to 1, the pin goes to 5 volts, and when the bit is set to 0, the pin goes to 0 volts. Some applications they are used for are data acquisition, logical voltage supply, control system, interfacing and so on.

Common microcontrollers that are easily available in Rawalpindi College road, Pakistan are 8051, 8052 etc and these are the ones you would probably like to use and create an additional charm in your project. And they are not expensive as you might think, they cost about Rs. 50-70 only. Google 8051 or 8052 and you would come across so many resources about them. Some useful readings and resources are mentioned below.

[1] http://www.rentron.com/8051.htm

[2] http://www.8052.com/

[3] http://web.mit.edu/rec/iap/microintro.html

[4] http://en.wikipedia.org/wiki/List_of_common_microcontrollers

MATLAB Functions in Communications - 1

1) taylorwin

Taylor window

Syntax:

w = taylorwin(L)
w = taylorwin(L,nbar)
w = taylorwin(L,nbar,sll)

calculates the amplitude coefficients for the antenna elements by applying a Taylor taper. L is the no. of antenna elements, nbar is the no. of side lobes, sll is the side lobe level in dB (-ive value).

Description: 

Taylor windows are similar to Chebyshev windows. While a Chebyshev window has the narrowest possible mainlobe for a specified sidelobe level, a Taylor window allows you to make tradeoffs between the mainlobe width (HPBW) and sidelobe level. Taylor window coefficients are not normalized. Taylor windows are typically used in radar applications, such as weighting synthetic aperature radar images and antenna design.

2) awgn

Add white Gaussian noise to signal

Syntax:

y = awgn(x,snr)

Description:

Additive white Gaussian noise (AWGN) is a channel model in which the only impairment to communication is a linear addition of wideband or white noise with a constant spectral density and a Gaussian distribution of amplitude. The command adds white Gaussian noise to the vector signal x. The scalar snr specifies the signal-to-noise ratio per sample, in dB. If x is complex, awgn adds complex noise. This syntax assumes that the power of x is 0 dBW.

For Details:

http://www.mathworks.com/help/toolbox/signal/taylorwin.html
http://www.mathworks.com/help/toolbox/comm/ref/awgn.html

How PCI Express Works (PCI Express Lanes)

When the computer starts up, PCIe determines which devices are plugged into the motherboard. It then identifies the links between the devices, creating a map of where traffic will go and negotiating the width of each link. This identification of devices and connections is the same protocol PCI uses, so PCIe does not require any changes to software or operating systems.

Each lane of a PCI Express connection contains two pairs of wires -- one to send and one to receive. Packets of data move across the lane at a rate of one bit per cycle. A x1 connection, the smallest PCIe connection, has one lane made up of four wires. It carries one bit per cycle in each direction. A x2 link contains eight wires and transmits two bits at once, a x4 link transmits four bits, and so on. Other configurations are x12, x16 and x32.

Figure below shows the different pci express slots and their datarates.

PCI Express is available for desktop and laptop PCs. Its use may lead to lower cost of motherboard production, since its connections contain fewer pins than PCI connections do. It also has the potential to support many devices, including Ethernet cards, USB 2 and video cards.

Often during interviews a question is asked how can one serial connection be faster than the 32 wires of PCI or the 64 wires of PCIx?and how PCIe is able to provide a vast amount of bandwidth in a serial format?

The answer of this question will be presented in the next post. Till then just admire and believe the unseen.

The Art of PCB Design

Most of the engineering students try their best to go for just simulation based final year project and a hardware based project sounds to be a nightmare for all of us. But this is a fact that students who do a hardware based project have a certain edge over others when they get into the job hunting process as they can apply for R&D jobs as well as traditional technical jobs. Mostly the skill of PCB designing is vital to learn if a student opts for a hardware based project.

Designing a simple PCB is not as difficult as it first seems to be. There are a few simple steps to follow and these steps are vital to the PCB designing process regardless of the complexity of the design, though additional work can be done to ensure the proper performance of more complex PCB. Following is a list of steps to design a PCB. Keep in mind that veteran PCB designers may skip some of these steps when designing simple PCBs but my reader here is an average and novice engineer. With the same reason in mind, only the basic process has been described to give a general idea.

1. Preliminary Process:

Before starting to work on PCB, make sure you have a complete design of what you want to put on the PCB. Minor changes can be done afterwards but a major change is never acceptable as you may have to work from scratch once again. Select a PCB design software which you find best. There is no major difference in the features of existing softwares as far as PCB designing is concerned.

2. Symbol and Foot Print Library:

Once you have the circuit diagram and the software to work with, you now need the basic building blocks of the PCB design: Symbols and Footprints. A symbol is a representation of a component of your design. It has pins, pins' directions (whether input, output or bidirectional), a label and a designation. Each symbol has a footprint associated with it which is a component's area of contact with PCB.Each PCB designing software has integrated libraries of symbols and footprints. Search through them. If the symbols and footprints for the components of your design are already available, you don't need to design new ones. But if you have to make new ones, consult the datasheets of those components. You will find all the necessary data to build symbols and footprints. First draw symbols and then associate footprints with them.

3. Schematic Design:

In this step, you draw the circuit diagram available with you in the software. Put the components of your design on the sheet and connect them through wires.

4. Conversion to PCB:

After drawing the circuit, convert it to PCB design. This simply means that the symbols will be replaced with their associated footprints. The connections will be jumbled up though.

5. Component Placement:

This step and the next one are the actual art of PCB designing. You have to put all the components (footprints) in an organized and orderly manner. The more efficiently you arrange components on the board, the easier the next step gets and it will also save you PCB space and money! Spend as much time as possible on this step in an effort to make the wires the least jumbled up.

6. Design Rules:

You have to define a set of rules which will be followed when routing the board. These rules include dimensions of routes and vias etc.

7. Routing:

After arranging the components on the PCB in the best possible manner, you will now connect them. Now you may be wondering that the components are already connected (as you can see a pile of connections in front of your eyes!) then how you can connect them again. The answer is that, that pile of connections is just for your reference! Obviously you don't want your PCB look that ugly with a lot of wires intersecting each other! And believe me, such a PCB is never going to work if even fabricated.There are a couple of methods to get your components connected: Auto routing and manual routing. As the names suggest, in auto routing, the software automatically makes connections between components according to predefined rules while in manual routing, you have to do that task. Manual routing is preferable for simple designs but can't work if complexity exceeds a particular threshold. In the case of more complex designs, the best approach is to have a round of manual routing after the auto routing to improve the quality.

8. Final Outputs:

After you are done with the routing process, you can generate gerber and drill files which are sent to the fabricator for PCB fabrication process. A number of other output files can also be generated like BOM (bill of materials) etc.