Đặng Thanh Bình. Chapter 3 Modulation Techniques

Transcription

1 Đặng Thanh Bình Chapter 3 Modulation Techniques

2 Contents Modulation Concept Definitions Wireless Modulation Components Bit/Baud Comparison Why Modulate? Advantages of Digital Modulation

3 Contents Modulation Techniques Analog Modulation AM, FM Digital Modulation ASK, FSK, PSK, QAM Spread Spectrum CSS, DSSS, FHSS, THSS Orthogonal Frequency-Division Multiplexing

4 MODULATION CONCEPT

5 Modulation for Wireless Modulation is the process of encoding information from a message source in a manner suitable for transmission How does it work? In modulation, a message signal, which contains the information is used to control the parameters of a carrier signal, so as to impress the information onto the carrier. In general it involves translating a baseband signal (source signal) to a modulated signal at a higher frequency (the carrier frequency, f c )

6 Digital Modulation Digital modulation is the process by which a sequence of pulses (message) of duration T is transformed into a sequence of sinusoidal waveforms, s(t) of duration T. The general form of the modulated signal is: Digital modulation can then be defined as the process whereby the amplitude, frequency, phase or a combination of them is varied in accordance with the information to be transmitted

7 Main Wireless Modulation Components

8 Main Wireless Modulation Components The Messages (Information source) The information source produces the contents of the message to be transmitted over the link. Information sources fall into two basic categories: Analog: Information takes the form of a continuous function of time. Digital: Information takes the form of a sequence (or file) of discrete values often 0 s and 1 s. The message signal could also be a multilevel signal, rather than binary; this is not considered here.

9 Main Wireless Modulation Components The Messages (Information source)

10 Main Wireless Modulation Components Media: Carrier The channel has certain types of signals that are easily transmitted - known as carriers. Basically, the modulator works by putting the source information onto a carrier. For physical channels, sinusoidal signals are the most suitable carriers.

11 Main Wireless Modulation Components Media: Carrier

12 Main Wireless Modulation Components Modulator/Demodulator The modulator converts the source information into a signal that can be sent through the channel At the other end of the channel, the demodulator reconverts the signal received through the channel into its original form. For two-way (i.e., duplex) communication, both ends of the link have a modulator and a demodulator, a combination known as a modem. What is half-duplex? Full-duplex?

13 Main Wireless Modulation Components Review Full-Duplex (Song công toàn phần) Half-Duplex (Bán song công)

14 Bit Rate / Baud Rate Bit rate is the number of bits per second. Bit rate is important in computer efficiency Baud rate is the number of signal units per second. Baud rate is less than or equal to the bit rate. Baud rate is important in data transmission. Baud rate determines the bandwidth required to send signal Baud rate = bit rate / # bits per signal unit

15 Bit Rate / Baud Rate- Example Example 1: An analog signal carries 4 bits in each signal unit. If 1000 signal units are sent per second, find the baud rate and the bit rate Baud rate = 1000 bauds per second (baud/s) Bit rate = 1000 x 4 = 4000 bps

16 Bit Rate / Baud Rate- Example Example 2: The bit rate of a signal is If each signal unit carries 6 bits, what is the baud rate? Baud rate = 3000/6 =500 bauds/sec

17 Why modulate? Ease of radiation Reduce antenna size: the size of an antenna is proportional to the signal wavelength. By increasing the carrier frequency, the wavelength decreases. The size of antenna λ/4 = c/4f e.g., 3 khz 50 km antenna e.g., 3 GHz 5 cm antenna Simultaneous transmission of several signals FDM (Frequency Division Modulation) Reduce the influence of interference Frequency Hopping

18 Advantages of Digital Modulation Spectral efficiency use of a narrow bandwidth to send a large amount of data Good privacy and security features Digital encryption techniques may be employed greater noise immunity and robustness to channel impairments Lower power consumption Repeatable, more easily produced, more flexibility Reduced device size Easier multiplexing

19 Hearing, Speech & Voice-band Channels

20 ANALOG MODULATION TECHNIQUES

21 Amplitude Modulation (AM) Amplitude of carrier signal is varied as the message signal to be transmitted. Frequency of carrier signal is kept constant

22 Frequency Modulation (FM) FM integrates message signal with carrier signal by varying the instantaneous frequency. Amplitude of carrier signal is kept constant

23 DIGITAL MODULATION TECHNIQUES

24 Basic Digital Modulation Techniques Types of digital-to-analog modulation:

25 Amplitude Shift Keying (ASK) The strength of the carrier signal is varied to represent binary 1 and 0. Frequency and phase remains the same.

26 Amplitude Shift Keying (ASK) Highly susceptible to noise interference.

27 Review: Narrowband vs Wideband

28 Frequency Shift Keying (FSK) Frequency of the carrier is varied to represent digital data (binary 0/1) Peak amplitude and phase remain constant. Avoid noise interference by looking at frequencies (change of a signal) and ignoring amplitudes. Limitations of FSK is the physical capabilities of the carrier.

