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.

Cell Size

The number of cells in any geographic area is determined by the number of MS subscribers who will be operating in that area, and the geographic layout of the area (hills, lakes, buildings etc).

Large Cells: If the cell site is on top of a hill, with no obstructions for miles, then the radio waves will travel much further than if the cell site was in the middle of a city, with many high rise buildings blocking the path of the radio waves.

Generally large cells are employed in:

• Remote areas.
• Coastal regions.
• Areas with few subscribers.
• Large areas which need to be covered with the minimum number of cell sites.

Small Cells: Small cells are used where there is a requirement to support a large number of MSs, in a small geographic region, or where a low transmission power may be required to reduce the effects of interference. Typical uses of small cells:

• Urban areas.
• Low transmission power required.
• High number of MSs.

The Trade Off – Large vs Small: There is no right answer when choosing the type of cell to use. Network providers would like to use large cells to reduce installation and maintenance cost, but realize that to provide a quality service to their customers, they have to consider many factors, such as terrain, transmission power required, number of MSs etc. This inevitably leads to a mixture of both large and small cells.

GNU Radio Companion - A useful software

Software radio is the technique of getting code as close to the antenna as possible. It turns radio hardware problems into software problems. GNU Radio is a collection of software that when combined with minimal hardware, allows the construction of radios where the actual waveforms transmitted and received are defined by software. The signal processing blocks are implemented in C++. Conceptually, blocks process infinite streams of data flowing from their input ports to their output ports.

To aid beginners, Josh Blum of Johns Hopkins University, has developed a graphical interface for GNU Radio. This GUI termed GNU Radio Companion (GRC) allows users to interact with GNU Radio signal blocks in a manner similar to Labview or Simulink. The entire interface encompasses over 150 blocks from the GNU Radio Project. Blocks are manually integrated into GRC via descriptive python definitions. The definitions are very flexible, and allow multiple GNU Radio blocks to be grouped into a single GRC super-block. A simple AM transmitter and receiver designed in GRC is presented below. The input sound can be given through a mic via sound card of your PC and heard on the receiver PC. Thus a complete and simple communication system can be realized in few minutes with no extra hardware at all.

Recommended Readings:

[1] http://www.comsec.com/software-radio.html

[2] http://savannah.gnu.org/projects/gnuradio

Simulink to Microcontroller!

The Simulink 'Blockset for Microchip dsPIC / PIC24 / PIC32 Microcontrollers' generates a C code from the Simulink model for a wide list of Microchip microcontrollers. The generated C code is compiled on the fly to obtain the executable binary file (.hex and .coff file), ready to download into the targeted microcontroller. The downloaded binary program behaves like the Simulink model. 

The Blockset for your version of MATLAB can be downloaded from

http://www.kerhuel.eu/wiki/Download

For a tutorial video on this, goto (please create login to view the video)

http://vimeo.com/6481193

For Easy Interfacing in Projects (Using Simulink/LabView)

NI USB-6009

14-Bit, 48 kS/s Low-Cost Multifunction DAQ

• 8 analog inputs (14-bit, 48 kS/s)
• 2 analog outputs (12-bit, 150 S/s); 12 digital I/O; 32-bit counter
• Bus-powered for high mobility; built-in signal connectivity
• OEM version available
• Compatible with LabVIEW, LabWindows/CVI, and Measurement Studio for Visual Studio .NET