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.