CS 6910 Pervasive Computing Spring 2007 Section 7

CS 6910 – Pervasive Computing Spring 2007 Section 7 (Ch. 7): Multiple Access Techniques Prof. Leszek Lilien Department of Computer Science Western Michigan University Slides based on publisher’s slides for 1 st and 2 nd edition of: Introduction to Wireless and Mobile Systems by Agrawal & Zeng © 2003, 2006, Dharma P. Agrawal and Qing-An Zeng. All rights reserved. Some original slides were modified by L. Lilien, who strived to make such modifications clearly visible. Some slides were added by L. Lilien, and are © 2006 -2007 by Leszek T. Lilien. Requests to use L. Lilien’s slides for non-profit purposes will be gladly granted upon a written request. 1

Chapter 7 Multiple Division (Access) Techniques Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved 2

Outline n n 7. 1. Introduction 7. 2. Concept and Models for Multiple Access n 7. 2. 1. Frequency Division Multiple Access (FDMA) n 7. 2. 2. Time Division Multiple Access (TDMA) n 7. 2. 3. Code Division Multiple Access (CDMA) n n n n 1) Introduction 2) Spread Spectrum 3) Direct Sequence Spread Spectrum (DSSS) 4) Frequency Hopping Spread Spectrum (HFSS) 5) Walsh Codes 6) Near-far Problem 7) Power Control 7. 2. 4. OFDM n 7. 2. 5. SDMA n 7. 2. 6. Comparison of FDMA, TDMA, and CDMA 7. 3. Modulation Techniques n 7. 3. 1. AM (Amplitude Modulation) n 7. 3. 2. FM (Frequency Modulation) n 7. 3. 3. FSK (Frequency Shift Keying) n 7. 3. 4. PSK (Phase Shift Keying) n 7. 3. 5. QPSK (Quadrature Phase Shift Keying) n 7. 3. 6. /4 QPSK n 7. 3. 7. QAM (Quadrature Amplitude Modulation) n 7. 3. 8. 16 QAM n n Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved (Modified by LTL) 3

7. 1. Introduction n Recall n Large # of traffic channels on each BS n n Bec. traffic channels used by 1 MS exclusively for call duration Bec. control channels shared by many MSs for short periods n nli o )c r wa r Fo do d( e ers v Re rse e v n c l o r n co ) lin n w do ( d rw Fo MS nk i l p ch n an h a tr k) lin p (u cc i f f a tr k) c ffi el h t (u ar l tro k w Small # of control channels on each BS n el n an ch el el n an Re BS Too expensive/inefficient to assign control channnel for call duration MS gets a traffic channel assigned for call duration n Once assigned, no need to compete for access to traffic channels n As was the case for control channels Copyright Dharma P. Agrawal and Qing-An Zeng. All rights reserved © 2007 © by 2003, Leszek T. Lilien 4

Introduction – cont. n n For control channels — we used contention-based protocols n Many MS’s competing for the same control channel For traffic channels — we use now contention-free protocols n Dedicated channel for each MS (not shared with other MSs) n Allocated by BS When “this” MS requests OR: n When another MS tries to reach “this” MS n n Allocated thanks to exchange of control messages over control channels using contention-based protocols Copyright Dharma P. Agrawal and Qing-An Zeng. All rights reserved © 2007 © by 2003, Leszek T. Lilien 5

7. 2. Concept and Models for Multiple Access (Multiple Division) n n Q: Why “multiple access”? A: Multiple MSs can share a radio channel (without interference) => we can have multiple access to the same channel n Multiple traffic channels used simultaneously n MS attached to a transmitter/receiver n Transmission from any MS is received by all MSs within its radio range n n MS communicates with its BS via a traffic channel Dedicated traffic channel not shared by other MSs Copyright Dharma P. Agrawal and Qing-An Zeng. All rights reserved © 2007 © by 2003, Leszek T. Lilien 6

7. 2. Concept and Models for Multiple Division – cont. 1 n MS can hear traffic on many channels n n MS must distinguish its own traffic from any other traffic n Ignore traffic from its own BSs to other MSs n Ignore traffic from “foreign” BSs n Ignore traffic from other MSs n n n From “foreign” BSs ‘ from other MSs Like a person picking up his conversation partner’s speech when many people have many independent conversations in a room Also BS must distinguish traffic from different MSs MS or BS can distinguish thanks to orthogonalization of signals on different traffic channels n OPTIONAL details – p. 144 Copyright Dharma P. Agrawal and Qing-An Zeng. All rights reserved © 2007 © by 2003, Leszek T. Lilien 7

