Chapter 7 Multiple Division Techniques for Traffic Channels

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1 Introduction to Wireless & Mobile Systems Chapter 7 Multiple Division Techniques for Traffic Channels

2 Outline Introduction Concepts and Models for Multiple Divisions Frequency Division Multiple Access (FDMA) Time Division Multiple Access (TDMA) Code Division Multiple Access (CDMA) Orthogonal Frequency Division Multiplexing (OFDM) Space Division Multiple Access (SDMA) Comparison of FDMA, TDMA, and CDMA Modulation Techniques Amplitude Modulation (AM) Frequency Modulation (FM) Frequency Shift Keying (FSK) Phase Shift Keying (PSK) Quadrature Phase Shift Keying (QPSK) π/4qpsk Quadrature Amplitude Modulation (QAM) 6QAM 2

3 Concepts and Models for Multiple Divisions Multiple access techniques are based on orthogonalization of signals A radio signal is a function of frequency, time, and code as s(f, t, c) = s(f, t)c(t) where s(f, t) is the function of frequency and time and c(t) is the function of code Use of different frequencies to transmit a signal: FDMA Distinct time slot: TDMA Different codes CDMA Multiple simultaneous channels: OFDM Specially separable sectors: SDMA 3

4 Frequency Division Multiple Access (FDMA) Orthogonality conditions of two signals in FDMA: F s i i = j ( f, t) s j ( f, t) df =, i, j =,2,..., k 0 i j Frequency User n User 2 User Time Single channel per carrier All first generation systems use FDMA 4

5 Basic Structure of FDMA MS MS 2 f f 2 f n f f 2 f n MS n Reverse channels (Uplink) Forward channels (Downlink) BS 5

6 Forward & Reverse Channels in FDMA & Guard Band f f 2 f n f f 2 f n Reverse channels Forward channels Guard Band W g Sub Band W c N Frequency Total Bandwidth W = NW c 6

7 Time Division Multiple Access (TDMA) Orthogonality conditions of two signals in TDMA: T s i i = j ( f, t) s f t dt { j (, ) =, i, j =,2,..., k 0 i j Frequency User User 2 User n Time Multiple channels per carrier Most of second generation systems use TDMA 7

8 2 Introduction to Wireless & Mobile Systems, 4 th Edition The Concept of TDMA Frequency f Slot Frequency f MS MS 2 t 2 2 t t t 2 MS n n n t t n n Frame Frame Frame Frame BS Reverse channels (Uplink) Forward channels (Downlink) 8

9 n n Introduction to Wireless & Mobile Systems, 4 th Edition TDMA: Channel Structure f Frame Frame Frame 2 n 2 2 n t (a). Forward channel f Frame Frame Frame 2 n 2 2 n t (b). Reverse channel 9

10 Forward and Reverse Channels in TDMA Frequency f = f Frame Frame 2 n 2 n 2 n 2 n Forward channel Reverse channel Forward channel Reverse channel Channels in TDMA/TDD 0

11 Frame Structure of TDMA Frequency Frame 2 n Frame 2 n 2 Frame n Time Guard time Head Data

12 Code Division Multiple Access (CDMA) Orthogonality conditions of two signals in CDMA: C i = j si ( t) s j ( t) dt = {, i, j =,2,..., k 0 i j Frequency User n... User 2 User Time Code Users share bandwidth by using code sequences that are orthogonal to each other Some second generation systems use narrowband CDMA Most of third generation systems use wideband CDMA 2

13 CDMA Encode and Decode Channel output Z i,m Sender Data bits Code d = d 0 = Slot Slot 0 Z i,m = d i. c m slot Channel output slot 0 Channel output Received input Code Receiver Slot Slot 0 M D i = S Z i,m. c m= m M d = Slot channel output d 0 = Slot 0 channel output 3

