Stallings, Wireless Communications & Networks, Second Edition, © 2005 Pearson Education, Inc. All rights reserved Multiple Access Techniques

Stallings, Wireless Communications & Networks, Second Edition, © 2005 Pearson Education, Inc. All rights reserved Outline FDMA TDMA CDMA Spread Spectrum

Stallings, Wireless Communications & Networks, Second Edition, © 2005 Pearson Education, Inc. All rights reserved 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?

Stallings, Wireless Communications & Networks, Second Edition, © 2005 Pearson Education, Inc. All rights reserved 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

Stallings, Wireless Communications & Networks, Second Edition, © 2005 Pearson Education, Inc. All rights reserved TDMA Time is partitioned into frames Each frame consists of N slot 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.

Stallings, Wireless Communications & Networks, Second Edition, © 2005 Pearson Education, Inc. All rights reserved TDMA

Stallings, Wireless Communications & Networks, Second Edition, © 2005 Pearson Education, Inc. All rights reserved 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.

Stallings, Wireless Communications & Networks, Second Edition, © 2005 Pearson Education, Inc. All rights reserved 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”)

Stallings, Wireless Communications & Networks, Second Edition, © 2005 Pearson Education, Inc. All rights reserved Code-Division Multiple Access (CDMA) Basic Principles of CDMA D = rate of data signal Break each bit into k chips Chips are a user-specific fixed pattern Chip data rate of new channel = kD

Stallings, Wireless Communications & Networks, Second Edition, © 2005 Pearson Education, Inc. All rights reserved CDMA

Stallings, Wireless Communications & Networks, Second Edition, © 2005 Pearson Education, Inc. All rights reserved CDMA Encode/Decode slot 1 slot 0 d 1 = Z i,m = d i. c m d 0 = slot 0 channel output slot 1 channel output channel output Z i,m sender code data bits slot 1 slot 0 d 1 = -1 d 0 = slot 0 channel output slot 1 channel output receiver code received input D i =  Z i,m. c m m=1 M M

Stallings, Wireless Communications & Networks, Second Edition, © 2005 Pearson Education, Inc. All rights reserved CDMA: two-sender interference

Stallings, Wireless Communications & Networks, Second Edition, © 2005 Pearson Education, Inc. All rights reserved CDMA Example If k=6 and code is a sequence of 1s and -1s For a ‘1’ bit, A sends code as chip pattern For a ‘0’ bit, A sends complement of code Receiver knows sender’s code and performs electronic decode function = received chip pattern = sender’s code

Stallings, Wireless Communications & Networks, Second Edition, © 2005 Pearson Education, Inc. All rights reserved CDMA Example User A code = To send a 1 bit = To send a 0 bit = User B code = To send a 1 bit = 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 for Direct Sequence Spread Spectrum

Stallings, Wireless Communications & Networks, Second Edition, © 2005 Pearson Education, Inc. All rights reserved 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

Stallings, Wireless Communications & Networks, Second Edition, © 2005 Pearson Education, Inc. All rights reserved 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

Stallings, Wireless Communications & Networks, Second Edition, © 2005 Pearson Education, Inc. All rights reserved 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

Stallings, Wireless Communications & Networks, Second Edition, © 2005 Pearson Education, Inc. All rights reserved Walsh Codes Set of Walsh codes of length n consists of the n rows of an n  n Walsh matrix: W 1 = (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

Stallings, Wireless Communications & Networks, Second Edition, © 2005 Pearson Education, Inc. All rights reserved 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

Stallings, Wireless Communications & Networks, Second Edition, © 2005 Pearson Education, Inc. All rights reserved Figure 6.27 Spread spectrum

Stallings, Wireless Communications & Networks, Second Edition, © 2005 Pearson Education, Inc. All rights reserved General Model of Spread Spectrum System

Stallings, Wireless Communications & Networks, Second Edition, © 2005 Pearson Education, Inc. All rights reserved 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

Stallings, Wireless Communications & Networks, Second Edition, © 2005 Pearson Education, Inc. All rights reserved 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

Stallings, Wireless Communications & Networks, Second Edition, © 2005 Pearson Education, Inc. All rights reserved 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

Stallings, Wireless Communications & Networks, Second Edition, © 2005 Pearson Education, Inc. All rights reserved Figure 6.28 Frequency hopping spread spectrum (FHSS)

Stallings, Wireless Communications & Networks, Second Edition, © 2005 Pearson Education, Inc. All rights reserved Figure 6.29 Frequency selection in FHSS

Stallings, Wireless Communications & Networks, Second Edition, © 2005 Pearson Education, Inc. All rights reserved Figure 6.30 FHSS cycles

Stallings, Wireless Communications & Networks, Second Edition, © 2005 Pearson Education, Inc. All rights reserved Figure 6.31 Bandwidth sharing

Stallings, Wireless Communications & Networks, Second Edition, © 2005 Pearson Education, Inc. All rights reserved Frequency Hopping Example

Stallings, Wireless Communications & Networks, Second Edition, © 2005 Pearson Education, Inc. All rights reserved FHSS (Transmitter)

Stallings, Wireless Communications & Networks, Second Edition, © 2005 Pearson Education, Inc. All rights reserved Frequency Hopping Spread Spectrum System (Receiver)

Stallings, Wireless Communications & Networks, Second Edition, © 2005 Pearson Education, Inc. All rights reserved Slow and Fast FHSS commonly use multiple FSK (MFSK) have frequency shifted every T c seconds duration of signal element is T s seconds Slow FHSS has T c  T s Fast FHSS has T c < T s FHSS quite resistant to noise or jamming with fast FHSS giving better performance

Stallings, Wireless Communications & Networks, Second Edition, © 2005 Pearson Education, Inc. All rights reserved Slow MFSK FHSS

Stallings, Wireless Communications & Networks, Second Edition, © 2005 Pearson Education, Inc. All rights reserved Fast MFSK FHSS

Stallings, Wireless Communications & Networks, Second Edition, © 2005 Pearson Education, Inc. All rights reserved 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

Stallings, Wireless Communications & Networks, Second Edition, © 2005 Pearson Education, Inc. All rights reserved Figure 6.32 DSSS

Stallings, Wireless Communications & Networks, Second Edition, © 2005 Pearson Education, Inc. All rights reserved Figure 6.33 DSSS example

Stallings, Wireless Communications & Networks, Second Edition, © 2005 Pearson Education, Inc. All rights reserved Direct Sequence Spread Spectrum Example

Stallings, Wireless Communications & Networks, Second Edition, © 2005 Pearson Education, Inc. All rights reserved Direct Sequence Spread Spectrum System

Stallings, Wireless Communications & Networks, Second Edition, © 2005 Pearson Education, Inc. All rights reserved DSSS Example Using BPSK

Stallings, Wireless Communications & Networks, Second Edition, © 2005 Pearson Education, Inc. All rights reserved Approximate Spectrum of DSSS Signal