
Outline • FDMA • TDMACDMA • Spread Spectrum
Multiple Access Techniques • Multiple users want to access the common BS or AP simultaneously • If two or more user signals arrive at the BS at the same time, there will be interferences, unless the signals are orthogonal • How can we achieve the orthogonality?
FDMA • The total bandwidth is divided into nonoverlapping frequency bands (channels) • Each user occupies a channel for the duration of the connection • waste of resources • Narrowband transmission • Forward and reverse links use FDD
TDMA • Time is partitioned into frames • Each frame consists of Nslot data slots plus a header and a trailer • Each slot is for transmission of one information unit • A user continues to use the same slot in every frame during call connection • waste of resources • TDMA systems require strict time synchronization.
TDMA • W-TDMA: Each user occupies the total frequency bandwidth during its slots • N-TDMA: The total frequency spectrum is divided into frequency subbands (channels); within each frequency channel, TDMA is used. −→Both time and frequency are partitioned.
Code Division Multiple Access (CDMA) • used in several wireless broadcast channels (cellular, satellite, etc) standards • unique “code” assigned to each user; i.e., code set partitioning • all users share same frequency, but each user has own “chipping” sequence (i.e., code) to encode data • encoded signal = (original data) X (chipping sequence) • decoding: inner-product of encoded signal and chipping sequence • allows multiple users to “coexist” and transmit simultaneously with minimal interference (if codes are “orthogonal”)
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
d0 = 1 1 1 1 1 1 1 d1 = -1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 M Di = SZi,m.cm m=1 M d0 = 1 d1 = -1 CDMA Encode/Decode channel output Zi,m Zi,m= di.cm data bits sender slot 0 channel output slot 1 channel output code slot 1 slot 0 received input slot 0 channel output slot 1 channel output code receiver slot 1 slot 0
CDMA Example • 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 • <d1, d2, d3, d4, d5, d6> = received chip pattern • <c1, c2, c3, c4, c5, c6> = sender’s code
CDMA Example • 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
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 • -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
Advantages of Cross Correlation • The cross correlation between an m-sequence and noise is low • This property is useful to the receiver in filtering out noise • The cross correlation between two different m-sequences is low • This property is useful for CDMA applications • Enables a receiver to discriminate among spread spectrum signals generated by different m-sequences
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 • Welsh codes • Variable-Length Orthogonal codes
Walsh Codes • Set of Walsh codes of length n consists of the n rows of an n ´ n Walsh matrix: • W1 = (0) • n = dimension of the matrix • Every row is orthogonal to every other row and to the logical not of every other row • Requires tight synchronization • Cross correlation between different shifts of Walsh sequences is not zero
Spread Spectrum • important encoding method for wireless communications • analog & digital data with analog signal • spreads data over wide bandwidth • makes jamming and interception harder • two approaches, both in use: • Frequency Hopping • Direct Sequence
Spread Spectrum Advantages • immunity from noise and multipath distortion • can hide / encrypt signals • several users can share same higher bandwidth with little interference • CDM/CDMA Mobile telephones
Pseudorandom Numbers • generated by a deterministic algorithm • not actually random • but if algorithm good, results pass reasonable tests of randomness • starting from an initial seed • need to know algorithm and seed to predict sequence • hence only receiver can decode signal
Frequency Hopping Spread Spectrum (FHSS) • signal is broadcast over seemingly random series of frequencies • receiver hops between frequencies in sync with transmitter • eavesdroppers hear unintelligible blips • jamming on one frequency affects only a few bits
Slow and Fast FHSS • commonly use multiple FSK (MFSK) • have frequency shifted every Tc seconds • duration of signal element is Ts seconds • Slow FHSS has Tc Ts • Fast FHSS has Tc < Ts • FHSS quite resistant to noise or jamming • with fast FHSS giving better performance
Direct Sequence Spread Spectrum (DSSS) • each bit is represented by multiple bits using a spreading code • this spreads signal across a wider frequency band • has performance similar to FHSS