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

Synthetic Aperture Radar

A Synthetic Aperture Radar (SAR), or SAR, is a coherent mostly airborne or spaceborne sidelooking radar system which utilizes the flight path of the platform to simulate an extremely large antenna or aperture electronically, and that generates high-resolution remote sensing imagery. Over time, individual transmit/receive cycles (PRT's) are completed with the data from each cycle being stored electronically. The signal processing uses magnitude and phase of the received signals over successive pulses from elements of a synthetic aperture. After a given number of cycles, the stored data is recombined (taking into account the Doppler effects inherent in the different transmitter to target geometry in each succeeding cycle) to create a high resolution image of the terrain being over flown.

Reference: http://www.radartutorial.eu/20.airborne/ab07.en.html

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.

GSM: Idle Mode Behavior

The flow diagram shows the overall idle mode processes. A powered on mobile station (MS) that does not have a dedicated channel allocated is defined as being in idle mode. While in idle mode it is important that the mobile is both able to access and be reached by the system. The idle mode behavior is managed by the MS. It can be controlled by parameters which the MS receives from the base station on the Broadcast Control Channel (BCCH). All of the main controlling parameters for idle mode behavior are transmitted on the BCCH carrier in each cell. These parameters can be controlled on a per cell basis. Moreover, to be able to access the system from anywhere in the network, regardless of whether the MS is powered on/off, it has to be able to select a specific GSM base station, tune to its frequency and listen to the system information messages transmitted in that cell. It must also be able to register its current location to the network so that the network knows where to route incoming calls.

The PLMN selection mechanism, the cell selection and reselection algorithms in addition to the location updating procedure are the core of the idle mode behavior. Its purpose is to always ensure that the mobile is camped on the cell where it has the highest probability of successful communication. In idle mode the MS will notify the network when it changes location area by the location updating procedure. Thus, the network will be kept updated concerning which location area the MS is presently in. When the system receives an incoming call it knows in which location area it should page the MS, and does not need to page the MS throughout the whole MSC service area. This reduces the load on the system. If the MS does not respond to the first paging message, then the network can send a second paging message.

Sometimes MS does not camp on the best cell and needs to perform a cell re–selection process before initializing the call. This could be related to wrong Cell Reselection parameters like CRO – Cell Reselect Offset, Cell Reselect Hysteresis (CRH), Temporary Offset (TO) or Penalty Time (PT).