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Chapter 7 Multiple Division Techniques for Traffic Channels

Chapter 7 Multiple Division Techniques for Traffic Channels. 曾志成 國立宜蘭大學 電機工程學系 tsengcc@niu.edu.tw. 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

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Chapter 7 Multiple Division Techniques for Traffic Channels

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  1. Chapter 7Multiple Division Techniques for Traffic Channels 曾志成 國立宜蘭大學 電機工程學系 tsengcc@niu.edu.tw EE of NIU

  2. 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) • 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 • Spacially separable sectors: SDMA EE of NIU

  3. Frequency Division Multiple Access (FDMA) • Orthogonality conditions of two signals in FDMA • NO overlapping frequency in the frequency domain F for the signals si(f,t) and sj(f,t). • Two signals do not interfere with each other. • Single channel per carrier. • All first generation systems use FDMA. • A frequency synthesizer is used to adjust and maintain the Tx and Rx frequencies. EE of NIU

  4. User n … User 2 User 1 Time The Concept of FDMA Frequency EE of NIU

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

  6. 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 Frequency Protecting bandwidth EE of NIU

  7. … Frequency User 1 User 2 User n Time Time Division Multiple Access (TDMA) • Orthogonality conditions of two signals in TDMA: • Multiple channels per carrier • Most of 2G systems use TDMA EE of NIU

  8. Frequency f … … … #1 #1 t … … … #2 #2 t … … … #n #n t Frame Frame BS Forward channels (Downlink) The Basic Structure of A TDMA System Slot Frequencyf ’ … … … #1 #1 t MS #1 … … … #2 #2 t MS #2 … … … … #n #n MS #n t Frame Frame Reverse channels (Uplink) EE of NIU

  9. f ’ Frame Frame Frame #1 #2 #1 #2 #n #2 #n #n … … … t (b). Reverse channel Structure of Forward and Reverse Channels in a TDMA/FDD System f Frame Frame Frame #1 #2 #1 #2 #n #1 #2 #n #n … … … t (a). Forward channel #1 EE of NIU

  10. Structure of Forward and Reverse Channels in a TDMA/TDD System Frequency f = f ’ Frame Frame #n #1 #2 #1 #2 #n #1 #2 #n #n #1 #2 … … … … Time Forward channel Reverse channel Forward channel Reverse channel EE of NIU

  11. Frame Structure of TDMA Frame Frame Frame Frequency #1 #2 #2 #n #2 #n #n #1 #1 … … … Time Header Data Guard time EE of NIU

  12. Code Division Multiple Access (CDMA) • Orthogonality conditions of two signals in CDMA • NO overlapping of signals in code axis C for signals si(t) and sj(t). • Signals do not have any common codes in the code space • Users share bandwidth by using code sequences that are orthogonal to each other • Some 2G systems use CDMA • Most of 3G systems use CDMA EE of NIU

  13. Frequency User 1 . User 2 . . User n Time Code The Concept of CDMA EE of NIU

  14. d0 = 1 1 1 1 1 1 1 d1 = -1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 - M Di = ΣZi,m.cm m=1 M d0 = 1 d1 = -1 CDMA Encode/Decode Channel output Zi,m Zi,m= di.cm Data bits Sender slot 0 Channel output slot 1 Channel output Code Slot 1 Slot 0 Received input Slot 0 channel output Slot 1 channel output Code Receiver Slot 1 Slot 0 EE of NIU

  15. CDMA: Two-Sender Interference EE of NIU

  16. 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’ Cj’ = 0, i.e., Ci’ and Cj’ are orthogonal codes Ci Cj = 0, i.e., Ci and Cj are orthogonal codes EE of NIU

  17. Spreading Digital signal s(t) Spreading signal m(t) Power Power Code c(t) Frequency Frequency Spread Spectrum • Spreading of data signal s(t) by the code signal c(t) to result in message signal m(t)as: EE of NIU

  18. Transmitter Receiver Spreading Despread Digital signal s(t) Digital signal s(t) Spreading signal m(t) Code c(t) Code c(t) Power Power Power Frequency Frequency Frequency Direct Sequence Spread Spectrum (DSSS) EE of NIU

  19. Transmitter Receiver Spreading Despread Digital signal s(t) Spreading signal Digital signal Hopping pattern Hopping pattern Power Power Power Frequency Frequency Frequency Frequency Hopping Spread Spectrum (FHSS) EE of NIU

  20. An Example of Frequency Hopping Pattern Frequency Time EE of NIU

  21. Example of Orthogonal Codes: Walsh Codes t Wal (0, t) Wal (1, 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 EE of NIU

  22. Near-Far Problem • The near-far problem stems from a wide range of signal levels received in wireless and mobile communication system. MS2 BS MS1 Received signal strength 0 Distance Distance d2 d1 MS2 BS MS1 EE of NIU

  23. Adjacent Channel Interference Power MS1 MS2 Frequency f1 f2 EE of NIU

  24. Interference baseband signals Spectrum spreading signal Despread signal Baseband signal Interference signals 0 f f Frequency Frequency Frequency Interference in Spread Spectrum EE of NIU

  25. Power Control in CDMA • Controlling transmitted power affects the CIR • Pr: Received power in free space • Pt: Transmitted power • d: Distance between receiver and transmitter • f: Frequency of transmission • c: Speed of light • a: Attenuation constant (2 to 4) EE of NIU

  26. Orthogoanl Frequency Division Modulation (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 • “*” means a complex conjugate relation EE of NIU

