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.