29 Frequency Shift Keying (FSK)

30 Phase Shift Keying (PSK) Phase of the carrier is varied to represent digital data (binary 0 or 1), i.e., Binary PSK (BPSK) Amplitude and frequency remains constant. Phases are separated by 180 degrees. If phase 0 deg. to represent 0, 180 deg. to represent 1. (2- PSK) PSK is not susceptible to noise degradation that affects ASK or bandwidth limitations of FSK Simple to implement, inefficient use of bandwidth. Very robust, used extensively in satellite communication.

31 Phase Shift Keying (PSK)

32 Phase Shift Keying (PSK)

33 Quadrature Phase Shift Keying (QPSK) The essence of quadrature modulation methods is the application of complementary pairs of amplitude to two simultaneous sinusoidal waves differing in phase by one-quarter of a cycle. Sinusoidal waves (of the same frequency) with a phase difference of a quarter (or three-quarters) of a cycle are said to be in a quadrature phase relationship. It is customary to refer to one of these waves as the I wave, or in-phase wave, and the other as the Q wave, or quadrature wave.

34 Quadrature Phase Shift Keying (QPSK)

35 Quadrature Phase Shift Keying (QPSK) In fact, each symbol uses the addition of an I wave and a Q wave, giving a total of four possible symbols. To each possible waveform is allocated one of the four, 2-bit binary combinations 00, 01, 10 or 11, so any binary bit-stream can be transmitted by an appropriate sequence of sinusoidal symbols.

36 Quadrature Phase Shift Keying (QPSK) QPSK can achieve twice the data rate of a comparable BPSK scheme for a given bandwidth

39 Constellation Diagrams A constellation diagram helps us to define the amplitude and phase of a signal when we are using two carriers, one in quadrature of the other. The X-axis represents the in-phase carrier and the Y-axis represents quadrature carrier

40 Constellation Diagrams

41 Constellation Diagrams

42 QPSK Constellation diagram

43 QPSK Constellation diagram

44 ASK, BPSK, QPSK Constellation Diagrams

45 π/4 QPSK Widely used in the majority of digital radio modems This variant of QPSK uses two identical constellations which are rotated by 45 (π/4 radians, hence the name) with respect to one another (two QPSK constellations offset by ±π/4). reduces the phase-shifts from a maximum of 180, but only to a maximum of 135 Eliminates Zero Crossings Usually, either the even or odd symbols are used to select points from one of the constellations and the other symbols select points from the other constellation

46 π/4 QPSK Example The binary data that is conveyed by this waveform is: The odd bits, highlighted here, contribute to the in-phase component: The even bits, highlighted here, contribute to the quadrature-phase component: Thus, the first symbol (1 1) is taken from the 'blue' constellation and the second symbol (0 0) is taken from the 'green' constellation

47 π/4 QPSK Example

48 Quadrature Amplitude Modulation PSK is limited by the ability of the equipment to distinguish between small differences in phases. Limits the potential data rate. If multiple pairs of Q and I amplitude (say 1 and -1; and 3 and -3) are allowed, then more symbols become available. This is the principle of quadrature amplitude modulation, or QAM, which you can think of as the application ofask to QPSK (or PSK). We can have x variations in phase and y variations of amplitude x y possible variation (greater data rates)

49 Quadrature Amplitude Modulation

52 Quadrature Amplitude Modulation 64-QAM 5 bits per symbol

53 Why Not Just Keep Going? Errors in IQ modulation create symbol errors in transmission Noise in the transmission channel create symbol errors Inaccuracies in the receiver creates errors Signal-to-noise ratio (SNR) requirements increase with higher order modulations

54 Why Not Just Keep Going?

55 SPREAD SPECTRUM

56 Spread Spectrum Principles The signal occupies a bandwidth much larger than is needed for the information signal The spread spectrum modulation is done using a spreading code, which is independent of the data in the signal Despreading at the receiver is done by correlating the received signal with a synchronized copy of the spreading code Developed initially for military applications Basis for CDMA

57 Spread Spectrum Principles In spread spectrum (SS), we combine signals from different sources to fit into a larger bandwidth, but our goals are to prevent eavesdropping and jamming. To achieve these goals, spread spectrum techniques add redundancy.

