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....