14 CDMA: Two-sender Interference 4

15 Structure of a CDMA System MS MS 2 Frequency f C C 2 Frequency f C C 2 MS n C n Reverse channels (Uplink) C n Forward channels (Downlink) BS C i x C j = 0, i.e., C i and C j are orthogonal codes C i x C j = 0, i.e., C i and C j are orthogonal codes 5

16 Spread Spectrum Spreading of data signal s(t) by the code signal c(t) to result in message signal m(t) as m( t) = s( t) c( t) Power Digital signal s(t) Spreading Spreading signal m(t) Power Frequency Code c(t) Frequency 6

17 Direct Sequence Spread Spectrum (DSSS) Transmitter Spreading Receiver Despread Digital signal s(t) Spreading signal m(t) Digital signal s(t) Power Code c(t) Power Code c(t) Power Frequency Frequency Frequency 7

18 Walsh Codes (Orthogonal Codes) Wal (0, t) Wal (, t) Wal (2, t) Wal (3, t) Wal (4, t) Wal (5, t) Wal (6, t) Wal (7, t) t t t t t t t t 8

19 An Example of Frequency Hopping Pattern Frequency Time 9

20 Frequency Hopping Spread Spectrum (FHSS) Transmitter Spreading Receiver Despread Digital signal Spreading signal Digital signal s(t) Power Hopping pattern Power Hopping pattern Power Frequency Frequency Frequency 20

21 Near-far Problem MS 2 BS MS Received signal strength Distance 0 Distance d 2 d BS MS 2 MS 2

22 Adjacent Channel Interference Power MS MS 2 f f 2 Frequency 22

23 Interference in Spread Spectrum Interference baseband signals Baseband signal Spectrum spreading signal Despread signal Interference signals 0 Frequency f Frequency f Frequency 23

24 Power Control in CDMA Controlling transmitted power affects the CIR P r P t = 4πdf P r = Received power in free space P t = Transmitted power d = Distance between receiver and transmitter f = Frequency of transmission c = Speed of light α = Attenuation constant (2 to 4) c α 24

25 Orthogonal Frequency Division Multiplexing (OFDM) Divide a channels in to multiple sub-channels and do parallel transmission Orthogonality of two signals in OFDM can be given by a complex conjugate relation indicated by *: *, i = j s( f, t) s j ( f, t) dt =, i, j =,2,..., k 0, i j F Spectrum of a single OFDM subchannel Spectrun of an OFDM signal with multiple subchannels 25

26 Modulation/Demodulation Steps in OFDM Low speed bit stream High speed data stream Serial to parallel conversion N N 2. IDFT Guard interval insertion Transmission of OFDM signal N n Modulation operation at the OFDM transmitter Received OFDM signal Guard interval removal DFT N N 2. Parallel to serial conversion High speed data stream N n Demodulation steps at the OFDM receiver 26

27 Space Division Multiple Access (SDMA) Space divided into spatially separate sectors s(f,t,c) Beam i Omni-directional transmission s(f,t,c) Beam 3 s(f,t,c) Beam 2 s(f,t,c) s(f,t,c) Beam n Beam The concept of SDMA 27

28 Transmission in SDMA Beam Beam 2 Beam 3 MS MS 2 BS MS 3 The basic structure of a SDMA system. 28

29 Comparison of various Multiple Division Techniques Technique FDMA TDMA CDMA SDMA Concept Active terminals Divide the frequency band into disjoint subbands All terminals active on their specified frequencies Divide the time into non-overlapping time slots Terminals are active in their specified slot on same frequency Spread the signal with orthogonal codes All terminals active on same frequency Divide the space in to sectors Number of terminals per beam depends on FDMA/ TDMA/CDMA Signal separation Filtering in frequency Synchronization in time Code separation Spatial separation using smart antennas Handoff Hard handoff Hard handoff Soft handoff Hard and soft handoffs Advantages Simple and robust Flexible Flexible Very simple, increases system capacity Disadvantages Inflexible, available frequencies are fixed, requires guard bands Requires guard space, synchronization problem Complex receivers, requires power control to avoid near-far problem Inflexible, requires network monitoring to avoid intracell handoffs Current applications Radio, TV and analog cellular GSM and PDC 2.5G and 3G Satellite systems, other being explored 29

