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Chapter 7

Chapter 7. Multiple Division and Signal Encoding (Modulation) Techniques. Outline. Introduction Concepts and Models for Multiple Divisions Frequency Division Multiple Access (FDMA) Time Division Multiple Access (TDMA) Code Division Multiple Access (CDMA)

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Chapter 7

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  1. Chapter 7 Multiple Division and Signal Encoding (Modulation) Techniques

  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) • p/4QPSK • Quadrature Amplitude Modulation (QAM) • 16QAM

  3. Signaling Criteria • RF communications with binary data has been a very tough problem, especially for wireless • What determines how successful a receiver will be in interpreting an incoming signal? • Signal-to-noise ratio (or Eb/No) • Data rate (R) • Bandwidth (B) • Encoding scheme • An increase in data rate increases bit error rate • An increase in SNR decreases bit error rate • An increase in bandwidth allows an increase in data rate

  4. Concepts and Models for Multiple Divisions of Signals • 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

  5. User n … User 2 User 1 Time • Single channel per carrier • All first generation systems use FDMA Frequency Division Multiple Access (FDMA) Frequency

  6. Basic Structure of FDMA f1’ MS #1 f1 f2’ MS #2 f2 … … … fn’ MS #n fn BS Reverse channels (Uplink) Forward channels (Downlink)

  7. f1’ f2’ fn’ f1 f2 fn … … Reverse channels Forward channels Guard Band Wg Sub Band Wc … 1 2 3 4 N Frequency Total Bandwidth W = NWc Forward and Reverse channels in FDMA and Guard Band Protecting bandwidth

  8. Frequency User 1 User n User 2 … Time • Multiple channels per carrier • Most of second generation systems use TDMA Time Division Multiple Access (TDMA)

  9. Frequency f … … … #1 #1 t … … … #2 #2 t … … … #n #n t Frame Frame BS Forward channels (Downlink) The Concept of TDMA Slot Frequency f ’ … … … #1 #1 t MS #1 … … … #2 #2 t MS #2 … … … … #n #n MS #n t Frame Frame Reverse channels (Uplink)

  10. f ’ Frame Frame Frame #1 #2 #1 #2 #n #1 #2 #n #n … … … t (b). Reverse channel TDMA/FDD: Channel Structure f Frame Frame Frame #1 #2 #1 #2 #n #1 #2 #n #n … … … t (a). Forward channel

  11. Forward and Reverse Channels in TDMA Frame Frame Frequency f = f ’ #n #1 #2 #1 #2 #n #1 #2 #n #n #1 #2 … … … … Time Reverse channel Forward channel Reverse channel Forward channel Channels in TDMA/TDD

  12. Frame Structure of TDMA Frame Frame Frame Frequency #1 #1 #1 #2 #2 #n #2 #n #n … … … Time Head Data Guard time

  13. Frequency User 1 User 2 . . . User n Time Code • Users share bandwidth by using code sequences that are orthogonal to each other • Some second generation systems use CDMA • Most of third generation systems use CDMA Code Division Multiple Access (CDMA)

  14. Structure of a CDMA System Frequency f ’ Frequency f C1’ C1 MS #1 C2’ C2 MS #2 … … … Cn’ Cn MS #n BS Reverse channels (Uplink) Forward channels (Downlink) Ci’ x Cj’ = 0, i.e., Ci’ and Cj’ are orthogonal codes, Ci x Cj = 0, i.e., Ci and Cj are orthogonal codes

  15. Code-Division Multiple Access(CDMA) • Basic Principles of CDMA (a multiplexing scheme used with spread spectrum) • D = rate of data signal • Break each bit into kchips • Chips are a user-specific fixed pattern • This pattern is called the User’s Code • The codes are orthogonal (limited set) • Chip data rate of new channel = kDchips/sec

