No usa una envolvente constante, con cambios de amplitud al cambiar la fase para pasar de un estado a otro.

Transcripción

1 Generalidades Modulación QPSK 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. 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. No existe un dominio i en el lti tiempo (solo para sincronización) i ió y existe el uso de canales lógicos. Dado que la reutilización de frecuencias no constituye problema alguno, añadir celdas a la red es substancialmente mas fácil. Esto hace fácil llenar huecos en la cobertura. Es fácil añadir celdas para cubrir eventos especiales. 16 Spread Spectrum (DSSS) 17 1

2 Multiply BPSK signal s d (t) = A d(t) cos(2π f c t) by c(t) [takes values +1, -1] to get s(t) = Ad(t)c(t) cos(2π f c t) A = amplitude of signal f c = carrier frequency d(t) = discrete function [+1, -1] At receiver, incoming signal multiplied by c(t) Since, c(t) xc(t) = 1, incoming signal is recovered DSSS Using BPSK 18 Code-Division Multiple Access (CDMA) Basic Principles of CDMA D = rate of data signal Break each bit into kchips 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 <c1, c2, c3, c4, c5, c6> For a 0 bit, A sends complement of code <-c1, -c2, -c3, -c4, -c5, -c6> Receiver knows sender s code and performs electronic decode function ( d ) = d1 c1 + d2 c2 + d3 c3+ d4 c4 + d5 c5 + d6 c6 S u <d1, d2, d3, d4, d5, d6> = received chip pattern <c1, c2, c3, c4, c5, c6> = sender s code 19 2

3 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) User A 1 bit: 6 -> 1 User A 0 bit: -6 -> 0 User B 1 bit: 0 -> unwanted signal ignored CDMA What can be gained from apparent waste of spectrum? Immunity from various kinds of noise and multipath th distortion ti Can be used for hiding and encrypting signals Several users can independently use the same higher bandwidth with very little interference 20 Importancia de la alineación de tiempo. CDMA 21 3

4 CDMA Fuerte interferencia para desplazamientos de 2 y CDMA 23 4

5 Definitions Correlation The concept of determining how much similarity one set of data has with another Range between 1 and 1 1 The second sequence matches the first sequence 0 There is no relation at all between the two sequences Exactly what is needed for separation of users They are orthogonal to each other -1 The two sequences are mirror images Cross correlation The comparison between two sequences from different sources rather than a shifted copy of a sequence with itself 24 Categories of Spreading Sequences Spreading Sequence Categories PN sequences Orthogonal codes For FHSS systems PN sequences most common For DSSS systems not employing CDMA PN sequences most common For DSSS CDMA systems PN sequences Orthogonal codes 25 5

6 Categories of Spreading Sequences Important PN (pseudorandom numbers or pseudonoise sequences)properties Randomness Uniform distribution Balance property In a long sequence, the fraction of binary ones should approach 1/2 Same number of 0s and 1s 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. Independence No one value in the sequence can be inferred from the others. Correlation property Unpredictability Important in Spread Spectrum 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 Orthogonal codes Orthogonal codes All pairwise cross correlations are zero Fixed- and variable-length codes used in CDMA systems For CDMA application, each mobile user uses one sequence in the set as a spreading code Provides zero cross correlation among all users Types Walsh codes Variable-Length Orthogonal codes 27 6