COURSE FOR COMMUNICATION ENGINEER

What is 0 dBm? 

Everyone who works with Telecom knows there is a relationship between Watts and dBm. But if the Power is expressed in Watts, why we must know - and use - this relationship in our day-to-day??

Watt (W) and miliWatt (mW)

First of all, to understand what it means for example 0 dBm, we at least have to know the basic unit of power, the Watt. By definition, 1 Watt means 1 Ampere (A) current in 1 Volt (V) voltage, or in mathematical terms P = VA. It is interesting to note that the amount of power radiated by an antenna is very small in terms of Watts, but it is enough to reach several miles.

And as the signs are very small, is more common to refer to them in terms of prefix, such as military or milliwatts (mW), which means 1 / 1000 (one thousandth) of Watts.

Mathematics

Besides the signals were rather small, it - as well as other quantities of physics such as electricity, heat or sound - propagate nonlinearly. It would be more or less like compound interest on a loan.
Or brought into our world of engineers, imagine a cable for transmitting 100 watts, with a loss of 10% per meter. If the spread was linear, the final 10 meters would have no more power!

Only it's not how it happens. In the first meters, have 10% less power, which is 90 watts. And this is the value that 'enter' on the cable until the next meter. Thus, the second meter, we would have 10% less of that power or 81 watts (= 90 - 9). Repeating this calculations, you see that in fact the power never reaches zero, as it would if calculations were linear. (At the end of the cable have actually 34.86 Watt)

To solve problems o deal with this - and make our lives easier - we need to know the logarithms. We saw this in school, but there are people who do not like to hear. The good thing is that we need not know much about them, just understand what they are.
Just understand that if we transformed the magnitudes in logarithms, the calculations become addition and subtraction rather than multiplication and division.

Of course, in order to do the calculations by adding and subtracting, we must make the necessary conversions. But with the help of a calculator or Excel, is not that complicated.

Decibels (dB)

By definition, we have:

Sure, we say that working with logarithms (or decibels) is much easier - and the common good. But by the formulas above, still can not understand. So the best way to understand why we use dB (decibels), is seeing how they help us through a practical example.
Consider a standard wireless link, where we have a transmitter (1) and a receiver (5), Antennas (3), Cables, Jumpers and Connectors (2) and Free Space (4).

Using real values, and without using the help of dB, let's do the math and see, from the transmitted power, how much power we have at receiver. So with dummy values, but close to reality, we have:

• Transmitter Power = 40 Watts
• Cables and connectors loss = - 0.5 (Half Power)
• Antenna Gain = 20 + times in the Power
• Free Space Loss = - 0 000 000 000 000 000 1 Power

Note: This amount of loss in free space is quite big. And it is obtained based on distance and other factors. For now, just accept that it is a practical value of loss of RF for a given distance of our link.
The link with the absolute values in Watt would then be as below.

We can work this way, of course. But you must agree that it is not very friendly.
Now, if we use the proper conversion of power, gain and loss for dB, we can simply add and subtract.

It was so much easier, isn't it?
Now we just need to know the formulas to do the conversions.

Converting with Formulas in Excel

Considering that the amount of wattage is in cell B3, the formula for convrting W in dBm is:
= 10 * ( LOG10 ( 1000 * B3 ) )
And the formula to reverse - convert dBm to Watt, considering that our power in dBm is in cell B6 is:
= 10 * ( LOG10 ( 1000 * B6 ) )
And as a result, we have calculated values.

Note that in case we are using the 1000 value in the formulas, for wearing the Watt, but we want the result in dBm.
To calculate (convert) db to ratio, or ratio to db, the formulas do not include the value of 1000.

Calculations without using a calculator

Of course, we will use calculators in the projects and programs such as Excel. But we also know how to do calculations (conversions) without using a calculator. If anyone tells you that the power is + 46 dBm, you need to know what that means in terms of Watts.
For this, there are certain tricks that can be used to arrive at least an approximate match.
For this, a good way is to memorize the equivalent to multiplying factors in dB, as in the table below (at least those that are in bold).

