Multiple access and multiplazione
1) Techniques to multiple access:
The techniques to multiple access have the scope to concur more customers dislocate in various places of the space the access to you to a resource placed in an only place of the space.
2) FDMA:
The various channels differ for the frequency bands that they employ.
3) TDMA:
The time is subdivided in wefts and every weft in slot, to every slot is associated a channel in which a customer can insert its data. In kind a guard interval is previewed also whom account of the various delays of propagation holds that they can endure marks them coming from from various places.
Weft TDMA is constituted from a preamble, of the bit of information and the trail bits.
The bit of information is constituted from Time slot, ciascuna di.le which contain of the trail bits, of the bit of synchronization, the information and of the bit of guard that as said they hold account of the various delays of propagation.
4) CDMA:
To every channel spectrum is associated a various orthogonal code spread, this is obtained by means of a generator PRBS
5) Techniques two-station communication:
There are technical two-station communication to division of time and division of frequency, in the FDD the up-link the down-link happens to a frequency and to an other frequency while in the TDD to the up-link one is associated slot and to down-link an other slot.
6) Multiplazione:
The multiplazione differs from the multiple access in how much in this case all the customers of the resources is locati in the same place and therefore they can be sincronizza you.
7) Typology of multiplazione:
TDM Time Division Multiplex
FDM Frequency Division Multiplex
Both but are not in a position to effectively dealing the case of wefts with bit variable installments, what that instead can very be implemented through the multiplex statistical in which the multiplex it is connected to the buffer of the various channels, before passing to the buffer successive, empties the buffer puts into effect them, in this way channels do not use see their resource you riallocata in constructive way. It is but necessary that the packages contain a identificativo of the channel to which they correspond.
8) Alternatives for the CDMA:
We have the pure CDMA and the hybrid CDMA these to they time is subdivided ulteriorly in other categories, in particular the pure CDMA is subdivided in DS, TH, FH while the hybrid CDMA is subdivided in MC and TD.
9) DS-CDMA:
DS is for Direct Sequence, tightens it to send comes multiplied for a having key a frequency of chip the much highest one, in this way is obtained marks them to extensive phantom that however is distinguished regarding those products with different keys, the information only is spalmata on one wider phantom. A disturbance to tight band comes increased from the demodulatore while it marks them useful ridiviene to tight band and can easily therefore be leaked.
10) TH-CDMA:
The time is subdivided in wefts, every channel has associated a sequence of knows to you therefore in a weft will come transmitted in correspondence of a Time data slot while for an other weft it is transmitted in correspondence of an other Time-slot, the sequence must naturally be famous is to the emitter that to the receiver. Being the transmission in CDMA, it is had more that also than a channel it can be transmitted to the inside of the same Time-slot.
11) FH-CDMA:
The band to disposition is subdivided in sottobande, on the base of a randomica sequence, in a data moment transmits in CDMA to a frequency and in a successive moment I always transmit in CDMA to an other frequency, in the same moment but an other emitter cannot transmit to the same frequency.
12) Sequence to maximum length:
Bit of which can be obtained as an example with a registry to sliding to 3 makes the sum module 2 of last the two bit and the filler in income.
13) orthogonal Codes:
Obtained orthogonal codes are as an example those of Walsh-Hadamard in ricorsiva way by means of the matrix:
.
14) CDMA, relationship andB/ N0 :
The noise is 
that is thermal noise
more which had noise to interference, this last one for a data
finishes has had them to all the other customers therefore (U-1)
everyone of which transmits an energy andS for every symbol, therefore the relationship between andS and the noise total are 
, considering then che 
obtains the
relationship between the energy for bit and the density of 
noise where 
is the factor of spreading
and also has replaced 
where RS is the binary rhythm to the escape of the modulator
while RSS is the binary
rhythm to the escape of the spreader.
15) Sistemi to access random:
In kind the number of channels is much inferior regarding the number of customers, therefore it is necessary to realize a mechanism to contesa in order adjudicating the channels available, two are the adopted main techniques:
To) Aloha
the package comes sended in a
moment whichever from finishes them mobile, if not there are
collisions then the radio station base sends marks them of received
otherwise evidently does not send it, to that point begins a randomico
Time-out after which the package comes sent back. The fraction
of packages transmitted with happening comes defined Throughput of the
system, it is worth: 
where G is the offered
traffic admitting distribution of the packages of Poisson and equal
length for all, the maximum percentage of packages transmitted with
happening is 18%.
B) Slotted Aloha
differs from the ALOHA in how much must be present a base weft
that is a system synchronization, in such a way it is succeeded to
make that if two packages collidono, collidono completely and
therefore the number of the collisions is reduced, in particular the
fraction of packages transmitted with happening is 
, in this case the obtainable maximum is 36%.
16) PRMA:
It is a system to contesa that to regimen it is reduced to a TDMA, in short has a weft to the inside of which the Time slot are not associate you to no finish them, therefore in correspondence of a free Time-slot, every finishes them can reserve, if the SRB the confirmation the reservation then that Time slot is to assigned it or until to the term of its transmission (…implicit reservation) or for a predetermined time (…reservation esplicita).pertanto in that Time-slot the others finishes them cannot send given sin when it does not come freed.
17) SDMA:
It is one technical to multiple access to space division, is to the base of the cellular concept, it takes advantage of the selettività of the antenna of the SRB that therefore can only serve customers who find themselves in a determined zone of the space.