SACCH Frame in GSM

SACCH Frame holds special importance in GSM as it contains vital control information regarding the network. The SACCH is transmitted after 12 frames (no 13) of a 26 Traffic Multiframe. 12 frames before and after SACCH carry voice traffic whereas the 26th frame is idle that distinguishes two traffic MFs. SACCH is thus called an associated channel as it traverses along with the voice traffic.

SACCH contains signaling information such as Power control information, timing advance (TA) and other control related data. 4 SACCH frames form a SACCH Multiframe (480 ms). SACCH frames are used in the triggering of Radio-Link Timer (RLT) Counter that is directly related with the quality of a call. If a SACCH frame is not decoded sucessfully, RLT Counter is decremented by 1, whereas successful decoding of the counter increments it by 2. The call drops when RLT Counter is reaches the value of zero.

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