With the corresponding values of dB and multiplier factor, we convert eg +46 dBm to mW.
Answer: First, we expressed 46 values that we already know by heart.
So 46 = 10 + 10 + 10 + 10 + 3 + 3
That is, we multiply the reference value (1 mW) for four times the factor of 10 and twice the factor of 2.
What gives us
1mW x 10 x 10 x 10 x 10 = 1 000 0 mW
1 000 0 mW x 2 x 2 = 40000 mW = 40 W
Ie, + 46 dBm is equal to 40 watts.

Conclusion

Well, I think now you have given to see that when we do the calculations in dB everything is easier. Moreover, the vast majority of Telecom equipment has specifications of its units in dB (Power, Gain, Loss, etc.).
In short, just use basic math to understand the values and reach the final figures.
When we say that such a signal is attenuated by 3 dB, means that the final power is half the initial power. Likewise, if a given power is increased by 3 dB means twice the power.
A good practice, irrespective of how you will work with the calculations is to store at least some values such as 0 dBm = 1 mWatt (our initial question), 30 dBm = 1 Watt, and in our example, 46 dBm = 40 Watt.
So you can quickly learn, for example, the equivalent for the calculations.
For example, 43 dBm = 46 dBm - 3 dB. That is, half the power of 46 dbm. Then, 43 dBm = 20 Watts!
Just finally, in our example, the received power was - 84 dBm, remember?

In this case, doesn't need memorizing. Just so you know which is equivalent to a very low power, but enough for a good example for GSM call.

 

What is VSWR? 

Everyone has heard at least. Many knows that a bad VSWR affects the performance of the Network. But what about you: do you know what it means, and why we need to know how to use these measures?

Today we'll try to see a very simple way what is VSWR.

VSWR

To understand what is VSWR, we need to talk a little bit about signal propagation in radio frequency systems.
Simply put, the radio frequency signals are driven by electric cables between transmitters / receivers to their respective antennas.

By its definition, VSWR or Voltage Standing Wave Ratio is a ratio of peak voltage on the minimum amplitude of voltage of standign wave. It does not help much, does it?
Okay, let's try to see how it relates ...
In radio frequency systems, the characteristic impedance is one of the most important factors to consider. In our case this factor is typically 50 Ohms. This is a constructive parameter, ie is some characteristic determined by its construction. In the case of a cable for example, depends on the size of the inner and outer conductors, and the type of insulation between them. All components of a link - cables, connectors, antennas - are constructed to have the same impedance.
When we insert an element in our system, we have what we call the Insertion Loss, which can be understood as something that is lost, taking into account what that actually went in and came out.
And this loss occurs in two ways - by Attenuation - especially on cable - and y Reflection.
As for the attenuation along the cables, there's not much we can do. Part of the signal is lost along the cable by the generation of heat and also by unwanted radiated off the handle. This loss is characteristic of the same, and defined in terms of dB per lenght unit - the longer the cable is, the greater is the loss. This attenuation also increases with increasing temperature and frequency. Unfortunately, these factors are not much scope of our control, since the frequency is already preset by the system we use, and the temperature will be exposed to climatic variations of where the cable has to pass .
The most we can do is try to use cable with less attenuation , ie, cableswith high quality materials used in its construction of the drivers internal and external and insulating dielectric . As a rule, the larger the diameter of the cable, the lower your attenuation. Typical values of diameters are 1/2 ", 7/8" and 1 5/8 ".
The choice of coaxial cable for the system is a process that requires a very comprehensive analysis, taking into account its characteristics (is it softer, etc ...) and costs of several options of existing cables, necessary cable length - and the consequent loss that it will introduce, the loading of the tower or brackets where cables  will be posted, among others.
 

But the other form of loss that we have in our system, and can be controlled a bit more is the loss by reflection, ie loss of the signal, which has just returned, lost by the end where it was injected. For this reason we call the Return Loss.
If there is any problem in the middle between the transmitter / receiver and antennas - such as a fold or infiltration of water - half ends with the impedance mismatch. So, part of the signal which ideally should leave by the antenna, then returns reflected!
Speaking in terms of the matching impedances, if the value of X, Y and Z are equal, we have the following.

Now with values close to the real impedance unmatched scenario, have the following.

If we consider an ideal transmission line, the VSWR would be 1:1, ie all the power to reach your destination, with no reflection (nothing lost).

And the worst means of transmission in the world, we would have infinite VSWR, ie all the power would be reflected (lost).