  27. Spectrun of an OFDM signal with multiple subchannels The Frequency Spectrum of An OFDM Signal Spectrum of a single OFDM subchannel If there is no crossing of other channels at the center frequency of each subcarrier in the frequency domain, the ISI is zero. EE of NIU

  28. Modulation/Demodulation Operations of The OFDM Transmitter/Receiver • The Tx converts high-speed data streams into n parallel low-speed bit streams. • Modulated and mixed the n parallel bit streams. • Insert the guard time to reduce ISI. • The receiver take the reverse actions. EE of NIU

  29. Modulation/Demodulation Steps in OFDM Low speed bit stream N1 Guard interval insertion IDFT Serial to parallel conversion Transmission of OFDM signal High speed N2 …. data stream Nn Modulation operation at the OFDM transmitter N1 Parallel to serial conversion Guard interval removal DFT High speed N2 Received OFDM signal data stream …. Nn Demodulation steps at the OFDM receiver EE of NIU

  30. s(f,t,c) Beam i s(f,t,c) Beam 3 s(f,t,c) Beam 2 s(f,t,c) s(f,t,c) Beam 1 Beam n Space Division Multiple Access (SDMA) • By using the smart antenna, the omi-directional communication space is divided into spatially separable sectors. EE of NIU

  31. The Basic Structure of A SDMA System Beam1 Beam 2 Beam 3 MS3 MS2 BS MS1 EE of NIU

  32. Comparison of various Multiple Division Techniques EE of NIU

  33. Modulation Techniques • Why need modulation? • Small antenna size • Antenna size is inversely proportional to frequency • 3 kHz  50 km antenna • 3 GHz  5 cm antenna • Limits noise and interference, • e.g., FM (Frequency Modulation) EE of NIU

  34. Amplitude 1 0 1 1 1 0 + Time 0 bit Analog and Digital Signals • Analog Signal (Continuous signal) • Digital Signal (Discrete signal) Amplitude S(t) Time 0 EE of NIU

  35. Hearing, Speech, and Voice-band Channels Human hearing Human speech Voice-grade Telephone channel .. Frequency (Hz) 100 10,000 Pass band Guard band Guard band Frequency cutoff point Frequency (Hz) 180 200 3,500 4,000 EE of NIU

  36. Amplitude Modulation (AM) (1/2) • AM is the first method ever used to transfer voice info. from one place to another. • The modulated carrier signal s(t) is: • Acos(2pfct) is the carrier signal with amplitude A and carrier frequency fc • x(t) is the modulating signal • x(t)cos(2pfct) represents a double sideband (DSB) signal • Disadvantage: power inefficient EE of NIU

  37. Amplitude Modulation (AM) (2/2) Figure source: http://en.wikipedia.org/wiki/Amplitude_modulation EE of NIU

  38. Frequency Modulation (FM) (1/2) • FM is a method of integrating the information signal with an alternating current wave by varying the instantaneous frequency of the wave. • The modulated carrier signal s(t) is: • fD is the peak frequency deviation from the original frequency and fD << fc. • BW=2(b+1)fm with b=fD/fm and fm is the maximum modulating frequencyused. EE of NIU

  39. Frequency Modulation (FM) (2/2) Message signal x(t) Time Carrier signal Time FM signal s(t) Time EE of NIU

  40. Frequency Shift Keying (FSK) • 1/0 represented by two different frequencies. Carrier signal ‘1’ for message signal ‘1’ Time Carrier signal 2 for message signal ‘0’ Time 1 0 1 1 0 1 Message signal x(t) Time FSK signal s(t) Time EE of NIU

  41. Phase Shift Keying (PSK) • Use alternative sine wave phases to encode bits • PSK has a perfect SNR but must be demodulated synchronously, which makes the demodulation circuit complex. Carrier signal Time Carrier signal Time 1 0 1 1 0 1 Message signal x(t) Time PSK signal s(t) Time EE of NIU

  42. Quadrature Phase Shift Keying (QPSK) • Four different phase shifts used are: or EE of NIU

  43. BPSK/QPSK Signal Constellation • I (in-phase) and Q (quadrature) modulation is used to implement the QPSK. Q Q 0,1 1,1 0,0 1 0 I I 1,0 (b) QPSK (Quadrature Phase Shift Keying) • BPSK • (Binary Phase Shift Keying) EE of NIU

  44. /4 QPSK • In π/4QPSK, the input sequence is encoded by the changes in the amplitude and direction of the phase shift and not in the absolute position in the constellation. • π/4QPSK uses 2 QPSK constellations offset by p/4. • Signaling elements are selected in turn from the two QPSK constellations so that each symbol has a phase change. • π/4QPSK can be noncoherently demodulated. EE of NIU

  45. Carrier Phase in π/4QPSK • The phase of the carrier k is: • kis carrier phase shift corresponding to input bit pairs. • If q0=0, input bit stream is [1011], then EE of NIU

  46. 8 Quadrature Amplitude Modulation (8-QAM) • Combination of AM and PSK: modulate signals using two measures of amplitudes and four possible phase shifts • A representative 8-QAM table EE of NIU

  47. Rectangular Constellation of 16-QAM • Splitting the signal into 12 different phases and 3 different amplitudes for a total of 16 different possible values, each encoding 4 bits. Q 1000 0000 1100 0100 0001 1001 1101 0101 I 0011 0111 1011 1111 1110 0110 0010 1010 EE of NIU

  48. Homework • P7.2 • P7.3 • P7.17 • P7.18 • P7.20 EE of NIU

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