58 Spread Spectrum Procedure Input fed into channel encoder Produces narrow bandwidth analog signal around central frequency Signal modulated using sequence of digits Spreading code/sequence Typically generated by pseudonoise/pseudorandom number generator Increase bandwidth significantly Receiver uses the same sequence to demodulate signal Demodulated signal fed in to channel decoder

59 Spread Spectrum Advantages Anti-jamming Interfrence rejection Message security and privacy Low probability of interception

60 Spread Spectrum Types Frequency-hopping spread spectrum (FHSS) Direct-sequence spread spectrum (DSSS) Chirp spread spectrum (CSS) Time-hopping spread spectrum (THSS)

61 FHSS Rapidly switching a carrier among many frequency channels, using a pseudorandom sequence known to both transmitter and receiver

62 FHSS

63 Frequency selection in FHSS

64 FHSS cycles

65 Bandwidth sharing

66 DSSS Each bit in the original signal is represented by multiple bits (chip code) in the transmitted signal The chipping code spreads the signal across a wider frequency band in direct proportion to the number of bits used

67 DSSS

68 DSSS example

69 ORTHOGONAL FREQUENCY- DIVISION MULTIPLEXING (OFDM)

70 OFDM and Multicarrier Transmission Single carrier transmission The concept of single-carrier is that each user transmits and receives data stream with only one carrier at any time. Multicarrier transmission The concept of multi-carrier transmission is that a user can employ a number of carriers to transmit data simultaneously.

71 OFDM and Multicarrier Transmission Single carrier transmission The concept of single-carrier is that each user transmits and receives data stream with only one carrier at any time. Multicarrier transmission The concept of multi-carrier transmission is that a user can employ a number of carriers to transmit data simultaneously.

72 OFDM and Multicarrier Transmission

73 OFDM and Multicarrier Transmission Orthogonal frequency division multiplexing (OFDM) technique is widely used in wireless communication nowadays. OFDM is a special case of a multi-carrier transmission technique, which : divides the available spectrum into many subcarriers, each one being modulated by a low data rate stream. modulated by a low data rate stream. splits data stream into N parallel streams of reduced data rate and transmit each on a separate subcarrier.

74 OFDM and Multicarrier Transmission OFDM carriers are frequency spaced by a multiple of 1/T, where T is the modulation period, and it is characterized by an overlap of the spectrum of the signals transmitted on different carriers.

76 OFDM and Multicarrier Transmission In radio wave bands, the carriers are typically separated by several kilohertz or more In OFDM, the subcarriers are typically only a few tens of hertz apart. Also: Conventional FDMA: each modulated carrier has data from a separate source OFDM: the modulated subcarriers usually carry data from a single source.

77 OFDM and Multicarrier Transmission OFDM modulation cannot be implemented via hardware with analog oscillators: it would be too much expensive and the imperfections of the oscillators (frequency drift, phase noise) would cause critical malfunctions. But it can easily implemented via software, in a totally digital way, using the FFT (Fast Fourier Transform).

78 OFDM and Multicarrier Transmission No N RF radio in both transmitter and receiver OFDM uses an efficient computational technique Discrete Fourier Transform (DFT) and its counterpart, the Inverse Discrete Fourier Transform (IDFT)- to replace sinusoidal generator. Implemented through Fast Fourier Transform (FFT) routines highly optimized Fast Fourier transform (FFT) is an efficient algorithm to compute the Discrete Fourier Transform (DFT) and its inverse.

81 OFDM Carriers OFDM = Orthogonal FDM OFDM symbol forms Rectangular Window of duration T Has a sinc(x)/x-spectrum with zeros at 1/ T Other carriers are put in these zeros sub- peak value of f1 is at zeroes of others carriers are orthogonal Difference between successive subcarrier is just one cycle

82 OFDM Carriers

83 OFDM Carriers

84 OFDM Spectrum Total Power spectrum is almost square shape Band width (W)= NΔf

85 OFDM Spectrum

86 OFDM In Time and Frequency Domain

87 OFDM: Multiplex or Modulation? OFDM can be viewed as either a modulation technique or a multiplex technique. Modulation technique Viewed by the relation between input and output signals Multiplex technique Viewed by the output signal which is the linear sum of the modulated signals

88 OFDM: Multiplex or Modulation? OFDM in its primary form is considered as a digital modulation technique, and not a multiuser channel access method, since it is utilized for transferring one bit stream over one communication channel using onesequence of OFDM symbols. However, OFDM can be combined with multiple access using time, frequency or coding separation of the users. Example OFDMA = OFDM + TDMA

89 Advantages of OFDM Allows carriers to overlap (no guard band as in FDMA), resulting in lesser wasted bandwidth without any Inter Carrier Interference (ICI) High data rate distributed over multiple carriers resulting in lower symbol rate (more immune to ISI) Permits higher data rate as compared to FDM Increased security and bandwidth efficiency possible using CDMA OFDM (MC-CDMA) Simple guard intervals make the system more robust to multipath effects.

90 OFDM Transmitter

91 OFDM Summary OFDM a multi-carrier modulation scheme Basic transmission unit OFDM symbol of a certain time duration An OFDM symbol consisting of multiple subcarriers Each subcarrier transports an information symbol (e.g., QPSK) The subcarriers are orthogonal Integral in time domain vanishing: Sampling at frequency domain: no mutual interference between subcarriers