  16. CDMA • If k = 6 and code is a sequence of 1’s and -1’s • For each ‘1’ bit, A sends user code as a chip pattern • <c1, c2, c3, c4, c5, c6> • For each ‘0’ bit (-1), A sends complement of user code • <-c1, -c2, -c3, -c4, -c5, -c6> • Receiver knows sender’s code and performs decode function (assume synchronized so that the receiver knows when to apply the user code ) • < d1, d2, d3, d4, d5, d6 > = received chip pattern • < c1, c2, c3, c4, c5, c6 > = sender’s code

  17. 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 decoded results: + 6 which translates into 1 • User A ‘0’ bit: - 6  binary 0 • User B ‘1’ or ‘0’ bit decoded result: 0  signal ignored, SA signal decode results in a value of 0 which is different from a decode value of +/- 6 for transmitted bits ‘1’ or ‘0’ from User A

  18. CDMA Example Continued SA(1,1,-1,-1,1,1) = 1 X 1 + 1 X (-1) + (-1) X (-1) + (-1) X 1 + 1 X (-1) + 1 X 1 = 0 SA(-1,-1,1,1,-1,-1) = (-1) X 1 + (-1) X (-1) + 1 X (-1) + 1 X 1 + (-1) X (-1) + (-1) X 1 = 0

  19. Top Case B sends a 1  SB = 8 Bottom Case B sends a 0  SB = - 4

  20. Transmissions from B and C, receiver attempts recovery using A’s codeword (an error situation) Decode result: SA = 0 for this case where B and C have sent data and we attempt to recover a transmission from A . Different receiver codeword can result in Sx that is non-zero but much less than the correct orthogonal result.

  21. Spreading signal m(t) Digital signal s(t) Spreading Power Power Code c(t) Frequency Frequency Spread Spectrum Spreading of data signal s(t) by the signal c(t)results in message signal m(t) as: Two Types of m(t) DSSS and FHSS c(t) is a pseudo-noise (PN) sequence or a pseudo-noise code. 1-bit of c(t) is a chip

  22. Spread Spectrum • Input is fed into a channel encoder • Produces analog signal with narrow bandwidth • Signal is then further modulated using a spreading scheme of binary digits • Spread by orthogonal codes or a spreading sequence • Spreading sequences are generated by PN (pseudonoise/pseudo-random number) generators • Both spreading types (codes & PN sequences) can be used in the same system as is the case in cell systems • Effect of modulation is to increase the bandwidth of the signal to be transmitted (as shown in the previous slide)

  23. Spread Spectrum (continued) • What can be gained from apparent waste of spectrum? • Immunity from various kinds of noise and multipath distortion • Can be used for hiding and encrypting signals • Several users can independently use the same bandwidth with very little interference (local separation) • Provides dynamic system load flexibility • Produces isolation of cell users from each other (separation), both locally (cell) and globally (cluster)

  24. Spread Spectrum (continued) • On receiving end, the same spreading scheme is used to demodulate the spread spectrum signal • Has the effect of narrowing the bandwidth of the desired signal (thus reducing the noise and improving the SNR) • The signal is then fed into a channel decoder to recover the data. • Originally used by the military for secure/secret comm while lowering the probability of jamming. • The original idea (FHSS) was conceived by a Hollywood star (HedyLamarr) in 1940; patent granted in 1942; realized in 1957 by Sylvania. First operational use during the Cuban missile crisis in 1962 after the patent had expired.

  25. Processing Gain of Spread Spectrum

  26. Digital Message s(t) Power Power Power Frequency Frequency Frequency Direct Sequence Spread Spectrum (DSSS) DSSS Receiver Output Receiver not shown Transmitted Signal Sss(t)

  27. Direct Sequence Spread Spectrum (DSSS) • Each bit in the original signal is represented by multiple bits in the transmitted signal • Spreading code spreads the signal across a wider frequency band • The amount of spreading is in direct proportion to number of pseudonoise (PN) bits used or in other words related to the PN bit rate  bandwidth • One technique combines digital information stream with the spreading code bit stream(PN bit stream) using exclusive-OR