7. 2. Concept and Models for Multiple Division – cont. 2 n n Duplex communications = simultaneous 2 -way communications Duplex communications requires n Forward (downlink) channel n Reverse (uplink) channel n o )c k lin n ow rd wa (d r Fo u e( ers v Re ard rw Fo MS n an ch l tro ) nk (d af ) tr a h cc fi li n ow l e nn ha lc o r nt co nk pli el c ffi l e nn n an el ch a tr k) n e rs ve li p u ( Re Copyright Dharma P. Agrawal and Qing-An Zeng. All rights reserved © 2007 © by 2003, Leszek T. Lilien BS 8

7. 2. 1. Frequency Division Multiple Access (FDMA) Frequency User n … User 2 User 1 Time • Separate (unique) carrier frequency per user • All 1 G (first-generation) systems use FDMA Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved 9

Basic Structure of a FDMA System - 1 BS and n MSs - fi’ and fi – for MS #i MS #2 f 2 ’ f 2 fn ’ fn MS #n … f 1 ’ … MS #1 Reverse channels (Uplink) Forward channels (Downlink) Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved BS 10

FDMA Channel Structure Reverse channels f 1’ f 2’ fn’ Protecting bandwidth Forward channels f 1 f 2 fn … … Frequency Guard Band Wg Subband Wc 1 2 3 4 … n Frequency Total bandwidth W = N * Wc (for reverse channels or forward channels) Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved (Modified by LTL) 11

7. 2. 2. Time Division Multiple Access (TDMA) … User n User 2 User 1 Frequency Time • Separate (unique) time slot per user • The same carrier (frequency) split into time slots • Each frequency efficiently utilized by multiple users • Most of 2 G systems use TDMA Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved 12

MS #n Frame … … Frame Reverse channels (Uplink) … t #1 … Frequency f … t … … … #2 #2 t … … #n … MS #2 … #n #2 MS #1 … #1 #1 … Slot #2 Slot #n Frequency f ’ #1 Basic Structure of TDMA … t Frame BS Forward channels (Downlink) Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved 13

Two Duplexing Modes for TDMA • Two duplexing modes for TDMA a) FDD = frequency division duplexing - Frequency forward channels differs from frequency for reverse channels e. g. , next slide: f used for all forward channels and f’ (not f) used for all reverse channel #1 b) TDD = time division duplexing - same frequency for all forward and all reverse channels e. g. , slide +2 f used for all forward channels and f (same) used for all reverse channels Copyright Dharma P. Agrawal and Qing-An Zeng. All rights reserved © 2007 © by 2003, Leszek T. Lilien 14

a) Channel Structure in TDMA/FDD System #n … t #n #2 #1 … Frame #n #2 #1 Frame #2 f t (a) All forward channels on frequency f f’ #2 #1 … Frame #n #2 #1 Frame … (a) All reverse channels on different frequency f ’ Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved 15

b) Channel Structure in TDMA/TDD System f … #n #2 #1 … #n #2 Frame #1 … #n #2 #1 Frame t Forward channel Reverse channel (a) All forward and all reverse channels on the same frequency f (b) (1 st half of each frame used forward channels, 2 nd half – for reverse channels) Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved 16

TDMA Frame Structure … #n #2 #1 … Frame #n #2 #1 Frequency Frame Time Notice Guard time between Time slots Head Guard time Data - Minimize interference due to propagation delays Time slot Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved 17

7. 2. 3. Code Division Multiple Access (CDMA) n Outline: 1) 2) 3) 4) 5) 6) 7) Introduction to CDMA Spread Spectrum Direct Sequence Spread Spectrum (DSSS) Frequency Hopping Spread Spectrum (FHSS) Walsh Codes Near-far Problem Power Control Copyright Dharma P. Agrawal and Qing-An Zeng. All rights reserved © 2007 © by 2003, Leszek T. Lilien 18

7. 2. 3. Code Division Multiple Access (CDMA) – cont. Frequency CSMA = carrier sense multiple access User n based) . . . User 2 User 1 Do not confuse CDMA (conflict-free) with CSMA (contention- 1) Introduction Time Code • Separate (unique) code per user • Code sequences are orthogonal => different users can use same frequency simultaneously (see Fig above) • Some 2 G systems use CDMA / Most of 3 G systems use CDMA Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved (Modified by LTL) 19