In practice

It is clear that there is an ideal system, one that is not the worst in the world. What happens is that there are maximum VSWR that each application can accept. The typical value in our case is 1.5:1.
So what are the problems that we can in a bad VSWR (very large)? Besides the power radiated effectively be much smaller than it should be, may also occur the burning of electronic components that have no protection for that unwanted reflected signal.
So as basic recommendations:

• Avoid bending the cables to the fullest - making turns as smooth as possible - and tighten the connectors: isolating the system that does not suffer problems like water seepage or poeira.
  • In addition, the connectors and cables must be made by professionals, and using professional equipment. It does not help tighten a connector evil feito.
• Use  always the best quality components possible: no equipment is perfect, and even the processes of production glitches arise. The quality of the material and manufacturing process of the elements is paramount so as to achieve a better quality of sinal.
• Check that all elements of the system have the same impedance.

Tables and Graphs

Is not the goal here to explain what are the standing waves, because understanding requires significant wave theory, but a simple and very interesting for you to see - and understand - as these waves are formed is shown on the site bessernet.com. Be sure to visit the link below. Enter a value of return loss, hit enter, and check!

http://www.bessernet.com/Ereflecto/tutorialFrameset.htm

The magnitudes of reflection VSWR, Return Loss dB and Power Reflected% are related, and can be converted into one another, using the formulas or tables below.
For the standing wave (please visit link above to understand first):

And for the power transmitted and reflected:

With some values we have tabulated the table below.

Here comes a good tip: Understand the return loss as 'How much weaker, in dB, the reflected unwanted signal is, compared with the transmitted signal? "
In the case of 1.5:1, power is 14 dB below the original value, or 4% was lost. Note that a VSWR of 1.9:1 almost 10% of energy is lost!
Conclusion
To conclude, we can then understand the VSWR as an indicator of signal reflected back to the transmitter radio frequency, always taking the value 1 in the denominator. And the lower this index, the better!
Thus, a radio frequency system with 1.4:1 VSWR is better than one with 1.5:1!
And another with 1:1 VSWR would have a perfect impedance matching. In other words, occurs only in theory.
Finally, the VSWR in a radio frequency system can be measured by special equipment.
One of them, and well known, is the Master Site. With mode "Distance-To-Fault" you can identify the location of problems in a damaged system.

Parameter Timing Advance (TA):

The Timing Advance parameter is very important in many applications and procedures of the GSM system.

Let us understand what it is today, and know its main uses.

What is Parameter Timing Advance (TA) in GSM?

The Timing Advance is a parameter that allows the GSM BTS to control the signal delays in their communication with the mobile.
More specifically, is calculated by the delay of information bits in Data Access Burst received by BTS.

Recalling a little: GSM uses TDMA with sequential designated timeslots to allow different users sharing the same frequency.
A burst represents the physical content of a timeslot and can be of 5 types: Normal, Frequency Correction, Synchronization, Access or Dummy.
Each burst can carry bits of different types: Information, Tail, Training Sequence.
We have eight timeslots, each user transmits within 1 / 8 of that time, periodically. The arrival time in each slot is then known.
Users are randomly located around the station, a closer and more distant, yet we can consider the propagation environment as being the same for everyone.
So if we know the time and speed that the signal travels, we calculate the distance!
And how to use this parameter, not only to just check how far we are from BTS?

Applications

A major application of this parameter, you control the time at which each mobile can transmit a burst of traffic within a timeslot in order to avoid collisions of transmissions of the other adjacent users.

The TA signal is transmitted in the SACCH as a number between 0 and 63, in units of bit periods (3.69 microseconds). If the signal travels at 300 meters per microsecond, each TA is a distance of approximately 1100 meters. Because this is the distance round, each increase in the value of TA corresponds to a distance 550 between the mobile and BTS.
For example, TA = 0 means that the mobile is up to 550 meters from the station, TA = 1 means it is between 550 and 1100 meters, TA = 2, from 1100 to 1650 meters and so on.