30 Modulation Techniques Why need modulation? Small antenna size Antenna size is inversely proportional to frequency (wavelength) e.g., 3 khz 50 km antenna 3 GHz 5 cm antenna Limits noise and interference, e.g., FM (Frequency Modulation) Multiplexing techniques, e.g., FDM, TDM, CDMA 30

31 Analog and Digital Signals Analog Signal (Continuous signal) Amplitude S(t) 0 Time Digital Signal (Discrete signal) Amplitude Time Bit 3

32 Hearing, Speech, and Voice-band Channels Human hearing Human speech 00 Voice-grade Telephone channel.. Frequency (Hz) 0,000 Pass band Guard band Frequency cutoff point Guard band ,500 4,000 Frequency (Hz) 32

33 Amplitude Modulation (AM) Message signal x(t) Time Carrier signal Time AM signal s(t) Time The modulated carrier signal s(t) is: s( t) [ A + x( t)] cos(2πf t) = Where c f is the carrier frequency c and A is amplitude 33

34 Frequency Modulation (FM) Message signal x(t) Time Carrier signal Time FM signal s(t) Time The modulated carrier signal s(t) is: t s( t) = Acos (2π f + + ct 2π f x( τ ) dτ θ0 t0 BW=2(b+)f m with b= f D /f m ; f m is the maximum modulating frequency used Where f D is the peak frequency deviation from the original frequency and f D << f c 34

35 Frequency Shift Keying (FSK) /0 represented by two different frequencies Carrier signal for message signal Time Carrier signal 2 for message signal 0 Time 0 0 Message signal x(t) FSK signal s(t) Time Time 35

36 Phase Shift Keying (PSK) Use alternative sine wave phases to encode bits Carrier signal sin( 2πf c t) Carrier signal sin( 2π f c t + π ) Message signal x(t) PSK signal s(t) 0 0 Time Time Time Time 36

37 Quadrature Phase Shift Keying (QPSK) Four different phase shifts used are: φ φ φ φ, 0,0 0,,0 = 0 = π / 2 = π = 3π / 2 or φ φ φ 0,0 0,,0 φ, = π / 4 = 3π / 4 = 3π / 4 = π / 4 I (in-phase) and Q (quadrature) modulation used 37

38 QPSK Signal Constellation Q Q 0, 0 I, 0,0 I,0 (a) BPSK (Binary Phase Shift Keying) (b) QPSK (Quadrature Phase Shift Keying) 38

39 π/4 QPSK The phase of the carrier is: θ + φ k = θk k Where θ k is carrier phase shift corresponding to input bit pairs. If θ k =0, input bit stream is [0], then: θ = θ + φ = π / 4 θ 2 0 = θ + φ = π / 4 + π / 4 = 2 0 All possible states in π/4 QPSK 39

40 Quadrature Amplitude Modulation (QAM) Combination of AM and PSK: modulate signals using two measures of amplitude and four possible phase shifts A representative QAM Table Bit sequence represented Amplitude Phase shift π/2 0 2 π/2 00 π 0 2 π 0 3π/2 2 3π/2 40

41 Quadrature Amplitude Modulation (QAM) Two carriers out of phase by 90 deg are amplitude modulated Q I Rectangular constellation of 6QAM 4

42 Other Constellations of 6QAM (a) 8 phases, 4 amplitudes (b) 8 phases, 2 amplitudes 42

Chapter 7. Multiple Division Techniques

Chapter 7. Multiple Division Techniques Chapter 7 Multiple Division Techniques 1 Outline Frequency Division Multiple Access (FDMA) Division Multiple Access (TDMA) Code Division Multiple Access (CDMA) Comparison of FDMA, TDMA, and CDMA Walsh