How modulation is done ??

It consists of a radio transmitter for digital data .The first step is the digital modulation of data into analog baseband signal according to one of the schemes presented in the following.The analog modulation then shifts the center frequency of analog signal up to the radio carrier .This signal is then transmitted via the antenna .

The model actually consist of digital modualtion box and analog modulation box.

The reciever receives the analog signal into analog baseband signal with the help of this known as carrier.This would be all that is needed for an analog radio tuned into the radio station .(the analog baseband signal would constitute the music)For digital data ,another step needed.Bits or frames have to be detected ,i.e the receiver must synchronise with the sender .How synchronisation is acheived ,depens on the digital modualtion scheme .After synchronistion ,the receiver has to decide the signal represents a digital 1 or 0 ,reconstructing the original data

The digital modulation schemes presented inthe following sections differ in many issues such as spectral efficeincy ,power efficiency ,and the robustness to multipath propagation

Modulation

This function has three parameters amplitude,frequency adn phase which may be varied in accordance with data or another modulating signal.For digital modulation which is the main topic in this section.Digital data 1 and 0 is translated into an analog signal (base band signal).Digital modualtion is required if digital data has to be transmitted over a medium that only allows for the system -to connect a computer to this system a modem is needed .The modem is needed ,the modem then performs the translation of didgital data to analog signals and vice versa.There are three basic methods for modulation - amplitude shift keying(ASK), frequency shift keying(FSK),phase shift keying(PSK).These will be discussed in more detail in following sections.

Apart from the translation of digital data into analog signal ,wireless transmission requires an additional modulation ,an anolog modulation that shifts the center frequency of the base band signal generated by the digital modulation upto the radio carrier.For example ,the digital modulation translates a 1 Mbps bitstream into a baseband signal cannot be directly transmitted in wireless systems

As for digital modulation ,three different basic schemes are known for analog modulation :amplitude modulation, frequency modulation, phase modualtion..The reader is reffered to Halsall(1996) ,for more details .

Modulation can be specified as:

g(t) = At * cos(2*3.142*t+ Pt)

Radio Interface

The most intresting interface in a GSM system is Um ,the radio interface ,as it coprises many mechanisms presented for multiplexing and media access.GSM implements SDMA using cells with BTS and assigns an MS to BTS .Futhermore ,FDD is used to seperate downlink and uplink as described.Medium access combines TDMA and FDMA .In GSM 900,124 channels ,each 200 khz wide are used for FDMA ,whereas GSM 1800 uses ,374 channels .Due to technical reasons ,channels are reserved for organisational data ; the reamaining 90 are used for customers .Each BTS then manages a single channel for organisational data and e.g ,upto 10 channels for used data .The following example based on GSM 900 systems ,but GSM works in a similar way at 1800 and 1900 Mhz.
Each of 248 channels is additionally seperated intime via a GSM TDMA frame ,i.e each 200 khz carrier is subdivided into frames that are repeated continously .The duration of a frame is 4.615 ms .A frame is again subdivided into 8 GSM time slots ,where each slot represnets a physcial TDM channel and lasts for 577 ms .Each TDM channel occupies the 200 khz carrier for 577 micro sec every 4.615 ms.
Data is transmitted in small portions ,called bursts , sometimes also called normal bursts as used for data transmission inside time slot.The bursts only of 546.6 micro sec long and contains 148 bits.The reamaning 30.5 micro sec are used as guard spaces to avoid overlapping with other bursts.

More on GSM architecture

Mobile station (MS):
The MS comprises all user equipment and software needed for communication with GSM network .An MS consist of user independent hard end software and of the subscriber identity module(SIM), which stores all the user-specific data that is relevant to GSM .While an MS can be identified via international mobile equipment idenetity (IMEI), a user can personalize any MS using his/her SIM.i.e user-specific mechanism like charging and authentication are based on SIM,not on the device itself.Device -specific mechanism,e.g theft protection ,use the device specific IMEI.Without the SIM,only emergency calls are possible .The SIM contains many identifiers and tables ,such as card type ,serial number ,a list of subscribed services , a personal identity number (PIN),a PIN unblocking key(PUK),an authenticatin key and international mobile subscriber identity (IMSI).The PIN is used to unlock the MS .Using the wrong PIN three times will lock the SIM.Insuch cases,the PUK is needed to unlock the SIM .The MS stores dynamic information while logged into the GSM systems,such as ,e.g the cipher key and location information consisting of temporary mobile subscriber identity (TMSI)and the loacation area identification (LAI).Typical MSs for GSM 900 have a transmit power of upto 2 W ,whereas the GSM for 1800 1 W is enough due to the smaller cell size .Apart from telephone interface ,MS can also offer other type of interfaces to users with display ,loudspeaker, microphone ,and programmable soft keys .Further interfaces comprise many more vendor specific functions and components ,such as cameras ,finger printsensors,calenders,address books ,games,internet browsers.Personal digital assistants (PDA) with mobile phone functions are also available .The reader should be aware that an MS could also be integrated into a car or be used for location tracking of a container.

Near and Far terminals

Consider the situation as ,A & B are both sending with the same transmission power .As the signal strength decreases proportionally to the square of distance between them,B s signal drowns A s signal.As a result ,C cannot receive A transmission.
Now think of C as being an arbiter for sending rights. In this case ,termiinal B would already drown ut terminal A on physcial layer .C in return would have no chance of applying a fair scheme as it would only hear B.
Then near/far effect is a sever problem of wireless networks using CDM.All sognals should arrive at receiver with more or less the same strength.Oherwise a person standing closer to somebody could always speak louder than a person further away .Even if the sender were seperated by code,the closest one would simply drown out others .Precise power control is needed to receive the senders with the strength as that of receiver.For eg ,the UMTS system presented in chapter adapts power 1500 times persecond

Medium access control

This includes medium access control(MAC) algorithms which were specificially adapted to the wireless domain .Medium access control consist all mechanism that regulate user access to a medium using FDM,TDM,SDM.MAC is thus similar to traffic regulations in the vehicals used in same street crossing TDM, it reqires rules to cross TDM.
IT includes hidden and exposed terminals:
Consider a scenario with 3 mobile phones as assumption .The transmission ranges of mobile A can reach to B but not C.On the other hand transmission range of C cant reach A ,it can only reach B .Finally transmission range of B can reach both A & C.
A starts sending to B , C does not receive any transmissoin .C also wants to send something to B.But it detects that the medium is busy,so carrier ssense fails .C also starts sending to B ,thus causing collision at B .But A cannot detect this collision and continues to send to B.
Thus A is hidden for C and vice versa
While hidden terminals may cause collision .the next effect only causes necessary delay .Now consider the situation that B sends something to A and want to trnsmit data to some other mobile phones outside the interferance range of A and B.C senses the carrier and detecs htat the carrier is busy .C postpones the its transmission until it detects the medium being idle again .But as A is outside the interferance of C ,waiting is not necessary .Causing collison at B does not matter much because the collison is too weak to resist to A.In this situation C is exposed to A.