The maximum distance allowed by the TA between the MS and BTS is 35 km (GSM 850 / 900) * 63 or 550 meters.
So, for example during a test drive, we can measure how far we are from the BTS through the value of TA. He does not give us the position exta, but gives an accurate range of 550 meters.
Controlling interference by continually adjusting the TA, we have less data loss, and improve the quality of our signal.
As this is a parameter directly related to distance, it is natural that the TA is also used in locating applications.
Another good application is the handover control.
Imagine you have a cell that uses two concentric bands. You can set as a condition to allow the handover from one band to another.
More specifically: if you have a cell with 850/1900, you can set the band 850 as BCCH, and 1900 only to traffic. The TA threshold to control the terminal so it does not make for the 1900 handover if you're far from the BTS.

Extended Range

Despite the limitation of the GSM standard is 35 km as we speak, you can enable a feature that allows the TA is greater than 63. For this, the station receives the uplink signal in two adjacent timelots, instead of just one.

Conclusion

 This was a brief explanation of the parameter TA in GSM.

 What is RTWP? 

If you work with UMTS,'ve probably heard someone talk about RTWP. Its definition can be found in a dictionary of acronyms, such as http://acronyms.thefreedictionary.com/RTWP: Received Total Wideband Power.

Represents a measure of UMTS technology: the total level of noise within the UMTS frequency band of any cell.RTWP is related to uplink interference, and its monitoring helps control the call drops - mainly CS. It also has importance in the capacity management, as it provides information for the Congestion Control regarding Uplink Interference.In UMTS, the uplink interference may vary due to several factors, such as the number of users in the cell, the Service, Connection Types and Conditions of Radio, etc..

As our goal is to always be as simple as possible, we will not delve in terms of formulas or concepts involved. We will then know the typical values, and know what must be done in case of problems.

Typical Values

Ok, we know that RTWP can help us in checking the uplink interference, then we need to know its typical values.In a network is not loaded, normal, acceptable RTWP Average value is generally around -104.5 and -105.5 dBm.

Values around -95 dBm indicate that the cell has some uplink interferers.

If the value is around -85 dBm, the situation is ugly, with strong uplink interferers.

Usually we have High, Low and Medium measures of RTWP. However, the maximum and minimum values are recommended only as auxiliary or reference, since they may have been caused by a peak of access, or even been forced to have a momentary value due to some algorithm i.e..

Thus, the value that helps us, and has the most accurate information is the same Mean RTWP!

For cases in which cell has two carriers, the difference between them RTWP should not exceed 6 dB.

Based on these typical values, most vendors have an alarm: RTWP "Very High. "

What to do in case of problems?

We have seen that RTWP can cause performance degradation, mainly CS Call Drops. Note: Actually, it's not RTWP that causes performance degradation. What happens is that when its value is 'bad', it's actually indicating the presence of interference - the latter being responsible for degradation.

But what can we do when we find bad values?

If RTWP is not at acceptable levels, some actions should be taken.

• The first thing to do is check if there is a configuration issue with the RNC or NodeB. This is the most common case, especially in cases of new activations.
• Once verified the parameter settings, the next step is the physical examination, especially jumpers and cables, often partially reversed. It also should be checked if there is faulty transmitters, or any other problem that could generate intermodulation between the NodeB and the antenna.
• If the parameter settings and hardware are ok, the chance is very high that we have external interference, such as a Interferer Repeater.

In cases where there may be external interference, we must begin to act after such a prioritization based on how much this is affecting the cell KPI's across the network, if it carry high traffic, major subscribers, etc..

Note: There are many forms of interference in the uplink, both internal and external. Only a few are listed above. The deepening of all possibilities is beyond the goal of being simple to teach the concepts, but this is a suggestion for whoever wants to deepen the study, identification and elimination of interference.

In practice

to find - and eliminate - problems of interference is one of the biggest challenges in our area. For being such a complex problem, we recommend that be collected enough data for each investigation. Insufficient data collected can lead to erroneous conclusions, further worsening the problem.

The uplink interference may appear only in specific periods. Thus, it is recommended that data be collected from at least one week (7 days) for every 24 hours. Usually this amount of data is sufficient. In the figure below, we see different days and times - colorful - a fictional example where the interference occurred.

Data should be collected for the suspicious cell, but also for its adjacent cells, allowing it to make a triangulation increasing the chances of locating the source of interference.

Another way to locate the source of interference is to do a test in field. An antenna guy must gradually change the azimuth of the antenna, while another professional do RTWP measurements. That is, through the information directing the antenna and the respective values of RTWP, you can draw conclusions very good.

