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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
