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.