Structure of a CDMA System (with FDD) Frequency f ’ Frequency f C 1 MS #2 C 2 ’ C 2 Cn ’ Cn … … C 1 ’ … MS #1 MS #n Reverse channels (Uplink) Forward channels (Downlink) BS Notes: 1) FDD (frequency division duplexing) since f for all forward channels, and f’ for all reverse channels 2) Ci = i-the code 3) Ci’ x Cj’ = 0, i. e. , Ci’ and Cj’ are orthogonal codes on f’ Ci x Cj = 0, i. e. , Ci and Cj are orthogonal codes on f Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved 20

Two Implementation Methodologies for CDMA • Two implementation methodologies for CDMA a) DS = direct sequence - next slide b) FH = frequency hopping - same frequency for all forward and all reverse channels e. g. , slide +2 f used for all forward channels and f (same) used for all reverse channels Copyright Dharma P. Agrawal and Qing-An Zeng. All rights reserved © 2007 © by 2003, Leszek T. Lilien 21

2) Spread Spectrum for CDMA n Concept of spread spectrum: n n Pseudorandom sequence c(t) phase-modulates data-modulated carrier of s(t), producing m(t) occupies broader bandwidth and has lower peak power than s(t) where: n s(t) - original signal / m(t) – xmitted signal derived fr. s(t) by spreading n c(t) – code signal (a parameter for spreading) n Results in better resistance to interference Transmitter Original digital signal s(t) Spreading Xmitted spread signal m(t) Power Code c(t) Frequency Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved (Modified by LTL) 22

3) Direct Sequence Spread Spectrum n Concept of DSSS for CDMA n Pseudorandom sequence c(t) phase-modulates datamodulated carrier of s(t), producing m(t) n m(t) occupies broader bandwidth & has lower peak power than s(t) Original digital signal s(t) Power Transmitter Receiver Spreading Despreading Xmitted spread signal m(t) Code c(t) Frequency Power Frequency Code c(t) Recreated digital signal s(t) Power c(t) is Synchronized! Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved Frequency (Modified by LTL) 23

4) Frequency Hopping Spread Spectrum n Concept of FHSS for CDMA n Pseudorand. hopping pattern sequence changes freq. of digital radio signal across broad freq. band in random way n n Radio xmitter freq. hops fr. channel to channel in predetermined pseudorandom way (cf. next slide) m(t) occupies broader bandwidth & has lower peak power than s(t) Transmitter Receiver Spreading Despreading Original digital signal Power Xmit spread signal Hopping pattern Power Frequency Recreated (“dehopped”) digital signal Hopping pattern Power Frequency Hopp. patt. Synchronized! Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved Frequency (Modified by LTL) 24

An Example of Frequency Hopping Pattern Frequency Time Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved 25

*** SKIP *** 5) Walsh Codes n n Each user in CDMA assigned ≥ 1 orthogonal waveforms derived from 1 orthogonal code Walsh Codes are an impor- Wal (0, t) tant set of orthogonal Wal (1, t) codes t t Wal (2, t) t Wal (3, t) t Wal (4, t) t Wal (5, t) t Wal (6, t) t Wal (7, t) t Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved 26

6) Near-far Problem Assume 1: xmission power of MS 1 = = xmission power of MS 2 => RSS of MS 1 at BS > > RSS of MS 2 at BS MS 2 Assume 2: MS 1 & MS 2 use adjacent channels => out-of-band radiation of MS 1’s signal interferes with MS 2’s signal in the adjacent channel (cf. next BS MS 1 RSS slide) RSS = received signal strength D = distance 0 D MS 2 d 2 BS d 1 MS 1 Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved D (Modified by LTL) 27

Adjacent Channel Interference in CDMA MS 1 f 1 n Power MS 2 f 2 Frequency Adjacent channel interference can be serious => must keep out-of-band radiation small Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved 28

Adjacent Channel Interference in Spread Spectrum System in CDMA Interference baseband signals Baseband signal Spread signal Despread signal Interference signals Frequency n Frequency Adjacent channel interference can be especially serious when spread spectrum technique used n n Frequency Cf. figure Simple solution: power control (next slide) Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved 29