It is obvious that changing the online system may not be a good practice, and tests can be made with a Yagi antenna and a Spectrum Analyzer.

Vendors offer several ways to measure RTWP, using the OSS, performance counters and logs.

Conclusion

In this brief tutorial, we learn what is RTWP, and that the ideal typical value is about -104.5 dBm and -105.5 dBm.As the RTWP is directly related to Uplink Interference - and we know that interference is the main cause of performance degradation - have concluded that improving RTWP, ie making is as close as possible to -105 dBm, improving the Call Drop Rate!

IMPORTANT : Seizing the opportunity, see what was stated at the start of this tutorial - dictionary - by describing RTWP. Remember that this site has been the subject of a very interesting tutorial in the Tips Section. If you have not visited this section of the portal yet , I strongly recommend, because it has many issues that help in our growth in telecom and IT area.

What is HSN and MAIO in GSM? 

Today let's understand what are the parameters MAIO and HSN in a GSM network.

The terms MAIO and HSN are also often used, but many people are confused about it's planning. That's right, HSN and MAIO are used in frequency planning of a GSM network, and know them well naturally will lead us to better results.

Quickly: The HSN is used to define the hopping sequence from one frequency list, and MAIO is used to set the initial frequency on this list.

It did not help? So come on and try to understand better ...

Note: The goal here is not to teach HSN and MAIO planning, since this task involves many possible configurations and scenarios, which would escape the scope of our tutorial. The main goal today is to understand, in a planning already deployed, what they mean values MAY HSN and assigns.

Frequency Hopping e MA List

To understand how HSN and MAIO are used in planning, we first need to know some brief concepts.

• Frequency Hopping – or FH: one of the great advantages of the GSM system, in the constant search to reduce interference. More on the FH due to a new tutorial.

• MA List: set of frequencies (channels) assigned to a particular sector, ie are those channels that can be used to attend calls from users.

To illustrate, let's consider a sector with 4 TRX, where the first TRX is used for BCCH and the others are TCH TRX.

The MA List with the channels of traffic then would be:

HSN e MAIO

Sure, with the example in mind, let us return to our parameters.

First, the definition of HSN: Hopping Sequence Number. It is a number that defines the frequency hopping algorithm, and can vary from 0 to 63, ie there are 64 hopping algorithms to be used in GSM.

If HSN is zero, the frequency hopping sequence is cyclic, ie without changes.

If HSN is greater than zero, then frequencies vary pseudo-randomly.

When we enabled the Hopping - our case - all TRX in the SAME SECTOR has the SAME HSN. And if the we have 1x1 SFH it is recommended to have the SAME HSN for ALL SECTORS of the BTS.

In our example, the MA List is small - just three frequencies. The size of the MA List should be taken into account in the planning of HSN: HSN should be the designated so as to minimize the average probability of collision, according to the designated MAIOs.

And how MAIO's are designed?

Well, first defining MAIO: Mobile Allocation Index Offset. It's MAIO that designate the initial position of frequency - among the frequencies available in MA List, that list with the frequency hopping. It is the frequency that TRX uses so get hopping.

MAIO planning is straightforward if the number of TRX is small compared to the length of the sequence of hopping.

For example, MAY 0 means that the TRX should use the first frequency, or f1.

GSM Automatic Frequency Planning Tools

The concept of HSN and MAIO is important, and when the number of TRX and frequencies is small, we can even do planning 'byt hand'.

However, the best way - and always recommended - is to use network planning tools suitable for this purpose, as the AFP, from Optimi, or Ultima Forte, from Scheme.

These tools can be configured with measurements collected from the network (via BSS and / or Drive Test ), and with predictions (calculations) built in that allow the creation of a Interference Matrix. Based on this matrix, along with other algorithms, it allow a better design of parameters based on such critical conditions in traffic load and access. According to characteristics of each sector, they then provide the final planning, including the possibility of simulations.

Conclusion

Knowing the concept of HSN and MAIO we can use them correctly in our plans, and/or do audits of our existing networks. For example, in two hopping sequences, if we have the same HSN and different MAIO, we guarantee that they never overlap, or in other words, are orthogonal.

