CDMA

Generalidades
• Modulación
™QPSK
¾ No usa una envolvente constante, con cambios de amplitud al cambiar la fase para pasar de un estado
a otro.
• Acceso múltiple
™Todos los usuarios pueden transmitir al mismo tiempo.
™Los usuarios se identifican en base a un código.
¾ Se asignan códigos ortogonales (Walsh) a los usuarios, de tal forma que el receptor pueda recuperar
la transmisión destinada a una unidad individual a partir de múltiples transmisiones.
™Los usuarios pueden transmitir y recibir en cualquier momento.
¾ No
N existe
i t un dominio
d i i en ell tiempo
ti
(solo
( l para sincronización)
i
i ió ) y existe
i t ell uso de
d canales
l lógicos.
ló i
™Dado que la reutilización de frecuencias no constituye problema alguno, añadir celdas a la red
es substancialmente mas fácil.
¾ Esto hace fácil llenar huecos en la cobertura.
¾ Es fácil añadir celdas para cubrir eventos especiales.
Marzo 2007
Iván Bernal, Ph.D.
16
Spread Spectrum (DSSS)
Marzo 2007
Iván Bernal, Ph.D.
17
1
DSSS Using BPSK
• Multiply BPSK signal
sd(t) = A d(t) cos(2π fct)
by c(t) [takes values +1, -1] to get
s(t) = A d(t)c(t) cos(2π fct)
¾ A = amplitude of signal
¾ fc = carrier frequency
¾ d(t) = discrete function [+1, -1]
• At receiver, incoming signal
multiplied by c(t)
™Since c(t) x c(t) = 1,
™Since,
1 incoming signal is
recovered
Marzo 2007
Iván Bernal, Ph.D.
18
Code--Division Multiple Access (CDMA)
Code
• Basic Principles of CDMA
™D = rate of data signal
™Break each bit into k chips
¾ Chips are a user-specific fixed pattern
™Chip data rate of new channel = kD
• If k=6 and code is a sequence of 1s and -1s
™For a ‘1’ bit, A sends code as chip pattern
¾ <c1, c2, c3, c4, c5, c6>
™For a ‘0’ bit, A sends complement of code
¾ <-c1, -c2, -c3, -c4, -c5, -c6>
• Receiver knows sender’s code and performs electronic decode function
S u (d ) = d1× c1 + d 2 × c 2 + d 3 × c3 + d 4 × c 4 + d 5 × c5 + d 6 × c6
¾ <d1, d2, d3, d4, d5, d6> = received chip pattern
¾ <c1, c2, c3, c4, c5, c6> = sender’s code
Marzo 2007
Iván Bernal, Ph.D.
19
2
CDMA
• User A code = <1, –1, –1, 1, –1, 1>
™To send a 1 bit = <1, –1, –1, 1, –1, 1>
™To send a 0 bit = <–1, 1, 1, –1, 1, –1>
• User B code = <1, 1, –1, – 1, 1, 1>
™To send a 1 bit = <1, 1, –1, –1, 1, 1>
• Receiver receiving with A’s code
™(A’s code) x (received chip pattern)
¾ User A ‘1’ bit: 6 -> 1
¾ User A ‘0’ bit: -6 -> 0
¾ User B ‘1’ bit: 0 -> unwanted signal ignored
• What can be gained from apparent waste of spectrum?
™I
™Immunity
it from
f
various
i
kinds
ki d off noise
i and
d multipath
lti th distortion
di t ti
™Can be used for hiding and encrypting signals
™Several users can independently use the same higher bandwidth with very little interference
Marzo 2007
Iván Bernal, Ph.D.
20
CDMA
• Importancia de la alineación de tiempo.
Marzo 2007
Iván Bernal, Ph.D.
21
3
CDMA
• Fuerte interferencia para desplazamientos de 2 y 6.
Marzo 2007
Iván Bernal, Ph.D.
22
CDMA
Marzo 2007
Iván Bernal, Ph.D.
23
4
Definitions
• Correlation
™The concept of determining how much similarity one set of data has with
another
™Range between –1 and 1
¾1 Thee second
seco d sequence
seque ce matches
c es thee first
s sequence
seque ce
¾0 There is no relation at all between the two sequences
9Exactly what is needed for separation of users
9They are orthogonal to each other
¾-1 The two sequences are mirror images
• Cross correlation
™The comparison between two sequences from different sources rather than
a shifted
hift d copy off a sequence with
ith itself
it lf
Marzo 2007
Iván Bernal, Ph.D.
24
Categories of Spreading Sequences
• Spreading Sequence Categories
¾PN sequences
¾Orthogonal codes
™For FHSS systems
¾PN sequences most common
™For DSSS systems not employing CDMA
¾PN sequences most common
™For DSSS CDMA systems
¾PN sequences
¾Orthogonal codes
Marzo 2007
Iván Bernal, Ph.D.
25
5
Categories of Spreading Sequences
• Important PN (pseudorandom numbers or pseudonoise sequences)Properties
™Randomness
¾ Uniform distribution
9 Balance property
‰ In a long sequence
sequence, the fraction of binary ones should approach 1/2
‰ Same number of 0s and 1s
9 Run property
‰ A run is a sequence of all 1s or a sequence of all 0s.
‰ One half of the runs of each type should be of length 1, one fourth of length 2, one eighth of length 3 and
so on.
¾ Independence
9 No one value in the sequence can be inferred from the others.
¾ Correlation property
9 Important in Spread Spectrum
9 If a period of the sequence is compared term by term with any cycle shift of itself, the number
of terms that are the same differs from those that are different by at most 1.
™Unpredictability
Marzo 2007
Iván Bernal, Ph.D.
26
Orthogonal codes
• Orthogonal codes
™All pairwise cross correlations are zero
™Fixed- and variable-length codes used in CDMA systems
™F CDMA application,
™For
li ti
each
h mobile
bil user uses one sequence in
i the
th sett as a spreading
di
code
¾ Provides zero cross correlation among all users
™Types
¾ Walsh codes
¾ Variable-Length Orthogonal codes
Marzo 2007
Iván Bernal, Ph.D.
27
6