7) Power Control in CDMA Two alternatives: a) Control transmit power Pt of MS 2 => received power Pr of adjacent channel interference from MS 2 is controlled => CCIR is controlled (CCIR = cochannel interference ratio) OR: b) Control transmit power Pt of MS 1 => received power Pr of MS 1 is controlled (kept strong enough) Copyright Dharma P. Agrawal and Qing-An Zeng. All rights reserved © 2007 © by 2003, Leszek T. Lilien 30

** SKIP ** 7) Power Control in CDMA –cont. Pt = Pr = d = f = c = a= Transmit power Received power in free space Distance between receiver and transmitter Frequency of transmission Speed of light Attenuation constant (2 to 4) Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved 31

** SKIP** 7. 2. 4. OFDM ** SKIP** 7. 2. 5. SDMA Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved 32

** SKIP ** 7. 2. 6. Comparisons of FDMA, TDMA, and CDMA (Example) Operation FDMA TDMA CDMA Allocated Bandwidth 12. 5 MHz 7 7 1 0. 03 MHz 1. 25 MHz 12. 5/0. 03=416 12. 5/1. 25=10 416/7=59 12. 5/1. 25=10 Control channels/cell 2 2 2 Usable channels/cell 57 57 8 Calls per RF channel 1 4* 40** Voice channels/cell 57 x 1= 57 57 x 4= 228 8 x 40= 320 3 3 3 57/3=19 228/3=76 320 1 4 16. 8 ? Delay Depends on the number of slots ? ? Frequency reuse Required channel BW No. of RF channels Channels/cell Sectors/cell Voice calls/sector Capacity vs. FDMA * ** Depends on the number of codes Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved 33

7. 3. Modulation Techniques n Why need modulation? n “Transferring” from signal freq. to carrier frequency allows to use small antenna size n Antenna size is inversely proportional to frequency n E. g. , 3 k. Hz 50 km antenna 3 GHz 5 cm antenna n n Limits noise and interference n E. g. , FM (Frequency Modulation) Multiplexing techniques (efficient use of spectrum) n E. g. , FDM, TDM, CDMA Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved (Modified by LTL) 34

Analog and Digital Signals n Analog signal (continuous signal) Amplitude S(t) Time 0 § Digital signal (discrete signal) Amplitude 1 0 1 + Time 0 _ Bit Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved 35

Hearing, Speech, and Voice-band Channels Human hearing Human speech Voice-grade Telephone channel . . 100 10, 000 Frequency (Hz) Pass band Guard band Frequency cutoff point 0 200 3, 500 4, 000 Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved Frequency (Hz) 36

7. 3. 1. AM (Amplitude Modulation) Message signal x(t) Time Carrier signal Time AM signal s(t) Time - Amplitude of carrier signal is varied as the message signal to be transmitted - Frequency of carrier signal is kept constant Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved 37

7. 3. 2. FM (Frequency Modulation) Message signal x(t) Time Carrier signal Time FM signal s(t) Time FM integrates message signal with carrier signal by varying the instantaneous frequency Amplitude of carrier signal is kept constant Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved 38

7. 3. 3. FSK (Frequency Shift Keying) • 1/0 represented by two different frequencies slightly offset from carrier frequency Carrier signal 1 for message signal ‘ 1’ Time Carrier signal 2 for message signal ‘ 0’ Time 1 0 1 Message signal x(t) Time FSK signal s(t) Time Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved 39

7. 3. 4. PSK (Phase Shift Keying) • Use alternative sine wave phase to encode bits Carrier signal 1 Time Carrier signal 2 Time 1 0 1 Message signal x(t) PSK signal s(t) Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved Time 40

** SKIP ** 7. 3. 5. QPSK (Quadrature Phase Shift Keying) Signal constellation n Q Q 0, 1 1 0 I 1, 1 0, 0 I 1, 0 (a) BPSK (b) QPSK BPSK = binary phase shift keying (p. 161/-1) Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved 41

** SKIP ** 7. 3. 6. /4 QPSK n All possible state transitions in /4 QPSK Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved 42

** SKIP ** 7. 3. 7. QAM (Quadrature Amplitude Modulation) A combination of AM and PSK Two carriers out of phase by 90 deg are amplitude modulated Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved 43

7. 3. 8. 16 QAM Splitting signal into 12 different phases and 3 different amplitudes – the total of 16 different possible values Rectangular constellation of 16 QAM Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved 44

The End of Section 7 (Ch. 7) 45