Another conclusion is that two channels with different HSN, but with the same MA List and at the same time slot, will interfere with 1 / n of bursts, where n equals the number of different frequencies in the hopping sequence. This conclusion is somewhat more complex to see, and is due to feature pseudo randomly from HSN. So if you have interest, deepen their studies of MAY and HSN. Otherwise, just understand that it is why we say that the Frequency Hopping somehow averages the interference across the network.

 What is Rake Receiver?

 

Have you ever heard of "Rake Receiver"? Surely you've heard of Receiver (Receiver in English), and you probably have heard of Rake (Rake in English).

So now you can imagine what can be a Rake Receiver?

If the analogy does not help much, come on.

In a wireless communication system, the signal can reach the receiver via multiple distinct pathways.

In each path, the signal can be blocked, reflected, diffracted and refracted. The signal of this many routes reach receivers faded. The Rake receiver is used to correct this effect, selecting  the correct / stronger signals, bringing great help in CDMA and WCDMA systems.

Okay, but what is the Rake Receiver, and how it does it?

Definition

The Rake Receiver is nothing more than a radio, whose goal is to try to minimize the effects of the signal fading due to multipath suffers when he travels. In fact, we can understand a set of Rake Receiver sub-radios, each lagged slightly, to allow the individual components of the multipath can be tuned properly.
Each of these components is decoded completely independently, but are combined in the final. It is as if we took the original signal, and adicionássemos other copies of the original signal reaching the receiver with different amplitudes and arrival times. If the receiver knows the amplitude and arrival time of each of these components, it is possible to estimate the channel, allowing the addition of components.
Each of these sub-radios Rake Receiver is called Finger. Each finger is responsible for collecting the energy of bit or symbol, hence the analogy with the groomer that we use in the garden, where each branch of the rake collecting twigs and leaves!

To ease some of the understanding, imagine two signal components arriving at the mobile unit as seen in the previous figure, with a lag ∆t among them.

Notice how each Finger works:

• The first with component g1 and time reference t;
• The second with component g2, but with the time reference t - ∆t.

The Fingers are so receptors that works independently with the function of demodulating the signal, ie, receive and remove the RF components from the information.

The big idea behind the methodology of combining multiple copies of the transmitted signal to obtain a better signal is that if we have multiple copies, probably at least one must be in good condition, and we have more chance of a better decoding!

Key Benefits

The main advantage of Rake Receiver is that it improves the SNR (or Eb / No). Naturally, this improvement is observed in larger environments with many multipaths than in environments without obstruction.
In simplified form: we have a better signal than we would have without using Rake Receiver! This is already a sufficient argument, isn't it?

Disadvantages and Limitations

The main disadvantage of Rake Receiver is not necessarily technical, and is not as problematic. This disadvantage is primarily because the cost of the receivers. When we insert one more radio receiver, we need more space and also increase complexity. Consequently, we increase costs.

The greater the number of multipath components supported by the receiver, the more complex is the algorithm. As we always do here, we will not be deducting formulas involved, but the complexity increases almost exponentially.

And in the real world, the amount of multipath components that arrive at the receiver is quite large, there is not a 'limit'. Everything will depend on the environment.

The threshold number of fingers in a mobile unit is determined by each technology standard, which for example in CDMA is 6, corresponding to the maximum number of channels to direct traffic that can be processed by the mobile unit at once (Active Set).

However, in cellular environments, most of the CDMA mobile units need only actually 3 of demodulators (WCDMA uses 4). More than that would be a waste of resources, and an additional cost to manufacture the phone.

Searcher

An important detail in the CDMA and WCDMA systems is the use of one finger of the Rake Receiver as a 'Searcher'. It is so called because of its function of seeking pilot signals being transmitted by any station (BS) in the system. These pilot signals can be understood as beacons used to alert the mobile, the presence of a BS.

Thus, in the UMTS UE(User Equipment), we have a simplified form of the configuration of the Rake Receiver as below.

Fingers on BS and UE

To conclude, the number of Rake Fingers used in the BS and the UE is generally different. That's because we saw that to have more fingers, the physical size of the receiver increases, as their power requirements. This can be a problem for the UE but not a problem for the BS, since it is able to offer more space and power for new fingers. It is only the criterion of cost to be taken into account in the BS.

Anyway, the only critical issue is with UE. But the current three/four fingers ensure excellent gain proven in practice (CDMA/WCDMA).