Cellular systems

Cellular systems for mobile communications implement using SDM.

• Each transmitter is called base station ,covers a certain area of cell.
• Cell radii can vary from tens of meters in buildings,100 mtrs in cities ,upto some kilometers in the country side.
• The shape of cells are never perfect circles or hexagons , but depend on envoirnment(buildings,mountains,valleys,etc), on weather conditions , sometimes on system -load.

Here a mobile station within a cell around a base station communictes with the base station and vice versa.

Advantages of cellular systems with small cells are the following:-

• Higher capacity:-Implementing SDM allows frequency reuse.If one transmitter is far away from another ,i.e outside the interferance range it can reuse the same frequency.As most of mobile phone systems assign frequencies to certain users ,this frequency is blocked for other users .But as frequency is a scarce resourse the number of concurent users per cell is also very limited.
• Less transmission power :- while power aspects are not a big problem for base stations , they are indeed a problemetic for mobile stations.A receiver far away from the base station would need much more transmit power than the current few watts.But energy is a serious problem for mobile handling devices.
• Local interferance only:- Having long distances between sender and receiver results even more interferance problems.With the small cell mobile stations and base stations only have to deal with local interferance.
• Robustness:- celllular systems are decentralized and so ,more robust again the failure of single components.If one antenna fails,this only influence communication within small area.

Small cells also have some disadvantages:

• Infrastructure needed: Cellular systems need a complex infrastructure to connect all base stations .This inculdes many antennas ,switches for forwarding .location registers to find a mobile station etc,which make the wholw system quite expensive.

• Handover needed :The mobile station has to perform a handover when changing from one cell to another.Depending on the cell size and the speed of movement ,this can happen quite soon
  

Working of CDMA

The following steps are generally performed in the process of CDMA technique:-

• Two senders ,A and B want to send data.CDMA assigns the following unique and orthogonal key sequences: key Ak =010011 ,key Bk = 110101 for sender B .Sender A wants to send the bit Ad = 1, sender B sends Bd= 0.To illustrate this example ,let us assume that we code a binary 0 = -1 ,a binary value 1 as +1.We can apply the standard addition and multiplication rules.
• Both the senders spread their signals using their key as chipping sequence (the term spreading here refers to the simple multiplication of the data bit with the whole chipping sequence.).In reality ,parts of much longer chipping sequence applied to single bits for spreading .Sender A then sends the signal As = Ad * Ak = +1 *(-1,+1,-1,-1,+1,+1) = (-1, +1,-1,-1,+1,+1).Sender NB does the same with its data to spread the signal with code : Bs = Bd *Bk = -1 *(+1,+1,-1,+1,-1,+1) = (-1, -1,+1,-1,+1,-1).
• Both the signals are then transmitted at the same time using the same frequency ,so the signals superimpose in space (analog modulation is neglected in this example).Discounting interferance from other sender and envoirnmental noise from the simple example,and assuming that the signals have the same strength at the receiver : C = As + Bs =(-2, 0, 0,-2,+2,0).
• The receiver now wants to receive the data from the sender A and , therefore tunes into the code of A, i.e it applies A's codes for de-spreading .As a result is much larger than 0, the receiver detects a binary 1.Tuning in to sender B, i.e applying B's code gives C * Bk = (-2,0,0,-2,+2,0)*(+1,+1,-1,+1,-1,+1) = -2 + 0+0-2-2+0 =- 6.The result is negative, so 0 has been detected.
• This is the way in which one CDMA technique is to be performed ,this example involved several simplifications.

CDMA technique

Finally ,codes with certain characterstics can be applied to the transmission to enable the use of code division multiplexing (CDM).Code division multiplexing access(cdma) systems use exactly these codes to seperate different users in code space and to enable acess to a shared medium without interferances .The main problem is how to find good codes and how to seperate the signal from noise generated by other signals and the envoirnment.
Earlier ,how the codes for spreading a signal could be used.The code directly controls the chipping sequence .But is a god code for CDMA ?.A code for a certain user should have a good autocorreleation and should be orthogonal to other codes.Orthogonal in code space has same meaning as in standard space(in three- dimensional space).Think of systems of co-ordinates and vectors starting at origin ,i.e in (0, 0 ,0).Two vectors (2,5,0)and (0,0,17) = 0+0+0.But also vectors like (3,-2,5) and (-2, 3,3) are orthogonal :(3,-2,5)*(-2,3,3) = -6 -6 +12 =0, thus they are orthogonal.
They must also be or hav an auto- correleation between them.
The barker code for example has(+1,-1,+1,-1,+1,-1,-1,-1), has a good auto co-releation i.e the inner product with intself is large, the result is 11.This code is used for ISDN and IEEE 802.11 .But as soon as this Barker code is shifted 1 chip further, the co-releation drops to an absolute value of 1. It says at this low value until the code matches itself again perfectly.This helps ,for example , to synchronize a receiver with incoming data stream.The peak in matching process helps the receiver to reconstruct the original data precisely,even if noise distorts the original signal up to certain level.