  28. Direct Sequence Spread Spectrum (DSSS)

  29. CDMA for Direct Sequence Spread Spectrum BPSK Demodulator CDMA User #1’s Code Signal frequency Spreading Code of User 1 BPSK The use of the code at the receiving end has the effect of narrowing the bandwidth for the specific user. User n’s Code Including noise

  30. Frequency Hopping Spread Spectrum (FHSS) Digital Data Power Power Power Frequency Frequency Frequency FHSS Receiver Output Receiver Not Shown Frequency-hopping Signal

  31. Frequency Hoping Spread Spectrum(FHSS) • Signal is broadcast over seemingly random series of radio frequencies • A number of channels allocated for the FH signal • Width of each channel corresponds to bandwidth of input signal • Signal hops from frequency to frequency at fixed intervals • Transmitter operates in one channel at a time • Bits are transmitted using some encoding scheme • At each successive interval, a new carrier frequency is selected (IEEE 802.11 standard uses 300 mS intervals)

  32. Frequency Hoping Spread Spectrum • Channel sequence dictated by spreading code • Receiver, hopping between frequencies in synchronization with transmitter using the same spreading code, receives the message using the same (known to user) encoding scheme as the transmitter • Spreading code = c(t) also known as chipping code • Frequency changes can be slow(equal or greater than the signal element time) or fast(changes within the signal element itself) • Advantages • Eavesdroppers hear only unintelligible blips • Attempts to jam signal on one frequency succeed only at knocking out a few bits

  33. An Example of Frequency Hopping Pattern Frequency Time

  34. Categories of Spreading Sequences • Spreading Sequence Categories • PN sequences (pseudonoise) • 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 • Spreading codes result in a higher transmitted data rate  increased bandwidth; increased system redundancy (jamming resilient);the spreading codes are noise like in their appearance

  35. Near-far Problem MS2 BS MS1 Received signal strength Distance Distance 0 d2 MS2 BS MS1 d1

  36. Received Signals at BS Reception of CDMA signals at BS (reverse link) requires equal power levels from all MSs in the cell. MS1 is causing adjacent channel interference to MS2 Power MS1 MS2 Solution ? Frequency f1 f2

  37. Power Control in CDMA Controlling transmitted (effective) power affects the CIR Pr= Received power in free space (Units of Pr and Pt must be the same) Pt = Transmitted power (usually dB or dBm) d = Distance between receiver and transmitter f = Frequency of transmission c = Speed of light (3 X 108 m/s if d is in meters and f is in Hz) a = Attenuation constant (2 to 4)

  38. CDMA Advantages for a Cellular Network • Transmission is in the form of a Direct Sequence Spread Spectrum (DSSS) which provides • Frequency diversity – frequency-dependent transmission impairments (e.g. fading) have less effect on signal which is spread over a large bandwidth • Multipath resistance – chipping codes used for CDMA exhibit low cross correlation and low autocorrelation thus a signal delayed by more than 1 chip interval does not interfere with it’s own stronger/direct signal. • Privacy – privacy is inherent. For DSSS, each user has a unique code resulting in spread spectrum/noise-like signals • Graceful degradation – system only gradually degrades (SNR  error rate increase) as more users access the system up to the point of an unacceptable error rate

  39. Drawbacks of CDMA Cellular • Self-jamming – unless all of the MS are perfectly synchronized, the arriving transmissions will not be perfectly aligned on chip boundaries  interference. Requires very accurate timing sources (GPS receiver disciplined clock). No time or frequency guard bands as in TDMA and FDMA. • Near-far problem – signals closer to the receiver are received with less attenuation than signals farther away and given lack of complete orthogonality, distant stations more difficult to recover – power control very important • Soft handoff – requires that the mobile acquire the new cell before it relinquishes the old; this is more complex than hard handoff used in FDMA and TDMA schemes

  40. Comparison of various Multiple Division Techniques

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