Spread spectrum:-
As the name suggest ,spread spectrum techniques involve spreading the band width needed to tansmit the data - which does not make sense at the first sight.Spreading the bandwidth has a several advantages.The main advantage is the resistance to narrow band interferance .Idealized narrowband signal from a sender of user data .The sender now spreads the signal ..i.e converts the narrow band signal to broad band signal.The energy neededto transmit the signal is the same , but it is now spread over a large frequncy range.The power level of the spread signal without losing data .Depending on the generation and reception of the spread signal, the power level of the signal can even be low as background noise.This makes it difficult todistingiush the user signalfrom background noise and thus hard to detect
During transmission , narrowband and broad band interferance add to signal.The sum of interferance and user signal is received .The receiver now knows how to de-spread the signal,converting the spread signal into narrowband signal again,while spreading the narrowband interferance and leaving the broadband interferance.The receiver can reconstruct the original data because the power level of the user signal is high enough ,ie the signal is much stronger than the reamining interferance.The folowing sections show how spreading can be performed.
The seperation of channels ,CDM is now used insted of FDM .This application shows the tight coupling of CDM and spred spectrum .Spreading the narrowband signal is acheived using a special code.
This blog is about mobile & wirless communication ,the working and basic requirements in mobile communications.Please fell free to put your suggestions or you can mail me @ rocky1171986 at gmail.com. Feel free to ask and comment...i lll be giving important topics on the subject as per university of mumbai.In this site some precise news and develpoments will also be put up, all will be in updates sections.All the updates will be done soon.
Regards,
Rocky
I will be putting the updates regarding mobile communications subject for semester 7 ,IT & com sci. I am in the process of scaning some important questions which ill be soon uploading!!!!!!!
CHAPTER NO :-1
INTRODUCTION
QUE 1:- Explain the need of wireless communication with its application ?
OUE 2:- mobile & wireless devices .
CH NO :- 2
WIRELESS TRANSMISSION
OUE 1: - Explain suitable waveforms BFSK without abrupt phase changes which belong to CPM scheme
QUE 2: - SPREAD SPECTRUM , cellular systems (if possible )
Security systems:
Wireless technology may supplement or replace hard wired implementations in security systems for homes or office buildings .They are also used to keep a track on visitors to the nearby area or as per the wish and settings.
Television remote control:
With the help of wireless communications a user can be able to switch television channels by just sitting on his place
Mobile telephone:
A latest gizmo which helps us to communicate with the desire unit/person anywhere ,it is the device which has made communication much easier.These instruments use radio waves to enable the operator to make phone calls from many locations world-wide. They can be used anywhere that there is a cellular telephone site to house the equipment that is required to transmit and receive the signal that is used to transfer both voice and data to and from these instruments.
Bluetooth:
One of the device which actual works on radiation ,helpful in sending and receiving data .The only drawback of this unit is the range is limited...
The term "wireless" has become a generic and all-encompassing word used to describe communications in which electromagnetic waves or RF (rather than some form of wire) carry a signal over part or the entire communication path. Common examples of wireless equipment in use today include:
Professional LMR (Land Mobile Radio) and SMR (Specialized Mobile Radio) typically used by business, industrial and Public Safety entities
Consumer Two Way Radio including FRS (Family Radio Service), GMRS (General Mobile Radio Service)
Amateur ("Ham") and Citizens band ("CB") radios
Consumer and professional Marine VHF radios
Cellular telphones and pagers: provide connectivity for portable and mobile applications, both personal and business.
Global Positioning System (GPS): allows drivers of cars and trucks, captains of boats and ships, and pilots of aircraft to ascertain their location anywhere on earth.
Cordless computer peripherals: the cordless mouse is a common example; keyboards and printers can also be linked to a computer via wireless.
Cordless telephone sets: these are limited-range devices, not to be confused with cell phones.
Satellite television: allows viewers in almost any location to select from hundreds of channels.
Wireless networking (i.e. the various flavors of unlicensed 2.4 GHz WiFi devices) is used to meet a variety of needs. Perhaps the most common use is to connect laptop users who travel from location to location. Another common use is for mobile networks that connect via satellite. A wireless transmission method is a logical choice to network a LAN segment that must frequently change locations. The following situations justify the use of wireless technology:
- To span a distance beyond the capabilities of typical cabling,
- To avoid obstacles such as physical structures, EMI, or RFI,
- To provide a backup communications link in case of normal network failure,
- To link portable or temporary workstations,
- To overcome situations where normal cabling is difficult or financially impractical, or
- To remotely connect mobile users or networks.
The "polarization" of an antenna is the orientation of the electric field (e-plane) of the radio wave with respect to the Earth's surface and is determined by the physical structure of the antenna and by its orientation. It has nothing in common with antenna directionality terms: "horizontal", "vertical" and "circular". Thus, a simple straight wire antenna will have one polarization when mounted vertically, and a different polarization when mounted horizontally. "Electromagnetic wave polarization filters" are structures which can be employed to act directly on the electromagnetic wave to filter out wave energy of an undesired polarization and to pass wave energy of a desired polarization.
Reflections generally affect polarization. For radio waves the most important reflector is the re-ionosphere signals which reflect from it will have their polarization changed unpredictably. For signals which are reflected by the ionosphere, polarization cannot be relied upon. For line of sigth for communication for which polarization can be relied upon, it can make a large difference in signal quality to have the transmitter and receiver using the same polarization; many tens of dB difference are commonly seen and this is more than enough to make the difference between reasonable communication and a broken link.
Polarization is largely predictable from antenna construction but, especially in directional antennas, the polarization of side lobes can be quite different from that of the main propagation lobe. For radio antennas, polarization corresponds to the orientation of the radiating element in an antenna. A vertical omni-directional wi-fi antenna antenna will have vertical polarization (the most common type). An exception is a class of elongated waveguide antennas in which vertically placed antennas are horizontally polarized. Many commercial antennas are marked as to the polarization of their emitted signals.
Polarization is the sum of the E-plane orientations over time projected onto an imaginary plane perpendicular to the direction of motion of the radio wave. In the most general case, polarization is elliptical (the projection is oblong), meaning that the antenna varies over time in the polarization of the radio waves it is emitting. Two special cases are linaer polariation (the ellipse collapses into a line) and circular polarization (in which the ellipse varies maximally). In linear polarization the antenna compels the electric field of the emitted radio wave to a particular orientation. Depending on the orientation of the antenna mounting, the usual linear cases are horizontal and vertical polarization. In circular polarization, the antenna continuously varies the electric field of the radio wave through all possible values of its orientation with regard to the Earth's surface. Circular polarizations, like elliptical ones, are classified as right-hand polarized or left-hand polarized using a "thumb in the direction of the propagation" rule. Optical researchers use the same rule of thumb, but pointing it in the direction of the emitter, not in the direction of propagation, and so are opposite to radio engineers' use.
In practice, regardless of confusing terminology, it is important that linearly polarized antennas be matched, lest the received signal strength be greatly reduced. So horizontal should be used with horizontal and vertical with vertical. Intermediate matchings will lose some signal strength, but not as much as a complete mismatch. Transmitters mounted on vehicles with large motional freedom commonly use circularly polarized antennas so that there will never be a complete mismatch with signals from other sources. In the case of radar, this is often reflections from rain drops.