Multi-carrier modulation

Special modulation scheme that stands some what apart from others are multi-carrier modulation scheme(MCM),orthogonal frequency division multiplexing (OFDM)or coded OFDM (COFDM) that are used in contxt of European digital radio system DAB and the WLAN standards ,IEEE 802.11a and hiperLan2 .The main attraction of MCM is its good ISI mitigation property.As explained,higher bit rates aare more vulnerable to ISI . MCM splits the high bit rate stream into many lower bitrate streams,each stream being sent using an independent carrier frequency. If for example,n symbols/s have to be transmitted, each subcarrier transmits n/c symbols/s with c being the number of subcarriers. One symbol could for example represent 2 bit as in QPSK .DAB ,foe example ,uses in between 193 and 1538 of these sub-carriers. The physcial layer of HiperLan2 and IEEE 802.11a uses 49 subcarriers for data.
In this there is actually the superposition of orthogonal frequencies.The maximum of one subcarrier frequency appers exactly ata frequency where all other sub-carriers are equal to zero.
Using this scheme , frequency selective fading only influences some sub-carriers and not the whole signal - an additional benefit on MCM. Typically ,MCM transmits the symbol with guard spces between single symbols or groups of symbols .This helps the receiver to handle multi-path propagation .OFDM is a special method of implementing MCM using orthogonal carriers.Computationally , it is a very efficeint algorithm based on fast fourier transform (FFT) for modulation or demodulation.

Advanced frequency shift keying(AFSK)

A famous scheme used in many wireless systems is minimum shift keying (MSK).MSK is basically BFSK without abrupt phase changes i.e it belongs to CPM schemes .In this data bits are first seperated into even and odd bits ,the duration of each bit being doubled.This scheme also uses two frequencies :f1- lower frequency and f2-- higher frequency,the higher frequency is twice that of higher frequency,f2= 2 f1.
According to this scheme ,the lower or higher frequency is chosen to generate the MSK signal:
1.If the even and the odd bit are both 0 ,then higher frequencies f2 is inverted.(i.e f2 is used with phase shift of 180 deg).
2. If the even bit is 1 ,odd bit is 0 then the lower frequency f1 is inverted .This is the case ,eg ,in the fifth to seventh columns.
3. If the even bit is 0 and the odd bit is 1 , f1 is taken without changing the phase .
4. If both bits are 1 then original f2 is taken.
A high frequency is always chosen if even and odd bits are equal.The signal is inverted if odd bit equals 0.This scheme avoids all pahse shift in resulting MSK signal.

Phase shift keying(PSK)

Finally ,phase shift keying (psk) usese shifts in the phase of signal to represent the data.There is a phse shift of of 180 degress or pi as 0 foolows the 1.Same happens when 1 follows the 0.This simple scheme shifting the pahse by 180 degres each time the value of data changes,is also called binary PSK(BPSK).A simple implementation of a BPSK modulator could multiply ,a frequency f with +1 if the binary data is 1 and with -1 if the binary data is 0.
To receive the signal correctly, the receiver must synchronous in frequently and phase with transmitter.

Frequency shift keying(FSK)

Modulation scheme often used for wireless transmission is frequency shift keying(FSK).The simplest form of FSK ,also called binary fsk(BFSK),assigns one frequency f1 to binary 1 and another to frequency f2 to binary 0.A very simple way to implement FSK is to switch between oscillators ,one with the frequency f1 and other with frequency f2,depending on the input.To avoid sudden changes in phase ,special frequency modulation with continous pahse modulations (cpm) can be used .Sudden changes in phase cause high frequencies ,which is an undesired side - effect.
A simple way to implement demodulation is by using two bandpass filters ,one for f1and the other for f2.A comparator can then compare the signal evels of the filter outputs to decide which of them is stronger.FSK needs a larger band-width compared to ASK but is much less suscepilble to errors.
It has a wide sinusoidal graph in it....

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.