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Access Scheme
FDMA
- In Frequency Division Multiple Access, the frequency band is divided in slots. Each user gets one
frequency slot assigned that is used at will. It could be compared to AM or FM broadcasting radio
where each station has a frequency assigned. FDMA demands good filtering.
TDMA
In Time Division Multiple Access, the frequency band is not partitioned but users are allowed to use
it only in predefined intervals of time, one at a time. Thus, TDMA demands synchronization among
the users
Access Scheme (CDMA)
CDMA
- CDMA, for Code Division Multiple Access, is different than those traditional ways in that it
does not allocate frequency or time in user slots but gives the right to use both to all users
simultaneously. To do this, it uses a technique known as Spread Spectrum****. In effect, each
user is assigned a code which spreads its signal bandwidth in such a way that only the same
code can recover it at the receiver end. This method has the property that the unwanted
signals with different codes get spread even more by the process, making them like noise to
the receiver.
- CDMA (Code-Division Multiple Access) refers to any of several protocols used in so-called
second-generation (2G) and third-generation (3G) wireless communications. As the term
implies, CDMA is a form of multiplexing, which allows numerous signals to occupy a single
transmission channel, optimizing the use of available bandwidth. The technology is used in
ultra-high-frequency (UHF) cellular telephone systems in the 800-MHz and 1.9-GHz bands.
- CDMA employs analog-to-digital conversion (ADC) in combination with spread spectrum
technology. There are trillions of possible frequency-sequencing codes, which enhances
privacy and makes cloning difficult.
Idea of Communication with Coding Technique
Communication Channel
d
1
.C
1
2
.c
2
3
.c
3
4
.c
4
d
d
d1 d
Data Representation in CDMA
Data Bit 0 → -1 Data Bit 1 → +1 Silent -> 0
D4.c
D2.c
d1.c
D3.c
Idea of Communication with Coding
Technique
Communication Channel
[ -1 , -1, -3 , +1 ]
Silent
Bit 1
Bit 0 Bit 0
Data Representation in CDMA
Data Bit 0 → -1 Data Bit 1 → +1 Silent -> 0
Chip Sequence
[+1, +1, +1 , +1] [+1, -1, +1 , -1] [+1, +1, -1 , -1] [+1, -1, -1 , +1]
C
1
C
2
C
3
C
4
[ -1, -1, -1 , -1] [-1, +1, -1 , +1]
[0 , 0 , 0 , 0 ] [+1 , -1 , -1 , +1]
CDMA Coding scheme
If sender0 has code (1,–1) and data (1,0,1,1), and sender1 has code (1,1) and data
(0,0,1,1), and both senders transmit simultaneously, then this table describes the
coding steps:
Step Encode sender0 Encode sender
0 vector0=(1,-1),data0=(1,0,1,1)=(1,-1,1,1) vector1=(1,1),data1=(0,0,1,1)=(-1,-1,1,1)
1 encode0=vector0.data0 encode1=vector1.data
2 encode0=(1,-1).(1,-1,1,1) encode1=(1,1).(-1,-1,1,1)
3 encode0=((1,-1),(-1,1),(1,-1),(1,-1)) encode1=((-1,-1),(-1,-1),(1,1),(1,1))
4 signal0=(1,-1,-1,1,1,-1,1,-1) signal1=(-1,-1,-1,-1,1,1,1,1)
- Because signal0 and signal1 are transmitted at the same time into the air, they add to
produce the raw signal: (1,-1,-1,1,1,-1,1,-1) + (-1,-1,-1,-1,1,1,1,1) = (0,-2,-2,0,2,0,2,0)
CDMA Coding scheme
This raw signal((0,-2,-2,0,2,0,2,0)) is called an interference pattern. The receiver
then extracts an intelligible signal for any known sender by combining the sender's
code with the interference pattern, the receiver combines it with the codes of the
senders. The following table explains how this works and shows that the signals do
not interfere with one another:
Step Decode sender0 Decode sender
0 vector0=(1,-1), pattern=(0,-2,-2,0,2,0,2,0) vector1=(1,1), pattern=(0,-2,-2,0,2,0,2,0)
1 decode0=pattern.vector0 decode1=pattern.vector
2 decode0=((0,-2),(-2,0),(2,0),(2,0)).(1,-1) decode1=((0,-2),(-2,0),(2,0),(2,0)).(1,1)
3 decode0=((0+2),(-2+0),(2+0),(2+0)) decode1=((0-2),(-2+0),(2+0),(2+0))
4 data0=(2,-2,2,2)=(1,0,1,1) data1=(-2,-2,2,2)=(0,0,1,1)
Further, after decoding, all values greater than 0 are interpreted as 1 while all
values less than zero are interpreted as 0. For example, after decoding, data0 is (2,-
2,2,2), but the receiver interprets this as (1,0,1,1).
Synchronous CDMA Asynchronous CDMA
They use orthogonal codes. It use unique "pseudo-random" or
"pseudo-noise" (PN) sequences.
completely reject arbitrarily strong signals
using different codes, due to the
orthogonality of these systems
This is not true for Asynchronous CDMA;
rejection of unwanted signals is only partial.
It can’t use the spectrum more efficiently in
mobile telephony applications.
It can use the spectrum more efficiently in
mobile telephony applications.
No such flexibility in allocation of
resources.
offers a key advantage in the flexible
allocation of resources
There are a fixed number of orthogonal
codes, timeslots or frequency bands that
can be allocated for CDM,
There is no strict limit to the number of
users that can be supported in an
Asynchronous CDMA system
Synchronous CDMA is ideally not suited to
a mobile network where large numbers of
transmitters each generate a relatively
small amount of traffic at irregular intervals
Asynchronous CDMA is ideally suited to a
mobile network where large numbers of
transmitters each generate a relatively
small amount of traffic at irregular intervals
Advantages
Can share a common bandwidth without
interfering each other.
Flexible network planning (planning is no longer needed)
Greater coverage (larger area for a given amount of power )
High capacity (greater coverage capacity)
Cost (larger profit for providers due to increased capacity, less infrastructure)
Clarity
Customer satisfaction (privacy, better call quality longer battery life due to
less power consumption, prevent cross talks)
Compatibility (dual mode analog and digital)
CDMA Applications
A p p lic a t io n s :
G P S O n e ® P o s it io n L o c a t io n T e c h n o lo g y
G P S O n e ® P o s it io n L o c a t io n T e c h n o lo g y
P o s itio n lo c a tio n c o n c e p t a p p lic a tio n s.
G e t d ir e c tio n s o n th e m o v e
G e t d ir e c tio n s o n th e m o v e
L o c a te a lo s t p e t
L o c a te a lo s t p e t
- P r o v id e tr a ff ic a n d n a v ig a tio n s e r v ic e sP r o v id e tr a ff ic a n d n a v ig a tio n s e r v ic e s
L o c a te V e h ic le s a n d A s s e ts c o n c e p ts
L o c a te V e h ic le s a n d A s s e ts c o n c e p ts
a p p lic a tio n s
a p p lic a tio n s
Q t v ™ S t r e a m in g V id e o a n d A u d io
Q t v ™ S t r e a m in g V id e o a n d A u d io
S u p e r io r p la y b a c k r a te s in a f u lly in te g r a te d
s o f tw a r e s o lu tio n
2 D a n d 3 D G a m in g E n g in e
2 D a n d 3 D G a m in g E n g in e
L if e - lik e a n im a tio n in 2 D a n d 3 D e n v ir o n m e n ts.
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